(588c) Properties of Amphiphilic Dendritic Polymers in Solutions, at the Air/Water Interface and on Solid Substrates

J. Gregorowicz,^a A. Brzozowska,^a, J. Paczesny,^a M. Luszczyk,^a K. Nikiforov,^a V. Sashuk,^a P. Parzuchowski,^b G. Rokicki,^ba Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland

^b Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland

Dendritic polymers with a tree-like architecture are considered promising candidates for functional supramolecular materials. These nanoscale structures have a variety of prospective applications, e.g. adhesives, functional coatings, chemical sensors, drug delivery systems, separation processes and biomimetic systems. To achieve this, a comprehensive understanding of structure â?? property relationships is required. Dendritic macromolecules are represented by two main classes of structures: dendrimers and hyperbranched polymers. Dendrimers are macromolecules with highly regular branching, having a three-like structure that emerge from a central core. The synthesis of dendrimners is well controlled and as a result dendrimer macromolecules have very well defined structure reflecting complete branching and regularity. As opposed to dendrimers, random hyperbranched polymers are in fact mixtures of species differing in molecular weight and the degree of branching. Nevertheless, hyperbranched polymers have more possibilities of potential applications, since they are less expensive and simple to produce. There are still ongoing discussions as to the extent to which dendrimers and hyperbranched polymers possess similar properties.

In this contribution, behaviour of amphiphilic hyperbranched polyesters and poly(propylene imine) dendrimers at the air/water interface, on solid substrates and in solutions were investigated. All macromolecules used in the experiments have alkyl chains as terminal functionalities.

The phase equilibria in binary systems formed by the hyperbranched polyester H3200 (synthesized from Boltorn H30 through the esterification of OH-end groups with a mixture of eicosanoic (arachidic) and docosanoic (behemic) acids leading to C20/C22 (3:1) alkane chains as end groups) and alcohols are reported. Both liquid â?? liquid and solid â?? liquid equilibria were investigated. The influence of the presence of water on the phase behaviour of selected H3200 solutions is elaborated and the properties of the solid phase precipitated from alcohol solution were investigated. It was shown that a small amount of water present in these systems has a significant impact on the extent of the liquid â?? liquid phase split. At temperatures below 325 K alcohol solutions of H3200 form gels over a wide concentration range of the polymer. It seems that in these systems gelation results from arrested phase instability.

Despite defected structure and dispersity HBPs with terminal hydroxyl groups substituted with alkyl chains form well-organized monolayers at the air/water interface and on solid substrates. The monolayers were studied using surface pressure (static and dynamic modes) and Kelvin potential measurements, Brewster angle microscopy, relaxation measurements and compression/decompression experiments. It was shown that the HBPs molecules arranged into monolayers have polyester interior in contact with the water subphase and the alkyl chains are all pointing toward the air, forming a parallel-packed hydrophobic layer. The structure of this hydrophobic layer changes from soft liquid â?? like to stiff solid â?? like depending on the length of the terminal functionalities and temperature. The HBPs monolayers were transferred onto silicon and gold substrates using the Langmuir â?? Blodgett technique. Detailed structural analysis of monolayers deposited at solid substrates reveled that alkyl tails remained arranged in an up-right orientation with dense liquid-crystalline ordering.

Poly(propylene imine) (PPI) dendrimers of third and fifth generations with terminal amine groups substituted with alkyl chains also form well-organized monolayers at the air/water interface and on solid substrates. It was shown that the dendrimer molecules arranged into monolayers have core amine part in contact with the water subphase and the alkyl chains are all pointing toward the air, forming a parallel-packed hydrophobic layer. Also here, the structure of this hydrophobic layer changes from soft liquid â?? like to stiff solid â?? like depending on the length of the terminal alkyl chains and temperature. In comparison to the HBPs dendrimes macromolecules form more stiff and less temperature sensitive monolayers at the air/water interface. HBPs macromolecules in the monolayer interact mainly through alkyl chains. In case of PPI dendrimers interaction of terminal alkyl chains is strengthen by hydrogen bonds between secondary amine groups. This effect makes the monolayers stiffer and more resistant to temperature changes.