Melting and Crystallization Behavior of Syndiotactic Polystyrene

Abstract

The melting and crystallization behavior of new syndiotactic polystyrene (s-PS) with high tactisity were investigated and compared with those of isotactic polystyrene (i-PS). The weight average molecular weights (Mw) of the s-PS samples were 3.5×105 (s-PS 35), 1.14×106 (s-PS 114) and 2.00×106 (s-PS 200), respectively, and that of i-PS was 2.6×106. The melting temperatures of s-PS and i-PS with various mole-cular weights were measured by dilatometry and DSC. The melting temperature of s-PS 35 measured by dilatometry was 275°C, and that of i-PS was 233°C. The difference in temperature between the melting point and the crystallization point in the thermal hysteresis of s-PS 35 was 25°C to 30°C, which was the half of the i-PS value. The equilibrium melting temperature of s-PS 35 obtained after the Hoffman-Weeks method was 275°C, that of s-PS 114 was 279°C and that of s-PS 200 was 282°C. The spherulites of s-PS samples were obtained by isothermal crystallization from the melt with thin thickness. All these spherulites displayed positive birefringence. The number of spherulites was unchanged through out the observations and the spherulites were nucleated at the same place during every crystallization experiment. The Avrami exponent of the isothermal crystallization was 3 for each s-PS sample with various molecular weights. Thus the s-PS spherulites were believed to grew by heterogeneous nucleation with three dimensional growth, which was the same for the i-PS isothermal crystallization. The maximum crystallization rate constants of s-PS were at least 105 times larger than those for a i-PS within the present experimental conditions. The crystallization rate constant of the s-PS samples at a given crystallization temperature decreased with increasing Mw, of the s-PS samples.