Effect of variation in the chemical structure on the rate of crystallisation of polymers

Abstract

Seven linear polyesters having similar molecular weights and closely related structures were prepared. The crystallisation rates of these polyesters were measured by using the Du Pont thermal analyzer and DSC cell. Additional rate measurements were made using dilatometry. The relative merits of the two techniques are discussed. The results obtained by DSC and dilatometry were analysed in terms of the Avrami equation using a computer program. The variation in the Avrami integer n and its possible significance are considered in detail. Parameters which represented the readiness of a polymer to crystallise, and the maximum crystallisation rates obtainable were selected. These were used to compare the crystallisation behaviour of the different polyesters. Chain flexibility was found to be an important factor governing nucleation. Crystallisation was inhibited by the presence of bulky groups restricting chain movement. Melting temperatures of the polymers were measured using optical microscopy. An estimate of their thermodynamic melting temperatures wags made using crystallisation half times. The effect of polymer structure on melting temperature is discussed. Spherulite growth measurements were made wherever possible. These results are related to the overall crystallisation rates. X-ray powder photographs and fibre diagrams were obtained. Results showed that crysallisation rates were not affected by crystal structure. The degree of crystallinity of samples of some of the polyesters was olttained from measurement of heats of fusion. An explanation of the crystallinites obtainable in the different polyesters is given.