Ion-Specific Control over Transparency Transition and Mechanical Properties of Dual Thermoresponsive Hydrogels

Thermoresponsive hydrogels, due to their reversible optical properties that change with temperature, hold great promise for applications in smart windows and wearable sensors. However, traditional strategies for modulating the lower or upper critical solution temperature (LCST/UCST) transition typically involve complex synthetic processes and struggle to control the transition temperature and transparency window width while also presenting a trade-off between transparency and mechanical properties. Here, we report a dual-responsive hydrogel (PNMN) constructed by dispersing poly(N-isopropylacrylamide) (PNIPAm) microgels within a poly(N-acryloylglycamide) (PNAGA) network. Ion-specific modulation provides a simple and reversible strategy to simultaneously regulate its transparency and mechanical properties. Using SO₄^2-, the transparency window (T_700nm > 50%) can be reduced from 22 to 7 °C, while using SCN^- expands it to 26 °C. Simultaneously, the mechanical state of this hydrogel can transition from soft/elastic to tough/energy-dissipating, exhibiting tensile strengths of 22-695 kPa and moduli of 15-387 kPa while maintaining flexibility at -20 °C. Spectroscopic analysis revealed that strongly hydrated ions enhance hydrogen bonding between ordered polymers, while weakly hydrated ions disrupt interchain bonds and promote solvation of the polymer with water. This work demonstrates a feasible method for synergistically modulating thermal responsiveness and mechanical strength, providing a pathway for developing multifunctional adaptive hydrogels for next-generation windows and wearable devices.