Structure and properties of homogeneous copolymers of propylene and 1-hexene

The solid state structure and properties of homogeneous copolymers of propylene and 1-hexene were studied by examining melting behavior, dynamic mechanical response, and morphology primarily with atomic force microscopy, wide- (WAXS) and small-angle X-ray scattering, and tensile deformation. Chain microstructure was analyzed by C-13 NMR. The results indicate that copolymers used in this study have an essentially random distribution of comonomer. For copolymers with less than 10 mol% hexane, crystallinity decreases with increasing comonomer content, as expected for exclusion of comonomer from the polypropylene crystal. The peak melting and crystallization temperatures also decrease with increasing hexene content. Copolymers with more than 10 mol% hexane crystallize with a new crystal structure that permits incorporation of hexene units. This is inferred from a higher level of crystallinity than would be expected if comonomer were excluded from the crystal and better development of the crystals as the hexene content increases. Copolymers with the new crystal structure crystallize slowly. After an incubation period, long fibrous lamellae form sheaf-like arrays that develop into small spherulites. The corresponding enthalpy change as a function of time assumes an S-shape characteristic of a phase transition described by the Avrami process. The new crystallographic form has not been reported for either polypropylene or for poly(l-hexene). It follows from WAXS studies of highly oriented films that the crystallographic unit cell has orthorhombic symmetry with a = 1.9860 nm, b = 1.7176 nm, and c = 0.6458 nm. The most intense diffracting planes are identified as the (210) plane reflecting at 2theta = 10.30degrees, the (230) plane reflecting at 2theta = 17.65degrees, the (040) plane reflecting at 2theta = 20.60degrees, the (031) plane reflecting at 2theta = 20.73degrees, and the (112) plane reflecting at 2theta = 28.52degrees for Cu Kalpha radiation. On the basis of pole figures, it is evident that the easiest slip during plastic deformation of the new crystal form occurs along (0k0) planes.