Reactive Molecular Dynamics Simulations of Self-Assembly of Polytwistane and Its Application for Nanofibers

We investigated the self-assembly and mechanical properties of polytwistane (PT), particularly a seven-strand PT rope, using reactive force field based molecular dynamics simulations at different temperatures. We show that upon self-assembly due to strong van der Waals interaction among PT units, PTs form a twisted structure (ropelike) with a twisting angle of similar to 0.16 rad/nm at 300 K, which makes them mechanically stronger. The PT rope has high Young's modulus (similar to 0.45 TPa) at 300 K. Interestingly, Young's modulus increases with temperature for the seven-strand PT rope, whereas it decreases with temperature for a single-strand PT. This is because in the case of the seven-strand PT, the twisting angle also contributes to the elastic property of the PT rope and twisting depends on the temperature. We estimate a maximum load transfer of similar to 1.1 and similar to 3.3 nN to the central unit at 100 and 300 K, respectively. Hence, the amount of load transfer critically depends on the twisting in the rope. The fracture behavior of the single-strand PT and seven-strand PT rope is also investigated. We find two major mechanisms of PT fracture: in the first case, an acetylene-like structure is attached with one of the twistane units at the breaking region. In the second case, all the three sp(3) C-C bonds of the participating twistane units break, creating three sp(2) C sites along the breaking region. In the case of the PT rope, the fracture in the individual strands occurs in a sequential manner. We predict that the self-assembled twisted PT rope is a promising candidate for carbon fiber applications where mechanical properties are of interest.