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In thin polymer films, little is known about the dynamics of the adsorbed layers, despite their importance in innovative applications. Here, Broadband Dielectric Spectroscopy (BDS) was utilized to investigate the glassy dynamics of thin films of a low MW Poly (vinyl methyl ether) (PVME) (thicknesses: 7 – 160 nm). A recently developed nano-structured capacitor arrangement was employed; where a silicon wafer with nanostructured SiO2 nano-spacers, with heights of 35 nm and 70 nm, is placed on top of a thin film spin coated on an ultra-flat highly conductive silicon wafer. Further, PVME/SiO2 interactions was confirmed by contact angle measurements, hence an adsorbed layer is allowed to form. For films with thicknesses smaller than 50 nm, BDS measurements showed two relaxation processes. The first process coincided, in its position and temperature dependence, with the -relaxation of bulk PVME, thus it was assigned to the -relaxation of a bulk-like layer. The second process showed a different temperature dependence and was ascribed to the relaxation of polymer segments adsorbed at the substrate. Both processes showed no thickness dependence. The results will be discussed in detail. To our knowledge, this is the first study of the segmental dynamics of an adsorbed layer in thin films.
For many years now, the so-called three layer model (free-surface, bulk-like, and adsorbed layers) has been commonly used, along with other parameters, to explain the deviations seen in glass transition and glassy dynamics for polymers confined into thin films, compared to their bulk value. Nevertheless, due to the hard accessibility of the adsorbed layers in supported films, little is known about the nature of their dynamics and how they really influences the overall dynamics of the thin films. Here, the irreversibly self-assembled adsorbed layer of a low MW Poly (vinyl methyl ether) (PVME) is solvent-leached from a 200 nm film. The thickness and topography of this layer is checked with Atomic Force Microscopy (AFM), to insure no dewetting and low roughness. Further, the dynamics of the adsorbed layer is then in-situ probed with Broadband Dielectric Spectroscopy (BDS). A recently developed nano-structured capacitor arrangement was employed; where a silicon wafer with nanostructured SiO2 nano-spacers, with heights of 35 nm, is placed on top of a thin film spin coated on an ultra-flat highly conductive silicon wafer. All results will be discussed in detail and quantitatively compared to our recent work on the glassy dynamics of PVME thin films (50 nm- 7nm), where BDS measurements showed two thickness-independent relaxation processes. The first process was assigned to the -relaxation of a bulk-like layer. Whereas the second process showed a different temperature dependence and was ascribed to the relaxation of polymer segments adsorbed at the substrate. To our knowledge, this is the first in-situ study of the dynamics of an irreversibly adsorbed layer.
Thin polymeric films are of great importance of high number of high-tech applications for instance in sensors and nanoelectronics. Form the scientific point of view thin films with thickness below 100 nm are ideal model systems to study confinement effects on its properties for instance on the molecular relaxation processes. In this contribution an overview is presented about the behavior of different systems as investigated by nanosized relaxation spectroscopy like broadband dielectric spectroscopy employing nano structured capacitors and AC chip calorimetry complimented by ellipsometry. The systems considered are PVME1, PVME/PS blends2,3 P2VP4, PBAC5 and polysulfone6. Besides the film also the adsorbed layer on the substrate prepared by a leaching approach and investigated by AFM is considered.1,4-7. For these investigationsss it is found that the adsorbed layer itself exhibits a relaxation dynamics which might be assigned either to molecular motions or to adsorptions desorption kinetics.