Predictive Phase Diagrams for RAFT Aqueous Dispersion Polymerization: Effect of Block Copolymer Composition, Molecular Weight, and Copolymer Concentration

Abstract

Polymerization-induced self-assembly (PISA) of poly(glycerol monomethacrylate)–poly(2-hydroxypropyl methacrylate) (PGMA–PHPMA) diblocks is conducted using a RAFT aqueous dispersion polymerization formulation at 70 °C. Several PGMA macromolecular chain transfer agents (macro-CTAs) are chain-extended using a water-miscible monomer (HPMA): the growing PHPMA block becomes increasingly hydrophobic and hence drives in situ self-assembly. The final copolymer morphology in such PISA syntheses depends on just three parameters: the mean degree of polymerization (DP) of the PGMA stabilizer block, the mean DP of the PHPMA core-forming block, and the total solids concentration. Transmission electron microscopy is used to construct detailed diblock copolymer phase diagrams for PGMA DPs of 47, 78, and 112. For the shortest stabilizer block, there is essentially no concentration dependence: spheres, worms, or vesicles can be obtained even at 10% w/w solids simply by selecting the DP of the PHPMA block that gives the appropriate molecular curvature. For a PGMA DP of 78, the phase diagram is rich: and the copolymer morphology depends strongly on the total solids concentration. There is also a narrow region where spheres, worms, and vesicles coexist, which may be due to the effect of polydispersity. For a PGMA 112 macro-CTA, the phase diagram is dominated by spherical morphologies. This is probably because the longer core-forming block DPs required to reduce the molecular curvature are significantly more dehydrated and hence less mobile, which prevents the in situ evolution of morphology from spheres to higher order morphologies. This hypothesis is supported by the observation that addition of ethanol to aqueous PISA syntheses conducted using the longer macro-CTAs allows access to diblock copolymer worms or vesicles, since this cosolvent solvates the core-forming PHPMA chains and hence increases their mobility at 70 °C. Elucidation of such phase diagrams is vital to ensure reproducible targeting of pure phases, rather than mixed phases.