Structure and Properties of Segmented Poly(urethane-urea) Elastic Fibers. Part 2: Hysteresis of Stress-Strain Curves.:Part2 : Hysteresis of Stress-Strain Curves

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The effects of pre-elongation ratio, stress relaxation time and recovery time on the hysteresis behavior of the stress-strain curve are discussed for segmented poly (urethane-urea). The loss of stress and extension energy was found to be much larger than unrecovered strain with pre-elongation. In the range of low extension ratio of α<2, the large loss in shear modulus was shown for unloading and re-elongation curves. However the breaking energy of the linkages was lower than 0.75 kJ/mol. As for the range of 2<α<3.5, the linkages having the bond energy of 20kJ/mol were broken by pre-elongation. The pre-elongation of α>4 led to break the linkages having the strong bond energy of 75kJ/mol. This energy corresponds to the break down of intermolecular hydrogen bond of hard segment. The molecular weight between crosslink points (Mc) estimated by shear modulus of unloading or re-elongation curves was almost the same as that of the prepolymer. The strong hydrogen bond of 75kJ/mol between hard segments re-bonded and recovered faster than other weaker linkages. The Mooney-Rivlin plots of unloading and re-elongation curves had much smaller values of C2 than those of the pre-elongation curves. This suggests that the strain softening effect decreased by pre-elongation. It is especially interesting that the unloading curve for elongation of α>4 had the value of C2_??_0 and showed ideal rubber elasticity.
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