Isothermal Titration Calorimetry to Probe the Coil-to-Globule Transition of Thermoresponsive Polymers

Isothermal titration calorimetry (ITC) is introduced as a new way to study the effect of cosolutes on the coil-to-globule transition of thermoresponsive polymers. From isothermal titrations, critical cosolute concentrations can be identified, at which a coil-to-globule transition occurs. The concept of a temperature-dependent critical cosolute concentration is proven employing different isomers of dihydroxybenzene (DHB) and one isomer of hydroxy benzaldehyde (mHBA) in solutions of two thermoreversible polymer, namely poly(N-isopropylacrylamide) (PNiPAM) and poly(N,N-diethylacrylamide) (PDEAM). It is shown that the temperature-dependent critical cosolute concentration, determined by ITC, and the cosolute concentration-dependent critical temperature, probed via differential scanning calorimetry (DSC), yield the same phase diagram. The advantage of employing ITC is the ability to probe even critical concentrations at very low temperatures, whereas the corresponding critical temperatures are not easily accessible in DSC. In addition, kinetic information about the coil-to-globule transition in different systems is obtained, and the effect of the DHB isomers on the transition temperature is found to scale as ortho > para > meta.