Nonisothermal crystallization of the polypropylene/organosilica nanocomposite

Nonisothermal crystallization of the neat isotactic polypropylene homopolymer (PP-0) and of the nanocomposite containing 4.68 wt.% of organosilica (PP-4.68) was studied in the standard differential scanning calorimetry (DSC) mode during constant-rate cooling from the melt state. Analysis of the nucleation parameters derived from cooling rate dependencies of the temperatures for the onset of crystallization exotherms suggested a slight increase of the nucleation barrier for lamellar crystallization of PP within a confined space between neighboring nanoparticles of an infinite cluster of the nanocomposite, concomitant to stronger restrictions to transport of PP segments across the melt/lamellar crystal interface. The overall crystallization rate data for PP-4.68 were consistent with the assumption of two separate contributions from the initial (unconstrained) and the subsequent (constrained) nucleation mechanisms, respectively. The obtained results were considered as evidence for a coexistence in an undercooled PP melt of the nanocomposite of initial nucleation sites characteristic for the neat PP-0, and the basically different nucleation sites (presumably, PP chains anchored by both ends to the surfaces of two adjacent nanoparticles).