Modelling the morphology evolution of polymer materials undergoing phase separation

Abstract

This contribution models the morphology evolution of hetero-phase polymers that undergo phase separation and inversion during their formation. High impact polystyrene (HIPS) is a selected representative of hetero-phase polymers with double-emulsion (or ‘salami’) morphology, because it consists of the continuous polystyrene (PS) phase with dispersed micron-sized polybutadiene (PB) particles containing sub-micron occlusions of the partially grafted copolymer (PB-g-PS). Our modelling effort builds on the work of Nauman and He (2001) [1], but addresses the weakest point of the Cahn–Hilliard model applied to HIPS evolution: the Ostwald ripening, which is in reality suppressed or reduced by grafting. Phase inversion is the most critical but so far least-modelled step in double-emulsion morphology evolution. Therefore we demonstrate the modelling of all three experimentally observed mechanisms of double-emulsion formation: (i) encapsulation of PS particles into PB domains by shear forces, (ii) thermodynamically consistent reaction-induced phase separation, and (iii) preservation of graft-stabilized PS particles through the phase inversion. We stress out the requirement of proper setting of three-component (polystyrene–polybutadiene–styrene) thermodynamics as the basis for the realistic description of the phenomena occurring during the evolution of hetero-phase morphology. We thus present the first phenomenological model capable to describe all principal steps in the HIPS double-emulsion morphology evolution and can thus conclude with the discussion of future efforts aimed at its quantitative refinement and modelling of industrial reactors.