Shear-induced alignment of block copolymer worms in mineral oil

Poly(stearyl methacrylate)-poly(benzyl methacrylate) [PSMA-PBzMA] diblock copolymer worms were synthesized directly in mineral oil via reversible addition-fragmentation chain transfer (RAFT) dispersion polymerization at 90 °C. Free-standing gels were obtained from this polymerization-induced self-assembly (PISA) formulation when targeting PSMA 13 -PBzMA 65 dispersions at 5% w/w to 20% w/w copolymer concentration. Gel permeation chromatography (GPC) studies indicated that almost identical copolymer chains were obtained in all cases, while transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) studies confirmed that highly anisotropic worms were formed with mean cross-sectional diameters of 11.9-13.1 nm. These worms undergo a thermoreversible worm-to-sphere transition on heating up to 150 °C. Rheological studies were conducted to characterize the shear rate- and concentration-dependent behaviour caused by this change in copolymer morphology, where the initial shear-thinning worm gels form spheres ( i.e. a Newtonian fluid) on heating up to 150 °C. Complementary shear-induced polarized light imaging (SIPLI) experiments confirmed the formation of aligned linear worms under applied shear between 80 °C and 110 °C, with high-viscosity dispersions of branched worms being obtained at 20-60 °C and low-viscosity spheres being produced at 150 °C. This study informs the use of such block copolymer worms as rheology modifiers for non-polar oils, which is of potential interest for the automotive industry. Optorheology studies of thermoresponsive diblock copolymer nano-objects dispersed in mineral oil indicate reversible transformation from randomly oriented branched worms at 20 °C to aligned linear worms at 80-110 °C and isotropic spheres at 150 °C.