Xiaowu Fan

Intercalation of polymerization initiators into montmorillonite nanoparticle platelets: free radical vs. anionic initiator clays

We have investigated the properties of clay surface bound initiators for the preparation of polymer-layered silicate nanocomposite materials: one for anionic and two for free radical polymerization. These organic cations were intercalated into the galleries of montmorillonite clay by cation exchange. The structural, thermal, and compositional properties of the three types of initiator clays were studied and compared. X-ray diffraction studies showed that all three systems were swelled after initiator intercalation and the increase of interlayer d-spacing was dependent on the structure and molecular chain length of the initiators. Thermal gravimetric analysis, differential scanning calorimetry, and X-ray photoelectron spectroscopy traces indicated the presence of the initiator on clay and a complete cation exchange process was achieved only by adding excessive amounts of initiator. Infrared spectra compositionally confirmed the successful initiator intercalation into the clay platelets. Important differences were found between surface bound mono and dicationic free radical initiators.

Surface structural characterization and mechanical testing by nanoindentation measurements of hybrid polymer/clay nanostructured multilayer films

Nanostructured montmorillonite/poly(diallyldimethylammonium chloride) multilayer thin films were fabricated up to 100 layers thick by stepwise alternating polyelectrolyte and clay deposition from solution. The structure and morphology of the films were characterized by x-ray diffraction, ellipsometry, atomic force microscopy, and quartz crystal microbalance ex situ and in situ measurements. The mechanical properties were tested by nanoindentation. The hardness of the multilayer thin film was 0.46 GPa. The thin films modulus was correlated to its ordering and anisotropic structure. Both hardness and modulus of this composite film were higher than those of several other types of polymer thin films.

Fabrication and electrochromic properties of layer-by-layer self-assembled ultrathin films containing water-soluble phthalocyanine

Alternate ultrathin films of water-soluble copper (II) phthalocyanine-3,4′,4″,4″′-tetrasulfonic acid (CuPcTs) and poly(diallyldimethylammonium chloride) (PDADMAC) were fabricated using the electrostatic layer-by-layer self-assembly technique. Linear depositions of the bilayers were observed from the UV-vis absorption spectra and ellipsometry. The average thickness of the CuPcTs monolayer obtained from ellipsometry was about 1.2 nm and CuPcTs molecules were estimated to be tilted to the film surface. The cyclic votammogram (C-V) of the fabricated films in aqueous solution containing 0.1 M KCl suggested the oxidation and reduction of the CuPcTs at around +0.9 V and 0.05 V respectively. UV-vis absorption spectra were observed for the films at applied voltages of 0.8 V and 0 V in the KCl aqueous solution and reversible absorption changes were observed.

Supramolecular ultrathin film strategies for DNA assemblies: Substrates for optobioelectronics, gene therapy, and microarrays

We describe our strategies and results in the preparation of supramolecularly ordered ultrathin films of DNA assemblies using the layer-by-layer (LbL) alternate polyelectrolyte adsorption technique. The properties of DNA are intimately associated with their polyelectrolyte behavior in solution. Deposition at interfaces is governed by conformation, orientation, and charge density of these biomolecules in relation to the physisorption phenomena in oppositely charged surfaces. Thus, controlling the nature of surfaces (polymer charge density, ionic strength, other non-covalent interactions, etc.) is important in modifying the adsorption phenomena. In this work, differences in adsorption and incorporation of DNA with dyes, linear polymers and dendrimers are highlighted. A number of surface sensitive spectroscopic and microscopic techniques were used to probe the adsorption and multilayer assembly phenomena, e.g. surface plasmon resonance spectroscopy (SPS), AFM, quartz crystal microbalance (QCM) and ellipsometry. These studies are important for future applications such as the use of polycations as non-viral gene transfection vectors for drug-delivery and DNA adsorption on microarray surfaces. By combining with the alternate assembly of azobenzene and phthalocyanine dyes, we have been able to prepare optobiolelectronic substrates where the phenomena of irradiation and electrochemistry can be used to probe the ordering and response of these films.