Blends of propylene-ethylene and propylene-1-butene random copolymers: I. Morphology and structure

Samples of propylene-ethylene (EP) and propylene-(1-butene) (BP) random copolymers with various comonomer content (2-3.1 wt% ethylene, 9.9 wt% 1-butene), were melt-mixed in Brabender internal mixer at various compositions (25/75, 50/50, 75/25). Films of copolymers and blends, as well as of a homopolymer sample (iPP), obtained by compression moulding and with different thermal history were characterized by optical and scanning electron microscopy (OM, SEM), small-angle light scattering (SALS), small- and wide angle X-ray scattering (SAXS, WAXS) and differential scanning calorimetry (DSC). It was found that all copolymers and blends studied crystallized exclusively in monoclinic α-modification forming spherulitic structure in a very broad undercooling range. The average size of spherulites is smaller in the copolymer containing 1-butene as compared to those containing ethylene or to iPP homopolymer, due to enhanced heterogeneous nucleation in BP copolymer. SEM microscopic observations demonstrated that EP and BP copolymers were miscible at all examined compositions and form homogeneous blends. Structural and morphological analysis indicated that the comonomer units are incorporated into growing crystallites in both EP and BP copolymers, while the non-crystallizing material is rejected out of the crystallites. For small concentrations of comonomer some of non-crystallizing species are pushed ahead of the front of growing spherulite into interspherulitic regions. For higher comonomer concentration these species are mostly trapped in intraspherulitic regions. Melting behavior of copolymers reflects the incorporation of comonomer into crystalline phase: melting temperature and crystallinity degree decrease in copolymers and blends as compared to plain iPP.