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We show that when a self-avoiding polymer chain is pulled off a stickysurface by force applied to the end segment, it undergoes a first-orderthermodynamic phase transition albeit without phase coexistence. This unusualfeature is demonstrated analytically by means of a Grand Canonical Ensemble(GCE) description of adsorbed macromolecules as well as by Monte Carlosimulations of an off-lattice bead-spring model of a polymer chain. Theoreticaltreatment and computer experiment can be carried out both in the constant-forcef statistical ensemble and in the constant-height h ensemble. We find that theforce-assisted desorption undergoes a first-order dichotomic phase transitionwhereby phase coexistence between adsorbed and desorbed states does not exist.In the f-ensemble the order parameter (the fraction of chain contacts with thesurface) is characterized by huge fluctuations when the pulling force attains acritical value f_D. In the h-ensemble, in contrast, fluctuations are alwaysfinite at the critical height h_D. The derived analytical expressions for theprobability 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 uponforce-assisted detachment. In addition, one finds that the hithertocontroversial value of the universal critical adsorption exponent \phi dependsessentially on the extent of interaction between the loops adsorbed chain sothat \phi may vary within the limits 0.39 < \phi < 0.59.

Polymer desorption under pulling: first order phase transition without phase coexistence