Polymerized liquid crystals as actuators and sensors

Because of their broad range of properties, polymeric materials play an important role in our society. In most cases, however, these materials are static and their functional properties do not change. For a number of applications, it would be useful if these properties could be switched autonomously in response to environmental changes or via the application of external fields, depending on the requirements of the user. The fabrication of functional hierarchical materials that undergo changes to their structure and properties in response to external stimuli represents a grand materials challenge.

Nature provides many examples of complex shape deformations and color changes that rely on the organization of components on a molecular, supramolecular, and mesoscopic scale. These examples range from the opening of pine cones to the cellular ensembles that form muscles. Hierarchical structuring such as this has inspired the creation of novel polymers that undergo a predetermined deformation or color change upon exposure to a stimulus (e.g., temperature, humidity, light, or pH) without the need for multiple-component assembly.¹ These polymers are of great interest for the development of low-cost future technologies such as actuators for smart surfaces and battery-free sensors for use in healthcare and wastewater management.² To achieve appropriate sensitivity and selectivity, devices based on hierarchically ordered polymers must contain a well-defined trigger that induces a structural change in response to the stimulus.³

In our work, we exploit the self-assembly property of liquid crystals (LCs) to create well-defined, hierarchically ordered responsive materials. LCs are a state of matter with properties that lie between those of liquids and solids. The crystals exhibit different mesophases (e.g., smectic and nematic phases) depending on the degree of lateral orientation and the rotational order of the molecules: see Figure 1(a). In the nematic phase, the molecules are directionally ordered. Further organization of the molecules into a helical, rotational arrangement results in a chiral-nematic phase. The orientation of LCs can be manipulated via a variety of techniques, such as the use of alignment layers: see Figure 1(b). In cases where the LC monomers have photopolymerizable groups, the arrangement can be fixed by exposure to light. LCs can be further arranged by using top-down structuring techniques such as photolithography, inkjet printing, and photoembossing: see Figure 1(c).

Figure 1. (a) Schematic of (i) nematic, (ii) chiral-nematic, and (iii) smectic liquid crystalline phases. (b) Example of bottom-up structuring. Monodomains are produced via the use of alignment layers and reactive mesogens (RMs) are polymerized with light (hν) to construct a liquid-crystal (LC) network. (c) Examples of LC networks fabricated by top-down structuring using (i) photolithography and (ii) inkjet printing.¹

Using these techniques, we have created responsive LC networks with functional properties that are defined by our choice of order and structure. The response in these anisotropic materials is caused by a small reversible change to the local molecular order. This change leads to shrinkage in the direction along the long axis of the LC molecules and expansion in all other directions.¹ Disorder such as this can be induced by trigger molecules that undergo a structural change when exposed to a stimulus (e.g., azobenzene for light-responsive materials or hydrogen-bonding moieties for humidity responsive systems).⁴ We are therefore able to modify the properties of an LC network for a specific application by altering its shape and by varying the LC mixture.

We have prepared flat polymer films that fold into cones, spirals, and accordions as a result of their complex 3D hierarchical order. To induce the accordion-fold actuation shown in Figure 2(a), we used a photo-alignment method in combination with a mask exposure to pattern the nematic director in the polymer actuator in discrete stripes, producing a twisted nematic LC director of alternating orientation. To enhance the folding process, the twist in all stripes has uniform handedness. We also added an IR absorber to the nematic LC mixture to improve the response of the actuator to IR heating. When heated, the material in each stripe expands along the long axis on one side of the film and contracts in the same direction on the other side, leading to the formation of accordion-style folds.

Figure 2. (a) Deformation of an LC polymer film with a 3D hierarchical order via irradiation with infrared light.⁶(b) Color change of a green-reflecting chiral-nematic LC coating after exposure to calcium.⁵

Chiral-nematic LC networks are also able to reflect light as a result of self-organizing molecular helices within the material.⁷ Adsorption of an analyte by the photonic material causes the film to swell or shrink, resulting in a change to the reflected color. Sensors constructed in this way can achieve a selective high optical response without the use of specific binding moieties. Responsive optical materials such as these therefore hold promise for the fabrication of cheap, easy-to-use, battery-free sensors for use in clinical diagnostics, and are also appealing for use in environmental monitoring and wastewater management. We have fabricated an optical calcium sensor based on a chiral-nematic LC polymer network containing benzoate metal-binding ligands. We treated this polymer with potassium hydroxide to yield a responsive green-reflecting film. Our sensor shows a remarkably selective response to calcium with a color change from green to blue that, as shown in Figure 2(b), is visible to the naked eye. This change occurs as a result of the LC order in the photonic polymer. The order leads to a preorganized optimal binding geometry, and the dehydration properties of the calcium complex result in a high optical response. Our calcium sensor could be used as a medical test strip for the qualitative detection of calcium in serum. Using spectroscopy, we were able to distinguish the optical responses to normal serum and samples mimicking hypocalcemia and hypercalcemia.

We have developed a variety of stimulus-responsive materials based on polymerized LC monomers. These materials show great promise for application in the fields of sustainable energy, health monitoring, and water and light management.³ We are currently adjusting the multicomponent LC mixtures to tune the responsive functional properties of these materials. Specifically, we intend to create actuators that can perform a significant amount of work. We also plan to develop responsive IR reflectors that reject IR light during peak periods of illumination but permit its passage when light conditions are lower without affecting the response to light in the visible region. We hope that these devices will also provide a means of saving energy by reducing the amount of heating or cooling required in buildings and cars.

The author would like to acknowledge the many discussions with, and contributions of, all former and current colleagues. A special word of thanks to colleagues Dick Broer, Michael Debije, and Cees Bastiaansen. Special thanks also to Laurens de Haan and Monali Moirangthem, who fabricated the accordion actuator and calcium sensor, respectively. Our research has been supported by the Eindhoven University of Technology, the Dutch Polymer Institute, and the Netherlands Organization for Chemical Research (CW) with financial aid from the Netherlands Organization for Scientific Research (NWO) and the Technologiestichting STW.