Direct observation of modulated structure upon cold-crystallization of syndiotactic polystyrene

Isothermal and non-isothermal crystallization of melt-quenched syndiotactic polystyrene (sPS) films from the glassy state were studied using complementary tools of optical microscopy (OM), depolarized light scattering (DPLS), time-resolved Fourier transform infrared (FTIR) spectroscopy, and simultaneous wide-angle and small-angle X-ray scattering (WAXD/SAXS). On slow heating at a rate of 1 °C/min, modulated structures characterized by periodic density fluctuations with a wavelength of 7 μm are distinctly observed under phase contrast OM to abruptly emerge at 129 °C−130 °C, spanning entire sample dimensions, before the initial crystallization (T _i) at 130.5 °C, as-determined from time-resolved FTIR. The contrast of the modulated structures is initially low but is gradually enhanced as the sample crystallinity increases at higher temperatures. The interdomain distance of the modulated structures increases during the crystal melting at T > 260 °C and the modulated structures remain discernible up to 282 °C, a temperature higher than the maximum melting temperatures of the sPS crystals at 279 °C. DPLS detected the subtle increase in depolarized light intensity at T < T _i and disclosed the classic four-leaf-clover scattering pattern at T > T _i to feasibly probe the spherulite growth from 5 to 9 μm before crystal melting. In addition, similar modulated structures are observed in the amorphous film subjected to isothermal crystallization at 130 °C. Simultaneous WAXD/SAXS results showed that the crystallized film possesses the mesomorphic form, and a profound SAXS intensity at the low q region (< 0.03 Å⁻¹) is seen before the emergence of the interference scattering peak located at the position (q _m) of 0.05 Å⁻¹. The low-q scattering becomes more significant as crystallization time is increased, whereas q _m shifts to the high q region. The spinodal-assisted crystallization is proposed to occur in the sPS sample, and some details are provided based on these results.