The direction of influence of specific ion effects on a pH and temperature responsive copolymer brush is dependent on polymer charge

Abstract

The effect of electrolyte identity on the conformation of homopolymer brushes of poly(2-(2-methoxyethoxy) ethyl methacrylate) (PMEO2MA) and poly(2-(diethylamino)ethyl methacrylate) (PDEA), as well as a P(MEO2MA-co-DEA) copolymer brush of 90:10 mol% composition has been examined. PMEO2MA is a thermoresponsive polymer with a lower critical solution temperature of ~28 °C, PDEA a weak polybase with an apparent pKa of ~7.5, while the copolymer brush exhibits both pH and temperature responsive behaviour. Brushes were synthesised using surface initiated ARGET ATRP. The effect of temperature at low and high pH in the presence of different electrolytes on the thickness of the P(MEO2MA-co-DEA) 90:10 mol% brush was tracked with ellipsometry and the polymer volume fraction profile was elucidated with neutron reflectometry. The effect of each electrolyte on the copolymer brush conformation as the temperature changes could be switched by changing the pH from low (charged DEA segments) to high (uncharged DEA segments). The overall behaviour of the copolymer in each salt was PDEA-like at low pH, and MEO2MA-like at high pH. At low pH a reverse Hofmeister effect was observed, while at high pH a direct Hofmeister effect was observed.