Viscosity Maximum with Temperature in Solutions of Methacrylate-Based Comb Copolymers

Thermo-thickening behavior in polymer solutions has been reported, but most cases are restricted to aqueous media. We report non-monotonic viscosity-temperature relationships in concentrated (10 – 20 wt%) oil solutions containing methacrylate-based comb copolymer. Viscosity increases then decreases with increasing temperature. Statistical copolymers containing four different methacrylate repeat units functionalized with aromatic, aliphatic (C4 and C12), and long saturated hydrocarbon (around C220) side groups with variable overall polarity were prepared by radical polymerization (M_n ≈ 2.3×10⁵ g/mol, Ð ≈ 2.1). Rheological measurements show that the maximum viscosity (location and magnitude) can be tuned by copolymer concentration and backbone polarity. Small-angle X-ray scattering and cryogenic transmission electron microscopy reveal aggregation at low temperatures, swelling with heating and dissolution into dispersed molecules at high temperatures. We propose an aggregation-expansion-dissolution model to explain these results.