Relationship between segment structures and elastic properties of segmented poly(urethane‐urea) elastic fibers

Studies on segmented poly(urethane‐urea) (SPUU) elastic fibers having various segment structures were done in terms of elastic recovery and stress‐strain relationship (S‐S). Three kinds of segment structures were used: 1) the same composition having different sequences of segment units, 2) the same length of soft segments having different molecular weights of polyol, and 3) different segment structures having almost the same stress at 350% elongation. The SPUU elastic fibers having higher sequence numbers of both soft and hard segment units, that is, greater block structures, show better elastic recovery properties, especially delayed elastic recovery. The SPUU elastic fibers showing better elastic recovery take an optimum value for the number‐average molecular weight (M_n) of soft segments jointed with urethane bonds. Here the optimum M_n depends on the molecular weight of polytetramethyleneglycol (PTMG) as a starting material. The hysteresis loss in S‐S for the pre‐elongation decreases with an increase of M_n of PTMG. The SPUU elastic fibers having greater block structures show lower stress with lower 2C₁ and 2C₁ + 2C₂ of Mooney‐Rivilin plot constants for elastic fibers having the same composition. This indicates a lower density of crosslinks for finite deformation. An increase of the urea bonds or the molar ratio of urea bond to urethane bond raises the stress. It is found that the polymerization process, as well as composition, is important for design structures of SPUU elastic fibers.