Miscible blends of syndiotactic polystyrene and atactic polystyrene.Part 2.Depolarized light scattering studies and crystal growth rates

摘要

Crystallization kinetics and morphology in miscible blends of syndiotactic polystyrene (sPS) and atactic postyrene (aPS) have been investigated by means of time-resolved depolarized light scattering (DPLS),polarized optical microscoipy (POM) and scanning electron microscopy (SEM).Two different weight-average molecualr weight of aPS,i.e.M_w=100k and 4.3k,were used to prepared the blends and denoted sPS/aPS (H) and sPS/aPS(M),respectively.Owing to a dilution effect,addition of aPS reduces the crystal growth rate and the overall crystallization rate of sPS;the reduction is more significant in sPS/aPS(M)of which a depression of equilibrium melting temperature is found due to the enhanced mixing entropy.Linear crystal growth is always observed in sPS/aPS(H) at the temperatures studied (240-269 deg C) and results in an interfibrillar segregation morphology revealed by SEM,whereas sPS/aPS(M) with high aPS content exhibits non-linear growth behavior at low supercooling and gives an interspherulitic segregation morphology.Based on the Lauritzen-Hoffman theory,the fold surface free energies (sigma_e) of sPS lamellae derived from DPLS and POM are in fair agreement,being 15.1 erg/cm~2 from the former and 12.6 erg/cm~2 from the latter.The peculiarly low values of sigma_e and the derived work of chain folding are discussed briefly.On addition of aPS,the lateral surface free energy of lamellae remains intact (9.9 erg/cm~2) regardless of aPS molecualr weight sued,which is ascribed to the absence of specific interaction between sPS and a PS components.Moreover,it seems that the activation energy for sPS chains to diffuse from the miscible melt to the crystal growth front is slightly increased in sPS/aPS (M),plausibly attributable to the extra energy required for the demixing process.