Preparation and Spectral Properties of Supramolecular Aggregates of β-Cyclodextrin and Polypropylene Glycol
|School||University of Science and Technology of China|
|Course||Polymer Chemistry and Physics|
|Keywords||Cyclodextrin Polypropylene glycol Polypseudorotaxane Media Thermal Degradation Aggregate|
β-Cyclodextrin (CD) is an cyclic oligosaccharide consisting of seven glucose units connected byα-1,4 linkages. It has a hydrophobic cavity and a hydrophilic exterior, which could form polypseudorotaxanes with polypropylene glycol (PPG). Although the preparation of the polypseudorotaxanes ofβ-CD and PPG was reported, no one focus on the influence of preparation methods on the properties of the polypseudorotaxanes. In the work, several polypseudorotaxanes were prepared from different methods, and characterized by a series of experimental measurements. The main results are as follows:1. Two polypseudorotaxanes were obtained from magnetic stirring method and hydrothermal method, and characterized using elemental analysis, nuclear magnetic resonance spectroscopy (NMR), powder X-ray diffraction, infrared spectroscopy and thermogravimetric analysis (TG). Based on the results of elemental analysis and 1H NMR, the host–guest stoichiometries of the two polypseudorotaxanes were determined to be five-to-one for magnetic stirring method and seven-to-one for hydrothermal method. Their stacking forms and thermal properties were significantly changed. These results show that preparation methods not only change the stoichiometries of host-to-guest, but also influence the properties of the complexes.2. The difference in thermal degradation behavior between PPG and its complex ofβ-cyclodextrin was carefully compared by a gas chromatography coupled to time-of-flight mass spectrometry (GC-TOF-MS) as well as in situ Fourier transformation infrared spectroscopy and TG analyses. The present data revealed a very interesting phenomenon that there was a variation in the order of thermal stability of PPG and its complex between the conditions of TG and GC-TOF-MS. In addition, the relative abundances of released fragments in the inclusion complex PPG–(β-CD)5 were changed with different programmed temperatures in GC-TOF-MS measurements. Moreover, our results also show thatβ-CD could protect PPG chains from heat shock duing temperature rising process.3. The present work reveals a significant influence of a series of inorganic salts on stoichiometries, spectral properties and thermal behaviors of binary aggregates formed by PPG andβ-CD. First, the presence of lithium carbonate and lithium chloride, sodium carbonate and sodium chloride, and sodium arsenite in aqueous solution leads to three different stoichiometric ratios (6:1, 7:1 and 8:1) ofβ-CD to PPG, even though these salts do not appear in the aggregates obtained in current conditions. This important finding demonstrates that the self-assembling process of a binary mixture of a few of CD molecules and a polymer chain can be mediated by additions and alterations of inorganic salts. Second, powder X-ray diffraction patterns and field emission scanning electron microscopy images indicate that the molecule-ion interactions between inorganic ions and PPG especiallyβ-CD play a considerable role in fabricating three-dimensional microstructures of the binary aggregates. Third, gas chromatography coupled to time-of-flight mass spectrometry results indicate that there is a significant change in decomposition mechanisms of the aggregates formed by PPG andβ-CD in the absence and presence of lithium carbonate. Taken together, these observations provide a novel framework for understanding functions of inorganic salts in designing and constructing multicomponent aggregates.