{"id":{"repo_id":"freiburg-diss","oai_identifier":"oai:freidok.uni-freiburg.de:2066"},"canonical_url":"https://search.dev.ndltd.org/etd/freiburg-diss/oai:freidok.uni-freiburg.de:2066","repository":{"repo_id":"freiburg-diss","name":"University of Freiburg","base_url":"https://freidok.uni-freiburg.de/oai/oai2.php"},"display":{"title":"A biaxial nematic phase in a thermotropic liquid crystalline side-chain polymer : a deuterium NMR study","abstract":"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.","abstract_html":"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.","abstract_has_math":false,"creators":["Severing, Kirsten"],"institution":null,"degree_name":null,"degree_level":null,"degree_discipline":null,"degree_department":null,"school":null,"contributors":["Saalwächter, Kay"],"advisors":[],"committee_chairs":[],"committee_members":[],"year":null,"date_issued":"","date_published":null,"updated_at":"2026-07-24T02:22:33Z","subjects":["Biaxial-nematische Phase","Spinsonde","Direktorfluktuation","heterogene Linienverbreiterung","Direktorrelaxation","biaxial nematic phase","spin probe","director fluctuation","heterogeneous line broadening","director relaxation"],"languages":[],"rights":[],"rights_urls":[],"identifier_entries":[]},"links":{"outbound_url":"https://freidok.uni-freiburg.de/data/2066","outbound_label":"Repository record","outbound_source":"source_url"},"metadata_groups":[{"id":"people","label":"People","entries":[{"key":"dc:contributor","label":"Contributor","values":["Saalwächter, Kay"]},{"key":"dc:creator","label":"Author","values":["Severing, Kirsten"]}]},{"id":"academic_context","label":"Academic Context","entries":[{"key":"dc:type","label":"Dc Type","values":["DoctoralThesis"]}]},{"id":"subjects_keywords","label":"Subjects and Keywords","entries":[{"key":"dc:subject","label":"Dc Subject","values":["Biaxial-nematische Phase","Spinsonde","Direktorfluktuation","heterogene Linienverbreiterung","Direktorrelaxation","biaxial nematic phase","spin probe","director fluctuation","heterogeneous line broadening","director relaxation"]}]},{"id":"additional","label":"Additional Metadata","entries":[{"key":"dc:description.abstract","label":"Abstract","values":["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.","Ziel der vorliegenden Arbeit war die Untersuchung der Phasensymmetrie von flüssigkristallinen Seitenkettenpolymeren mit Hilfe von Deuterium-NMR. Die laterale Anbindung eines brettförmigen Mesogens sollte in dem zuerst untersuchten System die Rotation um die Mesogenlängsachse unterbinden und die Ausbildung einer vor 35 Jahren theoretisch vorausgesagten biaxial-nematischen Phase unterstützen. Die Identifizierung dieser Phase, die durch eine dreidimensionale Orientierungsfernordnung gekennzeichnet ist, erfolgte durch winkelabhängige Messungen der quadrupolaren Aufspaltung einer der Probe zugemischten deuterierten Spinsonde. Mit Hilfe des sogenannten Klappexperiments, welches die Messung der Signalaufspaltung einer parallel zum Magnetfeld orientierten Probe sowie einer um 90° geklappten Probe beinhaltet, wurde der Biaxialitätsparameter (eta) für verschiedene Temperaturen innerhalb der flüssigkristallinen Phase bestimmt. Das untersuchte Polymer bildete demnach beim Abkühlen aus der isotropen Schmelze zunächst eine uniaxial nematische Phase (eta ~ 0), bevor bei tieferen Temperaturen eine Umwandlung in eine biaxial-nematische Phase erfolgte, welche sich durch einen ansteigenden Biaxialitätsparameter zeigte. Ein- und zweidimensionale transversale Relaxationsexperimente wurden durchgeführt, um das Ausmaß einer heterogenen Linienverbreiterung, welche durch eine quasi-statische Direktorverteilung hervorgerufen werden kann, und deren Effekt auf die zu bestimmende Aufspaltung abzuschätzen. In gleicher Weise wurden Seitenkettenpolymere untersucht, bei denen die geometrische Anbindung des Mesogens (von `side-on´ zu `end-on ´), die Länge des flexiblen Spacers zwischen der Polymerkette und dem Mesogen oder die Polymerdynamik durch Beimischen eines niedermolekularen Flüssigkristalls, gezielt variiert wurde. Neben der experimentellen Identifizierung einer biaxialen Phase in einem thermotropen Flüssigkristall war eine weitere Erkenntnis dieser Arbeit, dass die Spacerlänge eine wichtige Rolle bei der Ausbildung einer biaxialen Phase spielt. Während ein end-on Polymer mit langem Spacer im gesamten Bereich der nematischen Phase uniaxiales Verhalten aufwies, bildete ein end-on Polymer mit kurzem Spacer bei niedrigen Temperaturen überraschenderweise ebenfalls eine biaxiale Phase. Des Weiteren konnte gezeigt werden, dass die Ausbildung einer biaxial-nematischen Phase entscheidend an die Dynamik des Polymerrückgrades und damit an den Glasübergang gekoppelt ist. Um ein vollständigeres Verständnis der Direktordynamik in einem biaxial-nematischen System zu erhalten, sollen Experimente mit gekreuzten Feldern (Magnetfeld und elektrisches Feld) vorgenommen werden. Dazu wurden die Probenpräparation, der experimentelle Aufbau, und die Pulsprogramme des NMR-Spektrometers modifiziert., sowie bereits Vorversuche mit einem niedermolekularen Flüssigkristall durchgeführt."]},{"key":"dc:format.medium","label":"Dc Format Medium","values":["application/pdf"]},{"key":"dc:title","label":"Title","values":["A biaxial nematic phase in a thermotropic liquid crystalline side-chain polymer : a deuterium NMR study","Eine biaxial-nematische Phase in einem thermotropen flüssigkristallinen Seitenkettenpolymer : eine Deuterium-NMR Studie"]}]}],"canonical_facts":{"dc:contributor":["Saalwächter, Kay"],"dc:creator":["Severing, Kirsten"],"dc:description.abstract":["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.","Ziel der vorliegenden Arbeit war die Untersuchung der Phasensymmetrie von flüssigkristallinen Seitenkettenpolymeren mit Hilfe von Deuterium-NMR. Die laterale Anbindung eines brettförmigen Mesogens sollte in dem zuerst untersuchten System die Rotation um die Mesogenlängsachse unterbinden und die Ausbildung einer vor 35 Jahren theoretisch vorausgesagten biaxial-nematischen Phase unterstützen. Die Identifizierung dieser Phase, die durch eine dreidimensionale Orientierungsfernordnung gekennzeichnet ist, erfolgte durch winkelabhängige Messungen der quadrupolaren Aufspaltung einer der Probe zugemischten deuterierten Spinsonde. Mit Hilfe des sogenannten Klappexperiments, welches die Messung der Signalaufspaltung einer parallel zum Magnetfeld orientierten Probe sowie einer um 90° geklappten Probe beinhaltet, wurde der Biaxialitätsparameter (eta) für verschiedene Temperaturen innerhalb der flüssigkristallinen Phase bestimmt. Das untersuchte Polymer bildete demnach beim Abkühlen aus der isotropen Schmelze zunächst eine uniaxial nematische Phase (eta ~ 0), bevor bei tieferen Temperaturen eine Umwandlung in eine biaxial-nematische Phase erfolgte, welche sich durch einen ansteigenden Biaxialitätsparameter zeigte. Ein- und zweidimensionale transversale Relaxationsexperimente wurden durchgeführt, um das Ausmaß einer heterogenen Linienverbreiterung, welche durch eine quasi-statische Direktorverteilung hervorgerufen werden kann, und deren Effekt auf die zu bestimmende Aufspaltung abzuschätzen. In gleicher Weise wurden Seitenkettenpolymere untersucht, bei denen die geometrische Anbindung des Mesogens (von `side-on´ zu `end-on ´), die Länge des flexiblen Spacers zwischen der Polymerkette und dem Mesogen oder die Polymerdynamik durch Beimischen eines niedermolekularen Flüssigkristalls, gezielt variiert wurde. Neben der experimentellen Identifizierung einer biaxialen Phase in einem thermotropen Flüssigkristall war eine weitere Erkenntnis dieser Arbeit, dass die Spacerlänge eine wichtige Rolle bei der Ausbildung einer biaxialen Phase spielt. Während ein end-on Polymer mit langem Spacer im gesamten Bereich der nematischen Phase uniaxiales Verhalten aufwies, bildete ein end-on Polymer mit kurzem Spacer bei niedrigen Temperaturen überraschenderweise ebenfalls eine biaxiale Phase. Des Weiteren konnte gezeigt werden, dass die Ausbildung einer biaxial-nematischen Phase entscheidend an die Dynamik des Polymerrückgrades und damit an den Glasübergang gekoppelt ist. Um ein vollständigeres Verständnis der Direktordynamik in einem biaxial-nematischen System zu erhalten, sollen Experimente mit gekreuzten Feldern (Magnetfeld und elektrisches Feld) vorgenommen werden. Dazu wurden die Probenpräparation, der experimentelle Aufbau, und die Pulsprogramme des NMR-Spektrometers modifiziert., sowie bereits Vorversuche mit einem niedermolekularen Flüssigkristall durchgeführt."],"dc:format.medium":["application/pdf"],"dc:subject":["Biaxial-nematische Phase","Spinsonde","Direktorfluktuation","heterogene Linienverbreiterung","Direktorrelaxation","biaxial nematic phase","spin probe","director fluctuation","heterogeneous line broadening","director relaxation"],"dc:title":["A biaxial nematic phase in a thermotropic liquid crystalline side-chain polymer : a deuterium NMR study","Eine biaxial-nematische Phase in einem thermotropen flüssigkristallinen Seitenkettenpolymer : eine Deuterium-NMR Studie"],"dc:type":["DoctoralThesis"]},"updated_at":"2026-07-24T02:22:33Z"}