  We consider a self-avoiding walk model of polymer adsorption where the
adsorbed polymer can be desorbed by the application of a force. In this paper
the force is applied normal to the surface at the last vertex of the walk. We
prove that the appropriate limiting free energy exists where there is an
applied force and a surface potential term, and prove that this free energy is
convex in appropriate variables. We then derive an expression for the limiting
free energy in terms of the free energy without a force and the free energy
with no surface interaction. Finally we show that there is a phase boundary
between the adsorbed phase and the desorbed phase in the presence of a force,
prove some qualitative properties of this boundary and derive bounds on the
location of the boundary.
