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Colloidal Complexes of Perfluorooctadecanoic Acid with Cationic Copolymers Andreas F. Thu¨nemann*,† and Kai Helmut Lochhaas‡ Max Planck Institute of Colloids and Interfaces, Am Mu¨ hlenberg, D-14476 Golm, Germany, and Colloid Surface Technologies GmbH, Industriepark Kalle-Albert, Rheingaustrasse 190-196, 65174 Wiesbaden, Germany Received February 12, 1999. In Final Form: May 12, 1999 Five solid complexes of poly[(diallyldimethylammonium chloride)-co-(N-methyl-N-vinylacetamide)] with perfluorooctadecanoic acid were prepared at 1:1 stoichiometry. Remarkably low surface energies, as determined by contact angle measurements, were attributed to a strong enrichment of complex surfaces with CF3 groups. By increasing of the charge density of the polyelectrolyte, the surface energy decreased from 14 to 9 mN/m. As determined by small-angle X-ray scattering, the solid complexes show a mesomorphic structure. A columnar liquid crystalline-like structure with columns consisting of stacks of disklike micelles, two-dimensionally ordered in hexagonal arrays, is suggested. The thermal stability of the structures is explained by Coulomb interactions between diallyldimethylammonium groups and carboxylate ions
Introduction The systematic preparation of low-energy surfaces performed via the self-assembly of polyelectrolyte-surfactant complexes is potentially of high technological importance as well as of fundamental interest. Coatings, especially in this case, seem to have a great variety of applications for these new materials, because they are easy to prepare. Such coatings are very promising for selfcleaning surfaces or as very thin protective coatings for walls which are prone to contamination (graffiti1). The growing field of solid polyelectrolyte-surfactant complexes has recently been reviewed.2,3 Many of these complexes show the pronounced tendency of forming well-ordered liquid crystalline-like structures with high mechanical and thermal stability. Little is known in the field of fluoroand silicone-surfactant-containing polyelectrolyte-surfactant complexes. Up to now, systems with lamellar mesophase structures and with perforated lamellar structures and one example of a two-dimensional hexagonal mesophase have been found.4-7 Here we report on an ordered columnar mesophase with a hexagonal superstructure formed by the complexation of perfluorooctadecanoic acid (2) with cationic copolymers of diallyldimethylammonium chloride and N-methyl-N-vinylacetamide (1), resulting in poly[(diallyldimethylammonium chloride)-co-(N-methyl-N-vinylacetamide)] (3). The complexation is shown schematically in Figure 1. To investigate the influence of the charge density on the structure, five polymer complexes, varying in the amount of diallyldimethylammonium units, that is, 25, 47, 65, 83, and 100 mol %, were prepared. These salt-free binary * To whom correspondence should be addressed. † Max Planck Institute of Colloids and Interfaces. ‡ Colloid Surface Technologies GmbH. (1) Information supplied by the manufacturer, Colloid Surface Technologies GmbH, Wiesbaden, Germany. (2) Ober, C. K.; Wegner, G. Adv. Mater. 1997, 9, 17. (3) Antonietti, M.; Burger, C.; Thu¨nemann, A. F. Trends Polym. Sci. 1997, 5, 262. (4) Antonietti, M.; Henke, S.; Thu¨nemann, A. F. Adv. Mater. 1996, 6, 41. (5) Thu¨nemann, A. F.; Lochhaas, K. H. Langmuir 1998, 14, 4898. (6) Thu¨nemann, A. F.; Lochhaas, K. H. Langmuir 1998, 14, 6220. (7) Thu¨nemann, A. F.; Lieske, A.; Paulke, B. R. Adv. Mater. 1999, 11, 321.
Figure 1. Sketch of complex formation: (1) poly[(diallyldimethylammonium chloride)-co-(N-methyl-N-vinylacetamide)]; (2) perfluorooctadecanoic acid; (3) stoichiometric polyelectrolyte surfactant complex
systems with 1:1 stoichiometry present, similar to diblock copolymers, the simplest case for studying various aspects of microphase transition.5 Experimental Section A. Materials. The surfactant perfluorooctadecanoic acid was supplied by Aldrich (98%) and used as received. The copolymers poly[(diallyldimethylammonium chloride)-co-(N-methyl-N-vinylacetamide)] were synthesized by radical polymerization with the initiator 2,2′-azobis(2-amidinopropanedihydrochloride) (V50, Wako) in water at 50 °C. The experimental conditions have been described elsewhere.8 All polymers were purified by ultrafiltration (membrane with a molecular-mass cutoff of 10K) and subsequently freeze-dried. The composition of the polymers was analyzed by chloride titration and 13C NMR spectroscopy. The stoichiometry of diallyldimethylammonium chloride to N-methylN-vinylacetamide was 25:75 (a), 47:53 (b), 65:35 (c), and 83:17 (e) (see Figure 1). The molecular weight of the compounds was determined by GPC (0.5 mol/dm3 NaNO3, Tosohaas Progel-TSKPW column, refractive index and light-scattering detectors). The weight- and number-average molecular weights (Mw and Mn, (8) Ruppelt, D.; Ko¨tz, J.; Ja¨ger, W.; Friberg, S. E.; Mackay, R. A. Langmuir 1997, 13, 3316.
10.1021/la990149s CCC: $15.00 © 1999 American Chemical Society Published on Web 08/10/1999
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respectively) of the compounds in Figure 1 are (a) Mw ) 177 000 g/mol, Mn ) 128 000 g/mol, (b) Mw ) 162 000 g/mol, Mn ) 113 000 g/mol, (c) Mw ) 205 000 g/mol, Mn ) 151 000 g/mol, (d) Mw ) 238 000 g/mol, Mn ) 163 000 g/mol, and (e) Mw ) 623 000 g/mol, Mn ) 187 000 g/mol. B. Preparation. A 1:1 equivalent of perfluorooctadecanoic acid (0.5 g) was dissolved in 100 mL of water (Millipore) and adjusted to pH 9 with 10% (w/w) sodium hydroxide: At 90 °C the solution was stirred and a solution of 1.0 equiv of polyelectrolyte in 50 mL of water was added in droplets. The stoichiometry was calculated with respect to the charges. After the pH was adjusted to 3 with 10% HCl, a solid complex was obtained as a white precipitate. The solid was separated, washed three times with 20 mL of hot water (80 °C), and dried for 12 h at the reduced pressure 0.1 mbar. Elemental analysis for the complexes delivered [observed (calculated for a 1:1 stoichiometry)] 3a: C, 35.7 (36.84); H, 3.1 (3.24); N, 4.0 (4.19); Cl,
