Polymeric Pseudo-Liquid Membranes from Polymethacrylate Derivative Bearing Oligodimethylsiloxane Unit

Novel liquid membrane system, which is named polymeric pseudo-liquid membrane was constructed from polymethacrylate derivative bearing oligodimethylsiloxane (PDMSMA), showing rubbery state under operating conditions, as a membrane matrix. In the present study, dibenzo-18-crown-6 (DB18C6), dibenzo-21-crown-7 (DB21C7) or O-allyl-N-(9-anthracenylmethyl)cinchonidinium bromide (AMCC) was adopted as a carrier for KCl transport, CsCl transport or optical resolution of racemic mixture of phenylglycine (Phegly), respectively. The results of KCl and CsCl transports revealed that the membrane transport was attained by carrier-diffusion mechanism like conventional liquid membranes. The present study led the conclusion that PDMSMA can be applicable not only to membrane transport of alkali metal ions, such as K and Cs, but also to chiral separation.


INTRODUCTION
Membrane separation has been perceived as an environmentally benign and an economically promising separation technology among separation technologies, such as distillation, crystallization and so on [1][2][3][4][5]. Membranes have been already applied to various areas, such as seawater desalination by reverse osmosis (RO), ultrapure water production by nanofiltration (NF), separation or concentration of colloidal particles and macromolecules by ultafiltration (UF), reduction of water turbidity by microfiltration (MF) and elecrtodialysis (ED) by ion-exchange membranes, removal and recovery of gases, hemofiltration, hemodiafiltration, hemodialysis and so on. So far, membranes have been applied to separation by using the difference in dimension between the target molecule and others. When the difference in dimension between target molecule and others becomes narrower and finally a given mixtures show similar or same dimensions, such as constitutional isomers or enantiomers, will not be separated well by membrane separation techniques enumerated above. In order to separate mixtures showing similar or same dimension, a given membrane has to discriminate the target molecule by recognizing the difference in shape and the alignment of functional moiety (substituent). To this end, it is necessary to introduce molecular recognition sites or molecular recognition materials into a given separation membrane. organogel membranes [23,24]; (4) stabilization of supported liquid membranes [25,26]; (5) liquid membranes based on ionic liquids [27][28][29]; and (6) polymeric pseudo-liquid membranes (PPLMs) [30][31][32][33][34][35][36][37].
Polymeric pseudo-liquid membrane (PPLM) has been studied based on the knowledge of polymer science, especially, polymer physics. PPLM is a liquid membrane consisting of polymeric material with a rubbery state and a carrier. In PPLM, a polymeric material works as a membrane matrix retaining a carrier and a durable barrier, separating source and receiving phases. From above, PPLM is a different membrane system from a supported polymeric liquid membrane [38][39][40][41]. A supported polymeric liquid membrane is a microporous hydrophobic membrane, of which pore is loaded with a polymeric (oligomeric) liquid exhibiting an affinity toward organic compounds of interest.
Even though PPLM consisted of a rubbery polymer, which showed fluidity, as a membrane matrix, diffusivities of carrier and carrier/target molecule complex in it is lower than those in a usual liquid membrane, which consisted of organic liquid, such as chloroform etc. and a carrier. It is required to construct PPLM adopting a polymeric material, of which T g is lower than those previously studied [33][34][35][36][37] so that the novel PPLM can show a higher flux value than those observed previously. To this end, in the present study, polymethacrylate derivative bearing oligodimethylsiloxane (PDMSMA), of which T g is expected to be around -120 °C, was adopted as a candidate membrane matrix for PPLM. Herein, transport of KCl by dibenzo-18-crown-6 (DB18C6), that of CsCl by dibenzo-21-crown-7 (DB21C7) and chiral separation of racemic mixture of phenylglycine (Phegly) by O-allyl-N-(9-anthracenylmethyl)cinchonidinium bromide (AAMC) were studied.

Materials
Methacrylate derivative bearing oligodimethylsiloxane (DMSMA), of which average degree of constitutional repeating unit of dimethylsiloxane was determined to be 2.5 by 1 H NMR, was kindly provided by Shin-Etsu Chemical Co., Ltd. (Tokyo, Japan). The chemical structure of DMSMA can be seen in Figure 1. 2,2'-Azobis(2-methylpropionitrile) (AIBN) [45] and toluene [46] were purified by conventional methods. Lphenylglycine (L-Phegly) were obtained from commercial sources and used without further purifications. Water purified with an ultrapure water system (Simpli Lab, Millipores S. A., Molsheim, France) was used.

Polymerization of DMSMA
Prior to polymerization, the inhibitor in DMSMA was removed by passing the monomer through an alumina column.
Conventional radical polymerization of DMSMA initiated by AIBN was carried out as follows; 8. DMSMA was also polymerized by atom transfer radical polymerization (ATRP) [47,48] so that PDMSMA with the prescribed degree of polymerization (molecular weight) could be obtained. Polymerization scheme is shown in Figure 1. ATRP of DMSMA was carried out as follows: CuCl was degassed by vacuum followed by nitrogen backfill three times. The prescribed amounts of DMSMA, EBMP and ligand (PMDETA) which had been degassed by three freezepump-thaw cycles and then back-filled with nitrogen were added by syringe and placed in a thermostated oil bath at 70 °C for 12 h. The polymerization solution was dissolved into CHCl 3 and passed over an alumina (activated neutral) column to remove the catalyst. The solution was poured into methanol at around -90 °C and then the obtained precipitate was dried in vacuo at ambient temperature for 5 days.

Characterization of PDMSMA
Gel permeation chromatography (GPC) was performed on a JASCO liquid chromatography system composed of a PU-2089 HPLC pump and an 860-CO column oven (operated at 35 °C) equipped with a JASCO 870-UV (JASCO Co., Hachioji, Japan) and a Shodex RI-101 RI detector (Showa Denko K. K., Tokyo, Japan). Polystyrene standard (Tosoh Co., Tokyo, Japan) were used for calibration and THF as eluent at a flow rate of 1.0 cm 3 min -1 .
Differential scanning calorimetry (DSC) measurements were performed with Shimadzu DSC-60 (Shimadzu Co., Kyoto, Japan). The heating rate was fixed to be 10 °C min -1 and the sample was purged with nitrogen at a flow rate of 50 cm 3 min -1 .

Preparation or Polymeric Pseudo-Liquid Membranes
Polymeric pseudo-liquid membranes were prepared as follows; 100 mg of PDMSMA and the prescribed amount of DB18C6, of which amount was 2.50 mg, 5.00 mg or 7.50 mg, were dissolved in 1.0 cm 3 of CHCl 3 . In the case that DB21C7 was used as a carrier instead of DB18C6, 100 mg of PDMSMA and 2.50 mg of DB21C7 were dissolved in 1.0 cm 3 of CHCl 3 . The polymer solution was poured into a flat-laboratory-dish (48 mm diameter), followed by immersing a PTFE membrane filter (Omnipore Membrane Filter (Merck Millipore, Bellerica, MA, USA); diameter 47mm; pore radius 0.10 µm; porosity 0.80; thickness, 80 µm) into the cast solution. Then the flat-laboratory-dish was evacuated in a desiccator so that the cast solution could thoroughly penetrate into pores in the PTFE membrane filter. The solvent was allowed to evaporate at 25 °C for 5 h and then additionally at 60 °C for 24 h.
Control membrane was prepared as follows; about 100 mg of PDMSMA was dissolved in 1.0 cm 3 of CHCl 3 . Control membrane for PPLM was constructed from the solution thus prepared as described above. PPLM for chiral separation of Phegly was prepared as described above. Instead of carrier for alkali metal ions, 2.50 mg of O-allyl-N-(9-anthracenylmethyl) cinchonidinium bromide (AAMC) was used as a carrier for chiral separation of racemic mixture of Phegly.

Transport of Alkali Metal Salts
Transport of alkali metal salt, such as KCl and CsCl, was studied using apparatus schematically shown in Figure 2. The PTFE membrane filter impregnated with membrane components, such as PDMSMA and carrier or just PDMSMA, was secured tightly with Parafilm between two chambers of permeation cell. The thickness of the PTFE filter, 80 µm, was adopted as membrane thickness for the present study. In the present study, the membrane area for PTFE membrane filter was 3.0 cm 2 ; the effective membrane area was determined to be 2. Ltd., Kyoto, Japan) and CO-1305 oscilloscope (Kenwood Co., Hachioji, Tokyo), of which schematic diagram is shown in Figure 2.

Transport of Racemic Mixture of Phegly
Aqueous solution of racemic Phegly was placed in the left-hand side chamber (L-ide) and aqueous solution in the right-hand side chamber (R-side). Each concentration of racemic Phegly was fixed to be 1.0 x 10 -6 mol cm -3 . Transport experiments were carried out at 40 °C (313 K). The pH condition of the source phase (L-side) was kept to be 11 by Na 2 HPO 4 /NaOH and that of receiving phase was maintained at pH 3 by H 3 PO 4 /NaH 2 PO 5 .  (1)

Characterization of PDMSMA
Conditions and results of polymerization are summarized in Table 1. As expected from the fact that the polymerization initiated by EBMP was controlled/living radical polymerization [47,48], polydispersity indices for PDMSMA prepared by ATRP were narrower than that initiated by AIBN.
The glass transition temperatures (T g 's) for those polymers synthesized in the present study were determined to be -121.4 °C for PDMSMA-13, -120.8 °C for PDMSMA-27 and -121.1 °C for PDMSMA-168, respectively. In the case of poly(2-ethylhexyl acrylate) (P2EHA) [34], the glass transition temperature was dependent on its molecular weight and was lowered with the decrease in the number average molecular weight. Contrary to this, such molecular weight dependence of T g was hardly observed in the present study. DSC results revealed that three types of PDMSMA obtained in the present study showed rubbery state at the operating temperature of membrane transport studies (40 °C, 50 °C and 60 °C).

Transport of KCl through the PDMSMA Polymeric Pseudo-Liquid Membranes
KCl transport adopting dibenzo-18-crown-6 (DB18C6) as a potassium ion carrier [49] was studied so that the results obtained in the present study could be compared with results previously reported [33][34][35][36][37].   through the control membrane. In other words, KCl was transported through the PDMSMA liquid membrane by simple diffusion, which composed of the diffusion of free ion and uncomplexed ion pairs. In addition to the above results, the slope for the time-transport curves of K + transport was increased with the rise in the operating temperature.
When the membrane transport of uni-univalent salt, such as KCl and so on, is transported through a given liquid membrane simultaneously by simple diffusion and facilitated transport, the flux of uni-univalent salt can be represented by eq. (2) [50,51].
In Figure 4, the product J C,obs and membrane thickness δ, J C (mol cm cm -2 h -1 ), which is a flux per unit membrane thickness, per unit membrane area and per unit time is plotted as a function of carrier concentration. The relationships for the operating temperature of 40 °C, 50 °C and 60 °C are shown together in Figure 4. The flux J C gives a straight line, passing though that for each control experiment. The relationship shown in Figure 4 can be explained by eq.
(2). The relationship shown in Figure 4 revealed that the membrane transport was carried out by carrierdiffusion mechanism [50,51] not by fixed-site jumping one [17,52,53]. In other words, the present membrane matrix of PDMSMA was fluid enough so that carrier and carrier/substrate complex were able to freely diffuse within the membrane. As a result, membrane transport of K + was carried out like usual supported liquid membranes [50,51]. Following the K + flux, which is represented by eq.
(2), K + flux should be proportional to the square of the initial feed concentration of K + . To this end, the relationship between K + flux and the initial feed K + concentration through PDMSMA-168/DB18C6 liquid membrane at 50 °C was studied. In Figure 5, the K + flux facilitated by carrier and that of simple diffusion are plotted as a function of initial K + feed concentration. As expected, the plots of logarithm of both K + fluxes against the logarithm of the initial KCl concentration gave a slope of two. In the present study, three types of membrane matrix with different molecular weight were synthesized. Transport of KCl through PDMSMA-13/DB18C6 and PDMSMA-27/DB18C6 PPLMs at three different operating temperatures, such as 40 °C, 50 °C and 60 °C, were also studied. The activation energy of membrane transport can be determined from fluxes thus obtained. As shown in Figure 6, Arrhenius plot of K + flux vs. reciprocal of absolute temperature yields activation energy of K + transport. Figure 6 revealed that the flux of K + was increased with the decrease in the molecular weight of membrane matrix PDMSMA and increased with the rise in the operating temperature. From the slope of the straight line, the activation energy of K + transport for each membrane was determined. The determined activation energies are summarized in Table 2. In the table, number average molecular weights and glass transition temperatures for those membrane matrices are also given for convenience. Three types of membrane matrix PDMSMA studied in the present study showed rubbery state under the membrane transport conditions. The fluidity of membrane matrix was expected to increase with the decrease in molecular weight of PDMSMA. This will lead to enhancement in K + flux and to decrease in activation energy of diffusion within the membrane. As anticipated, the activation energy of membrane transport was decreased with the decrease in molecular weight. Figure 7 summarizes relationship between the activation energy of K + transport and the corresponding glass transition temperature. The numeral in the parenthesis shows the number average molecular weight. Unfortunately, molecular weights of membrane matrix are scattered and it is difficult to discuss the relationship all together. As mentioned above, among the polymeric pseud-liquid membranes from PDMSMA, the activation energy of K + transport was decreased with the decrease in molecular weight. Comparing the activation energy for PDMSMA-168 and PC18MA ( ), of which molecular weights were similar; PDMSMA-168 with lower glass transition temperature gave lower activation energy. Similar tendency can be found among PDMSMA-13, PDMSMA-27 and PC12MA (), and between P2EHA and P2EHMA ( ). From Figure   7, a membrane matrix with lower glass transition temperature and lower molecular weight will give lower activation energy of K + transport, which is not to mention. In Table 3, the fluxes studied in the present study are summarized together with previous results for polymeric pseudo-liquid membranes from P2EHMA [33], P2EHA [34], PC12MA [35], PC18MA [36], PC18AAm [37], supported liquid membrane [54] and polymer inclusion membrane [16] so that the present studies can be compared with those results. In the case of polymer inclusion membrane [16], dicyclohexyl-18-crown-6 (DC18C6) was used as a carrier instead of DB18C6. The membrane performances for those membranes are given as normalized fluxes, which are fluxes per unit membrane area, per unit membrane thickness, per unit carrier concentration and per square of unit substrate concentration.
As expected, polymeric pseudo liquid membranes consisting of PDMSMA showed higher normalized K + fluxes than a usual liquid membrane and a polymer inclusion membrane. However, against the anticipation from glass transition temperature of membrane matrix, the newly obtained membrane matrix, PDMSMA, did not give higher normalized K + fluxes comparing with previous results. So far the polymeric pseudo-liquid membrane consisting of poly(dodecyl methacrylate), PC12MA, and DB18C6 gave the highest membrane performance. The membrane performance of polymeric pseudo-liquid membrane might be governed by various factors, such as glass transition temperature of membrane matrix, molecular weight, partition coefficient of the solute between water and the membrane matrix, equilibrium constant for the association between substrate and carrier, concentration of carrier, diffusion coefficient of the complexed solute and so on. Due to factors enumerated above, the present membrane did not show better membrane performance.

Transport of CsCl through the PDMSMA Polymeric Pseudo-Liquid Membranes
The performance of polymeric pseudo-liquid membrane is exclusively dependent on the nature of carrier like usual liquid membranes. From this, dibenzo-21-crow-7 (DB21C7) [55,56]  Cs 2 years and so on [57,58]. Figure 8 shows time-transport curves of CsCl through PDMSMA-168/DB21C7 membranes and the corresponding control membrane at 60 °C. The flux dependence on carrier concentration is shown in Figure 9. Figure 9 revealed that CsCl was also transported through the membrane by a mobile carrier mechanism as KCl was transported through the PDMSMA/DB18C6 polymeric pseudo-liquid membrane.

Chiral Separation of Racemic Mixture of Phenyglycine (Phegly)
The results obtained in the previous section suggested that polymeric pseudo-liquid membrane consisting of PDMSMA and carrier with chiral environment was expected to show enantioselective transport ability. So far polymeric pseudo-liquid membranes from PC12MA [35], PC18MA [36] and PC18AAm [37] showed not only transport ability of alkali metal ions but also chiral separation ability adopting O-allyl-N-(9-anthracenylmethyl)cinchonidinium bromide (AAMC) as a chiral carrier. Cinchona alkaloids have been used as resolving agents for chiral binaphthols [59], chiral acids [60], amino acid derivatives [60,61] and oligopeptides [62]. Cinchona alkaloid was also used as a carrier for chiral separation with membranes [63]. From those studies, AAMC was adopted as a carrier for optical resolution. A racemic mixture of phenylglycine (Phegly) was adopted as a model racemate. Time-transport curves of racemic mixture of Phegly through PDMSMA-168/AAMC polymeric pseudo-liquid membrane at 40 °C is shown in Figure 10. L-Phegly was transported through the membrane in preference to the antipode as reported [35][36][37]63]. At the operating temperature of 40 °C, the permselectivity toward the L-isomer was determined to be 1.27.

CONCLUSIONS
Novel liquid membrane system, which is named polymeric pseudo-liquid membrane, was constructed from polymethacrylate derivative bearing oligodimethylsiloxane (PDMS), of which glass transition temperature was determined to be around -121 °C, as a membrane matrix. In the present study, dibenzo-18crown-6 (DB18C6), dibenzo-21-crown-7 (DB21C7) or O-allyl-N-(9-anthracenylmethyl)cinchonidinium bromide (AMCC) was adopted as a carrier for KCl transport, CsCl transport or optical resolution of racemic mixture of phenylglycine (Phegly), respectively. The results of KCl and CsCl transports revealed that the membrane  transport was attained by carrier-diffusion mechanism like usual liquid membranes. L-Phegly was selectively transported from racemic mixture of Phegly and the permselectivity toward the L-enantiomer of 1.27 was observed. The present study led the conclusion that PDMSMA can be applicable not only to membrane transport of alkali metal ions, such as K + and Cs + , but also to chiral separation.