Thin and Responsive Polymer Coatings: Insight into Polyelectrolyte Multilayer and their Composites with Polyelectrolyte Brushes and Halloysite Nanotubes
This thesis gives an insight into thin coatings consisting of polyelectrolyte multilayers, polyelectrolyte brushes and Halloysite nanotubes. Thin coatings are able to modify surfaces without affecting the underlying substrate. These coating can be used to protect the substrate and to introduce funct...
| Summary: | This thesis gives an insight into thin coatings consisting of polyelectrolyte multilayers, polyelectrolyte brushes and Halloysite nanotubes. Thin coatings are able to modify surfaces without affecting the underlying substrate. These coating can be used to protect the substrate and to introduce functionality, e.g. responsiveness to humidity or temperature.
The first part is based on polyelectrolyte multilayers (PEMs). PEMs are build up of oppositely charged polyions via the layer-by-layer technique and can be tailored with nanometer precision. These coating are not densely packed and contain air-filled holes (voids) in the dry state. The knowledge about the amount of voids is of crucial importance for future applications. Here, polystyrene sulfonate/poly(diallyldimethylammonium) chloride (PSS/PDADMAC) PEMs were studied at varying humidity to investigate the internal composition. Ellipsometry was used to determine thickness and refractive index as a function relative humidity. The relative swelling behavior is independent of the layer number. While increasing humidity, PEMs swell and air is replaced by water inside the voids. It was shown that above 30% RH the voids are fully filled with water. Additionally, the void fraction and refractive index of the pure polymer mater were determined to 5 ± 1% and 1.592 ± 0.002, respectively.
The second part describes the mutual interactions between PEMs and brushes. In contrast to PEMs, brushes are perpendicular orientated to the surface. Composites of brushes covered with PEMs retain their responsiveness to variation of external humidity. Therefore, this stimulus is used to study the internal composition of poly-2-(methacryloyloxy)ethyl-trimethylammonium chloride (PMETAC) brushes coated with PSS/PDADMAC PEMs. Ellipsometry measurements showed that the composites swell less than single brushes but more than single PEMs. Neutron reflectometry showed that PEMs penetrate fully a brush and stay mobile. While increasing humidity, brush and PEM start to separate and water accumulates in the transition region. The overlap of brush and PEM decreases down to 10% for a brush coated with a PEM of 17 layers. Swelling and separation are almost reversible.
The last part focuses on the orientation of Halloysite nanotubes (HNTs) on substrates coated with polyelectrolytes. HNTs are charged, tubular shaped alumosilicates and an orientation of them leads to anisotropic coatings. Here, spray coating was used to align HNTs on a surface. The influence of HNT-substrate interactions and different spraying parameters were investigated. Polyethylenimine (PEI) coated silicon wafer provided a sufficient strong interaction with HNTs for orientation on the surface. A low liquid flow rate (0.2 mL/min) and a high gas flow rate (40 L/min) led to best alignment. Increasing concentration and spraying time resulted in increasing coverage. The orientation was quantified by the nematic order parameter and was increased up to 0.81 by optimizing spraying conditions. Additionally, oriented bilayers of PEI and HNTs were produced. |
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