Adsorption of a directed polymer subject to an elongational force

We consider several different directed walk models of a homopolymer adsorbing at a surface when the polymer is subject to an elongational force which hinders the adsorption. We use combinatorial methods for analyzing how the critical temperature for adsorption depends on the magnitude of the applied force and show that the crossover exponent phi changes when a force is applied. We discuss the characteristics of the model needed to obtain a re-entrant phase diagram