Polymer-solid contacts: Interfaces and Interphases

Within the priority program "Polymer-solid contacts: Interfaces and Interphases" of the German Research Foundation (DFG) which was co-initiated by Prof. Faupel, we studied the structure and formation of interfaces between polymers and solid materials like metals and ceramics. Such interfaces are important in many applications including polymer nancomposites in which solid nanoparticles are embedded in a polymer matrix. In particular, we probed the free volume distribution across the polymer-solid interface. The free volume strongly affects many physical properties like viscosity, diffusivity and brittleness. With the technique of positron annihilation lifetime spectroscopy, the average hole size in a material can be directly determined at the atomic scale. A moderated beam of positrons with well defined implantation energy, available at the Research Reactor Munich and in Tsukuba, Japan, can even probe thin films and interphases, which are extended interfacial regions with properties different from the bulk polymer (see figure). As the positron beam broadens on the way from the polymer surface through the polymer to the interface region and the o-Ps intensity is reduced, the resolution and thus the accuracy decreases with increasing penetration depth. To avoid this problem, we also observed the interface region from the substrate side through a hole in the substrate with a focused beam. The results for the polymer Teflon AF on a SiN membrane, for example, reveal in interfacial region of about 10 nm thickness. In this interphase, the average density is approximately 5 % higher.

The above mentioned extended interphase has to be taken into account especially in polymer-nanocomposites, where the properties of the polymer matrix are significantly affected by the contribution of the interphase around the solid nanoparticles. Our investigations also revealed that the influence of a functionalized layer on the surface of nanoparticles, which is generally used in commercial nanocomposites to obtain a uniform dispersion of the nanoparticles, can be seemingly similar to that of an interphase. We showed that these two effects need to be strictly separated.

Fig. 1 Schematic of a polymer solid contact and the implantation and information depth of positron annihilation as function of acceleration energy.

In early work, we also investigated interfacial and surface effects for thermosetting polymers, particularly epoxies. Generally, compared to thermoplastic polymers, the length scale where confinement effects set in turned out to be much larger.

Selected publications

Ohrt, C.; Rätzke, K.; Oshima, N.; Kobayashi, Y.; O'Rourke, B.; Suzuki, R.; Uedono, A.; Faupel, F.; Free volume profiles at polymer-solid interfaces probed by focused slow positron beam, Macromolecules 48 (2015) 1493-1498.

Ohrt, C.; Koschine, T.; Harms, S.; Faupel, F.; Rätzke, K.; Egger, W.; Ravelli, L.; Willner, L.; Schneider; G.J.; Interphases in Polymer Solid-Contacts and Nanocomposites Probed by Positron Annihilation Lifetime Spectroscopy, Soft Materials Vol. 12 Supplement 1 (2014) 135-141.

Butt, H.J.; Duran, H.; Egger, W.; Faupel, F.; Harmandaris, V.; Harms, S.; Johnston, K.; Kremer, K.; Lin, F.Y.; Lue, L.; Ohrt, C.; Rätzke, K.; Ravelli, L.; Steffen, W.; Vianna, S.D.B.; Interphase of a polymer at a solid interface, Macromolecules47(23) (2014) 8459-8465.

Ohrt, C.; Koschine, T.; Rätzke, K.; Faupel, F.; Willner, L.; Schneider, G.J.: Free volume in PEP-silica nanocomposites with varying molecular weight, Polymer 55 (2014) 143-149.

Harms, S.; Rätzke, K.; Zaporojtchenko, V.; Faupel, F.; Egger, W.; Ravelli, L.: Free volume distribution at the Teflon AF®/silicon interfaces probed by a slow positron beam, Polymer Vol. 52(2) (2011) 505-509.

Harms, S.; Rätzke, K.; Faupel, F.; Schneider, G.; Willner, L.; Richter, D.: Free volume of Interphases in Model Nanocomposites Studied by Positron Annihilation Lifetime Spectroscopy, Macromolecules Vol. 43 Issue 24 (2010) 10505-10511.

Early work on thermosetting polymers

Kanzow, J.; Faupel, F.; Egger, W.; Sperr, P.; Kögel, G.; Wehlack, C.; Meiser, A.; Possart, W.: Depth-resolved analysis of the aging behavior of epoxy thin films by positron spectroscopy, Adhesion (2005) 465-477.

Kanzow, J.; Schulze-Horn, P.; Kirschmann, M.; Zaporojtchenko, V.; Dolgner, K.; Wehlack, C.; Possart, W.; Faupel, F.:Formation of a metal/epoxy resin interface, Applied Surface Science 239(2) (2005) 227-236.

Schmidtke, E.; Günther-Schade, K.; Hofmann, D.; Faupel, F.: The distribution of the unoccupied volume in glassy polymers, Journal of Molecular Graphics & Modelling 22(4) (2004) 309-316.

Faupel, F.; Kanzow, J.; Günther-Schade, K.; Nagel, C.; Sperr, P.; Kögel, G.: Positron annihilation spectroscopy, polymers Materials Science Forum 445-446 (2004) 219-223.