Microphase Separation and Gelation through Polymerization-Induced Self-Assembly Using Star Polyethylene Glycols

Polymerization-induced self-assembly (PISA) during the synthesis of diblock copolymers has garnered considerable interest; however, architectures beyond diblock copolymers have scarcely been explored. Here, we studied PISA using 4- and 8-arm star polyethylene glycol (PEG), as well as 2-arm (linear) PEG, wherein each terminus of PEG was functionalized with a chain-transfer agent, holding a constant molar mass for each arm. Styrene was polymerized from each PEG terminus through reversible addition-fragmentation chain-transfer (RAFT) polymerization in an ionic liquid (1-butyl-3-methylimidazolium hexafluorophosphate, [BMIM][PF₆]), with a total solute concentration of 40 wt %. While the styrene monomer is soluble in [BMIM][PF₆], polystyrene is not; thus, self-assembly and cross-linking (gelation) occur. Structural analysis by small-angle X-ray scattering revealed that a relatively ordered microphase-separated structure for PISA was observed. Two-arm PEG-PS formed hexagonally packed cylinders, whereas 4- and 8-arm PEG-PS exhibited hexagonal close-packed spheres and disordered spheres. The dynamics, studied by oscillatory rheology, were also influenced by the number of arms; the 4-arm star block copolymers showed the highest plateau modulus. This study demonstrates that the topology is an important factor in controlling the microphase-separated structure and mechanical properties when preparing gels through PISA.