The structure and properties of two laterally ordered (directionally crystallized) polymers are discussed, namely, polyacrylonitrile and poly(vinyl trifluoroacetate). Evidence from x‐ray diagrams, infrared measurements, thermal expansion data, and fiber properties are interpreted in terms of molecular structure. It is concluded that the existence of only chain‐to‐chain order (lateral order) in polyacrylonitrile and poly(vinyl trifluoroacetate) is principally due to steric and dipolar intramolecular repulsions. These lead to a chain stiffening effect, regardless of tacticity, which is so great in the former case that the backbone chain is forced to assume a twisted, kinked configuration. The stiff molecular chains may be thought of as rigid rods which pack in a lattice array. The side groups, nitrile or trifluoroacetate, assume varying angles with the respect to the rod axis in order to minimize the steric and dipolar repulsion forces. This variation gives rise to diffuse off‐equatorial scattering in the x‐ray diagram of the oriented polymers and low values of the dichroic ratios of selected bands in the infrared. The chain stiffening effect is greater in polyacrylonitrile and this polymer exhibits a higher “melting point”, higher fiber tenacity at equivalent orientation, less solubility, more general diffuse scattering in the x‐ray diagram, and lower dichroic ratios in the polarized infrared spectrum than does poly(vinyl trifluoroacetate). In fact, polyacrylonitrile, in contrast to poly(vinyl trifluoroacetate), acts as if it were all ordered and contained no amorphous regions in the usual sense, as judged from thermal expansion data and the lack of a definite amorphous halo in the x‐ray diagram. Poly(vinyl trifluoroacetate) is concluded to be no more syndiotactic than conventional poly(vinyl acetate), since laterally ordered poly (vinyl trifluoroacetate) can be derived from commercial poly (vinyl alcohol) by esterification. Therefore, the observed lateral order in poly (vinyl trifluoroacetate) is not due to a predominantly syndiotactic structure. These laterally ordered polymers have a properties intermediate between those of normally crystalline polymers and those of amorphous polymers.

The thickness of single crystal lamellae of polyethylene, crystallized from solution, is found to increase greatly during annealing at temperatures above 110°C. The change is observed with small‐angle x‐ray diffraction and electron microscopy. The increase in thickness of the lamellae takes place at the expense of their lateral perfection; holes develop within the lamellae. All evidence indicates that a major refolding of the molecules occurs; this emphasizes the need for a new concept of the amount of motion and freedom that polymer molecules can have in the solid state. A similar process apparently occurs during the annealing of bulk samples crystallized from the melt.

Long spacing order in the fiber axis direction is found for the majority of viscose rayon yarns, varying in repeat spacing from about 170 to 220 A. This order is interpreted as a regularity of crystallite placement such that the crystallites themselves act as diffracting centers to form a line lattice with long spacings. The diffraction found for different rayons varies in intensity and perfection as well as repeat spacing. This new structural feature can be related to the periodic length determined by the limiting degree of polymerization after hydrolysis. Current interpretations in the literature regarding the diffuse scattering at small angles are extended to include scattering due to micro‐voids or regions of low electron density. Since it is known that the materials are so dense that interparticle interference arises for the particle scattering, the reciprocity rule dictates that the voids are the source of the major amount of the scattering, not the crystallites. This interpretation is in agreement with the concepts of polymer structure from other techniques and can be used to explain the changes in scattering with swelling or the very small changes with crystallization. The micro‐voids are elongated in the fiber axes direction. For a dry cellulose fiber a distribution of sizes of the axes of the ellipsoidal voids can be obtained for relative comparisons.