Nanoscale Thin Film Ordering Produced by Channel Formation in the Inclusion Complex of α-Cyclodextrin with a Polyurethane Composed of Polyethylene Oxide and Hexamethylene

Abstract

A polyurethane consisting of six blocks of polyethylene glycol and five blocks of hexamethylene diisocyanate was synthesized. The influence of the addition of a-cyclodextrin (a-CyD) on the lower critical solution temperature behavior of the polyurethane was investigated by “cloud point” measurements, and the dependence of the phase state (solution, suspension, and gel) of a-CyD/polyurethane mixtures on the concentration of the two components was determined. The results suggest that the polyurethane forms inclusion complexes with a-CyD and that close to the maximum number of a-CyDs was included. The associative constant of the a-CyD/polyurethane inclusion complex was determined by 1H NMR shift titration using a modified Benesi-Hildebrand equation, and the complex was characterized in the solid state by 13C cross polarization/magic angle spinning NMR and X-ray diffraction. These studies showed that the complexes adopted a channel-like structure. Finally, the morphology of a-CyD/polyurethane complexes in the solid state was visualized by scanning electron microscopy and atomic force microscopy (AFM). AFM images of the inclusion complexes spun-cast on to silicon reveal the existence of ordered domains with heights commensurate with the existence of tetra- and higher-order a-CyD channels. The height quanta of these well-ordered, discrete plateaus point to the dominating influence of the size of the polyethylene glycol blocks within the polyurethane and suggest a route to the production of controlled subnanometer structured surfaces.