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

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) ....

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

List of materials used in this article

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