Amphiphilic Dendritic Molecules: Hyperbranched Polyesters with Alkyl-Terminated Branches X. Zhai, † S. Peleshanko, † N. S. Klimenko, ‡ K. L. Genson, † D. Vaknin, § M. Ya. Vortman, ‡ V. V. Shevchenko, ‡ and V. V. Tsukruk* ,† Department of Materials Science and Engineering, Iowa State University, Ames, Iowa 50011; Institute of Macromolecular Chemistry, Kiev, 02160, Ukraine; and Ames Laboratory and Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011 Received August 26, 2002 ABSTRACT: We report on the synthesis of a series of second-generation hyperbranched polyesters with a variable composition of alkyl-terminated groups. We observed that the chemical modification of the hyperbranched cores by substituting a controlled fraction of the terminal hydroxyl groups with hydrophobic alkyl chains is an effective method for a controlling amphiphilic balance of hyperbranched cores with a degree of branching of 50%. Even for imperfect cores, the chemical reaction of hydroxyl groups alkyl tails was very efficient. In fact, the number of attached alkyl tails was fairly close to the theoretical value based on the assumption that all targeted hydroxyl groups were available for the reaction despite their different interior/exterior location. Detailed microstructural analysis of the structure revealed that organized monolayers could be formed at the air-water interface if the number of alkyl tails was higher than two per core. Similar to regular dendrimers, the alkyl tails of hyperbranched molecules at high surface pressure form intramonolayer ordering of the quasi-hexagonal type. However, higher defectness and irregularities of the hyperbranched cores are responsible for poor intralayer ordering of alkyl tails in comparison with regular dendrimers. At high surface pressure, the alkyl tails became arranged in an up-right orientation. The highly water-swollen state of the hyperbranched cores of prolate shape and the partially submerged and standing-off alkyl tails is a characteristic of hyperbranched molecules with fewer alkyl chains in condensed monolayer state at the air-water interface. The core structure is transformed into the oblate, flattened state with preservation of standing-off orientation of the alkyl tails for hyperbranched molecules with crowded outer shells. Introduction Dendritic polymers have attracted significant interest due to their promising properties of combined function- alized macromolecules and nanoparticles. 1-3 Hyper- branched polymers and dendrimers represent two major and different classes of such materials. Contrary to highly regular dendrimers obtained in a multistep processes, hyperbranched polymers are synthesized in one pot. The morphology and overall shape of dendritic molecules and their interfacial behavior can be con- trolled through the internal chemical architecture, the nature and distribution of terminal groups, and the strength of the molecule-surface interactions. 4-9 De- spite significant polydispersity and inherit defectness of their chemical structure caused by internal cyclization and side reactions, hyperbranched polymers possess, to a great extent, all major elements, which are charac- teristics of compact nanoparticle-like structures with significant fraction of terminal groups located on the exterior of the molecules. 10-14 However, in contrast to the highly regular dendrimers, the hyperbranched polymers did not show sharp transitions and exhibited a macroscopic spreading behavior similar to that of isotropic liquids. 15 Discrete molecules and their surface aggregates were observed while studying adsorption of hyperbranched molecules formed from a four-functional core and AB 2 monomer. 16 Molecular dimensions were consistent with theoretical estimates and molecules sustained signifi- cant external stresses. To design dendritic molecules capable of forming organized aggregates and monolayers at interfaces, amphiphilic balance should be introduced by appropriate modification of terminal groups with e.g., hydrophobic tails. Several examples of such a modifica- tion focused on balance of hydrophobic and hydrophilic interactions have been reported for both regular den- drimers and hyperbranched polymers. 6,17 The fabrica- tion of stable Langmuir monolayers at the air-water interfaces and Langmuir-Blodgett (LB) monolayers on a solid substrate have been reported for dendrimers with polar cores and hydrophobic shells. Conformational flexibility of dendrimer branches allowed for the folding of the dendritic structure and forming a pancake shape of the polar cores at the air-water interface. Stable monolayers were formed with the alkyl chains aligned perpendicular to the water surface and the dendritic core in a pancake conformation facing the aqueous phase. 18 This model suggested that significant flexibility of the dendritic cores provides for conformational reor- ganization, resulting in an overall shape compatible with the planar air-water interface. Only higher gen- eration dendrimers showed surface irregularities of the monolayers, which were attributed to space constraints imposed by the shell-core branched structure. 19,20 Hy- perbranched polyesters with epoxy-containing alkyl tails were used for the fabrication of robust elastic monolay- ers with residual surface functionality. 21 Much less attention has been devoted to structural studies of the series of the hyperbranched polymers with † Department of Materials Science and Engineering, Iowa State University. ‡ Institute of Macromolecular Chemistry. § Ames Laboratory and Department of Physics and Astronomy, Iowa State University. * To whom correspondence should be addressed: e-mail vladimir@iastate.edu. 3101 Macromolecules 2003, 36, 3101-3110 10.1021/ma021383j CCC: $25.00 © 2003 American Chemical Society Published on Web 03/18/2003