A study has been carried out of the differences in mechanical properties of oriented fibers of poly(ethylene terephthalate) (2GT), poly(trimethylene terephthalate) (3GT), and poly(tetramethylene terephthalate) (4GT). The properties studied include the tensile stress–strain behavior, the recovery from strain, shrinkage at 100°C and the glass‐transition temperatures. The stress–strain curves of the three materials differ markedly. 2GT shows a monotonic increase in stress with increasing strain up to failure, which occurs at ∼20% strain, and the oriented fibers possess a comparatively high initial modulus. 3GT shows a much lower initial modulus and there is an inflection in the stress–strain curve at about 5% strain. The stress–strain curve of 4GT shows a number of distinct features. Although the initial modulus of 4GT is similar to that of 3GT, the stress–strain curve shows a pronounced plateau in the region between 4% and 12% strain. At higher strains the stresses rise rapidly before failure. These features of the stress–strain curves in the three polymers can be related to previous studies where the x‐ray diffraction spectrum and the Raman spectrum have been examined for fibers under stress. The ranking of both the recovery and shrinkage behavior of these materials is in the order 3GT > 4GT > 2GT. These results can also be understood in terms of the results of the previous structural studies, and it is concluded that the molecular conformations in both the crystalline and noncrystalline regions play a key role in determining the mechanical behavior.

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synopsis"he addition of very small amounts of carefully selected polymers by melt blending has been found to radically change the flow properties at spinning of conventional polymers such aa polyethylene terephthalate, nylon 66, and polypropylene. This has a large effect on spun properties, especially at high wind-up speeds. The additive polymers used have included liquid crystal polymers, polyethylene, polyethylene glycol, and nylon 66. A major phenomenon is a considerable lowering of spun orientation, or wind-up speed suppression. In order to activate the non-liquid crystal polymers to achieve this effect special spinning conditions are necessary. The decrease in orientation can potentially lead to large increases in spinning productivity.The mechanism is viewed as being nonuniform extensional flow in a twwphase system and not molecular interaction.

A theoretical model for a short fiber reinforced composite is proposed. The composite is assumed to consist of an aggregate of sub‐units, each sub‐unit possessing the elastic properties of a reinforced composite in which the fibers are continuous and fully aligned. The elastic constants of a partially oriented composite are then calculated by the Voigt and Reuss averaging procedures, giving upper and lower bounds respectively for the composite modulus. Comparison is made with experimental data for such composites. The measured modulus of glass and carbon fiber composites is found to be given by the Reuss or lower bound, to a good approximation compared with the difference between the bounds, for fiber orientations ranging from almost isotropic to highly aligned.