Regensburg 2025 – scientific programme

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CPP: Fachverband Chemische Physik und Polymerphysik

CPP 4: Crystallization, Nucleation and Self-Assembly I

CPP 4.2: Talk

Monday, March 17, 2025, 11:45–12:00, H34

Liquid crystalline ordering of conjugated polymers at the vacuum interface — Anton Sinner, Alexander Johannes Much, and •Oleksandr Dolynchuk — Experimental Polymer Physics, Martin Luther University Halle-Wittenberg, Germany

Structure formation in films of conjugated polymers (CPs) strongly affects their optoelectronic properties and is of fundamental interest in the context of sanidic liquid crystalline (SLC) phases discovered in board-like polymers. Recent experimental results on films of the model CP poly(3-hexylthiophene) (P3HT) revealed the edge-on crystal orientation in the top film layer and suggested that the vacuum interface induces it. Here, we use grazing-incidence wide-angle X-ray scattering to study in situ the crystallization and melting in films of P3HT and polydiketopyrrolopyrrole (PDPP), another board-like CP, at the vacuum interface. Upon cooling from the melt, the ordering of both P3HT and PDPP indeed starts at the vacuum interface as an edge-on oriented disordered SLC phase and undergoes multiple SLC transitions with decreasing temperature. The same thermal hierarchy of SLC phases is observed for both CPs during melting, demonstrating that the vacuum interface stabilizes the edge-on disordered SLC phase above its bulk transition temperature, similar to the surface freezing of short alkanes. Notably, the observed effects are much stronger for PDPP, also resulting in a thicker edge-on oriented layer of more than 100 nm. Thus, we show that surface-induced SLC ordering is a general phenomenon of CPs, presumably due to their board-like architecture, and it significantly influences the molecular orientation in CP films.

Keywords: Crystallization; Liquid crystals; Conjugated polymers; Surfaces; X-ray scattering