Synthesis of Polymer Silica Hybrid Xerogels and Nanofibers through Sol Gel Processing and Electrospinning

Abstract

Polymer silica hybrids are attracting a lot of attention owing to their ability to be used in a number of applications as they combine the properties of the organic phase (flexibility, processability, ductility) and those of the inorganic phase(thermal stability, rigidity). In recent years, polymer-silica hybrids with various tailored properties have attracted a lot of attention and have found applications in a variety of fields including catalysis, adsorption, pervepaoration, sensors, and enzyme encapsulation. The scope and utility of these polymer silica hybrids are further broadened by transforming them to nanosized materials i.e. nanoparticles and nanofibers. The work reported in this thesis covers various investigations carried out by modulating sol-gel process of silica and later on incorporating controlled sol-gel processing with electrospinning to generate particles and fibers with sizes in nanometers. It includes assimilation of two polymers Poly Acrylonitile (PAN) and Polyvinyl Alcohol (PVA) into the silica gel matrix to produce hybrid xerogels and/or nanofibers. Due to the variation in the techniques and nature of materials involved in the synthesis, this research work is divided into three parts. In the first part, we report on the synthesis of PVA-silica hybrid xerogels through acid catalyzed sol-gel processing of silica precursor Tetraethoxy Silane (TEOS) in a mixed solvent consisting of water and ethanol. We systematically investigate the effect of varying ratio of PVA and silica precursor on the surface structure, thermal properties, crystallinity and solubility of the resultant xerogels. All the xerogel samples are found to display mesoporous surface morphology and the pore size is found to increase with the increase in polymer content of the xerogel. Unlike highly water soluble nature of PVA, all the hybrid xerogels are found to display extremely reduced solubility in water. This anomalous behaviour of PVA hybrids can be attributed to stronger than expected interactions between PVA and silica. XRD and DSC analysis of the xerogels point towards the loss of crystallinity of the PVA in the hybrids. FTIR examination of the xerogels also provides evidence for a covalent bond between PVA and silica which results in crosslinking of PVA in the hybrids. Catalytic properties of the as- synthesized and calcined xerogels are analyzed by studying the sorption of a fluorescent active dye Rhodamine 6G on the xerogels. Entrapment of R6G in the xerogels structure is also studied by incorporating the dye in the hybrid xerogels during synthesis. The structure of the final hybrid is verified through SEM, FTIR and XRD studies while its optical properties are investigated through UV-visible spectroscopy. Of the various approaches used to synthesize polymer silica hybrid nanofibers, the one-step electrospinning process has received a lot of attention due to its simplicity, cost effectiveness and speed. Electrospinning is a decades-old technique which draws very fine fibers from a viscous liquid (usually a polymer or polymer solution) under the force of an electrostatic field. In the second part of research, Using electrospinning and sol-gel methodologies, we report a method to prepare silica-PVA nanofibers with reduced water solubility. Silica-PVA hybrid fibers are obtained by electrospinning a mixture of the silica precursor solution and aqueous PVA. We systematically investigate how the amount of TEOS, the silica-PVA ratio, the aging time of the precursor solution and the solution rheology influence morphology of the fibers. Just like the hybrid xerogels, PVA-silica nanofiber hybrids were water insoluble when soaked overnight in water. We believe that mixing of the silica precursor and PVA in solution in the presence of acid catalyst initiates the participation of the PVA polymer in the silica precursor crosslinking so that its –OH group becomes unavailable for H-bonding with water. Third part of the research involves application of a controlled sol-gel synthesis incorporated with electrospinning to produce polyacrylonitrile-silica (PAN-silica) nanofibers. Hybrid fibers are obtained with varying amounts of PAN, silica, acid catalyst and silica precursor aging time. Solution viscosity, conductivity and surface tension are found to relate strongly to the electrospinnability of PAN-silica solutions. Later, thermal stabilization of the hybrids at 280oC followed by carbonization at 800oC transformed fibers to carbon-silica hybrid nanofibers with diameter ranging from 400 to 700 nm. FTIR analysis of the fibers confirmed presence of carbon and silica content. Graphitic character of the carbon-silica fibers was confirmed through raman studies and fibers are found to contain almost even distribution of crystalline (graphitic) and amorphous (non-graphitic) characters.