Study On Preparation And Performance Of Thermo-Responsive And Molecular Recognizable "Smart" Track-Etched Membranes

Thermo-responsive gating membranes and molecular recognizable gating membranes have wide potential applications in the fields of controlled release, substance separation, and chiral resolution, etc. Up to now, few literatures have been reported on intelligent gating membranes possessing both thermo-responsive and molecular recognizable characteristics simultaneously. Moreover, investigations on chiral resolution employing such membranes are still lacking. Therefore, it has substantial significances to systematically investigate the preparation technology, the relationship between structure and performance, and stimuli-responsive mechanism of such thermo-responsive and molecular recognizable gating membranes, which are the key fundaments for designing and fabricating membranes with good performance for the separation of special molecules. In this study, a new type of thermo-responsive and molecular recognizable intelligent membranes was designed and successfully prepared based on the successful preparation of different modified cyclodextrin, copolymer of poly(N-isopropylacrylamide) (PNIPAM) and modified cyclodextrin, and PNIPAM grafted gating membranes. The preparation technology, the controllable property of microstructure, and the relationship between microstructure and separation performance of such membranes were investigated systematically. The molecular recognizable capability controlled by temperature and the chiral resolution capability of such membranes were also experimentally studied.PNIPAM is a thermo-responsive polymer which has lower critical solution temperature (LCST) around 30~34 oC and its responsiveness is fast. Therefore, PNIPAM is a widely-used thermo-sensitive material in the study of thermo- responsive gating membranes.β-Cyclodextrins (CDs) with seven glucose residues, are torus-shaped molecules composed of cyclicα-1,4-oligoglucopyranosides. Because of their structure of hydrophobic cavities with size suitable for naphthalene ring and of hydrophilic external surfaces, CDs can recognize many inorganic, organic and chiral compounds and selectively include them in their cavities to form host-guest or supramolecular complexes. Track-etched (TE) membrane with straight cylindrical pores perpendicular to the surface and narrow pore size distribution is a favorable porous substrate membrane used to study microstructure morphology and stimuli-responsive mechanism of smart membranes. In this study, smart membranes with both thermo-responsive and molecular recognizable characteristics were prepared by grafting both NIPAM and modified CD monomers onto polyethylene terephthalate (PET) TE membrane by employing plasma-induced grafting polymerization or the combined method of plasma-induced grafting polymerization and chemical reaction.Natural CDs do not have active groups to react with NIPAM, so they need to be modified before polymerization. Three modified CDs, mono-6-deoxy-6-(N-acryl- oxyethylenediamino)-β-cyclodextrin (AACD), mono-6-deoxy-6-(N-allylamino)-β- cyclodextrin (ACD) and mono-6-deoxy-6-(2-hydroxypropyl methacrylate-hexane- diamino)-β-cyclodextrin (GMA-HAD-CD), which have double bond and different length of substituted group, were synthesized. Compared with AACD and GMA- HAD-CD, ACD has less synthesis step and therefore its yield and purity were higher.The intermediate product ethylenediamino-β-cyclodextrin (EDA-CD) in the second step of preparing AACD could also be used as monomer to react with PNIPAM. Because it is difficult to analyze the properties such as composition, chain length and the LCST of polymer grafted on the substrate membrane, poly(N-isopropylacrylamide-co-2-hydroxypropyl methacrylate ethylenediamino-β- cyclodextrin) P(NIPAM-co-GMA/CD) polymer was synthesized, and thermo- responsive characteristics and molecular recognizable capability of such a polymer were experimentally studied. When GMA content in the copolymer was small (feeding molar ratio of NIPAM and GMA was 19.5: 1), P(NIPAM-co-GMA) copolymer had thermo-responsive characteristics similar to homopolymer PNIPAM, but its LCST became smaller than that of PNIPAM. The thermo-responsive characteristics of CD-immobilized copolymer P(NIPAM-co-GMA/CD) became a little worse, however, their molecular recognizable capability was still remained. The LCST of P(NIPAM-co-GMA/CD) copolymer was higher than that of P(NIPAM-co-GMA) copolymer in aqueous solution. When P(NIPAM-co-GMA/CD) copolymer was dissolved in ANS aqueous solution, its LCST became lower than that in aqueous solution but still higher than that of P(NIPAM-co-GMA) copolymer in aqueous solution.PNIPAM was successfully grafted on the surface and in the pores of the polycarbonate (PC) TE porous membrane by plasma-induced grafting polymerization, and the preparation technical conditions, including grafting technical conditions such as monomer concentration, polymerization temperature and time, and plasma treating technical conditions such as treating power and time, were experimentally investigated. The grafted PNIPAM polymers were formed onto the membrane surface and inside the pores throughout the entire membrane thickness, and the PNIPAM polymers were filled in the membrane pores gradually with the increase of the pore-filling ratio (F). For the PNIPAM-g-PCTE membranes with F is smaller than 44.2%, the water flux at 40 oC was always larger than that at 25 oC. With the F increasing, the water flux of PNIPAM-grafted membranes decreased at 25 oC as well as at 40 oC. However, when F was too large (> 44.2%), the water flux of PNIPAM grafted membranes became zero no matter what the environmental temperature was. The pore diameter of the PNIPAM-g-PCTE membrane with F = 23.9% increased dramatically when the temperature changed from 28 to 34 oC, but kept unvaried at the temperatures lower than 28 oC and/or higher than 34 oC (pore size at 34 oC was nearly two times as that at 28 oC). The contact angle of PNIPAM-g-PCTE membrane increased largely when the temperature increased, while that of substrate membrane became a little smaller at the same environmental temperature. The thermo-responsive gating characteristics of the water flux of PNIPAM-g-PCTE membranes were mainly dependent on the pore size change rather than the variation of membrane surface hydrophilicity.Poly(N-isopropylacrylamide-co-N-allylamino-β-cyclodextrin)-g-PET (P(NIPAM-co-ACD)-g-PET) membranes were successfully prepared by employing plasma-induced peroxide radical grafting polymerization method and plasma-induced free radical grafting polymerization method, respectively. The addition of sodium dodecyl sulfate, dimethylsulfoxide and N,N-dimethylformamide into monomer solution, was helpful to improve the reaction temperature and consequently obtain higher grafting yields (Y). Poly(N-isopropylacrylamide-co- 2-hydroxypropyl methacrylate ethylenediamino-β-cyclodextrin)-g-PET (P(NIPAM-co-GMA/CD)-g-PET) membranes were successfully prepared by the combined method of plasma-induced free radical grafting polymerization and chemical reaction. P(NIPAM-co-GMA/CD)-g-PET membranes with appropriate grafting yield (Y=1.47%) prepared by appropriate feeding molar ratio of monomers (e.g. feeding molar ratio of NIPAM and GMA was 2.9 : 1) showed good thermo-responsive characteristics, whose pore diameter at higher temperature (34~40 oC) was about 1.15 times as that at lower temperature (25~28 oC) compared with 1.29 times of PNIPAM-g-PET membrane (Y=1.42%). When feeding molar ratio of NIPAM and GMA was 1.6 : 1, P(NIPAM-co-GMA)-g-PET membrane with Y=2.67% showed thermo-responsive characteristics. However, because the water flux was small, the thermo-responsive gating effect was not significant. With the ratio of GMA in grafted polymer increasing, the thermo-responsive characteristics of P(NIPAM-co-GMA)-g-PET membranes became worse. To obtain better thermo- responsive characteristics, it is necessary to increase grafting yield to some extent. The gating temperatures of resultant thermo-responsive P(NIPAM-co-GMA)-g-PET membranes were still around 32 oC. The contact angle of P(NIPAM-co-GMA/CD)- g-PET membrane also showed thermo-responsive characteristics. When the temperature increased, the contact angle of P(NIPAM-co-GMA/CD)-g-PET membrane increased largely, whereas that of substrate membrane became smaller at the same time. The thermo-responsive characteristics of the water flux of P(NIPAM-co-GMA)-g-PET membranes were mainly dependent on the pore size change rather than the variation of membrane surface hydrophilicity.CD-immobilized PGMA/CD-g-PET and P(NIPAM-co-GMA/CD)-g-PET membranes could discriminate D,L-tryptophan (D,L-Trp), and the chiral resolution process was based on redarded transport methanism. PGMA/CD-g-PET membrane (Y=3.08%,CD content is 11.0μg/cm2) has better chiral resolution capability toward D-Trp than PGMA-g-PET (Y=2.53%) at 25 oC. The enantiomeric excess (e.e.%) firstly increased and then decreased as time increased, and the maximal e.e.% of PGMA/CD-g-PET membrane was 7 %. The e.e.% of P(NIPAM-co-GMA/CD)-g- PET membrane was also increased first and then decreased. The less the CD content was, the smaller the e.e.% of P(NIPAM-co-GMA/CD)-g-PET membrane. When CD content was 2.6μg/cm2, the maximal e.e.% was nearly 7%; while the maximal e.e.% was 10% for 11.5μg/cm2. ANS diffused faster through PGMA/CD-g-PET membrane (Y=3.07%, CD content was 5.6μg/cm2) than through PGMA-g-PET membrane (Y=3.27%) at both 25 oC and 40 oC, because of the facilitated transport function of CD. Because of the association constant of ANS with CD was smaller at higher temperature, the facilitated transport of CD toward ANS was enhanced at higher temperature. The difference between diffusion coefficient of ANS through PGMA/CD grafted membrane at 40 oC and that at 25 oC was larger than that through PGMA grafted membrane. The diffusion coefficient of ANS through PGMA/CD-g- PET membrane at 40 oC was even larger than that through substrate membrane. The adsorption of ANS onto P(NIPAM-co-GMA/CD)-g-PET membrane was mainly due to stronger recognition capability of CD toward ANS, while the adsorption of the substrate and P(NIPAM-co-GMA)-g-PET membrane toward ANS was weaker. ANS adsorbed onto P(NIPAM-co-GMA/CD)-g-PET membrane at lower temperature (i.e. 25 oC) and desorbed from P(NIPAM-co-GMA/CD)-g-PET membrane at higher temperature (i.e. 40 oC) with good repeatability. The reason mainly existed in both the"swollen-shrunken"configuration change of P(NIPAM-co-GMA) grafted chains around LCST and the stronger recognition of CD toward ANS. ANS was partially captured in the CD cavity with naphthalene group enclosed in the cavity and benzene ring residing outside the cavity. At lower temperature (T < LCST), the P(NIPAM-co-GMA) copolymer swells, and steric hindrance from the polymer near the cavities is small, guest molecules can easily enclose into the cavity, so that the associated constant is higher. However, at higher temperature (T > LCST), the P(NIPAM-co-GMA) copolymer shrinks, and the polymer chains agglomerate around the cavity, increasing the steric hindrance, which leads to a smaller binding constant. As CD content increased, the difference between adsorption amount of ANS onto P(NIPAM-co-GMA/CD)-g-PET membrane at 25 oC and that at 40 oC became larger. ANS adsorbed on the P(NIPAM-co-GMA/CD)-g-PET membrane could be washed away by water at higher temperature, and the membrane could be regenerated.In summary, a novel thermo-responsive and molecular recognizable gating membrane was designed and successfully prepared. The guest-molecule recognition and chiral-molecule discrimination capability were experimentally studied by thermo-responsive flux experiment, chiral resolution experiment and guest-molecule adsorption experiment. The results showed that guest molecules can be separated and the membrane can be regenerated simply by changing the environmental temperature.