Tuning the vesicle-to-worm transition for thermoresponsive block copolymer vesicles prepared via polymerisation-induced self-assembly†
We have previously reported the synthesis of thermoresponsive poly(stearyl methacrylate)-poly(benzyl methacrylate) [PSMA-PBzMA] diblock copolymer vesicles in mineral oil via polymerisation-induced self-assembly (PISA). Such vesicles undergo a vesicle-to-worm transition on heating, which provides an interesting new oil-thickening mechanism (see M. J. Derry, et al., Angew. Chem., 2017, 56, 1746–1750). In the present study, we report an unexpected reduction in dispersion viscosity when heating vesicles of approximately the same composition above a certain critical temperature. Transmission electron microscopy (TEM) studies indicate rich thermoresponsive behavior, with vesicles present at 20 °C, worms being formed at 130 °C and spheres generated at 180 °C, indicating that a worm-to-sphere transition occurs after the initial vesicle-to-worm transition. Moreover, we have also prepared a series of new thermoresponsive diblock copolymer vesicles by RAFT dispersion copolymerization of n-butyl methacrylate (BuMA) with benzyl methacrylate (BzMA) using a poly(stearyl methacrylate) precursor in mineral oil. This model system was developed to examine whether statistical copolymerization of a suitable comonomer (BuMA) could be used to tune the critical onset temperature required for the vesicle-to-worm transition. Indeed, oscillatory rheology studies confirmed that targeting membrane-forming blocks containing up to 50 mol% BuMA lowered the critical onset temperature required to induce the vesicle-to-worm transition to 109 °C, compared to 167 °C for the reference PSMA14-PBzMA125 diblock copolymer. Variable temperature small-angle X-ray scattering (SAXS) experiments confirmed a vesicle-to-worm transition, with the vesicles initially present at 20 °C being converted into worms when heated above 130 °C. Furthermore, a substantial reduction in dispersion viscosity was again observed when heating above the critical onset temperature. TEM and shear-induced polarized light imaging (SIPLI) studies indicate that linear worms are no longer present at 160 °C and 170 °C respectively, suggesting a subsequent worm-to-sphere transition. The thermal transitions studied herein proved to be irreversible on cooling on normal experimental timescales (hours).