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Project B3: Influence of nanofillers on the drying and properties of polymer latex films

Principal Investigators: E. Bartsch (Freiburg) / H. Meyer (Strasbourg)
Collaborators: Y. Holl, C. Gauthier (Strasbourg)
PhD Student: Vasilii Lesnichii

Current state of the research

High-performance coatings are increasingly produced via the drying of water-based polymer (“latex”) dispersions which replace polymer films from solvent-casting due to environmental reasons. An ongoing challenge in the field of latex film formation is posed by two conflicting requirements: sufficient viscoelasticity of the polymer material during film formation and high mechanical strength of the final film [1]. A recent solution to this problem has come from transfering the concept of nanofillers, i.e. reinforcing polymer films with nanoparticles, to latex films either via polymer-latex blends composed of high-Tg and low-Tg components or via core-shell polymer latexes with a high-Tg core and a low-Tg shell [2]. (The development of nanocomposite dispersions filled with silica particles has also been reported [3]). While several works on the dependence of the macroscopic performance of nanofilled polymer- latex films on size, shape or surface properties are available [4], studies addressing the molecular origin of the achieved improvements are scarce. One exception is the work of Ref. [5] where the effect of silica nanofillers on polymer interdiffusion was studied and a decrease of the polymer diffusion coeffcient was observed. Following Ref. [6] this effect was explained by the presence of a layer of reduced (polymer) mobility around the nanofillers.

In this project the film formation from core-shell latexes and the properties of the final films will be studied by a combination of tracer and polymer diffusion experiments, computer simulations of solvent and polymer motion near rigid spherical substrates, and experiments monitoring the distribution of surfactants and water during drying and in the final films as well as the mechanical properties of the films.

Contributions of the participating groups

The Freiburg group has recently shown by Forced Rayleigh scattering (FRS) that tracer diffusion studies on film forming core-shell latex dispersions can yield insight into the drying process and the properties of the final film (e.g., resistance to water penetration, presence of a polymer layer with reduced mobility around rigid cores) [7]. In addition, the FRS technique has been widely used to measure very slow polymer diffusion close to Tg [8]. On the other hand, the simulation group in Strasbourg has studied in the past model polymer films on cooling towards Tg  [9]. The films are either freely standing (two free interfaces) or  supported by a (smooth) substrate. In both cases the film dynamics is spatially heterogeneous: there is a smooth gradient from enhanced relaxation at the interfaces towards (slow) bulk dynamics with increasing distance from the interfaces. This gradient leads to a depression of Tg  relative to the bulk, in pure polymer films [9] and supported polymer-solvent mixtures [10].  Among mechanical properties, friction and scratch resistance are of particular practical and scientific interest as they depend on shear and compression properties at the surface. To understand friction measurements properly surface analysis and data on bulk mechanical properties are required. Thanks to previous experimental and modeling work in Strasbourg [11] true friction coefficients can be extracted from raw friction data. The well-designed polymeric films synthesized in Fribourg are perfectly suited to address some open theoretical problems, especially the question of whether the characteristic length scales for shear and compression are different, and the role of the surfactant segregated at the surface during film formation – an issue already addressed for simpler latex films in Strasbourg [12].


Research project and collaborations

b3We want to study the film formation of core-shell particles composed of a (essentially) poly-(tetrafluoroethylene) (PTFE) core and a poly(n-butylmethacrylate) (PBMA) shell (see figure). To systematically explore the influence of the core and shell on the film properties we vary the shell size and shell Tg (copolymerisation of BMA and methylacrylate) as well as the size and nature of the core (nanofiller in the final film) using also silica and surface-modified (compatibilized) silica particles as cores. As a control, we also produce some filled films from latex blends of the same materials for comparison. For these studies we use FRS with a variety of dyes which allow to probe the hydrophobic (polymer particle plus particle interface) and hydrophilic (interstitial water and interface) parts of the drying films as well as the small molecule and polymer diffusion. These studies are complemented by experiments on macroscopic film properties, especially on the mechanical performance (apparent and true friction coefficients, shear dynamic moduli; collaboration with C. Gauthier) and on the distribution of surfactant (via Confocal Raman Microscopy, XPS, ATR, contact angle hysteresis; collaboration with Y. Holl) and water (via Magnetic Resonance Profiling; collaboration with J. L. Keddie, University of Surrey).

In collaboration with the Freiburg group the Strasbourg simulation group will develop a simulation model to study solvent (the FRS dye) and polymer diffusion in viscoelastic polymer layers supported by a substrate and containing adsorbing filler particles arranged on an fcc lattice (see figure). The surface structure and attraction strength of the fillers, the experimental size ratios (filler radius to FRS grating spacing), and the properties of the filler and polymer shall be adapted as closely as possible to the experimental system. The simulation then aims to calculate the FRS signal and compare this with the self-diffusion of the components at different water content. Calculation of local mechanical properties would also be interesting [13]; this can profit from interactions with project C3.

We believe that the combination of FRS experiments and simulation, monitoring dynamic properties on molecular length scales, with methods like Confocal Microscopy, MRI and mechanical spectroscopy which probe film properties on a more or less macroscopic level, is a crucial advantage of the present project as compared to approaches using a single technique. Thus, we expect to obtain a more complete understanding of the mechanism and principles controling the performance of nanofilled coatings derived from latex dispersions.


[1]J. L. Keddie, Mater. Sci. Eng. R21, 101 (1997).
[2]P. A. Steward, J. Hearn, M. C. Wilkinson, Adv. Colloid Interf. Sci. 86, 195 (2000).
[3]F. Tiarks, J. Leuninger, O. Wagner, E. Jahns, H. Wiese, Surf. Coatings Intern. 5, 221 (2007).
[4]A. F. Routh, J. L. Keddie, Latex Film Formation: with Applications in Nanomaterials, Springer, (2009).
[5]M. Kobayashi, Y. Rharbi, L. Brauge, L. Cao, M. A. Winnik, Macromolcules 35, 7387 (2002).
[6]G. Tsagaropoulos, A. Eisenberg, Macromolecules 28, 6067 (1995).
[7]K.I. Suresh, A. Veniaminov, E. Bartsch, J. Polym. Sci. B 45, 2823 (2007).
[8]D. Ehlich, H. Sillescu, Macromolecules 23, 1600 (1990).
[9]S. Peter, S. Napolitano, H. Meyer, M. Wübbenhorst, J. Baschnagel, Macromolecules 41, 7729 (2008).
[10]S. Peter, H. Meyer, J. Baschnagel, J. Chem. Phys. 131, 014902 (2009).
[11]S. Lafaye, C. Gauthier, R. Schirrer, Tribology International 38, 113 (2005).
[12]F. Belaroui, M. P. Hirn, Y. Grohens, P. Marie, Y. Holl, J. Coll. Interf. Sci. 261, 336 (2003).
[13]G. J. Papakonstantopoulos, K. Yoshimoto, M. Doxastakis, P.F. Nealy, J.J de Pablo, Phys. Rev. E 72, 031801 (2005).


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