Small angle neutron scattering studies of composite latex film structure

Abstract

The mechanical properties and structure of composite films made of high T g polystyrene (PS) nodules dispersed in a low T g polybutylacrylate (PBuA) matrix were studied by means of dynamic mechanical spectrometry and small angle neutron scattering. For films cast from mixtures of PS and PBuA latexes, film mechanical reinforcement was obtained above a percolation threshold of about 30% PS volume fraction. A segregation of PS particles into dense clusters in the PBuA continuous matrix, reminiscent of a phase separation, was observed. For films cast from core-shell particles, this segregation phenomenon may be prevented, depending on the coverage of the PS core by the PBuA shell. An efficient core encapsulation in the core-shell morphology leads to poor contact between PS cores, and the elastic moduli are then close to that of the PBuA matrix. Upon annealing the films above the T g of PS, extensive coalescence of PS particles occurred when large contacts were already present in the dry film at room temperature, and a percolating network of coalesced PS domains provides large elastic moduli at temperatures between the T g of PBuA and PS. The coalescence was prevented when PS particles were taken apart by the PBuA shell.