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

Summary

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.

Abstract

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° 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.

Introduction

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 LCs^1,2. 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-order^3,4 or quasi-first-order transition^5,6,7,8. 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?

Protocol

1. Preparation of the Liquid Crystal Alignment Layer of a Perfluoropolymer onto Glass Substrates

1. Preparation of the perfluoropolymer solution
   1. 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 µm thick to be spin-coated.
      NOTE: Please see the Materials List for the solution and solvent used.
2. Coating of the perfluoropolymer onto clean glass substrates
   1. Wash the glass substrates (typical size: 1 cm x 1 cm) by sonication at 38 or 42 kHz in an alkaline detergent. Repeatedly rinse them with distilled water. Typically, rinse more than 10 times, with 5 min of sonication each time.
   2. Subject the substrates to UV-O₃ cleaner for 10 min.
   3. Drip 20 µL of the solution from step 1.1 onto the cleaned glass substrates. Immediately spin-coat the solution at 3,500 rpm and room temperature for 70 s. Bake the film at 80 °C for 60 min to remove the solvent and at 200 °C for 60 min for curing.

2. Preparation of LC Cells

1. Glue two glass substrates coated with film together using a photo-curable resin and an LED lamp with a wavelength of 365 nm (1.1 W/cm²). Adjust the thickness of the gap between the two substrates to within the range of 2-100 µm by using micrometer-size glass particles or polyethylene naphthalate films.
2. Introduce an LC material, 4'-butyl-4-heptyl-bicyclohexyl-4-carbonitrile (CCN47; 0.2-10 µL)⁹ to the prepared LC cells using a spatula under capillary force at a temperature higher than the isotropic liquid (I)-nematic (N) phase transition temperature.
   NOTE: CCN47 has a negative dielectric anisotropy, and the phase sequence is Cry 298.6 K SmA 301.3 K N 331.3 K I, where Cry and SmA stand for crystal and smectic A phases. Do not introduce CCN47 in the N phase or SmA phase, because flow-induced alignment would be promoted.

3. Sample Characterization

1. Texture observation by polarizing optical microscopy (POM)¹⁰
   1. Observe the LC cells under POM with 4-100X objective lenses in conjunction with a hot stage to control the sample temperature with ± 0.1-K accuracy. Record the textures in more than 5 frames, at even intervals per Kelvin. Use a digital color camera sequentially, both upon cooling and heating in the range of 291-343 K.
2. Dielectric spectroscopy (DS)¹¹
   1. Prepare LC cells, with ITO electrodes – which can have a square or circular shape and can be purchased commercially – on both substrates. Solder a lead wire to each substrate.
      NOTE: Please see the Materials List for the substrates used.
   2. Measure the capacitance or dielectric constant of the LC cells, exactly as used for POM, using a commercial impedance/gain-phase analyzer. Ensure that the state of the samples is equilibrated before each measurement. Measure the time-dependence of capacitance or the dielectric constant of the LC cells by measuring the capacitance of the LC cells manually every 5 min.
   3. Start the DS measurement only if the capacitance or dielectric constant of the LC cells becomes non-time-dependent.
3. High-resolution differential scanning calorimetry (HR-DSC)¹²
   1. Put the LC cells into a home-made HR-DSC to be examined, exactly as in POM (never use DSC pans). Refer to Reference 12 to design and build an HR-DSC and to learn how to use it. Perform measurements with scan rates of 0.05-0.10 K/min to enhance the minimum temperature resolving power.
4. Grazing incident X-ray diffraction (GI-XRD)¹³
   1. Put either the LC cell (used for POM or DSC) or a sample with a 2 to 5 µL droplet of CCN47 onto a coated substrate on the GI-XRD sample stage, which should be equipped with a temperature controller.
   2. Equilibrate the sample for more than 10 min at desired temperatures in the range of 291-343 K, both upon cooling and heating.
   3. Use an incident X-ray beam on the sample, with a minute incident angle around 0.05-0.10°, to extract surface information on molecular orientation and orderings/structures. Swing the incident angle of the X-ray beam to find the optimal incident angle at which the strength of the diffraction is the strongest. Take the measurements at the optimal incident angle.
      NOTE: Keep in mind that GI-XRD makes it possible to probe interfacial peculiarities on the nanometer scale, thus maximizing the signal from thin layers while minimizing the signal from the bulk. Note that normal XRD geometries, other than GI-XRD, are not surface-sensitive methods, since the X-ray radiation beam has a large penetration depth into materials.

Results

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)....

Discussion

The 10x POM images taken using a 5-µm 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°. The transition temperatures upon cooling and heating are 321.5 K and 325.3 K, respectively. Since CCN47 has a birefringence of ~0....

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
CYTOP	Asahi Glass Co. Ltd.	CTX-809A
Solvent for CYTOP	Asahi Glass Co. Ltd.	CT-180 Sol.
Alkaline detergent	Merck KGaA	Extran MA01
NOA61	Norland Products, Inc.	#37-322	Purchasable from Edmund Optics
AL1254	JSR Corporation		Planar alignment material in self-made cells
4’-butyl-4-heptyl-bicyclohexyl-4-carbonitrile	Nematel GmbH & Co. KG		Custom-made
UV-O3 cleaner	Technovision Inc.	UV-208
Hot-stage system	Mettler Toledo	HS82
High-Definition Color Camera Head	Nikon	DS-Fi1
Impedance/gain-phase analyzer	Solartron Analytical	1260
Indium Tin Oxide (ITO)-coated substrate	GEOMATEC Co. Ltd.		Custom-made