Surface Stress and Surface Tension in Polymeric Networks*

Understanding of how surface properties could change upon deformation is important for controlling adhesion, friction and lubrication of soft polymeric materials. We use a combination of the theoretical calculations and coarse-grained molecular dynamics simulations to study surface stress dependence on deformation in films made of soft and rigid polymeric networks. Simulations show that films of polymeric networks could demonstrate surface properties of both polymer melts and elastic solids depending on their deformation. In particular, at small film deformations the film surface stress Υ is equal to the surface tension obtained at zero film strains, γ₀, and surface properties of networks are similar to those of polymer melts. The surface stress begins to show a strain dependence when the film deformation ratio λ approaches its maximum possible value λ_max corresponding to fully stretched network strands without bond deformations. In the entire film deformation range the normalized surface stress Υ(λ)/γ₀ is a universal function of the ratio λ/λ_max. Simulation data for large film deformations point out that the significant increase in the surface stress is due to the onset of the bond deformation. In this deformation regime network films behave as elastic solids.