Hydrogen-Bonded Supramolecular Liquid Crystal Polymers: Smart Materials with Stimuli-Responsive, Self-Healing, and Recyclable Properties.

Hydrogen-bonded liquid crystalline polymers have emerged as promising "smart" supramolecular functional materials with stimuli-responsive, self-healing, and recyclable properties. The hydrogen bonds can either be used as chemically responsive (i.e., pH-responsive) or as dynamic structural (i.e., temperature-responsive) moieties. Responsiveness can be manifested as changes in shape, color, or porosity and as selective binding. The liquid crystalline self-organization gives the materials their unique responsive nanostructures. Typically, the materials used for actuators or optical materials are constructed using linear calamitic (rod-shaped) hydrogen-bonded complexes, while nanoporous materials are constructed from either calamitic or discotic (disk-shaped) complexes. The dynamic structural character of the hydrogen bond moieties can be used to construct self-healing and recyclable supramolecular materials. In this review, recent findings are summarized, and potential future applications are discussed.

Fully (Re)configurable Interactive Material through a Switchable Photothermal Charge Transfer Complex Gated by a Supramolecular Liquid Crystal Elastomer Actuator.

Charge transfer complexes (CTCs) based on self-assembled donor and acceptor molecules allow light absorption of significantly redshifted wavelengths to either the donor or acceptor. In this work, we demonstrate a CTC embedded in a hydrogen-bonded liquid crystal elastomer (LCE), which in itself is fully reformable and reprocessable. The LCE host acts as a gate, directing the self-assembly of the CTC. When hydrogen bonding is present, the CTC behaves as a near-infrared (NIR) dye allowing photothermal actuation of the LCE. The CTC can be disassembled in specific regions of the LCE film by disrupting the hydrogen bond interactions, allowing selective NIR heating and localized actuation of the films. The metastable non-CTC state may persist for weeks or can be recovered on demand by heat treatment. Besides the CTC variability, the capability of completely reforming the shape, color, and actuation mode of the LCE provides an interactive material with unprecedented application versatility.

One-Pot Synthesis of Melt-Processable Supramolecular Soft Actuators.

The application of reprocessable and reprogrammable soft actuators is limited by the synthetic strategies, 3D-shaping capabilities, and small deformations. In this work, melt-processable supramolecular soft actuators based on segmented copolymers containing thiourethane and liquid crystal segments have been prepared via sequential thiol addition reactions in a one-pot approach using commercially available building blocks. The actuators demonstrated immediate, reversible response and weightlifting capabilities with large deformations up to 32 %. Through exploiting the supramolecular cross-links, the material could be recycled and reprogrammed into 3D actuators and welded into an actuator assembly with different deformation modes. Our work offers a one-pot synthesis and straightforward melt-processable approach to prepare supramolecular soft actuators with large deformations that can be reprocessed and reprogrammed into arbitrary 3D shapes.

Pyranine Based Ion-Paired Complex as a Mechanophore in Polyurethanes.

A new mechanophore for polyurethane thermoplastic elastomers based on ion-paired complexes is developed. 8-(2-hydroxyethoxy)pyrene-1,3,6-trisulfonate (HEPTS) is incorporated into polyurethanes as an end-capper and aggregates in apolar media. Aggregation of the ionic HEPTS end groups in solution depends on concentration solvent polarity. The addition of dimethylformamide to a tetrahydrofuran solution of the polymer results in the dissociation of the aggregates and a significant shift in fluorescence emission from yellow to blue. The same shift in fluorescence emission is induced by stretching the solid polymer at strains larger than 1 and stresses above 7.5 MPa, with a clear increase above 12.5 MPa. Strain induced dissociation of HEPTS aggregates not attached to the polymer chain leads to fluorescence changes that are much less reproducible.

Vacuum Thermoforming of Optically Switchable Liquid Crystalline Elastomer Spherical Actuators.

Liquid crystal elastomer (LCE) actuators are generally limited in shape, size, and quantity by the need for aligning via stretching and fixing via photopolymerizing. A thermoplastic LCE is presented that may be vacuum thermoformed into centimeter-sized hemispheres. The scalable industrial process induces LCE alignment without requiring postfixing. The hemispheres display remarkable properties, actuating with strains around 20% and transitioning from opaque and scattering to highly translucent upon heating: both the physical and optical effects are fully reversible. Simulations reveal the LCE experiences biaxial strains during processing, the magnitude varying as a function of location on the hemisphere: the resulting alignment describing the hemisphere actuation well. The thermoplastic LCE hemispheres may be combined to form complete spheres by simply heating the joint. The hemisphere can also be physically deformed into a ball which can then unfold back into the hemisphere again. By doping the hemispheres with photoswitches, fluorescent or photothermal dyes, devices are formed for light collection and redistribution, addressable water containers that may pour at will, and light-responsive surfing devices. This is the first example of an LCE amenable to high-volume industrial vacuum thermoforming which may lead to intricate 3D-shaped actuators with new functional properties.

Direct Ink Writing of Recyclable Supramolecular Soft Actuators.

Direct ink writing (DIW) of liquid crystal elastomers (LCEs) has rapidly paved its way into the field of soft actuators and other stimuli-responsive devices. However, currently used LCE systems for DIW require postprinting (photo)polymerization, thereby forming a covalent network, making the process time-consuming and the material nonrecyclable. In this work, a DIW approach is developed for printing a supramolecular poly(thio)urethane LCE to overcome these drawbacks of permanent cross-linking. The thermo-reversible nature of the supramolecular cross-links enables the interplay between melt-processable behavior required for extrusion and formation of the network to fix the alignment. After printing, the actuators demonstrated a reversible contraction of 12.7% or bending and curling motions when printed on a passive substrate. The thermoplastic ink enables recyclability, as shown by cutting and printing the actuators five times. However, the actuation performance diminishes. This work highlights the potential of supramolecular LCE inks for DIW soft circular actuators and other devices.

Patterned Actuators via Direct Ink Writing of Liquid Crystals.

Soft actuators allowing multifunctional, multishape deformations based on single polymer films or bilayers remain challenging to produce. In this contribution, direct ink writing is used for generating patterned actuators, which are in between single- and bilayer films, with multifunctionality and a plurality of possible shape changes in a single object. The key is to use the controlled deposition of a light-responsive liquid crystal ink with direct ink writing to partially cover a foil at strategic locations. We found patterned films with 40% coverage of the passive substrate by an active material outperformed "standard" fully covered bilayers. By patterning the film as two stripes, a range of motions, including left- and right-handed twisting and bending in orthogonal directions, could be controllably induced in the same actuator. The partial coverage also left space for applying liquid crystal inks with other functionalities, exemplified by fabricating a light-responsive green reflective actuator whose reflection can be switched "on" and "off". The results presented here serve as a toolbox for the design and fabrication of patterned actuators with dramatically expanded shape deformation and functionality capabilities.