Thermal-induced crystal/conformation transformations in syndiotactic polystyrene films treated with different solvents

Abstract

Solution-crystallized or solvent-induced crystal forms and transitions of these crystals upon post-annealing in solvent-treated syndiotactic polystyrene (sPS) (films cast from o-dichlorobenzene or immersed in 1,2-dichloroethane, respectively) were investigated via wide angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), and Fourier-transform infrared (FTIR) spectroscopy. IR spectra in the frequency range of 720-820 cm-1, which is attributed to the C-H out-of-plane bending of the phenyl ring for neat syndiotactic polystyrene (sPS) and their crystals/solvent complexes (sPS/o-dichlorobenzene or sPS/1,2-dichloroethane), were examined for the freshly as-cast samples in comparison with those having been subjected to post-annealing by heating to high temperatures. Three bands centered at approximately 779, 769, and 750 cm-1 were observed, the intensity and wavenumber being sensitive to the helical structural changes as a function of the annealing temperature. The band at 769 cm-1, being associated with the intramolecular interactions of the helical chains of the γ-crystal, was used to confirm the formation of such a phase. The bands at 779 cm-1 and 769 cm-1 disappeared and the sharper band at 750 cm-1 remained upon further annealing above the γ → α′ transition temperature. Thus, by examining the temperature dependency as revealed in the IR spectra the thermal-induced phase transition from the δ- to an ordered γ- and then to an α′-phase was successfully proven for the sPS/o-dichlorobenzene complex. Similarly, the other sPS/1,2-dichloroethane complex underwent transition (from a δ- to a disordered γ- and then to an α′-pahse) upon the same thermal treatments, except that the intermediate phase is a disordered γ-phase.