Morphology and rheology of compatibilized polymer blends: Diblock compatibilizers vs crosslinked reactive compatibilizers

Reactive compatibilization is commonly used when blending immiscible homopolymers. The compatibilizers formed from the interfacial coupling of two types of reactive chains often have a graft copolymer architecture. Here we consider the case where both reactive chains are multifunctional, leading to a crosslinked copolymer at the interface. Experiments were conducted on a model blend of similar to 30% polydimethylsiloxane drops in a polyisoprene matrix. Compatibilizer was formed by an interfacial reaction between amine-functional polydimethylsiloxane and maleic anhydride-functional polyisoprene. Both species were multifunctional, and therefore capable of interfacial crosslinking. Optical microscopy showed some unusual features including drop clusters, nonspherical drops, and some drops with apparently nonsmooth surfaces. All these features suggest that a crosslinked "skin" covers the interface of the drops. Rheologically, the reactively compatibilized blend showed gel-like behavior in oscillatory experiments, enhanced viscosity and elastic recovery at low stresses, and strong viscosity overshoots in creep experiments, all of which are likely attributable to drop clustering. At the highest stress studied (400 Pa), the viscosity of the reactively compatibilized blend is comparable to that of a similar blend compatibilized by diblock copolymer. This suggests that, in practical processing operations that occur at even higher stresses, interfacial crosslinking by multifunctional chains will not adversely affect processability.