Characterization of dextrin hydrogels by FTIR spectroscopy and solid state NMR spectroscopy

Abstract

Fourier transform infrared (FTIR) and 13C solid state nuclear magnetic resonance (NMR) spectroscopy were used to study dextrin structural changes occurring upon hydrogel formation by vinyl acrylate (VA) grafting and subsequent free radical polymerization. The degrees of VA substitution (DS) and polymerization (DP) were quantified up to 40%VA by FTIR intensity measurements and partial least squares (PLS)/FTIR, the latter being a faster and less error-prone method. Above 40%VA, both parameters are underestimated by FTIR. A spin counting NMR experiment showed high carbon observabilities for hydrogels and improved PLS/NMR models were achieved for DS and DP determination. Alternative NMR integration methods are hindered by the broad VA peaks and need for area correction, due to their CP dynamics. NMR changes in C1 profile showed that a single helical conformation predominates at lower %VA, being replaced by disordered conformations as %VA increases. Furthermore, a correlation FTIR/NMR study indicated that ring conformations are significantly affected in hydrogels, compared to unpolymerized dextrin.