Electrochemical Cloud Point Temperature from Thermoamperometry: Real-Time Analysis for Phase Transition of Thermoresponsive Polymers

Cloud point temperature (T_cp) is a thermal index used to define the phase transition of thermoresponsive polymers. In this study, we used electrochemical techniques to obtain an electrochemical cloud point temperature (T_ecp) that exhibits the more accurate phase transition temperature and can replace T_cp. Thermoamperometry on an ultramicroelectrode was conducted with a poly(arylene ether sulfone) (PES₁₀) as a model system to obtain a current-temperature (i-T) curve in real time; the T_ecp of the PES₁₀ was determined from the i-T curve. The i-T curve shows an unprecedented current decrease in the PES₁₀ solution despite increasing temperature; on the other hand, the current increased linearly with increasing temperature in the solution without PES₁₀. This phenomenon was analyzed by considering the characteristics of PES₁₀ during phase transition, such as dynamic viscosity, temperature of the solution, and electrode impedance. It was confirmed that the current drops shown in the i-T curves were mainly due to the decrease of real electrode area. The comparison of T_ecp and T_cp showed that both depended similarly on the concentrations of the thermoresponsive polymer and the supporting electrolyte. The results reveal that by adjusting the concentration of polymer and electrolyte in an organic solution, T_ecp, as a new analytical method, can be used in electric circuit-based energy storage appliances such as Li-ion batteries and supercapacitors.