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The growth kinetics of the adsorbed layer of poly(2-vinylpiridine) on silicon oxide is studied using a leaching technique which is based on the Guiselin brushes approach. The adsorbed layer is grown from a 200 nm thick P2VP film for several annealing time periods at different annealing temperatures. Then the film is solvent-leached, and the height of the remaining adsorbed layer is measured by atomic force microscopy. At the lowest annealing temperature only a linear growth regime is observed, followed by a plateau. Here, the molecular mobility of segments is too low to allow for a logarithmic growth. At higher annealing temperatures, both linear and logarithmic growth regimes are observed, followed by a plateau. At even higher annealing temperatures, the growth kinetics of the adsorbed layer changes.
A linear growth followed by logarithmic growth kinetics is observed for short annealing time periods. For longer annealing time periods, an upturn of the growth kinetics is observed. At the highest annealing temperature, only a logarithmic growth regime is found. The change in the growth kinetics is discussed by an alteration in the structure of the adsorbed layer. Moreover, the interaction between the polymer segments and the substrate becomes weaker due to both enthalpic and entropic effects. Therefore, at high annealing temperatures the polymer segments might more easily desorb from the substrate.
The glass transition behavior of thin films of poly(bisphenol A carbonate) (PBAC) was studied employing ellipsometry. The glass transition temperature increases with the reduction of the film thickness. This result is attributed to the formation of an adsorbed layer with a reduced mobility compared to bulk PBAC. Therefore, for the first time, the growth kinetics of the adsorbed layer of PBAC was investigated, prepared by leaching samples from a 200 nm thin film which were annealed for several times at three different temperatures. The thickness of each prepared adsorbed layer was measured by multiple scans using atomic force microscopy (AFM). Additionally, an unannealed sample was measured. Comparison of the measurements of the unannealed and the annealed samples provides proof of a pre-growth regime for all annealing temperatures which was not observed for other polymers. For the lowest annealing temperature after the pre-growth stage only a growth regime with a linear time dependence is observed. For higher annealing temperatures the growth kinetics changes from a linear to a logarithmic growth regime at a critical time. At the longest annealing times the films showed signs of dewetting where segments of the adsorbed film were removed from the substrate (dewetting by desorption). The dependence of the surface roughness of the PBAC surface on annealing time also confirmed that the films annealed at highest temperatures for the longest times desorbed from the substrate.
In the course of miniaturizing modern technology down to the molecular scale, understanding the materials behavior has to be investigated and deviations from the bulk have to be understood. A combination of nano-sized relaxation spectroscopies (Broadband Dielectric Spectroscopy (BDS) and Specific Heat Spectroscopy (SHS)) employing AC nanochip calorimetry were utilized to investigate the glassy dynamics of ultra-thin films of blends of Poly (vinyl methyl ether) (PVME) / Polystyrene (PS) (50:50 and 25/75 wt-%), which are miscible in the bulk (thicknesses: 8nm - 200nm, film thickness was controlled by ellipsometry, film topography by AFM). Both methods are sensitive to different probes; where SHS senses entropy fluctuations BDS measures dipole fluctuations. For BDS measurements, a recently developed nano-structured electrode sample arrangement is employed. By measuring the dynamic glass transition in dependence of the film thickness, SHS showed that the Tg of the whole film was strongly influenced by a nanometer-thick surface layer at the polymer/air interface due to a self-assembling process. Compared to the SHS results the BDS measurements show a completely different behavior. At high temperatures the temperature dependence of the relaxation times of the films follows that of bulk-like PS/PVME obeying the VFT-law. With decreasing temperature the temperature dependence deviates from the VFT to an Arrhenius law where the apparent activation energy decreases with decreasing film thickness. This is the first example were confinement induced changes were observed by BDS for ultra-thin films. All results were analyzed in detail in a comprehensive discussion.
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.
Specific heat spectroscopy (SHS) employing AC nanochip calorimetry was used to investigate the glassy dynamics of ultra-thin films (thicknesses: 10 nm–340 nm) of a polymer blend, which is miscible in the bulk. In detail, a Poly(vinyl methyl ether) (PVME)/Polystyrene (PS) blend with the composition of 25/75 wt. % was studied. The film thickness was controlled by ellipsometry while the film topography was checked by atomic force microscopy. The results are discussed in the framework of the balance between an adsorbed and a free surface layer on the glassy dynamics. By a self-assembling process, a layer with a reduced mobility is irreversibly adsorbed at the polymer/substrate interface. This layer is discussed employing two different scenarios. In the first approach, it is assumed that a PS-rich layer is adsorbed at the substrate. Whereas in the second approach, a PVME-rich layer is suggested to be formed at the SiO2 substrate. Further, due to the lower surface tension of PVME, with respect to air, a nanometer thick PVME-rich surface layer, with higher molecular mobility, is formed at the polymer/air interface. By measuring the glassy dynamics of the thin films of PVME/PS in dependence on the film thickness, it was shown that down to 30 nm thicknesses, the dynamic Tg of the whole film was strongly influenced by the adsorbed layer yielding a systematic increase in the dynamic Tg with decreasing the film thickness. However, at a thickness of ca. 30 nm, the influence of the mobile surface layer becomes more pronounced. This results in a systematic decrease in Tg with the further decrease of the film thickness, below 30 nm. These results were discussed with respect to thin films of PVME/PS blend with a composition of 50/50 wt.%as well as literature results.