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 (T_b). This phenomenon is clearly demonstrated by the hysteresis of dynamic storage modulus G′(T) during repeated heating and subsequent cooling protocol at T < T_b 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 (T_gel) 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 T_gel. The temperature sequence for the phase transitions is T₁ + 2 °C ≅ T_gel < T_b, independent of concentration (ϕ_w). The spinodal temperatures of the a-PNIPAM solution (T_s,sol) and a-PNIPAM gel (T_s,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 T_s,gel is slightly higher than T_s,sol; the gap is ca. 0.9-1.6 °C depending on ϕ_w. The derived T_s,sol and T_s,gel marginally decrease with increased ϕ_w. Using time-resolved light scattering (TRLS), the nucleation and growth (NG) and spinodal decomposition (SD) of the gel are validated at temperatures above T_b. 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 T_s,gel in the present dynamically asymmetric system. On the basis of phase transformation from the pinned SD structure into the pinned NG structure, T_s,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 T_s,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 T_b, 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.