Thermal Degradation of Polymethyl Methacrylates


(3) J.H. Baxendale, S. Bywater and M. G. Evans, Trane. Faraday. Soe., 42, 675 (1946). (4) J. . Baxendale, S. Bywater and M. G. Evans, J. Polymer Sci.,...
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Dee., 1953

THERMAL DEGRADATION OF POLYMETHYL METHACRYLATES

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The present study shows clearly the experimental difficulties in obtaining true values of formation constants of metal complexes even in apparently simple systems. The interference of the MeY-Me and similar ion pairs is to be expected in all cases where polyvalent cations and (or) anions are presOur values 1.78 2.2 300 340 140 1.1 ent in a complex equilibrium. Although the inSohwaraenbach values’ 1.2 1.5 80 170 110 2.1 fluence of those ion pairs on the complex equilibria necessary, by similar extrapolation. The possible may be quite large, it has been, up to now, considerror of those “thermodynamic” constants is, of ered only in a few cases (see ref. 21). Still greater course, greater than the error of the “concentra- may be the interference of the “neutral” electrotion” constants given in Table I1 and valid at lytes as potassium chloride and nitrate and espe0.10 m total nitrate concentration only. This is cially of complexing admixtures as acetates, phosetc., commonly used as buffer solutions. essentially due to the nature of the approximations phates, Only few formation constants of weak ion pairs are introduced in eq. 13. However, even for systems known exactly. Their numerical application to exshowing a great variation of Kel with total nitrate perimental data obtained at high ionic strengths concentration (systems involving lead) the values (0.1 up to l), cambe, with our present knowledge of given in Table I11 should be correct within *25%. For comparison Table I11 gives also equilibrium thermodynamics of concentrated solutions, only constants as calculated from the concentration very approximate. Measurements a t low ionic formation constants of the individual complexes strengths ( p << 0.1) will be required to enable obtained by S~hwarzenbach~ by an entirely differ- the performance of the necessary extrapolations ent technique. The differences between both sets with a reasonable precision. Special care should be taken when using stability of values are in most cases greater than the experimental error of the methods employed. It is pos- constants other than thermodynamic ones for the sible that the agreement could be improved if discussion of numerical relations between formaSchwarzenbach values could be corrected for the tion constants a n 4 structural properties of complexes. formation of MeY-Me and similar ion pairs.

TABLE I11 THERMODYNAMIC EQUILIBRIUM CONSTANTS AT 20 COMPARED WITH SCHWARZENBACH’ VALUES AS OBTAINED AT 20 ’, p = 0.100, CHLORIDE SOLUTIONS KCUpb KCUCd KcUzn KPbzn Kcden K2i

THERMAL DEGRADATION OF POLYMETHYL METHACRYLATES BYS. BYWATER National Research Council, Applied Chemistry Division, Ottawa, Canada Received February io, 1065

Experiments are described on the thermal decomposition in vacuo of thin films of polymethyl methacrylate. The investigation covers a series of sharply fractionated samples over a large molecular weight range. The results are interpreted in terms of a free radical mechanism for thermal breakdown. Except for preliminary measurements carried out with a benzoyl peroxide-catalyzed polymer, the samples used were all Fe ++-HzOz initiated samples produced in aqueous solution. The higher molecular weight fractions were those described by Baxendale, Bywater and Evans3 (their Table IV) and were the results of a double fractionation. The lower fractions were produced by separating a lower molecular weight Fe ++-H202 catalyzed polymer into twentyfive fractions. Thin films were formed by dissolving the The rate of breakdown of the polymer was measured by polymer in benzene, placing the solution in a glass ring on a the increase in pressure in a closed system due to monomer mercury surface and allowing the benzene to evaporate. evolution. The pressure increase was measured on a Pirani Preliminary drying was carried out by heating in a vacuum gage, calibrated with monomer vapor at the same time as desiccator for two days at 80”. Viscosity measurements were made in reagent grade benvapor pressure measurements2 were being made. The total volume of the cell and Pirani to the first stopcock was 126 zene solution at 20” using a n Ostwald viscometer whose flow time for benzene was 380 seconds. Molecular weights cc. so that very small rates could be measured. The polymer in the form of thin film (0.0005” thick) were determined from the extrapolated (s.,/C) us. C curves was cut into 2 discs using a No. 12 cork-borer and was placed using the relation between molecular weight and intrinsic viscosity of Baxendale, Bywater and Evans.4 on each side of a co per block and held in place by copper I n all cases a preliminary rapid heating of the sample to rings. This assembpy was held in a glass cup inside a cylindrical cell around which a furnace could be lowered. Tem- 180-200” with the pumps running was found to be necessary to remove the last traces of solvent from the film. The peratures were measured by means of a copper-constantan thermocouple element in a well drilled into the block and measurements could then be taken reproducibly over a range held in contact by a small copper wedge. The leads were of temperatures. Since the Pirani detects minute traces brought out of the cell by soldered joints through kovar-glass of monomer, measurements of the rate of monomer evolution seals. The cell could be evacuated to a pressure of lo-& at a series of temperatures could be made without apprecimm. by means of a mercury diffusion pump backed by a (3) J. H. Baxendale, 8. Bywater and M. G. Evans, Trans. Faraday rotary oil pump.

Very little work of a quantitative nature has been reported on the thermal breakdown of polymers. This work describes the thermal breakdown of polymethyl methacrylate, a polymer which is known qualitatively to break down almost exclusively to monomer.‘ Experimental

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(1) 6. L. Madorsky, J . Polymer Sei.,9, 133 (1952). (2) 9. Bywater, dbid.. 9, 417 (1962).

Soc., 4 2 , 675 (1946). (4) J. H. Bexendale, 8. Bywater and M. G. Evans, J . Polymsr Scd.. 1, 237 (1946).

S. BYWATER

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ably changing the amount of polymer in the cell with most samples. For the investigation of rates at various extents of reaction, i t was necessary to heat the material to between 200-300" with the pumps running for a controlled time. On quickly cooling, the rate of monomer evolution could then be studied over a lower temperature range without further changing the residual amount of polymer. A preliminary heating to 180" caused no measurable loss in weight of the polymer (< l%),except in the case of very low molecular weight polymers. The sample was brought to a steady temperature with continuous pumping. For a rate measurement at each temperature, the umps were cut off and the pressure increase was followed by Firani measurements for several minutes. The sample was continuously pumped except for these short periods. During a measurement the pressure of monomer vapor never exceeded 2 X 10-3 mm. so that repolymerization should be negligible. The rate of monomer formation was always found to be a linear function of time over the small time ranges studied. I n order to check on the reliability of the apparatus rates of monomer evolution were measured with various amounts of a low molecular weight polymer film on the block. The rate of monomer production was directly proportional to the amount of material as expected. At the low temperature used in all these experiments, the uestion arises as to whether monomer diffuses out of the Zlm fast enough, that the results are not vitiated by repolymerization in the highly viscous medium. This problem has been treated by Cowley and Melville, who found that in the photochemical degradation of polymethyl methacrylate results below 160" were unreliable due to slow diffusion of monomer out of the film. The experimental arrangement used here was very similar to that of Cowley and Melvilles except that the films were gomewhat thinner and rates of monomer evolution much lower (2 X lo-* to 5 X 10-7 g./sec./ml. at the lowest temperatures). These factors make reliable measurements below 160" possible. Calculations using the formula given by Cowley and Melville show that the rates measured even at 100" (the lowest temperature ever used), where diffusion of the monomer will be slowest, should not be complicated by retention of monomer in the polymer film.

Results The over-all activation energy was found to be independent of extent of reaction (up to 20% monomer loss) for the whole range of molecular weights as measured on fractionated samples. The rate of reaction a t any temperature falls off rapidly with increasing conversion (Fig. 1). h

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The over-all rate and activation energy for thermal breakdown was found to be influenced markedly by the molecular weight of the fraction (Fig. 2). Since the rate drops rapidly with extent of reaction, attempts were made t o measure all rates at essentially zero conversion. This was

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322,000

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7Io3 PK). Fig. 2.-Arrhenius plots for the decomposition of fractionated polymers of different molecular weights. The numbers indicate the molecular weight of each fraction (10-mg. samples).

accomplished for all specimens except those of molecular weight 61,000 (95y0 residue) and 47,300 and 13,500 (90% residue) where reaction was so rapid that the preliminary heating to remove volatiles also produced appreciable degradation. For comparison with the other curves these should be transposed toward the right-hand side of the figure. The variation of activation energy with molecular weight is shown in Fig. 3. Two distinct regions of constant activation energy can be seen with an intermediate zone. 0

v3

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p:

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Per cent. polymer residue. Fig. 1.-Variation of rate of monomer evolution with extent of reaction a t a given temperature (M = 322,000). (5) P.R. E. J. Cowley and H.W. Melville, Proc. Roy. Soc. (London), AZ10, 461 (1951).

4.0 5.0 LOG,,(MOLECULAR WEIGHT).

6.0

Fig. 3.-Variation of activation energy with molecular weight (fractionated samples).

THERMAL DEGRADATION OF POLYMETHYL METHACRYLATES

Dec., 1953

The behavior of a sample- heterogeneous with respect to molecular weight (M, = 80,000) is to be contrasted with the above behavior (Fig. 4). Here the over-all rate and activation energy drop with increasing conversion. This, of course, is to be expected since the work on fractionated samples shows that the lower molecular weight fraction will react preferentially and, as it is removed, the higher molecular weight constituents will degrade with a progressively lower activation energy.

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PERCENTAGE POLYMER

RESIDUE.

Fig. 5.-Variation of intrinsic viscosity of the polymer residue a t various extents of decomposition.

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osmotic work of Grassie and Melville show that the initial bond break must occur a t the end of the chains. A free radical ending thus produced enables a rapid elimination of monomer to take place by the reverse of the normal polymerization propagation step. N

Fig. 4.-Arrhenius plots for the deconiposition of a heterogenous polymer at various extents of decomposition: 1, <1%; 2, 14%; 3, 29% (10-mg. samples).

Molecular weights of the polymer residue were measured viscometrically in several cases. There was insufficient specimen to measure viscosities over a range of concentrations but since the solution concentration used was of the order of 0.05%) the (I~~,/C)values do not differ appreciably from intrinsic viscosity values. Figure 5 shows the variation of viscosity average molecular weight (of the polymer residue in the cell) with increasing extents of reaction at different molecular weights, The highest fraction alone shows a drop in average molecular weight, but it can be safely assumed that since the viscosity average molecular weight is not unduly sensitive to the presence of a small amount of short chain material, then shorter chain polymers may be present in the polymer residue even with the medium molecular weight fractions. Discussion The results presented here show a number of contrasts with those presented by Grassie and Melville,6 but are still explainable in terms of the same general mechanism. The differences are due t,o the use of fractionated samples, the use of lower reaction temperatures and to the use of polymer ' containing a different end grouping. The molecular weight data presented here together with the (6) N. Grmsie and H.W. Melville, Proo. Roy. Soo. (London),AIQQ, 1 (1949).

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The radical is regenerated allowing the reaction to repeat many times, and this step is of low activation energy (ED) due to the energy gain on forming the monomer double bond. In fact, numerically, ED = AH Ep There AH is the heat of polymerization and Ep the activation energy for the normal polymerization propagation step. Thus if the molecular chain is short, it is possible for each polymer chain undergoing decomposition to degrade completely to monomer without the intervention of a chain termination step. The over-all rate would remain unchanged if a chain transfer reaction consisting of abstraction of a hydrogen atom a t random from a neighboring chain would occur. This process must occur very infrequently otherwise the mean molecular weight of the residue would drop rapidly due to the accumulation of shorter chains terminated in this way. With higher molecular weight polymers the molecular chain length is likely to be higher than the kinetic chain length and chain termination by radical interaction is likely to occur before the molecules are completely degraded. In this case the residual average molecular weight of the polymer would decrease, with increasing conversion, and the over-all rate would be lower because of a lower chain initiation rate due to the smaller number of chain endings per unit mass. This sharp change in mechanism which would

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occur when kinetic and molecular chain lengths were equal, depends on the final one unit radical produced by complete degradation of short chain polymers being incapable of a transfer reaction with a neighboring chain. This is only possible if the radical is capable of isomerization to a stable molecule or if it is rapidly removed from the seat of reaction. Isomerization does not seem likely in the present case, but it seems reasonable that the final one, and possibly two, monomer unit radicals would be volatile enough to be removed into the gas phase. No polymerization of the monomer already there is possible since polymer radicals are unstable a t the reaction temperature, so slow recombination is likely. The calculations mentioned above showed that monomer volatilizes rapidly enough so that interaction with radicals is not important; it seems possible that the small radicals could volatilize rapidly enough so as not to react with polymer. A definite proof would be difficult to obtain, but it is clear that a very low chain transfer rate is implied. Alternative mechanisms involving recombination of radicals with samples of all molecular weights make it necessary to assume a sharp change of initiation or termination mechanisms on increasing the molecular weight of the sample. It seems very reasonable to assume that the depolymerization propagation reaction is unchanged in all cases. This change would involve a difference in activation energy of about 36 kcal. between low and high molecular weight materials in either chain initiation or termination steps. It is difficult to suggest a plausible scheme which would produce this change. The present data conform to the above scheme if it is assumed that the final one unit radical does not react further due to one of the causes mentioned above. This implies that the kinetic chain length is around lo3 since the sharp mechanism change occurs a t molecular weights between 50,000 and 200,000. Application of the usual steady state treatment to the concentration of the various radicals, assuming that transfer or terminated polymer chains do not appreciably decompose since due to the low conversions used, the original polymer is in large excess, then the rate of monomer production at low conversions is given by the equations dIMl hi Low mol. wt. polymers = - X P, dt Mo dIM1 = k~ High mol. wt. polymers dt

where P, is the original weight of polymer, N is the molecular chain length, Mo is the monomer molecular weight and ki,k ~kt,, are the rate constants for chain initiation, depropagation and mutual termination, respectively. Use of these equations yields values of 48 kcal. for E, and 22 kcal. for ET using the extreme experimental over-all activation energies for low and high molecular weights and a The value of Ei apvalue of 18.5 kcal. for pears low in comparison with known bond dissociation energies in simpler molecules, but there exist no data on larger molecules for an accurate com-

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parison. The value of ET must correspond to a diffusion controlled recombination of polymer radicals in the viscous polymer melt. It is in reasonable agreement with the ET value found by Cowley and Melville in photochemical degradation studies a t about the same temperatures. The results of Grassie and Melville were obtained a t much higher temperatures where ET was apparently zero due to a melt viscosity of a different order of magnitude. This fact explains the main differences in experimental results. Thus, using the same scheme for the kinetics of depolymerization, due to the particular Ei and ET value's in their case, little change in activation energy is predicted bet ween low and high molecular weight samples, and none in fact was found. Since a limiting over-all activation energy is obtained a t high molecular weights in this work, this implies that ET does not change over a limited range of polymer viscosities, since the absolute viscosity of the polymer must have varied between the 322,000 and 982,000 molecular weight samples. This perhaps is understandable since Fox and Flory7 have shown that in a series of polystyrene melts of different molecule weights the absolute viscosity changes somewhat, but the activation energy for viscous flow is unchanged with molecular weight over roughly the same temperature interval. The diffusion of polymer radicals in the polymer is a related phenomenon and hence ET would probably change very little in these experiments. Chain initiation exclusively at the ends of the original polymer would explain the unchanged activation energy up to 20% conversion. Grassie and Melville observed an increase in activation energy as reaction proceeded and supposed this to be due to the presence of two types of chain ending in the polymer. No change would be expected with the polymers used here, since the studies on their polymerization3 suggested identical -OH groups on each end of the polymer chain. Recent on the kinetics of polymerization of methyl methacrylate by Arnett have corroborated this evidence. The rapid drop in rate up to 20% conversion is difficult to explain on the basis of the above simple mechanism. An extensive transfer reaction would rapidly reduce the number of original type chain endings, and hence could produce a rapid drop in over-all rate, but would also produce a noticeable drop in the molecular weight of the residue. No such effect was noticeable with low molecular weight polymers. It must be concluded that the mechanism is undoubtedly more complex than the simple scheme suggested above. The limitations imposed by working with a thin polymer film are great and, for example, make it impossible to investigate the effect of polymer concentration and produce uncertainties due to unknown diffusion rates. The simple scheme above does explain many of the observed experimental facts, but cannot be regarded as entirely satisfactory. (7) T. G. Fox and P. J. Flory, J . Am. Chem. SOC.,70, 2384 (1948). (8) L. M. Arnett and J. H. Peterson. ibid., 74.2031 (1952) (9) L. M. Amett, ibid., 74, 2027 (1952).

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