Studies of Thermal Stability of Polymers by Thermal Volatilisation Analysis

A great deal of work has been carried out on the degradation of polystyrene in order to elucidate the mechanism of the reactions involved. Most of the workers have assumed that transfer reactions are taking place and included them as a factor in the theoretical treatments of the kinetics of degradation. Other workers have shown, by indirect experimental work, that transfer reactions to take part in the degradation process of this polymer. However, this work is a direct attempt to show that intermolecular transfer is a major factor taking part in the degradation process. The technique used was the TVA where the small pressure which exists in a continously evacuated system between a hot sample, decomposing to volatile products, and a cold trap placed some distance away is recorded continuously as the temperature of the sample is increased in a linear manner. The TVA curve indicates the variation of rate of volatilization of the sample with temperature. The degradation method used was that of comparing the thermal stability of very thin films of polymer with that of the thicker films. Degradation was carried out under conditions of temperature rise of 10 C/min. or at isothermal temperatures. Thin films of polystyrene were observed to be thermally more stable than the bulk material. The molecular weight dropped more sharply in the case of thick films when compared with thin films of the same sample weight. More styrene monomer was produced from degraded thin films than from the thicker films. Also, the shape of the curve of the rate of volatilization against percentage conversion in thin films resembled that of poly (a- methyl styrene) where the zip length is much greater than the chain length. This behaviour was considered to be due to the reduction of intermolecular transfer reactions in thin films of polystyrene. Polymers where transfer reactions are well established such as polypropylene, poly (isobutene) and some elastomers were also examined and showed an effect of thin film stability similar to that observed in polystyrene. Poly (methyl methacrylate) was also examined in the form of thin and thick films and in powder form. The results suggested that transfer reactions of the secondary hydrogen atoms might be taking place to some extent. Poly (methyl acrylate) which contains a tertiary hydrogen atom showed an effect similar to that observed in polystyrene; while there was no change in thermal stability of thin and thick films of poly (tert. butyl methacrylate). Very few useful conclusions could be drawn from results obtained from the degradation of thin and thick films of PVC, PVA and poly (vinyl alcohol)