Liquid crystal elastomers (LCEs) are shape-morphing materials that demonstrate reversible actuation when exposed to external stimuli, such as light or heat. The actuation’s complexity depends heavily on the instilled liquid crystal alignment, programmed into the material using various shape-programming processes. As an unavoidable part of LCE synthesis, these also introduce geometrical and output restrictions that dictate the final applicability. Considering LCE’s future implementation in real-life applications, it is reasonable to explore these limiting factors. This review offers a brief overview of current shape-programming methods in relation to the challenges of employing LCEs as soft, shape-memory components in future devices.
The combination of forming and shape-programming methods is guided by geometrical and mesogen alignment limitations, impacting both material shaping and subsequent shape-change. Each LCE production method permits specific modes of actuation, thereby restricting the potential applications of the final material.
Polymer-dispersed liquid crystal elastomers as moldable shape-programmable material
M. Bobnar, N. Derets, S. Umerova, V. Domenici, N. Novak, M. Lavrič, G. Cordoyiannis, B. Zalar and A. Rešetič
The current development of soft shape-memory materials often results in materials that are typically limited to the synthesis of thin-walled specimens and usually rely on complex, low-yield manufacturing techniques to fabricate macro-sized, solid three-dimensional objects. However, such geometrical limitations and slow production rates can significantly hinder their practical implementation. In this work, we demonstrate a shape-memory composite material that can be effortlessly molded into arbitrary shapes or sizes. The composite material is made from main-chain liquid crystal elastomer (MC-LCE) microparticles dispersed in a silicone polymer matrix. Shape-programmability is achieved via low-temperature induced glassiness and hardening of MC-LCE inclusions, which effectively freezes-in any mechanically instilled deformations. Once thermally reset, the composite returns to its initial shape and can be shape-programmed again. Magnetically aligning MC-LCE microparticles prior to curing allows the shape-programmed artefacts to be additionally thermomechanically functionalized. Therefore, our material enables efficient morphing among the virgin, thermally-programmed, and thermomechanically-controlled shapes.
Shape programmability of PDLCEs with various deformation modes.
S. Umerova, D. Kuscer, M. Bobnar, N. Derets, B. Zalar and A. Rešetič
Thermomechanically active shape-programmable elastomer microparticles are very promising for development of particulate composites with unique operational properties, which could be attractive for various applications, e.g., in 3D printing. Liquid crystal elastomers are one of the suitable candidate materials due to their remarkable spontaneous shape change response. Performed rheological and thermomechanical tests demonstrate that shear stress can be used to efficiently manipulate the nematic order-driven morphology of liquid crystal elastomer microparticles (μLCEs). Specifically, by exploiting the soft-elasticity character of the material through manipulation of shear amplitude time profile, the nematic director can be oriented along the flow, which results in alignment of microparticles. Using this method, a suspension of well-aligned, thermomechanically elongated monodomain μLCEs can be created by shear stress-assisted cooling. Although the alignment can be lost in absence of persistent flow, it is restored instantaneously on re-application of the flow. The reason for this is the preservation of particle elongation at room temperature in zero flow. This shape memory can be erased by heating the system to the isotropic phase. Our work represents an important step forward in the development of a new generation of shape-programmable materials, which are potentially suitable for additive manufacturing of artefacts with anisotropic physical properties.
Schematics of shear-driven deformation and temperature ‘shape resetting’ of µLCEs (top) and viscosity relaxation measurements at 25 °C (bottom).
Polymer-dispersed liquid crystal elastomers
A. Rešetič, J. Milavec, B. Zupančič, V. Domenici and B. Zalar
The need for mechanical manipulation during the curing of conventional liquid crystal elastomers diminishes their applicability in the field of shape-programmable soft materials and future applications in additive manufacturing. Here we report on polymer-dispersed liquid crystal elastomers, novel composite materials that eliminate this difficulty. Their thermal shape memory anisotropy is imprinted by curing in external magnetic field, providing for conventional moulding of macroscopically sized soft, thermomechanically active elastic objects of general shapes. The binary soft-soft composition of isotropic elastomer matrix, filled with freeze-fracture-fabricated, oriented liquid crystal elastomer microparticles as colloidal inclusions, allows for fine-tuning of thermal morphing behaviour. This is accomplished by adjusting the concentration, spatial distribution and orientation of microparticles or using blends of microparticles with different thermomechanical characteristics. We demonstrate that any Gaussian thermomechanical deformation mode (bend, cup, saddle, left and right twist) of a planar sample, as well as beat-like actuation, is attainable with bilayer microparticle configurations.
Programmable shape memory of PDLCEs with bilayer director field.
Thermomechanical response of liquid crystal elastomers: role of crosslinker density
N. Derets, V. Domenici, A. Rešetič and B. Zalar
In this work thermomechanical properties of main-chain liquid crystal elastomers (MC-LCEs) with different degrees of crosslinking were investigated, and gradual loss of thermomechanical response was observed on repetitive measurements. Specifically, six samples of MC-LCEs were prepared, with crosslinker-to-mesogen relative concentration ranging from 5% to 10% in steps of 1%. The obtained results were then compared to thermomechanical response of side-chain liquid crystal elastomers (SC-LCEs). Additionally, thermomechanical response of polymer dispersed main-chain liquid crystal elastomers (MC-PDLCEs) was investigated. Results indicate that in MC-LCEs the concentration of crosslinker defines thermomechanical response and affects stability of the system. The loss of thermomechanical response is negligible in the case of crosslinker to mesogen ratio being the smallest, namely in 5% sample, and it is unaffected by glasslike to nematic phase transition. SC-LCEs do not show any sign of such behaviour and remain stable after several cycles of thermomechanical measurements.
Thermomechanical response of MC-LCEs with different crosslinker to mesogen ratio (a) and the change of the thermomechanical response of several MC-LCEs depending on crosslinker to mesogen ration in the system after three cycles of measurements (b).
New Liquid Crystalline Elastomeric Films Containing a Smectic Crosslinker: Chemical and Physical Properties
A. Rešetič, J. Milavec, A. Bubnov, D. Pociecha, V. Hamplova, E. Gorecka, B. Zalar and V. Domenici
Side-chain liquid crystal elastomers (SC-LCEs) have been designed by using a new smectic crosslinker. Two types of monodomain films were prepared based on polysiloxane chains, with a different relative concentration of both crosslinker and mesogenic comonomers. The mesomorphic behavior of the two SC-LCE systems was investigated by differential scanning calorimetry and polarized optical microscopy showing a different mesomorphic behavior: in one case, we obtained a nematic SC-LCE film, in the other case, a Smectic A SC-LCE film. In both systems, the mesophases were stable in a wide temperature range. Moreover, the SC-LCE films possess a relatively high orientation at room temperature. The physical-chemical properties, such as the local orientational ordering, structural organization, and dynamics of SC-LCEs’ constituents were studied by means of static and dynamic 2H NMR experiments, small-angle X-ray, and wide-angle X-ray diffractions. The relevant physical properties, such as the thermo-elastic and thermo-mechanic behaviors, are reported and discussed in view of the practical applications.
Selection of 2H NMR spectra of the deuterated side-chain LCE recorded on cooling from the isotropic phase to room temperature.
Deuteron NMR investigation on orientational order parameter in polymer dispersed liquid crystal elastomers
A. Rešetič, J. Milavec, B. Zupančič, V. Domenici and B. Zalar
Polymer-dispersed liquid crystal elastomers have been recently introduced as a thermomechanically active composite material, consisting of magnetically oriented liquid crystal elastomer particles incorporated in a cured polymer matrix. Their thermomechanical properties are largely governed by the degree of imprinted particle alignment, which can be assessed by means of deuterium perturbed 2H-NMR. Spectra of samples with various degrees of imprinted particle alignment were recorded and the results simulated using the discrete reorientational exchange model developed for determining the dispersion of liquid crystal elastomer's domain orientational distribution. We show that the model can be applied to measure the orientational distribution of embedded liquid crystal microparticles and successfully determine the orientational order parameter in the composite system. Thermomechanical measurements correlate well with the obtained results, thus additionally confirming the validity of the applied method.
Experimentally determined orientational order parameter Q(σθ) (top) and thermomechanical response (bottom) of PDLCE samples oriented in different magnetic fields.
Stress-strain and thermomechanical characterization of nematic to smectic A transition in a strongly-crosslinked bimesogenic liquid crystal elastomer
A. Rešetič, J. Milavec, V. Domenici, B. Zupančič, A. Bubnov, B. Zalar
Bimesogenic liquid crystal elastomers, composed with controlled concentrations of two mesogenic constituents, specifically a nematogen and a smectogen, exhibit either enhanced nematic or smectic structure. Here we present a thorough examination of thermomechanical and elastic properties of such a system at high crosslinking density, with ensuing disordered smectic structure which prevents the observation of smectic-nematic phase transition by means of thermomechanical response measurements. Yet, the onset of local smectic order is clearly observable in stress-strain measurements via an anomaly in the elastic modulus. Breakdown of the local smectic structure is detected at higher mechanical loads. Bimesogen composition – temperature phase diagram is as well constructed, consistent with the one obtained by complementary experimental techniques. Stress-strain temperature dependence measurements can thus serve as efficient and simple phase diagram characterization method that is able to detect formation of smectic clusters in strongly crosslinked and thus highly disordered smecto-nematic liquid crystal elastomers.
“Bimesogen concentration-external stress-temperature” phase transition behavior of M11/x. Left: Experimentally determined values of the critical stress for the transition from the smectic A into the nematic phase (orange solid circles). Right: Points at zero stress (orange solid circles with black center dot) define the SmA-N phase transition line in the bimesogen composition vs. temperature phase diagram.
Dynamic investigations of liquid crystalline elastomers and their constituents by 2H NMR spectroscopy
J. Milavec, A. Resetic, A. Bubnov, B. Zalar and V. Domenici
Several side-chain liquid single crystalline elastomers (LSCEs) were investigated by means of 2H NMR spectroscopy focusing on the temperature dependence of the spectral line width and spectral shape, as well as the T-dependence of spin–lattice and spin–spin relaxation times (T1 and T2). All measurements were performed at the Larmor frequency of 76.753 MHz for 2H. The LSCEs were prepared in the form of monodomain films by using either 2H-labelled monomers (or co-monomers), 2H-labelled cross-linkers or 2H-labelled molecular probes free to diffuse in the LSCE matrix. In all cases, deuterons are on thephenyl moiety. The main features of the measured 2H NMR properties (static and dynamic ones) in these different cases were discussed and compared with those of low-molecular-weight mesogens (i.e.the same molecules used as monomers or co-monomers in the side-chain LSCEs) in the nematic and smectic A phases. Among all investigated LSCE samples, those 2H-labelled on the cross-linker sites present distinct properties either on the 2H NMR spectral line width and spectral shape in the paranematic and in the nematic phases, which can be interpreted in terms of director reorientational dynamics in the intermediate–slow motional regime, taking into account the inherent local director misalignment of LSCEs.
Deuteron NMR resolved mesogen vs. crosslinker molecular order and reorientational exchange in liquid single crystal elastomers
J. Milavec, V. Domenici, B. Zupančič, A. Rešetič, A. Bubnov and B. Zalar
2H quadrupole-perturbed NMR spectra of crosslinker-labelled (a) and mesogen-labelled (b) LSCE.
Differences in the temperature behaviour of orientational ordering of structurally equivalent side-chain liquid single crystal elastomers (LSCEs) with 2H-labelled crosslinker and mesogen have been studied by deuteron quadrupole-perturbed NMR. The impact of nematic director reorientations on the deuteron NMR spectral shapes was analyzed in terms of a discrete reorientational exchange model. This provided for the determination of the degree of nematic director alignment and for the quantification of the influence of the reorientational exchange on the 2H NMR spectra in terms of two parameters, the nematic director orientational dispersion parameter σθ and the motional effectiveness parameter α. A comparative analysis of model simulations and experimental spectra reveals that mesogenic molecules in LSCEs exhibit faster reorientational dynamics as compared to crosslinker molecules and that mesogens and crosslinkers exhibit a similar and rather substantial static director orientational disorder.
