Solid state blending of poly(ethylene terephthalate) with polystyrene: Extent of PET amorphization and compositional effects on crystallizability

Polystyrene (PS) and poly(ethylene terephthalate) (PET) were blended together in the solid state via cryogenic mechanical attrition (CMA) and in the melt through conventional twin-screw extrusion. CMA PS/PET blend morphologies were characterized both qualitatively and quantitatively through microscopy and thermal analysis. Specifically, CMA reduced the dispersed-phase domain size and its distribution relative to simple melt extrusion, although not to the extent attained with added chemical compatibilizers. CMA also amorphized the PET phase and depressed the PET cold crystallization rate, which was measured by post-CMA nonisothermal MDSC analysis. The PET amorphization efficiency and crystallizability for CMA PS/PET blends were the highest and lowest, respectively, at the PS/PET phase inversion. These concomitant phenomena are known to be caused by CMA-induced PET crystal defect formation and subsequent entropic stabilization. Such behaviors are linked to the enhanced presence of an uncrystallizable rigid amorphous PET phase, and the weight fraction of this rigid amorphous fraction (RAF PET) was quantified and also maximized near the PS/PET phase inversion. Moreover, the increased compatibilization and amorphization efficiencies and reduced PET crystallizability were determined to be interdependent. These studies have verified that CMA of PET with PS is more efficient than extrusion due to the formation of nonequilibrium, metastable morphologies that can be more precisely controlled and better stabilized with an interesting, composition-dependent interplay between PET crystallizability and the extent of PS/PET compatibilization.