Structure and properties of segmented poly(urethane-urea) elastic fibers. Part 1: Dependence of elastic recovery on chemical structure and molecular weight of soft segment

The time dependence of elastic recovery was measured in the range of -30°C to 100°C for segmented poly (urethane-urea), which consists of soft and hard segments. The dependence of elastic recovery on the soft segment and its molecular weight and the improvement of elastic recovery at low temperatures is discussed. The instantaneous and delayed unrecovered strain at room temperature showed a minimum value at a molecular weight of about 2000 of poly (tetramethylene glycol) (PTMG). But the segmented poly (urethane-urea) based on the high molecular weight of soft segment easily crystallized due to stress-induced crystallization at low temperatures and had high elastic unrecovered strain. X-ray diffraction data suggest the formation of crystallites of soft segment such as PTMG or poly (caprolactone) (PCL), which did not melt even after release of stress at a temperature of 0°C. Adding low molecular weight of fractionated PTMG was effective in preventing stress-induced crystallization. Copolymerization of two kinds of soft segment, such as PTMG and poly (propylene glycol) (PPG) or PTMG and PCL, in the stage of prepolymer reaction was very effective on elastic recovery at low temperatures. The experimental result shows that the block copolymerized PCL which was polymerized by using polyether of low molecular weight as an initiator improved elastic recovery at a wide range of low temperatures. Elastic recovery rate at low temperatures depended on both micro-Brownian motion in relation to glass transition temperature and stress-induced crystallization of the soft segment. It was found that the rubber point Tr at which the material behaved most rubber-like related to the transition temperature Tαa at which the material showed the maximum dynamic loss modulus by the following formula. Tr tαa + 30.