David L. Green, Department of Chemical Engineering, University of Virginia, 102 Engineers Way, Charlottesville, VA 22904, Jeremy Fowler, Chemical Engineering, University of Virginia, 102 Engineers Way, Charlottesville, VA 22904, Timothy E. Long, Department of Chemistry, Virginia Tech, Blacksburg, VA 24061, and Tomonori Saito, Chemistry, Virginia Tech, Blacksburg, VA 24061.
Our overall goal is to quantify how the interfacial wetting of diblock copolymers governs the emulsification of polymer droplets in immiscible homopolymer blends. This system in which the diblock copolymer migrates to the interface between the immiscible 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 phase 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 polymer droplets with diblock copolymers at their surfaces will show similar dispersion characteristics as the polymer-grafted nanoparticles. To this end, well-defined polyisoprene (PI) matrices and diblock copolymers of poly(isoprene)-b-poly(dimethylsiloxane) (PI-b-PDMS) were synthesized with living anionic polymerization. PDMS droplets were homogenized with the matrix and diblock copolymers to formulate well controlled model systems of PDMS/PI-b-PDMS/PI in which the diblock copolymer concentration was increased while the amounts of PDMS and PI were adjusted to maintain 7% PDMS and 93% PI. Using optical microscopy with a Linkam shear stage, shear flow was used to manipulate droplet sizes. At high diblock copolymer coverages and low shear rates, we observed that PDMS droplets that were once emulsified and dispersed, aggregated and formed large clusters. The point at which the droplets aggregate depends on the matrix molecular weight; in shorter polymer matrices, more diblock copolymer must be added to induce droplet aggregation. This behavior 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 match the predicted scaling behavior for autophobic dewetting, indicating that a wetting phase diagram can also be used to predict the regions over which diblock copolymers emulsify polymer droplets in immiscible homopolymer matrices.