Crystallization of a miscible propylene/ethylene copolymer blend

The crystallization behavior and morphological patterns of a miscible blend of two propylene/ethylene (P/E) copolymers that differed in ethylene content were studied. Metallocene-catalyzed P/E copolymers containing 3.1 and 11.0 mol% ethylene were chosen for blending. The difference in ethylene content was small enough to ensure miscibility of the pair in the melt, and the ethylene content was low enough to ensure that both were crystallizable. The blends were characterized by differential scanning calorimetry (DSC), wide angle X-ray diffraction (WAXD), optical microscopy (OM) and atomic force microscopy (AFM). The complex melting endotherm of the blends consisted of a broad low temperature peak at Tm1, a high temperature peak at Tm2, and an intermediate peak at Tm′ which was not characteristic of either constituent and depended on blend composition. The multiple melting peaks arose from distinct crystal populations. All the blends exhibited a mixed morphological texture of α-radial lamellae with short, densely packed γ-overgrowths, interspersed with areas of α-crosshatch. The high temperature peak at Tm2 was assigned to the melting of the α-radial lamellae which formed from chains of the lower comonomer constituent. The broad low temperature peak at Tm1 was attributed to the melting of γ-crystal overgrowths on the radial lamellae. The new peak at Tm′ was thought to arise from the melting of the α-crosshatch lamellae. The lamellar thickness, and hence Tm′, correlated with the crystallization temperature, which decreased as the blend was made richer in the higher comonomer constituent.