Effects of polymerization method on structure and properties of thermoplastic polyurethanes

The effects of the dynamic polymerization method and temperature on the molecular aggregation structure and the mechanical and melting properties of thermo-plastic polyurethanes (TPUs) were successfully clarified. TPUs were prepared from poly (ethylene adipate) glycol (M_n = 2074), 4,4′-diphenylmethane diisocyanate and 1,4-butanediol by the one-shot (OS) and the prepolymer (PP) methods in bulk at dynamic polymerization temperatures ranging from 140 to 230°C. Glass-transition temperatures (T_gs) of the soft segment and melting points (T_ms) of the hard segment domains of OS-TPUs increased and decreased, respectively, with increasing polymerization temperatures, but those of PP-TPUs were almost independent of the polymerization temperature. T_gs of the soft segment and T _ms of the hard segment domains of these TPUs polymerized above 190°C were almost the same regardless of the polymerization method. Solid-state nuclear magnetic resonance spectroscopy (NMR) analyses of OS-and PP-TPUs showed that the relative proton content of fast decay components, which corresponds to the hard segment domains, in these TPUs decreased with increasing polymerization temperatures. These results clearly show that the degree of microphase separation becomes weaker with increasing polymerization temperatures. The temperature dependence of dynamic storage modulus and loss tangent of OS-TPUs coincided with those of PP-TPUs at polymerization temperature above 190°C. The apparent shear viscosity for OS- and PP-TPUs polymerized above 190°C approached a Newtonian behavior at low shear rates regardless of the polymerization method. These results indicate that TPUs polymerized at higher temperatures form almost the same molecular aggregation structures irrespective of the dynamic polymerization method.