Stimulus response and molecular structural modification of polyacrylamide gel in nitric acid: a study by Raman, FTIR, and photoluminescence techniques

Abstract

Stimulus response of photopolymerized 1% and 0.5% N,N'-methylene bisacrylamide (MBA) crosslinked 10% polyacrylamide (PAAm) hydrogels was studied in nitric acid. The hydrogels swelled exponentially to saturation in 13 h due to the osmotic pressure arising from diffusion of ions in to the gel. MBA (0.5%) gels swell more with larger time constant than 1% MBA gels due to lower bulk modulus. Diffusion coefficient of nitric acid in the hydrogel and polymer-solvent interaction parameter were estimated from the swelling behavior and discussed. At longer times, the hydrogels deswelled linearly in nitric acid due to molecular modification of amide group by acidic hydrolysis. Degree of swelling and deswelling increase with nitric acid concentration. Raman and FTIR investigations revealed the formation of carboxylic acid due to acidic hydrolysis.