Molecular Control over the Mechanism of Crystal Growth

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Non-technical Abstract:Control of crystalline structure is important to the application of materials, whether to control the activity of pharmaceuticals or the electronic conductivity of advanced materials. However, deciphering the physical/chemical processes that determine growth of specific crystal structures is complex. This project systematically measures the influence of molecular shape and the strength of intermolecular interactions on the kinetics of crystal growth to investigate molecular level control of the mechanism of melt crystallization. In addition to advancing the fundamental science, this work is invaluable to all fields of science for which crystal growth is critical, such as polymers, pharmaceuticals, food products, geology, and atmospheric science. In addition, professional development for high school chemistry teachers is made available across the state of North Carolina. This discipline based professional development helps teachers shift curricula from a standardized test-based content/algorithm focus to critical-thinking/problem-solving based teaching and learning.Technical Abstract:The research supported by this award uniquely presents a melt crystallization strategy that directly probes the influence of intrinsic factors of molecular shape and intermolecular forces on the mechanism of crystal growth. Experimental measurement of the crystallization kinetics of oligoacenes and alkylammonium metal halide salts, as a function of the number of aromatic rings and length of alkyl chains, respectively, is being used to probe the impact of molecular anisotropy and crystal dimensionality on the rate of crystallization. Comparative kinetic measurements from isotropic vs. oriented melts of liquid crystals is utilized to articulate the impact of molecular pre-orientation on the rate of crystallization. Also, the crystallization kinetics across the family of substituted benzenes creates a geometric and energetic matrix with which to evaluate functional group effects. The PI's recent discovery of Transition Zone Theory, the condensed matter analog of Eyring's transition state theory, enables for the identification of entropic and enthalpic activation parameters of crystallization reactions. This award facilitates a continuing offering of the summer teacher workshops, and also supports hiring a high school chemistry teacher to work with the PI during summers to write and publish components of this professional development, which affords greater dissemination of the problem-solving/discovery based materials.