Particulate Beta-Sheet Nanocrystal-Reinforced Supramolecular Elastomers

Many synthetic supramolecular thermoplastic elastomers (TPEs) are analogs of natural silks in the sense that both types of materials contain b-sheet crystalline domains that serve to crosslink and reinforce the elastic network. However, one of the key differences between the synthetic and natural analogs is the aspect ratio of b-sheet crystals. The nanocrystals in silks are particulate, with the sizes in all three dimensions less than 10 nanometers (e.g., 2.1x2.7x6.5 nm³ in spider dragline), while the crystalline domains in synthetic supramolecular TPE display fibrous morphology with the longest dimension ranging from hundreds of nanometers to micrometers.

To provide reinforcement, these crystalline domains must be strong and stiff. In order to attain the strength and stiffness at such a small length scale, it is necessary to use strongly associating molecular motifs. Thus, those capable of forming multiple cooperative hydrogen bonds are adopted by both nature and man. However, the strong association also provides a thermodynamic incentive for a high degree of association that exceeds the aforementioned nanometer scale. In silk, the size of the crystallites is generally believed to be attributable to their specific amino acid sequences. In the absence of specific amino acid sequences, how to achieve the nanometer size is an interesting challenge for synthetic TPEs.

In this presentation, I will first show that particulate b-sheet nanocrystals with the longest dimension well-below 100 nm can be attained without an elaborate amino acid sequence. The small size is attributed to the grafting topology. The topology renders a rapid increase of entropic loss as the degree of association increases to stop the growth of the b-sheet. Second, I will show that the particulate nanocrystals display a remarkable ability to simultaneously provide stiffness, extensibility, and strength to the synthetic elastic network and do so highly efficiently at a low volume fraction of the material. The herein studied butyl rubber-based thermoplastic elastomers containing 3.6 volume % of b-sheet nanocrystals are stiffer, stronger, and more extensible than vulcanized butyl rubber reinforced by 20 volume % of carbon black and poly(styrene-b-isobutylene-b-styrene) reinforced by >33 volume % of polystyrene domains. The high reinforcing efficacy of the b-sheet crystals is attributable to two phenomena associated with their small sizes, a stick-slip mechanism for energy dissipation and an auxiliary layer of polymer brush that contributes to increasing the modulus.