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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.

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