Fig. 1.

Two distinct forms of the effective site deformation energy E(Δl) of an individual monomeric unit (site) that lead to qualitatively different stretching scenarios of the linear polymer chain of N sites. The site extension is Δl. Curve 1 is a nonconvex energy function (between Δl_a and Δl_b) that leads to nonuniform, two-phase stretching of the chain. Some sites extend weakly by Δl_a, and some extend strongly by Δl_b. The chain extension follows the convex hull (red line 2) of E(Δl), with only the relative fraction of weakly extended sites changing as the chain extends. The average (per site) energy of nonuniform extension is less than that of the corresponding uniform E(Δl) extension. Curve 3 is a convex function E(Δl) that leads to uniform extension of all of the sites.