Photomechanical effects in liquid crystalline polymer networks and elastomers

Abstract

Liquid crystals are widely employed as stimuliresponsive materials. Liquid crystallinity can be retained in polymeric form. Photoinduced mechanical effects in liquid crystalline polymer networks and elastomers have been a topic of considerable recent research. This review details the historical underpinnings and recent advances in the synthesis and the corresponding photomechanical response of these materials. In nearly all cases, the conversion of light into mechanical work has employed azobenzene as either a guest additive or covalently attached to the network. V C 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018, 56, 695-705 KEYWORDS: azobenzene; liquid crystal; liquid crystal elastomer; liquid crystal polymer; photochemistry; shape change; shape programing INTRODUCTION Liquid crystalline materials are synonymous with their use in the displays that surround us in our daily lives. The widespread application of liquid crystals in display applications is enabled by their unique assimilation of properties: the long range fluidity of liquids combined with the anisotropic optical properties of crystalline solids. In displays, the relationship of orientation of the liquid crystal director and birefringence has been leveraged by concurrent advances in thin film transistors to locally control the transmission and spectral output (color) of light visualized in a pixel. The display community, in response to seemingly insatiable consumer demand, is currently focused on improvements in resolution and speed as well as realization of flexible form factors.

Light‐Responsive Shape‐Changing Polymers

“…They can be referred to by several names, the most frequent of them being liquid crystal polymers (LCPs), liquid crystal polymer networks (LCNs), or liquid crystal elastomers (LCEs). The differences between these classes of materials are highlighted in Figure . LCPs are generally described as linear main‐chain or comb‐like polymers that are typically un‐crosslinked and can self‐organize to form LC phases through stiff rod‐like or disk‐like molecular structures and intramolecular interactions (hydrogen bonding, dipole–dipole, and aromatic interactions) .…”

“…On the other hand, LCNs maintain some of the high‐performance properties of LCPs but possess a crosslinked network architecture . LCNs are almost exclusively prepared from monomer precursors that are liquid crystalline . They can be prepared to be glassy (glass transition temperature, T g , is higher than ambient temperature) or elastomeric ( T g is lower than ambient temperature), where elastomeric liquid crystalline polymer networks are typically referred to as LCEs (Figure ).…”

Photoreversible Soft Azo Dye Materials: Toward Optical Control of Bio‐Interfaces

“…This effect is especially appealing for interfacing with neural cells, many of which possess an inherent mechanotaxis trigger response to minute physical stimulation. Various light‐induced changes in shape have been achieved in shape‐memory polymers, carbon nanotube‐containing composites and bilayers, and crosslinked polymers and elastomers incorporating photochromic molecules, but herein we focus on azobenzene‐driven actuators, that can produce motion reversibly on a size scale and timescale of interest to living cells. Photomechanical effects in azobenzene‐containing materials were first observed more than 50 years ago in azo‐dyed textile fibers curling slightly while drying in the sun, but it was not until 20 years later that much notice was taken of this curious effect.…”

“…Azobenzene‐conjugated polymers have been also used as smart coatings for inducing biofilm disruption . As described in the previous section, liquid crystalline elastomers have been broadly implemented as actuators, soft microrobots, and sensors . Phase transition temperatures in LCEs are typically much higher than physiological temperature; so their implementation as cell culturing substrates has been so far modest and restricted to nondynamic applications.…”

“…From the biomaterial development perspective, some of the most powerful properties of azo photoswitches emerge when they are combined into polymers or other host material matrices, to fine‐tune mechanical and surface biocompatibility. In particular, photo‐orientation and photomechanical effects in azobenzene‐based soft material systems ranging from photoinduced surface patterns in thin films of amorphous azo polymers to photoactuation in liquid crystalline elastomers (LCEs), and photoinduced mechanical changes in 3D matrices, offer an attractive potential for biomaterial development …”