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 s....

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 a....

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