The Elasticity and Related Properties of Rubbers

Abstract

Abstract In Section I the relation of rubbers to other classes of polymers and the molecular basis of rubber elasticity are briefly examined. In Section II the methods used in the quantitative development of the statistical-thermodynamic theory of a molecular network are outlined, and the main conclusions, in the form of stress—strain relations, etc., are presented and compared with experimental data. Section III examines the photoelastic properties of rubbers from both theoretical and experimental standpoints and discusses in detail the evidence derived from photoelastic studies on the statistical segment length in the molecular chain and its relation to intramolecular energy barriers. Section IV is concerned with the thermodynamic analysis of stress—temperature data for rubber and other polymers, with particular reference to the internal energy and entropy changes during extension under constant pressure or constant volume conditions. Methods of deriving these quantities are compared and the results related to the modified molecular network theory due to Flory. Section V deals with the phenomena of swelling in liquids and considers both the effect of swelling on the mechanical properties and the effect of different types of stress or strain on the swelling equilibrium. Sections I–V are concerned mainly with the statistical theory and its applications. The final section examines in considerable detail the formulation of more general theories of large elastic deformations on a purely empirical or phenomenological basis, so as to overcome some of the limitations of the statistical theory. The logical basis of these formulations is presented and the conclusions are discussed in relation to the available experimental evidence. Some common pitfalls to be avoided are emphasized.