Category: Talk

Frank Cichos

Molecular Nanophotonics Group, Universität Leipzig

Holding single molecules in liquids is still a challenge as Brownian motion fueled by thermal energy is randomizing molecular position quickly and common forces to counteract this erratic motion commonly scale with the volume. The benefits of holding and manipulating a single molecule are however huge as this would allow for long time observations of molecular conformation detecting rare events, molecular reactions and true bi-molecular interactions. Here we report on a method which employs the fuel of Brownian motion – thermal energy – to confine Brownian motion of single molecules in liquid. By generating time-dependent feedback controlled inhomogeneous temperature fields with a laser heated gold structure, we create thermophoretic drive fields which allow the confinement of single molecules in solution. The feedback control and the inhomogeneous character of the temperature field even allow for a trapping a well controlled number of multiple molecules. We expect that this simple method and an extension to large arrays of traps will pave the way for controlled molecular interactions studies.

C. Sungkapreecha¹, N. Iqbal¹, A.M. Gohn², W.W. Focke³, and R. Androsch¹

¹Interdisciplinary Center for Transfer-oriented Research in Natural Sciences, Martin Luther University Halle-Wittenberg, 06099 Halle/Saale, Germany
²School of Engineering, Penn State Behrend, 4701 College Drive, Erie, PA 16563, USA
³Institute of Applied Materials, Department of Chemical Engineering, University of Pretoria, Private Bag X20, Hatfield 0028, South Africa

The phase diagram of the binary polymer/drug system poly (lactic acid) (PLA) and N,N-diethyl-3-methylbenzamide (DEET) has been established for the purpose of screening the potential use of biocompatible and biodegradable PLA as a drug-delivery reservoir for the insect repellent DEET. Crystallization-induced solid-liquid (S-L) phase separation occurs on sufficiently slow cooling of liquid solutions containing crystallizable PLA at temperatures slightly higher than ambient temperature. Non-isothermal crystallization starts spherulitically at point-like nuclei, followed by dendritic crystal growth due to the depletion of crystallizable polymer in the solution in the late crystallization stage. Analysis of vacuum-dried crystallized systems reveals formation of intra-spherulitic scaffold-like structures with a fine structure which is tunable by the conditions of crystallization. These scaffolds then may serve as a reservoir for liquid drugs (here DEET), which are slowly released to the environment.
If crystallization of PLA is suppressed due to fast cooling or high concentration of stereodefects in racemic random PLA copolymers then liquid-liquid (L-L) phase separation into an almost pure DEET phase and a PLA/DEET solution occurs at temperatures slightly below room temperature. As such, the investigated system shows a subambient upper critical solution temperature (UCST), with the critical temperature decreasing and critical polymer concentration increasing with decreasing molar mass of the polymer. It was confirmed that the enhanced miscibility of the system components in case of low molar mass PLA is due to increased entropy of mixing. Furthermore, it appears that L-L transition temperatures are independent on the PLA stereochemistry.

References
[1] C. Sungkapreecha, N. Iqbal, A.M. Gohn, W.W. Focke, R. Androsch, Phase behavior of the polymer/drug system PLA/DEET, Polymer 126, 116–125 (2017).
[2] C. Sungkapreecha, M.J. Beily, J. Kressler, W.W. Focke, R. Androsch, Phase behavior of the polymer/drug system PLA/DEET: Effect of PLA molar mass on subambient liquid-liquid phase separation, Thermochim. Acta 660, 77–81 (2018).

F. Kremer

Institute of Experimental Physics I, University of Leipzig, Linnéstr. 5, 04103 Leipzig, Germany

The question on what length-scale molecular and especially glassy dynamics of polymers takes place is of fundamental importance and has multifold practical implications as well. Continue reading Molecular dynamics at nanometric length scales

S. Förster,¹ A. Sander,¹ R. Hammer,¹ K. Duncker,¹ E. Kohl,¹ and Wolf Widdra,^1,2

¹ Institute of Physics, Martin-Luther-Universität Halle-Wittenberg, Halle, Germany
² Max Planck Institute of Microstructure Physics, Halle, Germany

Poly(3-hexylthiophene) (P3HT) adsorption on an atomically well-ordered Au(001) surface via electrospray deposition is reported [1]. In-situ scanning tunneling microscopy reveals a complex adsorption behavior of P3HT. Weakly adsorbed polymer chains exhibit a truly 2D random coil like chain configuration. Their end-to-end distance and their radius of gyration is reported as function of the polymer length. Additionally, a fraction of the P3HT molecules is constraint into a fully stretched configuration along the high-symmetry [110] crystal direction indicating a strong molecule-substrate interaction. This adsorption is accompanied by local lifting of the Au(001) surface reconstruction [2] underneath the polymer chains.
Continue reading Polythiophene Adsorption on Noble Metal Surfaces via Electrospray Deposition