Evaluation of the fractionated crystallization of dispersed polyolefins in a polystyrene matrix

Abstract

When crystallisable polymers like isotactic poly(propylene) (iPP) or linear low density polyethylene (LLDPE) are finely dispersed in an incompatible matrix like atactic polystyrene (PS) a fractionated crystallisation process will develop if the number of dispersed droplets is greater than the number of active heterogeneities originally present in the bulk polymer. In this work, several PS/iPP and PS/LLDPE blends were prepared in a composition range where PS was always the matrix component and in some cases compatibilizers were used to enhance dispersion. By applying a self-nucleation procedure we were able to corroborate that what causes the fractionated crystallisation is the lack of highly active heterogeneous nuclei (i.e., those normally active at low supercoolings in the bulk polymer) in every droplet. A detailed characterisation of the particle size distribution was carried out by SEM and the validity of using a Poisson distribution to calculate the concentration of heterogeneities present in one blend system was examined. The calculation of the concentration of heterogeneities can qualitatively explain the presence or absence of particular exotherms in the complex DSC cooling behaviour of some compositions of the PS/LLDPE/SEBS blends. However, the effect of the dispersity of the particle size distribution was found to greatly influence the results. When sufficient amount of a compatibilizer is used to obtain the minimum possible particle size the iPP crystallises exclusively at 45°C. The origin of such crystallisation and the possibility that it may be interpreted as arising from homogeneous nucleation is discussed along with analogous data for dispersed LLDPE.