Time-Resolved Light Scattering of Atactic Poly(N-isopropylacrylamide) in Water: Physical Gelation and Spinodal Temperature of the Hydrogel

Aqueous solutions of atactic poly(N-isopropylacrylamide) (a-PNIPAM) exhibiting lower critical solution temperature behavior can form macroscopic gels at the gel temperature T_gel, which is lower than the binodal temperature T_b (Macromolecules 2023, 56, 6354). This study applied time-resolved light scattering (TRLS) for the first time to explore the structure evolution of the transparent gel at T_i (T_gel < T_i < T_b). In addition, via the subsequent T-jump from T_i to some selected temperatures T_x’s (≥T_b), the TRLS intensity profiles of the phase-separated a-PNIPAM gel were also acquired until the steady state is reached, at which point, the phase-separated structures, induced either by nucleation and growth (NG) or by spinodal decomposition (SD), were finally pinned. At each T_x, the scattered intensity profiles of a-PNIPAM gels with either pinned NG or pinned SD structures were further analyzed to obtain the scaled structure factor F(x). Judging from the T_x-dependent F(x) profiles, a novel approach is proposed to determine the spinodal temperature (T_s,gel) of the macroscopic gel. For the well-characterized 7 wt % a-PNIPAM aqueous solution with T_b = 30.6 °C, the TRLS intensity profile of the macroscopic gel at T_i (=29.2 °C) exhibits a mass-fractal dimension of 2.0 ± 0.1. The derived T_s,gel by TRLS in this study is 31.2 ± 0.1 °C, which is also in good agreement with that derived previously from small-angle X-ray scattering based on the Ornstein–Zernike scattering function. Remarkably, an extremely small temperature gap (∼0.6 °C) exists between T_b and T_s,gel for the specific a-PNIPAM/H₂O hydrogels studied.