Physical gelation and spinodal decomposition of poly(N-isopropylacrylamide) aqueous solutions measured by rheometry

Aqueous solutions of poly(N-isopropylacrylamide) (PNIPAM) undergoes sophisticated phase transitions at 20–33 °C. At this temperature range, the solution exhibits a phase behavior of lower critical solution temperature and physical gel formation. The transition temperatures of PNIPAM solutions of various concentrations were determined using two different methods, namely, turbidity and rheological measurement. A quasi-equilibrium phase diagram was then constructed. Analyses of loss modulus G″ and storage modulus G′ give rise to the spinodal temperature (T_s), macroscopic gel temperature (T_gel) and the temperature at which G′ starts to rise (T_onset,G’) due to the thermal concentration fluctuations when the spinodal is approached. T_gel was obtained from the crossover temperature of the G′ and G″ curves, and T_s was derived based on the Fredrickson–Larson–Ajji–Choplin mean field theory. Depending upon solution concentration, the measured T_onset,G’ is approximately 3–10 °C lower than the derived T_s. Upon heating, the one-phase solution undergoes pronounced concentration fluctuations at a temperature above T_onset,G’. The concentration fluctuations subsequently triggered the physical gelation process to develop a macroscopic-scale gel network at T_gel before spinodal decompsotion at T_s.