Supramolecular hybrid structures and gels from host-guest interactions between alpha-cyclodextrin and PEGylated organosilica nanoparticles

Abstract

Polypseudorotaxanes are polymer chains threaded by molecular rings that are free to unthread; these "pearl-necklace" can self-assemble further, leading to higher-order supramolecular structures with interesting functionalities. In this work, the complexation between alpha-cyclodextrin (alpha-CD), a cyclic oligosaccharide of glucopyranose units, and poly(ethylene glycol) (PEG) grafted to silica nanoparticles was studied. The threading of alpha-CD onto the polymeric chains leads to their aggregation into bundles, followed by either the precipitation of the inclusion complex or the formation of a gel phase, in which silica nanoparticles are incorporated. The kinetics of threading, followed by turbidimetry, revealed a dependence of the rate of complexation on the following parameters: the concentration of alpha-CD, temperature, PEG length (750, 4000, and 5000 g mol(-1)), whether the polymer is grafted or free in solution, and the density of grafting. Complexation is slower, and temperature has a higher impact on PEG grafted on silica nanoparticles compared to PEG free in solution. Thermodynamic parameters extracted from the transition-state theory showed that inclusion complex formation is favored with grafted PEG compared to free PEG and establishes a ratio of complexation of five to six ethylene oxide units per cyclodextrin. The complexation yields, determined by gravimetry, revealed that much higher yields are obtained with longer chains and higher grafting density.