Adsorption and localization of random copolymers subject to a force: The Morita approximation

We discuss directed walk models of random copolymers, either adsorbed at a surface or localized at an interface between two immiscible liquids. We consider the response to an applied force which can cause desorption or delocalization into a bulk phase, and calculate the critical force as a function of temperature. The randomness is quenched and, even for directed models, we cannot usually treat the quenched system analytically so we resort to an approximation in which the quenched average is approximated by an annealed average with a side condition which ensures the correct proportion of the types of comonomers. We argue that this approach gives the exact result for the quenched system in some cases and a bound in others