Tailoring PNIPAM hydrogels for large temperature-triggered changes in mechanical properties

N-isopropylacrylamide (NIPAM)-based hydrogel films are used for touch-controlled applications, where the temperature-induced change in the mechanical properties is utilized to create tactile feedback. N,N^′-methylenebisacrylamide (BIS) and poly(ethylene glycol)diacrylate (PEGDA) are used as cross-linkers to study the influence of their size and concentration on the viscoelastic properties in a temperature-controlled rheology setup. The changes in water content between swollen and collapsed state of the hydrogel samples increase with decreasing cross-linking density and increasing size of the cross-linker resulting in bigger meshes in the network. The difference in the viscoelastic properties of the hydrogels increases with increasing deswelling ratio and is highest for the P(NIPAM-PEGDA) hydrogels with low cross-linking density with a 50-fold increase in the storage modulus. The deswelling ratio of these P(NIPAM-PEGDA) hydrogels is up to five times higher compared to the P(NIPAM-BIS) hydrogels of the same cross-linking density. The mesh sizes are estimated from the mechanical properties.