Melting behavior and identification of polymorphic crystals in syndiotactic polystyrene

Melting peaks and resulted crystal forms in syndiotactic polystyrene (s-PS) subjected to various thermal treatments were thoroughly investigated using DSC and X-ray diffraction. This study concluded that the four major crysta l forms were individually generated and no transformation occurred between alpha', alpha ", beta' and beta " forms during thermal treatments. Thickeni ng of these crystals, however, was likely during either slower heating scan (i.e. dynamic annealing) or extended isothermal annealing at relatively hi gh temperatures. Different polymorphs were found to dominate the observed m ultiple melting endotherms of a thermally treated s-PS sample, which was er ased of the thermal history in advance. Through cross-reference between the thermograms and X-ray crystallographs, each of the multiple melting peaks in DSC thermograms of s-PS was individually assigned and associated with th e identified crystal forms. The melting peak of beta-form is always located at lower temperature than those of alpha-form. Meanwhile, beta-form is les s thermally stable than alpha-form. (C) 1999 Elsevier Science Ltd. All righ ts reserved.