Chair for Multicomponent Materials

Polymer surface modification

The modification and of polymer surfaces with low energy ions is not only of fundamental interest but has various applications. Examples are

• improvement of adhesion of metals and other materials,
• control of cell adhesion,
• tuning of surface energy and hydrophobicity.

Low-energy ions can be produced by ion beams and by plasmas. For industrial applications atmospheric plasmas are particularly interesting. For fundamental studies, we often use ion beams of well defined energy.

We have been working on polymer surface modification partly together with partners from industry and within the framework of BMBF projects.

The following processes dominate the interaction of low energy ions with polymers (see figure):

• formation of new functional groups,
• increase in surface energy,
• sputtering of low molecular weight species,
• surface cross-linking, and
• removal of contaminants.

fig1

Fig. 1 Sketch of the main processes occuring in polymer surface modification by low-energy ions.

Experimental methods

Our experimental methods include:

• Contact angle measurements to coarsely and quickly reveal changes in surface energy.

• X-ray photoelectron spectroscopy (XPS, ESCA) to study the formation of new functional groups. Angle resolved XPS allows us to obtain information on the treatment depth.

• Atomic force microscopy (AFM) to detect changes in the surface roughness.

• Peel-test to measure adhesion of metals and other materials.

• A novel method introduced by our group to measure the condensation (sticking) coefficient, for details see:

-  Condensation coefficients of silver on polymers; A. Thran, M. Kiene, V. Zaporojtchenko, and F. Faupel, Phys. Rev. Lett. 82, 1903 (1999).

- Condensation coefficients and initial stages of growth for noble metals deposited onto chemically different polymer surfaces; V. Zaporojtchenko, K. Behnke, A. Thran, T. Strunskus, and F. Faupel, Appl. Surf. Sci. 144-145, 355 (1999

- Condensation coefficients of noble metals on polymers: A novel method of determination by X-ray photoelectron spectroscopy; V. Zaporojtchenko, K. Behnke, T. Strunskus, F. Faupel, Surf. Interf. Anal., 30, 439 (2000)

• A novel method introduced by our group to measure surface cross-linking.
This method is based on our technique to measure the change in the surface glass transition temperature and an empirical relation between glass transition temperature and degree of crosslinking. For details see:
- Investigation of the drastic change in the sputter rate of polymers at low ion fluence; J. Zekonyte, V. Zaporojtchenko, and F. Faupel, Nuclear Instruments and Methods in Physics Research B (NIMB) 236, 241 (2005).

 

Dependence of the sputter rate on the polymer structure

We found huge variations in the sputter yield of polymers of about two orders of magnitude which we could attribute to the competition between crosslinking and chain scission. This competition strongly depends on the polymer structure. Details are reported in:

• 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)
• Investigation of the drastic change in the sputter rate of polymers at low ion fluence; J. Zekonyte, V. Zaporojtchenko, and F. Faupel, Nuclear Instruments and Methods in Physics Research B (NIMB) 236, 241 (2005).
• Effects of ion beam treatment on atomic and macroscopic adhesion of copper to different polymer materials; V. Zaporojtchenko, J. Zekonyte, F. Faupel, Nucl. Instr. and Meth. in Phys. Res. B. 265 139-145 (2007).

The largest sputter rate was found for Teflon. Based on this finding, we now use Teflon for sputter deposition of organic films and preparation of polymer-metal nanocomposites.

Initial time dependence of the polymer sputter rate

We also investigated the initial time dependence of the polymer sputter yield based on two new techniques which we developed to measure in situ the removal of material from the polymer surface with extreme precision. The sputter yield of polymers turned out to exhibit an initial huge drop for most polymers (see figure) which we could attribute to the time needed to form a steady state between crosslinking and chain scission. Details are reported in:

• Mechanisms of Argon ion-beam modification of polystyrene J. Zekonyte, J. Erichsen, V. Zaporojtchenko, F. Faupel, Surface Science 532-535, 1040 (2003).
• 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).

Our investigations also have consequences for the interpretation of SIMS (secondary ion mass spectroscopy) measurements for polymers.

fig2

Fig.2 Initial dependence of the sputter yield and the crosslink density on the ion fluence for treatment of polystyrene with 1 keV Ar ions.

Macroscopic adhesion and atomic sticking

The figure below shows a typical example of the effect of oxygen ion beam treatment on the adhesion of Cu on different polymers. Depending on the polymer, one observes a strong enhancement of the peel strength by several orders of magnitude or almost no effect. Particularly striking is the comparison between PS and PaMS which have almost the same structure. Generally, a maximum in the fluence dependence is observed. Above a certain fluence the adhesion becomes worse again. The details also depend on the type of ion and exposure to gas atmospheres.

fig3

Fig. 3 Peel strength as function of ion fluence (1 keV O2+) for different polymers.

Our investigations explain the effect of the polymer structure on the adhesion enhancement in terms of the different ability of the polymers to form crosslinks. A particularly striking result is that the maximum peel strength is reached at a fluence which is about two orders of magnitude below the fluence where atomic sticking saturates (see figure below). This means that newly arriving ions create new functional groups and thus give rise to a further increasing contact angle but macroscopic adhesion is weakened due to chain scission processes. In particular, this demonstrates that contact angle measurements can be very misleading for the improvement of adhesion.

The reduction of the peel strength above the maximum is attributed to the formation of a weak boundary layer where, among other processes, low molecular weight species resulting from chain scissions accumulate (see layer model below).

fig4

Fig. 4 Cu sticking (condensation) coefficient on Ps as function of Ar+ ion fluence. Atomic adhesion (and contact angle) saturate at a fluence which is about two orders of magnitude above the fluence of maximum peel strength.

Layer Model

fig5

Fig. 5 Sketch of the layer model resulting from our investigations. The model in particular explains the microscopic origin of the weak boundary layer and allows one to predict its depth below the surface.

Details are reported in:

• Influence of thermal treatment on the morphology and adhesion of gold films on TMC-Polycarbonate C. v. Bechtolsheim, V. Zaporojtchenko, and F. Faupel, Appl. Surf. Sci. 151, 119 (1999).
• Structural and chemical surface modification of polymers by low-energy ions and influence on nucleation, growth and adhesion of noble metals J. Zekonyte, V. Zaporojtchenko, S. Wille, U. Schürmann, and F. Faupel, in: Polymer Surface Modification: Revelance to Adhesion, K.L. Mittal (Ed.), 3, 243 (2004).
• Formation of a metal/epoxy resin interface J. Kanzow, P. Schulze Horn, M. Kirschmann, V. Zaporojtchenko, K. Dolgner, C. Wehlack, W. Possart, and F. Faupel, Applied Surface Science 239, 227 (2005).
• Tailoring of thermoplastic polymer surface with low energy ions: relevance to growth and adhesion of Cu J. Zekonyte, V. Zaporojtchenko, and F. Faupel, Proc. 5th Int. Symposium on Polymer Surface Modification: Relevance to Adhesion, Toronto, Kanada, Juni 2005, Ed. K. Mittal, Adhesion Aspects of Thin Films, Vol. 3, pp. 235-262 (2007).
• Effects of ion beam treatment on atomic and macroscopic adhesion of copper to different polymer materials, V. Zaporojtchenko, J. Zekonyte, F. Faupel, Nucl. Instr. and Meth. in Phys. Res. B. 265 139-145 (2007).

Nanostructuring of polymers with low-energy ions

We also applied low-energy ions for nanostructuring of polymer surfaces. The following figure gives an example.

fig6

Fig. 6 Formation of nanoneedles via ion-beam treatment of Teflon AF. Each needle has a Ag on top which was used as a nanomask.