New Functional Supramolecular Organogel Systems Based On Cyclodextrin

The functional materials of supramolecular organogels based on β-cyclodextrin (β-CD) were designed and synthetized depending on the viewpoints of supramolecular chemistry in this thesis. Supramolecular Chemistry is the chemistry involving molecular assemblies with non-covalent bonds. In brief, Supramolecular Chemistry is a science for various functional systems formed by non-covalent bonds among multiple molecules.The basic research ideas in this thesis depended mainly on the three academic sectors from Supramolecular Chemistry, Cyclodextrin Chemistry and Sol-gel Chemistry. According to the definition provided by J.-M. Lehn, supramolecular complex is a combination beyond the molecules of covalent bonds. Supramolecular Chemistry is aimed at developing complex and ordered aggregates with certain function and property based on non-covalent bonds among multiple molecules in a system. Therefore, some supramolecuar materials can be designed and self-assembled depending on specific non-covalent bonds from the structural characteristics of molecules in a system. Cyclodextrin is a kind of macrocyclic polysaccharides with unique structures as the investigated candidates of supramolecular chemistry. Therefore, we have chosen P-cyclodextrin as basic material to construct supramolecular organogels. The Sol-gel Chemistry is one branch of Colloidal Chemistry. It focused more on a state exchange from solution to gel state, or in reverse. It is a new field to study the low-molecular weight organogels. The characterization of new gel system and explaination of formation mechanisms are all new subjects.The systems of supramolecular gels here mostly are some β-CD solutions with a small quantity of salts such as LiCl or K2CO3, etc. Facing physical stimuli or chemical additives, β-CD molecules in the systems constructed to form gel fibers or colloidal particles as subunits of gel network by self-assembling. The network trapped solvent molecules by capillary force or face adsorption to form a gel with specific structural characteristics and performance. The supramolecular interactions within this kind of gel materials mainly are hydrogen-bonds, molecule-ion interaction and van der Waals force, etc. And these supramolecular interactions influenced the stimuli-responsive performance of gels and were employed to design various functional gel materials.This thesis mainly consists of three parts as follows:i) It had not been found ever before that β-Cyclodextrin (β-CD) itself has gelling abilities under heating, without the help of the guest molecules embedding in cavities. The experiments showed that P-CD can self-assemble an organogel with a very small amount of lithium chloride (LiC1) in N,N-dimethylformamide (DMF) by heat. The experiments also showed that the β-CD organogel microstructures were β-CD molecular clusters composed of many channel-type β-CD fibers extending from a cage-type β-CD core in various directions. The morphology of the organogel was characterized by OM and SEM. The process of the gel formation and the structure were suggested based on FTIR, TR-FTIR,’H NMR and XRD results. The main supramolecular interactions among the various molecules in the system were discussed for gelation. A molecular dynamics simulation of the gel structure was performed in agreement with the results. The experiments showed that the concentration of P-CD for the gel formation ranged from0.13to0.28mol/L. The approximate heat effect of the phase transition was28kJ/mol. This investigation results will be of great significance to develop new temperature-controlling materials with native P-CD of low cost in application.ii) The β-CD/LiCl/DMF solution can be developed to become supramolecular organogels by introducing formic (FA), acetic (AA) or propanoic acid (PA) into the system without heating. However, the experiments showed that the gel morphologies were diverse depending on different additives of FA, AA or PA into the system. The polarities and H-bond abilities of FA, AA or PA influenced micro-structures and physicochemical properties of these gels. More orderly β-CD self-assembly was in PA gel, while lower crystallinity of colloidal particles was in FA gel, which was revealed by SAXS, WAXS, FT-IR and XRD. The diversity of the gels in morphology, thermostablility and mechanical strength were studied by OM, SEM, DSC and rheology. The phase diagram of theβ-CD system with FA was described for such gel formation. A common mechanism of the gels formation was clarified, namely, H-bonds exchange from DMF-β-CD complex to DMF-carboxylic acid complex resulted in destruction of solvent shell on β-CD gelator and β-CD self-aggregation in the co-solvent system. By introducing an additive to manipulate sol-gel transitions, this approach would be a potential implication in controllable stimuli-responsive materials.iii) For supramolecular "gel A" of β-CD/K2CO3/1,2-propylene glycol system, it can be activated by the addition of HCOOH to evolve into another "gel B". This manipulation achieved the gel-sol-gel’phase transition. In the process, the release of CO2dissociated the network of the "gel A" bridged by K2CO3and the new network of the "gel B" can be reorganized with the help of the newborn HCOOK. The two gels,"A and B", were investigated by OM, SEM, SAXS, WAXS, FTIR and XRD. The"gel B" with greater elastic modulus (G’,1×105Pa) could endure higher applied stress (the yield point300Pa) than the "gel A"(G’,3X104Pa; the yield point180Pa). The "gel B" also exhibited the higher phase transition temperature (161.75℃) than that (34.4℃) of the "gel A" in DSC analysis. The reason behind the gel-sol-gel’phase transition was the salt bridge evolution. This work is the first report on gel evolution with phase reorganization triggered by chemical additive (HCOOH), which may be of great significance to develop drug controlled release materials, and other more complex stimuli-responsive materials.In addition, physical techniques often are used in characterizing gel structures and self-assembly process. The main physical techniques are small-angle X-ray scattering (SAXS), wide-angle X-ray scattering (WAXS), Fourier transform-infrared spectroscopy (FT-IR), X-ray diffraction (XRD), optical microcopy (OM), scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and rheology. These techniques had also been used to design and describe the new gel systems in this thesis.In all, the main methods with supramolecular interactions we used were the utilization of hydrogen-bonds between β-CD molecules for gelation of β-CD gel, the use of molecule-ion interaction from salts, and destruction of solvent shell of β-CD molecules, etc. These findings may be of great significance not only to develop gel materials but also to extend CD chemistry and supramolecular science.The innovated points in this thesis mainly include:i) It is firstly proposed that β-Cyclodextrin (β-CD) itself has gelling abilities under heating. Without the help of the guest molecules embedding P-CD cavities, the reversible organogel of β-CD/LiCl/DMF system can be formed.ii) We proposed that the organogel microstructures were P-CD molecular clusters composed of many channel-type β-CD fibers extending from a cage-type β-CD core in various directions. This suggests the gelation mechanism of the β-CD/LiCl/DMF system.iii) The β-CD/LiCl/DMF solution can also be developed to become supramolecular organogels by introducing formic, acetic or propanoic acid into the system without heating. The mechanism of the gels formation involved in hydrogen-bonds interaction from the carboxyl group of carboxylic acids. The DMF-P-CD complex for salvation with hydrogen exchange became DMF-carboxylic acid complex, resulted in destruction of solvent shell on P-CD molecules and P-CDs self-aggregation in the co-solvent system.iv) For theβ-CD/LiCl/DMF system, the destruction of solvation forβ-CD resulted in β-CD gel formation under chemical additive of small carboxylic acid. The polarities and H-bond abilities of FA, AA or PA influenced micro-structures and physicochemical properties of these gels.v) For "gel A" of P-CD/K2CO3/1,2-propylene glycol system, it can be activated by adding HCOOH to evolve into "gel B". This manipulation achieved the gel-sol-gel’ phase transition. This gave us an unprecedented opportunity.vi) The reason behind the gel-sol-gel’phase transition was the salt bridge evolution. This approach may be of great significance to develop drug controlled release materials, and other more complex stimuli-responsive materials.