Preparation and chromatographic evaluation of thermoresponsive N-isopropylacrylamide copolymers stationary phases containing amino groups

Two cationic thermoresponsive stationary phases were designed and prepared containing poly[N-isopropylacrylamide-co-(2-dimethylamino)ethylmethacrylate] and poly [N-isopropylacrylamide-co-(2-diethylamino)ethylmethacrylate] via a simple method, the direct copolymerization of monomers with double bonds on silica surfaces. The two copolymers were synthesized by radical polymerization and then characterized using Fourier transform infrared and gel permeation chromatography. The thermoresponsive property and amounts of copolymers grafted on silica were determined through transmittance measurements and thermogravimetric analysis, respectively. The copolymers grafted silica particles were then applied as high-performance liquid phase (HPLC) stationary phases for chromatographic separation. Chromatographic properties of mobile phases at different pH values were evaluated by changing temperatures and using benzene and hydrocortisone as the test analytes. Retention time of the analytes was prolonged with increasing temperature on both thermoresponsive columns due to enhanced hydrophobic interaction between analytes and stationary phases. The resolution increased with increasing pH of mobile phase. The optimal separation was obtained at phosphate buffer solution (10 mM, pH 8.0) and at 50°C. The pH of mobile phase had a crucial effect on separation efficiency. The results illustrated that poly[N-isopropylacrylamide-co-(2-diethylamino)ethylmethacrylate] copolymer modified silica was more advantageous for the temperature-responsive chromatographic separation because its lower critical solution temperature was relatively lower compared to the poly[N-isopropylacrylamide-co-(2-dimethylamino)ethylmethacrylate] stationary phase.