Understanding of the Plasticizing Effects of Glycerol and PEG 400 on Chitosan Films Using Solid-State NMR Spectroscopy

Differences were found in the plasticizing effects of two commonly used softening materials on amorphous chitosan films. The plasticizing efficiencies for glycerol and PEG 400 were found to be similar in mechanical tests, but the changes in the three-dimensional H-bonded structure monitored by solid-state NMR spectroscopy were different. The analysis of Lee−Goldburg cross-polarization build-up curves demonstrated that, while glycerol decreases the mobility of the acetamide groups, PEG 400 increases it. Further, while glycerol molecules are immobilized in chitosan films, PEG 400 remains mobile in them. The results of two-dimensional (2D) 1H−13C frequency-switched Lee−Goldburg (FSLG) HETCOR experiments supported the mobility difference of the chitosan chains and suggested a specific interaction between the glycerol and the glucosamine units. Our findings were supported by density functional theory calculations. Overall, PEG 400 acts as an external plasticizer, while glycerol acts as an internal plasticizer.