Rheological studies of thermo-responsive diblock copolymer worm gels

Abstract

A series of diblock copolymer worms are prepared via aqueous dispersion polymerization using reversible addition–fragmentation chain transfer (RAFT) polymerization chemistry. More specifically, a poly(glycerol monomethacrylate) (PGMA) RAFT chain transfer agent is used to polymerize a water-miscible monomer, 2-hydroxypropyl methacrylate, at 70 °C. The poly(2-hydroxypropyl methacrylate) (PHPMA) chains become increasingly hydrophobic as they grow, which leads to nanoparticle formation. Careful control of the diblock composition is achieved by fixing the mean degree of polymerization (DP) of the PGMA stabilizer block at 54 and systematically varying the DP of the core-forming PHPMA block. This strategy enables the worm phase space to be targeted reproducibly. These worms form soft free-standing gels in aqueous solution due to inter-worm entanglements. Rheological studies enable the influence of the diblock copolymer composition on the gel strength, critical gelation concentration (CGC) and critical gelation temperature (CGT) to be assessed. A maximum in gel strength is observed as the DP of the PHPMA block is increased from 130 to 170. The initial increase is due to longer worms, while the subsequent decrease is associated with worm clustering, which leads to more brittle gels. The gel strength is reduced from approximately 100 Pa to 10 Pa as the copolymer concentration is lowered from 10 to 5 w/w%, with a CGC being observed at around 3 to 4 w/w%. The CGT is relatively concentration-independent, but sensitive to the diblock copolymer composition: longer (more PHPMA-rich) chains lead to the CGT being reduced from 20 °C to 7 °C. This is because longer PHPMA blocks require a greater degree of hydration to induce the worm-to-sphere transition, which can only achieved at progressively lower temperatures. Reversible de-gelation also occurs on cooling, since there can be no entanglements between isotropic particles. This RAFT formulation also provides a rare example of thermo-responsive diblock copolymer worms.