Improved homopolymer separation to enable the application of 1H NMR and HPLC for the determination of the reaction parameters of the graft copolymerization of acrylic acid onto starch

Graft copolymers of starch with acrylic acid are a promising green, bio based material with many potential applications. The grafting of acrylic acid onto cassava starch in an aqueous medium initiated by Fenton’s reagent has been studied. Common grafting result parameters are add-on (yield) and graft efficiency (selectivity). However, the analysis of the reaction products and an accurate determination of these parameters stand or fall with a complete separation of the entangled but ungrafted homopolymer from the grafted product. Therefore, this separation is the core of the newly developed analytical procedure. An appropriate solvent has been selected with dedicated testing from the range methanol, ethanol, acetone, dioxane, 2-propanol, and 1-propanol. Acetone showed the best performance in many respects. It has a high dissolving power for the homopolymer, as well as the highest yield of precipitation for the starch derivatives and it is the most economical in use. After the successful separation, the precipitated graft copolymers could be analyzed quantitatively by nuclear magnetic resonance. The liquid with homopolymer and unreacted monomer was analyzed by high pressure liquid chromatography. Proof of grafting has been found by FTIR and TGA analyses. The mass balance calculation shows a systematic error which appears fairly consistent: 18.0 ± 2.5 wt %. This was used as a correction factor in the calculation of the grafting parameters but more importantly, it means that the method we developed has a high level of repeatability, in the order of 97%.

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► Quantitative analysis to determine add-on and selectivity of starch grafting.
► A highly effective separation of the homopolymer from the grafted starch was developed.
► HPLC and 1H NMR were used for quantitative analysis of the separated fractions.
► FTIR, TGA, and SEM indicate the occurrence of true grafting.