[4] J. Kopp, H. Bannasch, C. Andree, G. B. Stark, Langenbecks Arch. Chir. 1996, S1, 229. [5] K. Kubo, N. Tsukasa, M. Uehara, Y. Izumi, M. Ogino, M. Kitano, T. Sueda, J. Oral Rehabil. 1997, 24, 70. [6] L. T. Canham, Adv. Mater. 1995, 7, 1033. [7] S. C. Bayliss, L. Buckberry, P. J. Harris, C. Rousseau, Thin Solid Films 1997, 297, 308. [8] S. C. Bayliss, P. J. Harris, L. Buckberry, C. Rousseau, J. Mater. Sci. Lett. 1997, 16, 737. [9] A. Curtis, C. Wilkinson, Biomaterials 1998, 18, 1573. [10] S. C. Bayliss, P. J. Harris, L. D. Buckberry, J. Porous Mater., in press. [11] L. D. Buckberry, H. J. Adcock, J. Adler, I. S. Blagbrough, P. J. Gaskin, P. N. Shaw, ATLA, Altern. Lab. Anim. 1994, 22, 72. Low Surface Energy Coatings from Waterborne Nano-Dispersions of Polymer Complexes By Andreas F. Thünemann,* Antje Lieske, and Bernd- Reiner Paulke Fluorocarbon materials are unique in their ability to repel both oil and water. There has been great develop- mental work optimizing fluorocarbon polymers as ultra low-adhesive materials since the discovery of poly(tetra- fluoroethylene) [1] over fifty years ago. Commercially avail- able fluorinated side-chain acrylic and methacrylic poly- mers are typical low surface energy coating materials. [2] Amongst the numerous molecular structures investigated, a close-packed uniform CF 3 surface was found to possess the lowest surface tension ever measured (6 mN/m). [3] A polymeric material that can maintain a stable, uniform CF 3 surface may play a key role in producing non-adhesive ma- terials with surface energies much lower than that of poly- (tetrafluoroethylene) (20 mN/m). Applications of such a material are manifold, ranging from protection from graf- fiti [4] to coatings with anti-fouling properties. [5] Promising strategies for realizing surfaces that are highly enriched with CF 3 groups include the development of self-organizing materials such as Langmuir-Blodgett monolayers, [6,7] fluori- nated block copolymers, [8,9] semifluorinated side-chain ionenes, [10] fluorinated poly(a,L-glutamate)s, [11] and solid complexes of polyelectrolytes with fluorinated surfac- tants. [12,13] Of these promising new low surface energy materials, the one that successfully combines low cost and ecologically friendly synthesis has the best chance of being introduced for daily use. A key factor here is avoidance of organic solvents. In earlier articles we showed that fluori- nated polyelectrolyte surfactant complexes could be pre- pared from aqueous solution. [12,13] However, to date, coat- ings of these materials are prepared from organic solutions. In this paper we report a waterborne nano-dispersion of a new complex which can be applied to smooth surfaces, re- sulting in thin films with surface energies significantly be- low that of poly(tetrafluoroethylene). Imperatively, the op- tical appearance of treated surfaces is not affected by such a coating. For complexation we used two commercially available compounds: Poly(ethyleneimine) (PEI), a branched, water- soluble polymer that is widely used in the paper industry. [14] At low pH, PEI has the highest known charge density of all polyelectrolytes. The second compound, Fluowet SB, is a surfactant made from sulfosuccinic acid, which is esterified with fluorinated side chains. Fluowet SB is regularly used as a spreading agent. [15] The complexation of PEI with Fluowet SB (stoichiometric complex with respect to charge) was performed in water by adding solutions of PEI to a dilute micellar solution of Fluowet SB. In contrast to the procedure described earlier, [12,13] macroscopic precipi- tation of the complex was avoided by using an excess of Fluowet SB to aid dispersion. The diameters of the dis- persed complex particles were about 200 nm, as deter- mined by dynamic light scattering. A dilute (0.3 % w/w) dispersion of PEI-Fluowet SB was deposited on smooth, but chemically different, surfaces such as glass, aluminum, car, and airplane sheets. The coated surfaces were dried, washed extensively with water, and finally polished. By this procedure, transparent, thin coatings of about 400 nm thickness were achieved that were invisible to the human eye. These coatings exhibited highly enhanced contact an- gles of the test liquids deposited on each surface, indicating a significant reduction of the surface energies (Table 1). The surface free energy of a solid can be used as a guide to its relative adhesive properties. However, it is not a straightforward exercise to measure the surface free energy of solids directly. For practical reasons, procedures based on contact angle measurements are regularly used. [10,16±20] Dynamic contact angles (y) of test liquids with different surface tensions were measured by means of the sessile drop method (Table 1). [21] It is found, in particular, that all non-polar liquids that spread readily on the pristine sur- faces show high contact angles on complex-coated surfaces. From the contact angles of a homologous series of non- polar liquids the critical surface tension g c of a surface can be determined as introduced by Zisman. [16] g c defines the wettability of a solid by denoting the lowest surface tension a liquid can have and still exhibit a contact angle greater than zero degrees. The g c of a surface is usually obtained from a Zisman plot of contact angles; [16] i.e., a plot of cos y versus the surface tensions of non-polar liquids g l , with the extrapolation of g l to cos y = 1 giving g c . Zisman plots result in critical surface tensions varying from 13.1 mN/m to 5.7 mN/m depending on the coated material (Fig. 1 and Table 2). The latter value is identical to the lowest critical surface tension encountered to date, [22] which was obtained for an oriented close-packed monolayer of perfluorododec- anoic acid. It is widely accepted that such low critical surface tensions characterize the resulting properties of a surface of closed-packed perfluoromethyl CF 3 groups. [8,23] Adv. Mater. 1999, 11, No. 4 Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim,1999 0935-9648/99/0403-0321 $ 17.50+.50/0 321 Communications ± [*] Dr.A. F. Thünemann Max Planck Institute of Colloids and Interfaces Kantstrasse 55, D-14513 Teltow (Germany) Dr.A. Lieske, Dr. B. R. Paulke Fraunhofer Institut für Angewandte Polymerforschung Kantstrasse 55, D-14513 Teltow (Germany)