Influence of melt structure on Form II to I phase transition of Polybutene-1 under shear flow

Abstract

For decades, the influences of chemical structures and compositions, blend components and ambient environments on phase transition of polybutene-1 (PB-1) from Form II to I have been reported. Nevertheless, in terms of polymer processing, how to expedite the phase transition with a facile way is still a challenging task. In this study, four times the phase transition rate can be achieved by simply extruding the near melting point melt (NMP melt) of PB-1 through a capillary die. The melting survived Form I nanocrystals within the amorphous melt at the partially melting temperature (Tp) between the nominal melting temperature (Tm) and the equilibrium melting temperature (Tm0) allow accelerating the phase transition which was investigated by means of in situ small/wide angle X-ray scattering (SAXS/WAXS). Upon extruding of NMP melt, Form II transformed even faster than in the quiescent NMP melt. The number of flow-induced oriented crystalline structures (cylindrites) increased by reducing Tp. Kinetically, the half-transformation time (τ1/2) increased with rising Tp. Furthermore, τ1/2 of the core layer is large compared to τ1/2 of the skin layer. Our results suggest that both melting survived Form I nanocrystals and shear flow play integral roles in accelerating the phase transition of PB-1. We tentatively concluded that the higher nucleation probability of Form I during phase transition can be generated by more tie molecules distributed in the oriented lamellae caused by shear flow and the introducing extra internal stresses depended on the mismatch in the thermal expansion coefficients of Form I nanocrystals and Form II lamellar crystals.