Molecular dynamics of nanometric layers of glass formers in interaction with solid substrates

• Main Author: Mapesa, Emmanuel Urandu
• Other Authors: Universität Leipzig, Fakultät für Physik und Geowissenschaften
• Format: Doctoral Thesis
• Language: English
• Published: Universitätsbibliothek Leipzig 2014
• Subjects: dünne Schichten; dynamischer Glasübergang; Dynamics; confinement; glass transition temperature; thin films; nanopores; segmental mode; normal mode; terminal subchains; dielectric spectroscopy; ddc:530
• Online Access: http://nbn-resolving.de/urn:nbn:de:bsz:15-qucosa-155709; http://nbn-resolving.de/urn:nbn:de:bsz:15-qucosa-155709; http://www.qucosa.de/fileadmin/data/qucosa/documents/15570/PhD%20Dissertation_Herr%20Mapesa_online.pdf

Broadband Dielectric Spectroscopy (BDS) in combination with a nanostructured electrode arrangement – which circumvents the conventional need to evaporate metal electrodes onto soft matter – is used to study the molecular dynamics of several glass forming materials confined in nanometric (> 5 nm) layers. Other complementary experimental tools employed in this work include spectroscopic vis-Ellipsometry (SE), AC-chip calorimetry (ACC), X-ray reflectrometry (XRR), Differential Scanning Calorimetry (DSC) and Atomic Force Microscopy (AFM). The latter is used to characterize the topography of the samples and to determine their thicknesses. Under the conditions of annealing samples (Tg + 50K) in high oil-free vacuum (10E-6 mbars) for at least 12 h and carrying out measurements in inert (dry nitrogen or argon) atmosphere, it is found for all studied thin layers that the structural relaxation, and hence the dynamic glass transition – in its mean relaxation times – remains within a margin ±3 K from the respective bulk behaviour. It is revealed, inter alia, that the one-dimensional confinement of thin films introduces restrictions on other (slower) molecular relaxation processes which manifest, depending on the specific system under investigation, as (i) an interruption of the end-to-end (normal mode) fluctuation of the chains, or (ii) a slowing down of the delta-relaxation when the system is cooled towards glass-formation. Furthermore, (iii) evidence is provided to show that the dimensionality of confinement plays a significant role in determining the resulting dynamics. A molecular understanding of these findings is given, and the discussion presented with respect to the on-going international debate about dynamics in confinement.