Localization of micro- and nano-silica particles in heterophase poly(lactic acid)/poly(butylene adipate-co-terephthalate) blends

Abstract

This work studies the thermodynamic and kinetic parameters influencing the localization of micro- and nano-silica particles in multiphase mixtures of poly (lactic acid), (PLA), and poly(butylene adipate-co-terephthalate), PBAT. The surface energies of PLA and PBAT were measured at ambient and high temperature using contact angle and pendant drop techniques respectively. Based on this data, as well as that for silica, Young's model, which estimates the preferred lowest interfacial energy condition, predicts that silica particles should be located in the PBAT phase. Atomic force microscopy and scanning electron microscopy results confirm that when micro- or nano-silica particles are added to a PLA/PBAT melt, the silica particles are selectively localized in the PBAT phase irrespective of the PLA phase viscosity. The preferential encapsulation of silica particles by PBAT is shown to occur at the early stages of mixing. However, when micro- or nano-silica is initially distributed into the PLA melt prior to PLA/PBAT mixing, two distinct morphological behaviours are observed that persist over long mixing times. Nano-silica in PLA locates perfectly at the PLA/PBAT interface independently of the PLA viscosity while micro-silica locates at the interface with low viscosity PLA and stays distributed within the PLA when it is of high viscosity. It will be shown that the observed localization of micro- and nano-silica particles at the interface is due to the slow migration velocity of the particles at the interface that originates from the very low interfacial tension in this system. The localization of micro-silica particles in high viscosity PLA is related to the slow PLA film draining step close to the PLA-PBAT interface.