Novel Molar Design Concept for Low Molar Mass Glass-Forming Liquid Crystals

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ABSTRACT Proposal NO. CTS-9500737 University of Rochester Shaw H. Chen Liquid crystals have found potential applications to various optical and optelectronic technologies. With the exception of active components based on the change in molecular orientation in response to an applied field, practical applications of liquid crystals are limited by the intrinsic fluid nature and tendency to crystallize. To provide mesophase stability and environmental durability, liquid crystalline polymers have been explored as an alternative in view of the potential for vitrification with a glass transition temperature above the ambient. However, polymeric materials are inherently difficult to process into uniform films because of a generally high melt viscosity. It is most desirable to combine the attributes of these two classes of materials, i.e., ease of material processing and ability to preserve a high degree of mesomorphic ordering in a glassy environment. To achieve this ultimate goal, a two-year program is proposed for research on low molar mass glass-forming liquid crystals. In principle, all liquids should vitrify given a sufficiently rapid cooling rate. Nevertheless, the question of why some materials are more prone to glass formation than others has remained unanswered to date. Since theoretical or computational approaches do not appear to promise a rational molecular design tool in the foreseeable future, it is proposed that a molecular design concept be pursued from an empirical standpoint. In essence, low molar mass liquid crystals represent a subtle balance between two seemingly opposing factors: one to encourage liquid crystal mesomorphism and the other to discourage crystallization. It is proposed that two diametrically different structural features, mesogenic moiety and excluded-volume core, be chemically bonded to each other via a flexible spacer. The idea is to enable the chemically combined system to form and optically anisotropic, morphologically stable glass, even though both mesogenic moiety and excluded-volume core may crystallize on their own. Our technical approach consists of molecular design, chemical synthesis, and relevant characterizations including morphology, theology, and the kinetics of thermally activated phase transformation. In addition, characterizations with a direct bearing on practical applications will also be performed. Results from the proposed research will furnish new insight into vitrification, liquid crystallinity, morphology and processing of low molar mass compounds. In addition, through judicious molecular design, multifunctional materials capable of optical and optoelectronic applications are expected to come forth. Specific examples may include light polarization, nonlinear optical activity, photochromizm, and electroluminescence, which constitute a broad base for emerging display, imaging, and communications technologies.