Examination of the infrared absorption of three nylons, polyhexamethylene sebacamide, polyhexamethylene adipamide, and polycaproamide (nylon 6,10, 6,6, and 6, respectively) revealed the existence of bands associated with crystalline order. These discoveries were utilized in the study of crystallinity of nylon 6,10 and nylon 6 in conjunction with x‐ray and specific volume methods. The work falls into three parts. Part I deals with elementary consideration of dichroism and deduces a method of comparing the intensity of an infrared band in a drawn sample of polymer with that of a disoriented sample. A theory of dichroism in materials that have rotational symmetry about one direction is formulated. The question of measuring the absolute intensity of an infrared band is briefly considered. In Part II the crystallinity content of nylon 6,10 (a polymer chosen for its suitability for testing the method) is determined by using the 940 cm.−1 band and specific volume measurements. The effects of annealing and drawing are studied. Proportionality between change in specific volume and change in infrared absorption is confirmed by experiment up to an annealing temperature of 180°C. Above this temperature specific volume measurements give too high a crystallinity figure. Cold‐drawing is found to be connected with a reduction of crystallinity, and can produce voids. The crystallinity of our samples varied between 20 and 55%. The significance of these figures—absolute and relative—is discussed. An absolute figure of 1.052 for the amorphous density of nylon 6,10 is deduced. Part III deals with nylon 6. The infrared spectrum in the 7.5–12.5 μ region is examined and several of the bands are classified as crystalline or noncrystalline. Variation of these bands is studied when the polymer is annealed, drawn, and melted. Both annealing and cold‐drawing increase crystallinity content. In the drawn samples the crystallites are found to be better oriented than the noncrystalline parts. The direction of the transition moment of the 935 cm.−1 band is determined. X‐ray results reveal a new type of ordering corresponding to a state of packing intermediate between randomness and full crystalline order. Infrared bands at 990 and 720 cm.−1 are found to be connected with this state of order. Direct evidence is obtained that the molecules possess an order in the quenched amorphous state. The density of amorphous nylon 6,10 calculated from this ordering is in complete agreement with the figure deduced in Part II.

A quantitative analysis of the 725 cm.−1 doublet in the infrared spectrum of polyethylene is given and used to check the “row” model of polyethylene, as it naturally crystallizes from drawn melt. This analysis depends on a new interpretation of the 725 cm.−1 doublet, according to which the 730 cm.−1 component has its transition moment along the a crystallographic axis, and the 720 cm.−1 component along the b axis. The amorphous polymer gives only a band at 720 cm.−1. The analysis in certain cases enables the amount and orientation of the amorphous component of the polymer to be determined. From similar considerations, variations in the relative intensity of the two peaks in the course of drawing unoriented polyethylene are explained.

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