How a Chain Can Be Extended While Its Bonds Are Compressed

Extending polymer chains results in a positive chain tension, f_ch, primarily due to conformational restrictions. At the level of individual bonds, however, tension, f, is either negative or positive and depends on both chain tension and bulk pressure. Typically, the chain and bond tension are assumed to be directly related. In specific systems, however, this dependence may not be intuitive, whereby f_ch increases while f decreases; i.e., the entire chain is extended while bonds are compressed. Specifically, increasing the grafting density of a polymer brush results in chain extension along the direction perpendicular to the grafting surface while the underlying bonds are compressed. Similarly, 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 f is dominant over a wide range of network deformations and brush grafting densities.