Poly(ethylene imine) n-Alkyl Carboxylate Complexes Andreas F. Thu ¨ nemann* Institute of Theoretical Physics II, Heinrich Heine University Du ¨ sseldorf, Universita ¨ tsstrasse 1, D-40225 Du ¨ sseldorf, Germany, and Max Planck Institute of Colloids and Interfaces, Am Mu ¨ hlenberg, D-14476 Golm, Germany Sascha General Max Planck Institute of Colloids and Interfaces, Am Mu ¨ hlenberg, D-14476 Golm, Germany Received July 13, 2000. In Final Form: August 14, 2000 Polyelectrolyte complexes of high-molecular-weight poly(ethylene imine) (PEI) and n-alkyl carboxylic acids were prepared. They were investigated with the use of differential thermal analysis, small- and wide-angle X-ray scattering techniques, and fluorescence spectroscopy. The chain lengths of the carboxylic acids varied from decanoic acid (C10) to hexacosanoic acid (C26). Order-order transitions from smectic C to smectic A structures were found, whose enthalpies increase with 3 kJ/mol per CH2 group. The transitions result from the melting of n-alkyl side chains, which are packed into a two-dimensional hexagonal lattice. The transition temperatures, Tm, increased from -2 to 83 °C when the chain lengths were increased from C12 to C26. Thomson’s rule was used to arrive at a quantitative description of the dependency of Tm on the chain length. Pyrene was incorporated into the complexes, and the apparent dielectric constant of its surroundings was determined to decrease stepwise with the increasing length of the side chains. Introduction Poly(ethylene imine) (PEI) is a branched, water-soluble polymer that is widely used in the paper industry. 1 At low pH values, PEI has the highest known charge density of all polyelectrolytes. A further characteristic of PEI is its high capability in forming a number of different complexes with metal ions, 2 anionic polyelectrolytes, 3,4 and surfac- tants. 5 Rabolt et al. have shown that PEI forms a strong polyion complex with monolayers of perfluorostearic acid. 6 Such complexes are useful for making Langmuir-Blodgett films more robust. Kunitake et al. were able to prepare surfaces made of a PEI-surfactant complex which show an excellent blood compatibility. 7 The optimization of PEI- DNA complexes as gene vectors with an excellent safety profile 8 is another example of the numerous efforts being made in food, cosmetic, and pharmaceutical research to develop new applications for PEI. In an earlier study we reported on the complexation of PEI with retinoic acid, which serves as a possible new colloidal drug formulation. 9 In this work, we report on complexes of high-molecular- weight PEI and carboxylic acids in the solid state. The goal of this study is to understand the influence of the chain lengths of the carboxylate moieties on the thermal properties of the complexes and on the supramolecular structures. Experimental Section Materials. Water-free, high-molecular-weight poly(ethylene imine) (Mw ) 25 000 g/mol) was supplied by BASF (Ludwigs- haven, Germany). This polymer is highly branched, with molar ratios of 34:40:26 primary to secondary to tertiary amino groups). The carboxylic acids, pyrene and tetrahydrofurane (HPLC grade), were supplied by Aldrich and were used as received. The purity of the carboxylic acids was checked by determining their melting points and melt enthalpies using differential calorimetry. It was found that the melting points and the melt enthalpies are identical with the data reported in the literature. 11 Complex Preparation. For complex preparation, 22 mmol of PEI was dissolved in 20 mL of an ethanol-water mixture. The ratio of ethanol to water was 4:1 (v/v). Next, the PEI solution was slowly added to 11 mmol of the carboxylic acid, which was dissolved in 20 mL of hot ethanol (65 °C). The transparent solution was stirred for a further 30 min at 65 °C and then cast into films of the complex as described earlier. 12 Pyrene was incorporated into the complexes by dissolving the complexes in tetrahydro- furane and adding 0.05% (w/w) of pyrene. The homogeneous complex/pyrene solution was then cast into films. Methods. The FTIR spectra were recorded on a Nicolet Impact 400 spectrometer. The differential scanning calorimetry (DSC) measurements were performed on a Netzsch DSC 200. The samples were examined at a scanning rate of 10 K min -1 by applying two heating scans and one cooling scan. The onsets of the exothermic peaks in the second heating cycles were used to determine the melt transitions of the carboxylic acids and the PEI-carboxylate complexes. Wide-angle X-ray scattering (WAXS) measurements were carried out using a Nonius PDS120 powder diffractometer by transmission geometry. A FR590 generator was used as the source for Cu KR radiation, monochromatization * Corresponding author. E-mail: andreas.thuenemann@ mpikg-golm.mpg.de. (1) Information supplied by the manufacturer, BASF, specialty chemicals, Ludwigshafen, Germany. (2) Bekturov, E. A.; Mamutbekov, G. K. Macromol. Chem. Phys. 1997, 198, 81-88. (3) Kramer, G.; Buchhammer, H. M.; Lunkwitz, K. Colloids Surf., A 1998, 137, 45-56. (4) Dissing, U.; Mattiasson, B. J. Biotechnol. 1996, 52,1-10. (5) Stroeve, P.; Os, M.; Kunz, R.; Rabolt, J. F. Thin Solid Films 1996, 284-285, 200-203. (6) Ha, K.; Kim, J.-M.; Rabolt, J. F. Thin Solid Films 1999, 347, 272-277. (7) Uchida, M.; Kunitake, T.; Kajiyama, T. New Polym. Mater. 1994, 4, 199-211. (8) Nguyen, H. K.; Lemieux, P.; Vinogradov, S. V.; Gebhart, C. L.; Guerin, N.; Paradis, G.; Bronich, T. K.; Alakhov, V. Y.; Kabanov, A. V. Gene Therapy 2000, 7, 126-138. (9) Thu ¨ nemann, A. F.; Beyermann, J. Macromolecules 2000, 33, 6878-6885. (10) Deleted on revision. (11) Steiner, J. In Handbook of Lipid Research; Small, J. D., Ed.; Plenum Press: New York, 1986; Vol. 4 (From Alkanes to Phospholipids), p 587. (12) Antonietti, M.; Conrad, J.; Thu ¨ nemann, A. F. Macromolecules 1994, 27, 6007-6011. 9634 Langmuir 2000, 16, 9634-9638 10.1021/la000991u CCC: $19.00 © 2000 American Chemical Society Published on Web 10/14/2000