STUDIES ON CARBOXYLIC ACID CROSSLINKED POLYVINYL ALCOHOL AND THEIR APPLICATIONS

Abstract

Chemical crosslinking is a widely used method to alter the properties of polymers like polyvinyl alcohol (PVA). The physicochemical properties of PVA are greatly affected by the type of crosslinking agent used. This thesis work gives detailed insights into the effect of molecular structures of various carboxylic acids employed as crosslinking agents on the physicochemical properties of PVA. The carboxylic acids crosslinked PVA films were synthesized using the solution casting method. Various carboxylic acids were compared for PVA crosslinking and effects of varying the number of carbon atoms (odd-even), number of carboxyl groups and taking account of single and double bonds in carboxylic acids. The prepared films were characterized by techniques such as ATR-FTIR, XRD, SEM, AFM, DSC, TGA, Rheology, Tensile strength, Gel content, Percent swelling, and Sessile drop contact angle. PVA films crosslinked with dicarboxylic acids containing an even number of carbon atoms displayed translucency, roughness, and an irregular matrix. In contrast, films crosslinked with dicarboxylic acids incorporating an odd number of carbon atoms were transparent, homogenous, and densely packed. Furthermore, films crosslinked with acids containing an even carbon atom exhibited relatively lower crystallinity, and lower melting temperature (Tm). They displayed an unexpected irregular trend in swelling as crosslinker concentration increased compared to films containing dicarboxylic acids with an odd number of carbon atoms, exhibiting a consistent swelling with increasing crosslinker concentration. These results are attributed to the difference in the solubility and crystal structures of the odd-even series of dicarboxylic acids. On proceeding further, based on the degree of crosslinking and thermal properties, unsaturated carboxylic acid was found to be more effective compared to saturated acid. This was evident in the comparison between succinic acid (single bond) and maleic acid (double bond), where the crosslinking degree of the maleic acid crosslinked film was determined to be 6.47 times higher than that of the PVA-SA film. Also, malic acid crosslinked film displayed lower crystallinity and a higher vi contact angle (80.60) in contrast to succinic acid (76.0) crosslinked film and uncrossed PVA (30.4) film. When compared amongst di, tri and tetra carboxylic acid crosslinked PVA film, the degree of crosslinking, tetra acid demonstrated superior crosslinking performance and resulting in the highest mechanical strength and thermal stability. The highest strength was observed in the tetra acid crosslinked film, which exhibited a 127% increase over neat PVA. Regarding thermal stability, the decomposition temperature followed the order of tetraacid > triacid > diacid. Lastly, the synthesized crosslinked films were used to assess their efficacy in the adsorptive dye removal process using a cationic dye, namely crystal violet as a model dye. The results showed that the adsorption capacity of the films increases with an increase in the number of carboxylic groups of crosslinkers and surface roughness. A high dye removal efficiency (>90%) and adsorption capacity (38.53 mg/L) were observed for 30 wt% of tetracarboxylic acid crosslinked film for the dye concentration of 20 ppm with a contact time of 105 min at room temperature. Overall, the thesis emphasizes the critical importance of considering the molecular structure of carboxylic acids during the crosslinker selection process. These considerations play a crucial role in tailoring PVA films to achieve specific properties for diverse applications.