Fiber recovery

The recovery portions of the stress–strain cycles of poly(ethylene terephthalate), poly(tetramethylene terephthalate), nylon 66, poly(vinyl chloride), and rayon were analyzed and shown to be superposable if appropriate horizontal and vertical shifts are made. The recovery curves are postulated to represent the stress–strain behavior of an elastomeric portion of the fiber which controls recovery, and the scaling factors necessary for superposition are interpreted as indicating changes in the “length” and “modulus” of the elastomer. The elastomer modulus correlates very well with the slope of the stress–strain curve, showing that the elastomer governs the fiber stress during extension. Temporary set is assumed to be due to a temporary localized ordering of the elastomer modulus, which disappears when stress is completely removed. This local ordering lowers the elastomer modulus, which leads to yield points in the stress–strain curves. Permanent set is initiated just before the temporary set approaches a constant value and increases at almost the same rate as the rate of extension. It is completely recovered by heating to near the T g .