Nanostructured Polymer Films

Dewetting of thin polymer films

Nanostructured polymer surfaces are of strong interest with respect to basic research as well as for future applications. With the increasing demand on miniaturization of electronic devices the evolved polymer structures have to be decreased in their spatial dimensions as well. Due to the natural size of polymers, e.g. given by the radius of gyration of the unperturbed polymer chain, which is on the order of 10 nm, a simple scaling down assumption is limited. With structural sizes approaching the nanoscopic range significant changes in the underlying physical process occur. Examples are the slowing down of chain diffusion near surfaces, the modification of the chain conformation in ultra thin films and the shift in the glass transition temperature in thin films. In addition, the stability of these films is changed and a destabilization by dewetting will give rise to destruction of an initially homogeneous film. On the other hand this dewetting process can be utilized for the creation of nanostructured polymer surfaces. It is the aim to build-up controlled internal strcutures of periodically arrange diblock copolymer molecules which are confined into super-structures like droplets (originated from a second process, e.g. dewetting) acting as a matrix. By a variation of the preparation conditions, defined by the annealing temperature (above or below the micro-phase separation temperature), the volume of the polymeric material (confined into the super structure) and the parameters of the diblock copolymer itself (molecular weight, block ratio) different types of structures will become installed.

The model system polystyrene (PS) on top of silicon substrates (Si) covered with an oxide layer (SiOx) turned out to be preferentially examined. With respect to the long-ranged part of the effective interface potential as long as only van der Waals contributions are taken into account, PS is stable on Si and unstable on SiOx. The acting short ranged interaction gives rise to a more complex behavior which results in a two step dewetting process and structures on two hierarchical levels (see figure 1).

Within this article we focus on possible preparation routes of nano-patterns following these ideas. Thus we restrict to the regime of confined thin films with initial film thickness smaller as compared to twice the radius of gyration of the unperturbed molecule. Films are prepared by spin-coating to access this regime. Confined homopolymer films, polymer blend films as well as diblock copolymer films are subjected to a destabilization via a solvent vapor storage. As a consequence, the thin film morphology results from an interplay between dewetting, phase separation and micro-phase separation. Solvent vapor storage has developed to an alternative with respect to annealing above the glass transition of the polymers. After the initial preparation of the polymer film, exposure to a solvent vapor lowers the glass transition temperature. The polymer layer is swollen by the incorporated solvent (e.g. toluene) molecules from the surrounding atmosphere, which act as a plasticizer. As a consequence of different solubility of various polymers in the used solvent the swelling differs for two polymers in a binary blend or two blocks of a block copolymer. The original homogeneous polymer film is replaced by a highly concentrated polymer-solvent solution layer. Therefore the viscosity and surface tension are reduced. In addition the long range interaction changes. The polymer-substrate van der Waals interaction is replaced by the polymer/solvent-substrate interaction. This changes the effective Hamaker constant of the system. By extracting the solvent (e.g. toluene) molecules in a solvent quench the produced polymeric structure build-up by the remaining homo- or diblock copolymers is frozen-in and stabilized.

To determine a statistical description parallel to the sample surface, the power spectral density (PSD) function needs to be calculated. From a two-dimensional Fourier transformation an intensity distribution in reciprocal space is obtained as shown in figure 3b. In case a ring of intensity appears (as demonstrated within the particular example chosen here), after a radial averaging the PSD results. Repeating these steps with SFM data collected for different scan ranges yields a set of PSD functions covering different ranges in reciprocal space (given by the resolution and the maximal scan range of the individual SFM data). To enlarge the total accessible range in reciprocal space in a final step these PSD functions are merged into one (so called) master curve as shown in figure 3c. The position of the intensity peak corresponds to the most prominent in-plane length statistically describing the surface structure parallel to the sample surface.

Selected Publications:

  • 1. P.Müller-Buschbaum, P.Vanhoorne, V.Scheumann, M.Stamm
    Observation of nano-dewetting structures;
    Europhys.Lett. 40, 655 (1997)
  • 2. P.Müller-Buschbaum, S.A.O'Neil, S.Affrossman, M.Stamm
    Phase separation and dewetting of weakly incompatible polymer blend films;
    Macromolecules 31 ,5003 (1998)
  • 3. P.Müller-Buschbaum, M.Stamm
    Dewetting of thin polymer films: An x-ray scattering study;
    Physica B 248, 229 (1998)
  • 4. P.Müller-Buschbaum, J.S.Gutmann, M.Stamm
    Control of surface morphology by an interplay between phase separation and dewetting;
    J.Macromol.Sci. B38, 577 (1999)
  • 5. P.Müller-Buschbaum, J.S.Gutmann, M.Stamm
    Dewetting of confined polymer films: An x-ray and neutron scattering study;
    Phys.Chem.Chem.Phys. 1, 3857 (1999)
  • 6. P.Müller-Buschbaum, J.S.Gutmann, M.Stamm, R.Cubitt
    Surface structure analysis of thin dewetted polymer blend films;
    Macromol.Symp. 149, 283 (2000)
  • 7. P.Müller-Buschbaum, J.S.Gutmann, M.Stamm, R.Cubitt, S.Cunis, G.von Krosigk, R.Gehrke, W.Petry
    Dewetting of thin polymer blend films: Examined with GISAS;
    Physica B 283, 53 (2000)
  • 8. P.Müller-Buschbaum, J.S.Gutmann, M.Stamm
    Influence of blend composition on phase separation and dewetting of thin blend films;
    Macromolecules 33, 4886 (2000)
  • 9. P.Müller-Buschbaum, J.S.Gutmann, C. Lorenz-Haas, O.Wunnicke, M.Stamm, W.Petry
    Dewetting of thin diblock copolymer films;
    Macromolecules 35, 2017 (2002)
  • 10. C.Lorenz-Haas, P.Müller-Buschbaum, J.Kraus, D.G.Bucknall, M.Stamm
    Nucleated dewetting of thin polymer films;
    Applied Phys A 74, S383 (2002)
  • 11. P.Müller-Buschbaum, S.V.Roth, M.Burghammer, A.Diethert, P.Panagiotou, C.Riekel
    Multiple-scaled polymer surfaces investigated with micro-focus grazing incidence small-angle x-ray scattering;
    Europhys.Lett. 61, 639 (2003)
  • 12. P.Müller-Buschbaum
    Dewetting and pattern formation in thin polymer films as investigated in real and reciprocal space (invited review);
    J.Phys.Condens.Matter 15, R1549 (2003)
  • 13. P.Müller-Buschbaum, R.Cubitt, W.Petry
    Nano-structured diblock copolymer films: A grazing incidence small-angle neutron scattering study;
    Langmuir 19, 7778 (2003)
  • 14. P.Müller-Buschbaum, E.Bauer, O.Wunnicke, M.Stamm
    Control of thin film morphology by an interplay of dewetting, phase separation and micro-phase separation;
    J.Phys.Condens.Matter 17, S363 (2005)
  • 15. P.Müller-Buschbaum, E.Bauer, E.Maurer, K.Schlögl, S.V.Roth, R.Gehrke
    A new route to large-area ordered polymeric nano-channel arrays;
    Appl.Phys.Lett. 88, 083114 (2006)