Polymer, Vol.54, No.26, 6954-6960, 2013
Swelling and de-swelling of gels under external elastic deformation
We show that the equilibrium Poisson ratio of electrically neutral gels depends on their shear modulus. When immersed in a good solvent, gels increase their volume on imposed external deformation, but stiffer gels swell less and exhibit a larger Poisson ratio, closer to 0.5, while the gels with a higher solvent content (and correspondingly lower shear modulus) approach a Poisson ratio of 0.25. We monitor the full process of stress and shape relaxation after an instantaneous deformation by using the technique of digital image correlation (DIC), and show that the amount of stress relaxation in uniaxially strained gels is proportional to the shear modulus of the free swollen state and a change in effective strain. Experiments were conducted on polyacrylamide (PAAm) gels in a custom built setup to give the Poisson ratio to high accuracy and time resolution, as well as verification of homogeneous deformation in equilibrium. In addition to water, hydrophilic gels were stretched in three poor solvents: silicone oil, mineral oil, and in air. All three exhibited water loss on imposed deformation and a resulting increase in stress, with mineral oil presenting the smallest change due to its lower permeability to water. Mineral oil and silicone oil are of particular interest as they are often used in mechanical testing to prevent solvent loss. (C) 2013 Elsevier Ltd. All rights reserved.