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

Summary

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.

Abstract

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’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.

Introduction

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 hydrogels^1,2,3, polymers^{4,5,6,7,8,9,10,11}, liquid crystals [LCs]^{9,10,11,12,13,14,15,16,17}, pH-responsive micelles^{18,19,20,21,22}, 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 reported^{9,10,11,12,13,14,15}.

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.

Protocol

1. Preparation of rubbed surfaces for aligning LC molecules planarly

1. Prepare clean glass substrates.
   1. 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).
   2. Subject the substrates to ultraviolet-ozone (UV-O₃) cleaner (Table of Materials) for more than 10 min.
2. Coat planar alignment layer onto clean glass substrates.
   1. Drip 20 µL 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.
   2. Bake the coated glass substrates at 80 °C for 60 min to remove the solvent and at 180 °C for >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.

2. Preparation of LC cells

1. 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.
2. Place 100 µL of a photoreactive adhesive (Table of Materials) and 0.1 mg of micrometer-sized glass particles (diameter = 5 µm) 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.
3. 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 °C). Transfer the LC material (1-[4-butoxyazobenzene-4’-yloxy]-6-[4-cyanobiphenyl-4’ yl]hexane; CB6OABOBu; 0.2−10.0 µL) 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 °C TB 105.2 °C N 151.7 °C I on heating and I 151.4 °C N 104.5 °C TB 83 °C Cry on cooling. Do not introduce CB6OABOBu into the N phase or TB phase because flow-induced alignment is promoted.

3. Texture characterization by polarizing optical microscopy

1. Observe the LC cells placed on the hot stage to control the sample temperature (40−180 °C) with ± 0.1 K accuracy under a polarizing light microscope (POM, Table of Materials) using 4x−100x objective lenses. Record textures using a digital color camera sequentially during cooling and heating.
2. Use a UV epi-illuminator (Table of Materials) equipped on the POM with a wavelength of 365 nm (50 mW/cm²).

4. Photorheological measurements

1. Prepare of rheological measurements.
   1. 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’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.
   2. Set the temperature of the sample chamber to a value above the I-N phase transition point (>160 °C). Set a gap value for approaching the measuring plate to the base quartz plate to sandwich the sample (typical gap value used = 20 µm). 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 µm 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.
2. Perform rheological measurements.
   1. Irradiate UV light at 365 nm (1−100 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.
   2. 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’s instructions.

5. Photo-differential scanning calorimetry

1. Weigh 10 mg of CB6OABOBu powder sample and load it into a gold differential scanning calorimetry (DSC) pan. Heat the sample to 170 °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.
2. Perform photo-DSC measurements according to the manufacturer’s instructions (Table of Materials). Measure DSC data at a scan of 10 °C/min.
   NOTE: The photo-DSC machine is equipped with a UV light intensity of 50 mW/cm².

6. X-ray diffraction characterization

1. Heat the powder CB6OABOBu sample using the hot stage at 170 °C and suck the sample into an XRD capillary (diameter = 0.5 mm) by capillary force.
2. Attach the capillary to a sample holder equipped with a temperature controller. Set the chamber temperature (60 °C, 70 °C, 80 °C, 90 °C, 100 °C, 110 °C, 120 °C, 130 °C, 140 °C, 150 °C, 160 °C, and 170 °C for each X-ray diffraction measurement).
3. 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 Å.

Results

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 conforma...

Discussion

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 2

Materials

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