Doctoral students seminar (July 9, 2019)

Muhammad Tariq on “Interface Controlled Thermodynamics of First-Order Prefreezing”
and
Anika Wurl on “Polymer crystallization under anisotropic confinement in liquid crystals”

Location: UL, Linnéstr. 5, SR 532
Time: 3.30pm-5.00pm

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Abstracts

Muhammad Tariq

Heterogeneous nucleation and prefreezing are two possible mechanism by which a solid surface can induce crystallization of liquids. In contrast to heterogeneous nucleation, first-order prefreezing is an equilibrium phenomenon that refers to the reversible and abrupt formation of a crystalline layer of thickness ?_??? at an interface to a solid surface at the temperature ?_max well above the bulk melting temperature  ?_m [1, 2]. Thickness of the prefrozen layer ?_eq(?) diverges upon further cooling to bulk ?_m [1]. Most recently developed phenomenological theory of prefreezing [3] predicts ?_max to be a function of the difference of interfacial free energies ∆? = ?_???,???? − (?_???,??? + ?_???,????) and ?_???,????, whereas ?_?? and ?_??? are determined by ?_???,???? and ∆?/ ?_???,???? respectively.

Here, I will be presenting an experimental test of the theory by extending our investigations of prefreezing of poly(ε-caprolactone) (PCL) on graphite to a MoS₂ substrate. Using in-situ atomic force microscopy AFM measurements at elevated temperatures, we determine equilibrium properties of the prefrozen PCL layer on MoS₂. These properties include the temperature range of prefreezing ?_???, prefrozen layer thickness ?_eq (?) above the bulk ?_m, and discontinuous jump in thickness l_min at T_max. In comparison with PCL-HOPG, we find a clear decrease in l_eq(T) and l_min but T_max remains nearly unaffected. We analyze our experimental results with the phenomenological theory [3] and estimate the material parameters at the solid interface. A comparison of the material parameters for prefrozen PCL on MoS₂ and HOPG shows that above mentioned experimental observations are in good agreement with the predictions of the phenomenological theory.

References:
[1] A.-K. Flieger, M. Schulz, T. Thurn-Albrecht, Macromolecules, 51, 189 (2018).
[2] A.-K. Löhmann, T. Henze, T. Thurn-Albrecht, PNAS, 111, 17368 (2014).
[3] O. Dolynchuk, M. Tariq, T. Thurn-Albrecht, J. Phys. Chem. Lett. 10, 1942 (2019).

Anika Wurl

With increasing amounts of polymer nanoparticles in earth’s oceans and rivers, the study of interactions between lipid membranes and synthetic polymers is receiving a growing interest. In this project, we investigate if hydrophobic polymer chains, e.g. poly(ε-caprolactone) (PCL), can crystallize in anisotropically confined environments such as lipid bilayers or synthetic liquid-crystalline systems. In a system of PCL, dipalmitoylphosphatidylcholine (DOPC) and water, ¹³C INEPT NMR and differential scanning calorimetry (DSC) results showed that the system seperated into a phase of pure PCL as well as a mixed phase of DOPC and PCL, where crystallization of PCL possible. We now extend our research to liquid-crystal phases of block-copolymers of type (EO)_n-(PO)_m-(EO)_n. In this way, we aim to study the effects of different confinement variables on crystallization properties such as nucleation, growth and crystal morphology using a combination of ¹H, ²H and ¹³C NMR, polarized optical microscopy and molecular dynamics simulations.