Atomistic Nature of Stress in Polymer Networks

Abstract

The principles of macroscopic thermodynamics applied to experimental observations of the thermomechanical behavior of rubber clearly demonstrate the entropic character of rubber elasticity. That is, the restoring force acting on a piece of stretched rubber is primarily due to its change in entropy, rather than to its change in internal energy. This was established by the work of Kelvin¹ and Joule² in the middle of the last century. After the nature of macromolecules was recognized in the 1920–s, primarily through the work of Staudinger³, it became clear that an isolated long-chain molecule in thermal motion would behave as an entropic spring; a tensile applied force is required to maintain a given end-to-end distance of the chain since its configurational entropy increases as this distance decreases⁴. It appeared to be natural, then, to regard the entropic spring concept as the basis for the observed macroscopic entropic behavior of rubber and rubber-like solids. In fact, this concept has played the central role in the development of the classical molecular theory of rubber elasticity and continues to play a central role in current theories.