(528c) Development of Advanced Membranes Based On Molecularly Imprinted Polymeric Particles

Molecular imprinting of synthetic polymers is a process where a functional monomer and a cross-linker are co-polymerized, via a free-radical polymerization mechanism, in the presence of a target molecule (i.e., the imprint molecule) that acts as a molecular template. The functional monomer molecules initially form a complex with the imprint molecules via covalent or non-covalent bonding. This is followed by the polymerization of the functional monomer with a bifunctional or trifunctional cross-linker. The resulting highly cross-linked polymeric structure keeps in position the template molecules through covalent or non-covalent molecular interactions between the monomer and the template functional groups. Subsequent removal of the imprint molecules reveals the specific binding sites that are complementary in size and shape to the template molecule. In the present study, the chemically and mechanically stable molecularly imprinted polymers (MIPs) are employed for the development of an innovative water purification system, exhibiting a highly selective separation mechanism, at molecular level, based on affinity interactions.

Precipitation, suspension and miniemulsion polymerization are employed for the synthesis of MIP micro- and nanoparticles to be used as synthetic receptors selective to specific pesticides. In particular, MIP micro- and nanoparticles were prepared for the selective separation of atrazine from aqueous or organic solutions. Methacrylic acid (MAA) was used as functional monomer and ethylene glycol dimethacrylate (EGDMA) as cross-linker. The molar ratio of the cross-linker to the functional monomer was equal to 4:1.

Precipitation polymerization was carried out in a borosilicate glass tube equipped with a screw cap. The template molecule (i.e., atrazine) was added into the solvent (i.e., dichloromethane) followed by the addition of the functional monomer and the cross-linker. The free-radical co-polymerization of MAA with the cross-linking agent was carried out in the presence of a chemical initiator (i.e., AIBN, 2% w/w relative to the monomers mass). The total monomer mass concentration was typically 2% w/v relative to the solvent volume. The solution was first purged with dry nitrogen for 5 min and the tube was sealed under a nitrogen atmosphere. The precipitation polymerization was then initiated by placing the tube in a preheated water bath at 60^oC. The polymerization was carried out for about 16hrs.

Suspension polymerization was carried out in a laboratory scale, water-jacketed glass reactor of 100 ml working volume, equipped with a six-blade impeller, an overhead condenser and a nitrogen purge line. The reaction mixture was thermostated to within ±0.05^oC with the aid of a constant temperature bath. Initially, the template molecule, the functional monomer, the cross-linker and the initiator (i.e., AIBN, 2% w/w on the total monomers mass) were dissolved in chloroform. Accordingly, the organic phase (15ml in volume, containing the functional monomer, the cross-linker, the porogen, the template molecule and the initiator) was dispersed into an aqueous PVA solution (35ml in volume, 1% w/w) under the action of a mechanical agitator and a nitrogen atmosphere. Subsequently, the polymerization was carried out, at the specified temperature (i.e., 60^oC), for 24hrs.

Finally, miniemulsion polymerization was carried out in a laboratory scale, water-jacketed glass reactor. The organic phase consisted of the template molecule, the functional monomer, the cross-linker, the initiator (i.e., AIBN, 2% w/w on the total monomers mass) and the co-surfactant (i.e., hexadecane) was dispersed into an aqueous SDS solution (50ml, 2% w/w) via vigorous stirring for 1hr at room temperature, followed by sonication for 5 min. Subsequently, the polymerization was carried out, at the specified temperature (i.e., 60^oC) for 24hrs.

In all cases the polymer particles were collected by centrifugation. Subsequently, the template molecule was removed from the imprinted polymer particles by means of successive washing cycles with a methanol-acetic acid solution (9:1 v/v). Finally, the washed polymer microparticles were conditioned in methanol. UV spectroscopy was employed to monitor the removal of the print molecules. Non-imprinted polymeric nanoparticles (NIP) were also prepared under the same conditions in order to compare their affinity to the template molecule with that of the MIP particles. Batch-wise guest binding experiments were conducted to evaluate the rebinding capacity of polymers using either organic (i.e., acetonitrile) or aqueous (i.e., water/acetonitrile 9/1 v/v) solutions of atrazine (0.1-1mM). The MIPs were found to exhibit 30-60% higher affinity towards the imprint molecule than that of the non-imprinted polymers (NIPs), prepared in the absence of the template, thus proving their binding specificity and consequently, their capability to be used as highly-selective adsorbing materials.

Additionally, the imprinted polymers synthesized by precipitation polymerization were used for the development of hybrid ceramic membranes. The polymeric nanoparticles were impregnated in the ceramic filters employing vacuum filtration, with the aid of a stainless-steel device, using a small quantity of particles dispersed in distilled water. SEM images proved that the nanoparticles have been successfully impregnated into the filters, while the rebinding experiments in aqueous solutions showed that the hybrid ceramic filters with MIPs adsorb more atrazine than those with NIPs.