When is the Poisson Ratio of Polymer Networks and Gels Larger than 0.5?

The mechanical properties of elastic materials are characterized by Young's modulus and Poisson ratio (ν) defining a sample shape transformation under applied external stress. A Poisson ratio falls within the range -1≤ν≤0.5 with a specific value determined by Young's modulus and bulk modulus characterizing materials’ compressibility. We use coarse-grained molecular dynamics simulations of networks and gels made of linear and brush-like strands to obtain deformation-dependent Youngs’ modulus and Poisson ratio in both linear and nonlinear deformation regimes. Simulations show that the Poisson ratio of polymer networks and brush gels exceeds 0.5 value in the nonlinear deformation regime. This unusual behavior is due to the ability of the network and gel strands to sustain large reversible deformations, which are impossible to achieve in hard materials. In combination with the finite strand extensibility, this results in strand alignment and monomer density increase with increasing strand’s elongation. We developed a nonlinear network deformation model which defines conditions for the Poisson ratio to exceed 0.5 value. Model predictions are in agreement with the results of coarse-grained simulations of networks and gels.