Crystallization and Mechanical Properties of Isotactic Polypropylene/Calcium Carbonate Nanocomposites with a Stratified Distribution of Calcium Carbonate

摘要

Isotactic polypropylene (iPP)/calcium carbonate (CaCO_3) nanocomposites with a stratified distribution of CaCO_3 were prepared by two-step molding. The iPP and CaCO_3 nanoparticles were first mixed by a batch mixer and then compressed into thin layers. Thin iPP/CaCO_3 layers alternating with thin neat iPP layers were finally compressed together to form the stratified samples. The transcrystals were observed in the stratified samples by polarized optical microscopy and scanning electron microscopy. The transcrystals grew from the surfaces of the filled layers and occupied most of the space in the neat iPP layers. The b-form crystals were found in the stratified samples with thick transcrystalline layers, whereas the thickness of the transcrystalline layer was dependent on the content of CaCO_3 nanoparticles and the cooling rate of the processing. The relative crystallinity index of the conventional samples first increased and then decreased with the content of CaCO_3 nanoparticles. However, the relative crystallinity index of the stratified samples deceased slightly with the content of CaCO_3 nanoparticles because of the stratified distribution of the CaCO_3 nanoparticles. The stratified samples, except for the samples with high b-form contents, became more brittle than the conventional samples because of the transcrystal formation in the iPP layers. The stratified samples with high b-form contents showed much better mechanical properties than the conventional samples.