Temperature effects in the mechanical desorption of an infinitely long lattice chain: re-entrant phase diagrams.

We consider the mechanical desorption of an infinitely long lattice polymer chain tethered at one end to an adsorbing surface. The external force is applied to the free end of the chain and is normal to the surface. There is a critical value of the desorption force f(tr) at which the chain desorbs in a first-order phase transition. We present the phase diagram for mechanical desorption with exact analytical solutions for the detachment curve: the dependence of f(tr) on the adsorption energy epsilon (at fixed temperature T) and on T (at fixed epsilon). For most lattice models f(tr)(T) displays a maximum. This implies that at some given force the chain is adsorbed in a certain temperature window and desorbed outside it: the stretched state is re-entered at low temperature. We also discuss the energy and heat capacity as a function of T; these quantities display a jump at the transition(s). We analyze short-range and long-range excluded-volume effects on the detachment curve f(tr)(T). For short-range effects (local stiffness), the maximum value of f(tr) decreases with stiffness, and the force interval where re-entrance occurs become narrower for stiffer chains. For long-range excluded-volume effects we propose a scaling f(tr) approximately T(1-nu)(T(c)-T)(nu/phi) around the critical temperature T(c), where nu=0.588 is the Flory exponent and phi approximately 0.5 the crossover exponent, and we estimated the amplitude. We compare our results for a model where immediate step reversals are forbidden with recent self-avoiding walk simulations. We conclude that re-entrance is the general situation for lattice models. Only for a zigzag lattice model (where both forward and back steps are forbidden) is the coexistence curve f(tr)(T) monotonic, so that there is no re-entrance.