Tris- and tetra-(β-alanine)-grafted polyisobutylenes (A3 and A4) with the same grafting density have been synthesized and studied. IR, DSC, SAXS, and WAXD show that the β-alanine tetramer segments in A4 form β-sheet crystals similar to those in A3. A3 and A4 are melt-processable TPEs. The less than 5% by volume of β-sheet crystals in A3 and A4 bring about mechanical properties significantly better that those of a vulcanized butyl rubber composite optimally reinforced with carbon black (CBV). A4 is somewhat stiffer and less extensible than A3. Overall, only a small gain in toughness is achieved for A4 over A3. Importantly, A3 and A4 also show hysteretic properties and dynamic mechanical properties superior to CBV. Both A3 and A4 are less hysteretic than CBV at low strains and more hysteretic at high strains. Their loss factors are substantially lower than that of CBV in the temperature range of 60–100 °C, and their modulus dependence on strain amplitude at very low strains (Payne effect) are significantly subdued in comparison to CBV. In fact, A3 is slightly better than A4 in all respects from the view point of hysteresis and dynamic mechanical properties. Dichroic IR study indicates that the oligo(β-alanine) segments in A3 and A4 statistically prefer to orient perpendicular to the extension axis due to shear stress exerted on the elongated β-sheet crystals at <250% strain. At higher strains, the grafting molecular architecture and the random location of the load-bearing chain on the surface of the β-sheet crystals dictate the orientation of the β-sheet crystals to be random. No fragmentation of the β-sheet crystals in the hydrogen-bonding direction is detectable during the entire process of extension until break. The residual preference of the oligo(β-alanine) segments for the orientation perpendicular to the extension axis is attributed to the elongated β-sheet crystal morphology, which prevent the crystals from fully comply to the load-bearing chains.
Yan, X., Jin, H., Fahs, G. B., Chuang, S., Moore, R. B., & Jia, L. (2017). Supramolecular elastomers. Particulate β-sheet nanocrystal-reinforced synthetic elastic networks. Polymer, 121, 97–105. https://doi.org/10.1016/j.polymer.2017.06.005