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Neutron and dielectric spectroscopy - taking advantage of the strengths of each technique in confinement studies: Vibrational and molecular dynamics of nanoconfined liquid crystals

  • The effect of a nanometre confinement on the molecular dynamics of liquid crystals was studied by broadband dielectric and neutron spectroscopy. As confining host the molecular sieve Al-MCM-41 (Si/Al=60) with a mean pore diameter of 3 nm was selected. As guest the liquid crystals 8CB and E7 were chosen where the latter does not crystallize but undergoes a glass transition forming a nematic glass. Dielectric spectroscopy shows that for the confined systems one relaxation process is observed. Ist characteristic relaxation time is much lower compared to that of the bulk. No signature of the Phase transition characteristics of the bulk liquid crystal is detected. The temperature dependence of the relaxation time of this relaxation process has to be described by the Vogel-Fulcher-Tammann equation and thus shows similarities to glassy dynamics. The vibrational density of states measured by neutron spectroscopy (IN6, ILL; TOFTOF, MLZ) Shows excess contributions with respect to the DebyeThe effect of a nanometre confinement on the molecular dynamics of liquid crystals was studied by broadband dielectric and neutron spectroscopy. As confining host the molecular sieve Al-MCM-41 (Si/Al=60) with a mean pore diameter of 3 nm was selected. As guest the liquid crystals 8CB and E7 were chosen where the latter does not crystallize but undergoes a glass transition forming a nematic glass. Dielectric spectroscopy shows that for the confined systems one relaxation process is observed. Ist characteristic relaxation time is much lower compared to that of the bulk. No signature of the Phase transition characteristics of the bulk liquid crystal is detected. The temperature dependence of the relaxation time of this relaxation process has to be described by the Vogel-Fulcher-Tammann equation and thus shows similarities to glassy dynamics. The vibrational density of states measured by neutron spectroscopy (IN6, ILL; TOFTOF, MLZ) Shows excess contributions with respect to the Debye density of states (boson peak). For the confined systems the low frequency contributions of the boson peak are suppressed, an effect which is also found for conventional glass forming systems. In addition elastic scans were carried out at the backscattering spectrometer (BS) IN10 at ILL. Such measurements provide an overview about the temperature dependence of the microscopic dynamics. Assuming a Gaussian form for the elastically scattered intensities the effective mean square displacement was calculated. For the nanoconfined samples the mean square displacement is strongly reduced in comparison to the bulk. Also these measurements show the signature of a glass Transition indicated by a change in the temperature dependence of the mean square displacement. For confined E7 the extracted glass transition temperature is shifted by more than 20 K to higher temperatures. Broadband inelastic neutron scattering was carried out by combining time-of-flight (IN6, ILL; TOFTOF, MLZ) and backscattering (IN10, ILL; SPHERES, MLZ) experiments in the time domain. The time dependence of the incoherent intermediate scattering function Sinc(q,t) shows two Relaxation processes which were quantitatively analyzed by fitting the KWW function to the data. The process at shorter times shows a close similarity the methyl group rotation found for polymeric systems. The process at longer times was assigned to glassy dynamics. The obtained data are compared in detail in their temperature dependence to the dielectric results.zeige mehrzeige weniger

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Metadaten
Autor*innen:Andreas SchönhalsORCiD
Koautor*innen:Wibke Lohstroh, Michaela Zamponi, R. Zorn, Bernhard Frick
Dokumenttyp:Vortrag
Veröffentlichungsform:Präsentation
Sprache:Englisch
Jahr der Erstveröffentlichung:2016
DDC-Klassifikation:Naturwissenschaften und Mathematik / Chemie / Analytische Chemie
Technik, Medizin, angewandte Wissenschaften / Ingenieurwissenschaften / Ingenieurwissenschaften und zugeordnete Tätigkeiten
Freie Schlagwörter:Broadband dielectric spectroscopy; Neutron scattering
Veranstaltung:Deutsche Neutronenstreutagung 2016
Veranstaltungsort:Kiel, Germany
Beginndatum der Veranstaltung:20.09.2016
Enddatum der Veranstaltung:22.09.2016
Verfügbarkeit des Dokuments:Datei im Netzwerk der BAM verfügbar ("Closed Access")
Datum der Freischaltung:27.09.2016
Referierte Publikation:Nein
Eingeladener Vortrag:Nein
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