Bond Compression upon Chain Extension

Extending polymer chains entails a strictly positive chain tension. On the level of individual bonds, tension can be either negative or positive, and depends both on chain tension and bulk pressure. In specific polymeric systems this dependence may not be intuitive, whereby chain tension increases while bond tension decreases, or in other terms: the entire chain is extended, while individual bonds are compressed. For example, increasing grafting density of a polymer brush results in chain extension along the direction perpendicular to the grafting surface, while the individual bonds are compressed. A similar phenomenon is also predicted in deformed polymer networks. Upon compression of polymer networks, the extension of chains oriented in the “free” direction increases, while their bonds are getting more compressed. We demonstrate this phenomenon in molecular dynamics simulations and explain it by the fact that the pressure contribution to bond tension is dominant over a wide range of network deformations and polymer brush grafting densities, while chain conformations are still almost Gaussian.