When Phase Separation Is Not the End: Hidden Interfacial Dynamics of Aqueous Poly(N-isopropylacrylamide).

Phase separation is not the endpoint of the lower critical solution temperature (LCST) transition in aqueous poly(N-isopropylacrylamide) (PNIPAM). Even when globular morphology appears stabilized, the system continues to reorganize over longer timescales. To follow this hidden post-transition evolution, we combined calorimetry, optical microscopy, dynamic light scattering, and broadband dielectric spectroscopy (BDS) at ambient and high pressure. Above the LCST, poorly conducting polymer-rich globules are dispersed in a more conducting water-rich phase, creating contrasts in dielectric permittivity and electrical conductivity. Charge accumulation at these interfaces gives rise to Maxwell-Wagner-Sillars (MWS) relaxation, making BDS sensitive to interfacial and transport dynamics. At ambient pressure, microscopy and calorimetry show that the morphology approaches apparent stabilization within ~30 min. In contrast, the dielectric response continues to evolve for hours: the MWS relaxation shifts toward higher frequencies, while its relaxation strength and dc conductivity decrease. These changes indicate slow reorganization of the polymer–water interface and redistribution of mobile charge carriers after phase separation. High-pressure measurements extend this picture by showing that the hidden equilibration process can be modulated by compression. Using the pressure-temperature phase diagram to select comparable conditions near the LCST, time-resolved BDS experiments reveal shorter equilibration times with increasing pressure. This demonstrates that hydrostatic pressure can tune the phase behavior, globule morphology, and interfacial charge transport of PNIPAM solutions. More broadly, BDS reveals slow interfacial dynamics that remain active after apparent structural stabilization, which is relevant for PNIPAM-based smart windows, responsive membranes, and sensing platforms requiring stable and reproducible switching behavior.