The effect of mechanical constraints on the anti-polyelectrolyte behavior of zwitterionic polymers was studied systematically in this work via viscosity examination of uncross-linked polymer solutions, swelling of cross-linked polymer gels and swelling of elastomers compounded with these gels. Zwitterionic polymers, poly(sulfobetaine methacrylate) (polySBMA), poly(2-methacryloyloxyethyl phosphorylcholine) (polyMPC) and copolymers of polySBMA with a polyelectrolyte, poly(sodium methacrylate) (polyNaMA) were used in this study. Anti-polyelectrolyte effects were observed both in solution and as gels. However, when the zwitterionic gels were compounded with hydrogenated acrylonitrile butadiene rubber (HNBR), the anti-polyelectrolyte behavior of the native polymer was lost. This is attributed to mechanical constraints imposed by the elastomeric HNBR matrix that dominate Coulombic interactions. Without the manifestation of chain relaxation and extension afforded by the charge screening, the zwitterionic polymers serve as hydrophilic agents that facilitate water diffusion into rubber. (c) 2013 Elsevier Ltd. All rights reserved.