Phase changes in fibrous macromolecular systems and associated elasticity. Model phase diagrams

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Abstract
In order to illustrate the concepts of solid–solid and solid–melt first‐order transitions and associated elasticity for an ideally homogeneous fiber where different conformational arrangements of the macromolecules in the crystalline state are possible, phase diagrams were arbitrarily constructed on the basis of reasonable estimates of the variables involved. With some limitations, these diagrams illustrate several situations which might be expected to occur in real systems and the conditions under which one crystalline modification can be expected to be stable or metastable and under which a transition to another crystalline form or to the amorphous rubbery state can be observed. The variables which determine the state of the system were considered to be the external tensile force and temperature, pressure being regarded as a constant. Thermodynamic concepts and relationships used under such conditions were only a generalization of those established by Flory and Gee. Large changes of length of the fiber were assumed to occur during the reversible phase changes. A unified schematic picture of the elastic behavior of a fiber in its different physical states (single phase: crystalline or amorphous; two co‐existing phases: crystal–crystal or crystal–amorphous) is afforded by the construction, on the basis of the phase diagram, of stress–strain isotherms, length–temperature curves at constant load, and stress–temperature curves at constant length.

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