title: Synthetic Hectorite in Solvent Mixtures and resulting Nanocomposites
Creators: name: Mayr, Lina
Publisher: name: University of Bayreuth
Year: 2021
Subjects: subject: Hectorite
subject: nanocomposites
subject: osmotic swelling
subject: emulsions
subject: foams
subject: 540

description: In this thesis, hectorite, a layered silicate, was investigated in different solvent mixtures and was, then, used for the preparation of nanocomposites with different application possibilities. Osmotic swelling of hectorite in water offers an easy way to gain single 1 nm thick layers with a huge aspect ratio. For many applications like gas barrier, composites with such layers are desired. As many polymers are insoluble in water, organic solvents or solvent mixtures that enable osmotic swelling are needed. In the first part of this thesis, the swelling of sodium hectorite (NaHec) in different ternary mixtures consisting of methanol, acetonitrile, ethylene glycol, glycerol carbonate or water was investigated. It was found that in ternary mixtures, less water was necessary to trigger osmotic swelling than in corresponding binary mixtures. In this way, the water content could be reduced to 10 vol.-% using a mixture of methanol (70 vol.-%), water and acetonitrile (20 vol.-%). In a system with glycerol carbonate, a solvent that can also be polymerized, and methanol, only negligible amounts of water were necessary for osmotic swelling. Thus, hydrophilic NaHec could be osmotically swollen almost without water which extends its application possibilities. Furthermore, it was found that in binary mixtures of water with one organic solvent, the dipole moment of the latter determines the swelling behavior. The smaller the dipole moment, the more water was necessary to allow for osmotic swelling. However, for ternary systems, this simple correlation does not exist. A quantitative study of the osmotic swelling of NaHec in a ternary solvent mixture revealed a similar behavior like the osmotic swelling in water. The layer separation increased with increasing amount of solvent indicating complete osmotic swelling. Whereas completely miscible solvents were used for the swelling studies, the other two parts of this thesis are dealing with emulsions of immiscible solvents. In the second part, high internal phase emulsions (HIPEs) in which the external phase consisted of polymerizable molecules were used as templates for polymer foam composites. Hectorite was modified with a custom-made organo-cation (HecPEHMA) to disperse it in the external phase of a HIPE. Upon polymerization of this phase, open-cell composite foams with relative densities of 4 – 7 % could be synthesized. The mechanics of the foams were studied by compression tests depending on the filler content and the relative density of the foams. With only 2 % HecPEHMA, the Young’s modulus could be increased up to a factor of four. To investigate the mechanism of the strengthening, HecPEHMA was also incorporated in bulk polymer plates and their tensile properties were tested. However, the filler led to a weakening of the polymer concluding that the strengthening of the foams was not due to a reinforcement of the material. Instead, the foam mechanics were determined by the foam morphology. The foams with 2 % HecPEHMA had larger cells, thicker struts and a more consistent structure. Most probably, HecPEHMA acted as a Pickering emulsifier stabilizing the emulsion in addition to the molecular surfactant. This then led to an improved foam structure which simultaneously improved the mechanics. Contrary, a commercial organophilized montmorillonite (O-MMT) filler could not enhance the foam mechanics, most likely as its surface modification could not provide a Pickering effect. In the third part, hectorite was used to encapsulate a fragrance mixture in aqueous emulsions. In many applications, not a single fragrance is used but a mixture of different volatile substances. These fragrances usually differ in their vapor pressures and therefore, the composition of the mixture changes upon evaporation. Thus, unselective barriers are needed to maintain the original scent impression. As nanocomposite films with hectorite provide excellent gas barriers, it seems promising to use hectorite for barrier capsules as well. However, pristine hectorite is too hydrophilic to stabilize emulsion droplets. Therefore, hectorite was in-situ modified with polycationic poly(ethylene imine) (PEI) during the emulsification. The evaporation of a model fragrance mixture (eucalyptol, limonene, α-pinene and ethyl-2-methylbutyrate) from such emulsions was investigated. By applying hectorite layers at the water-oil-interface, the release could be significantly retarded in comparison with an emulsion that was solely stabilized with PEI. The release rates were no longer determined by their vapor pressures. All fragrances are hydrophobic but differ in their water solubility. As the capsule wall which consisted of hectorite and PEI was most probably swollen with water, the relative solubility of the fragrances in the capsule wall might be similar to the water solubility. Fragrances which have a higher water solubility diffuse faster through the capsule wall and were thus, released faster. By comparing emulsions made with different ratios between PEI and hectorite, it could be shown that the release was further retarded by lowering the amount of PEI. This is due to an increased tortuosity in the capsule wall. An additional chemical cross-linking of PEI could further improve the retardation of the fragrances. This work is a cumulative thesis. A detailed description of the results can be found in the attached publications.