Enthalpy relaxation in the cooling/heating cycles of polypropylene/organosilica nanocomposites: I: Non-isothermal crystallization

Non-isothermal crystallization of the neat isotactic polypropylene homopolymer (PP-0) and of a series of nanocomposites (PNC) containing up to 4.68 vol.% of organosilica was studied in the standard 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 revealed a slight but systematic increase of the nucleation barrier for lamellar crystallization of PP in the PNC concomitant to stronger restrictions to transport of PP segments across the melt/lamellar crystal interface. The overall crystallization rate data for the PNC were consistent with the assumption of two separate contributions from the initial (unconstrained), and the subsequent (constrained) growth mechanisms, respectively. The obtained results were considered as evidence for the coexistence in undercooled PP melts of the PNC of initial crystal nucleation and growth sites characteristic for the neat PP-0, and the basically different sites (presumably, PP chains anchored by both ends to the surfaces of two adjacent nanoparticles).