Force–extension relations in macromolecules of variable excluded volume and flexibility: energy and entropy changes on stretching

Abstract

Lattice chains, chain length 200 monomers, generated by the Monte Carlo method, have been studied on a tetrahedral lattice. Real-chain features such as conformational restrictions, variable segment interaction and chain thickness were assumed in the model. The chain vector density distribution function W(R) and the related changes in the Helmholtz energy were calculated as a function of the displacement of the chain ends, R. The force–displacement curves derived by this approach are non-linear with the shape depending on the solvent quality. The energy changes, accompanying the chain deformation, due to (a) conformational isomerization and (b) loss of segment contacts, were computed. In the model used (a) leads to a negative and (b) to a positive energetic contribution, f_u, to the total tensile force, f. The mixed energy–entropy character of the single-chain deformation was demonstrated for some representative chains differing in excluded volume and flexibility.