Mechanochromic Pure Polymer Hydrogels with Dual Stress Response, Tunable Interference Colors, and Superior Environmental Tolerance

Abstract

Mechanochromic hydrogels typically rely on complex nanostructures and additives, limiting their simplicity and stability. In this study, a series of pure polymer hydrogels (MPPHs) were synthesized by adjusting polymer concentration and cross-linking density, without the need for additional functional components. These hydrogels exhibited dual-response color changes under both compressive and tensile strains. The optimal 25PAM_0.5 hydrogel showed vibrant interference color shifts from grayish white to pastel violet under 60% compressive strain, while the 25PAM_0.04 hydrogel changed from deep purple to bright yellow at tensile strains up to 1500%. Birefringence analysis confirmed a strain-induced structural transformation with a birefringence value of 1.4 × 10^–4, surpassing most pure polymer hydrogels. Notably, the MPPHs maintained stable mechanochromic responses after 1000 compression cycles and 500 tensile cycles. To enhance environmental stability, a transparent hydrophobic poly(lauryl acrylate) coating was applied, significantly improving resistance to swelling and dehydration. These results highlight the unique combination of tunable optical properties, mechanical flexibility, and environmental stability, positioning MPPHs as promising materials for smart applications.