Viscoelastic Relaxation of HEMA-DMAEMA Responsive Hydrogels

Responsive hydrogels are a class of shape memory materials that undergo a large elastic volumetric change when interacting with a stimulus and can return to their original shape when that stimulus is removed. Important to their use in these device applications are the fundamental mechanical properties of these materials. The pH-sensitive 2-hydroxyethyl methacrylate, 2-(dimethylamino) ethyl metha-crylate (HEMA-DMAEMA) hydrogel is a linear-viscoelastic polymer that is used in microfluidics because it can be easily polymerized within these devices and their special stimuli responsive capabilities making them ideal candidates for sensors and actuators. In the experiments described in this paper the stress relaxation due to a step strain is studied. The rise time for our experiments was 0.3 s and the step-strains ranged from 3 to 7 % strain. Relaxation was recorded over three decades of time (1, 10, 100 and 1000 s). It was found that within this range the HEMA-DMAEMA hydrogel displayed linear-viscoelastic behavior.