Microcalorimetric investigation on association and dissolution of poly(N-isopropylacrylamide) chains in semidilute solutions

The concentration (C) effect on association and dissolution of linear poly(N-isopropylacrylamide) (PNIPAM) chains in dilute and semidilute aqueous solutions was investigated by using ultrasensitive differential scanning calorimetry (US-DSC). When C is below the overlap concentration (C*), both the phase-transition temperature (T-p) and enthalpy change (Delta H) in the heating process decrease as the solution becomes more dilute, while T-p and Delta H become independent of C in the semidilute regime (C > C*). Such a difference can be attributed to different contributions of interchain association and intrachain contraction in dilute and semidilute solutions. In the cooling process, T-p nearly remains a constant, but Delta H decreases as C increases in dilute solutions, but both of them slightly vary in the range C > C*. A hysteresis was observed in one heating-and-cooling cycle. Such a hysteresis becomes more obvious as C increases. The effect of scanning rate on both T-p and Delta H further confirms that the hysteresis is due to incomplete disruption of additional hydrogen bonds formed when the chains are in the collapsed state.