Polymorphic behavior of copolymers of syndiotactic polystyrene with m-methylstyrene

The influence of the presence of different amounts m-methylstyrene (m-MS), in the composition range 4-21 mol %, on the polymorphic behavior of syndiotactic polystyrene (s-PS) is presented. A comparison with the effect of the presence of p-methylstyrene is also presented. The behavior of copolymers subjected to solvent extraction or annealing procedures indicates that the presence of m-MS stabilizes the gamma form of s-PS, which remains stable in a large temperature range, up to 210-220 degreesC. The helical mesoporous delta form of s-PS has not been observed in copolymers with m-MS. The presence of low contents of m-MS induces crystallization of the beta form from polymer solution (by casting at high temperatures), whereas high concentrations of m-MS tend to stabilize the gamma form. The copolymer samples crystallize from the melt in alpha and beta forms depending on the concentration of m-MS, which stabilizes the beta form for composition around 11 mol %, and induces crystallization in the a form for lower (4 mol %) and higher (21 mol %) concentration. Moreover, even when the memory of the alpha form is maintained in the melt, the presence of m-MS in a concentration around 11 mol % induces the crystallization of the beta form, whereas, even though no memory of the alpha form is present in the melt, samples with 4 and 21 mol % of m-MS crystallize in the a form. The a. form obtained in these copolymers, by melt-crystallization or annealing of the gamma form, does not correspond to neither the disordered alpha', nor the ordered alpha modifications observed in s-PS. The presence of m-MS comonomeric units induces structural variation in the crystal packing of the trans-planar chains, and a new crystalline form is obtained.