Preparation And Characterization Of Molecularly Imprinted Membrane Based On Self-assembled System Of Lipoic Acid Thermoresponsive Molecularly Imprinted Polymers

The quantum chemistry simulation was applied to investigate the self-assembled system of thermo-sensitive molecularly imprinted polymers.Based on the density functional theory(DFT),the M06-2X/6-311+G(d,p)and M05-2X/6-31G* level were used to study the molecularly imprinted self-assembled system of ?-lipoic acid(ALA)and N-vinylcaprolactam(NVCL).The molecularly imprinted polymer(MIP)was prepared through a non-covalent bulk polymerization method using ethylene glycol dimethacrylate(EGDMA)as cross-linkers,the azodiisobutyronitrile(AIBN)as the initiator and acetonitrile as solvent.The thermo-sensitivity,micro-morphology,FT-IR spectra and ALA adsorption properties of ALA-MIPs were characterized as well as the adsorption and release characteristics of ALA-MIPs,which provided the basis for the development of MIM based on thermo-sensitive molecular imprinted polymers.Polyacrylonitrile(PAN)and Polyvinylidene fluoride(PVDF)were used as the main raw material to synthesis molecularly imprinted membranes using the heterogeneous particle-filling method by adding ALA-MIPs.The morphology of the molecularly imprinted membrane was investigated by scanning electron microscopy(SEM).Mechanical properties,hydrophilic properties,water absorption abilities,and ALA release properties of the membrane were investigated.The thermo-responsive release characteristics of MIM were also studied.NVCL was selected as a functional monomer through the molecular electrostatic potential(MEP)and reduced density gradient(RDG)isosurface visualization analysis.The results show that the active sites of ALA are mainly the O atom of carboxyl group and the H atom of hydroxyl group,while the active sites of NVCL are mainly the O atom of carbonyl group and the H atom connected to C atom next with N atom.ALA and NVCL are bound mainly via hydrogen bonding interaction in the prepolymerization system.When the molar ratio of ALA and NVCL raise to 1:4,the energy of system is-3030.32 kcal·mol-1 and the binding energy is-27.89 kcal·mol-1 which indicates the most stable structure.Acetonitrile shows the lowest free energy of dissolution(-22.23 kcal/mol)and the shortest hydrogen bond length between ALA and NVCL,which is the most suitable solvent for the complex system.The template and functional monomer were expected to form a more stable complex in bulk polymerizationat at the molar ratio of 1:4 based on the results of quantum chemistry calculations.The maximum adsorption amount of ALA imprinted was 6.53 mg/g.The result of scanning electron microscope(SEM)showed that MIPs had more three-dimensional imprinted cavities and greater specific surface area that could specifically bind to ALA than non-printed polymers(NIPs).FT-IR spectroscopy indicated that the binding sites were formed between the hydroxyl group and carboxyl group of ALA and the carbonyl group of NVCL in MIPs.Furthermore,the isothermal adsorption and specific adsorption experiment of MIPs and NIPs show that the adsorption capacity of NIPs was significantly smaller than that of MIPs.Scatchard plotting revealed that there are two types of binding sites for the binding of ALA in MIPs,whereas there is only one kind of binding site in NIPs.And compared with NIPs,MIPs have better adsorption selectivity for ALA.The adsorption of MIPs on ALA was dramatically influenced by the different temperatures.The adsorption amount reached to its lowest level at 20?,and peaked at 40?.The LCST of ALA-MIPs was about 25.7 ?.Ultimately,by adding MIPs to the casting solution,we made the thermo-responsive molecularly imprinted blend membrane.The results show that the PVDF membrane is a hydrophobic membrane has better mechanical properties than the PAN membrane which is a hydrophilic membrane.The addition of MIPs will increase the moisture absorption rate of the PAN-MIM,however,the moisture absorption rate of the PVDF-MIM will decrease.SEM photographs exhibit a denser structure of PVDF-MIM with a large number of independent small cavities,and MIPs are embedded inside the membrane,which is not conducive to the release of ALA.The PAN-MIM surface has a cavity with a larger pore size,and the MIPs are buried on the inner wall and lead to better release performance.Both PAN and PVDF membranes have a greater release of ALA at around 20?,and the PAN-MIM has a greater total release amount.Due to the retention of the internal structure of the PVDF-MIM,the release of ALA relies heavily on the molecular acceleration by heating and has a maximum release at the highest temperature at 60?.