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Study on the synthesis and properties of high strength and small molecular physical gel system

Author: XuZhiYan
Tutor: FangYu
School: Shaanxi Normal University
Course: Physical and chemical
Keywords: LMMGs Cholesterol NBD Fluorescent sensing VOCs
CLC: O648.17
Type: Master's thesis
Year: 2013
Downloads: 72
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Abstract

Physical gels based on low-molecular-mass organic gelators (LMOGs) have attracted considerable attention during the last few decades due to their marvelous properties and numerous potential applications. The LMOGs self-assemble into physical gels with continuous three-dimentional (3D) networks through supramolecular weak interactions like hydrogen bonding, π-π stacking, van der Waals interaction, electrostatic interaction, dipole-dipole interaction, coordination interaction and host-guest interaction, etc.. Then, the formed networks encapsulate and immobilize a large volume of organic solvents via surface tension and capillary force. The weak interactions endow supramolecular gel with a superior thermal reversibility which make it possible that the sol-gel phase transition progress can be easily controlled by adjusting the ambient temperature. It’s worth to be noted that the viscosity of the solution phase is extremly close to that of pure organic solvent, makes the medium can be wiped up easily when it was not needed. The features mentioned above make supramolecular gels possesse unparalleled advantages over chemical gels in applications of defect-free crystal growing, soft materials devices creation, templates for preparing micro/nano-materials, mild separation of bioactive substances and drug controlled release, etc..As a kind of natural products, cholesterol derivatives universally exist in organisms and play an important role in the progress of life. Furthermore, it has been widely used in self-assembling process due to its outstanding performance of aggregating into ordered spiral one-dimensional structures. So far, supramolecular gels based on cholesterol derivatives with excellent properties have been extensively published, however, the highly strong supramolecular gel was reported very rare.Considering the above research situation and the previous work of our group, a series of fluoresent LMOGs which adopt cholesterol segment and NBD unit as basic building blocks were synthesized and systematically studied. Fortunately, the compounds as synthesized show a high gelling efficiency to both protic/aprotic solvents and polar/nonpolar solvents, moreover, the obtained gels show high mechanical strength and unpresedented self-healing property. The gel properties and formation mechanisms were lucubrated via SEM, FTIR,1H NMR and XRD studies. In addition, a new "turn-on" type fluorescent sensing film which is highly sensitive to the organic solvent vapor was fabricated through spin-coating, the film was expected to detect the volatile organic compounds (VOCs). The research work mainly consists of the following two sections.In the first section, four nitrobenzoxadiazole (NBD)-containing cholesteryl (Chol) derivatives were prepared, the gelation behaviors and gel properties were systematically studied. It was found that a subtle change in the length of the spacers connecting the two structural units in the compounds, which are NBD and Chol, respectively, produced a dramatic change in the gelation ability and the gel properties of the compounds. As for gelling ability, compound1is much more powerful than others, especially in methanol-containing organic mixtures. Morphology study indicates that the aggregated structures change from fibers to balls along with the growth of gelator concentration and volume ratio of methanol which was regarded as a poor solvent for compound1. Furthermore, it was found that the mechanical property of the gel is closely related to the geometrical shape of the aggregated structures. Rheology study manifests that the gel of compound1in pyridine/methanol possesses high mechanical strength and excellent self-healing property, which lays a good foundation for further applications. FTIR,1H NMR, XRD and molecular mimicry demonstrate that the gelator aggregated into layer structure and then formed the original fiber through intermolecular van der Waals interaction and π-π stacking.In the second section, a novel di-NBD derivative has been designed and synthesized. It was found that the compound shows weak fluorescence in dry state but the emission is enhanced significantly when exposed in organic vapor. Accordingly, a "turn-on" type fluorescent film was fabricated through spin-coating. Interestingly, the film can enrich some solvent molecules such as methanol and ethanol due to the hydrogen bonding between the compound and solvent molecules, which makes the film show different response to different organic solvent vapors, and furthermore, the response is fully reversible, which lays a good foundation for practical applications. In conclusion, it is highly possible that the film can be developed into a sensing film for the descriptive and sensitive detection of volatile organic compounds (VOCs).

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CLC: > Mathematical sciences and chemical > Chemistry > Physical Chemistry ( theoretical chemistry ),chemical physics > Colloid chemistry ( physical and chemical dispersion system ) > Colloid > Gel and soft
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