Smart actuation of liquid crystal elastomer elements: cross-link density-controlled response

Abstract

Liquid crystal elastomers (LCEs) exhibit some remarkable physical properties, such as the reversible large mechanical deformation induced by proper environmental stimuli of different nature, such as the thermal stimulus, allowing their use as soft actuators. The unique features displayed by LCE are originated from their anisotropic microstructure characterized by the preferential orientation of the mesogen molecules embedded in the polymer network. An open issue in the design of LCEs is how to control their actuation effectiveness: the amount of mesogens molecules, how they are linked to the network, the nematic order degree, the cross-link density are some controllable parameters whose spatial distribution, in general, cannot be tuned except for the last one. In this paper, we develop a theoretical micromechanical-based framework to model and explore the effect of the network cross-link density on the mechanical actuation of LCE elements. In this context, the light-induced polymerization (photopolymerization) for obtaining the elastomers’ cross-linked network is of particular interest, being suitable for precisely tuning the cross-link density distribution within the material. This technology enables to obtain a molecular-scale architected LCEs, allowing the optimal design of the obtainable actuation. The possibility to properly set the cross-link density arrangement within the smart structural element (LCE microstructure design and optimization), represents an intriguing way to create molecular-scale engineered LCE elements having a material microstructure encoding the desired actuation capabilities.

Multiphysics modelling of light-actuated liquid crystal elastomers

“…The distribution tensor, presented in the previous section for standard polymers, is hereafter extended to consider the preferential chain orientation induced by the nematic order of the embedded mesogen units. It is worth mentioning that the network description based on the chain distribution tensor can be extended to describe other mechanisms affecting the material responsiveness, such as the cross-link density coming from the LCE synthesis [40].…”

“…Further, the light absorbance of the polymer has been determined by equation (4.3) with A pol = 400 m −1 , and molar absorptivity of the photo-absorber units is equal to ϑ = 30 m 2 mol −1 , while their concentration in the network has been assumed to be C n = 200 mol m −3 . The actuation capacity of LCEs is here assumed not to be dependent on the cross-link density of the polymer matrix, despite playing a general role [40]. The light source intensity distribution is assumed to be described by a Gaussian law as Ifalse(X,tfalse)=αfalse(tfalse)I0exp⁡false(−false|bold-italicXfalse(tfalse)−bold-italicX0false|2d2false), where I 0 is the maximum intensity occurring at X = X 0 , and the parameter d defines the width of the distribution; in the case of a moving light source, the position of the maximum light intensity is expressed through the translation speed v of the source as | X 0 | = v t .…”

“…When the statistical mechanics approach is considered, one of the main issues relies on understanding how the statistics conformation of an individual chain is related to the mechanics of the entire polymer network and, in turn, on how to upscale it to the macroscopic level (continuum mechanics). To this end, various models have been proposed in the literature [35], such as the three-chain model [36], the four-chain model [37], the eight-chain model [38] and the full network model [39][40][41]. In this work, we adopt a mechanical description of the polymer network based on a full network model.…”

“…Further, the light absorbance of the polymer has been determined by equation (4.3) with A pol = 400 m −1 , and molar absorptivity of the photo-absorber units is equal to ϑ = 30 m 2 mol −1 , while their concentration in the network has been assumed to be C n = 200 mol m −3 . The actuation capacity of LCEs is here assumed not to be dependent on the cross-link density of the polymer matrix, despite playing a general role [40]. The light source intensity distribution is assumed to be described by a Gaussian law as…”

Discontinuous Metric Programming in Liquid Crystalline Elastomers

“…Much like in other non-liquid crystalline systems, programming of crosslink density is an appealing route to introduce discontinuous actuation metrics. It has been established that properties relating to liquid crystalline behavior in these elastomers, particularly the nematic-to-isotropic transition temperature ( T NI ) and the magnitude of deformation, are dependent upon the polymer network structure. − Given that LCEs are commonly fabricated by photopolymerization reactions, one way to locally control crosslink density is through spatial variation of light exposure time or intensity. However, this approach leaves regions of unreacted crosslinking monomers, which could be unstable and give rise to inconsistent material properties over time.…”