The role of copolymer composition on the specific ion and thermo-response of ethylene glycol-based brushes

Thermoresponsive ethylene glycol based comb-polymer brushes were studied as a function of copolymer composition in aqueous electrolyte solutions. Homopolymer poly oligo(ethylene glycol) methyl ether methacrylate (POEGMA₃₀₀) and statistical copolymer brushes with 60 mol% OEGMA₃₀₀ and 40 mol% (di(ethyleneglycol) methyl ether methacrylate) (MEO₂MA) underwent a monotonic swollen to collapsed transition with increasing temperature. Neutron reflectometry and ellipsometry measurements showed that the higher ethylene glycol content of the homopolymer brush led to a higher degree of swelling for a given temperature, with the transition occurring over a wider temperature range than the copolymer brush. Dynamic atomic force microscopy force measurements revealed minimal impact of probe velocity on normal interaction forces for the homopolymer brush. In contrast, a significant increase in both repulsion and adhesion was observed for the copolymer brush. Exposure to potassium thiocyanate solutions increased the degree of swelling at any given temperature while potassium acetate showed the opposite behaviour for both brush compositions. However, the homopolymer brushes displayed a greater increase in swelling due to thiocyanate and reduced collapse due to acetate.