NON-Isothermal crystalization and melting behavior of poly (propylene)-poly (ethylene vinyl acetate) [PP/EVA] and poly (propylene)-(ethylene-propylene) [PP-EP/EVA] blends

摘要

The melting behavior of PP/EVA and PP-EP/EVA blends non-isothermally crystallized was studied. The morphology of blends was evaluated in terms of crystal habits, spherulitic structure and melting characteristics. For PP and EVA homopolymers the observed crystal habits were alpha-monoclinic and orthorhombic corresponding to PP and PE crystals since this last is a part of the EVA copolymer. Two type of alpha-crystals were associated to the experimental data and they were designated as low (LPalpha-C) and high (HPalpha-C) perfection alpha-crystals. These crystals were relatively unaffected in nature after blending the PP or the PP-EP with the EVA copolymer. Nevertheless, crystallinity decreased and the spherulitic structure changed. In differntial scanning calorimetry [DSC] heating traces two partially overlapped endothermic signals were observed during heating of the non-isothermally crystallized specimen. The low temperature endotherm (I) was associated to melting of LPalpha-C structures while the high temperature endotherm (II) was related to two different melting processes, one corre-sponding to HPalpha-C structures and the other to recrystallized or reorganized material. The shifting of endotherm I to lower temperatures was considered an effect of interfacial interactions in the crystalline zones between the LPalpha-C of PP and the orthorhombic crystals of EVA. Modulated differential scanning calorimetry [MDSC] traces corroborated that recrystallization is higher in PP/EVA blends the opposite being the case in PP-EP/EVA blends. Differences in the multiple melting between PP/EVA and PP/EP/EVA blends are discussed in terms of molecular interactions.