Mechano-optical behavior of loosely crosslinked double-network hydrogels: Modeling and real-time birefringence measurement during uniaxial extension

•Real-time birefringence measurement during uniaxial deformation of hydrogels is reported for the first time.•A loosely cross-linked Double network hydrogel synthesized from acrylamide obeys the stress-optical law.•A DN hydrogel synthesized from N,N-dimethyl(acrylamide) deviates from the stress-optical law at large deformations.•The stress-optical coefficient of hydrogels was obtained from birefringence-true stress plots.•A constitutive equation was proposed to describe the photo-elastic behavior of the hydrogels.

Real-time uniaxial strain-induced birefringence in loosely crosslinked double-network (DN) hydrogels synthesized from acrylamide (AAm) and N,N-dimethyl(acrylamide) (DMA) was measured. Tensile tests were performed at different extension rates from quasi-static conditions to very rapid tests followed by a holding for relaxation. Both DN hydrogels exhibited negative birefringence whose absolute value increased with extension and remained constant during relaxation. DN hydrogel synthesized from AAm displayed a linear stress optical (birefringence – true stress) behavior, however, a nonlinear trend was observed for the DN gel synthesized from DMA. A simple photoelastic model was developed based on the mechanical behavior of the gels using Fung elastic potential and Brewster's stress-optical law, and the model was compared with the experimental data.

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