Gelation of microsphere dispersions using a thermally-responsive graft polymer

Dispersions of microspheres (MSs) that form self-supporting particle gels are fundamentally interesting from the viewpoints of gel formation and mechanical properties. Here, we investigate model mixed MS/thermally responsive polymer dispersions that exist as particle gels at 37°C. The MS comprised poly(caprolactone) (PCL) and was prepared by solvent evaporation. The thermally responsive polymer contained a cationic backbone and poly(2-(2-methoxyethoxy)ethyl methacrylate) side chains and is abbreviated as PMA. Mixed PCL/PMA dispersions formed weak gels due to depletion at 20°C. At higher temperatures they formed stronger gels due to a combination of bridging of PCL MS by PMA and reinforcement by a PMA network. A key parameter controlling the mechanical properties of the reinforced MS particle gels was the volume fraction of PMA with respect to total polymer present (ΦPMA). Self-healing behaviour was observed for the gels using dynamic rheology and this depended on ΦPMA. The MS particle gel mechanical properties were conceptually described in terms of isostress and isostrain blending laws. At ΦPMA less than or greater than 0.057 the gels were dominated by the PCL or PMA networks, respectively. The latter value is suggested to be analogous to a phase inversion point.