Enthalpy-Entropy Compensation in Aggregation of Poly(styrene-co-sodium methacrylate) Ionomers in Aqueous Solution

Spectroscopic methods were employed to evaluate the thermodynamics of aggregates formation of P(S‐co‐MANa) in aqueous solution containing a trace of DMF(0.2 mol%) using pyrene as an optical probe. The mol %s of methacrylate unit in poly(styrene‐co‐sodium methacrylate)[P(S‐co‐MANa)] were 3.6, 5.0, 6.2 and 9.4. Measurements were made at a series of temperatures in the range 10–80 °C. Based on the temperature dependence of the critical aggregation concentration (CAC) and by the use of van't Hoff equation, thermodynamic parameters were calculated. At low temperature the aggregation is accomplished by a large positive entropy change and a small negative enthalpy changes. As the temperature increases, the magnitude of entropy change becomes smaller and the enthalpy change is more exothermic. Moreover, the changes in enthalpy and entropy compensate each other for all samples we have studied, and the compensation temperature was ∼320 K.