Stress relaxation of deformed gel in a good solvent

A method for obtaining elastic moduli and frictional properties of gel chains, diagonal and off-diagonal elements of a friction tensor, from a tensile force relaxation of cylindrical gel that is stretched in a good solvent is presented. The theory to describe the relaxation is developed on the assumption that an off-diagonal element of the friction tensor is nonzero and compared with experimental results obtained for the poly(acrylamide) gel in water. The experiments revealed that the tensile force relaxation, F(t) at a long t, was described by a function: F(t)=A(Fi-FS) x exp(-t/tau)+FS, where Fi, FS, and A, respectively, were tensile forces at the initiation and the steady state, and a constant. The tau was found to be proportional to a square of radius of the gel. The experiments also revealed that the elastic force relaxation was synchronized with a change of the gel diameter. The theory well elucidates the experimental results to reveal the bulk and shear moduli, and the diagonal and off-diagonal elements of friction tensor.