Beginning with fundamental conservation postulates, we construct a generic free energy balance for systems containing polymer interphases undergoing reversible, isothermal deformations. At constant mass, the differential Helmholtz free energy (and the total differential work) equals the total stress times strain in the system. We identify work potentials corresponding to different modes of deformation for a polymer interphase confined between two parallel planar surfaces. For deformations that change the interphase area at constant intersurface separation, the work potential is the surface tension times the differential change in area. However, the work potential for compression of the interphase at constant area is not given by the variation of the surface tension. Instead, it equals the stress (i.e. disjoining pressure) evaluated at the midpoint between the surfaces as required by direct application of conservation of linear momentum. Calculations using continuum-based self-consistent theory show that the work per area for compression 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 interaction of polystyrene layers adsorbed on mice from cyclopentane at temperatures near the theta point.