Rubber elasticity and chain configuration

Abstract

Various theories accounting for the existence of a front factor in the equation of state for rubber elasticity are reviwed. It is shown that a front factor which may be different from unity exists whether or not intramolecular energy effects are present. A unified theory is presented which is applicable both to Gaussian chains and to chains with intramolecular folding energy. Stress‐temperature measurements were used to obtain values of the intramolecular folding energy ϵ′ for numerous crosslinked polyacrylates and polymethacrylates. The values of ϵ′ for almost all of the polyacrylates were found to be approximately zero; the values of ϵ′ for the polymethacrylates varied generally between ‐350 and ‐700 cal./mole, indicating a preference for folded configurations. The front factors were also determined for these crosslinked polymers at 60°C. The front factor appears relatively independent of the degree of crosslinking. There appears to be a general trend to smaller front factors as the size of the alkyl chain increases in both series. An interpretation in terms of network interpenetration is given in the text. The variation of ϵ′ with copolymer composition has been determined for a series of butyl acrylate‐butyl methacrylate copolymers. The value of ϵ′ for isotactic amorphous poly(1‐pentene) was also measured and found to be zero. Finally, the effect of diluent on the front factor is discussed for the case where diluent is present during the crosslinking process and for the case where it is added after crosslinking.