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Authors: Humayun, Muhammad
Keywords: Natural Sciences
Chemistry & allied sciences
Phyical chemistry
Issue Date: 2010
Abstract: Poly (Vinyl Alcohol)-silica and Poly (Vinyl Alcohol)-alumina hybrid materials were synthesized in aqueous medium by the sol-gel method using hydrochloric acid(HCl) as catalyst and silicon ethoxide and aluminum butoxide as silica and alumina precursor respectively. Transparent, flexible thin film composites of homogenous thickness were obtained at room temperature after drying for 5 days. The hybrid composites were characterized by Ultra Violet- Visible (UV-Vis), Fourier Transform Infrared (FTIR) spectroscopy, Scanning Electron Microscopy (SEM), Energy Dispersive X-rays (EDX), X-Rays Difractometry (XRD), Thermogravimetric/ Differential Thermal analysis (TG/DTA), Thermal Conductivity (TC), mechanical properties and water uptake measurements. SEM micrographs revealed that silica and alumina was dispersed in the PVA matrix without the large aggregation of particles. The PVA-silica or alumina composites offer an outstanding balance of toughness. Tensile results indicate that there is a compromise in properties when compositing silica or alumina with PVA as the addition of silica or alumina makes the material stronger and more useful. This is related to an increased modulus and decreased elongation at break with increasing silica or alumina content. FTIR spectroscopy failed to identify any perturbation because of blending of silica or alumina with PVA, but only specific interaction of the hydrogen bonding between different groups of PVA and alumina/silica was observed. The comparison of UV-Vis spectroscopic analysis, of PVA, PVA - silica and PVA- alumina composite films showed almost similar spectrum except that peak of 335 nm shown in pure PVA shifted towards lower wavelength with both silica and alumina. This change in peak position is due to the interacting particles of silica or alumina. This change may be attributed to aggregation of silica /alumina particles. Between the silica and alumina, silica is causing more effect on this peak while alumina is to a lesser extent, which we think is due to bigger size of silica molecules. XRD pattern of pure PVA and PVA- silica or alumina composite point out the wide bulge among 16 and 22 of 2-θ degree, and this is connected to amorphous field in Poly (Vinyl Alcohol) thin film polymer. It was discovered that the extent of crystalline domain of composite Poly (Vinyl Alcohol) has improved with addition of silica or alumina. The crystalline movement is amplified with segment wise shift of Poly (Vinyl Alcohol) mass among sites of coordination and relaxation of confined structure raise the crystallinity with increase of silica or alumina. Thermal Conductivity (TC) of PVA composites showed decrease in initial stage until the amount of silicon ethoxide and aluminum butoxide was small (up to 20% silicon ethoxide and 10% aluminum butoxide). However, beyond 20% for silicon ethoxide and 10% for aluminum butoxide the Thermal Conductivity (TC) showed increase. This means that there is a threshold value for Thermal Conductivity (TC) depending upon the added filler. Up to this value, decrease is shown while beyond this there is increase. The TGA curve of the PVA composites showed that there is the thermal stability of the PVA film up to 370 K (97 oC). There was a mass drop from 370 K to 500 K, which may be due to evaporation of surface and structure water. The further drop of weight between 500 K and 738 K is attributed to decomposition and combustion of PVA film. In the temperature range of 738 K to 820 K, a weight loss was due to the transformation from carbon black to carbon dioxide. The whole process is supported by DTA curves. Water uptake swelling index measurements were done which shows the hydrophilic nature of PVA- alumina and hydrophobic nature of PVA- silica. It can be concluded that we have successfully prepared thin Film of PVA-silica and PVA- alumina, which are thermally stable and with good strength.
Appears in Collections:PhD Thesis of All Public / Private Sector Universities / DAIs.

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