(649g) Investigation of Mechanical Properties of a Triblock Copolymer Gel Using Cavitation Rheology and Laos

Soft materials often display non-linear mechanical behavior at large deformation, i.e., these materials either stiffen or soften with the application of strain. Here, we use cavitation rheology, an experimental technique for measuring mechanical properties of soft materials, to investigate the deformation behavior of a swollen, physically cross-linked polymer gel. The gel was obtained by dissolving poly methylmethacrylate-poly n-butylacrylate-poly methylmethacrylate (PMMA-PnBA-PMMA) in a mid-block selective solvent, 2-ethyl 1-hexanol. Different polymer volume fractions (5%, 7%, and 10% v/v) were selected to study the effect of crosslink density on the critical pressure of cavitation. Cavitation is a snap-through instability caused by the pressurization of a defect inside a gel. Similar mode of instability was observed for the cavitation experiments conducted at 6 °C and 22 °C. The experiments were also conducted at different compression rates and a decrease of the critical pressure was observed at a low compression rate likely caused by the relaxation of the polymer chains. Both neo-Hookean and Gent models were applied to capture the elastic contribution in pressure at different expansion ratios before and at instability point.  In addition to cavitation experiments, large amplitude oscillatory shear (LAOS) was used to characterize the gel deformation behavior. The Fourier coefficients and the Chebyshev coefficients were calculated from the stress responses to elucidate the strain stiffening behavior of these gels.