Name: high-contrast and fast photorheological switching of a twist-bend nematic liquid crystal
Uploaded: 2019-10-31T14:45:00.000+00:00
Duration: 6 min 24 s
Description: Watch this Scientific Journal Video about High-Contrast and Fast Photorheological Switching of a Twist-Bend Nematic Liquid Crystal at JoVE.com

This work was supported by the HAS-JSPS bilateral joint research project. Financial support from grants NKFIH PD 121019 and FK 125134 is acknowledged.

1. Preparation of rubbed surfaces for aligning LC molecules planarly
<ol>
	<li>Prepare clean glass substrates.
	<ol>
		<li>Cut glass substrates using a diamond-based glass cutter (Table of Materials) into small square pieces with averages sizes of 1 cm x 1 cm. Wash them by sonication at 38 kHz or 42 kHz in an alkaline detergent (Table of Materials, diluted in water at a detergent:water volume ratio of 1:3) and rinse with distilled water repeatedly (typically, more than 10x with 5 min of sonication for each rinse).</li>
		<li>Subject the substrates to ultraviolet-ozone (UV-O3) cleaner (Table of Materials) for more than 10 min.</li>
	</ol>
	</li>
	<li>Coat planar alignment layer onto clean glass substrates.
	<ol>
		<li>Drip 20 &#181;L of 1 mL of a polyimide planar alignment solution (Table of Materials, used as is) with a pipette onto the cleaned glass substrates. Immediately spin-coat the solution, using a spin coater (Table of Materials) at 3,000 rpm and room temperature (RT) for 70 s. 
		NOTE: The typical thickness of the alignment layer is about 20 nm.</li>
		<li>Bake the coated glass substrates at 80 &#176;C for 60 min to remove the solvent and at 180 &#176;C for &#62;60 min for curing. Rub the substrates using a rayon-cloth rubbing machine (Table of Materials) with the following parameters: rotation speed = 300 rpm, plate speed = 20 mm/s, and impression = 0.3 mm to realize uniaxial alignment of LC materials.</li>
	</ol>
	</li>
</ol>
2. Preparation of LC cells
<ol>
	<li>Place a glass substrate coated with the alignment layer onto another substrate, with the alignment layers face-to-face, and ensure that they are 80% overlapped to form a cell. 
	NOTE: The 20% nonoverlapped surfaces are to be used for introducing LC materials into the cell.</li>
	<li>Place 100 &#181;L of a photoreactive adhesive (Table of Materials) and 0.1 mg of micrometer-sized glass particles (diameter = 5 &#181;m) onto a clean glass substrate and mix them manually using the tip of a paper clip. Move the mixed material to four corners of the cell to adjust the cell gap and illuminate the cell using a low-pressure mercury vapor short arc lamp (Table of Materials) with a wavelength of 365 nm (1.1 W/cm2). Place the cell under the LED lamp at a distance of 1 cm for 5 min.</li>
	<li>After illumination, place the cell onto a hot stage and set the target temperature of the stage to heat the cell to a temperature above the isotropic liquid (I)-nematic (N) phase transition (typically at 160 &#176;C). Transfer the LC material (1-[4-butoxyazobenzene-4&#8217;-yloxy]-6-[4-cyanobiphenyl-4&#8217; yl]hexane; CB6OABOBu; 0.2&#8722;10.0 &#181;L) onto one open surface of the cell and push the materials towards the cell entrance using a microspatula to obtain contact between the LC material and entrance of the cell. Wait for the LC materials to be filled in the cell by capillary force. 
	NOTE: CB6OABOBu has a phase sequence: Cry 100.3 &#176;C TB 105.2 &#176;C N 151.7 &#176;C I on heating and I 151.4 &#176;C N 104.5 &#176;C TB 83 &#176;C Cry on cooling. Do not introduce CB6OABOBu into the N phase or TB phase because flow-induced alignment is promoted.</li>
</ol>
3. Texture characterization by polarizing optical microscopy
<ol>
	<li>Observe the LC cells placed on the hot stage to control the sample temperature (40&#8722;180 &#176;C) with &#177; 0.1 K accuracy under a polarizing light microscope (POM, Table of Materials) using 4x&#8722;100x objective lenses. Record textures using a digital color camera sequentially during cooling and heating.</li>
	<li>Use a UV epi-illuminator (Table of Materials) equipped on the POM with a wavelength of 365 nm (50 mW/cm2).</li>
</ol>
4. Photorheological measurements
<ol>
	<li>Prepare of rheological measurements.
	<ol>
		<li>Before placing any sample onto the stage of the rheometer (Table of Materials), perform geometry inertia calibration and zero gap calibration controlled by a software according to manufacturer&#8217;s instructions to ensure accuracy of the rheological study. Weigh 250 mg of the CB6OABOBu powder sample and load it onto the base quartz plate of the rheometer. 
		NOTE: For the present study, a plate with a diameter of 50 mm is used.</li>
		<li>Set the temperature of the sample chamber to a value above the I-N phase transition point (&#62;160 &#176;C). Set a gap value for approaching the measuring plate to the base quartz plate to sandwich the sample (typical gap value used = 20 &#181;m). Trim excess sample (e.g., by using paper wipes) that is outside of the gap when the measuring plate stops at the trimming position, which is 25 &#181;m above the targeted gap. 
		NOTE: Do not allow excess amount of CB6OABOBu to be introduced to the sample chamber, as this makes the measurements inaccurate.</li>
	</ol>
	</li>
	<li>Perform rheological measurements.
	<ol>
		<li>Irradiate UV light at 365 nm (1&#8722;100 mW/cm2), measuring photorheological switching of CB6OABOBu using the high-pressure mercury vapor short arc lamp. 
		NOTE: The light will be guided from beneath the sample container through the base quartz plate.</li>
		<li>Perform measurements in 1) oscillatory mode for extracting dynamic restoring information of the material and 2) steady rotational mode for obtaining effective rotational viscosity. For measurements in rotational mode, apply a constant shear stress of 13 Pa to the sample to ensure that the measurement is made in the Newtonian regime. 
		NOTE: The selection of the modes is performed by a software according to the manufacturer&#8217;s instructions.</li>
	</ol>
	</li>
</ol>
5. Photo-differential scanning calorimetry
<ol>
	<li>Weigh 10 mg of CB6OABOBu powder sample and load it into a gold differential scanning calorimetry (DSC) pan. Heat the sample to 170 &#176;C in the isotropic phase and ensure that there is no inhomogeneous sample distribution in the DSC pan as observed by the naked eye. Cover the DSC pan with a quartz plate.</li>
	<li>Perform photo-DSC measurements according to the manufacturer&#8217;s instructions (Table of Materials). Measure DSC data at a scan of 10 &#176;C/min. 
	NOTE: The photo-DSC machine is equipped with a UV light intensity of 50 mW/cm2.</li>
</ol>
6. X-ray diffraction characterization
<ol>
	<li>Heat the powder CB6OABOBu sample using the hot stage at 170 &#176;C and suck the sample into an XRD capillary (diameter = 0.5 mm) by capillary force.</li>
	<li>Attach the capillary to a sample holder equipped with a temperature controller. Set the chamber temperature (60 &#176;C, 70 &#176;C, 80 &#176;C, 90 &#176;C, 100 &#176;C, 110 &#176;C, 120 &#176;C, 130 &#176;C, 140 &#176;C, 150 &#176;C, 160 &#176;C, and 170 &#176;C for each X-ray diffraction measurement).</li>
	<li>Irradiate the sample by X-ray and detect the diffracted X-ray beams by a detector without UV irradiation and under a UV light intensity of 10 mW/cm2 for 1 min and 10 min. 
	NOTE: The current study was conducted in RIKEN beamline BL45XU. The light source was the SPring-8 standard in-vacuum undulator. A liquid nitrogen-cooled Si double crystal monochromator was used to monochromatize the beam. The wavelength was 1 &#197;.</li>
</ol>

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The authors have nothing to disclose.

As revealed in Figure 1, CB6OABOBu is a photo-responsive material with I, N, TB, and Cry phase sequences upon cooling. Since local ordering of these phases differs significantly, the photo-driven switching of rheological properties is expected to exhibit good viscoelastic contrast. To quantitatively investigate this, photo-rheology measurements were performed.
First, we consider the rheological data measured in the dark (Figure 2a, red open circles). At the I-N phase transition, the effective viscosity (&#951;eff) decreases, which is attributed to a shear-induced flow alignment. In the N phase, the viscosity is practically independent of the shear stress, indicating a Newtonian fluid behavior (Figure 2b). Transitioning to the TB phase results in an increase of the effective shear viscosity by one order of magnitude. Considering that the TB phase has a local nematic ordering but exhibits pseudo-layer structure analogous to the smectic ordering, the increase of the effective shear viscosity is attributed to formation of the pseudo-layer structures.
In the TB phase, strong shear thinning is observed with clear threshold values as a result of realignment of the pseudo-layer structures (Figure 2b). Subsequent solidification of the sample results in a sharp jump in &#951;eff (<img alt="Icon" class="xfigimg" src="/files/ftp_upload/60433/proportional.jpg" /> shear stress if shear rate is kept constant) by five orders of magnitude. The large scattering of the shear viscosity data in the crystal phase is a result of the sample&#8217;s large resistance exerted on the rotating cone. The sample, in this regime, is a solid characterized by storage modulus instead of a fluid characterized by viscosity. Results under UV intensities of 32.7 mW/cm2 and 59.6 mW/cm2 are shown as black filled circles and blue open diamonds. Three main differences are observed between this data and that measured in darkness: 1) a downshift of transition temperatures, 2) a decrease in &#951;eff in each phase, and 3) no significant viscosity variation among the original N-TB transition temperature for illuminated samples, which is explained by the disappearance of the TB phase under UV light.
It is clear that the rheological properties are indeed significantly distinct in different phases. To test the photo-driven rheological switching, rheological measurements were performed by shining UV light onto the sample. Figure 2c reveals that the photo-driven rheological switching has different contrast values at different temperatures: nearly 1 in the I and N phases, 10 in the TB phase, and 106 in the Cry phase. The ON and OFF switching times are also very short (~100 s, ON and OFF switching times shown in Table 1) both in the TB and Cry phases. The switching time is defined as the transient time for the variation of effective viscosity from 90% to 10% of its original value (that before the UV irradiation). Since the contrast in different phases is different, the switching time cannot be fairly compared between different phases. It is worth noting that for other molten liquids, the initial crystal phase typically recovers within several hours to several days, since their high viscosity prevents backward reaction in bulk, even at high temperatures9,14.
To determine the reason for the absence of slow nucleation, POM observation, photo-DSC, and XRD measurements were conducted. As the POM images in Figure 3 show, shining UV in the Cry phase triggers melting to the I phase at 80 &#730;C (cis-state rich). Maintaining the UV irradiation while decreasing the temperature causes crystallization of the cis-state molecules to occur at different temperatures than those of the trans-state. This suggests a microsegregation of trans- and cis-states. Photo-DSC data give direct evidence for this. As Figure 3c,d shows, exposure to UV light results in splitting of the phase transition peaks for both the I-N (on cooling) and crystal melting (on heating). These confirm that trans- and cis-state molecules form different phase structures. Thus far, most of the explored photo liquefactions owe their origins to the photo-induced temperature shift of glass transition. In contrast, this work demonstrates a novel working mechanism in realizing fast photo liquefaction processes, except for some recent discoveries27,28.

Smart mechanical materials with the capability to change their viscoelastic properties in response to environmental variation have generated tremendous interest among researchers. Switchability is considered to be the most important material factor, which offers robustness of repetitive mechanical response in living organisms. To date, artificial switchable materials with versatile functions have been designed by utilizing soft matter (i.e., photoresponsive hydrogels1,2,3, polymers4,5,6,7,8,9,10,11, liquid crystals [LCs]9,10,11,12,13,14,15,16,17, pH-responsive micelles18,19,20,21,22, and surfactants23). However, these materials suffer from more than one of the following problems: lack of reversibility, low switching contrast ratio of viscoelasticity, low adaptivity, and slow switching speed. In conventional materials, a tradeoff exists between the switching contrast ratio of viscoelasticity and switching speed; thus, designing materials covering all of these criteria with high performance is challenging. To realize materials with the aforementioned omnicapability, selecting or designing molecules that carry emergent natures of both high fluidity (viscous property) and rigidity (elastic property) is essential.
Liquid crystals are ideal systems with a potentially large number of liquid crystalline and solid phases that can be tuned by molecular design. This allows for self-assembled structures at different length scales in particular LC phases. For example, while high-symmetry nematic LCs (NLCs) exhibit low viscosity and elasticity because of their short-range spatial order, low-symmetry columnar or smectic LCs show high viscosity and elasticity due to one- and two-dimensional long-range periodicities. It is expected that if LC materials can be switched between two phases with large differences in their viscoelastic properties, then a viscoelastic smart material with high performance can be achieved. A few examples have been reported9,10,11,12,13,14,15.
This article demonstrates the preparation of a photorheological LC material with a phase sequence of isotropic (I)-nematic (N)-twist-bend nematic (TB)24-crystal (Cry) upon cooling (and vice versa upon heating), which exhibits fast and reversible viscoelastic switching in response to light. Presented here are the methods for measuring viscoelasticity and an illustration of the microscopic structure-viscoelasticity relationship. Details are described in the representative results and discussion sections.

<table><tbody><tr><td>21-401-10</td><td>AS ONE</td><td></td><td>Microspatula</td></tr><tr><td>AL1254</td><td>JSR</td><td></td><td>Planar alignment agent for liquid crystals</td></tr><tr><td>BX53P</td><td>Olympus</td><td></td><td>Polarising microscope with transmission/epi-illumination units</td></tr><tr><td>Discovery DSC 25P</td><td>TI instruments</td><td></td><td>Photo-DSC equipment</td></tr><tr><td>Glass cutter PRO-1A</td><td>Sankyo</td><td></td><td>A diamond-based glass cutter</td></tr><tr><td>HS82</td><td>Mettler Toledo</td><td></td><td>hot stage</td></tr><tr><td>MCR502</td><td>Anton Paar</td><td></td><td>A commercial rheometer</td></tr><tr><td>MRJ-100S</td><td>EHC</td><td></td><td>Rubbing machine</td></tr><tr><td>Norland Optical Adhesive 65, 81</td><td>Norland Products</td><td></td><td>Photoreactive adhesions</td></tr><tr><td>OmniCure S2000</td><td>Excelitas Technologies</td><td></td><td>A commericial high-pressure mercury vapor short arc lamp. Maximum 70 mW/cm^2.</td></tr><tr><td>PILATUS 6M</td><td>Dectris</td><td></td><td>Hybrid photon counting detector for X-ray diffraction dectection</td></tr><tr><td>S1126</td><td>Matsunami Glass</td><td></td><td>Glass substrate</td></tr><tr><td>SC-158H</td><td>EHC</td><td></td><td>Spin coater</td></tr><tr><td>SCAT-20X</td><td>DKS</td><td></td><td>Alkaline detergent</td></tr><tr><td>SLUV-4</td><td>AS ONE</td><td></td><td>Low-pressure mercury vapor short arc lamp</td></tr><tr><td>UV-208</td><td>Technovision</td><td></td><td>Ultraviolet-ozone (UV-O3) cleaner</td></tr></tbody></table>

high-contrast and fast photorheological switching of a twist-bend nematic liquid crystal

Smart viscoelastic materials that respond to specific stimuli are one of the most attractive classes of materials important to future technologies, such as on-demand switchable adhesion technologies, actuators, molecular clutches, and nano-/microscopic mass transporters. Recently it was found that through a special solid-liquid transition, rheological properties can exhibit significant changes, thus providing suitable smart viscoelastic materials. However, designing materials with such a property is complex, and forward and backward switching times are usually long. Therefore, it is important to explore new working mechanisms to realize solid-liquid transitions, shorten the switching time, and enhance the contrast of rheological properties during switching. Here, a light-induced crystal-liquid phase transition is observed, which is characterized by means of polarizing light microscopy (POM), photorheometry, photo-differential scanning calorimetry (photo-DSC), and X-ray diffraction (XRD). The light-induced crystal-liquid phase transition presents key features such as (1) fast switching of crystal-liquid phases for both forward and backward reactions and (2) a high contrast ratio of viscoelasticity. In the characterization, POM is advantageous in offering information on the spatial distribution of LC molecule orientations, determining the type of liquid crystalline phases appearing in the material, and studying the orientation of LCs. Photorheometry allows measurement of a material&#8217;s rheological properties under light stimuli and can reveal the photorheological switching properties of materials. Photo-DSC is a technique to investigate thermodynamic information of materials in darkness and under light irradiation. Lastly, XRD allows studying of microscopic structures of materials. The goal of this article is to clearly present how to prepare and measure the discussed properties of a photorheological material.

POM images, photorheometric data, photo-DSC data, and XRD intensity profiles were collected in darkness during temperature variation and while shining UV light. Figure 1a,b represents the structure of CB6OABOBu, with its phase sequence and possible conformations optimized by the MM2 forcefield in the modeling program (e.g., ChemBio3D).
When CB6OABOBu is in the trans-state, two energy-plausible conformational states appear, and the twisted conformation is the most stable one that promotes formation of the TB phase. When CB6OABOBu is excited to the cis-state when exposed to UV light, a kink-conformation is realized. Though the current conformational optimization made by the modeling program is useful for determining the conformation of a single molecule, it cannot be used for simulating conformational state of multiple molecules that are interacting, or even for self-assemblies of larger molecular clusters.
Figure 1c,d shows the POM textures in darkness and under 30 mW cm&#8722;2 UV irradiance, during cooling of the sample in a 2 &#956;m-thick LC cell with uniformly rubbed planar alignment. In the N phase, uniaxial alignment of molecules is realized (Figure 1c, top). When decreasing the temperature to the TB in darkness, a striped pattern forms, at which the stripes run parallel to the rubbing direction of the LC cell (Figure 1c, middle). This stripe pattern arises as a result of buckling instability and is recognized as a symbol of the TB phase, first reported by Panov et al.25. Further decreasing of the temperature leads to crystallization (Figure 1c, bottom). Irradiation of the UV light alters conformation from the trans- to cis-state, resulting in phase variation and thus texture variation. If starting from the TB phase, the UV light transforms the striped texture to the uniaxially aligned state of the N phase (Figure 1d, top-middle). Turning off the UV light allows the molecules to relax and reenter the trans-state, and the striped texture of the TB phase forms again.
Figure 2 shows the effective viscosity of CB6OABOBu under various conditions measured by the rheometer. Figure 2a shows the temperature dependence of the effective shear viscosity. The reason for calling the measured viscosity the effective shear viscosity is that the real components of viscosity in liquid crystals are orientation-dependent, and the measured viscosity is an orientation-averaged value in the current study. Figure 2b shows the shear stress dependence of the effective shear viscosity at different temperatures during the first and second runs. Figure 2c presents variation among the effective shear viscosity triggered by UV irradiation at different temperatures. Figure 2d demonstrates switching curves of the effective shear viscosity in a log scale at two different temperatures, (i.e., one in the N phase and the other in the TB phase). The detailed temperature dependence of the switching times is summarized in Table 1.
Figure 3a,b shows the textures of CB6OABOBu in a nonaligned sample under 50 mW/cm2 UV irradiance at 80 &#176;C (Figure 3a) and after cooling to 60 &#176;C (Figure 3b). Photo-DSC curves of Figure 3c demonstrate that upon cooling, I-N phase transitions of the trans- and cis-isomers are different. Though the photo-DSC is useful for detecting differences between the dark- and light-stimulated states, it should be noted that photo-DSC usually makes it difficult to quantitatively compare the real heat flow of the differences, since the baseline of the DSC curves changes significantly due to light absorption by the sample and the metal surface of the DSC pan. Figure 3d demonstrates that upon heating, the melting of the crystal phase of the trans- and cis-isomers are different, as measured by conventional DSC. Figure 3e,f shows XRD diffraction plots of the diffracted intensity as a function of d-spacing without and with UV irradiation, respectively. It can be seen that the intensity at each peak changes drastically when UV light is irradiated, mainly attributed to the crystalline structural transformation and local melting.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/60433/60433fig01.jpg" /> 
Figure 1: Chemical structure of CB6OABOBu and the evolution of textures on cooling.(a) Chemical structure of CB6OABOBu and its phase sequence. (b) Space-filling molecular models of CB6OABOBu optimized by the MM2 forcefield in the modeling program. (c) POM textures of CB6OABOBu under crossed polarizers in a 2 &#181;m-thick cell with uniformly rubbed planar alignment; during cooling without UV illumination. Top: in the N phase at 140 &#176;C; middle: in the TB phase at 104 &#176;C; bottom: at the TB-Cry phase transition. (d) POM textures at 90 &#176;C illustrating the photoswitching process. Top: before UV; middle: coexistence of N and TB phases shortly after 30 mW/cm2 UV irradiance at 365 nm; bottom: relaxed TB texture after switching off the UV illumination. Scale bars represent 100 &#181;m. This figure has been adapted with permission from Aya et al.26. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/60433/60433fig01large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/60433/60433fig02.jpg" /> 
Figure 2: Rheological properties and photodynamics of photoswitching of the rheological properties of CB6OABOBu.&#160;(a) Temperature dependence of the effective shear viscosity measured at a constant shear stress of 13 Pa in rotational mode with different UV irradiances: 0 mW/cm2 (red circles), 32.7 mW/cm2 (black circles), and 59.6 mW/cm2 (blue diamonds). (b) The effective shear viscosity as a function of increasing shear stress at selected temperatures. Black filled circles (100 &#176;C) and green filled diamonds (102 &#176;C) are data measured on the first scan, while black open circles (100 &#176;C) and green open diamonds (102 &#176;C) are the data measured on the second scan. (c) Repeatable photoswitching of the effective shear viscosity at 59.6 mW/cm2 irradiance. High and low values in each temperature correspond to UV-OFF and UV-ON states. (d) Photoswitching of the effective shear viscosity shown on a log scale at 97 &#176;C in the TB phase and 90 &#176;C in the Cry phase. Blue and red solid lines for the TB phase are best-fitting curves using simple exponential function upon UV-ON and UV-OFF states. The UV intensity is 59.6 mW/cm2. This figure has been modified and adapted with permission from Aya et al.26. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/60433/60433fig02large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/60433/60433fig03.jpg" /> 
Figure 3: Evidence of existence of micro-segregated domains with different crystal structures in the Cry phase.&#160;(a,b) POM textures through blue filter under 50 mW/cm2 UV irradiance in an octagonal spot of the shape of the field iris diaphragm in the middle at (a) 80 &#176;C and (b) 60 &#176;C. (c) Temperature dependences of the heat flow of the sample during cooling at a rate of 10 &#176;C/min without UV (black dots) and under UV (blue dots). (d) Temperature dependences of the heat flow of the trans-rich sample during heating at 2 &#176;C/min and 10 &#176;C/min rates (black and blue curves, respectively) without UV, and of the cis-rich sample at 2 &#176;C/min rate (red curve). (e,f) Shown is d-spacing dependence of the wide angle X-ray diffraction intensity. (f) Magnified view of the small d-value region of panel e. Blue dashed, red solid, and black long-dashed lines indicate the X-ray diffraction profiles without UV illumination, under 10 mW/cm2 irradiance for 1 min and 10 min, respectively. Open upward and filled downward triangles show increases and decreases in the diffracted intensity of each peak. This figure has been modified and adapted with permission from Aya et al.26. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/60433/60433fig03large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about High-Contrast and Fast Photorheological Switching of a Twist-Bend Nematic Liquid Crystal at JoVE.com

High-Contrast and Fast Photorheological Switching of a Twist-Bend Nematic Liquid Crystal

This protocol demonstrates the preparation of a photorheological material that exhibits a solid phase, various liquid crystalline phases, and an isotropic liquid phase by increasing temperature. Presented here are methods for measuring the structure-viscoelasticity relationship of the material.

Smart viscoelastic materials that respond to specific stimuli are one of the most attractive classes of materials important to future technologies, such ...

high-contrast-fast-photorheological-switching-twist-bend-nematic

Nanyang Technological University

Research

JoVE Journal

Chemistry

6.4K Views.  RIKEN Center for Emergent Matter Science (CEMS). Dit protocol legt uit hoe de visco-elastische stretching eigenschappen van vloeibare kristallen te karakteriseren, en het helpt om belangrijke vragen over hoe nieuwe fotorheologische materialen te ontwikkelen te beantwoorden. Het belangrijkste voordeel van deze methode is dat de rheologische eigenschappen en relevante structurele eigenschappen in realtime kunnen worden gemeten onder lichtstimuli, waardoor het fotorheologisch schakelgedrag kan worden opgenomen. Gebruik voor het begin van de pr...

Video: High-Contrast and Fast Photorheological Switching of a Twist-Bend Nematic Liquid Crystal

Untargeted Metabolomics from Biological Sources Using Ultraperformance Liquid Chromatography-High Resolution Mass Spectrometry (UPLC-HRMS)

Here we present a workflow to analyze the metabolic profiles for biological samples of interest including; cells, serum, or tissue. The sample is first separated into polar and non-polar fractions by a liquid-liquid phase extraction, and partially purified to facilitate downstream analysis. Both aqueous (polar metabolites) and organic (non-polar metabolites) phases of the initial extraction are processed to survey a broad range of metabolites. Metabolites are separated by different liquid chromatography methods based upon their partition properties. In this method, we present microflow ultra-performance (UP)LC methods, but the protocol is scalable to higher flows and lower pressures. Introduction into the mass spectrometer can be through either general or compound optimized source conditions. Detection of a broad range of ions is carried out in full scan mode in both positive and negative mode over a broad m/z range using high resolution on a recently calibrated instrument. Label-free differential analysis is carried out on bioinformatics platforms. Applications of this approach include metabolic pathway screening, biomarker discovery, and drug development.

Untargeted Metabolomics from Biological Sources Using Ultraperformance Liquid Chromatography-High Resolution Mass Spectrometry UPLC-HRMS

Untargeted metabolomics provides a hypothesis generating snapshot of a metabolic profile. This protocol will demonstrate the extraction and analysis of metabolites from cells, serum, or tissue. A range of metabolites are surveyed using liquid-liquid phase extraction, microflow ultraperformance liquid chromatography/high-resolution mass spectrometry (UPLC-HRMS) coupled to differential analysis software.

Here we present a workflow to analyze the metabolic profiles for biological samples of interest including; cells, serum, or tissue. The sample is first ...

Isolation and Preparation of Bacterial Cell Walls for Compositional Analysis by Ultra Performance Liquid Chromatography

The bacterial cell wall is critical for the determination of cell shape during growth and division, and maintains the mechanical integrity of cells in the face of turgor pressures several atmospheres in magnitude. Across the diverse shapes and sizes of the bacterial kingdom, the cell wall is composed of peptidoglycan, a macromolecular network of sugar strands crosslinked by short peptides. Peptidoglycan&#8217;s central importance to bacterial physiology underlies its use as an antibiotic target and has motivated genetic, structural, and cell biological studies of how it is robustly assembled during growth and division. Nonetheless, extensive investigations are still required to fully characterize the key enzymatic activities in peptidoglycan synthesis and the chemical composition of bacterial cell walls. High Performance Liquid Chromatography (HPLC) is a powerful analytical method for quantifying differences in the chemical composition of the walls of bacteria grown under a variety of environmental and genetic conditions, but its throughput is often limited. Here, we present a straightforward procedure for the isolation and preparation of bacterial cell walls for biological analyses of peptidoglycan via HPLC and Ultra Performance Liquid Chromatography (UPLC), an extension of HPLC that utilizes pumps to deliver ultra-high pressures of up to 15,000 psi, compared with 6,000 psi for HPLC. In combination with the preparation of bacterial cell walls presented here, the low-volume sample injectors, detectors with high sampling rates, smaller sample volumes, and shorter run times of UPLC will enable high resolution and throughput for novel discoveries of peptidoglycan composition and fundamental bacterial cell biology in most biological laboratories with access to an ultracentrifuge and UPLC.

The bacterial cell wall is composed of peptidoglycan, a macromolecular network of sugar strands crosslinked by peptides. Ultra Performance Liquid Chromatography provides high resolution and throughput for novel discoveries of peptidoglycan composition. We present a procedure for the isolation of cell walls (sacculi) and their subsequent preparation for analysis via UPLC.

The bacterial cell wall is critical for the determination of cell shape during growth and division, and maintains the mechanical integrity of cells in the ...

Synthesis of Programmable Main-chain Liquid-crystalline Elastomers Using a Two-stage Thiol-acrylate Reaction

This study presents a novel two-stage thiol-acrylate Michael addition-photopolymerization (TAMAP) reaction to prepare main-chain liquid-crystalline elastomers (LCEs) with facile control over network structure and programming of an aligned monodomain. Tailored LCE networks were synthesized using routine mixing of commercially available starting materials and pouring monomer solutions into molds to cure. An initial polydomain LCE network is formed via a self-limiting thiol-acrylate Michael-addition reaction. Strain-to-failure and glass transition behavior were investigated as a function of crosslinking monomer, pentaerythritol tetrakis(3-mercaptopropionate) (PETMP). An example non-stoichiometric system of 15 mol% PETMP thiol groups and an excess of 15 mol% acrylate groups was used to demonstrate the robust nature of the material. The LCE formed an aligned and transparent monodomain when stretched, with a maximum failure strain over 600%. Stretched LCE samples were able to demonstrate both stress-driven thermal actuation when held under a constant bias stress or the shape-memory effect when stretched and unloaded. A permanently programmed monodomain was achieved via a second-stage photopolymerization reaction of the excess acrylate groups when the sample was in the stretched state. LCE samples were photo-cured and programmed at 100%, 200%, 300%, and 400% strain, with all samples demonstrating over 90% shape fixity when unloaded. The magnitude of total stress-free actuation increased from 35% to 115% with increased programming strain. Overall, the two-stage TAMAP methodology is presented as a powerful tool to prepare main-chain LCE systems and explore structure-property-performance relationships in these fascinating stimuli-sensitive materials.

A novel methodology is presented to synthesize and program main-chain liquid-crystalline elastomers using commercially available starting monomers. A wide range of thermomechanical properties was tailored by adjusting the amount of crosslinker, while the actuation performance was dependent on the amount of applied strain during programming.

This study presents a novel two-stage thiol-acrylate Michael addition-photopolymerization (TAMAP) reaction to prepare main-chain liquid-crystalline ...

Orientational Transition in a Liquid Crystal Triggered by the Thermodynamic Growth of Interfacial Wetting Sheets

In liquid crystal (LC) physical chemistry, molecules near the surface play a great role in controlling bulk orientation. Thus far, mainly to achieve desired molecular orientation states in LC displays, the "static" surface property of LCs, so-called surface anchoring, has been intensively studied. As a rule of thumb, once the initial orientation of LCs is "locked" by specific surface treatments, such as rubbing or treatment with a specific alignment layer, it hardly changes with temperature. Here, we present a system exhibiting an orientational transition upon temperature variation, which conflicts with the consensus. Right on the transition, the bulk LC molecules experience the orientational rotation, with 90&#176; between the planar (P) orientation at high temperatures and the vertical (V) orientation at low temperatures in the first-order transitional manner. We have tracked thermodynamic surface anchoring behavior by means of polarizing optical microscopy (POM), dielectric spectroscopy (DS), high-resolution differential scanning calorimetry (HR-DSC), and grazing incidence X-ray diffraction (GI-XRD) and reached a plausible physical explanation: that the transition is triggered by a growth of surface wetting sheets, which impose the V orientation locally against the P orientation in the bulk. This landscape would provide a general link explaining how the equilibrium bulk orientation is affected by surface-localized orientation in many LC systems. In our characterization, POM and DS are advantageous by offering information on the spatial distribution of the orientation of LC molecules. HR-DSC gives information about the precise thermodynamic information on transitions, which cannot be addressed by conventional DSC instruments due to limited resolution. GI-XRD provides information on surface-specific molecular orientation and short-range orderings. The goal of this paper is to present a protocol for preparing a sample that exhibits the transition and to demonstrate how the thermodynamic structural variation, both in the bulk and on surfaces, can be analyzed through the abovementioned methods.

Here, we present a protocol to trigger an orientational transition of a liquid crystal in response to temperature. Methodologies are described for preparing a sample in order to observe the transition and the detailed transitional evolution.

In liquid crystal (LC) physical chemistry, molecules near the surface play a great role in controlling bulk orientation. Thus far, mainly to achieve ...

Engineering

Preparation of Liquid Crystal Networks for Macroscopic Oscillatory Motion Induced by Light

A strategy based on doped liquid crystalline networks is described to create mechanical self-sustained oscillations of plastic films under continuous light irradiation. The photo-excitation of dopants that can quickly dissipate light into heat, coupled with anisotropic thermal expansion and self-shadowing of the film, gives rise to the self-sustained deformation. The oscillations observed are influenced by the dimensions and the modulus of the film, and by the directionality and intensity of the light. The system developed offers applications in energy conversion and harvesting for soft-robotics and automated systems.
The general method described here consists of creating free-standing liquid crystalline films and characterizing the mechanical and thermal effects observed. The molecular alignment is achieved using alignment layers (rubbed polyimide), commonly used in the display manufacturing industry. To obtain actuators with large deformation, the mesogens are aligned and polymerized in a splay/bend configuration, i.e., with the director of the liquid crystals (LCs) going gradually from planar to homeotropic through the film thickness. Upon irradiation, the mechanical and thermal oscillations obtained are monitored with a high-speed camera. The results are further quantified by image analysis using an image processing program.

The goal of the protocol is to create liquid crystalline polymer films that can mechanically oscillate under continuous light irradiation. We describe in great detail the conception of free-standing films, from the liquid crystal alignment method to the photo-actuation. The experimental protocol applied to prepare this material is broadly applicable.

A strategy based on doped liquid crystalline networks is described to create mechanical self-sustained oscillations of plastic films under continuous ...

Novel Techniques for Observing Structural Dynamics of Photoresponsive Liquid Crystals

We discuss in this article the experimental measurements of the molecules in liquid crystal (LC) phase using the time-resolved infrared (IR) vibrational spectroscopy and time-resolved electron diffraction. Liquid crystal phase is an important state of matter that exists between the solid and liquid phases and it is common in natural systems as well as in organic electronics. Liquid crystals are orientationally ordered but loosely packed, and therefore, the internal conformations and alignments of the molecular components of LCs can be modified by external stimuli. Although advanced time-resolved diffraction techniques have revealed picosecond-scale molecular dynamics of single crystals and polycrystals, direct observations of packing structures and ultrafast dynamics of soft materials have been hampered by blurry diffraction patterns. Here, we report time-resolved IR vibrational spectroscopy and electron diffractometry to acquire ultrafast snapshots of a columnar LC material bearing a photoactive core moiety. Differential-detection analyses of the combination of time-resolved IR vibrational spectroscopy and electron diffraction are powerful tools for characterizing structures and photoinduced dynamics of soft materials.

Here, we present the protocols of differential-detection analyses of time-resolved infrared vibrational spectroscopy and electron diffraction which enable observations of the deformations of local structures around photoexcited molecules in a columnar liquid crystal, giving an atomic perspective on the relationship between the structure and the dynamics of this photoactive material.

We discuss in this article the experimental measurements of the molecules in liquid crystal (LC) phase using the time-resolved infrared (IR) vibrational ...

Microfluidic Preparation of Liquid Crystalline Elastomer Actuators

This paper focuses on the microfluidic process (and its parameters) to prepare actuating particles from liquid crystalline elastomers. The preparation usually consists in the formation of droplets containing low molar mass liquid crystals at elevated temperatures. Subsequently, these particle precursors are oriented in the flow field of the capillary and solidified by a crosslinking polymerization, which produces the final actuating particles. The optimization of the process is necessary to obtain the actuating particles and the proper variation of the process parameters (temperature and flow rate) and allows variations of size and shape (from oblate to strongly prolate morphologies) as well as the magnitude of actuation. In addition, it is possible to vary the type of actuation from elongation to contraction depending on the director profile induced to the droplets during the flow in the capillary, which again depends on the microfluidic process and its parameters. Furthermore, particles of more complex shapes, like core-shell structures or Janus particles, can be prepared by adjusting the setup. By the variation of the chemical structure and the mode of crosslinking (solidification) of the liquid crystalline elastomer, it is also possible to prepare actuating particles triggered by heat or UV-vis irradiation.

This article describes the microfluidic process and parameters to prepare actuating particles from liquid crystalline elastomers. This process allows the preparation of actuating particles and the variation of their size and shape (from oblate to strongly prolate, core-shell, and Janus morphologies) as well as the magnitude of actuation.

This paper focuses on the microfluidic process (and its parameters) to prepare actuating particles from liquid crystalline elastomers. The preparation ...

Measuring Magnetically-Tuned Ferroelectric Polarization in Liquid Crystals

Materials showing coupling phenomena between magnetism and (ferro)electricity, i.e., magnetoelectric effects, have attracted a great deal of attention due to their potential applications for future device technologies such as sensors and storage. However, conventional approaches, which usually utilize materials containing magnetic metal ions (or radicals), have a major problem: only a few materials have been found to show the coupling phenomena at room temperature. Recently, we proposed a new approach to achieve room-temperature magnetoelectrics. In contrast to conventional approaches, our alternative proposal focuses on a completely different material, a &#34;liquid crystal&#34;, free from magnetic metal ions. In such liquid crystals, a magnetic field can be utilized to control the orientational state of constituent molecules and the corresponding electric polarization through magnetic anisotropy of the molecules; it is an unprecedented mechanism of the magnetoelectric effect. In this context, this paper provides a protocol to measure ferroelectric properties induced by a magnetic field, that is, the direct magnetoelectric effect, in a liquid crystal. With the method detailed here, we successfully detected magnetically-tuned electric polarization in the chiral smectic C phase of a liquid crystal at room temperature. Taken together with the flexibility of constituent molecules, which directly affects the magnetoelectric responses, the introduced method will serve to allow liquid crystal cells to acquire more functions as room-temperature magnetoelectrics and associated optical materials.

In this report, we present a protocol to examine direct magnetoelectric effects, i.e., induction of ferroelectric polarization by applying magnetic fields, in liquid crystals. This protocol provides a unique approach, supported by the softness of liquid crystals, to achieve room-temperature magnetoelectrics.

Materials showing coupling phenomena between magnetism and (ferro)electricity, i.e., magnetoelectric effects, have attracted a great deal of attention due ...

An Electrochemical Cholesteric Liquid Crystalline Device for Quick and Low-Voltage Color Modulation

We demonstrate a method for fabricating a prototype reflective display device that contains cholesteric liquid crystal (LC) as an active component. The cholesteric LC is composed of a nematic LC 4&#39;-pentyloxy-4-cyanobiphenyl (5OCB), redox-responsive chiral dopant (FcD), and a supporting electrolyte 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (EMIm-OTf). The most important component is FcD. This molecule changes its helical twisting power (HTP) value in response to redox reactions. Therefore, in situ electrochemical redox reactions in the LC mixture allow for the device to change its reflection color in response to electrical stimuli. The LC mixture was introduced, by a capillary action, into a sandwich-type ITO glass cell comprising two glass slides with patterned indium tin oxide (ITO) electrodes, one of which was coated with poly(3,4-ethylenedioxythiophene)-co-poly(ethylene glycol) doped with perchlorate (PEDOT+). Upon application of +1.5 V, the reflection color of the device changed from blue (467 nm) to green (485 nm) in 0.4 s. Subsequent application of 0 V made the device recover the original blue color in 2.7 s. This device is characterized by its fastest electrical response and lowest operating voltage among any previously reported cholesteric LC device. This device could pave the way for the development of next generation reflective displays with low energy consumption rates.

A protocol for the fabrication of a reflective cholesteric liquid crystalline display device containing a redox-responsive chiral dopant allowing quick and low-voltage operation is presented.

We demonstrate a method for fabricating a prototype reflective display device that contains cholesteric liquid crystal (LC) as an active component. The ...

Preparation of Monodomain Liquid Crystal Elastomers and Liquid Crystal Elastomer Nanocomposites

LCEs are shape-responsive materials with fully reversible shape change and potential applications in medicine, tissue engineering, artificial muscles, and as soft robots. Here, we demonstrate the preparation of shape-responsive liquid crystal elastomers (LCEs) and LCE nanocomposites along with characterization of their shape-responsiveness, mechanical properties, and microstructure. Two types of LCEs &#8212; polysiloxane-based and epoxy-based &#8212; are synthesized, aligned, and characterized. Polysiloxane-based LCEs are prepared through two crosslinking steps, the second under an applied load, resulting in monodomain LCEs. Polysiloxane LCE nanocomposites are prepared through the addition of conductive carbon black nanoparticles, both throughout the bulk of the LCE and to the LCE surface. Epoxy-based LCEs are prepared through a reversible esterification reaction. Epoxy-based LCEs are aligned through the application of a uniaxial load at elevated (160 &#176;C) temperatures. Aligned LCEs and LCE nanocomposites are characterized with respect to reversible strain, mechanical stiffness, and liquid crystal ordering using a combination of imaging, two-dimensional X-ray diffraction measurements, differential scanning calorimetry, and dynamic mechanical analysis. LCEs and LCE nanocomposites can be stimulated with heat and/or electrical potential to controllably generate strains in cell culture media, and we demonstrate the application of LCEs as shape-responsive substrates for cell culture using a custom-made apparatus.

We demonstrate the preparation of siloxane-based and epoxy-based liquid crystal elastomers (LCEs) and LCE nanocomposites. The LCEs are characterized with respect to reversible strain, liquid crystal ordering, and stiffness. As a potential application, we demonstrate their use as shape-responsive substrates in a custom device for active cell culture.

LCEs are shape-responsive materials with fully reversible shape change and potential applications in medicine, tissue engineering, artificial muscles, and ...

High-Contrast and Fast Photorheological Switching of a Twist-Bend Nematic Liquid Crystal

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