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.