Recently, it has been shown that poly(stearyl methacrylate)-poly(benzyl methacrylate) diblock copolymer vesicles can be prepared via RAFT dispersion polymerization of benzyl methacrylate in mineral oil at 90 °C. This is an example of polymerization-induced self-assembly (PISA). Such vesicles undergo a vesicle-to-worm transition when heated up to 150 °C. This morphological transition leads to a significant increase in oil viscosity and can be explained in terms of surface plasticization of the oil-insoluble PBzMA block by hot solvent, which causes a subtle reduction in the fractional packing parameter for the diblock copolymer. In principle, this PISA formulation offers a new high-temperature oil-thickening mechanism for automotive engine oils. Herein we revisit this system with the aim of reducing the critical temperature at which the vesicle-to-worm transition occurs. A series of poly(stearyl methacrylate)-poly(benzyl methacrylate-stat-n-butyl methacrylate) diblock copolymer vesicles were prepared by RAFT dispersion copolymerisation in mineral oil. Oscillatory rheology studies confirmed that increasing the mole fraction of n-butyl methacrylate in the membrane-forming block significantly lowers the onset temperature required for the vesicle-to-worm transition. More specifically, introducing 47 mol % n-butyl methacrylate lowered the critical temperature required for the vesicle-to-worm transition by nearly 60 oC compared to the equivalent diblock copolymer prepared using solely benzyl methacrylate for the membrane-forming block. Moreover, an unexpected reduction in viscosity was observed above 125 °C, suggesting that a worm-to-sphere transition occurred. TEM studies confirm that worms are no longer observed at 140 °C. Preliminary variable temperature small-angle x-ray scattering (SAXS) experiments suggest rich thermoresponsive behavior, with spheres being formed at 180 oC and molecularly dissolved copolymer chains at 200 °C.

Synthesis of thermoresponsive block copolymer vesicles via polymerisation-induced self-assembly for oil thickening applications

2019 Fall National Meeting

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