Thermal Stability, Degradation, and Stabilization ... - ACS Publications

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Downloaded by PENNSYLVANIA STATE UNIV on June 6, 2012 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/ba-1996-0249.ch002

Thermal Stability, Degradation, and Stabilization Mechanisms of Poly(vinyl chloride) Béla Iván

1

Central Research Institute for Chemistry of the Hungarian Academy of Sciences, H-1525 Budapest, Pusztaszeri u. 59-67, Hungary

This survey concerns the major features tion, and stabilization crostructure

of poly(vinyl

of stability, thermal

chloride) (PVC).

and chain defects of PVC on the stability of the resin is

analyzed and reviewed in the light of recent findings. New results of thermal degradation

role of PVC stabilizers

chlorine substitution ing degradation.

These results indicate that the most is not preventing

(reversible

consumption,

polyenes.

-catalyzed thermolysis after consumption with degradation

of HCl dur-

the blocked

blocking mechanism).

action of blocking is the attachment the propagating

initiation by labile

but blocking the fast zip-elimination

After stabilizer

undergo reinitiation

experimental

of PVC in dilute solution in the presence

of stabilizers are also summarized. important

degrada-

The effect of mi-

of ester or thioglycolate

Reinitiation

structures

The possible regroups to

is explained by the rapid acid-

of these structures by the appearance of free HCl

of stabilizers.

Comparison

of experimental

findings

kinetics expected on the basis of the Frye-Horst,

Minsker, and the Michell

stabilization

the

mechanisms indicates that none

of these mechanisms is able to explain stabilization

of PVC.

P O L Y ( V I N Y L CHLORIDE) (PVC) is still one of the major commodity polymers

produced and consumed worldwide, despite some recent concerns. Because of the attractive economy of production and processing and the ease of prop erty variations from elastomeric to hard final products, this polymer is ex pected to be one of the significant members of the commercial plastics arena Current address: University of Mainz, Institute of Physical Chemistry, Welderweg 11-15, D55099 Mainz, Germany

0065-2393/96/0249-0019$12.00/0 © 1996 American Chemical Society In Polymer Durability; Clough, R., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1996.


20

POLYMER DURABILITY

for the foreseeable future. Despite its advantageous properties, P V C exhibits relatively low thermal, thermooxidative, and light stability (see reference 1 for a comprehensive review). Therefore, this resin cannot be used without effi cient stabilization; processing and use also require the suppressing of degra dation. As a consequence, the stability, degradation, and stabilization of this resin have been the subject of research and development projects since its appearance on the market more than half a century ago. Even though the identification and use of suitable stabilizers has occurred, the fine details of the mechanism and kinetics of P V C degradation and sta bilization have remained largely unknown. In the last 10 years, increased in terest and research efforts have revealed several new aspects of these proc esses. The focus of these new efforts include studying the microstructure of P V C with the newest powerful analytical techniques (1-6), the relation be tween chain defect structures and stability (1, 2, 7-15), the major character istics of the kinetics of thermal dehydrochlorination of P V C (J, 8, 15-23), degradation of dilute P V C solutions in the presence of stabilizers (1, 19, 2429), and the formulation of the reversible blocking mechanism (15, 24-27) of P V C stabilization. This chapter surveys thermal stability, degradation, and stabilization of P V C in fight of these recent results. A n overview of the Frye-Horst (30, 31), Minsker (32), and Michell (33, 34) stabilization mechanisms and the reversible blocking (15, 24-27) mechanism will also be included.

Microstructure and Thermal Stability of PVC Irrespective of the type of degradation (thermal, thermooxidative, or light), the major chain degradation process of P V C is zip-elimination of H C l and the simultaneous formation of conjugated double-bond-containing sequences (polyenes) in the polymer chain (Scheme I). The main dehydrochlorination

CH^

~ " W

C

H

CHs

CH" 2 CH^ 2 CH^ ι ι « CI CI CI i - HCl

^ C H "

CH

C

H

N

x

CH

C H ' ι

C

H

2

s

X

Cl

C H ' " I

Ci

j-HCl Scheme I. Zip-elimination of HCl from PVC and simultaneous formation of sequences containing conjugated double bonds (polyenes) in the chain during degradation.

X

C H * ^ C H ^ ^ C H * ^ CH'" ι CI X

N

ι

In Polymer Durability; Clough, R., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1996.

N


2.

IVAN

Stabilization of PVC

21

process of P V C degradation yields a corrosive gas, H C l , and leads to highly reactive polyenes absorbing light in the U V and visible region. This absorption results in undesired coloration from yellow to nearly black in some cases. The oxygen sensitivity of polyenes and their secondary processes such as crosslinking (35) result in further undesired deterioration and changes in chemical resistance and physical properties of the resin. Because heat is one of the major causes of degradation during processing, thermal degradation and its prevention have attracted the most attention. The primary dehydrochlorination process during thermal degradation of this poly mer involves three steps (J): • initiation of H C l loss • rapid zip-elimination of H C l and simultaneous formation of polyenes in the P V C chain • termination of the zipping process On the basis of experiments with low-molecular weight model com pounds, allylic and tertiary chlorine containing defects in the polymer chain were thought to be exclusively responsible for the low thermal stability of P V C (I). However, kinetic studies (36) and some mechanistic considerations (2) indicated random initiation by regular - C H C H C l - repeat units. Thermal degradation experiments (8) with P V C enriched with allyl chlorine (37) proved that random initiation of H C l loss also occurs during thermal degradation of P V C . The rate constant of random initiation was four orders of magnitude lower than that of initiation of degradation by defect structures such as allylic or tertiary chlorines during degradation of dilute P V C solutions at 200 °C (8). One of the most interesting aspects of these results is that the difference between the concentrations of labile and regular secondary chlorines in P V C yields similar dehydrochlorination rates either by random or labile site initi ations. In a practical sense, the low thermal stability of P V C can be viewed as its inherent property because of the existence of random initiation. There fore, PVCs having high thermal stability probably cannot be prepared by freeradical polymerization of vinyl chloride (1, 8, 15). 2

Degradation studies were performed with PVCs containing increased con centrations of tertiary (7, 11, 12) or allylic (8-12) chlorines and with polymers having at least one of the labile structures: polychloroprene (38-40), chlori nated butyl rubber (39), chlorinated ethylene-propylene copolymers (39), and tertiary chlorine-terminated polyisobutylenes (41). These studies have un doubtedly proved the importance of chain defects of P V C containing allylic and tertiary chlorine and their effect on thermal dehydrochlorination. Substi tution of labile chlorines with alkyl groups by treatment with alkylaluminum compounds such as trimethylaluminum (39, 42-44) and dimethylcyclopentadienyl aluminum (7, 38, 40, 43-A6) yielded significant decreases in the rate



22

POLYMER DURABILITY

of H C l loss in every case. These results provided direct additional evidence for the thermal instability of moieties containing allylic and tertiary chlorine. Detailed microstructure investigations (J, 4, 12) revealed that allylic chlo rines can be present at chain ends (I in Chart I) and in the main chain (II in Chart I). Recent studies {4, 47) indicated that the majority of allylic chlorines are located at chain ends. However, on the basis of Ή N M R studies with low-molecular weight P V C fractions, chain end I in Chart I is thermally stable (48). Tertiary chlorines can be present at branching points for short- and longchain branches in P V C . Ethyl branches, considered previously as one of the major sources of tertiary chlorines, have tertiary hydrogens at the branch points (6, 47). Thus, tertiary chlorines are mainly located at the starting points of η-butyl (III, Chart I) and long-chain branches (IV, Chart I) (I). Minsker and co-workers (13, 14) claimed that keto-allyl groups (V in Chart I) are the most important labile structures in P V C , and these chain defects are solely responsible for the low thermal stability of the resin. How ever, conclusive experimental evidence for the presence of keto-allyl groups in P V C has not been reported to date. Data obtained by studying the thermal decomposition of model compounds containing keto-allyl groups and their derivatives (49) indicated that only the cis isomer of V in Chart I leads to rapid dehydrochlorination, whereas the trans isomer is more stable than the secondary chlorine-containing regular monomer units of the polymer.

— C H = CH-CH C! 2

— C H = CH-CH-

2

I CI II

CH -CHCI-CH -CH CI

CH -CHCI-CH -CHCI-

I

I

2

2

2

2

-CH —C—CH — *** 2

2

**· —CH —C-~CH — *·

2

2

2

I

I

CI

CI

III

IV

—C-CH=CH-CH—

I) ο

I ci V

Chart I.


2.

IVAN


23

The source of random initiation of H C l loss has not been satisfactorily revealed. O n the basis of model experiments with secondary chloroalkanes, one might conclude that spontaneous unimolecular elimination of H C l from regular - C H - C H C 1 - repeat units by heat occurs. This process yields an allylic chlorine moiety and leads to subsequent fast zip-ehmination of H C l . Forma tion of cyclic chloronium ions by two neighboring monomer units resulting in initiation of H C l loss was also proposed for random initiation (2). The effects of tacticity on the thermal stability of P V C were studied i n tensively by Millân and co-workers (50, 51). The rate of H C l loss increased with increasing concentrations of G T T G " isotactic triads (50, 51). The length of polyenes in thermally degraded PVCs also depends on the tacticity. O n the basis of these findings the stereostructure of the polymer can be considered as one of the potential sources of random initiation. Although a double loga rithmic plot exhibited correlation between the rate of dehydrochlorination and the concentration of isotactic triads in P V C (12), several effects that occur simultaneously, such as initiation by chain defects, effects induced by the mor phology of the resins and HCl-catalysis, should be separated to draw reliable conclusions on the role of tacticity in the thermal stability of P V C .


2

Stabilization Mechanisms T h e Reversible Blocking Mechanism. Primary heat stabilizers of PVC, such as metal carboxylates and organotin compounds, usually possess two major roles: scavenging H C l and reacting with the polymer to prevent H C l loss and simultaneous discoloration (J). The major problem of P V C sta bilization mechanisms is determining how the stabilizers influence primary degradation processes such as initiation of H C l loss, fast zip-ehmination of H C l , and termination of unzipping. However, most of the stabilization studies with P V C have been carried out in bulk (with polymer melts, films, and pow ders), and the reaction conditions were far from ideal for investigating the kinetic and mechanistic details of the stabilization process. The effects of sam ple preparation, sample inhomogeneity, stabilizer distribution in the matrix, morphological differences and changes, diffusion problems, HCl-catalysis, and the influence of oxygen in air cannot be excluded when using solid samples. To overcome the problems connected with solid P V C samples, systematic thermal (J, 15, 24, 25) and thermooxidative (I, 15, 25, 27) experiments with dilute P V C solutions in the presence of a variety of stabilizers were performed in our laboratories during the last 10 years. P V C solutions (1%) in 1,2,4trichlorobenzene were degraded at 200 °C. The kinetics of free H C l evolution, formation of double bonds in the main chain, the average length of polyenes, and the initiation of zip-elimination were investigated in the absence and pres ence of industrial stabilizers such as lead stéarate, barium stéarate, cadmium


24

POLYMER DURABILITY

stéarate, calcium stéarate, zinc stéarate, dibutyltin distearate, dibutyltin maleate, and dioctyltin bisthioglycolate. Our stabilization studies with dilute P V C solutions have allowed us to gain new information on the kinetics and mechanism of P V C stabilization. The following major experimental findings were obtained (see Figures 1-4 for the characteristics of these results):


1. All the stabilizers yielded induction periods for the evolution of free H C l .

TIME Figure 1. Extent offree HCl evolution as a function of time obtained in thermal degradation experiments for unstabilized PVC and stabilized PVC (solid lines) and constructed by Frye-Horst, Minsker, and Michell mechanisms (dashed line) for stabilized PVC. Axes are in arbitrary units.

è (4-12) p

Figure 2. Number of double bonds in polyenes with length 4-12 [^(4-12)] as a function of time in experiments (solid lines) and constructed on the basis of different stabilization mechanisms (dashed lines). Axes are in arbitrary units.


2.

IVAN

25



η

I

TIME

Figure 3. Average length of polyenes as a function of time obtained by experiments and constructed by the different stabilization mechanisms. Axes are in arbitrary units. 2. The initial rates of free H C l evolution at the end of the induction periods were significantly higher in the presence of stabilizers. 3. Detectable amounts of short polyenes were formed during the induction periods. 4. The formation of double bonds and longer polyenes increased sharply at the end of the induction periods. 5. The average length of polyenes as a function of time exhibited a maxi mum in the vicinity of the end of the induction periods. 6. In some cases the concentration of labile structures in P V C estimated by kinetic analysis was higher at the end of the induction period than that in virgin P V C . 7. Initiation of H C l loss was not prevented during the induction periods, and the rate of initiation did not change at the end of the induction periods. This unexpected and strange set of observations—the rapid increase in the rates of free H C l evolution and in the corresponding double bonds (poly enes) but no change in the rate of formation of internal segments containing double bonds (new initiating sites) at the end of the induction periods—cannot be explained by the widely cited Frye-Horst (30, 31), Minsker (32), or Michell (33, 34) mechanisms. The unchanged rate of initiation at the end of the in duction period indicates that the sudden increase in the rate of H C l loss and double bond formation is due to reinitiation at already existing polyenes formed during the induction periods. During the induction periods, initiation of H C l loss is not inhibited by the stabilizers. However, shortly after initiation the propagating polyenes formed by unzipping of H C l are blocked by the


26

POLYMER

DURABILITY

REVERSIBLE BLOCKING

CONC.OF INTERNAL POLYENES

js^MINSKER FRYE-HORST -MICHELL


1

Figure 4. Concentration of internal polyenes (initiating sites) as a function of time obtained by experiments and constructed by the different stabilization mechanisms. Axes are in arbitrary units. stabilizers. Therefore, only short sequences containing conjugated double bonds are formed until stabilizer is present in the degrading systems. This mechanism, the reversible blocking mechanism, together with the primary process of thermal degradation of P V C is exhibited in Scheme II. For com parison, the Frye-Horst and Minsker mechanisms (see next section) of re placement by stabilizers of labile chlorines originally present in the resin are also shown in Scheme II. The most plausible explanation for the blocking reaction is the attachment of ester (or thioglycolate) groups to the propagating polyenes. Indeed, some literature data indicate that the organic groups of stabilizers incorporate into the P V C chain in the course of degradation in dilute solution (28, 29, 52) and in bulk samples (30, 31). The rapid degradation (dehydrochlorination and si multaneous double bond formation in the P V C chain) obtained at the end of the induction periods is due to the rapid acid-catalyzed thermolysis (Ei elim ination) of the allylic ester during the appearance of free H C l (Scheme III). This reaction yields a polyene terminated with an allylie chlorine; the thermal stability of this polyene is significantly lower than that of the corresponding ester in the absence of an acid. Interestingly, the acid-catalyzed thermolysis of labile (tertiary, ailylic, and benzylic) esters and carbonates was used to develop a large variety of photoresists in which the proton (acid) was gener ated by photochemical means (53-55).

Critical Evaluation of PVC-Stabilization Mechanisms. The widely accepted (56, 57) Frye-Horst hypothesis (30, 31) assumes that primary P V C stabilizers (metal carboxylates and organotin compounds) act simply by substituting labile (tertiary and allylic) chlorines (I-IV in Chart I) in the resin


2.

IVAN


PVC

27

\

substitution ^ S U B S T I T U T E D

'

of labile chlorines

*

LABILE SITES

(Frye-Horst and Minsker hypotheses)


initiation

HCl

zipelimination

blocking by stabilizer ^

GROWING ACTIVE POLYENES

BLOCKED POLYENES

fast reinitiation by free HCl appearance

termination

TERMINATED "INACTIVE" POLYENES

Scheme II. The primary processes of thermal degradation and the reversib blocking mechanism of PVC stabilization (Frye-Horst and Minsker mechanisms are also shown).

H

I —CH—CH—-CH-r-C——CH—CHA

O

Λ

I

Wv )

H

/

CI

I

Η

—CH=CH—CH=CH—CH—CH — 9

+ RCOOH

+

H®

I CI

Scheme III. Acid catalyzed E elimination of allylic ester from PVC chain after appearance offree HCl t


POLYMER DURABILITY


28

for more stable groups (ester or thioglycolate). The Minsker mechanism is analogous to the Frye-Horst proposition: Minsker and co-workers (32) claimed that the sole function of stabilizers is an exchange reaction between the ketoallyl groups (V in Chart I) and the ester or thioglycolate groups of stabilizers. Thus, suppressing or preventing degradation by stabilizers is due to the fast substitution of labile chlorines in P V C . This process subsequendy inhibits initiation of H C l loss during degradation according to the Frye-Horst and Minsker mechanisms. The Michell mechanism (33, 34) assumes radical chain dehydrochlorina tion of P V C in which chlorine radicals are the chain carriers. In the presence of stabilizers, the chlorine atoms are thought to react with the stabilizer and to yield metal chlorides and stéarate free radicals. This reaction terminates the chain propagation of H C l loss by trapping chlorine atoms and by esterifying P V C macroradicals that have unpaired electrons with stéarate free rad icals. Experimental results obtained in the course of our systematic studies con cerning degradation of dilute P V C solutions in the presence of stabilizers offer a unique opportunity for a critical evaluation of these different stabilization mechanisms. O n the basis of these stabilization mechanisms, kinetic curves (i.e., the expected shape of free H C l evolution), concentration of double bonds, average polyene length, and number of internal polyene sequences (initiating sites of degradation) as a function of time can be constructed and compared to experimental findings. Although random initiation was not known at the time the Frye-Horst mechanism (30, 31) was proposed, this process is also considered in constructing the kinetic curves. Figures 1-4 summarize the results of comparing the expected kinetics of the various stabilization mech anisms with experimental findings obtained during degradation of dilute P V C solutions in the presence of stabilizers (1, 15, 19, 24-27). In these curves, experimental findings are drawn by solid curves, whereas the kinetic curves expected on the basis of the Frye-Horst, Minsker, and Michell mechanisms are displayed by dotted fines. When the experimental results and the predic tions according to one or other mechanisms are in agreement, solid fines are used. As shown in Figure 1, the Frye-Horst, Minsker, and Michell mechanisms do not lead to increased rates of free H C l evolution at the end of the induction periods. According to all of these mechanisms, dehydrochlorination should occur by a constant rate (by random initiation on the basis of the Frye-Horst and Minsker mechanisms) after consumption of stabilizers. The concentration of double bonds in polyenes with conjugation length 4-12 [ξ (4-12)] versus time plots are exhibited in Figure 2. Because the ex clusive role of stabilizers is substitution of labile chlorines originally present in P V C by more stable groups according to the Frye-Horst (30, 31) and M i n sker (32) mechanisms, the concentration of double bonds as a function of time should increase monotonously because of random initiation. A rate increase ρ


2.

IVAN

29


and thus a break point is expected by the Michell mechanism at the end of the induction period in the | (4-12) versus time plot. This increase is due to the raise of the kinetic chain length of dehydrochlorination in the radical mechanism proposed by Michell (33, 34) after the consumption of stabilizers. As this analysis indicates, neither of these mechanisms can provide an expla nation for the sharp increase of ξ (4-12) at the end of the induction periods. According to the trends in Figure 3, only the Michell mechanism would yield maximum curve for the average length of polyenes (n) as a function of time. However, because of the absence of inhibition of random initiation by stabilizers, the shape of the η versus time plots should be similar to that of the unstabilized P V C on the basis of the Frye-Horst and Minsker mechanisms. As discussed in the previous section, estimation of the concentration of labile sites leading to fast H C l loss at the end of the induction periods (t ) gave higher concentrations in some cases than the labile chlorine concentra tion in the virgin P V C . However, the concentrations of any labile sites at t should not be higher than that in the starting material according to the F r y e Horst, Minsker, and Michell mechanisms. The concentration of internal polyenes (initiating sites) as a function of time is shown in Figure 4. By considering random initiation, the Frye-Horst and Minsker mechanisms give the same plot as observed for unstabilized and stabilized P V C . However, if the concentration of tertiary chlorine in P V C is not negligibly low (which is the case in most PVCs), then rates of polyene formation in stabilized P V C are expected to be lower than the rates of for mation in the unstabilized resin during the induction period according to the Frye-Horst mechanism. This difference is due to substitution of these labile chlorines. According to the Michell mechanism, the initiation should be lower for the stabilized P V C during the induction period. However, this mechanism would result in an increase of initiation rate after the end of the induction period, and this result is contrary to experimental findings. p


ρ

{

{

Concluding Remarks According to our analysis, the Frye-Horst, Minsker, and Michell mechanisms do not fully explain the currently available experimental results concerning the stabilization of P V C . The grounds of the Michell mechanism—a radical deg radation and corresponding stabilization—are entirely questionable. One shortcomings of this mechanism is that it is still not completely clear whether thermal degradation of P V C occurs by molecular, ionic, or radical mechanisms (I). O n the other hand, most of the primary P V C stabilizers have ionic char acter, and the exclusive formation of carboxylate radicals in a simple exchange reaction between chlorine atoms and stabilizers composed of ionic bonds is hardly believable. Chlorine radicals are highly reactive species possessing low selectivity in their reactions.


POLYMER DURABILITY


30

As the experimental data of our systematic studies indicate, the main role of stabilizers is not the substitution of labile chlorines present i n PVCs (as claimed by the Frye-Horst and Minsker mechanisms). Instead, the main role is the blocking of the propagating ("active") polyenes, as concluded by the reversible blocking mechanism (see Schemes II and III). In a recent paper, Starnes (58) argued that the reversible blocking mechanism should be viewed as the Frye-Horst hypothesis. H e claimed that the reversible blocking mech anism should also result i n replacement of allylic chlorines adjacent to poly enes (58). However, this replacement would occur only if degradation of P V C proceeded exclusively by a step-by-step molecular mechanism (59). As analyzed in a recent review (1 ), experimental findings exist to support either a radical or ionic mechanism of thermal dehydrochlorination of P V C . Allylic chlorines adjacent to polyenes would not exist in the extremely fast zipelimination process by either of these mechanisms. As shown earlier by ther mal degradation of PVCs containing increased concentrations of allylic chlo rines adjacent to short polyenes, the activity of these labile chlorines is significantly lower than that of the unzipping (propagating) active polyenic structures (8). This difference also indicates that allylic chlorines most likely do not exist in the zip-elimination process at all. Therefore, the blocking re action by thermal stabilizers of P V C cannot be viewed as a simplified displace ment of labile allylic chlorines adjacent to polyenes i n degrading P V C , as claimed by the Frye-Horst mechanism (59). As exhibited in Scheme II, the reversible blocking mechanism (15, 2427) also leads to a significant shift i n our views concerning stabilization of P V C . Therefore, the focus of P V C stabifization is changed from preventing initiation to the second major step of the primary degradation of P V C , that is, to the unzipping of H C l loss and thus to the fast interaction between the propagating polyenes and stabilizers according to the reversible blocking mechanism.

Acknowledgments Support by the Hungarian Scientific F u n d ( O T K A 4453) is gratefully acknowl edged.

References 1. Iván, B.; Kelen, T.; Tüdös, F. In Degradation and Stabilization of Polymers; Jellinek, H. H. G.; Kachi, H., Eds.; Elsevier Science: Amsterdam, Netherlands, 1989; Vol. 2, pp 483-714. 2. Starnes, W. H., Jr. Dev. Polym. Degrad. 1981, 3, 35. 3. Hjertberg, T.; Wendel, A. Polymer 1982, 23, 1641. 4. Llauro-Darricades, M. F.; Michel, Α.; Guyot, Α.; Waton, Α.; Petiaud, R.; Pham, Q. T. J. Macromol. Sci. Chem. 1986, A23, 221. 5. Benedikt, G. M. J. Macromol. Sci. Pure Appl. Chem. 1992, 129, 85.



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6. Starnes, W. H., Jr.; Wojciechowski, Β. J. Makromol. Chem. Macromol. Symp. 1993, 70/71, 1. 7. Iván, B.; Kennedy, J. P.; Kende, I.; Kelen, T.; Tüdös, F. J. Macromol. Sci. Chem. 1981, A16, 1473. 8. Ivan, B.; Kennedy, J. P.; Kelen, T.; Tüdös, F.; Nagy, T. T.; Turcsànyi, Β.J.Polym. Sci. Part A: Polym. Chem. 1983, 21, 2177. 9. Braun, D.; Böhringer, B.; Iván, B.; Kelen, T.;Tüdös,F. Eur. Polym. J. 1986, 22, 1. 10. Braun, D.; Böhringer, B.; Ivan, B.; Kelen, T.; Tüdös, F. Eur. Polym. J. 1986, 22, 299. 11. Hjertberg, T.; Sörvik, Ε. M. Polymer 1983, 24, 685. 12. Rogestedt, M.; Hjertberg, T. Macromolecules 1993, 60, 26. 13. Minsker, K. S.; Berlin, Α. Α.; Lisitskii, V. V.; Kolesov, S. V.Vysokomol.Soedin. Ser. A 1977, 19, 32. 14. Minsker, K. S.; Kolesov, S. V.; Yanborisov, V. M.; Berlin, Α. Α.; Zaikov, G. E. Polym. Degrad. Stab. 1986, 16, 99. 15. Iván, B.; Kelen, T.; Tüdös, F. Makromol. Chem., Macromol. Symp. 1989, 29, 59. 16. Simon, P. Polym. Degrad. Stab. 1992, 35, 45. 17. Simon, P. Polym. Degrad. Stab. 1992, 35, 157. 18. Simon, P.; Valko, L. Polym. Degrad. Stab. 1992, 35, 249. 19. Iván, B.; Turcsányi, B.; Kelen,T.;Tüdös,F. Izv. Chim. 1985, 18, 287. 20. Danforth, J. D. J. Am. Chem. Soc. 1971, 93, 6843. 21. Danforth, J. D.; Takeuchi, T. J. Polym. Sci. Part A: Poly. Chem. 1973, 11, 2091. 22. Danforth, J. D.; Spiegel, J.; Bloom, J. J. Macromol. Soi. Chem. 1982, Al7, 1107. 23. Danforth, J. D.; Indiveri, J. J. Phys. Chem. 1983, 87, 5376. 24. Iván, B.; Kelen, T.; Tüdös, F. Polym. Mater. Sci. Eng. 1988, 58, 548. 25. Iván, B.; Kelen,T.;Tüdös,F. Polym. Prepr. 1988, 29(1), 132. 26. Iván, B.; Turcsányi, B.; Kelen,T.;Tüdös,F. J. Vinyl Technol. 1990, 12, 126. 27. Iván, B.; Turcsányi, B.; Kelen, T.;Tüdös,F. Angew. Makromol. Chem. 1991, 189, 35. 28. Ayrey, G.; Hsu, S. Y.; Poller, R. C. J. Polym. Sci. Part A: Polym. Chem. 1984, 22, 2871. 29. Bartholin, M.; Bensemra, M.; Hoang, V. T.; Guyot, A. Polym. Bull. 1990, 23, 425. 30. Frye, A. F.; Horst, R. W. J. Polym. Sci. 1959, 40, 419. 31. Frye, A. F.; Horst, R. W. J. Polym. Sci. 1960, 45, 1. 32. Kolesov, S. V.; Berlin, Α. Α.; Minsker, K. S. Vysokomol. Soedin., Ser. A 1977, A19, 381. 33. Michell, E. W. J. Br. Polym. J. 1972, 4, 343. 34. Michell, E. W. J. J. Mater. Sci. 1985, 20, 3816. 35. Kelen, T.; Iván, B.; Nagy, T. T.; Turcsányi,B.;Tüdös,F.; Kennedy, J. P. Polym. Bull. 1978, 1, 79. 36. Tüdös, F.; Kelen, T. Macromol. Chem. 1973, 8, 393. 37. Iván, B.; Kennedy, J. P. ; Kelen, T.;Tüdös,F. J. Polym. Sci., Part A: Polym. Chem. 1981, 19, 679. 38. Iván, B.; Kennedy, J. P. ; Planthottam, S. S.; Kelen, T.; Tüdös, F. J. Polym. Sci., Part A: Polym. Chem. 1980, 18, 1685. 39. Iván, B.; Kennedy, J. P. ; Kelen, T.; Tüdös, F. Polym.Bull.1980, 2, 461. 40. Iván, B.; Kennedy, J. P. ; Kelen, T.; Tüdös, F. Polym.Bull.1980, 3, 45. 41. Iván, B.; Kennedy, J. P. ; Kelen, T.;Tüdös,F.J.Macromol.Sci.Chem. 1981, A16, 533. 42. Rogestedt, M.; Hjertberg, T. Macromolecules 1992, 23, 6332. 43. Iván, B.; Kennedy, J. P. ; Kelen, T.;Tüdös,F. J. Polym. Sci. Part A: Polym. Chem. 1981, 19, 9.



32

B.;

RECEIVED

POLYMER DURABILITY

44. Tüdös, F.; Ivan, B.; Kelen, T.; Kennedy, J. P. Dev. Polym. Degrad. 1985, 6, 147. 45. Iván, B.; Kennedy, J. P. ; Kelen, T.; Tüdös, F. Polym.Bull.1981, 6, 147. 46. Iván, B.; Kennedy, J. P. ; Kelen, T.;Tüdös,F. J. Macromol. Sci. Chem. 1982, A17, 1033. 47. Stames, W. H., Jr.; Chung, H.; Wojciechowski, Β. J. Polym. Prepr. 1993, 34(2), 114. 48. Van den Heuvel, C. J. M.; Weber, A. J. M. Makromol. Chem. 1983, 184, 2261. 49. Panek, M. G.; Villacorta, G. M.; Starnes, W. H., Jr.; Plitz, I. M. Macromolecules 1985, 18, 1040. 50. Millán, J.; Martinez, G.; Jimeno, M. L.; Tiemblo, P.; Mijangos, C.; Gómez-Elvira, J. M. Makromol. Chem. Macromol. Symp. 1991, 48/49, 403. 51. Martínez, G.; Gómez-Elvira, J. M.; Millán, J. Polym. Degrad. Stab. 1993, 40, 1. 52. Starnes, W. H., Jr.; Plitz, I. M. Macromolecules 1976, 9, 633. 53. Wilson, C. G.; Ito, H.; Fréchet, J. M. J.; Tessier, T. G.; Houlihan, F. M.J.Electrochem. Soc. 1986,133,181. 54. Fréchet, J. M. J.; Bouchard, F.; Eichler, E.; Houlihan, F. M.; Iizawa, T.; Kryczka, Wilson, C. G. Polym. J. 1987, 19, 31. 55. Shirai, M.; Kinoshita, H.; Tsunooka, M. Eur. Polym. J. 1992, 28, 379. 56. Andreas, H. In Plastics Additives Handbook; Gachter, R.; Müller, H., Eds.; Hanser: Munich, Germany, 1985; p 193. 57. Jennings, T. C.; Fletcher, C. W. In Encyclopedia of PVC; Nass, L. I.; Heiberger, C. Α., Eds.; Dekker: New York, 1988; Vol. 2, 2nd ed., p 45. 58. Starnes, W. H., Jr. Polym. Prepr. 1994, 35(1), 425. 59. Iván, B., submitted for publication. 29, 1994.

for review December 6, 1993.

ACCEPTED

revised manuscript November


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