Project C4: Frictional and adhesive properties of polymer surfaces and films of controlled structure and function

Principal Investigators: G. Reiter (Freiburg) / V. Le Houérou (Strasbourg)
Collaborators: C. Gauthier (Strasbourg)

Current state of the research

In many applications, the frictional and adhesive properties of polymer surfaces represent key features controlling performance and durability. For example, in polymer composites between polymeric and filler components, or for coatings by polymer films [1], the frictional and adhesive behavior of polymers at interfaces is decisive. In these cases, it is often crucial to fine-tune the adhesive properties, especially if adhesion should be weak to allow for easy attachment and detachment. A possible solution to this problem, which has recently been pursued by several authors [2,3], is micro-structuration of the polymer surfaces, inspired by the current understanding of how insects and lizards can adhere reversibly to many substrates. For instance, Lamblet et al. demonstrated that the adhesion of an acrylic adhesive at a PDMS substrate can be enhanced through micropatterning of the substrate [2]. Under similar conditions, the influence of different geometric variables of the surface patterns was investigated by Greiner [3]. Both works reveal the complex interplay of the geometry, softness, and aspect ratio of the patterns. The present project intends to integrate these considerations in a larger study which addresses not only adhesive but also frictional properties of micro- or nanopatterned polymer surfaces.

Contributions of the participating groups

The Strasbourg group has recently developed a novel experimental device for “dynamic JKR” tests [4,5], which allows the study of the dynamics of adhesive contacts under cyclic normal load. Besides surface dissipation (related to adhesion and friction), bulk dissipation (related to viscous properties of the material) can also be explored by an analysis of the hysteretic behavior during loading and unloading. In addition, the group has also thorough expertise in the analysis of the friction phenomena at microscopic and nanoscopic scales (scratch/sliding machine, nano-indenter) and in the development of Finite-Element methods to model experimental results. The Freiburg group has long-standing experience in the fabrication of thin polymer films, their characterization, and analysis of their structural and dynamical properties at the nanoscale [5-8]. In previous works, by using the same polymer on both sides of the interface, the Freiburg group focused on purely entropic factors, which control the interfacial zone and its frictional behavior. The particular interest was on friction caused by the penetration of polymers from a well defined layer into a crosslinked body of the same polymer, paying attention to the deformation of the attached polymers (i.e., changes in chain conformations) at high sliding velocities [5,6].

Research project and collaborations

The goal is to identify how polymer surfaces, which are modified in terms of topography, chemical functionality and macromolecular conformations, influence adhesive and frictional properties. We want to establish a correlation between rheological properties, linked to geometry, confinement and chemistry, and energy dissipation processes at polymer surfaces and films. The relevant scales for the considered contact area range from 1 μm to 1 mm for the contact diameter and from few 10 nm to 10 μm in penetration depth. The observed behavior will be resulting from a complex compound of different parameters: adhesion energy, roughness, viscous flow, plastic strain, chemical heterogeneities surfaces, etc. Each of these parameter will be influenced by temperature and chemical environment.

A central idea underlying the proposal is sketched in the figure below. The preparation of (ultra-) thin polymer films by spin coating creates residual stresses [8], possibly related to frozen-in, not well entangled polymer conformations. We expect that such nanoscopic features of the patterned polymer substrate impact the mechanical response at the microscale and beyond. The aim of this project is to explore a possible correlation between surface topography,  chain conformations, their relaxational behavior, and the resulting adhesive and frictional properties. The Reiter group will prepare polymer films with functionalized and well characterized surfaces. Various geometric patterns and chemistry (micrometer topography) having controlled nano- and microscale features and thin film coatings with diverse conformational states will be fabricated.

The mechanical response of these surfaces to contact forces and sliding motion shall then be studied in Strasbourg. Surface dissipation due to friction will be analyzed by standard contact angle measurements and contact hysteresis measurements through dynamic JKR experiments. Previous work [4] will be completed by two complementary steps: (i) It is necessary to identify the bulk energy dissipated during loading and unloading for the patterned surface. A means to achieve this could be the analysis of the effect that the ratio between contact area and edge length of the contact has on the JKR signal. (ii) The effect of surface treatment onto the surface energy dissipation will be analyzed for a flat and well-known hemisphere. At last, the surface of the topographic pattern must be coated or chemically treated. Furthermore, friction tests at the microscale will be carried out within the full range of temperature (–70 to +120 °C) and sliding speed (1 to 104 µm/s) of interest. The home-built microscope will be used to perform in-situ observation of the indenter/sample contact for transparent samples. These experiments shall be further complemented by nano-friction and nano-indentation tests to elucidate the behavior of the “limit layer” at the polymer surface, involved in the friction phenomenon (i.e., the layer affected by the complex interplay of adhesion, elastic response, plastic behavior, and viscous flow). These experimental studies will be complemented by numerical modeling using Finite-Element methods .

This project heavily relies on the complementary expertise of the participating groups – surface fabrication and characterization in Freiburg and experimental/numerical analysis of mechanical properties at multiple length scales in Strasbourg. The project also has strong thematic links with other projects of the IRTG, such as C1 or C3.
 

References
[1]R. A. L. Jones, R. W. Richards, Polymers at Surfaces and Interfaces, Cambridge, 1999. T. Baumberger, C. Caroli, Advances in Physics. 2006. pp. 279-348.
[2]M. Lamblet, E. Verneuil, T. Vilmin, A. Buguin, P. Silberzan, L. Léger, Langmuir, 2007, 23, 6966.
[3]C. Greiner, A. del Campo, E. Arzt, Langmuir, 2007, 23, 3495.
[4]E. Charrault, C. Gauthier, P. Marie, R. Schirrer, Langmuir, 2009, 25, 5847.
[5]A. Casoli, M. Brendlé, J. Schultz, P. Auroy, G. Reiter; Langmuir,  2001, 17, 388
[6]A. Casoli, M. Brendlé, J. Schultz, P. Auroy, G. Reiter; Tribology Lett., 2000, 8,  249.
[7]T. Vilmin, E. Raphaël, P. Damman, S. Sclavons, S. Gabriele, M. Hamieh, G. Reiter; Europhys. Lett., 2006, 73, 906.
[8]G. Reiter, M. Hamieh, P. Damman, S. Sclavons, S. Gabriele, T. Vilmin, E. Raphaël, Nature Mat., 2005, 4, 754.