Molecular dynamics simulations of the solubility and conformation change of chitosan grafted polyacrylamide: Impact of grafting rate

Graft modification is an effective approach to improve the water solubility and molecular weight of chitosan. This work investigated the effect of grafting rate on water solubility of chitosan-grafted polyacrylamide (Chi-gPAM) using molecular dynamics simulations. The intramolecular and intermolecular interaction between Chi-gPAM with the grafting rate of 100%-250% and water together with the conformation change were analyzed by radial distribution function (RDF), radius of gyration, solvation free energy (Delta GSFE), mean square displacement, diffusion coefficient and shear viscosity. The results showed that the intramolecular hydrogen bonding of Chi-gPAM was the dominant factor affecting its water solubility. While the grafting rate of Chi-g-PAM increased from 100% to 250%, the RDF of Chi-g-PAM decreased from 5.39 to 3.57 between O-H at 2.0 angstrom and 2.60 to 1.30 between N-H at 2.1 angstrom. The interaction between Chi-g-PAM and water increased with grafting rate as the absolute value of Delta GSFE for Chi-g-PAM increased from 432.917 kcal/mol to 858.749 kcal/mol. The solubility of Chi-gPAM in acidic solution was much better than that of Chi-g-PAM in aqueous solution. Our work on the fundamental insights into the effect of grafted molecular structure of chitosan-based copolymer on its solubility will benefit the development of environmentally friendly chitosan-based flocculants.