A study of the thermal degradation of methyl methacrylate polymers and copolymers by thermal volatilization analysis

European Polymer Journal, 1968. VoL 4. pp. 21-30.

Pergamon Press

Printed in England.

A STUDY OF THE T H E R M A L D E G R A D A T I O N OF METHYL M E T H A C R Y L A T E POLYMERS A N D COPOLYMERS BY T H E R M A L VOLATILIZATION ANALYSIS I. C. MCNEILL Chemistry Department, University of Glasgow, Glasgow, Scotland

(Received 28 July 1967) Abstract--Polymers of methyl methacrylate, of various molecular weights and prepared by radical and anionic mechanisms, have been examined. The thermal stability has been found to be highly dependent both on the method of preparation and on the molecular weight of the polymer. Comonomers in the methyl methacrylate chain in general have a profound effect on the stability. The effects of the presence of phenyl acetylene, methyl, ethyl, n-propyl and n-butyl acrylates, styrene, and ethyl methacrylate have been studied.

INTRODUCTION THE MECHA~S.~ of thermal degradation of poly(methyl methacrylate) has been studied extensively.(1) Scission of the polymer chain leads to depolymerization to give monomer as the sole product, the zip length of the depolymerization being large. All samples of the polymer break down rapidly above 300 ° but at lower temperatures the rate of degradation can be profoundly affected by a variety of factors, such as mode of polymerization, molecular weight, and the presence of small amounts of a co-monomer in the methacrylate chain. In this paper, the influence of these variables is studied for a number of methyl methacrylate homopolymers; some copolymers containing high proportions of methyl methacrylate have also been examined. It has been found convenient to compare the behaviour of the various materials using the new technique of thermal volatilization analysis. (-,. 3) In TVA the polymer sample is heated at a linear rate in a continuously evacuated system, and the small pressure of volatile degradation products distilling to a liquid nitrogen trap some distance from the heated sample is measured by Pirani gauge and recorded continuously as a function of temperature (time). The resulting thermogram gives a measure of the variation in rate of production of volatile products while the sample is being heated. The polymers and copolymers in this investigation are particularly suitable for study by TVA since all degrade almost completely to volatile products, of which the major component is methyl methacrylate monomer. Differences in the character of the thermograms can therefore be taken to reflect changes in the mechanism of production of monomer. The materials studied and discussed below are listed in Table 1, which also indicates their origin and number-average molecular weights. 21

22

I. C. McNEILL TABLE 1. LISTOF POLYMERSANT)COPOLYMERSSTUDIED

Material

Numberaverage tool. wt.

Footnote

Poly(methyl methacrylate) Poly(methyl methacrylate) Poly(methyl methacrylate) Poly(methyl methacrylate) Poly(methyl methacrylate) 5/1 copolymer of methyl methacrylate and phenyl acetylene 26/I copolymer of methyl methacrylate and methyl acrylate 8/1 copolymer of methyl methacrylate and methyl acrylate 4/1 copolymer of methyl methacrylate and ethyl acrylate 4/1 copolymer of methyl methacrylate and n-propyl acrylate 4/1 copolymer of methyl methacrylate and n-butyl acrylate 4/1 copolymer of methyl methacrylate and styrene 1/1 copolymer of methyl methacrylate and styrene 4/1 copolymer of methyl methacrylate and ethyl methacrylate

480,000 100,000 20,000 1,500,000 60,000 93,000 600,000 425,000 575,000 422,000 617,000 280,000 107,000 444,000

1 1 1 2 2 1 3 3 4 4 4 5 5 4

Ref. A B

C D E F G H I

J K L M N

1. For preparative details see experimental section. 2. An industrial research sample, prepared by an anionic mechanism. 3. Kindly supplied by Drs. N. Grassie and B. J. D. Torrance. Prepared by a free radical mechanism at 60 ° .

4. Kindly supplied by Dr. N. Grassie and Mr. J. D. Fortune. Prepared by a free radical mechanism at 60 °. 5. Kindly supplied by Drs. N. Grassie and E. Farish. Prepared by a free radical mechanism at 60 °.

EXPERIMENTAL

Polymers A, B, C Methyl methacrylate monomer (I.C.I. Ltd.) was washed with alkali to remove inhibitor, then with distilled water, and dried over calcium chloride. It was distilled three times in vacuo, finally into dilatometers containing different amounts of azobisisobutyronitrile. Dilatometers were sealed under vacuum and polymerizations were carried to 10% conversion at 60 °. Polymers were isolated by precipitation with methanol, purified by several reprecipitations from chloroform solution, dried under vacuum at room temperature and powdered.

Copolyraer F Methyl methacrylate was purified as above. Phenyl acetylene (B.D.H.) was twice distilled under vacuum. 21.0 ml of methacrylate and 5"0 ml of phenyl acetylene were distilled on the vacuum line into a dilatometer containing sufficient azobisisobutyronitrile to give 0-3% w/v in the reaction mixture. The dilatometer was sealed under vacuum and polymerization was carried to 10~o (1~ hr) at 60°. The copolymer was isolated and purified as in the previous paragraph. The composition of the copolymer was estimated approximately from the microanalysis for carbon and hydrogen.

Molecular weights Number-average molecular weights for polymers A, B and C, and copolymers, were measured in toluene solution at 25 °, using the Mechrolab Model 501 Membrane Osmometer, fitted with a Sylvania 300 grade cellophane membrane.

Thermal volatilization analysis All the thermograms were obtained from 25 mg samples in the form of fine powders, using a heating rate of 10° per min. The temperatures quoted are sample, not oven, temperatures. The apparatus used was as already described in detailJ 3) The rate (pressure) scale on the thermograms is non-linear.

The Thermal Degradation of Methyl Methacrylate Polymers and Copolymers

23

RESULTS AND DISCUSSION The temperatures at which the rate of volatilization reaches a maximum, designated Tm~x are listed in Table 2 for the various samples studied. Peaks below 220 °, which can be attributed to trapped volatile materials released as the polymer softens, are not included. Temperatures shown in brackets refer to peaks which appear as small shoulders on larger peaks. Thermograms are reproduced and are referred to in the following discussion, for all the samples except copotymers I, J, K and N. The thermograms for copolymers I, J and K were closely similar to that for copolymer H; that for copolymer N resembled the thermogram for polymer A. TABLE 2. VALb~ESor Tmax OBTAINED

Sample A B C D E F G H I J K L M N

FROM THE

TVA

THERMOGRAMS

Tmax values (+ 5°) (°C) 298 290 285 (310) (290) (305) (308) (302) (298) (300) 294

353 362 378 356 380 333 357 361 381 379 379 399 358 365

Effect of molecular weight on thermal stability, for poly(methyl methacrylate) samples prepared by the free radical mechanism Since monomer is the sole product, the two peaks observed in each of the thermograms (Fig. 1) correspond to different mechanisms of initiation of the chain depolymerization process. The evidence from the thermograms regarding these two reactions is as follows. At the first peak, the rate of degradation at a given temperature between 220 and 300 ° is lower for samples of higher initial molecular weight. Tmax is higher for samples of higher molecular weight. At the second peak, the rate of degradation at a given temperature between 320 and 350 ° is higher for samples of higher initial molecular weight. Tmaxis lower for samples of higher molecular weight. These observations are consistent with the accepted view that the first stage in the degradation of poly(methyl methacrylate) is end-initiated and the second is initiated by random chain scissions. The rate dependence on initial molecular weight shows an apparent inversion in the course of the degradation. The rates observed at the second peak are somewhat influenced by the different extents of reaction at the first stage for the three samples; the results discussed in the next section for the samples which were made by an anionic

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I.C. McNEILL

mechanism and for which this complication does not arise, confirm that this inversion is a genuine effect. It has been observed previously, and has been discussed by Grant and Bywater, (4) and by MacCallum, (~) in connection with the results of a number of isothermal degradations. The shift in the position of the second Tm~xfor samples of different initial molecular weight is a consequence of the mechanism of degradation. The significance of the shift at the first Tmax, however, is obscured by the overlap of the two stages of reaction; this would result, for polymers B and C, in a shift in the direction observed, independently of any mechanistic effect which might be present. ...-% ." /.

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Fro. 1. The effectof initial molecular weight on the thermal stability of poly(methylmethacrylate) samples prepared by the free radical mechanism at 60'. Dotted line, polymer A, I~n=480,000; dashedline, polymerB, i~ n= 100,000; full line, polymerC, IVln= 20,000.

Poly(methyl methacrylate) samplespreparedby an anionic mechanism In Fig. 2, thermograms for high and low molecular weight poly(methyl methacrylate) samples prepared by an anionic mechanism are compared; the corresponding trace for the low molecular weight polymer C, prepared by the free radical mechanism, is also given. The total absence of any first stage is evident for the high molecular weight sample D, which shows a similar Tmax value to a high molecular weight sample (A) prepared by the free radical route (see Fig. 1 and Table 2). More striking, however, is the absence of the first peak in the low molecular weight anionic sample, E. This indicates that the unstable ends present in the free radical sample are absent in material prepared by the anionic process. The temperature of the rate maximum for polymer E corresponds to that for the second stage of the degradation of low molecular weight polymer made by the free radical mechanism, as would be expected for a reaction initiated by random scission in two samples of comparable molecular weight.

Copolymer of methyl methacrylate withphenyl acetylene Most of the workers who have studied the mechanism of thermal poly(methyl methacrylate) have agreed with Grassie and Melville's that the unstable ends present in the polymer prepared by a radical unsaturated ends resulting from termination by disproportionation.

degradation of original view(5) mechanism are Since terminal

The Thermal Degradation of Methyl Methacrylate Polymers and Copolymers

25

.o..., ; " ' ~ t "h .' , t ~, : / I ' / ".:.'

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FIG. 2. Comparison of TVA thermograms for high and low molecular weight poly(methyl methacrylate) samples prepared by an anionic mechanism with the thermogram for a low molecular weight free radical sample. Dotted line, polymer D, l~l,= 1,500,000; full line, polymer E, ~n=60,000; dashed line, polymer C, ~n=20,000. unsaturation leads to instability, and the effect is dearly observed in the TVA thermogram (Fig. 1), it is of interest to observe the effect of main chain unsaturation. There are various ways in which double bonded structures might be introduced into the polymer chain. Copolymerization with acetylenic compounds offers one route. In the 5/I methyl methacrylate/phenyl acetylene copolymer prepared and studied, the copolymer would be expected to include - - C H = C P h - - units: CH3 I

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The bonds indicated might be expected to constitute weak points. The thermogram for the copolymer (F), Fig. 3, shows two interesting effects of the presence of the main chain double bonds. Firstly, there is scarcely any end-initiated reaction, but what little

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FIG. 3. Comparison of TVA thermograms for a 5/1 methyl methacrylate-phenyl acetylene copolymer, F, Mn=93,000 (full line) and poly(methylmethacrylate), sample B, M.= 100,000.

26

I.C. McNEILL

there is gives rise to a rate maximum at the same temperature as for a homopolymer of similar molecular weight (compare with the thermogram for polymer B shown on the same figure). The reason for the suppression of end initiation is probably similar to that discussed subsequently for acrylate copolymers. Secondly, the main peak on the thermogram appears not in the region of 360 °, as would be expected (Table 2) for a polymer of molecular weight around 100,000, but at 333 °, and there is no evidence of any further peak beyond this. The most likely explanation of this effect is that all initiation above 300 ° in the copolymer is the result of scissions at unsaturated units. If this view is correct, then double bonds in the polymer backbone, like those at the chain ends, can also be regarded as weak structures at which reaction is initiated.

Copolymers of methyl methacrylate with methyl acrylate It is well known that the incorporation of small quantities of certain acrylates into the polymer chain during polymerization of methyl methacrylate increases the thermal stability. It is therefore of interest to see how this effect is reflected in the TVA thermograms for such materials. Two methyl acrylate copolymers (G, H) have been examined;

/ t': / I00

200

/i .l .,' 300

400

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FIG. 4. Effect of copolymerizationwith methyl acrylate on the thermal stability. TVA thermogramsfor 26/1 (dashedline),and 8/1 (dottedline) copolymers,comparedvdththat for poly(methylmethacrylate),sampleA, of comparable molecularweight(fuUline). in Fig. 4 the TVA thermograms for these copolymers are compared with that for a methyl methacrylate homopolymer (A) of similar molecular weight. As the acrylate content is increased from zero to 1/26 and then to 1/8, the low temperature end-initiated reaction is progressively reduced; in the I/8 copolymer, it has been almost completely eliminated. The observed results are explained if the acrylate units in the chain exert a "blocking effect" on the depolymerization process, particularly at lower temperatures, so that unzipping either cannot pass through the foreign unit or does so much less readily. Thus the zip length of the depolymerization, which is large for the homopolymer, would become considerably shorter as the acrylate content of the copolymer is increased. Copolymers G and H would therefore only unzip to a small extent from the chain ends at low temperatures, and further degradation would depend on initiation by chain scissions occurring at higher temperatures. A detailed degradation study of the

The Thermal Degradation of Methyl Methacrylate Polymersand Copolymers

27

methyl acrylate-methyl methacrylate system over the full range of compositions has been carried out using isothermal techniques by Grassie and Torrance. (6)

Copolymers of methyl methacrylate with higher acrylates Copolymers I, J and K contained 2 0 ~ of ethyl, n-propyl, and n-butyl acrylate respectively. The molecular weights (Table 1) were comparable with those of copolymer H (12 ~o methyl acrylate) and polymer A, with which they may therefore be compared. The thermograms for these three copolymers are not reproduced since they were similar in shape to that for copolymer H (Fig. 4), showing the almost complete elimination of the end-initiated monomer-producing reaction. It appears probable, therefore, that the property of blocking this reaction is common to straight chain acrylate monomers in general. Although all these acrylates stabilize the poly(methyl methacrylate) chain, the mode of action may not be the same, thus the lower Tm.~ value for the methyl acrylate copolymer suggests the possibility of some difference in mechanism in this case.

Copolymers of methyl methacrylate withstyrene The degradation behaviour of these copolymers is interesting in view of the fact that both poly(methyl methacrylate) and polystyrene undergo depolymerization to yield monomer. Because of transfer reactions in the latter case, however, dimer, trimer, etc. are also obtained. Polystyrene shows higher thermal stability. The TVA thermo~am for pure polystyrene is independent of molecular weight over a wide range and, for the standard heating rate of 10°/min used in the present investigation, shows a sin~e rate maximum at 418 °. In Fig. 5, TVA thermograms for copolymers L and M are compared with that for poly(methyl methacrylate) of similar molecular weight (sample B). All three materials were prepared by a radical mechanism. The presence of 20 ~ of styrene in the methacrylate polymer chain results in a considerable increase in stability. The end-initiated reaction is eliminated. Styrene units in the chain might not be expected to exert the

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FtG. 5. Effectofcopolymerizationwithstyreneon the thermal stability. TVAthermogratmfor 4/1 (dashedline)and 1[I (fullline) copolymers,comparedwiththat for poly(methylmethacrylate), sample A, of comparable molecular weight.

28

I.C. McNEILL

same sort of blocking effect on the depolymerization as the acrylates considered above, since both poly(methyl methacrylate) and polystyrene depolymerize to monomer, whereas polymethyl acrylate and the higher polyacrylates do not. Such an effect would, however, account for the result observed. An alternative explanation is that given by Grassie and Farish (7~ in a study of this system by isothermal degradation techniques. They pointed out that copolymers of this pair of monomers would be expected, in view of the work of Bevington, Melville and Taylor, (s) to have scarcely any terminal unsaturation since the cross termination reaction is favoured in the polymerization process and occurs almost exclusively by radical combination rather than disproportionation. The temperature of the rate maximum for the 4/1 copolymer (L) is comparable with the second/'max for the methyl methacrylate homopolymer B, suggesting that scissions of the main chain between pairs of methyl methacrylate units is likely to be the initiation process in the degradation of this copolymer. The thermogram for the 1/1 methyl methacrylate/styrene copolymer shows a further considerable increase in stability, with a rate maximum at 399 °. Its behaviour is closer to that of polystyrene itself, in which initiation resulting from scission of the main chain gives rise to a/'max value of 418 °. By contrast, the depolymerization resulting from main chain scission in poly(methyl methacrylate) gives rise to a rate maximum in the region of 360 °. A 1/1 copolymer of these monomers must contain quite a large proportion of sequences of methacrylate units of length 2 or more. It would be anticipated that scissions of bonds between pairs of MM units could therefore occur in this copolymer as in the homopolymer and the 4/1 copolymer. The evidence is clear, however, that such scissions do not lead to significant amounts of depolymerization until the temperature of breakdown of polystyrene itself is nearly reached. A possible explanation of the unexpectedly high stability of the I/1 copolymer is as follows. The radical pair resulting from an initial scission of the chain at temperatures around 270°-320 ° depolymerize only as far as the nearest styrene unit. The styryl radical ends thus obtained do not depolymerize to give volatile products at this temperature, and may possibly recombine or disproportionate. It is only in the temperature range for breakdown of polystyrene that depolymerization occurs. If this is the true explanation, then polystyryl radicals produced in other systems by scissions of a polymer or copolymer chain at abnormally low temperatures (< 300 °) would not depolymerize. This is consistent with the observations of Grant and Grassie (9) in the case of copolymers of styrene with small amounts of ~-chloroacrylonitrile. The co-monomer forms weak points at which scissions occur around 250 °, but Grant and Grassie concluded that the polystyryl radicals produced did not depolymerize at this temperature. The results of Richards and Salter (1°) appear at first sight to conflict with this view. These workers found that depolymerization of polystyrene could be brought about at temperatures below 300" by heating in presence of poly(~-methyl styrene). The mechanism suggested for this reaction, however, involves initial attack on the polystyrene molecule by an a-methyl styrene monomer radical, resulting in a single polystyryl radical rather than a radical pair. The apparent stability of polystyryl radicals in the cases already considered was observed in situations where apair of polymer radicals was initially produced. It may be that in these cases a cage effect occurs; Richards and Salter, in another paper, (m have produced evidence for such an effect.

The Thermal Degradation of Methyl Methacrylate Polymers and Copolymers

29

Copolymer of methyl methacrylate with ethyl methacrylate In all the copolymers considered above, the presence of the co-monomer has been found to result in a considerable change in thermal stability compared with a methyl methacrylate homopolymer of similar molecular weight. Provided a suitable comonomer were found which did not interfere significantly either with the termination reaction in the polymerization or with the depolymerization process, it is conceivable that a methyl methacrylate copolymer could behave in the same way as the homopolymer. Such a substance is the structurally similar monomer, ethyl methacrylate. The 4/i copolymer N was found to behave in almost exactly the same way as polymer A of similar molecular weight. The only minor differences observed were a slight broadening of the main peak in the thermogram and the production of a minute quantity of non-condensable material, probably ethylene, at temperatures around 400 ° . CONCLUSIONS A study of the TVA thermograms for these polymers and copolymers, along with information on their composition and molecular weights, enables conclusions to be drawn regarding the reactions involved in the depolymerizations. In methyl methacrylate polymers and in copolymers containing high concentrations of methyl methacrylate, the following effects can be observed. 1. Unsaturated chain ends, present in polymers made by the free radical mechanism, introduce instability. Polymers prepared by the anionic mechanism do not possess these ends and are stable up to much higher temperatures. 2. Foreign units in the methacrylate chain can constitute weak points. An example is provided by the unsaturated units resulting from copolymerization with phenyl acetylene. 3. Other foreign units stabilise polymers made by the radical mechanism against breakdown at low temperatures. This property is common to methyl, ethyl, propyl and butyl acrylates, and is also shown by styrene. 4. Small amounts of ethyl methacrylate as co-monomer appear not to interfere with the depolymerization reactions initiated either at chain ends or by chain scissions. The behaviour of the 1/1 copolymer of methyl methacrylate with styrene is particularly interesting since it suggests that polystyryl radicals, produced at temperatures below those at which polystyrene itself degrades, are unable to depolymerize. Acknowledgements--Thanks are due to Mr. R. G. Perrett who obtained the thermograms, and to Mr. R. G. Perrett and Mr. G. McCulloch who carried out the osmotic molecular weight determinations. REFERENCES (1) J. R. MacCallum, Makromolek. Chem. 83, 137 (1965). (2) I. C. McNeill, J. Polym. Sci. A1, 4, 2479 (1966). (3) I. C. McNeill, Europ. Polym. 3".3, 409 (1967). (4) D. H. Grant and S. Bywater, Trans. Faraday Soc. 59, 2105 (1963). (5) N. Grassie and H. W. Melville,Proc. R. Soc. A199, 1 (1949). (6) N. Grassie and B. J. D. Torrance, unpublished. (7) N. Grassie and E. Farish, Europ. Polym. J. 3, 305 (1967). (8) J. C. Bevington, H. W. Melville and R. P. Taylor, J. Polym. Sci. 12, 449 (1954). (9) N. Grassie and E. M. Grant, Europ. Polym. J. 2, 255 (1966). (10) D. H. Richaxds and D. A. Salter, Polymer 8, 127 (1967). (1 I) D. H. Richards and D. A. Salter, Polymer 8, 139 (1967).

30

I . C . McNEILL

R6sum6---On a examin~ des poly(methacrylate de m~thyle) ayant des masses mol6culaires diff6rentes et pr6par~s par vole radicalaire et anionique. On a trouv~ clue la stabilite therrnique de ces polym~:res d~:pend ~ la fois du mode de preparation et de la masse mol~culaire. On sait que la presence de comonom~res clans la chaine du poly(m~thacrylate de m~thyle) affecte profond~ment la stabilitY. On a examin6 l'inftuence des motifs ph~nyl ac~tyl6ne, acrylates de m6thyle, d'~thyle, de n-propyle et de n-butyle, styr6ne et m6thacrylate d'6thyle. Sommario---Sono stati esaminati i polimeri del metil metacrilato a diverso peso molecolare e preparati con meccanismo anionico e radicalico. Si ~ visto chela stabilit& termica dipende notevolmente sia dal metodo di preparazione che dal peso molecolare del polimero. La presenza di comonomeri nella catena del metil metacrilato ha in generale un notevole effetto sulla stabiliD.. E' stato studiato l'effetto del fenil acetilene, del metil, etil, n-propil e n-butil acrilato, del!o stirene e dell'etil metacrilato. Zusammenfassung--Polymere des Methylmethacrylats verschiedener Molekulargewichte hergestellt nach radikalischen und anionischen Mechanismen, wurden untersucht. Es zeigte sich, dab die thermische Stabilittit sowohl yon der Herstellungsart als auch vom Molekulargewicht des Folymeren stark abhtingig ist. Co-Monomere in der Methylmethacrylatkette haben im allgemeinen einen erheblichen EinfiuB auf die Stabilitat. Der EinfluB yon Phenylacetylen. Methyl-, ,~.thyl-, n-Propyl- und n-Butylacrylaten wurde untersucht.