Name: orientational transition in a liquid crystal triggered by the thermodynamic growth of interfacial wetting sheets
Uploaded: 2017-05-15T13:00:00
Description: Watch this Scientific Journal Video about Orientational Transition in a Liquid Crystal Triggered by the Thermodynamic Growth of Interfacial Wetting Sheets at JoVE.com

This work was supported by JSPS KAKENHI grant Number 16H06037. We sincerely thank Dr. Yuji Sasaki in Hokkaido University for technical assistance for HR-DSC.

1. Preparation of the Liquid Crystal Alignment Layer of a Perfluoropolymer onto Glass Substrates
<ol>
	<li>Preparation of the perfluoropolymer solution
	<ol>
		<li>Prepare 1 mL of the perfluoropolymer solution by dissolving a perfluoropolymer solution (9 wt% polymer) in a commercial solvent at a ratio of 1:2; this ensures uniform films 0.5-1 &#181;m thick to be spin-coated. 
		NOTE: Please see the Materials List for the solution and solvent used.</li>
	</ol>
	</li>
	<li>Coating of the perf...

<ol>
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The 10x POM images taken using a 5-&#181;m LC cell (Figures 1a and b) clearly show that the orientational state of the bulk LC molecules transits between the P and the V orientations upon temperature variation in a first-order manner. This is marked by the domain nucleation and growth processes, with a new orientation differing from the initial orientation by 90&#176;. The transition temperatures upon cooling and heating are 321.5 K and 325.3 K, respectively. Since CCN47 has a birefringe...

In recent years, there has been growing interest in learning how dynamic molecular features and structures of surface molecules in response to external stimuli might affect the bulk orientation of materials in LC states. One example is to use LC biosensors as a new application of LCs1,2. To quantify how many target bio-species are detected, it is important to know how the interfacial LCs that contact adhering target molecules change and evolve, while also detecting and how they transfer/translate their properties into the bulk.
Using models to pursue these answers, we started with systems that have their surface molecular orientation and short-range orderings varying thermodynamically. These systems allow us to correlate the changes in surface orientation and orderings with the resulting bulk orientation in a systematic way. Recently, we found several LC systems that exhibit orientational transitions, where a spontaneous bulk molecular orientation changes with temperature. In principle, orientational transitions can be categorized into either quasi-second-order3,4 or quasi-first-order transition5,6,7,8. The former is accompanied by a continuous bulk molecular reorientation upon changes in temperature, whereas the latter demonstrates a discontinuous one. In this article, we describe an orientational transition in the quasi-first-order manner between the P and the V orientational states. It proceeds in the single nematic (N) phase by changing the temperature. Details will be provided in the Representative Results and the Discussion.
Since orientational change in the bulk should be governed by a change in the surface molecular orientation and short-range orderings, it is apparent that this system can potentially offer insights into how the thermodynamic variation in surface molecular orientation and short-range orderings affects the bulk orientation. In this article, with the goal of understanding the abovementioned issues, we tackled three problems using four complementary methods (i.e., POM, DS, HR-DSC, and GI-XRD): (1) What does the orientational transition look like? (2) Is the orientational transition thermally detectable? (3) Why and how does the orientational transition occur?

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>CYTOP</td>
			<td>Asahi Glass Co. Ltd.</td>
			<td>CTX-809A</td>
			<td></td>
		</tr>
		<tr>
			<td>Solvent for CYTOP</td>
			<td>Asahi Glass Co. Ltd.</td>
			<td>CT-180 Sol.</td>
			<td></td>
		</tr>
		<tr>
			<td>Alkaline detergent</td>
			<td>Merck KGaA</td>
			<td>Extran MA01</td>
			<td></td>
		</tr>
		<tr>
			<td>NOA61</td>
			<td>Norland Products, Inc.</td>
			<td>#37-322</td>
			<td>Purchasable from Edmund Optics</td>
		</tr>
		<tr>
			<td>AL1254</td>
			<td>JSR Corporation</td>
			<td></td>
			<td>Planar alignment material in self-made cells</td>
		</tr>
		<tr>
			<td>4&#8217;-butyl-4-heptyl-bicyclohexyl-4-carbonitrile</td>
			<td>Nematel GmbH &#38; Co. KG</td>
			<td></td>
			<td>Custom-made</td>
		</tr>
		<tr>
			<td>UV-O3 cleaner</td>
			<td>Technovision Inc.</td>
			<td>UV-208</td>
			<td></td>
		</tr>
		<tr>
			<td>Hot-stage system</td>
			<td>Mettler Toledo</td>
			<td>HS82</td>
			<td></td>
		</tr>
		<tr>
			<td>High-Definition Color Camera Head</td>
			<td>Nikon</td>
			<td>DS-Fi1</td>
			<td></td>
		</tr>
		<tr>
			<td>Impedance/gain-phase analyzer</td>
			<td>Solartron Analytical</td>
			<td>1260</td>
			<td></td>
		</tr>
		<tr>
			<td>Indium Tin Oxide (ITO)-coated substrate</td>
			<td>GEOMATEC Co. Ltd.</td>
			<td></td>
			<td>Custom-made</td>
		</tr>
	</tbody>
</table>

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.

POM images, DS data, HR-DSC data, and GI-XRD patterns were collected during temperature variation, especially in the vicinity of the orientational transition upon both cooling and heating.
Figure 1 represents the evolution of the texture made by POM and DS measurements during the POM observation of the orientational transition from the P (V) to the V (P) orientational state during cooling (heating)....

Watch this Scientific Journal Video about Orientational Transition in a Liquid Crystal Triggered by the Thermodynamic Growth of Interfacial Wetting Sheets at JoVE.com

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

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 ...

The overall goal of this procedure is to trigger an orientational transition of a liquid crystal in response to temperature. And to characterize the transition. This method can help answer key questions in the liquid crystal field, such as thermodynamic surface anchoring behaviors.And the advantage of this protocol is that the orientational transition can be clearly visualized in this continuous manner. And if the involved physical properties can be systematically probed. To begin this procedure, prepare one milliliter of perfluoropolymer solution by dissolving the desired perfluoropolymer in a commercial solvent at a ratio of one to two to ensure uniform films 0.5 to 1 micrometer thick for spin coating.Next, wash glass substrates by sonication at 42 kilohertz in an alkaline detergent. Then, repeatedly rinse each substrate with distilled water. After rinsing and drying the substrates, subject them to UV ozone cleaner for 10 minutes.Following this, drip 20 microliters of the perfluoropolymer solution onto each cleaned glass substrate. Immediately spin coat the solution at 3, 500 rpm and room temperature for 70 seconds. Then, bake the film at 80 degrees Celsius for 60 minutes to remove the solvent, and at 200 degrees Celsius for 60 minutes for curing.Apply a photo-curable resin pre-mixed with micrometer sized glass particles to one of the film-coated glass substrates. Then, place a second film coated glass substrate on top of the first substrate And use an LED lamp with a wavelength of 365 nanometers to glue the two substrates together. Next introduce 0.2 to 10 microliters of CCN47 to each of the prepared LC cells using a spatula under capillary force at a temperature higher than the isotropic nematic phase transition temperature.Now, observe the LC cells under polarizing optical microscopy with four to 100X objective lenses in conjunction with a hot stage to control the sample temperature with plus or minus 0.1 Kelvin accuracy. Using a digital color camera, record the textures in more than five frames, at even intervals per Kelvin, both upon cooling and heating in the range of 291 to 343 Kelvin. For dielectric spectroscopy, prepare an LC cell with ITO electrodes by soldering lead wire to each substrate.To measure the capacitance of the LC cells, wire the cells to the terminals of a commercial impedance gain phase analyzer. Then, measure the time dependence of capacitance of the LC cells every five minutes with the analyzer. For HR-DSC analysis place the LC cells into a homemade HR-DSC to be examined.Perform measurements with scan rates of 0.05 to 0.10 Kelvin per minute to enhance the minimum temperature resolving power. The evolution of the texture made by polarizing optical microscopy and dielectric spectroscopy measurements during the orientational transition from the planar to vertical orientational state during cooling and heating is depicted here. Upon cooling, the planar orientation is right below the isotropic nematic transition temperature.With the appearance of two and four brush Schlieren textures. By annealing or further cooling, the field of vision becomes completely dark, suggesting the completion of the orientational transition. HR-DSC data representing the heat flow through the sample as a function of temperature and time is shown here.The variation of heat flow with time data was used to analyze the Avrami exponent after the orientational transition. Grazing incidents x-ray diffraction patterns in droplet geometry and in C2 LC cell geometry at various temperatures. Demonstrate short range ordering of quasi-smectic a-wetting sheets.With layer structures formed near the surface. Even in the temperature range of the planar orientational states, quasi-smectic a-wetting sheets persist, indicating that the surface orientational condition is frustrated. The hysteresis range is consistent with the hysteresis range confirmed by polarizing optical microscopy and dielectric spectroscopy, which suggests that the orientational transition is triggered by quasi-smectic a-wetting sheet growth.So once mastered, the observation of the orientational transition can be done in four hours if the preparation procedures are performed properly. Following this procedure, other methods like fluorescent confocal microscopy and non-linear optical analysis can be performed in order to answer additional questions, like the special distribution of orientations of the liquid crystal director. After its development, this technique paved the way for researchers in the field of surface science of liquid crystals to explore links between the microscopic structures near surfaces and the microscopic structures and properties.After watching this video you should have a good understanding of how to prepare a sample to observe the orientational transition and how to characterize the important physical properties.

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