A biaxial nematic phase in a thermotropic liquid crystalline side-chain polymer : a deuterium NMR study

The goal of this thesis was the investigation of the phase symmetry of liquid crystalline side-chain polymers by means of deuterium NMR spectrodcopy. The lateral attachment of a book-shaped mesogen should - in the case of the system investigated in the beginning - prevent the rotation about the long molecular axis and therefore support the formation of a biaxial nematic phase, which had been theoretically predicted 35 years earlier. The identification of this special phase being characterized by a three-dimensional orientational order, was accomplished by angular dependent measurements of the quadrupolar splitting of a deuterated spin probe mixed into the polymeric sample. The biaxiality parameter (eta) was obtained for various temperatures within the liquid crystalline phase using the so-called tilt-experiment, which involves measurements of the quadrupolar splitting of a sample oriented along the magnetic field as well as a 90° tilted one. When cooling down from the isotropic phase the side-on polymer first entered a conventional uniaxial nematic phase (eta ~ 0), before at a lower temperature a phase transition to a biaxial nematic phase occurred, which was identified by an increasing biaxiality parameter. A director distribution which is stable on the time scale of the NMR experiment leads to a heterogeneous line broadening. One- and two-dimensional transverse relaxation experiments were conducted in order to estimate the magnitude of this broadening and to investigate the influence on the quadrupolar splitting. Other side-chain polymer systems, in which the geometry of the attachment (from `side-on´ to `end-on ´), the length of the flexible spacer between the polymer backbone and the mesogen or the polymer dynamics by the addition of a low molar mass liquid crystal, were systematically changed, were investigated in a similar way. Besides the experimental proof of a biaxial nematic phase itself the most important result of this work is the importance of the influence of the spacer length for the formation of a biaxial nematic phase. While an end-on polymer with a long spacer exhibited in the whole range of the liquid crystalline phase merely uniaxial behavior, a biaxial nematic phase was surprisingly observed at lower temperatures in the case of a shorter spacer. Furthermore, it could be shown that the dynamics of the polymer backbone, and thus the glass transition is the key issue for the formation of a biaxial nematic phase. NMR experiments in the presence of two crossed fields (magnetic field and electric field) should be carried out in order to obtain a more complete understanding of the director dynamics in a biaxial nematic system. For that purpose, the sample preparation, the experimental set-up as well as the NMR pulse programs have been modified and preliminary tests using a standard low molar mass liquid crystal have been carried out.

Author dc:creator

• Severing, Kirsten

Contributors dc:contributor

• Saalwächter, Kay

• Biaxial-nematische Phase
• Spinsonde
• Direktorfluktuation
• heterogene Linienverbreiterung
• Direktorrelaxation
• biaxial nematic phase
• spin probe
• director fluctuation
• heterogeneous line broadening
• director relaxation

Repository record source_url

https://freidok.uni-freiburg.de/data/2066

OAI identifier oai:identifier

oai:freidok.uni-freiburg.de:2066