Synthetic Hectorite in Solvent Mixtures and resulting Nanocomposites

Abstract

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…