Crystallization and morphological features of syndiotactic/atactic polystyrene (sPS/aPS) mixtures with a constant weight ratio of 5/5 but different weight-average molecular weights (MW) of aPS, i.e., 1880K (aPS-U), 100K (aPS-H), 4.3K (aPS-M), and 1.3K (aPS-L), have been investigated. For these athermal blends cold-crystallized at temperatures near the glass transition, MW effects of aPS on the spherulitic growth rates (G) have been examined using small-angle light scattering to detect the structural evolution at various isothermal temperatures (Tc). In addition, the crystallinity and segregation morphology of the cold-crystallized samples were also acquired using Fourier transformation infrared spectroscopy (FTIR) and scanning electron microscopy, respectively. Compared with neat sPS at a given Tc, the retardation of G was evident in the sPS/aPS-U and sPS/aPS-H blends, whereas G was increased in the other two blends. By considering both effects of dilution and Tg variation, a master curve of the normalized G vs Tc - Tg was successfully constructed for the neat sPS and all blends except the sPS/aPS-L. For the sPS/aPS-L blend, an extra energy was required for the sPS demixing and transportation to the crystal growing front, resulting from the enhanced mixing entropy associated with the aPS oligmers used. Owing to the high mobility of aPS-L chains, moreover, the sPS/aPS-L blend exhibited interspherulitic segregation morphology, whereas the sPS/aPS-U blend showed an interlamellar segregation at low 120-160 °C, but a mixed segregation of interlamellar/interfibrillar morphology at 160-200 °C. Under optical microscope observations on the crystallized samples, a regularly bicontinuous structure with periodic wavelength of about 2-5 μm, depending on T c, was found for all blends as well as the neat sPS, suggesting the occurring of liquid-liquid (SD-like) phase separation coupled with crystallization. Regardless of the Tc used, the signature of SD-like structure was always preserved within the developed spherulites; its wavelength was smaller than the spherulitic diameter (6-10 μm). For all crystallized blends, Tc dependence of the Hv scattering invariant (QHv) exhibited a pronounced minimum and the corresponding temperature (T min) was relevant with the chain mobility in the blends. Despite the differences in Tc (100-180 °C) and MW of aPS (1.3-1880K) used, FTIR results showed that the blend crystallinity was relatively unchanged, being ca. half of that for the pristine sPS, and the crystal modification was either mesomorphic or α-form crystals. These facts suggest that the Q Hv minimum is attributed to the irregular packing of lamellar crystallites within spherulites, resulting from the comparable competition between the phase separation and cold crystallization at Tmin. In contrast, phase separation and cold crystallization become the dominant phase transition respectively at the lower and higher temperature regimes, giving rise to the development of more ordered symmetry within spherulites.
All Science Journal Classification (ASJC) codes
- Materials Chemistry