Supramolecular Elastomers: Self-Assembling Star–Blocks of Soft Polyisobutylene and Hard Oligo(β-alanine) Segments

Abstract

A series of novel self-assembling star-blocks consisting of Mw = 29 000 g/mol 3-arm polyisobutylene (PIB) stars and oligo(β-alanine) end segments were synthesized and characterized. Star–blocks containing β-alanine dimers are viscous liquids, while those with tri-, tetra-, and penta(β-alanine)s are elastic solids. According to IR spectroscopy, the β-alanine dimer is partially hydrogen-bonded, while the trimer, tetramer, and pentamer are fully hydrogen-bonded and form β-sheets. DSC suggests crystalline β-alanine trimer tetramer and pentamer domains phase separated from the rubbery PIB. The melting temperature of the crystalline domains increases with the length of the oligo(β-alanine) segment. Transmission electron microscopy, wide-angle X-ray diffraction, and small-angle X-ray scattering of star-blocks containing tetra(β-alanine) indicate stacks of hydrogen-bonded β-sheets dispersed in a soft continuous PIB phase. The crystalline phases form fibrous lamellae with lengths up to ∼200 nm, widths up to ∼20 nm, and thicknesses of ∼2 nm, which is the length of β-alanine tetramer. Although the oligo(β-alanine) contents are very low (from 1.5 to 3.6 wt % in the series), the static and dynamic mechanical properties of the star–blocks are very different. The elastic moduli of the TPEs increase 5-fold with increasing β-alanine content. Evidently, the oligo(β-alanine) domains provide not only physical cross-links but also act as fillers.