Diethelm Johannsmann

Viscoelastic Analysis of Organic Thin Films on Quartz Resonators

By analyzing the frequency shifts and bandwidths of coated quartz resonators the viscoelastic coefficients of the coatings may be derived. The paper gives an overview of the experimental procedure and the data analysis. Some examples of the application of this technique in the range of soft condensed matter are described. The swelling and plastification of polymer films in solvent vapor was monitored in-situ. We reported on measurements of the acoustic thickness of polymer brushes, the thickness of which varies with the solvent quality. The acoustic thickness is significantly larger than the optical thickness because the dilute outer portions of the swollen layer affects the acoustic behavior of the brush much more than the optical behavior. Sandwiching polymer layers between quartz plates and rigid overlayers expands the range of the applicability of the technique at the end of low film thicknesses, because the rigid overlayer adds inertia to the composite film and therefore enhances the stress acting on the layer of interest. Finally, the combination with optical birefringence measurements in Fabry-Perot configuration gives access to the stress-birefringence inside the polymer film. Normal stress components proportional to the square of the oscillation amplitude have been measured for the first time.

Synthesis and swelling behavior of a weak polyacid brush

We describe the synthesis and the swelling behavior of a weak polyacid brush attached to a solid surface. Monolayers of poly methacrylic acid are generated by using self-assembled monolayers of an azo initiator and radical chain polymerization of methacrylic acid monomer at the surface of a planar substrate in situ. The thickness of the resulting surface-attached polyelectrolyte brush can be adjusted between 5 and 400 nm in the dry, collapsed state. The swelling behavior in water as a function of pH and salt concentration is investigated by multiple-angle null-ellipsometry. At high salt concentrations the brush thickness decreases due to electrostatic screening. At low concentration, however, an increase of thickness with ion concentration is found.

Surface-rheological measurements on glass forming polymers based on the surface tension driven decay of imprinted corrugation gratings

We report on surface-rheological experiments on glass forming polymers making use of the decay of imprinted surface corrugation gratings. Whereas the grating is frozen in at temperatures below the glass temperature Tg, surface tension drives its decay once the sample is heated to above Tg. From the balance between surface tension and viscous stress, the near-surface shear modulus G(ω) is derived. For polystyrene a marked dependence on chain length is found. While the near-surface moduli agree with the bulk values for highly entangled samples, an increase of the apparent stiffness by a factor of 10 compared to the bulk is found for chains shorter than the entanglement length.

Viscoelastic properties of thin films studied with quartz crystal resonators

We discuss the possibility of determining shear moduli for thin polymeric films coated on quartz resonators by analysing the variation of normalized frequency shift δf/f with different harmonics. For sufficiently thick films, the elastic compliance can be deduced from the increase in δf/f with overtone order. However, a correction must be applied to account for a frequency dependence of the Sauerbrey factor relating frequency shift to film mass. Since the elastic effect scales with the square of the film mass, the correction is most important for thin films. Possible sources of frequency dependence of the Sauerbrey factor are the finite thickness of the electrodes, lateral stress components, and insufficient control of the electrical boundary conditions. In order to obtain the shear compliance, the frequency dependence of the Sauerbrey factor was measured and subtracted from the data on polymeric thin films. The correction procedure can be avoided by using electrodeless quartzes and exciting the vibration with external electrodes across an air gap. Since, in this case, the Sauerbrey factor is approximately constant, the shear compliance can be obtained directly from the variation of δf/f with overtone order.

High frequency tribological investigations on quartz resonator surfaces

We report on the application of quartz crystal resonators for friction measurements in the MHz regime. The dissipative processes are studied by approaching a small sphere to the quartz surface and measuring the shifts in frequency and bandwidth as a function of the sphere–sample distance. Once elastic contact is established, the frequency and the bandwidth can be well described by an increased stiffness of the quartz–sphere system. In the range of distances where contact is just being formed, we observe strongly different behavior for high-friction metal–metal interfaces and low-friction fluoropolymer interfaces. For high-friction interfaces there is an excess dissipation of energy which we attribute to frictional processes in the contact zone.

Simultaneous determination of optical and acoustic thicknesses of protein layers using surface plasmon resonance spectroscopy and quartz crystal microweighing

The optical and the acoustic thicknesses of protein layers during adsorption were simultaneously determined by a combination of surface plasmon resonance spectroscopy and quartz crystal microweighing. Coupling of the surface plasmon was achieved by etching a diffraction grating into the quartz surface prior to the deposition of the metal electrode. The evolution of the acoustic and the optical thickness was markedly different, which is attributed to surface roughness and partial coverage.

Light-Induced Softening of Azobene Dye-Doped Polymer Films Probed with Quartz Crystal Resonators

Employing quartz crystal resonators, we have measured the elastic compliance of thin polymers films doped with an azobenzene dye. From a comparison of the shifts of resonance frequency on different acoustic overtones of the resonator, the films’ elastic compliance was derived. Upon irradiation with ultraviolett light at low intensity, the compliance decreases by a few percent. Irradiation with visible light at high intensity, on the contrary, softens the material. We assume that, when irradiated with high-power visible light, the azobenzene molecules undergo rapid cycling through the trans- and cis-states, thereby softening the polymer matrix.

Segment density profiles of polyelectrolyte brushes determined by Fourier transform ellipsometry

We describe a method for the explicit determination of the segment density profile φ(z) of surface-attached polymer brushes with multiple angle of incidence null-ellipsometry. Because the refractive index contrast between the brush layer and the solvent is weak, multiple reflections are of minor influence and the ellipsometric spectrum is closely related to the Fourier transform of the refractive index profile, thereby allowing for explicit inversion of the ellipsometric data. We chose surface-attached monolayers of polymethacrylic acid (PMAA), a weak polyelectrolyte, as a model system and determined the segment density profile of this system as a function of the pH value of the surrounding medium by the Fourier method. Complementary to the Fourier analysis, fits with error functions are given as well. The brushes were prepared on the bases of high refractive index prisms with the “grafting-from” technique. In water, the brushes swell by more than a factor of 30. The swelling increases with increasing pH because of a growing fraction of dissociated acidic groups leading to a larger electrostatic repulsion.

Derivation of the Shear Compliance of Thin Films on Quartz Resonators from Comparison of the Frequency Shifts on Different Harmonics: A Perturbation Analysis

Viscoelastic effects contribute to the shift in resonance frequency of quartz crystal resonators induced by deposition of thin films on the resonator surface. In turn, the mechanical stiffness of the film can be experimentally determined from a comparison of the resonance shifts on different harmonics. When the film is much thinner than the wavelength of shear sound, a series expansion of the viscoelastic effects to third order in film thickness leads to rather simple equations. When plotting the normalized frequency shift δf/f versus the square of the overtone order n2 one finds a linear relationship, where the slope is determined by the film’s elastic compliance. When the same analysis is carried out on the resonance bandwidths rather than the frequency shifts the viscous compliance is obtained. The effects of asymmetric coatings, electrodes, and liquid media are discussed.

Measurements on the surface glass transition of PMMA from the decay of imprinted surface corrugation gratings: the influence of molecular weight

Abstract The near-surface mobility of poly-methylmethacrylate (PMMA) chains is probed by means of optical diffraction. Particularly, we monitor the surface tension-driven decay of shallow surface corrugation gratings. The probe depth of the technique is about 100 nm. We report on quasi-static experiments, in which the sample temperature was ramped slowly from room temperature to a temperature well above the glass temperature, Tg. An estimate of the near-surface Tg is derived from the temperature at which the decay sets in, Tdec. For high molecular weight PMMA this decay temperature is much less than the bulk Tg as determined by calorimetry on the same material. For low molecular weight PMMA, this effect is absent.