No abstract

ABSTRACT:In this work, the melt crystallization of immiscible blends of isotactic polypropylene (iPP) and branched polyethylenes (PE) was followed by oscillatory shear measurements during controlled cooling. All the blends contained 20% iPP finely dispersed in several ethylene/ a-olefin copolymer matrices (with and without a nucleating agent) with densities ranging from 0.88 to 0.92 g/cm 3 (linear low, very low, and ultra low density polyethylenes: LLDPE, VLDPE, and ULDPE). The rheological results were compared with parallel differential scanning calorimetry (DSC) measurements at the same cooling rate. During preliminary evaluations of the neat resins, no effect was found of the variation of the frequency of oscillation or the applied shear strain on their crystallization (at least in the range explored in this work). In the case of the blends, when the iPP crystallized in a fractionated fashion, only one sudden increase in the storage modulus ( G) was observed during cooling due to the partial coincident crystallization of both iPP and the PE matrix. In the presence of a nucleating agent, an almost complete separation between the crystallization of both components in the blend was achieved and two increases in G were clearly observed upon cooling. A close match between the dynamic crystallization kinetics obtained by DSC and torsion rheometry was demonstrated by a direct comparison between calorimetrically measured solid conversion and G during cooling from the melt.