| Time: | December 4, 2023 |
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Prof. Dr. Regine von Klitzing
Institut für kondensierte Materie, TU Darmstadt
Montag, 4. Dezember 2023, 14:00 Uhr
ICP Seminarraum 1.079, Allmandring 3
“Nanomechanics and nanorheology of microgels at interfaces”
Abstract:
Coating surfaces by microgels instead of using macrogels has many advantages with respect to ease in preparation, versatility and speed in response to external stimuli as temperature, pH or external fields like light or magnetic fields. Microgels can be used e.g. for coatings of solid surfaces for sensorics or for stabilizing foams and emulsions due to their strong ability to deform at liquid interfaces. The response to outer stimuli corresponds a volume phase transition and the swelling abilities are significantly affected by the internal structure and mechanics of the microgels.
Mechanics of (micro)gel coatings can be studied by AFM indentation experiments. Often the Hertz model is sufficient to analyze the data [1]. Due to suitable spatial resolution (high but above the range of microgel mesh sizes) the indentation with a tip allows mapping of the mechanical properties of adsorbed microgels, i.e. gel particles with a diameter of several 100s of nm [2]. In case of heterogeneous microgels the soft shell can be distinguished from the stiffer core, while homogeneous microgels show a constant apparent elastic modulus [3]. In order to separate the storage modulus E’ from the loss modulus E’’ dynamic indentation measurements are carried out. Within the usual range
of cross-linker content, the elastic properties dominate the internal rheological behavior of the microgels. Increasing temperature across the volume phase transition temperature (VPTT) of PNIPAM microgels leads to an expected increase in the elastic modulus due to shrinking of the microgels above the VPTT. In contrast to this, the shrinking and swelling due to the cononsolvency effect in water/alcohol mixtures does not induce a systematic increase and decrease of the elastic modulus [4]. This is interpreted by specific solvent-polymer interaction.
Examples of foam films stabilized by microgels show that the film thickness is related to the elastic modulus of the microgels and that the correlation between swelling and elastic modulus follows the Flory-Rehner theory [5]. The latter is also valid for microgels containing the mussel-inspired dopamine which leads to additional stiffening and adhesive properties [6].
[1] S. Backes, R. von Klitzing. Polymers, 2018, 10, 978.
[2] A. Burmistrova, M. Richter, C. Üzüm, R.v.K., Coll. Polym. Sci. 2011, 289, 612.
[3] M.U. Witt, S. Hinrichs, N. Möller, S. Backes, B. Fischer, R. von Klitzing, JPCB, 2019, 123, 2405.
[4] S. Backes, P. Krause, W. Tabaka, M.U. Witt, M. R. von Klitzing, Langmuir, 2017, 33, 14269.
[5] M. Kühnhammer, K. Gräff, E. Loran, O. Soltwedel, O. Löhmann, H. Frielinghaus, R. von Klitzing, Soft Matter, 2022, 18, 9249.