Molecular dynamics at the glass transition: One dimensional and two dimensional nuclear magnetic resonance studies of a glass-forming discotic liquid crystal

Abstract

The molecular dynamics of the aromatic core as well as the hydrocarbon side chains have been studied in a new kind of glass-forming discotic liquid crystal, based on a triphenylene core. It serves as a model compound in which only a limited number of degrees of freedom is activated at the glass transition. Slow motion with non-Arrhenius temperature dependence (α process) is associated with the axial motion of the discs around the column axis. The geometry of this rotation is found to be neither a simple threefold jump as suggested by the pseudo threefold symmetry of the substituted triphenylene core nor does it correspond to small-step rotational diffusion, established as a prominent feature of the rotational motions of polymers at the glass transition. Instead ill-defined angular displacements with pseudo threefold symmetry are detected by two-dimensional exchange NMR. A simple model combining small-step diffusion and rotational jumps is presented that quantitatively accounts for the angular displacements observed. The motional behavior and the conformational order of the side chains were also studied. This shows that the high frequency β process detected previously by dielectric spectroscopy indeed can be associated with side chain motions. Moreover evidence is presented that the axial motion of the discs is a cooperative process, where adjacent molecules within the column rotate in a correlated fashion without changing the conformation of the side chains in the immediate neighborhood of the core.

High-Resolution 1H NMR Spectroscopy in the Solid State: Very Fast Sample Rotation and Multiple-Quantum Coherences

“…In the case of the HBC derivatives, 1 H spectra do not only provide clear evidence for the presence of a LC phase, they also allow the persistence of the columnar packing as well as the underlying processes of molecular rotations to be identified (18,19,20). In contrast to previous NMR investigations performed on this class of materials (69,55), all the results presented here are directly obtainable from as-synthesized samples, requiring experimental times on the order of a few hours, so that 1 H MAS methods are particularly well suited to routine spectroscopic investigations in the course of the synthesis of solid-state materials.…”

Alignment of Discotic Liquid Crystals

“…They are known to increase the temperature range of discotic columnar mesophases [39], and disc-shaped aromatic molecules that do not form columnar mesophases on their own may display columnar liquid crystalline phases when linked together. Oligomers do not crystallize on cooling but rather form an anisotropic glass of the preceding mesophase structure [40]. Finally, the oligomeric approach provides a wide flexibility in molecular design.…”

Multiple glass transitions in the plastic crystal phase of triphenylene derivatives

“…Apart from their relevance for novel functional materials, DLC have also attracted attention as an interesting, low-dimensional glass-forming system [5,6]. Since glass formation, i.e.…”

“…In spite of their high degree of positional order in the plastic crystal phase, these systems show a temperature dependent primary relaxation time that obeys the Vogel-Fulcher-Tamman (VFT) law which is characteristic for glass forming systems [7][8][9]. 2 H NMR and dielectric spectroscopy studies on triphenylene derivatives revealed that the glass transition is related to the hindered rotation of rotationally disordered discs around their columnar axis and is not coupled with the side chain mobility [5,10,11]. For another hexasubstituted triphenylene derivative, where one of the side chains is a chloroester, it was found that the glass transition is highly anisotropic, having a much more pronounced effect along the columnar axis [6].…”