Effect Of Preparation Conditions On Structures And Properties Of Organogels

Low molecular weight organogels are a type of soft matter with nano-scaled three-dimensional network formed through weak non-covalent interactions, such as hydrogen bonding, π–π stacking, hydrophobic interactions and/or van der Waals interactions. External stimuli, such as temperature, p H, mechanical strain, UV-visible light and ion, may bring about some changes in the chemical structure, molecular configuration or the self-assembly process of the gelators, which enable its potential applications in template materials, biomedical tissue engineering and drug delivery. Most studies focused on the design of new gelators and the gel properties. However, gel is a complicated system consisting of nano-scaled structures and a large amount of liquid. Factors that affect the structures and interactions between gelators and liquids might affect the properties of the gel. O n this account, our present thesis aimed at the investigation of the effects of preparation conditions: sonication, solvent, incorporation of carbon nanomaterials and cooling process, on the packing structures and properties of the resulting gels. Combining the gelation kinetic investigation as well as self-assembled structures, the effects of preparation conditions on the gel properties, and thus the gel properties will be evaluated. This work will provide basic data for future gel preparations and property regulations.Introduction of sonication helps to evaluate gelation ability and accelerate gelation velocity through influencing the self-assembly of the original un-aggregated gelator by using the extreme chemical and physical conditions created by acoustic cavitation. Furthermore, ultrasound irradiation time and water bath temperature applied play a crucial role in regulating the molecules orderly a nd forming new aggregates. SEM images provide evidence of different self-assembled structures from entangled fibers of gels formed through a traditional thermal nucleation process to tube-like structures and nanoparticles of gels formed through a combined thermal nucleation and ultrasonic nucleation process. The change of the growth pattern from one-dimension to two-dimension induced by sonication is also confirmed by kinetic studies. Results of FTIR and XRD revealed structural changes before and after sonication. In addition, gels obtained from ultrasound treatment possessed relatively worse rheological properties and lower thermo-stability due to less entanglement among the assembled structures of the resulting gels. These results may be useful to help to obtain a deeper understanding of the kinetics of ultrasound- induced aggregation. The reversible sonication triggered gelation and easily manipulated morphologies in ethanol may provide a new way for achieving the application of these soft materials in the template area and responsive and shape-memory materials.The compound 4D16 showed strong gelation ability in benzene, toluene and 1,2-dichloroethane. The xerogels formed in these solvents consisted of bundles of ribbons with an average diameter of 5 μm. The XRD investigation indicates 4D16 exhibited cubic structure in benzene, whereas a layer structure in toluene. Both SEM and POM morphology provide visual information of fiber growth during the self-assembly processes of the two structures. In benzene, radius fibers with a fractal dimension of approximately 1.4 were observed, whereas less open fibrillar structures with smaller fractal dimension in toluene. These results provide a deeper understanding of the gelation process and provide valuable insights into the factors influencing the microscopic structure of self-assembly systems.The incorporation of carbon nanomaterials showed significant effects on not only the fluorescence spectra of BOXD- T8 in DMF but also gel behaviors. An interesting aggregation mode was found for BOXD- T8 in DMF: transformation of H- into J-aggregates could be manipulated simply by increasing the concentration. When carbon nanomaterials were added into the solution, energy transfer from H-aggregates to outputting carbon nanomaterials may have been involved. Furthermore, the BOXD-T8 prefer to aggregate into aggregates(H- and J-aggregates) in the existence of SWNTs, while J-aggregation resulted in case of GO, which resulted in a lower critical gelation concentration and different aggregation morphologies compared to those of the corresponding BOXD- T8 organogel. The rheological study revealed that both G’ and G" were higher for hybrid gels compared to the BO XD-T8 gels, indicating the formation of a stronger solid- like network due to the physical enhancement effects of the incorporated carbon nanomaterials as rigid materials. While SWNTs could affect the gel in a more mechanical way, owing to the one-dimensional tube-like structures.The hydrogen-bonding based amide derivatives(HMMDAs) can form stable organogels in several organic solvents. 1-HMMDA is the most efficient gelator among the three ones in alkyl liquids, while 2-HMMDA in benzene and other liquids, like ethyl acetate and acetonitrile. 2-HMMDA gels are the most thermo and mechanical stable among them. More tailed hydroxylmethyl groups will induce a more disorder packing structure. Gels of 1-HMMDA and 2-HMMDA formed in different liquids showed fiber structure. And different cooling processes had little effects on the assembled structure. A typical branching difference due to different cooling methods was observed for the gels of 3-HMMDA. The gels prepared by an ice-cooling method showed a spherical structure, while long fibers for the fast and slow-cooled gels. The gels of 1-HMMDA, 2-HMMDA and 3-HMMDA formed in nitrobenzene show good thixotropic properties. For example, for the 2 wt% 3-HMMDA /nitrobenzene gel, the values of G’ after five circles can recover up to 93%. Recovery ratio is not much affected by destructive strain applied, while is greatly influenced by gelator concentration: recovery ratios of 1-HMMDA and 3-HMMDA gels decreased with increasing concentration, while 2-HMMDA gels show an opposite trend, recovery ratio increases as the concentration increases. Destructive strain values and gelator concentrations don’t affect largely on recovery time.