Localization of micro and nano- silica particles in a high interfacial tension poly(lactic acid)/low density polyethylene system

Abstract

This work studies the effects of thermodynamic and kinetic parameters on the localization and migration of spherical micro- and nano-silica particles in the high interfacial tension blend system of poly (lactic acid), (PLA)/low density polyethylene (LDPE). The surface modification of micro-silica particles from a high to a low energy surface was carried out by the grafting of (2-Dodecen-1-yl) succinic anhydride to the surface of micro-silica particles using a new gas-phase reaction approach. The surface modification was confirmed by X-ray photoelectron spectroscopy analysis and surface energy measurements. Young's model predicts that the thermodynamic equilibrium localization of unmodified and modified silica particles in PLA/LDPE blends should be in the PLA phase and at the PLA-LDPE interface, respectively. Scanning electron microscopy results confirm that when unmodified micro- or nano-silica particles are added to a PLA/LDPE melt, the silica particles are selectively localized in the PLA phase even in the blend sample with only 5 vol.% of PLA. However, the modified micro-silica particles were found to be located principally in the LDPE phase. The influence of kinetic parameters was imposed by premixing modified and unmodified micro-silica particles with a high viscosity LDPE phase (H-LDPE). In that case both silica types remain in the H-LDPE phase independent of shear rate and mixing time. When the viscosity of the LDPE phase is reduced, unmodified and modified micro-silica migrate to their thermodynamically predicted locations in the PLA phase and at the PLA-LDPE interface respectively. In the case of unmodified nano-silica particles premixed in the H-LDPE phase, individual well dispersed nano-silica particles migrate to the PLA phase while aggregates remain in the H-LDPE phase. These results have important implications in the field of nanocomposites and indicate that the localization of well-dispersed nanoparticles in a high interfacial tension multiphase system will not likely be influenced by kinetic effects. Kinetic effects are much more dominant in micro-scale silica systems and the kinetic effects are found to depend on a film-draining mechanism at the PLA-LDPE interface region.