Chair for Multicomponent Materials

Glass transition in organic thin films

It is well known that the dynamics of polymers change under confinement. This particularly holds for the glass transition. Consequently, one expects deviations from bulk behavior

- at the free surface,
- at interfaces, and
- in thin films.

The characteristic length scale where these deviations occur depends on various aspects, especially on the type of polymer. In thermoplastic polymers, the length scale is of the order of the radius of gyration. In thermosetting polymers, the crosslinking behavior can be affected over more than 100 mm. As a result, interfaces between solids and thermosetting polymers can extend over a large range and are therefore often called interphases.

Our research in this field includes the following aspects:

Investigation of the glass transition at the surface and in thin films of thermoplastic polymers

In this connection we developed a novel technique to measure the glass transition at polymer surfaces. The technique is based on the embedding of inert nanoparticles (see figure). The embedding process is monitored in situ with high resolution of about 0.1 nm, e.g., by means of X-ray photoelectron spectroscopy (XPS). At the surface glass transition, the polymer surface softens, and the nanoparticles are embedded. This leads to a reduction of the photoelectron intensity of the nanoparticles (typically Au clusters).

fig1

Fig.1 Sketch for the measurement of the surface glass transition temperature by our cluster embedding technique. The embedding process is monitored in situ by XPS (or other surface sensitive techniques).

Our investigations show a small depression of the glass transition temperature at the surface of polystyrene and other thermoplastic polymers. Measurements on polystyrene mixtures with a bimodal molecular weight distribution show that the depression of the surface glass transition temperature cannot be attributed the enrichment of short chains but is rather an intrinsic effect related to the increase in the degrees of freedom.

 

Selected publications

Embedding of noble metal nanoclusters into polymers as probe of the surface glass transition V. Zaporojtchenko, T. Strunskus, J. Erichsen, and F. Faupel, Macromolecules 34, 5, 1125 (2001)

Investigation of the surface glass transition temperature by embedding of noble metal nanoclusters into monodisperse polystyrenes, J. Erichsen, J. Kanzow, U. Schürmann, K. Dolgner, K. Günther-Schade, T. Strunskus, V. Zaporojtchenko, and F. Faupel, Macromolecules 37, 1831 (2004).

Glass transition temperature in thin polymer films determined by thermal discharge in X-ray photoelectron spectroscopy. J. Erichsen, K. Dolgner, V. Zaporojtchenko, and F. Faupel, Macromolecules 37, 8813 (2004) Communication.

 

Glass transition temperature at the surface of ion-beam treated thermoplastic polymers

Etching rate and structural modification of polymer films during low energy ion irradation V. Zaporojtchenko, J. Zekonyte, J. Erichsen, and F. Faupel, Nuclear Instruments and Methods in Physics Research B (NIMB) 208, 155 (2003)

 

Molecular weight dependence of the glass transition

Molecular weight dependence of the surface glass transition of polystyrene films investigated by the embedding of gold nanoclusters, J. Erichsen, K. Günther-Schade, K. Dolgner, V. Zaporojtchenko, T. Strunskus, and F. Faupel, Mat. Res. Soc. Symp. Proc. 710, DD14.5.1 (2002).

 

Polystyrene mixtures with a bimodal molecular weight distribution

Surface glass transition in bimodal polystyrene mixtures J. Erichsen, T. Shiferaw, V. Zaporojtchenko, and F. Faupel, Europ. Phys. J.E, (2007) DOI 10.1140.

 

Dynamics of thermosetting polymers under confinement

We also investigated the glass transition in thin films and at surfaces of thermosetting polymers, particularly of epoxies. These investigations are partly related to applications in the field of adhesives and were carried out with industrial partners in the framework of BMBF projects. Among other subjects, the following points were studied:

- the effect of film thickness on the polymer dynamics, particularly the glass transition
- the surface glass transition
- the effect of confinement on the chemical glass transition during crosslinking
- the glass transition at interfaces.

A central technique was positron annihilation in conjunction with a tunable low energy positron beam.

Generally, compared to thermoplastic polymers, the length scale where confinement effects set in turned out to be much larger.

Selected publications

Depth-resolved analysis of the aging behavior of epoxy thin films by positron spectroscopy J. Kanzow, F. Faupel, W. Egger, P. Sperr, G. Kögel, C. Wehlack, A. Meiser, and W. Possart, in: Adhesion - Current research and applications, W. Possart (Ed.), Wiley-VCH, 465 (2005).

Positron annihilation spectroscopy in polymers F. Faupel, J. Kanzow, K. Günther-Schade, C. Nagel, P. Sperr, and G. Kögel, Mat. Sci. Forum 445-446, 219 (2004), plenary talk at the Int. Conf. Positron Annihilation (ICPA-13), Kyoto, September 7-13, 2003.

J. Kruse, J. Kanzow, K. Rätzke, F. Faupel, M. Heuchel, J. Frahn, and D. Hofmann, Macromolecules 38, 9638 (2005).

Investigation of the surface glass transition temperature by embedding of noble metal nanoclusters into monodisperse polystyrenes J. Erichsen, J. Kanzow, U. Schürmann, K. Dolgner, K. Günther-Schade, T. Strunskus, V. Zaporojtchenko, and F. Faupel, Macromolecules 37, 1831 (2004).

In-situ investigations on the cross-linking process of the epoxy resin system DGEBA-DETA by means of positron annihilation lifetime spectroscopy in comparison with infrared spectroscopy J. Kanzow, V. Zaporojtchenko, H. Nabika, M. Mizuhata, S. Deki, and F. Faupel, Mat. Sci. Forum 445-446, 313 (2004).

Surface glass transition in bimodal polystyrene mixtures J. Erichsen, T. Shiferaw, V. Zaporojtchenko, and F. Faupel, Europ. Phys. J., 31.05.07. - Glass transition temperature in thin polymer films determined by thermal discharge in X-ray photoelectron spectroscopy J. Erichsen, K. Dolgner, V. Zaporojtchenko, and F. Faupel, Macromolecules 37, 8813 (2004) Communication.

Investigation of the surface glass transition temperature by embedding of noble metal nanoclusters into monodisperse polystyrenes J. Erichsen, J. Kanzow, U. Schürmann, K. Dolgner, K. Günther-Schade, T. Strunskus, V. Zaporojtchenko, and F. Faupel, Macromolecules 37, 1831 (2004).
Molecular weight dependence of the surface glass transition of polystyrene films investigated by the embedding of gold nanoclusters J. Erichsen, K. Günther-Schade, K. Dolgner, V. Zaporojtchenko, T. Strunskus, and F. Faupel, Mat. Res. Soc. Symp. Proc. 710, DD14.5.1 (2002).

X-ray reflectivity study on the surface and bulk glass transition of polystyrene R. Weber, K.-M. Zimmermann, M. Tolan, J. Stettner, W. Press, O.H. Seeck, J. Erichsen, V. Zaporojtchenko, T. Strunskus, and F. Faupel, Physical Review E 64, 061508 (2001).

 

Polymer-solid interfaces

Within the framework of the DFG (German Science Foundation) priority program “Polymer-solid junctions: interfaces and interphases”, which was co-initiated by F. Faupel, we investigated polymer-solid junctions. The main technique was positron annihilation in combination with a tunable low energy positron beam. More details are given under the topic “Polymer solid contacts”.