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Changes in the position, width, and shape of the amorphous halo with orientation in the X-ray diffraction patterns of amorphous and semicrystalline polymers are used to study the medium-range order in the amorphous phase. Analysis of the data shows that a fraction of the amorphous domains is preferentially oriented. The chain segments in these domains are more densely packed, and these changes in density are accompanied by changes in the conformation of the polymer chains. The variations in the amorphous scattering are simulated by varying the density of packing of a two-dimensional assembly of hard disks. The diameter of such a disk is ~4.3 A for polyethylene, which is larger than the Lennard-Jones diameter of 3.9-4.1 A. In polymers such as nylons and polyesters, the molecular packing is determined by varying contributions of more than one interchain interaction. The medium-range order in such oriented amorphous regions seems to arise from the same interactions that determine the crystal structure.

ABSTRACT:The orientation of the noncrystalline (amorphous) phase in poly (ethylene terephthalate) fibers is analyzed by separating the amorphous scattering in the wideangle x-ray diffraction patterns into isotropic and anisotropic components. Two parameters are used to characterize the amorphous orientation-the fraction of the anisotropic component and its degree of orientation. The x-ray amorphous orientation parameters are compared with the sonic modulus and the birefringence values. Our results illustrate that the intrinsic birefringence of the amorphous phase is not ''intrinsic'' but depends on its density. The role of the oriented amorphous phase in determining the strength (tenacity) and the dimensional stability (shrinkage) of the fibers is discussed. We conclude that although amorphous orientation determines the shrinkage, other factors such as the connectivity between the amorphous phase and the crystalline regions play an important role in determining the tenacity of the fibers.

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