CHAIN FORCE CONCEPT IN SYSTEMS OF INTERACTING CHAINS

The concept of the chain force, that is, the force that must be applied to the end atoms of a long-chain molecule in order to maintain its end-to-end distance at a fixed value, is well-defined for a system of noninteracting chains. We show here how this concept may be extended to chain systems with both intrachain and interchain noncovalent interactions and develop methods for its computation from the results of the molecular dynamics simulation of model systems. A simulation is made of a chain with fixed end atoms in interaction with a free melt of like chains with excluded-volume interactions modeled by a potential approximating hard spheres. For sufficiently long chains and sufficiently high reduced density of the system, it is found that the chain force-distance relation approaches Gaussian in accord with the Flory theorem. However, at high reduced density it is found that the chain force is due primarily to the screened excluded-volume interactions rather than to the covalent interactions as in an isolated ideal chain. For a chain in interaction with an anisotropic system of chains, as is the case in a stretched network, it is found that the chain force is no longer axial-that is, the line of action of the chain force is not parallel to the chain vector. The chain force is therefore equivalent to an axial force plus an applied moment; this moment is found to produce a significant softening effect on the stress-extension behavior.