Main-Chain Modification as a Result of Polyolefin Functionalization by Different Techniques [1 ed.] 9781617282522, 9781616687816

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Copyright © 2010. Nova Science Publishers, Incorporated. All rights reserved. Main-Chain Modification as a Result of Polyolefin Functionalization by Different Techniques, Nova Science Publishers,

POLYMER SCIENCE AND TECHNOLOGY

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MAIN-CHAIN MODIFICATION AS A RESULT OF POLYOLEFIN FUNCTIONALIZATION BY DIFFERENT TECHNIQUES

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Main-Chain Modification as a Result of Polyolefin Functionalization by Different Techniques, Nova Science Publishers,

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Main-Chain Modification as a Result of Polyolefin Functionalization by Different Techniques, Nova Science Publishers,

POLYMER SCIENCE AND TECHNOLOGY

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MAIN-CHAIN MODIFICATION AS A RESULT OF POLYOLEFIN FUNCTIONALIZATION BY DIFFERENT TECHNIQUES ROSESTELA PERERA CARMEN ROSALES CARMEN ALBANO AND

PEDRO SILVA

Nova Science Publishers, Inc. New York

Main-Chain Modification as a Result of Polyolefin Functionalization by Different Techniques, Nova Science Publishers,

Copyright © 2010 by Nova Science Publishers, Inc. All rights reserved. No part of this book may be reproduced, stored in a retrieval system or transmitted in any form or by any means: electronic, electrostatic, magnetic, tape, mechanical photocopying, recording or otherwise without the written permission of the Publisher.

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For permission to use material from this book please contact us: Telephone 631-231-7269; Fax 631-231-8175 Web Site: http://www.novapublishers.com NOTICE TO THE READER The Publisher has taken reasonable care in the preparation of this book, but makes no expressed or implied warranty of any kind and assumes no responsibility for any errors or omissions. No liability is assumed for incidental or consequential damages in connection with or arising out of information contained in this book. The Publisher shall not be liable for any special, consequential, or exemplary damages resulting, in whole or in part, from the readers’ use of, or reliance upon, this material. Independent verification should be sought for any data, advice or recommendations contained in this book. In addition, no responsibility is assumed by the publisher for any injury and/or damage to persons or property arising from any methods, products, instructions, ideas or otherwise contained in this publication. This publication is designed to provide accurate and authoritative information with regard to the subject matter covered herein. It is sold with the clear understanding that the Publisher is not engaged in rendering legal or any other professional services. If legal or any other expert assistance is required, the services of a competent person should be sought. FROM A DECLARATION OF PARTICIPANTS JOINTLY ADOPTED BY A COMMITTEE OF THE AMERICAN BAR ASSOCIATION AND A COMMITTEE OF PUBLISHERS. LIBRARY OF CONGRESS CATALOGING-IN-PUBLICATION DATA Main-chain modification as a result of polyolefin functionalization by different techniques / Rosestela Perera ... [et al.]. p. cm. Includes index. ISBN 978-1-61728-252-2 (eBook) 1. Polyolefins. 2. Graft copolymers. I. Perera, Rosestela. TP1180.P67M35 2009 668.4'234--dc22 2010016072

Published by Nova Science Publishers, Inc. © New York

Main-Chain Modification as a Result of Polyolefin Functionalization by Different Techniques, Nova Science Publishers,

CONTENTS vii 

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Preface

1 

Chapter 1

Introduction

Chapter 2

Techniques Used to Prepare Grafted Polythylenes

17 

Chapter 3

Secondary and/or Degradation Reactions in the Grafting Process

27 

Molecular Weight Distribution and Rheological Properties

35 

Thermal Properties and Thermogravimetric Analysis (TGA)

41 

Conclusion

55 

Chapter 4 Chapter 5 Chapter 6 References

57 

Index

61 

Main-Chain Modification as a Result of Polyolefin Functionalization by Different Techniques, Nova Science Publishers,

Copyright © 2010. Nova Science Publishers, Incorporated. All rights reserved. Main-Chain Modification as a Result of Polyolefin Functionalization by Different Techniques, Nova Science Publishers,

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PREFACE When polyolefins are functionalized to modify its chemical structure and polarity, some other secondary reactions take place as well. The grafting of a different monomer onto the main chain of the polymer involves the use of a free radical initiator, such as a peroxide, or irradiation of the sample (gamma rays or ultrasound), that not only provide the free radicals for the grafting but also for main chain modifications. Different reaction processes such as functionalization in solution, reactive extrusion or irradiation produce different structures. This work deals with modifications of the main chain as a consequence of polyolefin grafting through different techniques, such as functionalization in solution and reactive extrusion and through polymer irradiation with gamma rays and ultrasound. Different characterization methods such as Differential Scanning Calorimetry, Thermogravimetric Analysis, Gel Permeation Chromatography, Fourier Transform Infrared Spectroscopy, among others, are used to fully explain proposed mechanisms of polymer degradation and/or crosslinking.

Main-Chain Modification as a Result of Polyolefin Functionalization by Different Techniques, Nova Science Publishers,

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Chapter 1

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INTRODUCTION Polyolefins are polymers of high commercial interest due to characteristics such as low cost, easy processability and a wide range of physical-chemical properties. However, their use in blends with polar polymers and/or in the nanocomposites preparation has been limited due to their non-polar character. Polyolefins have been modified through grafting to overcome these inconveniences. Nevertheless, few systematic studies on this type of reactions have been conducted and problems such as low reaction efficiencies and grafting degrees (GD), changes in polymer processability, competition between monomer grafting versus homopolymerization and crosslinking or even degradation of polymer chains, arising from the complex coupling variables, are still unsolved. The grafting of maleic anhydride (MA), diethylmaleate (DEM), and dimethylamino ethyl methacrylate (DMAEMA) on polyolefins has been studied [1-7]. Such reactions have traditionally been carried out in solution with peroxides where a relatively homogeneous chemical environment is possible because the reactants are more easily mixed [1-2]. However, the effects of γ-radiation and ultrasonic- radiation on grafting polymers in solution have been less investigated [8-12]. Functionalization reactions carried out via reactive extrusion have several economic advantages, such as: functional polymers are produced without the construction of an entirely new facility, the process time is shortened and the cost of solvent recovery is avoided. In order to successfully use the extruders in reactive processing some important characteristics such as the residence time distribution, mixing and self-cleaning effects should be accurately controlled according to the polymer properties and usage. Consequently, the influence of temperature, mass flow rate, screw speed and screw

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2

Rosestela Perera, Carmen Rosales, Carmen Albano and Pedro Silva

configurations on the residence time distribution [13-16] must be taken into consideration. Besides, there are some important factors controlling the dispersion of chemical reactants such as diffusion and compatibility that should be also taken into account [17]. Secondary reactions are very important when the grafting of a monomer onto a polymer takes place, especially when the functionalization is carried out via extrusion with a peroxide or when γ-radiation and ultrasonic- radiation are used for producing radicals. Such reactions could be favored due to the high viscosity of the reaction media (which difficult the diffusion of the polar monomers towards the active sites), the high reactivity of the peroxide, the high temperatures during the process of functionalization in the extruder, the long residence time tails in the residence time distribution in reactive extrusion and the large amount of radicals formed through the radiation processes. This chapter deals with modifications of the main chain as a consequence of polyolefin grafting through different techniques, such as functionalization in solution and reactive extrusion and through polymer irradiation with gamma rays and ultrasound. Different characterization methods such as Differential Scanning Calorimetry, Thermogravimetric Analysis, Gel Permeation Chromatography, Fourier Transform Infrared Spectroscopy, among others, are used to fully explain proposed mechanisms of polymer degradation and/or crosslinking.

1.1. GAMMA RADIATION Radiation can produce several types of defects in solids. These defects are: (a) vacancies, (b) interstitial atoms, (c) impurity atoms and (d) ionization effects, among others. The ionization effects lie among the topics of this work and are related to the passage of charged particles or gamma-rays through a solid, causing extensive ionization and electronic excitation, which in turn lead to bond scission, free radical coloration, luminescence, etc., in many types of solids. These effects are important in insulators and dielectrics, ionic crystals, glasses, organic polymers, etc. The interaction of energetic radiation with matter is a complex phenomenon. Hence, it is useful to resolve it into primary and secondary stages. This analysis is based on the sequence of events following the arrival of a particle or quantum of energy, and implies nothing about the relative importance of the two stages in producing observable property changes.

Main-Chain Modification as a Result of Polyolefin Functionalization by Different Techniques, Nova Science Publishers,

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Introduction

3

The primary or direct effects consist in the displacement of electrons (ionization), the displacement of atoms from lattice sites, excitation of both atoms and electrons without displacement, and the transmutation of nuclei. Irradiation with gamma-rays produces only ionization as the important primary effect, atomic displacements sometimes resulting secondarily. Nuclear transmutations can, in principle, be produced by this kind of radiation, but occur to an appreciable extent. The secondary effects of the interaction of radiation with matter consist of further excitation and disruption of the structure by electrons and atoms which have been knocked on. The basic laws governing the secondary stages are in all cases the same as those governing the primary stage of charged particle bombardment. Gamma-rays produce ionization in all solids, and this is the most important effect. In insulators and polymers, chemical reactions may be promoted, some of which cannot be induced by other means. There is evidence that gamma-rays can produce displaced atoms in solids [18]. Ionizing radiation causes the formation of ions, and free radicals result upon neutralization of this ions. Both the ions and the free radicals may be chemically highly reactive. Displaced atoms, since they represent quite drastic disturbances in the solid, may have an important effect on any chemical reaction involving the solid itself, particularly on surface properties and surface reactions. Free radicals can be detected by measuring magnetic susceptibility and/or Electron Paramagnetic Resonance (EPR), which is the same as Electron Spin Resonance (ESR). The EPR technique can be used to study, the following effects of the ionizing radiations on the polymers, among others: polymerization processes, degradation processes in polymers, oxidation of polymers (antioxidants), graft copolymerization and crosslinking. The formation of free radicals in polymers exposed to the simultaneous action of γ-radiation and light has been studied by Russian scientists [19]. Free radical formation in polyethylene by high energy radiation (γ-rays) has been the subject of a large number of EPR studies [19-28]. Ionizing radiation of polyolefines induces excitation and ionization of the molecules. Irradiation of polyethylene gives way to the formation of an alkyl radical: •

− CH 2 − CH 2 − C H − CH 2 − CH 2 −

γ

β

α

β

γ

Main-Chain Modification as a Result of Polyolefin Functionalization by Different Techniques, Nova Science Publishers,

(1)

4

Rosestela Perera, Carmen Rosales, Carmen Albano and Pedro Silva

which is observed as a six-line EPR spectrum. Figure 1 shows the EPR spectrum of the high-density polyethylene (HDPE) irradiated at 900 kGy of integral dose. A multiple peak spectrum is observed due to different free radicals generated in the irradiation process. This spectrum can be interpreted as an overlapping of several spectra belonging to different paramagnetic centers and is a typical spectrum for a mixture of alkyl, allyl •

− CH 2 − C H − CH = CH − CH 2 −

(2)

and polyenyl free radicals •

− CH 2 − C H − (CH = CH )n − CH 2 −

(3)

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The peaks corresponding to the alkyl radical are marked with the number (1) in the spectrum of Figure 1, those corresponding to the allyl radical with the number (2) and the central and very intense peak is attributed to the polyenyl radical. The hyperfine splitting constant for alkyl radical is, in this case,

a1 = 17.5 G and that of the allyl radical a2 = 11.3 G [21]. The inset shows the spectra for 900 kGy, 30 days after irradiation. Most polymers show asymmetric single-line spectra attributed to

(

)

ROO• RO• radicals, which are formed after introduction of air to samples

( ) or irradiating the sample in presence of air.

containing free radicals R

•

There is some difficulty in distinguishing between alkyperoxy ROO • and alkoxy

(RO ) •

radicals, since both have the unpaired electron mainly

concentrated on oxygen atoms and neither shows a hyperfine interaction with alkyl protons. Oxidation and degradation of polyolefins proceed via a radical chain mechanism with initiation, propagation, branching and termination steps. This process is called “auto-oxidation” since it proceeds in a self-catalyzed manner when a natural or synthetic organic compound is exposed to oxygen. Under ionizing irradiation of solid polymers free radicals are formed which are trapped in a polymer matrix. Under certain conditions, these radicals can initiate graft polymerization. The grafting reaction is dependent on the physical state of the polymer and the properties of the free radicals formed in the polymer. Figure 2 is an example of the use of the EPR technique in the

Main-Chain Modification as a Result of Polyolefin Functionalization by Different Techniques, Nova Science Publishers,

Introduction

5

characterization of free radicals generated in the process of grafting polyethylene with diethyl maleate. In that figure, the total free radical concentrations after irradiation of two blends of HDPE/DEM are shown. In the time period 0 ≤ t ≤ 100 h the total free radical concentration falls for all the studied samples. This is the expected behavior in this kind of samples. After this period, the total free radicals concentration increases with time. A detailed study of the kind of radicals generated after the irradiation of the samples and their concentration is shown in Figure 3. A very noisy spectrum of six absorption lines is observed. The inset in the figure shows the effect of aging in the sample. As the time increases, the spectrum is better resolved, making separating the absorption lines in it possible. After a longer period of time, the spectrum is the typical one for a nitroxyl radical that corresponds to a hindered amine stabilizer. This is in agreement with an activation of the antioxidant under the effects of gamma irradiation [29].

1

dχ"/dH (A.U.)

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0

(1)

40000

dχ"/dH (A.U.)

(1)

2

20000

(a)

0

-20000

(1) (2)

(2)

-40000

330

335

340 H (mT)

(1)

345

350

(2)

(1) (2)

-1

(2)

(2)

(2)

-2

2 330 (b) 335

340

345

350

340

345

350

1 0

-1 -2

330

335

H (mT) Figure 1. EPR spectra of HDPE irradiated at: (a) 900 kGy, and (b) 150 kGy. The inset shows the spectra for 900 kGy, 30 days after irradiation.

Main-Chain Modification as a Result of Polyolefin Functionalization by Different Techniques, Nova Science Publishers,

Rosestela Perera, Carmen Rosales, Carmen Albano and Pedro Silva

6

15

Figure 4 shows the total free radical concentration (radicals/g) in highdensity, low-density and linear low-density polyethylenes (HDPE, LDPE, LLDPE) unirradiated, irradiated and post-irradiated after a storage time of one month. Figure 4a unfolds that the formation of radicals is small for small doses of radiation, the HDPE having the highest concentration. Upon increasing the dose of radiation, an increase in the concentration of radicals is observed, this phenomenon being most noticeable, as said, in the HDPE.

Spin/gr x 10

5

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Spin/gr x 10

15

4

16 14 12 10 8 6 4 2 0

0

2000

4000

6000

Time (h)

3 2

15% DEM 15 kGy 30% DEM 15 kGy 30% DEM 30 kGy

1 0 0

200

400

600

800

1000

1200

1400

Time (h) Figure 2. Total radical concentration for the blends studied. The inset shows the behavior of the blend HDPE/30% DEM irradiated at 15 kGy after up to 6000 hours.The initiation of a chain reaction via a single radical allows the formation of a sequence of crosslinks. Enhanced crosslinking occurs via a chain reaction involving both a polymer and a monomer. An EPR study of the damage over polymers led to the study of the total free radical concentration (TFRC) as a function of the integral dose supplied to the samples, as well as to the study of the TFRC decay and to the model that explains the generation and recombination of damages produced by irradiation.

After storage (Figure 4b), a decrease in the radical concentration in the polyethylenes (PEs) is obtained. However, a still rather high concentration is observed in the HDPE due to its higher degree of crystallinity, attributted to the fact that the radicals remain trapped in the crystalline regions. In LDPE and LLDPE, a very low radical concentration is observed due to very high

Main-Chain Modification as a Result of Polyolefin Functionalization by Different Techniques, Nova Science Publishers,

Introduction

7

reaction rates among themselves. Hence, it can be concluded that the radicals decay considerable more quickly in branched than in linear PEs. In summary, in all the homopolymers, the decrease in the radical concentration can be attributed to the easier recombination of those formed in the amorphous regions due to the higher chain mobility in those zones. The process of radical decay in crystalline polymers, especially in polyethylenes, involves two processes: one rapid (due mainly to the less stable radicals) and another slow. According to Dole et al. [30], the rapid decay in the radicals occurs in the amorphous phase and generally takes part at the beginning of the experiment. The slow process, on the other hand, is due to the diffusion of the radicals from inside the crystal to its surface where they react. That is why, after a month of storage, there is still a noticeable concentration of radicals in the sample of irradiated HDPE.

dχ"/dH [A. U.]

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One year later

330

330

340

350

340 H [mT]

360

350

370

H [mT] Figure 3. EPR spectra of HDPE/15% DEM in decalin irradiated at 15 kGy. The inset shows the spectrum one year later.

Main-Chain Modification as a Result of Polyolefin Functionalization by Different Techniques, Nova Science Publishers,

Rosestela Perera, Carmen Rosales, Carmen Albano and Pedro Silva

8

16 LLD P E LD P E H D PE

Radicals/g x 10

17

12

8

4

0 0

200

400

600

800

1000

Integral Dose (KGy)

LLDPE LDPE HDPE

17

Radicals/g x 10

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1.6

1.2

0.8

0.4

0.0 0

200

400

600

800

1000

Integral Dose (kGy) Figure 4. Radicals concentration at different radiation doses (a); and post-irradiated after a storage time of one month (b).The lines are a guide for the eyes.

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Introduction

9

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1.2. ULTRASONIC RADIATION Ultrasound is the name given to the study and application of the sound waves of frequencies higher that those human hearing can perceive (20 Kc/s). These applications include the technic of cleaning, drilling, chemical processes and production of emulsions. They take place either directly by the agitation of the waves or through the phenomenon of cavitation. High energy ultrasound waves can be considered those where a linear relationship does not exist between the applied stress and the resulting deformation. These waves exercise some effect on the medium through which they pass. The ultrasonically induced cavitation has the effect of promoting chemical reactions in the interior of certain liquids. This can be due to the electrolytic action caused by the appearance of opposed electric loads in the opposed ends of the bubbles. Other chemical effects are caused by the instantaneous and enormous increases of pressure, of the order of thousands of atmospheres, and of temperature, above hundreds of grades, in the vicinities of the cavities when their collapse takes place. It is also believed that they originate chemical changes as a result of the removal of energy from the resonant bubbles. Most studies of the effects of ultrasound on polymer systems have concentrated on the depolymerizing effects of ultrasound. However, polymerization due to ultrasound has been reported. Moreover ultrasound has been found to cause and/or accelerate emulsion polymerization of several monomers. This has been attributed to the formation of initiating radicals through depolymerization, monomer degradation or dissociation of water, as well as through the reduction in the size of emulsion droplets [31, 32].

1.3. INFRARED ANALYSIS One of the most popular forms of chemical modification has been the combined use of peroxides that generates macroradicals in the olefin chain and in an unsaturated monomer. Nevertheless, secondary reactions are also promoted in the free-radical mechanism. In order to study the different mechanisms of the grafting and secondary reactions, the presence of unsaturations in polyolefins backbone chains has to be known. These unsaturations and other groups present in polythylenes can be found by FTIR. In the region between 1500 and 1300 cm-1 of CH bending, the main difference

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Rosestela Perera, Carmen Rosales, Carmen Albano and Pedro Silva

Absorbance (a. u)

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among polyolefins lies in the intensity of the band associated with methyl groups in 1378 cm-1. Its presence is an indication of long-chain branching. In the region between 1300 and 1000 cm-1 carbon backbone stretching is associated with crystallinity. The presence of two bands in 730 cm-1 and 720 cm-1 are indicators of long aliphatic semicrystalline chains [33]. In linear low-density polyethylenes (LLDPEs) produced by the ZieglerNatta process, there are unsaturations of the vinyl type because a very intense band in 909 cm-1 is present. Also, there are unsaturations at 965 cm-1 corresponding to internal trans-unsaturations. For low-density polyethylene (LDPE), produced by high pressure, the vinylidene unsaturations are predominant over the vinyl ones, which could be seen in 888 cm-1. In this types of PEs, the unsaturations of the vinyledene type account for the 68 wt. % and the vinyl type for 15 wt. % of the total unsaturations. The absorption band also observed at 957 cm-1 may be due to allyl type unsaturations [33].

e d c b a

1800

1600

1400

1200

1000

-1

Wavenumber (cm ) Figure 5. Influence of the irradiation dose on FTIR spectra of LDPE grafted with 30 wt. % of DEM by γ-irradiation at different irradiation doses. a: neat LDPE, b: 50 kGy, c: 100 kGy, d: 200 kGy, e: 400 kGy.

FTIR spectra of neat and grafted LDPE with diethylmaleate (DEM) by γirradiation and ultrasonic radiation are presented in Figures 5 and 6. FTIR spectroscopy of functionalized polyethylenes confirmed the presence of diethyl-succinate (DES) units inserted in the PEs polymer backbone after grafting with DEM. At 1740 cm-1 there is a clear signal in the functionalized products that is absent in the neat materials and that is ascribed to the stretching of C=O bonds from ester groups of the DEM molecules attached to

Main-Chain Modification as a Result of Polyolefin Functionalization by Different Techniques, Nova Science Publishers,

Introduction

11

Absorbance (a. u)

the polyethylenes. At 720 cm-1 and 1460 cm-1 there are two more bands corresponding to the rocking of (CH2)n for n equal or higher than four, characteristics of polyethylenes [18].

1745 c b a

3500

2500

1500

500

-1

Wavenumber (cm )

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Figure 6. Influence of the exposition time on the FTIR spectra of LDPE grafted by ultrasonic radiation (a): neat LDPE, (b):0 wt. % of DEM and 20 min. of exposition time and (c): 30 wt. % of DEM, 2 wt. % of DP and 20 min. of exposition time.

1.4. GRAFTING DEGREES As it has been reported [2, 34-36], the mechanism of the functionalization reactions is based on the initial formation of free radicals of the alcoxy type due to the thermal decomposition of a peroxide or the free radical generation from other energy sources (γ-radiation or ultrasound). These radicals generate macroradicals through hydrogen abstraction from the macromolecules. Due to the increased reactivity of the double bond in the inserting monomer (DEM, for example) the macroradicals rapidly react with the unsaturated substrate, and the propagation reactions is quickly interrupted due to chain transfer with the polyolefin chain. To establish the mechanism of free radical grafting reactions on high molecular weight polyolefins, small molecules have been grafted in order to use them as models. The free radical bromine graft reactions on squalane, methylhexane and nonane were studied by Jois and Bronk [37]. On other hand, the free radical grafting of maleic anhydride (MA) on poliolefins was studied by Heinen et al. [38] using 13C NMR spectroscopy and specific isotope

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Rosestela Perera, Carmen Rosales, Carmen Albano and Pedro Silva

labeling of the MA. They found that grafting occurred on secondary and tertiary carbons depending on the composition of the polyolefin. When long methylene sequences were present (>3) grafting mainly occurred on secondary carbons. Otherwise, grafting also occurred on tertiary carbons. Usually, saturated monomeric MA grafted structures were formed, but it was also found that MA dimers and trimers were grafted in HDPE in the melt. Aglietto et al. [2] showed that solution DEM-grafted polyolefins contained mostly only one unit of DEM. Bremner and Rudin [39] proposed a mechanism by which allylic hydrogens in terminal vinyl groups were preferentially abstracted in peroxide modified HDPE. A combination of allylic radicals, which gave a crosslink, took place after rearrengement of the terminal vinyl group into a transvinylene group. Similar results were reported by Lachtermacher and Rudin in their peroxide modified LLDPEs [40]. In order to determine the grafting or functionalization degree, expressed as diethylsuccinate moles per 100 ethylene moles in linear low density polyethylenes, calibration curves could be used. Those curves are obtained from peak area ratios A1740cm-1/A1460cm-1 and A1740cm-1/A(720+730)cm-1 and the grafting degree from 1H NMR for grafted LLDPEs by solution and reactive extrusion [15]. The use of these curves allows determining quantitatively the functionalization degrees in a simpler, more rapid and exact way than by means of any other method of analysis described in the literature [1, 2, 7]. A similar calibration curve was obtained by Rojas et al. [41] in HDPEs using 13C NMR and the FTIR area ratio A1740cm-1/A1460cm-1. Another technique found in the literature is Fodor et al. [42] method for determining the grafting degree. It was followed by Aglietto et al. [2] and Rosales et al. [15] in preliminary studies. That method overestimates the grafting degree values probably due to a low compatibility between polyolefins and poly(diethyl fumarate). On the other hand, the calibration curve obtained from DEM solutions in octane underestimates those grafting degree values due to the difference between the extinction coefficients of the compression molded films and those of the solutions.

1.5. SECONDARY REACTIONS AND/OR DEGRADATION IN THE GRAFTING PROCESS To explain the differences in grafting degrees in varying polyethylenes, a detailed study of possible mechanisms of functionalization and secondary

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Introduction

13

reactions taking place must be included. The role of terminal unsaturations in increasing crosslinking efficiency or coupling reactions, has been studied previously in peroxide modified polyethylenes [39, 40, 43], although the exact mechanism is not always agreed. Bremner and Rudin [39] reported the major contributing reactions of terminal vinyl in a free-radical scheme that accounts for the changes in vinyl group concentrations in PE. These conclusions also fit in the work done by Lachtermacher and Rudin [40], i.e., chain extension reactions account for a decrease in the amount of terminal vinyls upon peroxide reaction. Allylic radicals are intermediate species in chain coupling reactions involved in peroxide modification and/or grafting reactions of LLDPE yielding trans or cis unsaturations. This is demonstrated by a shift in the molecular weight distributions curves of the modified polymers towards the high molecular weight end compared to the virgin resins, as it will be mentioned later. Simultaneously, the number of trans unsaturations (965 cm-1) tends to increase slightly with the grafting degree. Apparently, long-branching formation is due mainly to coupling allylic radicals. Such reactions cause a decrease in terminal unsaturations and an increase in internal unsaturations. Interconnection of such long branches would eventually lead to crosslinking in the peroxide modified polyethylenes. The exclusive production of Y-type branches noted by Lachtermacher and Rudin [40] in their work may not persist in other polyethylenes resins that have much lower unsaturation levels such as the HDPE. Figure 7 shows a possible mechanism of long-chain branching and/or crosslinking [44]. Smedberg et al. [43] found that the consumption of terminal vinyls in the crosslinking of LDPE with differing initial contents of terminal vinyl groups is directly proportional to its initial concentration, and that the formation of trans vinyls is independent of the initial vinyl groups consumed, which is not in agreement with results published by Bremner et al. [39]. Abraham et al. [45] increased the crosslinking density of a LDPE modified with peroxide blending it with a LLDPE which contributes terminal vinyl groups. There is no experimental evidence of benefit when adding tertiary radicals to allylic terminal radicals to produce long-chain branching in quaternary carbons. The generation of tertiary radicals through hydrogen abstraction is energetically favored over the production of secondary radicals, but the differences in tertiary and secondary C-H bond energies is rather small: 95 vs. 93 kcal/mol, compared to 86 kcal/mol of allylic hydrogens [44], and methylenic hydrogens are much more numerous than methines in LLDPE, LDPE and HDPE. Hence, it would not be surprising that the predominating reaction involving the formation of long-chain branching were the addition of

Main-Chain Modification as a Result of Polyolefin Functionalization by Different Techniques, Nova Science Publishers,

14

Rosestela Perera, Carmen Rosales, Carmen Albano and Pedro Silva

secondary radicals in the main chain of these polyethylenes to allylic radicals in chain ends. However, Cousin et al. [46] attributed the fact that peroxides are much more efficient producing radicals in LLDPE than in HDPE to the higher amount of tertiary carbons in α-olefin unities. H . R

CH 2

+

CH 2

C

CH 2

H RH

CH 2

+

CH

C .

CH 2

CH 2

CH 2

(a) . R

RH

CH 2

+

CH 2

. C

CH 2

+

CH 2

CH 2

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H

(b)

CH 2

H

H

C

C .

. C

CH 2

+

CH 2

CH 2

CH 2

H

CH 2

H

H

C

C

CH 2 CH 2

C

H

CH 2 CH 2

(c)

Figure 7. Mechanism of long-chain branching and/or crosslinking: (a) allyl hydrogen abstraction; (b) H abstraction to form a secondary radical; (c) reaction of products from (a) and (b) to form long branches(44). Main-Chain Modification as a Result of Polyolefin Functionalization by Different Techniques, Nova Science Publishers,

Introduction

15

In addition to combination, it is known that other secondary reactions with macroradicals can take place, which include disproportion or chain scissions, whose relative importance depends on the reaction temperature. The competition between crosslinking and chain scission reactions in LDPE and HDPE treated with peroxide has been reported in the literature [39, 47-49]. These works coincide in that chain growth prevails at temperatures lower than 250-270°C, whereas at temperatures higher than those values, chain scission reactions are favored. CH2

CH2

CH2

CH2

CH

. CH

CH2

CH2

CH2

+

. CH2

CH2

(a) CH2

. CH2

. CH2

CH

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H

CH2

CH3

+

CH2

CH

(b) CH2

CH2

. C

CH2

CH2

CH2

CH2

C

CH2

CH2

CH2

+

CH2

. CH2

CH2

CH2

CH2

(c)

Figure 8. Formation of terminal vinyl groups by: (a) β-scission of a secondary radical, (b) disproportionation of terminal primary radicals, and (c) formation of pendant unsaturation in tertiary radical(42).

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Rosestela Perera, Carmen Rosales, Carmen Albano and Pedro Silva

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Figure 8 shows a scheme of these reactions. As it is seen, the beta scission of the secondary radical produces new vinyl terminal groups and the decrease in the molecular weight. On the other hand, the disproportion of two primary terminal radicals can also increase the amount of terminal vinyl groups without modifying the molecular weight. Additionally, branch scissions in tertiary radicals produce new pendant unsaturations. This reaction leads to a decrease in the long-branch content and in the molecular weight. However, since the transvinyl unsaturation content is also increasing as a result of coupling reactions, it can be concluded that there are two competing mechanisms of chain growth and chain scission occurring in the melt. Similar results were found by Bremner and Rudin [39] with their dicumyl peroxide (DP) modified HDPEs.

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Chapter 2

TECHNIQUES USED TO PREPARE GRAFTED POLYTHYLENES

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2.1. GRAFTING IN SOLUTION WITH PEROXIDE In a reported study [7], the influence of 2,5 dimethyl 2,5 di (tert-butyl peroxy-hexane) (DBPH) as initiator, the DEM functionalization agent concentration, temperature and reaction time on the grafting degree in solution was established via a Plackett-Burman experimental design and a calibration curve obtained on the basis of studies of Fodor et al. [42]. It was found that the grafting degree is a function of the percentage of dissociated initiator, calculated from the initiator decomposition constants in mineral oil. When the functionalization monomer concentration was kept constant, an increase in the dissociated initiator concentration produced an increase in the grafting degree up to a limit, because a higher concentration of free radicals made them recombine among themselves at higher rates. On the other hand, an increase in DEM concentration produced an increase in the grafting degree. Some authors have proposed that the peroxide concentration reaches a limit because higher concentrations promote termination reactions by combination and/or disproportion that compete with the insertion of DEM in LLDPE and propitiate the formation of unsaturations, crosslinking and/or long chain branching [37, 38, 43]. A commercial LLDPE supplied by Resinas Industriales C.A from Venezuela (LLDPE1) was used in the grafting reactions in solution with

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Rosestela Perera, Carmen Rosales, Carmen Albano and Pedro Silva

peroxide. The properties, such as density (ρ), average weight molecular weight (Mw), molecular weight distribution (Mw/Mn), melt flow index (MFI) values, and melting temperatures are shown in Table 1. Grafting of this LLDPE1 with diethylmaleate (DEM) was carried out in solution under nitrogen atmosphere, in a four necked reactor, equipped with stirrer and thermometer. The polymer was dissolved in orto- dichlorobencene (DCB) at 129ºC. The temperature was raised up to the chosen values (140 and 160ºC) and a diethylmaleate solution in DCB (5 mL) was added. Afterwards, a solution of the initiator peroxide (2,5 dimethyl 2,5 di (tert-butyl peroxy-hexane), DBPH) was added in five portions. The two reaction times used (15 and 45 seconds) were measured from the first addition of the initiator. After a given time, the reaction was stopped and the product was precipitated and washed with acetone, filtered and dried for 16 h at 60ºC. The effects of DEM (2 and 20 phr) and initiator concentration (0.5 and 2 phr), temperature and time on graft content were established using an experimental design of four parameters. The grafting degree of the LLDPE1-g-DEM in solution with peroxide as a function of the dissociated initiator concentration is presented in Figure 9. The grafting degree was obtained by 1H NMR. These results confirm that the effect of the amount of initiator on the grafting degree is not linear and the efficiency of the process is very low. Then, polymeric radicals must have been used in secondary reactions such as coupling or β-chain scission because at a fixed dissociated initiator concentration with 20 phr of DEM and two initial initiator proportions (0.5 and 2 phr) the grafting degree is not the same. It was demonstrated by Agglietto et al. [2] that DEM does not homopolymerize and that a 20/1 ratio of DEM/initiator proportion has to be used in order to eliminate the secondary reactions during grafting. Also, two main factors may play an important role in the grafting reactions: the reactivity of the unsaturated monomer and the viscosity of the reaction medium. Diethylmaleate shows low reactivity due to the significant steric hindrance of the double bond. This monomer was supposed to exhibit steric hindrance because of the molecular configuration and pendant esterified groups [50]. The viscosity of the medium makes the diffusion of the DEM molecule to the active radical sites very difficult. The solubility test for all grafted products in solution indicated that no crosslinking was present as evidenced by complete dissolution. Therefore, it can be assumed other type of secondary reactions instead of crosslinking in the grafted LLDPE1 samples.

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Techniques Used to Prepare Grafted Polythylenes

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2.2. GRAFTING IN SOLUTION WITH γ-IRRADIATION AND ULTRASONIC RADIATION A commercial LDPE supplied by Resinas Industriales C.A in Venezuela was used in the grafting reactions by γ-irradiation and ultrasonic radiation. The properties of this material are reported in Table 1. Solutions of LDPE in decalin (10 % wt. /vol.) were prepared with different proportions of DEM. This functionalizing agent (DEM) was added at 100 ºC in different proportions (5, 15 and 30 wt. %) and the polymer was dissolved at 120ºC. Then, these solutions were exposed to different γ-irradiation doses (15, 30, 50, 100, 200 and 400 kGy) at a dose rate of 5 kGy/h, in a nuclear reactor located in the Venezuelan Institute for Scientific Research (IVIC). 1.00 b

GD (mol. %)

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0.80 a 0.60 0.40

c 0.20 0.00 0

1

2

Dissociated initiator concentration (phr) Figure 9. Grafting degree (GD) as a function of the dissociated initiator concentration for functionalized LLDPE1 in solution with peroxide; (a): 2 phr of DEM and 2 phr of initiator (DBPH), (b) 20 phr of DEM and 2 phr of DBPH, and (c) 20 phr of DEM and 0.5 phr of DBPH.

Solutions of LDPE in decalin were also exposed to ultrasonic radiation. DEM (15 and 30 wt. %) and dicumyl peroxide (0.2 and 2 wt. %) were employed as functionalizing agent and initiator in these grafting reactions, respectively. The piezoelectric rod was introduced into the solution. When the ultrasound produced the complete melting and dissolution of the sample (approximately 5 minutes), the time recording was started. It varied between

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Rosestela Perera, Carmen Rosales, Carmen Albano and Pedro Silva

10 and 30 minutes. In those samples where peroxide was used, it was added after the complete melting and/or dissolution of the polymer.

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Table 1. Material properties Material

ρ (kg/L)

Mw x10-3

Mw/ Mn

MFI ± 0.07 (dg/min.)

Tm ± 1 (°C)

LLDPE1

919

1.35

4.7

0.62

125

LLDPE2

931

0.79

3.0

4.56

120

LDPE

922

1.59

19

3.48

115

The materials grafted with γ-irradiation and ultrasonic radiation were precipitated with ethanol and washed with hexane and ethanol, filtered and dried under vacuum for 8 h at 70ºC. Gel content was determined by dissolution on boiling heptane. The influence of the DEM concentration on the grafting degrees for the samples functionalized by γ-irradiation and ultrasonic radiation are shown in Figures 10 and 11 at different radiation doses and/or different exposition times. Similar as found in grafting in solution with peroxides, an increase in the DEM concentration produced an increase in the grafting degrees. According to Ganzeveld et al [6], the increase in the functionalizing agent concentration increases the probability of reaction between the macroradicals and the molecules of monomer, thus increasing the possibility of grafting. This behavior is more evident at doses higher than 50 kGy (Figure 10). The grafting degree also increases when the γ-irradiation dose increase because more radicals are produced. The FTIR spectra of LDPE functionalized and not functionalized indicate that ultrasonic radiation does not produce significant changes in its structure. When the grafting degrees were determined, very low values were found (lower than 0.2%). Hence, a free radical generator such as dicumyl peroxide was needed, according to González et al [51], who used benzoyl peroxide at concentrations of 0.5 and 2 wt.% to enhance the functionalization of a polypropylene with maleic anhydride by ultrasonic radiation. When dicumyl peroxide was added to the system of LDPE and DEM, a significant increase in the grafting degree was obtained (Figure 12). For an exposition time of 20 minutes, the increase in the peroxide concentration in the reaction media contributed to an increase in the amount of radicals formed, and thus, in the grafting degree. This parameter reached a peak value of 0.54% when 2wt.% of DP was added. Similar results were reported by Zhang et al

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21

[52], who obtained a GD of about 0.60% in polyolefins functionalized with maleic anhydride.

0.6

G.D (mol. %)

0.4

DEM concen. (wt. %)

0.2

30 15 10 5

0.0

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15

30

50

100

200

400

Irradiation dose (kGy) Figure 10. Effects of the irradiation dose and DEM concentration on the grafting degree (GD) of LDPE grafted by γ-irradiation.

Figure 11 displays the GD values when the DEM concentration and ultrasound exposition times were changed for a DP concentration of 2 wt.%. The influence of the exposition time on the grafting degree is not very significant in ultrasonic radiation. No changes in the GD with exposition time were observed at 15 wt. % of DEM. This could be indicating that radicals favoring grafting are consumed before 20 minutes of reaction, reaching saturation at longer times at 15 wt. % of DEM. At 30 wt. % of DEM, a slight increase in the grafting degree was observed when the exposition time increased. The effect of initiator concentration is reported in Figure 12 for the functionalized products prepared by ultrasonic radiation. As it was mentioned before, the grafting degrees increase with the initiator concentration. Similar results were reported by Zhang et al [52], who obtained a grafting degree of about 0.60 mol. % in polyolefins functionalized with maleic anhidride.

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Rosestela Perera, Carmen Rosales, Carmen Albano and Pedro Silva

0.8

G.D (mol. %)

0.6 0.4

DEM conc. wt. %

0.2 30 0.0

15 20

30

Exposition Time (min)

0.6

G.D (mol. %)

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Figure 11. Effects of the exposition time and DEM concentration on the grafting degree (GD) of LDPE grafted by ultrasonic radiation.

0.4

0.2

0.0 0.0 0.2

D.P conc. (wt. %)

2.0

Figure 12. Effects of the dicumyl peroxide (DP) concentration on the grafting degree (GD) of LDPE grafted with 30 wt. % of DEM by ultrasonic radiation LDPE and 20 minutes of exposition time. Main-Chain Modification as a Result of Polyolefin Functionalization by Different Techniques, Nova Science Publishers,

Techniques Used to Prepare Grafted Polythylenes

23

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2.3. GRAFTING BY REACTIVE EXTRUSION In reactive extrusion, the process variable interactions must be known in order to study and fully understand it. These variables are mainly the screw speed, the mass flow rate and the temperature profile. Except for the material temperature, the other two variables influence the residence time distribution [14, 16]. The mean residence time is considered, to a first approximation, as the time required for the decomposition of 95% of the peroxide. Dicumyl peroxide decomposes when it is exposed to heat, and two cumyloxy radicals of equal reactivity are formed. Alkoxy radicals are known to be strong hydrogen abstracting species. The cumyloxy radical can also decompose via a β-scission reaction into acetophenone and a methyl radical. This last radical is very reactive and probably increases the secondary reactions [43]. In previous studies [53-56], the effects of screw configuration, extrusion conditions and initiator concentrations on the grafting degree were studied. The grafting of high-density polyethylenes (HDPEs), linear low-density polyethylenes (LLDPEs), low-density polyethylenes (LDPE) and ultra lowdensity polyethylenes (ULDPE) was carried out in internal mixers and corotating twin screw extruders. The grafting degree and the molecular weight distributions were measured and the extent of the effects of the grafting degree on the molecular characteristics was determined. It was found that the higher the initiator concentration, the higher the grafting degree of LLDPEs, no matter what screw configuration was assembled, peroxide or extruder was used. Additionally, as the grafting reaction is diffusion limited, high viscosity materials could lower the functionalization degrees [7]. On the other hand, the low compatibility between DEM and polyethylenes promoted monomer migration on the extrudates when a concentration higher than 10 phr of DEM was used in grafting by extrusion. Two commercial polyolefins such as LDPE and LLDPE2 supplied by Resinas Industriales C.A in Venezuela were used in the grafting reactions by reactive extrusion. The properties of these materials are reported in Table 1. The grafting reactions and DEM premix in LLDPE2 and LDPE were carried out in a Berstorff (ECS-2E25) corotating intermeshing twin screw extruder. A premix of 8 phr DEM in each PE was made at 75 rpm and 180ºC. A maximum of 10 phr of DEM was used in the grafting due to the migration effect that arises from PE/DEM incompatibility. The DEM was fed through the second port of the extruders.

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Rosestela Perera, Carmen Rosales, Carmen Albano and Pedro Silva

At an additional extrusion process, the grafting was carried out and the initiator (dicumyl peroxide) was then added with the small amount of DEM remaining (2 phr of DEM) at 35 rpm and 180ºC. The solid materials (pellets of polymer and DEM premix) were starved fed to the extruders by solid feeders and the extrudates were cooled in a water bath and pelletized afterwards. The liquid additives were introduced by a liquid injection pump in a liquid addition zone of the extruder. Also, reactions with DP were made at the same conditions of the grafting reactions. These reactions (grafting and with peroxide) were made in air and under nitrogen atmospheres. To eliminate the residual monomer, the functionalization products were dissolved in DCB, washed with acetone and then dried in a vacuum oven at 60°C for 16 h. The resultant materials were dissolved in n-heptane at 120°C and the insoluble polymer fractions were isolated. Dissolution of these fractions in DCB at 120°C was also carried out to verify the absence of crosslinked material by visual examination [7, 15]. The LLDPE2 and the LDPE were grafted and modified by extrusion using different conditions. Both neat materials were extruded in a nitrogen atmosphere and in air (extruded in N2 and extruded in air). Also, both materials were extruded with 0.5 phr of dicumyl peroxide (DP) under a nitrogen atmosphere and in air. These same materials were as well grafted with DEM and DP. The grafting degrees of the LLDPE2 and LDPE by reactive extrusion are presented in Table 2. Similar grafting degrees were obtained in both PEs in air because these materials have similar viscosities at the shear rates in the extruder. As it stands out from Table 2, the grafting degree of each polyethylene is higher when extruded in air. Similar results were obtained by Liu et al. [1] when LDPEs were functionalized with maleic anhydride in solution. These authors reach functionalizing the polyethylene without any peroxide in air at levels similar to those with peroxide in nitrogen, and explain their results saying that oxygen takes place in the initiation step of the free radical reaction. In studies to produce polypropylenes with controlled reology, the use of air introduced under pressure into the extruder as initiator of the degradation reactions via free radicals has been reported [57]. It is interesting to point out that the effect of air in the functionalization of LDPE is much more marked than in the functionalization of LLDPE2. In the qualitative analysis of gel contents through complete dissolution in DCB, the absence of crosslinking was assessed in all polyethylenes.

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Table 2. Grafting degrees, area ratios (A 909 cm-1/A 1460 cm-1 or A 965 cm-1/A 1460 cm-1) and complex viscosity (η*) at 0.1 rad/s and 200°C in reactive extrusion Type of modification

Material

Grafting degree (mol. %)

Extruded in Nitrogen (ein) Grafted in Nitrogen (gin) Extruded with DP in Nitrogen (pin) Extruded in air (eia) Grafted in air (gia) Extruded with DP in air (pia) Extruded in Nitrogen (ein) Grafted in Nitrogen (gin) Extruded with DP in Nitrogen (pin) Extruded in air (eia) Grafted in air (gia) Extruded with DP in air (pia)

LLDPE2ein LLDPEgin LLDPE2pin

0.15 ± 0.01 -

LLDPE2eia LLDPE2gia LLDPE2pia LDPEein LDPEgin LDPEpin LDPEeia LDPEgia LDPEpia

η* x 10-3 (Pa.s) at 0.1 rad/s

[A 909 cm-1/A 1460 3 cm-1] x 10 or [A 965 cm-1/A 1460 3 cm-1] x 10 5.6 ± 0.6 4.1 ± 0.2 5.5 ± 0.6

1.7 9.5 2.5

0.69 ± 0.04 0.02 ± 0.01 -

5.0 ± 0.6 1.8 ± 0.6 5.4 ± 0.4 0.0 0.56 ± 0.07 0.63 ± 0.09

1.7 28.0 2.5 2.8 3.5 2.9

0.68 ± 0.02 -

0.00 1.9 ± 0.1 0.80 ± 0.09

2.8 6.2 17.2

-

-

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Chapter 3

SECONDARY AND/OR DEGRADATION REACTIONS IN THE GRAFTING PROCESS

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3.1. GRAFTING IN SOLUTION According to Lachtermacher et al [40, 44] and Bremmer et al [39, 49], the determination of terminal unsaturations in polyethylenes is very important when studying secondary reactions of long-chain branching and/or crosslinking, because the reactions of vinyl terminal groups in presence of free radicals allow explaining the changes produced in the concentration of other vinyl groups such as internal unsaturations of the cis and trans vinyl type. In FTIR spectra of functionalized and non funcionalized LDPE subjected to γ-radiation, vibrational modes of the torsion type, characteristics of vinyl bonds were detected [58, 59]. Bands corresponding to out of plane deformation of the vinylidene group (R2C=CH2) at 887 cm-1 and of the trans vinyl group (RCH=CHR) in 965 cm-1 were identified. From peak area ratios of the 887 cm-1 band to the 1460 cm-1 band in functionalized and nonfunctionalized irradiated LDPE (Table 3), it can be suggested that irradiationgenerated radicals also participate in arylic coupling reactions, which consume the terminal unsaturations, giving rise to an increase in the long-chain branching and/or high molecular weight tails [39, 60]. Additionally, the presence of DEM contributes to the decrease in the terminal unsaturations, because they represent labile center where the functional monomer can insert into. This could justify the lower amount of terminal unsaturations that can be seen in functionalized LDPE. On the other hand, the increase in the amount of trans vinyl unsaturations is a consequence

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28

of the secondary reactions of chain branching due to arylic radical coupling with other radicals. These unsaturations could be attributed to the scission of C-H bonds in secondary carbons due to Gamma radiation, which activates βhydrogen atoms and renders them more susceptible to their abstraction by another adjacent radical, producing unsaturations of the cis and trans types. The last ones are favored due to conformational effects [43].

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Table 3. Area ratios (A 887 cm-1/A 1460 cm-1 and A 965 cm-1/A 1460 cm-1) and gel content of LDPE grafted by γ-irradiation [A 887 cm-1/ A 1460 cm-1] x 103

[A 965 cm-1/ A 1460 cm-1] x 103

Gel Content (wt. %)

DEM concentration (wt. %)

Radiation dose (kGy)

0

0

8.9

0.0

-

0

100

5.7

2.5

30

0

400

10.4

9.8

35

30

100

-

-

16

30

200

3.5

4.7

27

30

400

6.7

10.6

30

Another important parameter to analyze is the gel content, which indicates the formation of tridimensional chain networks (crosslinking reactions), impairing the dissolution of functionalized and no functionalized LDPE. Table 3 shows that the crosslinking reactions increase with the radiation dose, being this effect less pronounced in samples of LDPE without DEM. This fact evidences that grafting reactions compete with those of crosslinking. When the zone corresponding to unsaturations in LDPE’s FTIR spectra is analyzed (Figure 6), it can be said that no significant changes are produced in the chemical structure of the polymer by ultrasound exposition. Only a decrease in the band width at 887 cm-1 assigned to vinylene is observed. However, when LDPE is functionalized using ultrasound, two bands independent of the exposure time appear: one at 965 cm-1 assigned to trans vinyl (RCH=CHR) as a consequence of secondary reactions and another at 858 cm-1 attributed to the O-CH2-CH3 group of the grafted DEM unit [61]. The quantification of the unsaturation contents in non functionalized LDPE reveals a slight decrease in vinylidene groups as a consequence of exposure to ultrasound, which is in agreement with an increase in the molecular weight. The consumption of those groups is an indication of secondary reactions of long-chain branching and/or crosslinking. In LDPE functionalized with DEM there is also a decrease in vinylidene groups.

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Secondary and/or Degradation Reactions in the Grafting Process

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However, this decrease is very large, unfolding not only secondary reactions but also the grafting of the functional monomer. The increase in exposure time contributes to increase the feasibility of these types of reactions, because the consumption of unsaturations is higher. The inclusion of dicumyl peroxide induces the formation of trans vinyl or internal unsaturations, because alkoxy radicals are capable of generating both primary and secondary radicals. These radicals can react among themselves through disproportionation, where the primary radical can subtract a β hydrogen atom adjacent to the secondary radical [39]. Trans vinyl unsaturations are more numerous at low exposition times. This fact can be attributed to a cage affect, which is a consequence of the vicinity of alkoxy radicals just after the peroxide decomposition. When results from the different techniques (ultrasound and Gamma radiation) used in the functionalization of LDPE with DEM are compared, higher grafting degrees are obtained using ultrasound. This leads to inferring that the insertion is more efficient using this last method (Table 4).

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Table 4. Efficiency of the grafting reactions of LDPE using γ-irradiation and Ultrasonic radiation* γ-irradiation

DEM wt. % 15

15 2.4

Ultrasonic radiation*

Irradiation dose (kGy) 30 50 100 200 3.0 4.1 6.6 8.5

30 1.1 1.3 *2 wt. % of DP was added.

1.7

3.7

4.9

400 13.4

Exposition time (min) 20 min 30 min 13.2 13.2

7.5

7.5

8.5

The decomposition of dicumyl peroxide by ultrasonic radiation allows producing radicals with a lower energy requirement than when Gamma radiation is used to break the C-H bonds in the main polymeric chain, to produce radicals in the absence of a chemical initiator. Additionally, due to the increase in global temperature when ultrasound was used, there is an enhancement in DEM and macroradicals diffusion, producing a higher grafting of the functional monomer. This effect is favored through the vortex formation that increases the molecules mobility. This was not the case in Gamma radiation, where there were high viscosities in the reaction media and not stirring was performed.

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3.2. REACTIVE EXTRUSION In previous studies of extrusion grafted LLDPE1 [15], a premix of DEM in the polymer was made, the initiator was added through the extruder hopper and a temperature of 200°C in the last zone of the extruder was used. The high melt viscosity of this material and the conditions employed promoted crosslinking reactions that significantly lowered the processability of the functionalized resin. Additionally, the molecular weight distribution curve of LLDPE1-g-DEM fell below those of neat and grafted LLDPE1 in solution with peroxide, probably because the high molecular weight fractions did not dissolve and were lost in the filter prior to the GPC analysis [40, 62]. The functionalization variables used in the extruder at 200°C brought about the complete decomposition of the peroxide in polyethylene (99% of decomposition). However, the peroxide attack to the polymer in the melting zone of the extruder, the employed temperature and/or the tails at long residence times in the residence-time distribution curves promoted crosslinking reactions that lowered the processability of the functionalized product (LLDPE1-g-DEM) and originated the presence of gels. However, in other studies with a better control of the process, the crosslinking reactions were eliminated [15], but other secondary reactions (chain scission and/or long chain branching formation) could have taken place. When a low concentration of peroxide was used in the grafting of LLDPE1, the product obtained was free of gel because the peroxide was added when the monomer was well mixed in the melted polymer. Nonetheless, the grafting degree obtained was the lowest due to the high melt viscosity of the polymer and the presence of antioxidants on this material. The absence of gels in the grafted polyethylene was demonstrated in qualitative analysis through their complete dissolution in dichlorobenzene. A high transport delay time in the extruder means high stagnancy and channeling in the elements just below the initiator feed port because the same feed port was used to add the tracer agent in the residence time distribution curves (RTD) determination. When the grafting reactions are promoted due to stagnancy just below the peroxide feed port, secondary reactions such as thermal-oxidation or devolatilization of reactants could take place as well, because the initiator feed port zone is open to the environment. Long tails on the longer time side in RTD curves could also be promoting secondary reactions such as chain scission, crosslinking and/or long-chain branching. Furthermore, mixing elements in the extruder configuration affect the grafting and secondary reactions. Although the gear mixing elements

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incorporate a higher amount of liquids into the mass in non-reactive mixing [17], they are not that effective in functionalization reactions. The mechanical mixing intensity and the high transport delay time due to the liquid mixing elements had an adverse effect on the overall grafting yield because the free radicals formed could be lost in secondary reactions such as radical termination reactions, thermal-oxidation and/or mechanochemical degradation or through reactive devolatilization, which lowered the grafting efficiencies. These secondary reactions could be taking place just in the liquid mixing elements. The melt viscosity of the material and the grafting reaction rate are affected by the temperature in the reaction zone of the extruder. High temperatures in the initiator feed port and/or high transport delay times decreased the grafting degree due to thermal-oxidation, mechanochemical degradation reactions and/or devolatilization of chemical reactants. In order to establish the interdependence of the extrusion conditions employed in the grafting process, a combined experimental and simplified one-dimensional, non-isothermal model was implemented in the study of flow in the elements of the screw extruder. The theoretical results provided by the global model were validated by comparison with the experimental determinations of the mean residence time. The factors determining the efficiency of the process evaluated are the type of PE and monomer, the screw configuration, the pressure, degree of fill, temperatures in the screw elements, initiator concentration, screw speed, shear rate, shear strain and specific energy, mass flow rate and extent of mixing [56]. Table 3 displays the absorbance peak area ratios of bands at 909 cm-1 and 965 cm-1 (A909 cm-1/A1460 cm-1 and A965 cm-1/A1460 cm-1) for LLDPE2 and LDPE, respectively. The catalyst composition and polymerization mechanism of this LDPE have not been reported, but a growth reaction producing terminal carbon-carbon double bonds is inferred. Unfortunately, the band at 965 cm-1 could not be quantified due to resolution of the spectrometer, though it is present in all spectra of LLDPE2. When the spectra of samples of LLDPE2 reactively extruded under controlled atmosphere (nitrogen), a slight decrease in the vinyl terminal groups in the functionalized material is observed. On the other hand, the quantification of the concentration of the same groups in the same polymer functionalized in air was not possible, due to the detection limits of the spectrometer. Moreover, the characteristic band seems to be overlapped by a new one at 932 cm-1. As it was mentioned before, Bremner and Rudin [39], Suwanda and Balke [60] and Hendra et al. [63] have reported a significant decrease in the terminal unsaturations, due to long-chain branching and chain extension reactions when

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Rosestela Perera, Carmen Rosales, Carmen Albano and Pedro Silva

polyethylenes are reactively extruded with peroxides. It is very likely that in addition to the functionalization reactions, those mechanisms of chain modification are also taking place in the material functionalized under nitrogen atmosphere due to the coupling of two allylic radicals or of one allylic with another secondary radical. This increases the high-molecular weight tails and the amount of trans unsaturations. Nonetheless, this could not be corroborated due to lack of resolution of the band at 965 cm-1 in the spectra. The possibility of the insertion of the functional monomer in that allylic radical is not discarded. The production of a trans unsaturation is also possible through the cage effect or through disproportionation of a primary radical with a secondary one, but the steric hindrances tend to make a small contribution from the last option. The cage effect is possible when the peroxide has just decomposed and the free radicals are still very close to each other, a situation favored in high viscosity media, where the reactants diffusion is very slow. If one of those radicals abstracts a secondary hydrogen atom from the polymeric chain, then a macroradical is formed. Hydrogens in positions β to this radical are activated and abstracted easier. As the other peroxide radical is still in the vicinity, it could abstract this β-hydrogen to form a trans vinyl bond. Some of cis could also be formed, but the first conformation is energetically more favorable [43]. On the other hand, LDPE usually has a very low content of terminal unsaturations, yet vinylidene groups are the predominating unsaturations. In fact, when LDPE has to be crosslinked to be useful in some applications, propylene is used in the polymerization process as chain transfer agent, which produced terminal vinyls that increase the crosslinking efficiency [43]. It is worth noticing that a total absence of gels in all modified LDPEs was verified, though long-chain branching or chain extension through a macroradical combination mechanism as that reported by Smedberg et al. [43] are not discarded. However, the determination of band areas for vinylidene group was difficult due to its overlapping with those corresponding to ethyl, butyl and hexyl branches. Smedberg et al. [43] proposed that when peroxide is decomposed into radicals, a hydrogen is abstracted from the polymeric chain. There are different kinds of hydrogens in LDPE: primary, secondary, tertiary and allyl. Which one would be abstracted will depend not only on its reactivity but also on its concentration. It is well known that allylic hydrogens are the most reactive of them, followed by the tertiary, secondary and primary. On the other hand, LDPE regularly contains between 10 and 35 methyl groups for each 1000 carbons and a very low amount of unsaturations, which approximately

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Secondary and/or Degradation Reactions in the Grafting Process

33

renders between 10 and 35 tertiary hydrogen atoms, between 30 to 105 primary carbons, 2 allylic and nearly 1900 secondary carbons. Thus, the differences in reactivity of each type of carbons are diluted in this huge difference in concentrations. Hence, secondary hydrogen would most probably be abstracted. The same authors also reported that pendant vinyls or vinylidene produced a higher amount of crosslinks in LDPEs than terminal vinyls introduced in the chain, after the same amount of vinyls were consumed. In another work, Bremner et al. [49] concluded that the contribution of tertiary carbons to the crosslinking mechanism is very modest, which reinforces the importance of other reactive sites in this reaction mechanism. Heinen et al. [38], while studying the functionalization of polyethylenes, polypropylenes (PP) and ethylene/propylene copolymers (EPM) with 13C labeled maleic anhydride found that in polyethylenes and in EPM with low amounts of propylene, the abstraction of hydrogens and subsequent grafting takes place in secondary carbons. In EPM with high propylene contents and in PP the grafting proceeds only in tertiary carbons, which suggests the selective abstraction of hydrogens from those carbons. These authors also report that the grafted structure can participate in chain scissions in PP. Finally, they conclude that when long methylene sequences are present (>3), the insertion proceeds mainly in secondary carbons; otherwise, the grafting goes through tertiary carbons. In the case here discussed regarding the grafting of low-density and linear low-density polyethylenes via reactive extrusion, the feasibility of the following mechanism is considered. Initially, a secondary macroradical is formed due to the hydrogen abstraction by an alkoxy radical coming from the homolitic decomposition of the peroxide. As it is very reactive, it can be combined with a DEM molecule resulting in grafting or it can immediately be combined with another secondary radical and secondary reactions take place. If this does not happen, it will probably abstract a hydrogen atom. The result will be that a radical is still present but could change positions and move into the material until finding a lower energy site; in other words, a tertiary or allyl position. From there, it could be added to a vinyl group (which in the case of LDPE will be a pendant one) and start a chain reaction which leads to crosslinking, or be combined with another type of radical. A tertiary radical could of course lead to chain scissions, but if there is a considerable amount of unsaturations present in the polymer, and if the temperature is very low to favor chain scissions, then the addition to another vinyl group or to another secondary radical follows.

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Rosestela Perera, Carmen Rosales, Carmen Albano and Pedro Silva

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In conclusion, it can be said that the mechanisms of chemical modification depend on the type of polyethylene and could change if they take place in the presence or absence of oxygen.

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Chapter 4

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MOLECULAR WEIGHT DISTRIBUTION AND RHEOLOGICAL PROPERTIES The molecular weight distributions of the materials, before and after the functionalization reactions, were measured by a Waters 150 GPC equipped with a differential refractometer detector and Styragel packing columns, at 135°C and at a flow rate of 1 mL/min in DCB. Samples were previously dissolved in DCB or TCB and filtered. Solution concentrations were 0.1% wt./vol. A calibration curve was made with monodisperse polystyrene samples and the molecular weights were calculated using the universal calibration curve and the Mark-Houwink constants of polystyrene and linear polyethylenes [16]. On the other hand, compression-molded samples were used for oscillatory shear experiments with parallel plates. Frequency sweep experiments were performed in a Rheometrics RDA-II equipment in a range of 0.1 to 100 rad/s. In all cases the experiments were performed within the linear viscoelastic range. Also, melt flow index determinations were made using a Davenport melt index according to ASTM D-1238.

4.1. GRAFTING IN SOLUTION The influence of the irradiation dose (between 30 to 100 kGy) on the molecular weight distribution of the LDPE is presented in Figure 13. At high doses it was not possible determining the molecular weights of the LDPE, both

Main-Chain Modification as a Result of Polyolefin Functionalization by Different Techniques, Nova Science Publishers,

.

36

Rosestela Perera, Carmen Rosales, Carmen Albano and Pedro Silva

grafted and non-grafted but irradiated, due to their high viscosity and scarce solubility.

Fraction (wt. %)

0.6 Neat LDPE 30 kGy 50 kGy 100 kGy

0.4

0.2

0.0

2

4

6

8

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log (M) Figure 13. Effects of the irradiation dose on the molecular weight distribution of LDPE grafted with 30 wt. % of DEM by γ-irradiation.

Table 5. Number-average molecular weight (Mn) and weight-average molecular weight of the LDPE grafted by γ-irradiation Irradiation dose

30 wt. % DEM Mn x 10-3

Mw x 10-3

Mw/Mn

0 wt. % DEM Mn x 10-3

Mw x 10-3

0 kGy

-

-

-

8.2

159

19

30 kGy

13.1

239

18

10.9

332

30

Mw/Mn

50 kGy

20.1

388

19

11.1

420

37

100 kGy

29.0

410

14

11.6

632

54

Also, the influences of the irradiation dose and DEM concentration on the number and weight average molecular weights (Mn and Mw, respectively) and on the molecular weight distribution (Mw/Mn) are shown in Tables 5 and 6 for the LDPE modified by γ-irradiation and ultrasonic radiation, respectively. Figure 13 displays the fact that the molecular weight distribution curve is displaced towards higher molecular weights as the radiation dose is increased.

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Molecular Weight Distribution and Rheological Properties

37

This increase is also observed in more detail in the Mw and Mn values reported in Table 5. The fact that both long and shorter chains take part in secondary reactions can be inferred. Table 6. Number-average molecular weight (Mn) and weight-average molecular weight of the LDPE grafted by ultrasonic radiation Time (min.)

DEM Concentration (wt. %)

DP concentration (wt. %)

Mn x 10-3

0

0

0

8.2

159

19

20

0

0

11.3

225

20

15

2

11.2

208

19

30

2

10.1

191

19

0

0

11.2

224

20

15

2

9.8

212

22

30

2

9.8

198

21

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30

Mw x 10-3

Mw/Mn

On the other hand, the molecular weight distribution curves are narrower as the radiation dose increase. This can be corroborated by their polidispersity index values. This implies that the modifications originated in the polymeric chain not only reduce the amount of molecules, but also make them more homogeneous in length, bringing about a decrease in the polidispersity. It is worth mentioning that the negative effects of radiation are less pronounced in the grafting process, due to the presence of the grafting monomer, which competes with collateral reactions of crosslinking. However, in the absence of DEM there is a drastic increase in the polidispersity index, due to the fact that gamma radiation not only produces crosslinking, but also chain scissions. When the molecular weight distribution curves of LDPE grafted using ultrasonic radiation are analyzed, a displacement towards the high molecular weight fractions can be seen. This displacement is independent on the exposition time. Table 6 shows a lower Mw value for grafted LDPE. This behavior evidences a competition between the grafting of DEM and the secondary reactions and is more noticeable as the DEM concentration increases. When the molecular weights of γ-irradiation and ultrasonic radiation-modified LDPE are compared, it can be mentioned that ultrasonic radiation displays an additional advantage: smaller changes in the molecular weight distribution curves are observed. This fact is attributed to a lower amount of chain-branching and/or crosslinking in ultrasonic than in the gamma radiation process.

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Rosestela Perera, Carmen Rosales, Carmen Albano and Pedro Silva

DEM con. wt. %

MFI (dg/min)

4.0

0 15

5 30

3.0 2.0

1.0 0.0 15

30

100

50

Irradiation dose (kGy)

0.6

LLDPEeia LLDPEgia

Fraction (wt. %)

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Figure 14. Influence of the DEM concentration on the melt flow index values (MFI) of LDPE grafted by γ-irradiation.

0.4

0.2

0 3

4

5

6

Log M Figure 15. Influence of the type of modification on the molecular weight distribution of LLDPE2 grafted by reactive extrusion (eia: extruded in air, gia: grafted in air).

Main-Chain Modification as a Result of Polyolefin Functionalization by Different Techniques, Nova Science Publishers,

Molecular Weight Distribution and Rheological Properties

39

The rheological behavior of the polymers is very sensitive to their molecular weight distribution. An enhancement in molecular weight decreases the melt flow index values (MFI) of the polymer [54]. Figure 14 displays the influence of the irradiation dose and DEM concentration on the melt flow index values of the γ-irradiated LDPE. In all modified LDPEs a reduction of the MFI values were obtained when the irradiation dose increased because of the increase in their molecular weights, as was said before. In the modification of the LDPE without DEM, the reduction in the MFI value is lower because of the increase in the molecular weight distribution (Mw/Mn) with the irradiation dose (see Table 5).

Fraction (wt. %)

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0.6 LDPEeia LDPEgia LDPEpia

0.4

0.2

0.0 3

4

5

6

7

Log M Figure 16. Influence of the type of modification on the molecular weight distribution of LDPE grafted by reactive extrusion (eia: extruded in air, gia: grafted in air, pia: extruded with peroxide in air).

4.2. REACTIVE EXTRUSION In early studies [15], it was demonstrated that the functionalization reaction via melt-extrusion did not significantly change the molecular weight distribution in LLDPEs, when the initiator was added in the melted polymer. A broader distribution could be attributed to the branching sites in the polymer structure of the LLDPEs. The results suggest that the grafting occurs

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Rosestela Perera, Carmen Rosales, Carmen Albano and Pedro Silva

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preferentially on the longer or less substituted methylene sequences as it has been reported [38]. Additionally, there are several types of competing reactions in the extruder, i.e., thermal-oxidation, mechanochemical degradation, grafting and formation of long-chain branches. Similar results were obtained for the modified LLDPE2 and LDPE (Figures 15 and 16). However, in the reaction with peroxides in air, a larger increase in the molecular weight distribution curves was obtained. The dynamic viscosity (η*) at 0.1 rad/s of frequency is reported in Table 2 for the LLDPE2 and LDPE modified by reactive extrusion. A higher viscosity was obtained for the grafted materials modified in air than in those modified in nitrogen because of the increase in the long chain branches content in the former. The highest increase in viscosity was obtained in the LDPE modified with peroxide in air due to the crosslinking reactions that increased the molecular weight of this material.

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Chapter 5

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THERMAL PROPERTIES AND THERMOGRAVIMETRIC ANALYSIS (TGA) The melting and crystallization behavior of the neat and grafted polymers was studied by differential scanning calorimetry (DSC) using a Perkin Elmer DSC-2 and a Metller Toledo model 821 under nitrogen flow at cooling and heating rates of 10 ºC/min. Additionally, a technique known as successive selfnucleation and annealing (SSA) was employed to fully characterize the modified products. The development of this technique was reported by Müller et al. [64, 65] and has been used to characterize semicystalline polymers that are capable of undergoing molecular segregation during crystallization upon cooling from the melt. SSA has also been proven to be very useful in characterizing functionalized polyolefins [66-68]. The SSA characterization method was performed on the neat and grafted samples. The materials were melted at high temperature for 5 minutes (to erase any previous thermal history). Then, they were cooled at 10 ºC/min. to room temperature in order to create an initial standard thermal history. Subsequently, a heating scan at 10 ºC/min. was performed up to a selected self-seeding and annealing temperature (Ts), where the samples were isothermally kept for 5 minutes before cooling again at 10 ºC/min. down to room temperature. The first applied Ts is chosen so that the polymer will only self nucleate (Ts would be high enough to melt all the crystalline regions except for small crystal fragments and/or nuclei that can later self-seed the polymer during cooling). At the end of the first cooling from Ts, the polymer had been self nucleated as in the regime II defined by Fillon et al. [69] for selfnucleation. Then, the material is heated in the DSC once again at 10 ºC/min. but this time up to a Ts which is 5ºC lower than the previous Ts. This

Main-Chain Modification as a Result of Polyolefin Functionalization by Different Techniques, Nova Science Publishers,

Rosestela Perera, Carmen Rosales, Carmen Albano and Pedro Silva

42

procedure is repeated with each Ts being lowered at 5ºC intervals with respect to the previous step. Finally, the melting behavior was recorded at 10 ºC/min. when the thermal conditioning was over. Table 7. Thermal properties of the LDPE functionalized by γ-irradiation as a function of the DEM concentration: crystallization temperature (Tc), melting temperature (Tm) and crystallinity degree Irradiation dose

DEM concentration (wt. %)

Tc ± 1 (°C)

Tm ± 1 (°C)

Crystallinitydegree (wt. %)

0

0

96

109

41

50

0

95

110

40

10

94

109

36

15

94

110

34

30

95

110

37

0

96

109

40

10

96

109

39

15

95

109

38

30

94

109

32

0

94

109

39

10

94

109

39

15

93

109

34

30

92

109

35

30

91

109

32

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100

200

400

5.1. THERMAL PROPERTIES To fully study the effects of chemical modifications on the structure of the different polyethylenes once processed, their melting and crystallization thermograms were recorded. Tables 7 to 9 display the thermal properties of the materials modified by γ-irradiation and reactive extrusion. Table 7 shows a slight decrease in the crystallinity degree of LDPE as a consequence of DEM grafting, which is evident when the values corresponding to 15 and 30 wt.% of DEM are compared. Nonetheless, no significant differences were found in the values of melting and crystallization temperature, which is attributed to the low grafting degrees obtained.

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Thermal Properties and Thermogravimetric Analysis (TGA)

43

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Table 8. Thermal properties of the LLDPE2modified by reactive extrusion: crystallization temperature (Tc), melting temperature (Tm) and crystallinity degree Material

Tc (°C)

Tm (°C)

LLDPE2eia

112

122

Crystallinity degree (wt. %) 46

LLDPE2pia

112

122

46

LLDPE2gia

110

119

41

LLDPE2ein

112

121

47

LLDPE2pin

113

121

47

LLDPE2gin

112

120

46

Table 8 unfolds that a decrease in both the melting peak temperature (Tm) and the crystallization temperature (Tc) of linear low-density polyethylene grafted in air is evident. This decrease could be attributed to the interruption of ethylene sequences due to the DEM grafting and/or to chain branching. As a result, a decrease in the lamellar thickness will be produced [70]. The proposed grafting mechanism when peroxide is used indicates that first of all a radical is formed (when secondary hydrogen atoms are preferentially abstracted) and then, a DEM molecule is inserted in that carbon atom. It is well known that this group is excluded from the lamellar structure, as are those chain branches longer than pendant methylenes. This leads to the fact that only shorter methylene linear sequences can crystallize. Since the melting temperature is a function of the crystal thickness, a decrease in its value due to the increase in the chain branching means that they restrict or limit the lamellar thickness. In a more detailed fashion, when the branching degree increases, the length of CH2 linear sequences decrease and thinner lamellas result if those branches (or defects) are excluded from the crystalline phase. Various aspects are evident from the crystallization exotherms and melting endotherms of the LDPE modified by reactive extrusion: the widening of the melting and crystallization peaks of the LDPE modified in air, the decrease in both the melting peak temperature and crystallinity degree in the same polymer grafted in air, and the invariability of the crystallization exotherms and melting endotherms of the LDPE modified in nitrogen. These

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Rosestela Perera, Carmen Rosales, Carmen Albano and Pedro Silva

facts can be seen in Table 9, which displays the thermal characteristics of reactively extruded LDPE. Even though the sample modified with peroxide has more long-chain branching than the original one, it retains the melting enthalpy characteristic of LDPEs. That situation might not be the same in LLDPE, where the number of chain branching and/or crosslinks as byproducts of the functionalization reactions for each 100 carbon atoms may not be small compared to the amount of branching points for each 1000 carbon atoms present in the unmodified polymer.

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5.2. SUCCESSIVE SELF-NUCLEATION AND ANNEALING CHARACTERIZATION METHOD (SSA) Through the analysis of the results obtained using the SSA fractionation technique [64] it is easy to ascertain the heterogeneity in crystalline populations as a consequence of chemical modifications in the polyolefins, either after grafting and/or peroxide treatment or after the secondary or collateral reactions. The fractionated polymers show many separated melting peaks, corresponding to the melting of crystals of different lamellar thickness, which are limited by chain branches or functional groups that are excluded from the crystalline structure. Peaks with higher melting temperatures correspond to thicker crystals made of more linear molecules. The chain sections forming more stable crystals will crystallize in the first place, while other sections form unstable crystals or remain in the melt and crystallize during cooling, generating the subsequent peaks. The crystals can then thicken in the following heating cycles, excluding chain defects and can be reorganized internally to decrease their free energy, increasing their stability. The internal reorganization consists in, among other things, the expulsion of chain branching sites that could have been incorporated into the crystals in the initial crystallization process. The thickening and stabilization of the crystal produce an increase in the melting temperature, because this temperature is a function of the lamellar thickness. An annealing effect has even been reported in crosslinked low-density polyethylenes, where the chain mobility is very reduced [71].

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Thermal Properties and Thermogravimetric Analysis (TGA)

45

Table 9. Thermal properties of the LDPE modified by reactive extrusion: crystallization temperature (Tc), melting temperature (Tm) and crystallinity degree Material

Tc (°C)

Tm (°C)

LDPEeia LDPEpia LDPEgia LDPEein LDPEpin LDPEgin

98 99 98 99 99 99

107 107 105 107 107 107

Crystallinity degree (wt. %) 35 35 30 34 34 33

Heat Flow (a. u)

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Endo

f e

d c b

a

60

80 100 Temperature (°C)

120

Figure 17. Heating scan after SSA treatment of neat LDPE (a), and LDPE modified with 30 wt. % of DEM by γ-irradiation at different irradiation doses, b: 30 kGy, c: 50 kGy, d: 100 kGy, e: 200 kGy and f: 400 kGy.

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Table 10. Partial areas obtained after integration of each DSC scan as a function of irradiation dose and self-seeding temperature (Ts) in the LDPE grafted with 30 wt. % of DEM by γ-irradiation Irradiation dose (kGy) Ts

Area (%)

Melting temperature (°C)

0

30

50

100

200

400

0

30

50

100

200

400

112

32.2

25.8

22.5

26.4

25.6

20.0

112

111

111

111

112

112

107

15.0

21.0

18.3

19.1

19.3

22.3

106

106

106

106

107

107

102

12.5

13.9

13.2

13.4

12.2

15.2

101

101

101

101

102

102

97

9.7

10.4

9.8

9.8

10.0

11.6

96

96

96

96

102

97

92

7.2

7.1

8.6

7.8

5.8

8.9

91

91

91

91

91

92

87

5.9

6.2

6.3

6.0

5.8

6.1

86

86

86

86

87

87

82

4.8

5.2

4.5

5.0

5.4

4.7

82

81

81

81

82

83

77

3.9

3.5

4.8

4.2

4.2

3.8

77

76

77

76

77

78

72

3.4

2.7

3.2

2.5

3.5

2.9

72

72

72

72

73

73

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Rosestela Perera, Carmen Rosales, Carmen Albano and Pedro Silva

47

The low-melting temperature peaks represent the thinner crystal fractions, made of highly branched chains and with very short linear sequences between branching points. These crystals are stable only at low crystallization temperatures. All fractions have their particular crystallization temperature, depending on the branching content. The molecules are segregated as a function of branch distribution even at rapid cooling below their isothermal crystallization temperature, thus generating several endotherms. Figure 17 shows the heating scans obtained after applying the SSA treatments, where the presence of nine melting endotherms corresponding to chain segregation as a function of chain imperfections and branching can be seen. Grafted LDPE displays a significant decrease in the relative height of the endotherm with higher melting peak temperature, and an increase in the height of the endotherms with lower melting temperatures. This effect is more noticeable as the radiation dose increases and is attributed both to higher grafting degrees and to chemical modifications such as branching/crosslinking in the main chain, which proceed through secondary carbons [67]. Quantitatively, this behavior is evidenced in the peak areas under each endotherm (Table 10), where those under the higher melting temperature peaks decrease as new and less perfect crystalline populations are created. The largest area decrease of the higher temperature peak was obtained in the sample of LDPE with the higher grafting degree at 400 kGy. The melting peak temperatures of all peaks remained almost unchanged in all samples, because a change would only be a consequence of the employed self-seed temperature. SSA results of LDPE grafted using ultrasound (Figure 18) display that molecular segregation also brought about nine melting endotherms corresponding to each chosen self seeding temperatures. The heating scans of grafted LDPE show a shift towards the lower melting peak temperatures and a significant increase in the second peak height, which implies that the DEM grafting modifies the chain linearity and disrupts the longer crystallizing segments. It is evident from the same figure that the height of the second endotherm increases at the expense of that of the first one with exposition time. A decrease in the width of the peaks is also noticed. This could indicate that at longer exposition times not only a higher grafting degree is achieved but also the chain branching distribution is modified. The areas of the endotherms reported in Table 11 allow confirming that when using ultrasound, the DEM grafting proceeds effectively via secondary carbons, as the chain branching also does. This fact disrupts the more linear crystallizing segments, thus reducing the area of the peak with higher melting temperature. Additionally, higher exposition times increase the possibility of DEM grafting

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Rosestela Perera, Carmen Rosales, Carmen Albano and Pedro Silva

Endo

in tertiary carbons, in terminal allyl or onto the long branches. Melting peak temperatures only display significant decreases when the exposition times are increased.

Heat Flow (a. u)

a b

c

40

60

80

100

120

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Temperature (°C)

Figure 18. Heating scan after SSA treatment of neat LDPE (a), and LDPE modified with 30 wt. % of DEM by ultrasonic radiation, b: 20 minutes of exposition time, and c: 30 minutes of exposition time.

Table 11. Partial areas obtained after integration of each DSC scan as a function of exposition time and self-seeding temperature (Ts) in the LDPE grafted with 30 wt. % of DEM by ultrasonic radiation Ts

Exposition time (min) Area (%)

Melting temperature (°C)

0

20

30

0

20

30

112

32.2

24.3

17.4

112

112

110

107

15.0

20.6

21.1

106

107

106

102

12.5

14.8

13.0

101

102

100

97

9.7

11.1

11.7

96

97

95

92

7.2

7.4

7.2

91

92

90

87

5.9

6.5

6.9

86

87

86

82

4.8

4.7

4.4

82

82

81

77

3.9

3.8

4.5

77

77

76

72

3.4

3.2

4.7

72

73

76

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Heat Flow Endo

Thermal Properties and Thermogravimetric Analysis (TGA)

49

a b

2 mcal/s

c

40

60

80

100

120

140

160

180

Temperature (°C)

Heat Flow

a b

2 mcal/s

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Endo

(A)

40

c 60

80

100

120

140

160

180

Temperature (°C) (B)

Figure 19. Heating scans after SSA treatment for (A): LLDPE2 modified in air atmosphere, a: LLDPE2eia, b: LLDPE2pia and c: LLDPE2gia, and (B): LLDPE2 modified in nitrogen, a: LLDPE2ein, b: LLDPE2pin and c: LLDPE2gin.

Comparing Figures 17 and 18, it can be said that the grafting of LDPE with DEM using ultrasonic radiation produced a higher decrease in the partial areas of the peaks corresponding to the more linear sequences than when gamma irradiation a doses lower than 100 kGy is employed for the same purpose. This difference is attributed to higher grafting degrees and/or higher main chain modification (chain branching and/or crosslinking) when ultrasonic radiation is used.

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Rosestela Perera, Carmen Rosales, Carmen Albano and Pedro Silva

Heat Flow Endo

Figures 19 and 20 show the SSA curves of LDPE and LLDPE chemically modified via reactive extrusion. Those polyolefins underwent extrusion in two different atmospheres (nitrogen and air) and using peroxide alone, in one case, and peroxide and DEM in the other.

a

2 mcal/s

b c 40

60

80

100

120

140

160

180

Temperature (°C)

Heat Flow Endo

a

b

2 mcal/s

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(a)

c 40

60

80

100

120

140

160

180

Temperature (°C) (b)

Figure 20. Heating scans after SSA treatment for (A): LDPE modified in air atmosphere, a: LDPEeia, b: LDPEpia and c: LDPEgia, and (B): LDPE modified in nitrogen, a: LDPEein, b: LDPEpin and c: LDPEgin.

It is worth noticing that both unmodified and some modified linear lowdensity polyethylene display a small melting peak at high temperatures, corresponding to the longer linear sequences and small branch contents, whose importance is decreased in the polymer extruded in air with peroxide alone.

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Thermal Properties and Thermogravimetric Analysis (TGA)

51

On the other hand, this peak is absent in the functionalized material. It disappeared probably due to the DEM insertion and/or long-chain branching, that if randomly distributed, cause a disruption of the long crystallizing sequences. Additionally, the product grafted in air displays a thinner and shorter second peak, which can be attributed either to a higher grafting degree or a higher degree of chain modification through secondary reactions, produced or promoted by the presence of air. All this aims to the fact that chain reactions proceed in a selective fashion, preferentially through secondary carbons, though the possibility of them taking place through tertiary carbons or terminal allyls is not ruled out. Lachtermacher and Rudin [40] found a decrease in the intensity of the melting peak corresponding to the more linear fraction of a bimodal peroxide-treated LLDPE in conventional DSC scans. They also reported a widening of the peak with lower melting temperature and attributed their results to an increase in chain irregularities due to the formation of long branches during reactive extrusion. In conclusion, reactive extrusion with peroxide in air of LLDPE did not totally eliminate the more linear chains. Table 2 displays that practically there are not changes in the relative concentrations of terminal vinyls. Hence, chain branching is being produced in small amounts and only in secondary carbons. On the other hand, the peroxide and DEM combined action seems to be producing a mechanism of preferential attack to secondary carbons, but in a higher extension than when peroxide alone is used. Figure 20 shows the heating curves of LDPE after SSA treatments. LDPEs reactively extruded in nitrogen do not display changes in their thermal behavior when their heating scans are compared to that of LDPE extruded without chemical modification. This fact agrees with the absence of changes in its molecular structure. On the contrary, LDPEs reactively extruded in air unfold some changes. These changes include the shortening of the peak with higher melting temperature, which is more important in the grafted LDPE. Even though LDPE exhibits thinner crystals due to its high amount of branches, it is possible that when introducing a bulky polar group, even at low concentrations, the maximum attainable lamellar thickness is further limited. Another possibility is that the increase in the amount of chain branching disrupts even more the more linear sequences, limiting the chain length capable of being included into the crystal, and hence, its thickening. Chain branching in the amorphous phase of LDPE, that is, in tertiary and vinyl carbons is not ruled out, whatsoever. As a conclusion, it is worth noticing that in functionalization it is more important the relative amount of the different types of carbon atoms than their

Main-Chain Modification as a Result of Polyolefin Functionalization by Different Techniques, Nova Science Publishers,

52

Rosestela Perera, Carmen Rosales, Carmen Albano and Pedro Silva

relative C-H bond energies. Besides, the polar monomer grafting seems to exert more influence than the introduction of additional chain branching in the melting characteristics of modified LDPE.

5.3. THERMOGRAVIMETRIC ANALYSIS

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The thermal stability of the modified polyethylenes was measured by thermogravimetric analysis (TGA). A Mettler Toledo STDA-851 equipment was used at an scan rate of 10 ºC/min. in nitrogen atmosphere. Table 12 shows the decomposition parameters such as initial decomposition temperature (Ti) and activation energy (Ea) of LDPEs modified through γ-irradiation. The Ti values show no significant changes. However, an increase in the Ea values is observed with the irradiation dose in non-functionalized LDPE. This shows that crosslinks can confer higher thermal stability to polyethylene. On the contrary, no significant changes were obtained when the DEM concentration and/or radiation dose increase. Table 12. Decomposition temperatures, initial (Ti) and end (Tf), and activation energy (Ea) from TGA as a function of the irradiation dose and DEM concentration for LDPE grafted by γ-irradiation DEM concentration (wt. %)

Irradiation dose (kGy)

Ti ± 1 (ºC)

Tf ± 1 (ºC)

0

0

447

503

352

0

30

448

505

348

0

50

450

502

365

0

100

447

502

362

0

200

449

503

369

0

400

449

501

369

5

30

449

503

357

15

30

448

503

358

30

30

449

503

361

30

50

449

503

366

30

100

447

502

359

5

200

449

503

359

15

200

447

504

350

30

200

449

504

357

30

400

449

504

360

Main-Chain Modification as a Result of Polyolefin Functionalization by Different Techniques, Nova Science Publishers,

Ea ± 5 (kJ/mol)

Thermal Properties and Thermogravimetric Analysis (TGA)

53

Table 13 exhibits the thermal parameters of LDPE grafted using ultrasonic radiation. The initial decomposition temperature and activation energy of the non-grafted LDPE do not change with radiation and/or exposition time. Nonetheless, a very slight loss in the thermal stability of grafted LDPE is reported in the same table. Table 13. Decomposition temperatures, initial (Ti) and end (Tf), and activation energy (Ea) from TGA as a function of the irradiation dose and DEM concentration for LDPE grafted by ultrasonic radiation using 2 wt. % of DP DEM concentration (wt. %)

Exposition time (min.)

Ti ± 1 (ºC)

Tf ± 1 (ºC)

Ea ± 5 (kJ/mol)

0

0

447

503

352

0

20

449

503

362

30

448

503

358

15

20

447

505

342

30

20

448

505

346

30

30

438

501

346

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0

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Chapter 6

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CONCLUSION The modifications of the main chain as a consequence of polyolefin grafting through different techniques, such as functionalization in solution and reactive extrusion, and through polymer irradiation with gamma rays and ultrasound were described in this work. Some results were presented and some mechanisms were discused. In functionalization, it is more important the relative amount of the different types of carbon atoms in polyethylene chains than their relative C-H bond energies. Hence, chain reactions proceed in a selective way, preferentially through secondary carbons, though the possibility of them taking place through tertiary carbons or terminal allyls is not ruled out.

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[16] Vainio, T; Harling, A; Seppälä, JV. Polym. Eng. Sci. 1995, 35, 225-232. [17] Burbank, F; Brauer, F; Andersen, P. SPE ANTEC Tech. Papers. 1991, 149-152. [18] Dienes, GJ; Vineyard, G.H. Radiation Effects in Solids; Interscience INC: New York, 1957; Vol. II. [19] Rånby, B; Rabek, JF. ESR Spectroscopy in Polymer Research; SpringerVerlag: Berlin, 1977. [20] Albano, C; Perera, R; Silva, P; Sánchez, Y; Rosales, C; Becerra, M. SPE ANTEC Techn. Papers. 2003, 3739-3743. [21] Silva, P; Albano, C; Lovera, D; Perera, R. Revista Mexicana de Física, 2003, 49, S3, 192-194. [22] Albano, C; Perera, R; Silva, P; Sánchez, Y. Polymer Bulletin, 2003, 51, 135-142. [23] Sánchez, Y; Karam, A; Vargas, M; Albano, C; Perera, R; Silva, P. Memorias del VI Congreso Venezolano de Química, 2003, 306-310. [24] Albano, C; Perera, R; Silva, P; Sánchez, Y. Memorias del VI Congreso Venezolano de Química, 2003, 375-378. [25] Catarí, E; Albano, C; Karam, A; Perera, R; Silva, P; González, J. Nucl. Inst. Meth. Phys. Res. B, 2005, 236, 338-342. [26] Sánchez, Y; Albano, C; Karam, A; Perera, R; Silva, P; González, J. Nucl. Inst. Meth. Phys. Res. B. 2005, 236, 343-347. [27] Silva, P; Albano, C; Karam, A; Vargas, M; Perera, P. Revista Mexicana Física, 2006, S 52, 201-203. [28] Albano, C; Perera, R.; Silva, P; Sánchez, Y. Polymer Bulletin. 2006, 57, 901-912. [29] Kaush, HH. Macromol. Symp. 2005, 225, 165-178. [30] Dole, M. Polym. Plast-Technol. Eng. 1977, 13, 41-69. [31] Cruz, P. Ultrasonics. 1983, 21, 201-204. [32] Cruz, P; Patraboy, TJ. J. Phys. Chem. 1985, 89, 3379-3384. [33] Mitchell, J. J. Applied Polymer Analysis and Characterization. Hanser: New York, 1991; Vol. II. [34] Ruggeri, G; Aglietto, M; Petragnani, A; Ciardelli, F. Eur. Polym. J. 1983, 19, 863-866. [35] Aglietto, M; Bertani, R; Ruggeri, G; Segre, AL. Macromolecules. 1990, 23, 1928-1933. [36] Ciardelli, F; Aglietto, M; Ruggeri, G; Passaglia, E; Castelvetro, V. Macromol. Symp. 1997, 118, 311-316. [37] Jois, YH; Bronk, JM. Polymer. 1996, 37, 4345-4356.

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[38] Heinen, W; Van Duin, M; Rosenmöller, CH; Wenzel, C; De Groot, HJ. M; Lugtenburig, J. SPE ANTEC Tech. Papers. 1997, 2017-2021. [39] Bremner, T; Rudin, A. Plast. Rubb. Procc. Appl. 1990, 13, 61-66. [40] Lachtermacher, GM; Rudin, A. J. Appl. Polym. Sci. 1995, 58, 24332449. [41] Rojas, B; Fatou, JG; Martínez, C; Laguna, O. Eur. Polym. J. 1997, 33, 725-728. [42] Fodor, ZS; Iring, M; Tüdös, F; Kelen, T. J. Polym. Sci. Polym. Chem. Ed. 1984, 22, 2530-2550. [43] Smedberg, A; Hjertberg, T; Gustafsson, B. Polymer. 1997, 38, 41274138. [44] Lachtermacher, M; Rudin, A. J. Appl. Polym. Sci. 1995, 58, 2077-2094. [45] Abraham, D; George, KE. ; Francis, DJ. J. Appl. Polym. Sci. 1998, 67, 789-797. [46] Cousin, P; Bataille, P; Schreiber, HP; Sapieha, S. J. Appl. Polym. Sci. 1989, 37, 3057-3050. [47] Kim, KJ; Kim, BK. J. Appl. Polym. Sci. 1993, 48, 981-986. [48] Dontula, N; Campbell, GA., Connelly, R. Polym. Eng. Sci. 1993, 33, 271-278. [49] Bremner, T; Rudin, A; Haridoss, S; Polym. Eng. Sci. 1992, 32, 939-943. [50] Samay, G; Nagy, T; White, J. J. Appl. Polym. Sci. 1995, 56, 1423-1433. [51] González, E; González, M; González, M. J. Appl. Polym. Sci. 1998, 68, 45-52. [52] Zhang, Y; Li, H. Polym. Eng. Sci. 2003, 43, 774-782. [53] Sánchez, A; Rosales, C; Múller, AJ. SPE ANTEC Techn. Papers. 1997, 3761-3765. [54] Rosales, C; Perera, R; González, J; Ichazo, M; Rojas, H; Sánchez, A. J. Appl. Polym. Sci. 1999, 73, 2549-2567. [55] Sánchez, A; Rosales, C; Laredo, E; Müller, A. J; Pracella M. Macromol. Chem. Phys. 2001, 202, 2461-2478. [56] Rosales, C; Márquez, L; Perera, R; Rojas, H; Eur. Polym. J. 2003, 39, 1899-1915. [57] Brown, SB. Annu. Rev. Mater Sci. 1991, 21, 409-435. [58] Guadagno, L; Naddeo, C; Vittoria, V; Camino, G; Gagnani, C. Polym. Deg. Stab. 2001, 72, 175-186. [59] Mandelkern, L; Alamo, R. Polymer Data Handbook; Oxford University Press, Inc, 1999, 493-507. [60] Suwanda, D; Balke, ST. Polym. Eng. Sci. 1993, 33, 455-465.

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[61] Haslam, J; Willis, HA. Identification and Analysis of Plastics, Iliffe Books: London, 1965; 24-37, 72-77, 96-99, 154-167, 370. [62] Rudin, A; Grinshpun, V; O'Driscoll, KF. J. Liq. Chrom. 1984, 7, 18091821. [63] Hendra, PJ; Peacock, AJ; Willis, HA. Polymer. 1987, 28, 705-709. [64] Müller, AJ; Hernández, ZH; Arnal, ML; Sánchez, JJ. Polym. Bull. 1997, 39, 465-472. [65] Müller AJ; Arnal, ML. Prog. Polym. Sci. 2005, 30, 559-603. [66] Arnal, ML; Hernández, ZH; Matos, M; Sánchez, JJ; Méndez, G; Sánchez, A; Müller, AJ. SPE ANTEC Techn. Papers. 1998, 2007-2011. [67] Márquez, L; Rivero, I; Müller, A. J. Macrom. Chem. Phys. 1999, 200, 330-337. [68] Villarreal, N; Pastor, JM; Perera, R; Rosales, C; Merino, JC. SPE ANTEC Techn. Papers. 2002, 3812-3816. [69] Fillon, B; Wittmann, JC; Lotz, B; Thierry, A. J. Polym. Sci. Polym. Phys. 1993, 31, 1383-1393. [70] Wang, Y; Ji, D; Yang, C; Zhang, H; Qin, C; Huang, B. J. Appl. Polym. Sci. 1994, 52, 1411-1417. [71] Kao, YH; Phillips, P. J. Polymer. 1986, 27, 1669-1678.

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INDEX

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A abstraction, 14, 28, 33 acetone, 18, 24 acetophenone, 23 activation energy, 52, 53 active site, 2 additives, 24 annealing, 41, 44 antioxidant, 5 atoms, 2, 3, 4, 44 authors, 17, 24, 33

B behavior, 5, 6, 20, 37, 39, 41, 42, 47, 51 bending, 9 benzoyl peroxide, 20 blends, 1, 5, 6 bonds, 10, 27, 28, 29 branching, 4, 13, 37, 39, 43, 44, 47, 51 bromine, 11

C calibration, 12, 17, 35 carbon, 10, 31, 43, 44, 51, 55 carbon atoms, 44, 51, 55 catalyst, 31

chain branching, 10, 13, 14, 17, 27, 28, 30, 31, 32, 43, 44, 47, 49, 51, 52 chain mobility, 7, 44 chain scission, 15, 16, 18, 30, 33, 37 chain transfer, 11, 32 chemical properties, 1 chemical reactions, 3, 9 cleaning, 1, 9 collateral, 37, 44 compatibility, 2, 12, 23 competition, 1, 15, 37 composition, 12, 31 compression, 12, 35 concentration, 5, 6, 7, 8, 13, 17, 18, 19, 20, 21, 22, 23, 27, 28, 30, 31, 32, 36, 37, 38, 39, 42, 52, 53 conditioning, 42 configuration, 18, 23, 30, 31 construction, 1 consumption, 13, 28 control, 30 cooling, 41, 44, 47 copolymers, 33 coupling, 1, 13, 16, 18, 27, 28, 32 crystalline, 6, 7, 41, 43, 44, 47 crystallinity, 10, 42, 43, 45 crystallization, 41, 42, 43, 44, 45, 47 crystals, 2, 44, 47, 51 cycles, 44

Main-Chain Modification as a Result of Polyolefin Functionalization by Different Techniques, Nova Science Publishers,

Index

62

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D decay, 6, 7 decomposition, 17, 23, 29, 30, 33, 52, 53 decomposition temperature, 52, 53 defects, 2, 43, 44 deformation, 9, 27 degradation, ix, 1, 2, 3, 4, 9, 24, 31, 40 degradation process, 3 degree of crystallinity, 6 density, 4, 6, 10, 13, 18, 23, 33, 43, 44, 50 depolymerization, 9 detection, 31 dielectrics, 2 differential scanning, 41 differential scanning calorimetry, 41 diffusion, 2, 7, 18, 23, 29, 32 dispersion, 2 displacement, 3, 37 dissociation, 9 distribution, 1, 2, 23, 30, 36, 37, 39, 47 double bonds, 31 DSC, 41, 46, 48, 51 dynamic viscosity, 40

E electron, 4 electrons, 3 emulsion polymerization, 9 emulsions, 9 endotherms, 43, 47 energy, 2, 3, 9, 11, 29, 31, 33 environment, 1, 30 ESR, 3, 58 ester, 10 ethanol, 20 ethylene, 12, 33, 43 excitation, 2, 3 exercise, 9 experimental design, 17, 18 exposure, 28 expulsion, 44 extinction, 12

extrusion, ix, 1, 2, 12, 23, 24, 25, 30, 31, 33, 38, 39, 42, 43, 45, 50, 51, 55

F films, 12 fragments, 41 free energy, 44 free radicals, ix, 3, 4, 11, 17, 24, 27, 31, 32 FTIR, 9, 10, 11, 12, 20, 27, 28 FTIR spectroscopy, 10 functionalization, ix, 2, 11, 12, 17, 20, 23, 24, 29, 30, 31, 32, 33, 35, 39, 44, 51, 55

G gamma radiation, 37 gamma rays, ix, 2, 55 gel, 24, 28, 30 generation, 6, 11, 13 glasses, 2 GPC, 30, 35 grades, 9 groups, 9, 10, 12, 13, 15, 16, 18, 27, 28, 31, 32, 44 growth, 15, 16, 31

H HDPE, 4, 5, 6, 7, 12, 13, 15 heat, 23 heating, 41, 44, 47, 51 heating rate, 41 height, 47 heptane, 20, 24 heterogeneity, 44 hexane, 17, 18, 20 homopolymerization, 1 homopolymers, 7 hydrogen, 11, 13, 14, 23, 28, 29, 32, 33, 43 hydrogen abstraction, 11, 13, 14, 33 hydrogen atoms, 28, 33, 43 hyperfine interaction, 4

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Index

I

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inclusion, 29 incompatibility, 23 indication, 10, 28 indicators, 10 initiation, 4, 6, 24 injury, vi insertion, 17, 29, 32, 33, 51 insulators, 2, 3 integration, 46, 48 interaction, 2, 3 interactions, 23 interdependence, 31 ionization, 2, 3 ionizing radiation, 3 ions, 3 irradiation, ix, 2, 4, 5, 6, 10, 19, 20, 21, 27, 28, 29, 35, 36, 37, 38, 39, 42, 45, 46, 49, 52, 53, 55 isothermal crystallization, 47 isotope, 11

L labeling, 12 laws, 3 line, 4 linear molecules, 44 linearity, 47 liquids, 9, 31 low density polyethylene, 12 luminescence, 2

M macromolecules, 11 macroradicals, 9, 11, 15, 20, 29 media, 2, 20, 29, 32 melt, 12, 16, 18, 30, 31, 35, 38, 39, 41, 44 melt flow index, 18, 35, 38, 39 melting, 18, 19, 30, 41, 42, 43, 44, 45, 47, 50, 51, 52

63

melting temperature, 18, 42, 43, 44, 45, 47, 51 methyl groups, 10, 32 MFI, 18, 20, 38, 39 migration, 23 mixing, 1, 30, 31 mobility, 29 model, 6, 31, 41 models, 11 modified polymers, 13 molecular structure, 51 molecular weight, 11, 13, 16, 18, 23, 27, 28, 30, 32, 35, 36, 37, 38, 39 molecular weight distribution, 13, 18, 23, 30, 35, 36, 37, 38, 39 molecules, 3, 10, 11, 20, 29, 37, 47 monomers, 2, 9

N nanocomposites, 1 nitrogen, 18, 24, 31, 32, 40, 41, 43, 49, 50, 51, 52 NMR, 11, 12, 18 nonane, 11 nucleation, 41 nuclei, 3, 41

O octane, 12 oil, 17 order, 1, 9, 11, 12, 18, 23, 31, 41 organic polymers, 2 oxidation, 3, 4, 30, 31, 40 oxygen, 4, 24, 34

P parameter, 20, 28 parameters, 18, 52, 53 particles, 2

Main-Chain Modification as a Result of Polyolefin Functionalization by Different Techniques, Nova Science Publishers,

Index

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64

peroxide, ix, 2, 11, 12, 13, 15, 16, 17, 18, 19, 20, 22, 23, 24, 29, 30, 32, 33, 39, 40, 43, 44, 50, 51 peroxide radical, 32 polarity, ix polyethylenes, 6, 7, 10, 12, 14, 23, 24, 27, 32, 33, 35, 42, 44, 52 polymer, ix, 1, 2, 4, 6, 9, 10, 18, 19, 20, 24, 28, 30, 31, 33, 39, 41, 43, 44, 50, 55 polymer chains, 1 polymer matrix, 4 polymer properties, 1 polymer structure, 39 polymer systems, 9 polymerization, 3, 4, 9, 31, 32 polymerization mechanism, 31 polymerization process, 3, 32 polymerization processes, 3 polymers, 1, 3, 4, 6, 7, 39, 41, 44 polyolefins, ix, 1, 4, 9, 11, 12, 21, 23, 41, 44, 50 polypropylene, 20 polystyrene, 35 pressure, 9, 10, 24, 31 probability, 20 production, 9, 13, 32 propagation, 4, 11 propylene, 32, 33 protons, 4

R radiation, 1, 2, 3, 6, 8, 9, 10, 11, 19, 20, 21, 22, 27, 28, 29, 36, 37, 47, 48, 49, 52, 53, 58 radical formation, 3 radical mechanism, 9 range, 1, 35 reactants, 1, 2, 30, 31, 32 reaction mechanism, 33 reaction medium, 18 reaction rate, 7, 31 reaction temperature, 15 reaction time, 17, 18 reaction zone, 31

reactive sites, 33 reactivity, 2, 11, 18, 23, 32 recombination, 6, 7 recovery, 1 region, 9 relationship, 9 resins, 13 resolution, 31, 32 room temperature, 41

S saturation, 21 seed, 41, 47 seeding, 41, 46, 47, 48 segregation, 41, 47 shear, 24, 31, 35 shear rates, 24 solid polymers, 4 solubility, 18, 36 species, 13, 23 spectroscopy, 11 spectrum, 4, 5, 7 speed, 1, 23, 31 stability, 44, 52 stabilization, 44 stable radicals, 7 storage, 6, 7, 8 strain, 31 stress, 9 stretching, 10 surface properties, 3 surface reactions, 3 susceptibility, 3

T temperature, 1, 9, 17, 18, 23, 29, 30, 31, 33, 41, 42, 43, 44, 45, 46, 47, 48 TGA, vii, 41, 52, 53 thermal decomposition, 11 thermal properties, 42 thermal stability, 52, 53 thermograms, 42

Main-Chain Modification as a Result of Polyolefin Functionalization by Different Techniques, Nova Science Publishers,

Index thermogravimetric analysis, 52 torsion, 27 transport, 30, 31

U

65

V vacancies, 2 vacuum, 20, 24 variables, 1, 23, 30 viscosity, 2, 18, 23, 25, 30, 31, 32, 36, 40

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ultrasound, ix, 2, 9, 11, 19, 21, 28, 29, 47, 55

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