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Changes in nylon 6.6 and polyester filament yarns were determined after heat treatment with dry heat at various temperatures under constant length conditions. An attempt was made to relate structural changes and changes in physical properties due to heat setting. Density, obtained by the density gradient column technique, was used to calculate the degree of crystallinity as a structural parameter. Filament tensile strength and elongation at break were measured on a constant-rate-of-extension machine, and then toughness of the sample was obtained from the load elongation curve. The amount of dye uptake was estimated spectrophotometrically. Degree of crystallinity increased significantly as temperature increased for both nylon 6.6 and polyester fibers. Tenacity decreased substantially for nylon 6.6 and increased marginally for polyester. Elongation and toughness at break decreased for both nylon 6.6 and polyester. Tenacity of nylon 6.6 decreased despite an increase in degree of crystallinity. This suggests degradation of the fibers. Therefore, degree of crystallinity appeared to be of little importance as a contributor to change in tensile strength for degraded nylon 6.6. Tenacity of polyester was well predicted by degree of crystallinity. As crystallinity increased, tenacity of polyester increased. Elongation and toughness of both nylon 6.6 and polyester decreased as degree of crystallinity increased, but the relationship to crystallinity for polyester was not significant. Dyeability of both nylon 6.6 and polyester was well predicted by degree of crystallinity. In both cases, the amount of dye uptake decreased as crystallinity increased. This research suggests that determinations for structural changes such as degradation and orientation might be utilized in addition to crystallinity to predict tenacity of nylon 6.6 and elongation and toughness of polyester. / Ph. D.