The influence of water content on the mechanical responses of polyacrylamide hydrogels under stress-controlled cyclic loadings

Abstract

In this work, polyacrylamide (PAAm) hydrogels with different water contents (WCs) were prepared, and stress-controlled cyclic experiments were carried out. The effect of water content on the mechanical behavior of PAAm hydrogels was observed through stress–strain curves, apparent modulus, and dissipation energy across various loading cycles. It is concluded that with the increase in the WC, the peak and valley strains and the dissipation energy increase while the apparent modulus decreases. The WC significantly influences the evolution of dissipation energy with the increase of the loading cycles. For PAAm hydrogels with a relatively high WC (from 96% to 67%), the dissipation energy decreases appreciably between the first and the second loading cycles and then remains stable with increasing the loading cycles further. However, for PAAm hydrogels with a relatively low WC (50% and 34%), the dissipation energy decreases significantly between the first two cycles and then increases with increasing the loading cycles. Experimental results obtained under constant relative humidity and constant WC were compared, validating that the water loss leads to a decrease in the strain and an increase in the apparent modulus during the loading cycles.