Deformation Driven Deswelling of Brush Gels

We studied the effect of nonlinear strand deformation on the swelling of brush networks using Flory-Rehner and scaling models of gels. The model predictions are tested by coarse-grained molecular dynamics simulations of brush gels undergoing large uniaxial elongation. Our analysis showed that the swelling ratio of the brush gels Q_eq(λ) is a nonmonotonic function of the deformation ratio λ. It first increases with increasing gel deformation and then begins to decrease. This behavior is a manifestation of the optimization of the polymer/solvent interactions and conformational entropy of brush strands in the nonlinear deformation regime. The location of the maximum of Q_eq(λ) is directly related to the strand molecular architecture shifting to smaller λ values with increasing brush grafting density and degree of polymerization of the side chains. Analysis of the gel stress-deformation curves points out that the network firmness parameter β is identical in both dry networks and gels. However, the structural modulus of gels, G_s, appears to be reduced in comparison with that in the dry state by a factor equal to the ratio of the Kuhn lengths of brush strands in the dry and swollen states, G_s = G_dr b_K/b_K,s.