Ideal Probe Single Molecule Imaging for the Study of Small Molecule and Polymeric Glass Formers

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In this project funded by the Chemical Structure Dynamics and Mechanism (CSDM-A) program of the Chemistry Division, Professor Laura Kaufman of Columbia University is using advanced imaging techniques to study how molecules move in a class of materials known as glass formers. Glass formers may exist as supercooled liquids or glasses, materials that demonstrate unusual and poorly understood behaviors at both the microscopic and macroscopic scales. These materials are ubiquitous in nature and in modern technology, present in window glass, plastics, and medications delivered as pills and tablets. Understanding how molecules move in these systems not only addresses fundamental questions but also enhances understanding of how to stabilize these materials against various types of degradation. The PI develops and uses single molecule imaging approaches to detail the rotations and translations of molecules in these systems, teasing out key time scales and length scales related to molecular motions and relaxations in these environments. The single molecule imaging experiments in supercooled liquids underway employ newly designed and synthesized ideal probe molecules; these are probes that do not average in space or time over heterogeneities in a glassy host. As a result, these probes directly interrogate length and time scales of interest in these systems and test predictions of theories of the glass transition. The experiments detail dynamic heterogeneity as manifested in relaxation and exchange time distributions of rotational dynamics in both small molecule and polymeric glass formers. In addition, experiments that require sub-diffraction localization are being performed to follow translational dynamics and structural rearrangements in supercooled liquids to resolve questions regarding the poorly understood phenomenon of rotational-translational decoupling. Related techniques employing two types of probes are employed to measure dynamic correlations, allowing identification of length scales of heterogeneity in supercooled liquids. Ideal probe single molecule measurements are also being performed in thin polymeric films, with a focus on identifying the putative surface mobile layer that has practical ramifications for the stability of thin film devices and vapor-deposited glasses. To complement studies of glassy systems prepared through cooling, studies of such systems prepared through swelling are also underway. These studies both detail aspects of dynamic heterogeneity and directly visualize structural rearrangements underlying long time scale relaxations in these systems.