Self-crosslinkable superabsorbent hydrogels based on vinlalkoxysilanes as crosslinker
Aydın, Kinyas (2013) Self-crosslinkable superabsorbent hydrogels based on vinlalkoxysilanes as crosslinker. [Thesis]
Novel superabsorbent hydrogels were synthesized through the Free Radical Solution Precipitation polymerization technique. The synthesized materials were characterized by Differential Scanning Calorimetry (DCS), Simultaneous Thermal Analysis (STA), FTIR (Fourier Transmittance Infrared),1H Nuclear Magnetic Resonance Spectroscopy (NMR), 29Si Nuclear Magnetic Resonance Spectroscopy . These characterization methods provide valuable information to understand chemical structure and performance level of water uptake synthesized hydrogels, effect of synthesis parameters; change in cross linker agents, and thermal behavior of polymers. The effects of synthesis conditions such as monomer feed concentration of acrylamide/ 2-acrylamido-2-methylpropane-1-sulfonic acid (AMc/AMPS) on level of performance of superabsorbent hydrogels were investigated. Highest Equilibrium Swelling ratio (ESR) gravimetrically was obtained for smaller AMc/AMPS ratio when Vinyltrimethoxysilane (VTMS) ratio was kept constant. It was experimentally proved that increasing molar concentration of cross linking agent in polymerization mixture gave lowest ESR gravimetrically (g/g) in distilled water. Due to chemical structure and reactivity relationship, when changing the type of cross linking agent in polymerization reactions, while according to the characterization results we studied there was no important change, but different ESR values were obtained for each polymer synthesized with different vinylalkoxysilane. Based on three different chemical structures of vinylalkoxysilane cross linking agents, namely Triethoxyvinylsilane (TEVS), Vinyltrimethoxysilane(VTMS) and Tris(2- methoxyethoxy)vinylsilane (TMEVS), the highest ESR value was obtained for TEVS based samples. This highest ESR value for TEVS based samples is attributed to the higher rate of hydrolysis of alkoxy groups). An increase in electropositive density on Si atom was predicted to be responsible for higher rate of hydrolysis of alkoxy groups of TEVS leading ethanol as leaving group in aqueous medium. For TMEVS, when chemical structure and reactivity can be taken into account, it can be stated that for methoxyethoxy groups on Si atom, there are parallel two inductive powers cancelling each other and providing a weaker electropositive density on silicone atom, hence a slower rate of hydrolysis. According to thermal analysis, it was observed that the powder samples of superabsorbent hydrogels started to degrade thermally under N2 around 278 0C and three different common thermal behaviors were attributed to polymers based on chemical structure. The Monomer reactivity studies were analyzed according to 1H – NMR results for the synthesized powder samples in the same experimental conditions while excluding any cross linking agents. According to analysis, monomer reactivities of AMc and AMPS were found as 1.32 and 1.25 respectively. Based on monomer reactivity results, it is clear to conclude that for the Free radical solution precipitation polymerization of acrylamide and AMPS occurred randomly. Experimentally three different polar aprotic solvents were used as solvent medium in reactions,1,4-dioxane,Dimethylacetamide and Dimethylformamide. A significant difference result from using these three polar solvents was the comparable change in ESR under same conditions. Superabsorbent polymers (SAPs) synthesized in Dimethylformamide provided highest ESR values. Solution viscosity of prepared standard 6 %(wt/ml) of polymers were measured in Brookfield DV-III Rheometer while keeping torque value 15%. Viscosity of standard solutions of polymer based on three different vinylakoxysilanes were found as lowest for TEVS based polymer and highest for TMEVS based polymer. Molecular weight of synthesized polymers based on three different vinylalkoxysilanes were calculated according to the results obtained from Dilute Solution Viscometery experiment, while using the constants for polyacrylamide since about 90 % of synthesized polymers are acrylamide. According to dilute solution viscometery calculations, TEVS based polymer was found to have lowest MW, while TMEVS based polymer with highest MW. 29Si NMR analysis were done in order to check the hydrolysis and condensation behaviors of three different vinylalkoxysilanes. Simply, same molar concentration of vinylalkoxysilanes in an aqueous medium in constant temperature yielded different behaviors. According to results, the fastest hydrolysable cross linking agent was found to be TEVS. Two other cross linking agents have different responds in aqueous medium and corresponding hydrolysis products observed on Si29 NMR spectrums.
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