Polymer desorption under pulling a 1st — order phase transition without phase coexistence

Abstract

We show that when a self-avoiding polymer chain is pulled off a sticky surface by force applied to the end segment, it undergoes a first-order thermodynamic phase transition albeit without phase coexistence. This unusual feature is demonstrated analytically by means of a Grand Canonical Ensemble (GCE) description of adsorbed macromolecules as well as by Monte Carlo simulations of an off-lattice bead-spring model of a polymer chain.Theoretical treatment and computer experiment can be carried out both in the constant force statistical ensembl whereby at fixed pulling force f one measures the mean height 〈h〉 of the chain end above the adsorbing plane, and in the constant-height ensemble where for a given height h one monitors the resulting force 〈f〉 applied at the last segment. We find that the force-assisted desorption undergoes a first-order dichotomic phase transition whereby phase coexistence between adsorbed and desorbed states does not exist. In the f-ensemble the order parameter (the fraction of chain contacts with the surface) is characterized by huge fluctuations when the pulling force attains a critical value fD. In the h-ensemble, in contrast, fluctuations are always finite at the critical height hD.The derived analytical expressions for the probability distributions of the basic structural units of an adsorbed polymer, such as loops, trains and tails, in terms of the adhesive potential and f, or h, provide a full description of the polymer structure and behavior upon force-assisted detachment. In addition, one finds that the hitherto controversial value of the universal critical adsorption exponent ϕ depends essentially on the extent of interaction between the loops adsorbed chain so that ϕ may vary within the limits 0.34≤ϕ≤0.59.