Liquid crystal films on curved surfaces: An entropic sampling study

The confining effect of a spherical substrate inducing anchoring (normal to the surface) of rod-like liquid crystal molecules contained in a thin film spread over it has been investigated with regard to possible changes in the nature of the isotropic-to-nematic phase transition as the sample is cooled. The focus of these Monte Carlo simulations is to study the competing effects of the homeotropic anchoring due to the surface inducing orientational ordering in the radial direction and the inherent uniaxial order promoted by the intermolecular interactions. By adopting entropic sampling procedure, we could investigate this transition with a high temperature precision, and we studied the effect of the surface anchoring strength on the phase diagram for a specifically chosen geometry. We find that there is a threshold anchoring strength of the surface below which uniaxial nematic phase results, and above which the isotropic fluid cools to a radially ordered nematic phase, besides of course expected changes in the phase transition temperature with the anchoring strength. In the vicinity of the threshold anchoring strength we observe a bistable region between these two structures, clearly brought out by the characteristics of the corresponding microstates constituting the entropic ensemble.Comment: 14 pages, 5 figure

Molecular simulations elucidate soft elasticity in polydomain liquid crystal elastomers

Liquid crystal elastomers (LCE) — polymer networks with embedded 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, the resulting polydomain ma- terials exhibit an extraordinary soft elastic behavior under unidirectional pulling, with a plateau-like region in the stress-strain curve, before turning into a monodomain LCE where a standard elastic resistance is recovered. Here we investigate the microscopic origin of this behavior by performing large-scale molecular iso-stress Monte Carlo simula- tions of swollen polydomain main-chain LCE. Our simulations are based on the soft-core Gay-Berne interaction potential and reproduce the stress-strain experiment featuring the plateau-like behavior. Deeper insight into the molecular organization of our sim- ulated samples reveals that the underlying mechanisms are local domain rotation and growth, excluding orientational order destruction-reconstruction. It also suggests that these mechanisms may be assisted by a dissipation of elastic free energy stored in topo- logical defects created during the synthesis, which is compatible with the stress-strain irreversibility observed in some main-chain LCENon UBCUnreviewedAuthor affiliation: University of Ljubljana, Faculty of Mathematics and PhysicsPostdoctora

Biaxial liquid crystal elastomers: a lattice model

We present a simple coarse-grained lattice model for monodomain biaxial liquid-crystal elastomers and perform large-scale Monte Carlo simulations in the proposed model system. Orientational ordering \u2014uniaxial or biaxial\u2014 reflects in sample deformations on cooling the system. The simulation output is used to predict calorimetry data and deuterium magnetic resonance spectra

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

Liquid crystal elastomers (LCE)polymer networks constructed with liquid crystal unitsare 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 stressstrain 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 stressstrain 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

Main-chain swollen liquid crystal elastomers: A molecular simulation study

We have performed large-scale off-lattice computer simulations in a model system of swollen mainchain liquid-crystalline elastomers, constituted of weakly reticulated soft Gay\u2013Berne chains, investigating both nematic and smectic morphologies. We present isostress Monte Carlo results for temperature-scan and stress\u2013strain experiments, and connect to typical experimental observables, such as sample dimensions, specific heat, deuterium magnetic resonance spectra, and scattered X-ray patterns. We find that the results reproduce the main features of main-chain elastomers, e.g., a pronounced strain-alignment coupling and the existence of two (nematic\u2013isotropic and smectic\u2013nematic) order\u2013disorder phase transitions. The nematic\u2013isotropic transition in our system turns out to be weakly first-order

Controlling surface defect valence in colloids

We perform large-scale Monte Carlo simulations of orientational ordering in nematic shells and study the type and position of topological defects when an external electric field (homogeneous or quadrupolar) is applied. The field-induced variation of the defect number (and strength) can be used to change the valence of colloidal particles coated with a nematic layer

Molecular simulations elucidate electric field actuation in swollen liquid crystal elastomers

Swollen elastomer liquid crystals undergo significant deformations by application of an electric field perpendicular to their alignment axis, as shown in experiments by Urayama et al. [Urayama K, Honda S, Takigawa T (2006) Macromolecules 39:1943–1949]. Here we clarify this surprising effect at the molecular level using largescale Monte Carlo simulations of an off-lattice model based on a soft Gay–Berne potential. We provide the internal change of molecular organization, as well as the key observables during the actuation cycle

External field-induced switching in nematic elastomers: a Monte Carlo study

We present a Monte Carlo study of external field-induced switching in nematic elastomers, employing a coarse-grained shearable lattice model. In large enough systems a full-wavelength Freedericksz effect is observed as opposed to the half-wavelength effect seen in ordinary nematics that clearly reflects in simulated polarized light textures, as well as in deuterium magnetic resonance spectra. The reorientation of mesogenic units is accompanied by pronounced shear deformations

A microscopic lattice model for liquid crystal elastomers

We propose a simple coarse-grained lattice model for liquid crystal elastomers and show, through large scale Monte Carlo simulations, that it can reproduce stress\u2013strain, order, light transmission, and other experiments, including temperature effects. We focus both on homogeneously and inhomogeneously crosslinked materials

Defects and ordering in nematic coatings on uniaxial and biaxial colloids

We present a systematic lattice Monte Carlo simulation study of nematic ordering in thin nematic shells on uniaxial and biaxial colloidal particles. Typically, four mutually repulsive half-strength defect lines penetrating the shell are observed, as found for spherical particles. For shells of constant thickness, the defect lines tend to accumulate in the high curvature regions. If the thickness of the nematic coating varies across the surface, the defect lines tend to be located in the thinnest regions. On increasing the shell thickness, the defect lines transform into escaped structures with surface point defects