Abstract
Vitrimer chemistry of boronic transesterification was introduced into a triblock thermoplastic elastomer to allow associative bond exchange in its highly ordered nanoscale domains. The purpose of this localized dynamic cross-linking is to enable reprocessing while also improving creep resistance. Triblock poly(styrene-b-butadiene-b-styrene) (SBS) was used as an elastomeric vitrimer precursor to gain understanding of fundamental structure–property effects of localized dynamical cross-linking. The occurrence of the UV-induced thiol–ene reaction was corroborated by Fourier transform infrared (FTIR) spectroscopy and swelling experiments. Transmission electron microscopy (TEM) images and small-angle X-ray scattering (SAXS) profiles demonstrate a prominently lamellar morphology with domain spacings that decline with the increase of cross-linking degree. Short and long relaxation modes of triblock SBS vitrimers (vSBSs) were found to have opposite temperature dependence, which caused thermorheological complexity in linear viscoelasticity. As compared to its random analog, vSBS shows much slower stress relaxation and higher activation energy, as well as superior resistance to creep under a steady load. The enhanced rheological properties are attributed to the high concentration of localized cross-links in the soft phase and to additional constraints that hinder network strand diffusion in the microphase-separated structure. The mechanical properties of vSBSs at low strain were substantially enhanced by the associative dynamic cross-links between the soft segments, increasing the maximum stress to 7.5 MPa at 100% strain. Its Young’s modulus of 88.1 MPa is about 2.8 times that of neat SBS. Applications of vSBSs as hot-melt adhesives were explored, showing excellent lap shear strength on iron plates. Due to the dynamic exchange of boronic ester bonds, adhesive joints can efficiently rebind at least three times and the lap shear strength remains almost unchanged.
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