Functionalization and material investigation of acrylic acid-based hydrogels with controlled swelling behaviour

This paper is devoted to the formation and characterisation of medium-range swellable hydrogels of polyacrylic acid. Such polyacrylic acid (PAA) hydrogels were synthesised through polymerisation of acrylic acid (AA) monomers in aqueous solution of HNO3 as a medium, which also acts as an initiator and N, N-methylene bis-acrylamide (BAM) as cross linker in a mild concentration at 60 °C. After 2 to 4 hours, the polymerising mixture was transformed into a glassy, flexible and spongy mass, which was taken out and washed repeatedly to remove soluble and unreacted materials and dried. When the dried polymers so formed were subjected to alkaline saponification, the degree of swelling of hydrolysed hydrogels so formed was found to be nearly 115 times its mass. The degree of swelling of hydrolysed PAA hydrogels was highly affected by the variations of monomer concentration, NaOH concentration during saponification, initiator concentration, cross-linker density, and temperature of polymerisation. Such hydrogels is a reversible swelling behaviour. By the study of the above-mentioned factors affecting the degree of swelling of PAA hydrogels, it was revealed that the use of HNO3 as an initiator leads to the polymerisation of AA, resulting in hydrogels of a medium range of swelling capacity. This type of hydrogel has various applications in hygienic products, agriculture, drug delivery systems, sealing, pharmaceuticals, biomedical, tissue engineering and wound dressing, etc. The Study of temperature effect reveals that PAA hydrogels often display high swelling leading to deformation and deswelling, showing weaker mechanical properties. The TGA (Thermal Gravimetric Analysis) also reveals that the hydrogels has moderate moisture resistance and broad overlapping decomposition meaning, thereby, an appreciable amount of bound water in the hydrogel in HNO3 medium, indicating less densely cross-linked network and wide temperature range of weight loss, indicating gradual network breakdown rather than sharply defined stages. FTIR spectroscopy was carried out to show the chemical structure of the hydrogels. Morphology of hydrogels formed were also studied by FESEM. It would be probable to manufacture comparatively low swelling PAA hydrogels by carefully adjusting cross-linker concentration, polymerisation temperature and monomer content. The probable PAA hydrogels have good mechanical integrity and shape integrity.