COMPRESSION OF POLYMER INTERPHASES

Beginning with fundamental conservation postulates, we construct a gen eric free energy balance for systems containing polymer interphases un dergoing reversible, isothermal deformations. At constant mass, the di fferential Helmholtz free energy (and the total differential work) equ als the total stress times strain in the system. We identify work pote ntials corresponding to different modes of deformation for a polymer i nterphase confined between two parallel planar surfaces. For deformati ons that change the interphase area at constant intersurface separatio n, the work potential is the surface tension times the differential ch ange in area. However, the work potential for compression of the inter phase at constant area is not given by the variation of the surface te nsion. Instead, it equals the stress (i.e. disjoining pressure) evalua ted at the midpoint between the surfaces as required by direct applica tion of conservation of linear momentum. Calculations using continuum- based self-consistent theory show that the work per area for compressi on of adsorbed polymer layers is considerably more repulsive than the interaction based on the change in surface tension. Without using any arbitrary adjustment of parameters, we find satisfactory quantitative agreement between model predictions and experimental data for the inte raction of polystyrene layers adsorbed on mice from cyclopentane at te mperatures near the theta point.