Simulations of elongation of supercoiled and biopolymer networks

The unwinding of the compact, partially collapsed chains at their elongatio ns is studied by Monte Carlo (MC) simulations. The stretching of single cha ins below the theta point is described by elastic functions exhibiting regi on of mechanical instability specified by a concave curvature of the elasti c free energy and a van der Waals loop in the reduced force. The elastic fr ee energy and the stress-strain isotherms of supercoiled networks of the c* gel type formed by collapsed chains were evaluated by the three-chain mode l. The stress-strain isotherms exhibit a long plateau with a low elastic mo dulus and high ultimate drawing ratio (i.e., characteristics of the superel astic behavior outlined previously in deswollen networks). A substantial di fference in the elastic response was found between supercoiled networks and ordinary melt-crosslinked networks. The instability region was observed on some computed stress-strain isotherms of supercoiled networks, indicating the two-phase structure of a network, a phenomenon relevant also in the bio polymer gels having compact structures which unwind on stretching. (C) 1999 John Wiley & Sons, Inc.