Precursor of Shish-Kebab in Atactic Polystyrene/Isotactic Polystyrene Blend above Nominal Melting Temperature

In a previous paper [Polymer 2009, 50, 2095] we found that string-like objects were formed on the micrometer scale, after applying shear flow on melt of isotactic polystyrene (iPS) above its nominal melting temperature T_m, and assigned to precursors of shish-kebab (shear-induced precursor). In this paper we studied effects of a small amount of noncrystalline high molecular weight atactic polystyrene (aPS) in iPS on the shear-induced precursor formation above T_m (=223 °C). Polarized optical microscope (POM) measurements were mainly used to study the structure formation process of iPS and aPS/iPS blends under the chosen experimental conditions. It was found that there were more shear-induced precursors in aPS/iPS blend than in pure iPS at some shear conditions. This suggests that aPS can enhance the shear-induced precursor formation of iPS. The shear-induced precursors formed in the blends were as stable as those in pure iPS. Both could survive for more than 60 min at shear temperatures above T_m and melted at around 270 °C. An influence of aPS on the shear-induced precursor formation was also examined as functions of shear rate, shear strain and concentration of aPS. It was found that aPS enhanced the formation of the precursor at around 4 wt % of aPS most effectively in any shear conditions. These experimental findings suggest that the orientation and relaxation of noncrystalline high molecular weight aPS might have significant impact on the enhancement of precursor formation under shear flow. The corresponding mechanism presents new insights into the shear-induced crystallization.