Enhanced specific ion effects in ethylene glycol-based thermoresponsive polymer brushes

The thermoresponse of poly(di(ethyleneglycol) methyl ether methacrylate) (PMEO₂MA) brushes has been investigated in the presence of monovalent anions at either end of the Hofmeister series using ellipsometry, neutron reflectometry (NR) and colloid probe atomic force microscopy (AFM). NR measurements in deuterium oxide showed no evidence of vertical phase separation perpendicular to the grafting substrate with a gradual transition between a block-like, dense structure at 45 °C and an extended, dilute conformation at lower temperatures. All three techniques revealed a shift to a more collapsed state for a given temperature in kosmotropic potassium acetate solutions, while more swollen structures were observed in chaotropic potassium thiocyanate solutions. No difference was observed between 250 mM and 500 mM thiocyanate for a 540 Å brush studied by ellipsometry, while the lower molecular weight ∼200 Å brushes used for NR and AFM measurements continued to respond with increasing salt concentration. The effect of thiocyanate on the temperature response was greatly enhanced relative to PNIPAM with the shift in temperature response at 250 mM being five times greater than a PNIPAM brush of similar thickness and grafting density.