Diffusing-Wave Spectroscopy
This research is part of the NAPOLEON project. NAPOLEON stands for NAnostructured waterborne POLymEr films with OutstaNding properties. The goal of this project is the development of a technology platform based on waterborne nanocomposite particle dispersions.
Polymer Dispersions
Polymer dispersions play an important role in many branches of industry. Coatings, adhesives, and additives for paper production are typical products. The focus of our studies lies on the film formation process (see Fig.1).[1] For polymer dispersions, film formation is more complicated than for polymer solutions. Even though this process has been studied for about half a century, there still are many open questions.
- Fig. 1 Film Formation Process
The Routh-Russel model,[2],[3] which predicts different mechanisms of film formation in different regions of parameter space, provides the theoretical frame. This model takes surface tensions and viscoelastic properties into account, calculates the driving forces for drying, and predicts the mechanisms of film formation (dry/moist sintering, wet sintering, receding waterfront, or capillary deformation).
Diffusing-Wave Spectroscopy
We investigate the kinetics of drying films with Quasi-Elastic Light Scattering (QELS).[4] Dynamic Light Scattering (DLS) is a well-known technique to probe motion in soft condensed matter.[5]-[7] Drying films are an untypical subject of DLS investigations for several reasons. Typical samples in DLS are ergodic and highly diluted. They are usually contained in a glass tube. For drying films, the situation is different: The dispersions have a high solids content (10 to 50%) and are, at least in the final stage, non-ergodic. Even more important for the setup, the films cannot be investigated inside glass tubes. Therefore, a new setup for Diffusing-Wave Spectroscopy[8] has been constructed. The high dilution for DLS measurements comes from the condition of single scattering. DWS probes the other end of this range: multiply multiple scattering. The most important approximation for DWS is the diffusion approximation, which comes from the radiation transfer theory. In this approximation, the light is treated like diffusing photons (see Fig. 2). With this technique, it is possible to follow the cooperative diffusion of the polymer particles.
Current Research Activities
Variation of the softness of polymer particles
Some soft samples show fast relaxation processes after the gel point had been traversed (see Fig. 2). Ideally, the particles should be already close-packed at the gel point and should not move. In realistic systems, close-packing rarely is complete. The observed fast process could be caused by sliding of particles inside holes in order to optimize the packing and relax stress. Stress relaxation is a very important process in film formation, needed to avoid cracking. Further investigation is needed.
Addition of various salts
Salt has a large influence on the inter-particle forces because salt screens the electrostatic repulsion. The effects of salt on the different stages of the drying process are currently investigated. In collaboration with Dr. Joseph Keddie (University of Surrey, Physics Department, UK), parallel measurements with DWS and Magnetic Resonance Imaging (MRI) were performed. Both showed that the concentration as well as the kind of ions has an effect on the drying kinetics.
- Fig. ?? [figure caption]
- Fig. ?? [figure caption]
Methods
- Diffusing-Wave Spectroscopy
- Magnetic Resonance Imaging
Literatur
[1] Keddie J. L. Film Formation of Latex. Mater. Sci. Eng. 1997, 21, 101-
170.
[2] Routh A. F.; Russel W. B. Deformation Mechanisms during Latex Film
Formation: Experimental Evidence. Ind. Eng. Chem. Res. 2001, 40,
4302-4308.
[3] Routh A. F.; Russel W. B. A Process Model for Latex Film Formation:
Limiting Regimes for Individual Driving Forces. Langmuir. 1999, 15,
7762-7773.
[4] Berne B. J.; Pecora R. Dynamic Light Scattering : with Applications to
Chemistry, Biology, and Physics, Dover edition, John Wiley & Sons: New
York, 2000.
[5] Einstein A. Theorie der Opaleszenz von Homogenen Flüssigkeiten und
Flüssigkeitsgemischen in der Nähe des Kritischen Zustandes. Ann. Phys.
1910, 33, 1275-1298.
[6] Smoluchowski M. Molekular-Kinetische Theorie der Opaleszenz von
Gasen im Kritischen Zustande, sowie einiger Verwandter
Erscheinungen. Ann. Phys. 1908, 25, 205-226.
[7] Debye P. Light Scattering in Solutions. J. Appl. Phys. 1944, 15, 338-
342.
[8] David A. Weitz and David Pine, Diffusing-Wave Spectroscopy, Dynamic Light Scattering, Ed. Wyn Brown , 1992, O.U.P..
Financial Support
This project is funded by the NAPOLEON project.
The project is carried out by Alexander Martin König.
alexander.martin.koenig@tu-clausthal.de