POLYPROPYLENE DURING CRYSTALLIZATION FROM THE MELT AS A MODEL FOR THERHEOLOGY OF MOLTEN-FILLED POLYMERS

Polarized light microscopy shows that polypropylene crystallizes from the melt into a well distinguished spherulitic structure. Therefore, i t provides a useful model for molten-filled polymers, where the growin g spherulites are considered to be filler particles dispersed in a mat rix fluid. Although spherulites are randomly dispersed in the space, t wo dispersion models (simple cubic and centered cubic) are discussed t o correlate the transformed fraction alpha(t) with the volume fraction of filler phi(t). The combination of these results with those of diff erential scanning calorimetry (DSC) shows that the transformed fractio n alpha(t) is a direct indication of the volume fraction of filler phi (t). The rheological study, using oscillatory experiments coupled with DSC results, shows the relative sensitivity of the rheological functi ons to structural changes of the liquid during crystallization. Furthe rmore, they reveal the existence of a yield effect above a certain cri tical value of the filler content (phi(c) = 0.4). In the absence of th is yield effect; a model is proposed to predict the variation of the r heological functions with the filler content. This model shows not onl y a variation of the plateau modulus, but also the modification of the characteristic times of relaxation of the polymer matrix, whereas the shape of the relaxation spectrum remains unchanged. (C) 1996 John Wil ey & Sons, Inc.