Thermodynamics of perfect polymer fibers in tension: stress-induced enantiotropic phase transitions

Abstract The principles of the thermodynamic fusion theory [Smith Jr, KJ. Comp. Theor. Poly. Sci. 1997;7:139] are applied to solid–solid phase transitions affected by stress. Only perfect fibers of finite molecular weight polymers are considered. Transitions of one crystal form to another involves stretching work as well as that of constant force transition; the necessary equation is cast into an experimentally accessible form. Fiber volume and entropy are shown to pass through, respectively, a maximum and minimum as tensile force increases. Recovery of both the undeformed values is interpreted as the necessary criteria of phase transition under stress. The fundamental, Flory–Gee equation of phase equilibrium is integrated rigorously. Applications to actual fibers are deferred until sufficient empirical data is available.