Polymer Degradation and Stability 43 (1994) 125-129

Kinetics of polymer degradation involving the splitting off of small molecules: Part lO--

Thermal degradation of poly(vinylidene chloride)

Peter ~imon Department of Physical Chemistry, Faculty of Chemical Technology, Slovak Technical University,

Radlinskgho 9, 812 37 Bratislava, Slovak Republic

(Received 13 April 1993; accepted 29 April 1993)

High-conversion kinetic runs of the thermal dehydrochlorination of poly(vinylidene chloride) have been treated using the theory developed previously. The results suggest that the mechanism of immediate zip growth, with autocatalytic initiation of the zip reaction, takes place. Excellent agreement has been reached between experimental and theoretical kinetic runs. The preexponential factor and activation energy of random elimination of HCI appear to be independent of the type of polymer, their values being log(A/s- ~) = 12.8 and E~ = 160 kJ mol i. The average zip length and the ratio of the rate constants of catalysed and random initiation decrease with temperature.

INTRODUCTION

Dehydrochlorination of poly(vinylidene chloride) (PVDC) takes place in two stages, which can be almost completely separated and studied individually.~-4 In the first stage, which occurs at temperatures below 200°C, one HCI molecule is eliminated from each monomeric uni t? During this process the colour of the polymer changes rapidly from white to orange and finally to black. 5 In the second stage, which occurs at much higher temperatures, the remaining HCI is lost and a hard crosslinked carbon is formed. First it was concluded that HCI is the only product of PVDC degradation at temperatures below 220°C; 2 however, it was later found that dehydrochlorination is accompanied by demono- merization to some extent. 6

The dehydrochlorination in the first stage is accepted to be a radical chain reaction. 2'4'~9 Free radicals are generated by thermal t reatment as shown by retardation of dehydrochlorination by

Polymer Degradation and Stability 0141-3910/94/$07.00 © 1994 Elsevier Science Limited.

125

t r iphenylmethane and acceleration by 2,2- diphenyl-l-picrylhydrazyl and quinones. 2 The presence of radicals has been documented also by the ESR technique, s The effect of the atmos- phere in which dehydrochlorination takes place may also be interpreted in terms of the radical mechanism. Propylene has retarding effect on dehydrochlorination, presumably as a conse- quence of its ability to quench radicals. Nitric oxide and oxygen accelerate the de- hydrochlorination, which can be accounted for by their ability to produce active radicals. 7 The rate of degradation in the presence of oxygen is 1-5 times that in an inert a tmosphere? 7 Hydrogen chloride has no effect on dehydrochlorination either in the solid state 7 or in solution, l° However, the radical mechanism is not very well accommodated by the absence of H2 and Cl2 in the degradation products. 46 Indications exist that a polar mechanism may take place in the decomposition of PVDC in solution"' and the possibility of polar elimination of HCI from the surface of PVDC crystal has also been considered.

Prevailing opinion is that the dehydrochlorina-

126 Peter ~imon

tion of PVDC involves the zip mechanism, z- 4,7,8,11-19 It has been deduced that the influence of allyl activation is unidirectional. 2"3'~2 This has been demonstrated experimentally by the fact that the activation energy of elimination of HCI from head-to-head PVDC is much higher than from head-to-tail P V D C . TM Howeve r , the question of zip initiation has not been satisfac- torily resolved. Burnett et al. 2 found that the rate of initiation of dehydrochlorination is propor- tional to the reciprocal molecular weight, which would indicate that end groups are the main initiation centres. Other authors TM have found no correlation with molecular weight. Indications exist that the morphology of the polymer particles might play an important role in determining the rate of reaction, 1'4"7 the zip reaction probably originating in the chain-fold regions on the polymer crystal surface. ~ Danforth obtained an accurate representation of the kinetics of PVDC dehydrochlorination over the entire range of decomposition, assuming that the zip initiation occurs at random. ~8 Independently of the mechanism of zip initiation, the double bond formed has been established as a prominent defect site responsible for degradation.15 17 Considering zip growth, some of the mechanisms proposed implicitly involve zip termination. 2"4 On the other hand, Danforth's theoretical considera- tions have not taken zip termination into account~ ~8

In Part 3 of this series kinetic runs of PVC dehydrochlorination in an inert atmosphere were studied using the theory of polymer degradation by zip elimination based on a stochastic approach. TM In Parts 4 and 5, autocatalytic mechanisms of polymer degradation were analysed 2L22 and in Parts 6-9 the theory was applied to the study of PVC dehydrochlorination in HCI and oxidative atmospheres and to the degradation of polyvinyl esters and poly(vinyl bromide). 23-26 The mechanism described in Part 4 involves the immediate zip growth after random zip initiation, 2~ whereas that described in Part 5 comprises gradual zip propagation without

termination. 22 The aim of this paper is to employ the theory for the treatment of experimental data published in Papers 1, 2 and 5. We limit our- selves to the first stage of PVDC degradation, that is, to the loss of one HCI molecule from a vinylidene chloride monomeric unit.

SIMULATION OF THE KINETIC CURVES

For the simulation of kinetic curves we use the same procedure as in Parts 3, 6-9, 2°,2~26 i.e. the whole sets of kinetic runs measured for various temperatures are treated simultaneously. The rate constant of random initiation, the average zip length, the ratio of the rate constants of autocatalytic and random initiation and the ratio of the rate constants of zip propagation and random activation (B, m, fl, ),) are expressed using the Arrhenius equation. 2°'23 The kinetic parameters of degradation are obtained by minimizing the sum of squares between ex- perimental and theoretical kinetic runs.

For the model of immediate zip growth the parameters for minimization are the preexponen- tial factor and activation energy of random elimination of HCI, A and E,, the ratio of preexponential factors and the difference of activation energies corresponding to the rate constants of zip propagation and zip termination, AA and AEa, the ratio of preexponential factors and difference of activation energies correspond- ing to the rate constants of autocatalytic and random activations, AAt~ and AEt~, the probabil- ity of the occurrence of a structural irregularity, Po, and the maximum accessible conversion Xm. For the model of gradual zip growth, the parameters AAt~ and AEt~ obtained from preliminary calculations have always led to the value fl----~ 0, which means that the initiation step of the zip reaction is not autocatalytic. In this case the parameters for minimization are A, E,, P0 and Xm, and the ratio of preexponential factors and difference of activation energies correspond- ing to the rate constants of zip propagation and random elimination AA7 and AE~.

Table 1. Kinetic parameters for PVDC dehydrochlorination for the model of immediate zip growth

log(A/s l) E~ A logA AE~ log(AtJs J) Et~ PcJ x,, o Ref. (kJ mol-t) (kJ mol-~) (kJ mol-~)

12-89 159-5 -7.36 -71.4 13-23 156-2 0.076 0.980 0.0161 1 12-37 156.5 -4.86 -49.9 8.18 112.0 0-039 1-000 0.0062 2 13-05 164.0 -4.75 -56.2 10-87 132-9 0.006 0.930 0.0129 5

Kinetics of polymer degradation involving the splitting off of small molecules: Part 10

Table 2. Kinetic parameters for PVDC dehydrochlorination for the model of gradual zip growth

127

log(A/s 1) E,, log(A~/s ~) E~ p,, x,, o Ref. (kJ mol ') (kJ mol ')

10.90 134-8 8.39 106-8 0.050 0-889 0-0185 1 10.76 134-5 0.75 39.6 0.001 0- 999 0- 0068 2 9.44 147- 7 6.53 47- 9 0-000 1- 000 0- 0088 5

The values of At~ and Et~ are calculated using eqns (3) and (4) in Part 623 and A~ and E~ using eqns (1) and (2) in Part 9. 26 The results of the calculations are summarized in Tables 1 and 2.

R E S U L T S A N D D I S C U S S I O N

We have examined the possibility of the occurrence of the mechanisms of immediate and gradual zip growths in the dehydrochlorination of PVDC. As may be seen from the values of standard deviations per mesh point, o, the agreement between the experimental and theore- tical kinetic runs is excellent for both mechanisms (Tables 1 and 2). Figure 1 shows that the deviation diagrams for the model of immediate zip growth exhibit no regularity; the same is true in the case of gradual zip growth. Hence, neither the standard deviations nor the deviation diagrams provide an indication of which mechan- ism takes place in PVDC dehydrochlorination.

x

i

. 0 4

. 0 2

- . 0 2

- . 0 4

N

X X

× ~ 6. +

x ×

~ o ~ ,~, x • × 4

÷ e e ~,

~o ÷ ÷

+

" X e x p

Fig. l . Deviation diagram for PVDC dehydrochlorination for the model of immediate zip growth with autocatalytic zip initiation for the data in Ref. 1. Temperatures (°C): (0)

140; (O) 150; (x ) 160; (+) 170-3; (A) 180.

Considering thermal stability, the stability of PVDC lies between those PVBr and PVC. If one accepts the hypothesis that the rate limiting step in thermal degradation is the initiation of the zip reaction, the rate constant of random elimination for PVDC should lie between the constants for PVBr and PVC. Calculating B using eqn (1) in Part 3 TM and using data from Parts 3 and 9, 2o26 it can be seen that the values of the rate constants of random elimination increase in the order P V C < P V D C < P V B r for the data in both Tables 1 and 2. In Part 926 it has been ment ioned that the activation energy of random elimination for PVBr is less than for PVC owing to the lower C- -Br bond energy in comparison with the energy of the C--CI bond. In PVDC, a weak repulsion may be assumed to exist between the two bulky chlorine atoms in the ----CC12-- group. On the assumption that the random elimination for the three polymers occurs by the same mechanism, the activation energy for random elimination from PVDC should thus be slightly lower than for PVC and, most probably, should lie between the activation energies for PVBr and PVC. This relation is fulfilled if the model of immediate zip growth with autocatalytic initiation is assumed to occur in PVDC dehydrochlorina- tion (Table 1).

The occurrence of this mechanism is also supported by the results of a study of PVDC degradation in bibenzyl solution. ~5 Degradation in bibenzyi solution indicates the radical charac- ter of the reaction since bibenzyl is an efficient radical scavenger which is converted into stilbene. The author has found that the ratio of HCI evolved to stilbene is c. 35:1, whereas the limit for the pure free radical dehydrochlorina- tion is 2:1. If the radicals can react with the solvent, there is a possibility that they could also react with an adjacent chain during the degradation of solid PVDC, thus catalysing the initiation. It should be emphasized that we have not found any minimum corresponding to the autocatalytic initiation for the model of gradual

128 Peter ~imon

l m

100

75

50

25

i I I I I

140 150 160 170 180

= t]oC

Fig. 2. Dependences of the average zip length on temperature for various types of PVDC: (1) Ref. 1; (2) Ref.

2; (3) Ref. 5.

zip growth. The ratio 35:1 indicates that the zip length could be about 17 which is within the range of zip lengths for PVDC (Fig. 2).

Table 1 shows that the values of preexponen- tial factors and activation energies of random elimination are practically identical for all types of PVDC studied. This fact suggests that these kinetic parameters are independent of the type of PVDC and that their values are: log(A/s -]) = 12-8, E , = 160kJmol -~. This generalization is not so convincing as in the case of PVC where it has been made on the basis of treatment of a higher number of sets of experimental kinetic runs carried to higher conversions; 2° nonetheless, the agreement in A and E, is clear. Using our data, the ratio of rate constants for random dehydrochlorination of PVDC and PVC is 17 at 190°C. The activation energy for random elimination is identical with the value 159.4 kJ mol- ' found for the thermal degradation of head-to-head PVDC. ~3 For the unidirectional zip growth in head-to-head PVDC, the zip propagation taking place after initiation should lead to decomposition of one monomeric unit at any temperature since the tail-to-tail structure acts as an irregularity preventing the zip growth, and the average zip length is much higher than two double bonds. Consequently, the activation energy of degradation should be equal to the activation energy of random elimination.

The differences in activation energies for the propagation and termination steps of the zip reaction, AA and AEa, are negative. Thus, according to eqn (2) in Part 3, 20 the average zip length, m, decreases with temperatue. As Fig. 2 demonstrates, the zip length varies for different types of PVDC and this quantity may depend, inter alia, on the morphology of the polymer chain. 1'27 Since Ea is less than Ea, according to eqn (1) in Part 6, 23 fl also decreases with temperature. The values of fl are within the range 5-60, depending on temperature and on the type of polymer. The probability of the occurrence of a structural irregularity, Po, is low and the values of Xm are close to one as for P V C . 2°

A mechanism not involved in our models is that proposed by Bohme and Wessling' based on the assumption of zip initiation at chain folds. The fold length of lamellar crystals increases with annealing time, which could lead to acceleration in the rate of dehydrochlorination. Indeed, this mechanism could explain the autocatalytic course of PVDC dehydrochlorination. However, the excellent agreement between experimental and theoretical kinetic runs as well as the coherence with our previous results for other polymers 2°'26 favour the model of immediate zip growth where the zip initiation takes place both at random and in an autocatalytic way.

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Kinetics of polymer degradation involving the splitting off of small molecules: Part 10 129

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