Stiffening Liquid Crystal Elastomers with Liquid Crystal Inclusions

During the last few years, liquid crystalline elastomers (LCEs) have been systematically produced by cross-linking liquid crystalline side-chain polymers. In these networks, a liquid crystalline molecule is fixed at each monomeric unit. LCEs exhibit a novel combination of properties. Due to liquid crystalline groups, they show anisotropic liquid crystalline properties similar to conventional liquid crystals (LCs); but due to the three-dimensional network-structure of the polymer chains, they show typical elastomer properties, such as rubber elasticity or shape stability. One exceptional property of this combination is demonstrated when a mechanical deformation to the LCE causes macroscopically uniform orientation of the long molecular axis of the LC units (the so-called “director”).This response of the LC-phase structure to an applied mechanical field is similar to the effect of electric or magnetic fields on low molecular weight liquid crystals (LMLC), as illustrated in Figure 1. Figure la shows an undeformed LCE. Because of the non-uniform orientation of the director, the sample scatters light strongly so the elastomer is translucent like frosted glass. On the other hand, applying a mechanical field the director becomes uniformly aligned and the sample is transparent (Figure 1b). Such a macroscopically ordered rubber exhibits optical properties very similar to single crystals. These propertie s of LCEs offer new prospects for technical application, e.g., in nonlinear and integrated optics.

Liquid crystal elastomers (LCEs) are shape-morphing materials whose large and reversible shape transformations are caused by the coupling between the mobile anisotropic properties of liquid crystal (LC) units and the rubber elastic of polymer networks. Their shape-changing behaviors under certain stimuli are largely directed by the LC orientation; therefore, various strategies have been developed to spatially modulate the LC alignments. However, most of these methods are limited as they require complex fabrication technologies or have intrinsic limitations in applicability. To address this issue, programmable complex shape changes in some LCE types, such as polysiloxane side-chain LCEs, thiol-acrylate main-chain LCEs, etc., were achieved by using a mechanical alignment programming process coupled with two-step crosslinking. Here, we report a polysiloxane main-chain LCE with programmable 2- and 3D shape-changing abilities that were created by mechanically programming the polydomain LCE with two crosslinking steps. The resulting LCEs exhibited a reversible thermal-induced shape transformation between the initial and programmed shapes due to the two-way memory between the first and second network structures. Our findings expand on the applications of LCE materials in actuators, soft robotics, and smart structures where arbitrary and easily programmed shape morphing is needed.

Here, we study the influence of cross-linking density, X, on the swelling, thermal and electric field response of trifunctionally cross-linked liquid single crystal elastomers (V3 LSCEs) swollen with low molecular weight liquid crystals, 4-n-pentyl-4-cyanobiphenyl (5CB). The cross-linker concentrations are X3V3=3.3 mol %, X5V3=5 mol %, and X7V3=7 mol %. LSCE networks are characterized by frozen-in orientation order, P. Xc is a constitutional critical point for LSCE networks. When X<Xc (P=0), there are no shape changes and a nematic-isotropic transition takes place at TNI. When X>Xc (P>0), supercritical behavior prevails over a range of temperatures, ΔTs. Taking ΔTs∝(X-Xc)1/2, we find a critical cross-linking density, Xc≈3%, for V3 LSCEs. 5CB diffuses ⊥n into V3 similar to isotropic gels with typical times, τ⊥n in minutes, that decreases with increasing (X-Xc). Swelling with reorientation effects is initiated by 5CB propagating into V3 followed by relaxation of the front profile with τ⊥(2) (twist), and τ|| (13) (splay-bend) that both increase nearly parabolically with increasing (X-Xc). Front speeds are 50% faster in 5V3 than in 7V3 but no fronts were observed in 3V3. Compared to dry V3 LSCE volumes, Vd, the swollen volumes, Vs, increased as X→Xc: Vs/Vd∼(X-Xc)-0.27 reducing the frozen-in orientational order at X to \ ildeX=X(Vd/Vs). The maximum electromechanical effect found in swollen V3 was δMAX∼(\ ildeX-\ ildeXc) with \ ildeXc∼0.25%. The size of the effect is much smaller than observed in 8A2 which has a larger P. For 7V3 the effect is about 4 times smaller and for 5V3, it is an order of magnitude smaller.

Measurement of the orientational elastic constants and the twist viscosity of nematic side chain polymers

Synthesis-Processing-Property Relationships in Thermomechanics of Liquid Crystal Elastomers

ABSTRACTA novel liquid crystal elastomer (LCE) synthesized by melt polymerization, which exhibits the capacity of shape memory, is reported here for the first time. The method of synthesize the shape memory LCE has been explored. A facile two‐step method to synthesize these anisotropic materials to realize reversible shape change behavior is reported. The first reaction is the addition of nematic liquid crystal molecules to form a kind of liquid crystal polymer. Subsequently, the polymer is crosslinked to trap the order of the liquid crystal into a crosslinked LCE. The LCE exhibits liquid crystalline behavior which has shape memory with excellent fixity and recovery. Its shape memory and actuating properties also have been studied. When reheating the LCE to 165 °C, the shape will recover. The main chains and crosslinked bonds of the LCE contain ester groups, which are sensitive to alkaline and acidic condition. It turns out that the LCE is intact under acidic condition, but it can be degraded under alkaline condition. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 389–394

The dynamics of stringlike defects and the shrinkage and annihilation processes of individual defect loops in a nematic polymer liquid crystal have been experimentally investigated. In 40&amp;lt;t&amp;lt;500 s, the line density of defects ρ scales as ρ(t)∼t−1, as expected by theory and as found in the experimental studies of some low molecular weight (LMW) liquid crystals. For individual loops, they first change their complex shape into a circle, resulting in a rapid decrease in length, and then the circular loops continuously shrink. Before a complete annihilation, the relation between the radius R(t) of the circular loop and time t0−t, where t0 is the time to annihilation, is described by R2(t)=2Γ(t0−t)(t⩽t0). The kinetic constant Γ determined from this polymer liquid crystal is 0.34∼0.38 μm2/s, much smaller than those (200∼300 μm2/s) obtained in some LMW liquid crystals. The small Γ is ascribed to the high viscosity of this polymer liquid crystal, compared with LMW liquid crystals.

This paper summarises the properties of archiral and chiral liquid crystalline (LC) elastomers, the latter of which have found interest due to their piezoelectric properties. In addition the formation of new LC elastomers by a radical polymerization of acrylate groups covalently linked to LC polymers was investigated. This was done concerning the progress of the crosslinking reaction, concerning the influence of the crosslinking on the LC phases and concerning the network properties (swelling ratio and elastic modulus) of the resulting elastomers. It turns out that the networks prepared in this way are very soft and their crosslinking density is low. This can be explained assuming that the radical polymerization of the acrylates leads only to short oligomers, due to steric restrictions. Photo crosslinking initiated by a photoinitiator in the LC phase creates a memory of the orientation before crosslinking.

Modeling the thermo-responsive behaviors of polydomain and monodomain nematic liquid crystal elastomers

This dissertation demonstrates the design of reversibly self-morphing novel liquid crystalline elastomer (LCE) antennas that can dynamically change electromagnetic performance in response to temperature. This change in performance can be achieved by programming the shape change of stimuli-responsive (i.e., temperature-responsive) LCEs, and using these materials as substrates for reconfigurable antennas. Existing reconfigurable antennas rely on external circuitry such as Micro-Electro-Mechanical-Systems (MEMS) switches, pin diodes, and shape memory alloys (SMAs) to reconfigure their performance. Antennas using MEMS or diodes exhibit low efficiency due to the losses from these components. Also, antennas based on SMAs can change their performance only once as SMAs response to the stimuli and is not reversible. Flexible electronics are capable of morphing from one shape to another using various techniques, such as liquid metals, hydrogels, and shape memory polymers. LCE antennas can reconfigure their electromagnetic performance, (e.g., frequency of operation, polarization, and radiation pattern) and enable passive (i.e., battery-less) temperature sensing and monitoring applications, such as passive radio frequency identification device (RFID) sensing tags. Limited previous work has been performed on shape-changing antenna structures based on LCEs. To date, self-morphing flexible electronics, including antennas, which rely on stimuli-responsive LCEs that reversibly change shape in response to temperature changes, have not been previously explored. Here, LCE antennas will be studied and developed. Also, the metallization of LCEs with different metal conductors and their fabrication process, by either electron beam (E-Beam) evaporation or optical gluing of the metal film will be observed. The LCE material can have a significant impact on sensing applications due to its reversible actuation that can enable a sensor to work repeatedly. This interdisciplinary research (material polymer science and electrical engineering) is expected to contribute to the development of morphing electronics, including sensors, passive antennas, arrays, and frequency selective surfaces (FSS).

The monodomain nematic liquid crystal elastomer (LCE) is a nonporous material with one-dimension (1D) negative thermal expansion (NTE) along the direction of orientation. This stimuli-responsive size change is comparable with the contractile response of a muscle fibre. Recently, LCEs have become more and more popular and increasingly investigated as a full-fledged class of stimuli-responsive materials. However, the nematic LCE is limited to a single, unidirectional deformation unless complex hybrid architectures are made. We develop here a special LCE based on a liquid crystal polymer with an extraordinary phase transition sequence, “re-entrant nematic (NRe) – smectic A (SmA) – nematic (N) – isotropic (I) phase”, along with “prolate – oblate – weakly oblate – spherical” chain conformation evolution. In the aligned LCE film, these conformational changes of polymer chain are observed with the naked eye through their macroscopic translations into a unique sequence of “contraction – expansion – second expansion” deformations. Notably, a switch from NTE to positive thermal expansion (PTE) occurs upon heating. Moreover, a bilayer actuator composed of aligned and non-aligned LCE layers can perform 2D to 3D shape transformation with “curling – uncurling – second uncurling” actuation sequence. This LCE capable of multiple deformations in response to a single stimulus paves the way to multimodal single-material actuators. It provides a new strategy for the development of advanced materials with adjustable and multidirectional deformations.

Patterns perceived as regular by humans are ubiquitous and can be classified into non-biological patterns like cloud formations and sand dune stripes, biological patterns like zebra stripes, and artificial patterns like tiles. This paper focuses on three examples of non-biological self-organized patterns: wrinkles on material surfaces, alignment defect structures in liquid crystals, and a layered structure in liquid crystal elastomers. These non-biological patterns exhibiting useful functions are advantageous due to their low fabrication costs compared to biological and artificial patterns.

Liquid crystalline composite gels consisting of a low molecular weight gelator and a low molecular weight liquid crystal were prepared by two types of gelation method: (1) a continuous cooling method and (2) an isothermal gelating method, which provide different molecular self-assembling kinetics of the low molecular weight gelator in gelation processes. Optical microscopic and atomic force microscopic studies revealed that numerous fine strands of the one-dimensionally assembled low molecular weight gelators were formed in the composite gels in the case of the continuous cooling method as well as that of the isothermal gelating method. However, the thinner strands were more homogeneously dispersed in the composite gel prepared via the isothermal gelation at an appropriate temperature than those of the continuous cooling method. This difference on the dispersion state of the strands was shown, by polarizing optical microscopic observation, to have a significant influence on the molecular alignment state of the low-molecular-weight liquid crystal in theliquid crystalline composite gel. The electro-optical response, the light scattering-transmitting switching, of the liquid crystalline composite gel to an applied electric field was drastically dependent on the morphology of the gelators. High-contrast light switching was achieved for the composite prepared by the isothermal gelation. The response time of the electro-optical switching was less than 100 w s under 30 V rms .

The chemical response of liquid crystal elastomers (LCEs) offers substantial potential for applications in propulsion systems, micromechanical systems, and active smart surfaces. However, the shape-changing behaviors of LCEs in response to organic (isotropic) solvents remain scarcely explored, with most research focusing on liquid crystal (anisotropic) solvents. Herein, we prepared a series of aligned LCEs with varying crosslink densities using a surface alignment technique combined with an aza-Michael addition reaction, aiming to investigate their swelling behaviors in different isotropic solvents. We found that the rates of shape and volume variation modes, the elastic modulus of the LCEs, and the polarity of the solvent all significantly influence the swelling behavior. Specifically, when LCEs swell in acetone, dimethylformamide (DMF), and ethyl acetate, contraction occurs along the alignment direction. Conversely, extension along the alignment direction is observed when LCEs swell in toluene, anisole, and acrylic acid. Meanwhile, extension in the perpendicular direction is noted when LCEs swell in nearly all solvents. These shape changes can be attributed to the phase transitions of the LCEs. This research not only provides valuable insights into the swelling mechanisms of LCEs but also holds great promise for the development of solvent sensors and gas sensing applications.

Swelling behavior and deformations of main chain liquid crystal elastomers (MCLCEs) have been reported. The swelling agent used here is a low molecular weight liquid crystal (LMWLC), 4-n-pentyl-4-cyanobiphenyl (5CB). Monodomain MCLCE swells anisotropically with the swelling degree q mono = 11.9 and polydomain MCLCE swell isotropically in 3D with q poly = 12.1. These values are much larger than observed in swelling side-chain LCEs (SCLCEs). In the stress-strain measurement, the plateau area is found among two steep areas. The electromechanical effects of monodomain MCLCE is also observed, and the results show that the maximum contraction in ∥ is about 7%. The threshold for the onset of the electromechanical effect is about 10 V, which is larger than observed in SCLCEs.