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 Tgel, which is lower than the binodal temperature Tb (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 Ti (Tgel < Ti < Tb). In addition, via the subsequent T-jump from Ti to some selected temperatures Tx's (≥Tb), 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 Tx, 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 Tx-dependent F(x) profiles, a novel approach is proposed to determine the spinodal temperature (Ts,gel) of the macroscopic gel. For the well-characterized 7 wt % a-PNIPAM aqueous solution with Tb = 30.6 °C, the TRLS intensity profile of the macroscopic gel at Ti (=29.2 °C) exhibits a mass-fractal dimension of 2.0 ± 0.1. The derived Ts,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 Tb and Ts,gel for the specific a-PNIPAM/H2O hydrogels studied.