Hydrogels exhibit stress-relaxation when stretched. The decay in tensile stress can be ascribed primarily to strain-induced swelling of the polymer network. This assertion is formalized in a biphasic continuum model of the gel-solvent system. An equation of motion and a linear consti tutive law of the polymer network are solved with boundary and initial conditions appropriate for a stress-relaxation experiment The model predicts that the time constant of decay depends upon the square of the significant length of the sample (i.e., thickness of thin films, diameter of sphere or thin cylindrical fibers), a result which is confirmed by experiments. In addition, the values of the material constants (μ, the shear modulus; k, the bulk modulus; and 1/f, the hydraulic permeability) which are estimated using a non-linear regression algorithm, agree well with measurements of the material constants made by other methods
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