The development and integration of stimuli-response in materials is an emerging frontier of research. Reconfiguration of materials function, autonomously triggered by an environmental event, could transform system design, medicine, and human-machine interaction. Our research at CU seeks to harness molecular-level assembly of materials and composites to encode (“pixelate”) them with designed function.
Directed Self-Assembly of Liquid Crystalline Polymer Networks and Elastomers
A focus of our research efforts has been to realize shape reconfiguration in liquid crystalline elastomers. These materials, predicted from the theory of Nobel Laureate Pierre de Gennes (Physics, 1991), are organized polymers. The mechanical properties of these materials are highly directional, like muscle fiber. Considerable research, undertaken since these seminal predictions, have reported the synthesis of these materials, processing, and mechanical response of these materials to stimuli.
We have initiated a paradigm shift in the research of these materials, focused on spatially imprinted arbitrary organization in liquid crystalline elastomers. Adopting methods previously established in directing the molecular-level, self-assembly of liquid crystals in displays, through sustained effort we invented a materials chemistry to prepare liquid crystal elastomers that were amenable to these powerful processing techniques. This work detailed that through rational organization, these materials (of rubber-band consistency) generate nearly 3 J/kg of force output (video, Single Defect). The topological defects in the liquid crystal elastomers dictate this response:
We continue to demonstrate exceptional force outputs, including demonstrations that lift nearly 3,000 times their weight (video, LCE Laminate). The unprecedented power per weight ratio of these material actuators could transform robotics, prosthetics, or enable advanced medical devices. We are very interested in enhancing and optimizing the response of these materials to other stimuli, such as electric fields or light (video, Photoinduced deformation of LCE).
These materials also have considerable potential to transform the manufacture of flexible electronic devices. Design and construction of these devices is founded upon the flexibility of polymeric materials. Enabled by the material and processing advancements of these materials, we initiated an exploration into localizing the mechanical deformation of these engineered materials. Critical to this implementation, liquid crystalline elastomers maintain highly anisotropic mechanical properties and their mechanical deformation is strongly correlated to the orientation of the loading axis to the orientation of the liquid crystalline director. Further, the deformation of the material when the load is applied perpendicular to the liquid crystalline directorate is highly nonlinear. The promise of this approach to enabling the manufacture of robust, flexible devices was demonstrably confirmed. Here, we realized the synthesis and preparation of liquid crystalline elastomers with the organization in what is referred to as the homeotropic orientation (e.g. orthogonal to the sample surface). Accordingly, the nonlinear response of these materials becomes omnidirectional to in-plane loads. Employing advanced manufacturing techniques, we have prepared flexible devices and confirmed the hypothesis that marrying local control of deformation to the device design can considerably improve device robustness.
This guiding vision for our research has also allowed us to make contributions in optics. Modern optical systems utilize static optical elements to focus, reflect, absorb, or polarize light. We have championed the exploration of reconfigurable optical elements and are focused on realizing a new generation of optical elements. These efforts have largely focused on the cholesteric liquid crystalline phase, which inherently self-organizes into a periodic structure and accordingly, exhibits a selective reflection. Considerable research has examined the cholesteric phase, also seeking reconfiguration of the selective reflection. No prior report details direct control of the periodic organization of these materials in the planar orientation with electric fields. In a series of publications, we have detailed a new method to enable electric field control of the organization of these materials. Our approach incorporates small concentrations of crosslinked polymer networks to enforce “structural chirality” onto the liquid crystal host (as opposed to chirality transfer via molecular interactions). By mechanically distorting the polymer network via ion-mediated interactions, the periodicity of the organization of these materials is directly regulated by electric field. This manifests, for example, as the ability to make a selectively reflective mirror (red) into a true mirror (full spectrum) (video, PSCLC mirror).
We have reported unprecedented control of reflection – reconfiguring the position of the selective reflection over various spectral ranges (e.g., tuning) or adjusting the bandwidth of the selective reflection (e.g., broadening) by as much as an order of magnitude. In other words, these materials allow for electrical control of whether it is reflective as well as where it is reflective. The optical element can be pixelated and the reflective properties adjusted across the element
We are developing distinctive approaches to reconfigurable optical elements to support applications in national security and energy management.