Real time mechano-optical study on deformation behavior of PTMO/CHDI-based polyetherurethanes under uniaxial extension

Real time mechano-optical properties of two homologous segmented, thermoplastic polyurethanes (TPUs) obtained from the stoichiometric reactions of trans-1,4-cyclohexyl diisocyanate (CHDI) and poly(tetramethylene oxide)glycol (PTMO) were investigated. PTMO oligomers used had number average molecular weights 〈Mn〉 of 1020 and 2040 g/mol, resulting in TPUs with urethane hard segment contents of 14 and 7.5% by weight. AFM studies showed intertwined microphase morphology. Stress-strain measurements demonstrated the formation of very strong, elastomeric materials, with ultimate tensile strengths of 23-25 MPa and elongation at break values of about 1000%. Mechano-optical behavior of these polymers exhibited multiple regime behavior. The first strain optical regime is linear where primarily the soft segments orient. The transition between the first and second strain optical regimes was found to correspond to the saturation of straining of the soft segments that lead to start of rotation of hard segments in the stretching direction. The start of Regime II coincides with the onset of strain hardening and the distance between the hard segments increases appreciably with stretching in this regime. Increasing the soft segment length was found to promote its strain-induced crystallization.