Effect of in situ prepared silica nano-particles on non-isothermal crystallization of polypropylene

The crystallization behavior of PP/silica nanocomposites prepared in-situ via solid-state modification and sol-gel reaction is investigated. The crystallization behavior studied with DSC shows that in situ formed silica nano-particles act as nucleating agents. The non-isothermal crystallization kinetics of PP/silica nanocomposites is studied using a combined Avrami-Ozawa approach and shows a two-stage crystallization process: the primary stage is characterized by nucleation and spherulitic growth and the secondary stage is characteristic of the perfectioning of crystals. Silica speeds up the primary stage, resulting in a more narrow lamellar thickness distribution. The crystallization activation energy decreases with increasing silica content in the PP/silica nanocomposites. The nucleating efficiency of the in-situ prepared silica particles based on the scale as proposed by Lotz and co-workers is found to be 20% in the low concentration range and is higher compared to silica nano-particles as well as other nano-fillers studied. The melting behavior indicates the formation of more perfect crystals with a narrow lamellar thickness distribution and the WAXD patterns show that silica nano-particles induce the formation of crystals with the β-modification in PP at high silica content (ca. 5 wt%). DMTA analysis shows a marginal lowering of the Tg and an increased mobility of the amorphous phase.