Layer-by-Layer Deposition Of Polyelectrolyte Complexes For Multilayer Film Fabrication

In the past decade, the layer-by-layer (LbL) assembly technique has attracted extensive attentions of chemists, due to its valuable characteristics in composite film fabrication, such as independence of the geometry of the substrates capable of film deposition, abundance in materials used for LbL film fabrication, easiness in fine tailoring film composition and structures, and so forth. Advanced multilayer film materials with components such as polyelectrolytes, organic/inorganic particles , biomacromolecules such as proteins,enzymes,virus, oligocharged organic compounds and dentritic molecules have been successfully fabricated by the LbL assembly technique. Extension of materials for LbL film fabrication will certainly enrich the structures and therefore functionalities of the LbL assembled film materials. In recent decades, polymeric complexes have been under intense investigation for various potential applications, benefiting from their versatility in chemical composition and structures. The driving force for polymeric complexes formation is mainly electrostatic interactions, hydrogen bonding, coordination bond , guest–hostinteraction and the synergetic interaction of the above forces, etc. Polymeric complexes possess versatile structures in solution which are believed to be helpful to obtain polymeric films with well-tailored structures as well as functionalities. However, less attention has been paid to use polymeric complexes for LbL assembled multilayer fabrication. In this dissertation, we demonstrated that highly aggregated polymeric complexes can be used as building blocks for LbL film fabrication. The structures as well as the functionalities of the resultant LbL assembled films were fully tailored and explored.In chapter 1, we introduced the driving forces for LbL film fabrication which includ electrostatic interaction, hydrogen-bond, guest-host interaction, coordination bond, charge-transfer interactions, covalent bonds, the synergetic interaction of the above forces, etc. LbL assembled films have multiple applications in areas such as nonlinear optics, antireflection coatings, antifogging and self-clean film, superhydrophobic coating, antibacterial film, controlled releasing coatings and cell adhesion film. Recently, more and more attention has been paid to the fabrication of films with micrometre-thicknesses by using the LbL assembly technique because such thick films have enhanced mechanical robustness, high loading capacity for functional guest materials, adjustable hierarchical micro- and nanostructures, and so forth. We therefore introduced several methods currently being employed to speed up the LbL assembly process, such as spin LbL assembly, spray LbL assembly techniques and the exponential LbL assembly technique.In chapter 2, Positively charged poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA) complexes (noted as PAH-PAA) with a molar excess of PAH were layer-by-layer (LbL) assembled with polyanion poly(sodium 4-styrenesulfonate) (PSS) to produce multilayer films. The film structure and deposition behavior of the PAH-PAA/ PSS films were influenced by the structure of PAH-PAA complexes in solution. For the PAH-PAA complexes with a low ratio of PAA to PAH the PAH-PAA complexes have low-level cross-linking and are flexible. The resultant PAH-PAA/PSS films have a thin film thickness and smooth surface and exhibit a nonlinear deposition behavior where the amount of PAH-PAA complexes and PSS deposited in each deposition cycle are larger than in its previous cycle. The PAH-PAA complexes with a high ratio of PAA to PAH have high-level cross-linking and are rigid. The PAH-PAA/ PSS films constructed from the rigid PAH-PAA complexes have a large film thickness and rough surface and exhibit a linear deposition behavior. Deposition of the PAH-PAA/PSS films was well characterized by quartz crystal microbalance, atomic force microscopy, and scanning electron microscopy. The thermally cross-linked PAH-PAA/ PSS films can be released from substrate to form stable free-standing films by an ion-triggered exfoliation method. Meanwhile, positively charged PAH-PAA complexes can be LbL assembled with negatively charged PAH-PAA complexes with a molar excess of PAA to produce multilayer films. Use of polyelectrolyte-polyelectrolyte complexes as building blocks for LbL fabrication provides a facile way to tailor the structures of the resultant films by simply changing the structure of the complexes in solution.In chapter 3, we attempted to rapidly prepare a multilayer film with hierarchical micro- and nanostructures by alternately depositing sodium poly(styrene sulfonate) (PSS, containing different concentrations of NaCl) with PAH-PAA0.75 complexes with the feed monomer molar ratio of PAH to PAA being 1:0.75. After chemical vapor deposition of a layer of fluoroalkylsilane on top of the as-prepared PSS/PAH-PAA0.75 films, superhydrophobic coatings were successfully fabricated. We found that the addition of NaCl in aqueous PSS solution and the non-drying process for LbL PSS/PAH-PAA0.75 film fabrication are critical to fabricate superhydrophobic coatings with low sliding angles.In chapter 4, the evolution of the PSS/PAH-PAA0.75 films under post treatments was investigated. Multilayer films of polyelectrolyte complexes were assembled onto silicon substrates by the sequential adsorption of PAH-PAA0.75 complexes and poly(styrene sulfonate) (PSS) from aqueous solution. These films were then immersed briefly into acidic aqueous solution (pH2.5) followed with immersion in deionized water. The above treatments induced a two-step phase separation with the polymeric films, which finally resulted in microporous films. Control experiments confirm that the microporous structures originates from the large dimensions of the PAH-PAA0.75 complexes which cause large-scale chain movements during the post treatment of the films. Keywords:...