(298f) Evidence for the Autophobic Dewetting of Compatibilized Droplets in Immiscible Melts

Our goal is to quantify how the wetting of diblock copolymers governs the dynamic behavior of polymer droplets in immiscible homopolymer blends. This system in which the copolymer migrates to the interface between the homopolymers and emulsifies the dispersed phase is analogous to a system of nanoparticles grafted with and dispersed in the same homopolymer. Our previous work with nanoparticles shows that a wetting diagram of the graft and matrix homopolymers predicts that particles disperse in the complete wetting region and aggregate in the incomplete wetting region. Our hypothesis is that droplets with diblock copolymers at their surfaces will behave similarly to the nanoparticles. To this end, polyisoprene (PI) matrices and diblock copolymers of poly(isoprene)-b-poly(dimethylsiloxane) (PI-b-PDMS) were synthesized by anionic polymerization. PDMS droplets were homogenized with the matrix and copolymers to formulate model systems. Using optical microscopy with a shear stage, flow was used to manipulate droplet sizes and to probe dynamic behavior. At high copolymer coverages and low shear, we observed that droplets that were once emulsified and dispersed, aggregated and formed clusters. The point at which the droplets aggregate depends on the matrix molecular weight; in shorter matrices, more copolymer must be added to induce droplet aggregation. This is consistent with autophobic dewetting, which has been recently observed for highly grafted nanoparticles in homopolymer melts. Moreover, the surface coverages at which droplets aggregate correspond to the predicted behavior for autophobic dewetting, indicating that a wetting diagram can also be used to predict the regions over which diblock copolymers emulsify polymer droplets in homopolymer matrices. The occurrence and behavior of these aggregates can be explained and quantified by analyzing dynamic response to flow conditions.