Thermally responsive graft copolymer of soy protein isolate and N-isopropylacrylamide: synthesis and self-assembly behavior in aqueous solution

In recent years, soy protein isolate (SPI) has attracted great attention due to its biodegradability, biocompatibility, and wide availability. It has been used in food and pharmaceutical industry such as edible films and drug delivery systems. In this study, we report the synthesis and self-assembly behavior in aqueous solution of thermally responsive graft copolymer (SPI-g-poly(N-isopropylacrylamide) (PNIPA)) of soy protein isolate and N-isopropylacrylamide in aqueous solution. SPI-g-PNIPA was synthesized in the 8 mol/l urea cushioning solution, by using ammonium persulfate as the initiator and mercaptoacetic acid as the protein unfolding agent. Laser light scattering, transmission electron microscopy, and fluorescence spectroscopy have been used to study the self-assembly behavior of SPI-g-PNIPA in aqueous solution. Above the critical micelle concentration (cmc), SPI-g-PNIPA aggregates could assemble into different structures including the simple spherical structure, spherical core–shell structure, and random coil structure, depending on the graft copolymer concentration. The graft copolymer concentration, temperature, pH value, and ionic strength were found to influence the aggregate size and morphology of SPI-g-PNIPA in aqueous solution. With increasing ionic strength, the aggregate size increases. However, pH value, SPI-g-PNIPA concentration, and temperature have complicated influences on the aggregate size. The lower critical solution temperature of the SPI-g-PNIPA at pH 8.5 is 36 °C. The method of intrinsic fluorescence spectroscopy was used for the first time to determine the cmc value of SPI-g-PNIPA in aqueous solution.