High-performance thermoplastic polyurethane elastomers with enhanced mechanical properties prepared with chain extenders containing urea and carbamate moieties

Abstract

Chain extenders, usually small aliphatic diols, play a great role in thermoplastic polyurethanes (TPUs) by forming hard segments and increasing the molecular weight. In this study, two chain extenders, N,N-bis(2-hydroxyethyl)urea (MEA–DMC, containing a urea group) and N-(2-hydroxyethyl)-O-(2-hydroxyethyl)carbamate (MEA–EC, containing a single carbamate group), are synthesized to examine their effects on the resulting TPUs with the chain extender 1,4-butanediol (BDO) as a control. Specifically, the TPUs are prepared by reacting the extenders with a prepolymer composed of poly(tetramethylene glycol) (PTMG, M_n = 2000) and methylene-bis(4-cyclohexylisocyanate) (HMDI). The results show that the tensile strength and elongation at break of TPU-MEA-DMC-1, TPU-MEA-EC-1 and TPU-BDO-1 are 22.7 MPa, 12.0 MPa, and 8.2 MPa and 1400%, 1600% and 1200%, respectively. The influences of chain extender content, the molar ratio of NCO/OH (R-value), and prepolymer molecular weight on the properties of the as-prepared TPUs are also studied. The tensile strength of TPU-MEA-DMC is in the range of 22.7 MPa to 50.5 MPa, and that of TPU-MEA-EC is in the range of 12.0 MPa to 34.7 MPa, respectively. These improvements are primarily attributed to the presence of urea/urethane in the chain extenders or the hard segment density. Urea groups (two hydrogen-bond donors (–NH–)) or carbamate groups (one hydrogen-bond donor (–NH–)) enhance hydrogen-bonding density within the polymer, thus reinforcing its mechanical properties. Moreover, DSC analysis shows that TPU-MEA-DMC exhibits a PTMG cold crystallization exotherm near −25 °C, which weakens and shifts to higher temperatures with more hard segments or lower prepolymer molecular weight.