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

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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.