A study has been carried out of the changes in the x‐ray diffraction patterns which occur when oriented fibers or tapes of poly(trimethylene terephthalate) (3GT) and poly‐(tetramethylene terephthalate) (4GT) are subjected to mechanical tensile stress. Although the polymers show very different behavior in detail, in both cases comparatively large reversible lattice strains are observed (∼ several %). The diffraction pattern of 3GT changes monotonically with increasing macroscopic strain, suggesting that the lattice responds immediately to the applied stress, and deforms as though it were a coiled spring. In 4GT, on the other hand, there is no detectable change in the x‐ray diffraction pattern at low macroscopic strains, i.e., low values of the applied stress. At higher stresses, changes in the pattern occur which suggest a definite change in the crystal structure. Finally at the highest values of applied stress, the lattice deformations cease to increase. A preliminary discussion is presented of the relationship of these x‐ray diffraction results to the mechanical stress–strain behavior.

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SynopsisThe synthesis of poly(pentamethy1ene terephthalate) and its subsequent drawing into highly oriented fiber has proved difficult for two reasons. First, it was difficult to produce polymers of sufficiently high molecular weight to withstand subsequent drawing; and second, because of the slow rate of crystallization of the material, the spun fiber coalesced on the windup bobbin, becoming useless for drawing. Methods are described which overcome these difficulties. Two crystalline phases of the oriented fiber have been discovered. The a form is obtained by annealing under zero or small tension and has a crystallographic repeat along the fiber axis of 24.7 A compared with a chemical repeat of 14.5 A. There are evidently two monomers per crystallographic repeat, each appreciably contracted. The (3 form is obtained by annealing either under high tension or at constant extended length, but in the relaxed fiber it always exists in mixture with the a form. Its crystallographic repeat is 28.2 A, and so again consists of two monomers; but their conformation is more highly extended than that of the a form. An a-form fiber will transform reversibly to the (3 form under the influence of mechanical stress.

A method is described whereby integrated intensities can be obtained from X-ray diffraction photographs of polymeric fibres. Contour maps of optical density are produced using a digital microdensitometer and 'Photowrite' manufactured by Optronics International, Inc., and the total intensity obtained by integrating round contour lines. The method takes less time than making radial scans through the peak intensity of each reflexion arc with a scanning microdensitometer, and correcting for the spread in the circumferential direction.