Polyurethane tri-block copolymers - Synthesis, mechanical, elastic, and rheological properties

A series of polyurethane tri-block copolymers were synthesized by reacting a 4,4′-methylenebis(phenyl isocyanate) (MDI)-endcapped poly(tetramethylene oxide) (PTMO, Mn = 2,000 g/mol) with a monoamine-diamide (6T6m) hard segment (HS). The concentration of the HS in the copolymer was varied between 9 and 33 wt % by changing the length of the soft mid-block segment. The structure of the copolymers was analyzed by nuclear magnetic resonance, the amide crystallinity was investigated by Fourier transform infra-red and the thermal properties were studied by differential scanning calorimetry. The mechanical and elastic properties of the tri-block copolymer were subsequently explored by dynamic mechanical analysis, compression set and tensile experiments, and the melt rheological behavior was studied by a parallel plate method. The amide end groups displayed a high crystallinity and the modulus of the tri-block copolymers was relatively high. The fracture strain increased strongly with the molecular weight and the copolymers demonstrated a ductile fracture behavior for molecular weights above 6000 g/mol. Good compression set values were obtained for the tri-block copolymers despite their low molecular weight. In the molten state, the tri-block polymers displayed a gelling effect at low frequencies, which was believed to be a result of a clustering of the end-segments. POLYM. ENG. SCI., 2010.