Single Molecule Imaging for the Study of Heterogeneous Materials: Supercooled Liquids, Polymer Nanocomposites, and Ionic Liquids

We may think of a container of a pure liquid as uniform, the same properties (for example, density, refractive index and viscosity) throughout the sample container. At the molecular level however, the liquid is not uniform, but highly varied in how the molecules are arranged with respect to one another. At most temperatures, molecules are in motion. At any given moment, some regions of the liquid might be denser, or more viscous than other regions. This condition is called 'dynamic heterogeneity,' and it is an important consideration in discussing the mechanical properties of polymers, how proteins move through water, etc. Dynamic heterogeneity is difficult to measure precisely - most measurements look at averages within a sample. In this project, funded by the Chemical Structure, Dynamics, and Mechanisms-A (CSDM-A) program of the Chemistry Division, Professor Laura Kaufman of Columbia University and her students are using an advanced molecular imaging technique that can track the motion of a single (molecule probe) through a liquid or polymer material. The probe molecule is strongly fluorescent, meaning that when exposed to light of one wavelength, it emits light of another specific wavelength. The emitted light allows the molecule to be tracked as it moves through different regions of the material. The probe reveals dynamic heterogeneity as its motions are altered by changes in the structure of its environment. Professor Kaufman and her students are investigating dynamic heterogeneity in supercooled liquids (liquids colder than their freezing temperature but somehow still liquid), polymer nanocomposites (used in gas separations and electronic components), and ionic liquids. Most familiar ionic systems are solid, like table salt. Ionic liquids are composed of positive and negative ions but are liquid at room temperature. Ionic liquids may have potential for 'green' industrial processes. The students engaged in this research are learning a broad range of experimental techniques and analysis approaches relevant to the specific project, These approaches are also applicable to many areas of science and emerging technologies. In addition to providing training for graduate students, this project also includes outreach to K-8 students. The outreach events feature topics that provide an entry point to chemistry concepts such as phase diagrams and the concept of chemical stability.

Professor Kaufman and her students are refining and extending single molecule imaging approaches to detail aspects of dynamic heterogeneity in small molecule supercooled liquids and polymeric glass formers, with advances coming in the form of new probe molecules and new techniques to distinguish spatial and temporal heterogeneity. At the same time, the scope of this work is being extended to polymer nanocomposites and ionic liquids. Newly designed tethered probe molecules are prepared and used to complement free probe measurements and discriminate changes in dynamics that occur due to environmental rearrangement from those related to probe translocation. The tethered probes provide a new approach to characterizing spatial extent of regions of distinct dynamics in these complex systems. The project also employs super-resolution experiments that allow simultaneous tracking of rotations and translations; this study seeks to resolve questions regarding the poorly understood phenomenon of rotational-translational decoupling, clarifying if it emerges from experimental biasing. The study of polymer nanocomposites focuses on the dynamics of the interfacial layer, and how interfacial layer properties vary with changes in the host matrix. Molecular probes are being developed to clarify how the interaction between structural and dynamic heterogeneity influences the properties of ionic liquids.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.