Ingenieurwissenschaften und zugeordnete Tätigkeiten
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- Broadband dielectric spectroscopy (2) (entfernen)
Polymeric membranes represent a cost- and energy efficient solution for gas separation. Recently superglassy polymers with high free volume outperform many conventional dense polymers in terms of gas permeability and selectivity. However, such polymers are prone to pronounced physical aging, resulting in a dramatic reduction in the gas permeability. Molecular mobility of polymer segments plays an important role in the physical aging and the gas transport performance of polymeric membranes. Molecular mobility and physical aging of a representative superglassy polynorbornene with very high gas permeability, PTCNSi2g, was monitored by using dielectric spectroscopy with state-of-the-art high-resolution analyzers. This work helps to shed some light on the structure−property relationship of superglassy polymers on a molecular level and to provide practical “design rules” for the development of high performance polymers for gas separation.
A combination of nanosized dielectric relaxation (BDS) and thermal spectroscopy (SHS) was utilized to characterize the dynamics of thin films of Poly(vinyl methyl ether) (PVME) (thicknesses: 7 nm – 160 nm). For the BDS measurements, a recently designed nano-structured electrode system is employed. A thin film is spin-coated on an ultra-flat highly conductive silicon wafer serving as the bottom electrode. As top electrode, a highly conductive wafer with non-conducting nanostructured SiO2 nano-spacers with heights of 35 nm or 70 nm is assembled on the bottom electrode. This procedure results in thin supported films with a free polymer/air interface. The BDS measurements show two relaxation processes, which are analyzed unambiguously for thicknesses smaller than 50 nm. The relaxation rates of both processes have different temperature dependencies. One process coincidences in its position and temperature dependence with the glassy dynamics of bulk PVME and is ascribed to the dynamic glass transition of a bulk-like layer in the middle of the film. The relaxation rates were found to be thickness independent as confirmed by SHS. Unexpectedly, the relaxation rates of the second process obey an Arrhenius-like temperature dependence. This process was not observed by SHS and was related to the constrained fluctuations in a layer, which is irreversibly adsorbed at the substrate with a heterogeneous structure. Its molecular fluctuations undergo a confinement effect resulting in the localization of the segmental dynamics. To our knowledge, this is the first report on the molecular dynamics of an adsorbed layer in thin films.