Molecular simulations elucidate electric field actuation in swollen liquid crystal elastomers

Abstract

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.actuators | nematic rubbers | orientational stripe domains | microscopic modeling L iquid crystal elastomers (LCE) are rubbery functional materials consisting of weakly cross-linked polymer strands with embedded liquid crystalline (mesogenic) units. This unique microscopic design provides a pronounced coupling between macroscopic strain and the underlying mesogenic orientational order (1). As the latter can be controlled by a number of external stimuli such as temperature, electric field, or ultraviolet light in the case of photo-responsive azobenzene-based elastomers, LCE have great potential for application in various sensing devices, as well as for actuation (1, 2). The applications of such actuators are fascinating, ranging from artificial muscles, heart valves, and other biomedical applications (3-8), to micro-and nano-electromechanical (2), as well as electrooptical systems like adaptive lenses (3, 9), where their high deformability and low weight make them particularly promising. Among the possible actuation stimuli the external electric field is particularly appealing, even if quite difficult to implement since a rather strong field is required to induce deformations. This problem can be partly overcome by focusing on the so-called (semi)soft deformation modes, e.g., liquid crystalline director rotation accompanied by shear, where significant strain can be achieved at a rather low free energy cost (10-14).More recently, actuation has been implemented experimentally in unconstrained swollen samples by applying an external electric field perpendicular to the director, using optical and infrared spectroscopy techniques to detect changes in mesogenic ordering (15)(16)(17). These experiments were rationalized by a theoretical study based on the semisoft neoclassical free energy (18), as well as by a continuum treatment of the electro-opto-mechanical effect dynamics (19). On the other hand, little has been done so far to obtain a deeper molecular-level insight into the actuation-related phenomena in LCE. For example, we have carried out an early microscopic lattice simulation attempt (20), which, however, lacking translational molecular degrees of freedom, turned out to have only limited predictive power because of inevitable assumptions regarding the strain-alignment coupling. As this and several other issues can be safely bypassed within a more demanding microscopic description based on pairwise intermolecular interactions, we have recently implemented an off-lattice molecular simulati...

Emergent chirality in achiral liquid crystals: insights from molecular simulation models of the behaviour of bent-core mesogens

“…The model is sufficiently repulsive to promote liquid crystal behaviour through anisotropic excluded volume interactions but sufficiently soft to allow rapid movement through phase space, avoiding the jamming which is sometimes seen with harder particle coarsegrained models. The benefits of this type of model have been demonstrated by Zannoni and co-workers [36,37] and used to look at changes in molecular organisation in the actuation cycle of a liquid crystal elastomer [38]. We also note the similarity of our four-site model to the Monte Carlo model of Peroukidis et al [39].…”

From idealised to predictive models of liquid crystals

“…This has proved quite difficult and was only practically possible by employing a softer version of the GB pair potential that we had developed in the meantime with Mark Wilson, Roberto Berardi and Juho Lintuvuori [63]. The LCE work allowed us to perform virtual stress-strain MC experiments obtaining Young moduli and shedding some light on the mechanism of elongation-contraction leading to actuation driven thermally by going through the NI transition [64] or by the application of an electric field [65] in the swollen LCE systems experimentally prepared and studied by Urayama [66,67]. We performed stress-strain virtual experiments to examine the relation between mechanical properties and molecular structure and we arrived at proposing a mechanism for the phenomenon of supersoft elasticity [68], one of the most fascinating properties of some LCE, particularly the main chain ones polymerised in the isotropic phase .…”

Study of the optical, thermal, and mechanical properties of nematic liquid crystal elastomers

“…The different physical properties of LCE have been studied by various researchers [35][36][37][38][39][40][41][42][43][44][45], but the current applications of the material show that the research work that has so far been done on it is very insufficient. In the present study, an attempt was made to investigate the optical, thermal, and mechanical properties of monodomain NLCE so as to understand the physical behavior of this material and to develop new applications of it.…”