Preparation, characterization and application of a chiral thermo-sensitive membrane for phenylalanine separation of the racemic mixture

Poly(vinylidene fluoride)-grafting-poly(N-isopropylacrylamide) (PVDF-g-PNIPA), a thermo-sensitive polymer with a poly(vinylidene fluoride) (PVDF) backbone and poly(N-isopropylacrylamide) (PNIPA) side chains, was synthesized via radical copolymerization. Chiral micro-gels were synthesized with N-isopropylacrylamide (NIPA) and a chiral monomer that was obtained by an acrylation reaction with L-phenylalanine and acrylyl chloride. A novel chiral thermo-sensitive membrane for phenylalanine separation of the racemic mixture was prepared by a phase inversion method with a blend of chiral micro-gels and PVDF-g-PNIPA. Chemical composition and morphology of the membrane were characterized by Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM), respectively. The permeability, enantioselectivity, the resolution mechanism, the effects of temperature, blending ratio and type of enantiomer molecules on the chiral recognition properties were studied. The results showed the water flux of the membrane displayed a sensitivity to temperature and the permeability of D-phenylalanine was better than that of L-phenylalanine. Comparing with a chiral membrane at 40 °C the membrane at 25 °C obtained a larger percentage of enantiomeric excess (e.e.%). The values of e.e.% and the separation factor (α) at a temperature above the LCST were also much higher than that of membranes with no thermo-sensitivity at low temperature. The value of e.e.% and α increased with an increased blending ratio; when the blending ratio was 30 wt.% in the chiral blended membrane the value of e.e.% reached 10 %. The model of the chiral membrane could be described as a solution-diffusion model.