Acrylonitrile graft copolymerization of casein proteins for enhanced solubility and thermal properties

Casein proteins are soluble in 5% aq. ethanolamine, triehtylamine, and triethanolamine, but insoluble in organic solvents. Graft copolymerization of casein (40 g/L) with acrylonitrile (AN) was carried out in 5% w/v aq. triethanolamine at 60°C using potassium persulfate K2S2O8 as an initiator. Percent grafting and grafting efficiency increased with increasing initiator concentrations (up to 1.7 × 10−2 mole L−1) and reaction times, but decreasing [M]/[I] ratios. Fourier transform IR spectra confirmed the formation of the acrylonitrile-grafted-casein (AN-g-casein) copolymers. Under the reaction conditions studied, the grafted PAN side chains were characterized by gel permeation chromatography to have Mn between 1.58 and 5.88 × 104 dalton and polydispersities between 2.6 and 4.5. The AN-g-casein copolymers behaved more like a PAN homopolymer in terms of their thermal properties and solubilities. The decomposition temperatures of AN-g-casein copolymers were between 255 and 273°C, closer to the Td of the PAN homopolymer (275°C) and significantly higher than that of casein (180°C). The AN-g-casein copolymers are soluble in 50% aq. NaSCN and ZnCl2, but are insoluble in 32:28:40 wt % CaCl2/CH3CH2OH/H2O like PAN and dimethylformamide-like casein. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 2543–2551, 2000