This international collaboration, between scientists at the University of Hull, UK and Kent State University having complementary expertise, focuses on understanding phenomena associated with biaxiality - the spontaneous occurrence of two distinct optical axes - in the so-called nematic phase of liquid crystals (LCs). Although it was hypothesized about 38 years ago, biaxial nematics have, until recently, proven quite elusive in real materials - indeed, virtually all nematics are optically uniaxial and virtually all applications of LCs have been restricted to the manipulation of a single optical axis. The biaxial nematic phase is expected in LCs composed of molecules (or, molecular aggregates) that are intrinsically biaxial in shape, e.g., plank-like or bent-core (banana-shaped) molecules. Another important possibility for formation of a biaxial nematic, which has been the subject of several intense theoretical investigations, arises in mixtures of disk- and rod-shaped molecules. Depending on the aspect ratios of these component molecules and on the concentration, one can expect three nematic phases, two uniaxial and one biaxial, with a specific topology of the phase diagram and interesting evolution of the nature of phase transitions. The principal investigator in the UK, Professor Georg Mehl, and his team have pioneered the synthesis of compatible rod- and disk-like systems, which form stable solutions and exhibit distinct nematic phases (possibly including a biaxial phase). The US researchers will probe the structure, optical, and other physical properties, the dynamics, and critical behavior at several interesting phase transitions between different nematic phases, both in systems with biaxial-shaped molecules and in rod-disc mixtures.
The results and the data acquired under this project will help test the validity of the current theories and provide important insight into the physics and chemistry of novel LC materials. The researchers will employ state-of-the-art techniques such as dynamic light scattering, synchrotron x-ray diffraction, confocal and atomic force microscopies, and electro-optical measurements in their investigations. The results of this project are likely to have transformative impact on the industry via the emergence of a new display technology and other applications of optically biaxial fluids. Undergraduate and graduate students and postdocs will be trained in an area that has the potential of impacting the fields of photonics, telecommunications, and cyber infrastructure. Junior researchers will have opportunities to interact and forge long-term professional contacts with members of the research group in the UK and other international scholars at the Advanced Photon Source. US graduate students and postdocs will be able to diversify their skills and scientific experience and begin their careers with a profound appreciation for materials development as well as physical characterization.
The focus of this research has been to find and understand the properties of a new liquid crystal (LC) phase, referred to as the biaxial nematic LC. This is very similar to the uniaxial nematic LC materials used in flat panel computer and television displays, and cell phones. The new LC is predicted to be 100 times faster than the current technology, with unique optical properties that also make it very versatile and suitable for technological applications other than displays. The fundamental requirement is that their basic entity (molecules or nanoscale assembly of molecules) have the proper shape and symmetry to organize, on a larger scale, into the new phase. Molecules having banana or plank-like shape, or resembling an eating fork meet the basic requirements. During this project, especially synthesized and selected materials were used which included: tetrapodic systems in which four individual rod shaped molecules were tethered to an anchor atom giving them an effective rectangular shape, obliquely hydrogen bonding molecules giving them a statistical biaxial shape, and water solutions of (chromonic) plank-shaped dye molecules. The materials not commercially available were obtained from research collaborators at the University of Hull (UK), Center for Soft Materials Research (India), and from industry partners. State-of-the-art dynamic light scattering and synchrotron x-ray diffraction, and other experimental techniques were employed to obtain valuable information which revealed the connections between molecular architecture and the LC phases formed. The tetrapodic system showed the two nematic and one more complex phase with previously unknown features. The hydrogen-bonded molecules also formed a phase that resembled the new nematic phase. The work on chromonic materials revealed that several long held beliefs pertaining to their assembly into columns followed by the formation of LC phases were not substantiated and their theoretical description needs amendments. Our results provided a better quantitative understanding of them. The pharmaceutical systems were also found to form LC structures during the processes used in drug formulations. These were important findings that improved our scientific understanding of the new LC phase and these materials. This research project enabled the training of several highly skilled workers in the STEM area. Five graduate students and four postdoctoral scholars were mentored and trained in the conduct of research and preparation of manuscripts and professional presentations for dissemination of results. Four graduate students completed their PhD degrees. Three of the students and two postdocs were female. Two of the group members accepted faculty positions at the level equivalent to associate professor and now have their own teaching and research programs. Two are employed as researchers at a major company specializing in liquid crystal technology, and three are currently pursuing research as postdoctoral scholars. One student is in a process of completing his PhD. This research award by the National Science Foundation afforded the project team members an opportunity for international collaboration involving exchange of scientific materials, skills, and ideas. Results were disseminated via a total of 24 publications and presentations at professional conferences. One patent (No. 7,604,850) on "biaxial nematic liquid crystal electrooptical devices" was awarded by the US Patent and Trademark Offfice. The scientific outcomes of this project held significant importance for a major display manufacturer that provided a grant specifically to explore the potential of their use in fast display devices adding to the broader impact of the activities undertaken with support from the National Science Foundation.
