The effect of chain extender symmetry, branching and hydrogen bonding capability on the morphology and mechanical properties of thermoplastic polyurethanes

Abstract

The chemical architecture of chain extenders within thermoplastic polyurethanes (TPUs) has a marked effect on material properties. To evaluate the structure-property relationships within linear TPUs, three different chain extenders with varying chemical structure (i.e. symmetry, branching and hydrogen bonding capability) were used in the synthesis of three low T g semi-crystalline TPUs with low molecular weight (M n = 18 kDa). These copolymers were compared to a non-chain extended reference. Results showed that the architecture of the chain extender dictated the degree of hard-soft microphase separation and therefore the mechanical properties. A TPU without chain extender (PU-NC) was a brittle hard-soft phase mixed material. Incorporation of the branched asymmetrical chain extender, 1,2-propanediol (PU-PD), also resulted in a phase mixed material which was mechanically weak with low adhesive properties (lap shear max. stress = 1.0 ± 0.1 MPa). The use of the symmetrical linear chain extender 1,4-butanediol (PU-BD) afforded a hard-soft phase separated copolymer where the mechanical and adhesive performance obtained was as expected for such low molecular weight TPUs (E = 72 ± 8 MPa, lap shear max. stress = 2.1 ± 0.4 MPa). The copolymer with a symmetrical bisurea diol chain extender (PU-BU) also exhibited a hard-soft phase separated morphology, but with much higher tensile (E = 210 ± 16 MPa) and adhesive properties (lap shear max. stress = 4.4 ± 0.1 MPa) as a result of strong bidentate urea hydrogen bonding. Incorporating these non-covalent interactions into the chain extender provides a route to superior mechanical performance for TPUs with low molecular weight.