Spinodal Decomposition, Nucleation Growth, and Arrested Macrophase Separation of Physical Gels of Atactic Poly(N-isopropylacrylamide) in Water

Aqueous solutions of atactic poly(N-isopropylacrylamide) (a-PNIPAM) with 5–12 wt % concentrations exhibit physical gelation at temperatures lower than the binodal temperature (Tb). This phenomenon is clearly demonstrated by the hysteresis of dynamic storage modulus G′(T) during repeated heating and subsequent cooling protocol at T < Tb because of the difference in the T-dependent rate of interchain association and dissociation. Herein, we performed a temperature-sweep test at a low heating rate and at a low frequency to obtain dynamic rheological curves of the loss modulus and storage modulus. From these curves, two transition temperatures are derived, namely, the macroscopic gel temperature (Tgel) and the temperature at which G′ starts to rise (T1) owing to the enhanced interchain associations developing the self-similarly branched structures of associated chains. On heating, the one-phase a-PNIPAM solution undergoes pronounced concentration fluctuations at a temperature above T1 to enter the pregel regime followed by the formation of a macroscopic gel network at Tgel. The temperature sequence for the phase transitions is T1 + 2 °C ≅ Tgel < Tb, independent of concentration (ϕw). The spinodal temperatures of the a-PNIPAM solution (Ts,sol) and a-PNIPAM gel (Ts,gel) are obtained from small-angle X-ray scattering based on a theoretical model derived for the associated chains to account for the presence of gel junctions. The derived Ts,gel is slightly higher than Ts,sol; the gap is ca. 0.9–1.6 °C depending on ϕw. The derived Ts,sol and Ts,gel marginally decrease with increased ϕw. Using time-resolved light scattering (TRLS), the nucleation and growth (NG) and spinodal decomposition (SD) of the gel are validated at temperatures above Tb. The growth of the SD structure is dramatically hindered (or pinned down) within a very short period (<5 s). We find that Cahn’s linear theory for SD phase separation is inapplicable to derive Ts,gel in the present dynamically asymmetric system. On the basis of phase transformation from the pinned SD structure into the pinned NG structure, Ts,gel could be alternatively determined from the steady-state light-scattering profiles as a function of T obtained from TRLS with a stepwise cooling protocol. The estimated Ts,gel in this manner agrees with that derived from rigorous analyses of static SAXS as a function of temperature. On the basis of the present results, a phase diagram of semidilute a-PNIPAM/water solutions is constructed. At temperatures above Tb, the arrest of the macroscopic phase separation through NG and SD is found to develop pinned NG and SD structure depending on T and is thermoreversible. We propose that this temperature-dependent pinned NG and SD underlie the basic principle for the arrested macrophase separation and the thermoreversible change in the phase-separated structures of the physical gel as discussed in the text.