Morphology Development and Interfacial Erosion in Reactive Polymer Blending

This work studies morphology development in blends of polyamide (PA) with poly(isobutylene-p-methyl styrene) (IMSM) and brominated poly(isobutylene-co-p-methylstyrene) (BIMSM). The presence of pendent benzylic bromine in BIMSM facilitates the rapid in situ formation of BIMSM-PA graft copolymer. IMSM represents the nonreactive reference for this blend system. In this study several important anomalies have been observed for the reactive BIMSM/PA system as compared to classical interfacially modified blend systems. These anomalies are as follows: as much as a 37-fold reduction in volume average diameter for the reactive system as compared to the nonreactive one; high phase size distribution (d(v)/d(n)), at all blend compositions, with the fine droplets being in the 50-80 nm range scale; extensive droplet in droplet formation for BIMSM in PA in a BIMSM matrix; very high extents of reaction, i.e., 46 wt % of the total blend material reacts over a short time of mixing; and an emulsification study which demonstrates a linear drop in particle size and requires a very high concentration of copolymer (20 IMSM/80 BIMSM) to reach a plateau value. This is well beyond the amount of copolymer needed to saturate the interface even though static interfacial tension studies show that only 5% BIMSM in IMSM completely suppresses capillary breakup. These results are explained by a novel mechanism of reactive morphology development termed here as "interfacial erosion". The mechanism considers the formation of a very high viscosity graft copolymer right at the interface during dynamic mixing, resulting from the mutual contact of the BIMSM and PA molecules. The viscosity mismatch between the formed graft copolymer and the other constituents of the blend lead to the subsequent erosion of interphase material during dynamic mixing to form fine, nanometer-sized micelles in the bulk. The removal of the copolymer from the interface exposes nonreacted material and primes the interfacial region for further copolymer formation. In this fashion, most of the BIMSM can be made to react, and the resulting blend is a nanoscale dispersion with a number-average diameter of 50-80 nm and a volume average diameter of 300 nm. This work raises important considerations concerning the use of graft copolymers, in general, in polymer blend systems.