Layer-by-Layer Self-Assembly Of Conducting Multilayer Films And Research On Their Properties

In recent years, the self-assembled multilayer films have attracted much attention from the scientists in many fields. In general, self-assembled multilayer films exist in the interfaces between gas and liquid or liquid and solid. In 1991, professor Decher developed a simple route to fabricate multilayer films using the electrostatic interaction between cationic and anionic polyelectrolytes as driving force. This technique is believed to be a rapid and simple way to produce multilayer films where the film composition and thickness can be adjusted by controlling the preparation coditions. The method has been regarded as an important milstone for the multilayer film research. In this dissertation, we focus on the preparation of multilayer film with high conductivity by self-assembled method, investigate the relationship between electrical conductivity and structure of the multilayer film, and try to propose a theory to reveal the conductive mechanism, which will promote the development of the conducting self-assembly multilayer films.In chapter 2, a multilayer (PAA/GO)n consisting of poly(acrylic acid) (PAA) and graphite oxide (GO) nanoplatelets modified with cationic surfactant cetyltrimethylammonium bromide (C16TAB) is self-assembled successfully by using the electrostatic interaction. The results show that C16TAB modified GO nanoplatelets can adsorb PAA chains firmly, and the multilayer films show high conductivity. Because of the fact that PAA chains can diffuse into the interlayer of carbon molecular layers, the interlayer interaction is weakened and the interlayer height is expanded. When the C-C interlayer height reaches a threshold value (or n reaches a threshold value), the electrons in the carbon interlayer become free, and a percolation effect appears. The multilayer film has typical PTC/NTC effects if the PAA with an average molecular weight ranging from 2,000 to 4,000 is used to fabricate the multilayer films. The results also prove that the PAA chains with lower molecular weight can diffuse into the interlayer of carbon molecular layers easier, which will result in the fabrication of the multilayer films with higher conductivity and lower percolation threshold value. After reduction by hydrazine hydride, the conductivities of the conducting films are dramatically improved, and the percolation threshold occurs at high number of bilayers. By this assembling method, we develop an approach for self-assembly of the multilayer films with high conductivity and low percolation threshold value.In chapter 3, we successfully fabricate the (PSS/G)n conducting films consisting of C16TAB modifed graphite nanoplatelets and PSS polyelectrolyte. The results reveal that the reduced graphite nanoplatelets can be easily covered by C16TAB. Compared with (PAA/GO)n multilayer film, the conductivity of (PSS/G)n film is improved greatly. When the thickness of multilayer film is greater than 52 nm, a percolation effect appears, the multilayer film shows a percolation threshold at n = 4 and the highest conductivity of 204.1 S/cm.In chapter 4, we firstly propose a two-step method contained polymerization and growth process to prepare highly oriented PANI arrays without any templete. The results demonstrate that aniline concentration, acids dosage, acids concentration and species have great effects on the morphology of the PANI arrays, which realize the size and morphology controllability. We can obtain highly oriented PANI flake and fiber arrays at optimized preparation conditions. From the experimental results, we conclude anilinium cation micelles act as"template"in the formation process. Using XRD, UV-vis and FTIR, it is found that the PANI arrays have oriented molecular configuration and excellent crystallization. Furthermore, the relationship between conductivity of the PANI arrays and temperature follow the quasi one-dimensional variable range hopping model, which show that the highly oriented PANI arrays have semiconducting characters.In chapter 5, we obtain (PSS/PANI)n multilayer films from highly oriented PANI fiber array and PSS. The results show that the PANI exist in nanoparticles after PANI arrays dissolving in the solvent, the crystallization and molecular structure of the PANI arrays are still preserved. The multilayer films present higher conductivity and lower percolation threshold value than that prepared using traditional PANI particles. In chapter 6, the spin self-assembly of (PSS/G+/G-)n multilayer films are investigated, and the results show that the deposited amount against the bilayers exhibit a linear dependence at different PSS/G+/G- ratios, indicating a progressive and uniform deposition process. The multilayer films have thinner thickness at higher rotation speed (ω), and the deposition amount of surfactants (C16TAB and sodium dodecylsulfate) modified graphite nanoplatelets against theω-1/2 exhibit a direct proportion relationship. We can control the conductivity and percolation threshold value by adjusting PSS/G+/G- ratios. Generally, the films prepared at high PSS/G+/G- ratio have high conductivity and low percolation threshold. Furthermore, we also obtain the fact that smaller size of graphite nanoplatelets are expected to fabricate the spin self-assembly multilayer films with higher conductivity and lower percolation threshold value.Based on above, we prepare some kinds of highly conducting multilayer films by using self-assembly method. The relationships between the preparation conditions, structure and conductivity are investigated, and we find a percolation behaviour of conductivity for the multilayer film, and propose percolation mechanism. These results deepen and widen the research field of self-assembled conducting multilayer films in materials preparation, conductivity properties and conducting mechanism, and promote the development of the highly conducting self-assembled multilayer films.