Pulling absorbing and collapsing polymers from a surface

A self-interacting polymer with one end attached to a sticky surface has been studied by means of a flat-histogram stochastic growth algorithm known as FlatPERM. We examined the four-dimensional parameter space of the number of monomers ( up to 91), self-attraction, surface-attraction and pulling force applied to one end of the polymer. Using this powerful algorithm the complete parameter space of interactions and pulling force has been considered. Recently it has been conjectured that a hierarchy of states appears at low-temperature/poor solvent conditions where a polymer exists in a finite number of layers close to a surface. We find re-entrant behaviour from the stretched phase into these layering phases when an appropriate force is applied to the polymer. Of interest is that the existence, and extent, of this re-entrant phase can be controlled not only by the force, but also by the ratio of surface-attraction to self-attraction. We also find that, contrary to what may be expected, the polymer desorbs from the surface when a sufficiently strong critical force is applied and does not transcend through either a series of de-layering transitions or monomer-by-monomer transitions. We discuss the problem mainly from the point of view of the stress ensemble. However, we make some comparisons with the strain ensemble, showing the broad agreement between the two ensembles while pointing out subtle differences.