Dynamics Of Thin Polymer Films Research Articles

Fluorescence correlation spectroscopy as a tool to investigate single molecule probe dynamics in thin polymer films.

Fluorescence correlation experiments were performed on rhodamine 6G in PDMS spin-coated films on glass surfaces. With polarised excitation, ensemble bleaching of the dye and single molecule intensity fluctuations were observed. From the statistics of single molecule intensity data taken at different positions in the film, correlation functions were calculated. Two modes of motion with exponential decay shapes and correlation times of tau(c) = 0.15 s and tau(c) = 0.7 s could be detected. Potential origins of intensity fluctuations are lateral diffusion, rotational diffusion or intramolecular fluctuations of dyes involving spectral diffusion or photoinduced processes. From the experimental results, lateral diffusion can be ruled out as a motional mode. Single molecule fluctuations are assigned to changes of the molecular configuration of the dyes, which are rigidly bound to the glass. To assess the environmental influence on such molecular motions, the bulk viscosity of the PDMS was varied over two orders of magnitude, leading to changes of tau(c) of the slow mode by a factor of four. This result proves the sensitivity of the single molecule fluctuations to the molecular scale dynamics of the surrounding polymer matrix and makes the correlation time a measure of the local environment of dye probes.

Glass transitions and dynamics in thin polymer films: dielectric relaxation of thin films of polystyrene

The glass transition temperature T(g) and the temperature T(alpha) corresponding to the peak in the dielectric loss due to the alpha process have been simultaneously determined as functions of film thickness d through dielectric measurements for polystyrene thin films supported on glass substrate. The dielectric loss peaks have also been investigated as functions of frequency for a given temperature. A decrease in T(g) was observed with decreasing film thickness, while T(alpha) was found to remain almost constant for d>d(c) and to decrease drastically with decreasing d for d<d(c). Here, d(c) is a critical thickness dependent on molecular weight. The relaxation time tau(alpha) of the alpha process, which was measured as the frequency at which the dielectric loss realizes its peak value at a given temperature, was found to have a d dependence similar to that of T(alpha). The relaxation function for the alpha process was obtained by using the observed frequency dependence of the peak profile of the dielectric loss. The exponent beta(KWW), which was obtained from the relaxation functions, decreases as thickness decreases. This suggests that the distribution of relaxation times for the alpha process broadens with decreasing thickness. The thickness dependence of T(g) is directly related to the distribution of relaxation times for the alpha process, not to the relaxation time itself. The value of the thermal expansion coefficient normal to the film surface was found to increase with decreasing film thickness below T(g), but to decrease with decreasing film thickness above T(g). These experimental results are discussed in the context of a three-layer model in which within thin films there are three layers with different mobilities and glass transition temperatures.

The dynamics of thin polymer films

An increasing amount of experimental data now supports the idea that the dynamics of thin polymer films is different from bulk. An experimental consensus now supports the previously controversial view that glass transition temperatures of thin polymer films on weakly interacting substrates are reduced from bulk values, but evidence for whether the surface has a higher mobility than the bulk is still contradictory.

Electron transport dynamics in thin polymer films containing tris(2,2′-bipyridine)osmium(III/II) complexes

Rotating carbon disk electrodes were modified by a random copolymer (PSS-PVP) of styrenesulfonate and 4-vinylpyridine. The polymer was spincoated onto the electrodes, crosslinked by reaction with 1,2-dibromoethane, then loaded with Os(bpy) 2+ 3 by ion exchange. The oxidation of Fe(CN) 4− 6 contained in an aqueous contacting solution was effectively mediated by the osmium centers in the film. A study of the dynamics was carried out, and the results were analyzed via the theory of Savéant and Andrieux. When chronocoulometry was used to measure the electron diffusion rate in the film, the overall dynamics remained quantitatively inconsistent. The problem with self-consistency seems to arise from an intrinsic inadequacy of chronocoulometry for evaluating electron diffusion coefficients. A steady-state method was developed as an alternative. The polymer film was deposited across the face of a ring-disk assembly, then lateral current flow from disk to ring was measured by a modified form of square-wave voltammetry. The results suggest that the true rates of electron diffusion among osmium centers in PSS-PVP and in Nafion are almost two orders of magnitude larger than projected from chronocoulometry. As the ferrocyanide concentration is raised from 0.021 m M to 13.49 m M the dynamics of mediated oxidation change from SR to SR + E, then perhaps to S + E.