Isothermal crystallization-induced phase transition of syndiotactic polystyrene polymorphism

The growth of alpha and beta crystals for melt-crystallized syndiotactic polystyrene (sPS) has been studied by the morphological observations of transmission electron microscopy and the structural analysis of electron diffraction (ED). The crystallographic textures of alpha and beta single crystals were identified by the results of ED and polyethylene decorated experiments. Phase transformation from the alpha" phase to the beta' phase for bulk and thin film samples was observed during isothermal crystallization. The transition was found to occur in the later stage of crystallization as evidenced by the structural analysis of Fourier transform infrared spectroscopy experiments combined with the enthalpic measurements of differential scanning calorimetry experiments. The coexistent alpha " and beta' crystals exhibit three specific crystallographic correspondences as identified by the appearance of three different [00l] zonal ED patterns. The corresponding diffraction results indicate that the alpha" and beta' crystals are both grown as flat-on type and possess a common c-axis in these structures so as to form dual-layer lamellar texture. The rearrangement of molecular chains during the transition was thus proposed. The initial growth of the metastable alpha" crystals is attributed to the kinetic effect of crystallization while the later formation of the beta' crystals results from the tendency to reach a thermodynamically more stable state. We suggest that the occurrence of the isothermal transformation is the results of floating chain behavior for sPS crystalline superstructure.