Molecular Simulations Shed Light on Supersoft Elasticity in Polydomain Liquid Crystal Elastomers

Abstract

Liquid crystal elastomers (LCE)polymer networks constructed with liquid crystal unitsare functional materials characterized by a pronounced coupling between elastic strain and liquid crystalline orientational ordering. When prepared by polymerization and cross-linking in the isotropic phase, and then cooled (“isotropic genesis”), a polydomain LCE presents an extraordinary “supersoft” elastic behavior under unidirectional pulling, with a plateau-like low elastic modulus region in the stress–strain curve, before turning into a monodomain LCE where a standard elastic resistance is recovered. The physical mechanism of this behavior is of great interest, but still mysterious. This work investigates its origin, reproducing the experiment and succeeding to observe the plateau-like stress–strain behavior by means of large-scale molecular iso-stress Monte Carlo simulations. The results not only show that the basic molecular mechanism behind supersoft elasticity hinges on local domain rotation and growth but also suggest that this is assisted by a newly proposed dissipation of the elastic energy stored in the defects created by the isotropic genesis process, explaining also the irreversibility observed in main-chain LCE.

Elastocaloric effect in liquid crystal elastomers from molecular simulations

“…In total, the sample consists of N = 64000 particles; out of these, 33024 are swelling monomers. The crosslink fraction is close to ∼ 12% [16].…”

“…Uniaxial soft-core GB ellipsoids are used here to assemble LCE networks, representing bonded mesogenic molecules, as well as the non-bonded mesogenic swelling monomers. For compatibility with previous work [16,17], the ratio of potential well depths for a pair of aligned GB particles in the end-to-end and side-to-side relative positions is fixed at 0.2, and the length-to-width ratio of all GB ellipsoids is taken to be equal to 3. Bonding sites are assumed at the GB particle ends (head/tail) as well as on the equator wherever needed to provide cross-links creating a LCE network.…”

“…Further details regarding the potential parameters are given in Ref. [16] and in references therein. The elastomer network is generated using the 'isotropic genesis' procedure [16] in a cubic simulation box with periodic boundary conditions.…”

“…[16] and in references therein. The elastomer network is generated using the 'isotropic genesis' procedure [16] in a cubic simulation box with periodic boundary conditions. First, polydisperse (on average ∼ 8 GB units long) polymer chains are grown at low density in random directions, each starting from a random position within the simulation box.…”

Correlation functions of main-chain polymer nematics constrained by tensorial and vectorial conservation laws

“…(24), (26), and (29) is merely to match the continuous versions of Eqs. (25), (27), (30). They drop out again in Eqs.…”