Synthesis of soluble oligsiloxane-end-capped hyperbranched polyazomethine and their application to CO2/N2 separation membranes

ABSTRACT Three soluble hyperbranched polyazomethines containing oligosiloxane end group HBP-PAZ-SiOn were successfully synthesized. HBP-PAZ-SiOns were used as modifiers of ethyl cellulose (EC) and polysulfone (PS) membranes. Blend membranes, HBP-PAZ-SiOn/EC and HBP-PAZ-SiOn/PS were prepared by blending the THF solution of HBP-PAZ-SiOn with ethanol solution of EC and dichloromethane solution of PS, respectively. Surprisingly, the permeabilities for CO2 of the blend membranes were more than 15–16 times higher than those of pure EC and PS membranes without any drop of pemselectivity to N2. This unusual improvement has been achieved by both enhancement of diffusivity for carbon dioxide and nitrogen by the oligosiloxane groups and enhancement of affinity of the amino groups with carbon dioxide at the end groups of HBP-PAZ-SiOn.

In general, requirements for polymer materials as a practical gas permselective membrane are the following three: 1) a high gas permeability coefficient for a Gas A (P A such as P CO2 ), 2) a high gas permselectivity of Gas A to Gas B (P A /P B such as P CO2 /P N2 ), and 3) good membrane-forming ability giving an ability to yield a thin membrane. However, trade-off relationships between the permeability and permselectivity have been often observed [18]. In addition, polymers which may have high permeability and pemselectivity tend to have low membrane forming ability. To overcome these problems, more precise design of chemical structures of polymers used for permselective membranes is needed. Polyazomethines (PAZ) are known for their excellent thermal stability, good mechanical strength, environmental resistance, and optoelectronic property [19][20][21][22][23]. PAZs are usually synthesized from diamine and dialdehyde and therefore they have amine end groups. Since amine groups have strong interaction with CO 2 , they are useful for CO 2 separation [16]. However, due to the aromatic conjugated structures, the solubility of aromatic PAZ is very low. Hyperbranched polymers (HBP) have been reported, and they have high solubility compared with the corresponding linear ones. [24][25][26][27][28][29][30][31][32][33][34] Since insoluble polymers are not suitable for permselective membranes materials, we selected HBP of PAZ (= HBP-PAZ).
In this paper, in order to enhance solubility of HBP-PAZ, we introduced oligosiloxane chains with a different length by reaction with an oligosiloxane end capping reagent to the end of HBP-PAZ to give HBP-PAZ-SiO n . Their performances as CO 2 permeation membrane materials were estimated by using blend membranes of HBP-PAZ-SiO n with substrate polymers, ethyl cellulose (EC) and polysulfone (PS).

Materials
All the solvents used for monomer synthesis and polymerization were distilled as usual. Melamine and isophthalaldehyde were purchased from Aladdin Industrial Corporation. The silicon containing reagents purchased from TCI chemical Co., Inc., were used as received.

Measurements of carbon dioxide and nitrogen permeability
Carbon dioxide and nitrogen permeability coefficients (P CO2 and P N2 : cm 3 (STP)·cm·cm −2 ·s −1 ·cmHg −1 ) and the carbon dioxide separation factor (P CO2 /P N2 ) were measured by a gas chromatographic method by using YANACO GTR-11 MH according to our previous report. [30,32,35,36] The mixture of carbon dioxide and nitrogen (50/50 (v/v)) was used for the feed gas. The P CO2 and P N2 were calculated by the following equation: where Q, l, A, Δp, and t are the amount of the permeated gas, the thickness of the membrane, the permeation area of the membrane, the pressure difference across the membrane and the permeation time, respectively. Disc-type membranes were used. The A and l of the membranes were 1.77 cm 2 and around 120-310 μm, respectively. The Δp was 1 atm and the measurement temperature was 25°C.
The schematic view of the experimental setup is shown in Figure S1. A polymer film was placed in the membrane cell and exposed to vacuum to remove the gases from the polymer and membrane cell for 10 min by pull 'VAC', and then, the mixed CO 2 /N 2 test gas was feed to membrane cell through 'In'. After the retention time and wait for steady state, the permeate gas in the 'permeate storage tube' was measured by gas chromatography through the time lag. The residue gas in 'Membrane cell' was released through 'Retentate'. The amount of the permeated gas (Q) was calculated from the peaks of gas chromatogram.

2.2.2.
Other measurements 1 H NMR (600MHz) spectra were recorded on an AVANCE III spectrometer. The average molecular weights (M n and M w ) were evaluated by gel permeation chromatography (GPC) by using Polymer Laboratories (Varoam) liquid chromatography instruments (with MIXED-E, MIXED-B, MIXED-A, MZ-Gel SDplus columns, THF eluent, polystyrene calibration). The infrared spectra were recorded on Spotlight 400.

Synthesis of soluble oligosiloxane-end-capped hyperbranched polyazomethines HBP-PAZ-SIO n
Soluble oligosiloxane-end-capped polyazomethines HBP-PAZ-SiO n were synthesized according to Scheme 1. All the following reaction procedures were conducted under dry nitrogen.

Synthesis of HBP-PAZ
A typical procedure for synthesis of HBP-PAZ was as follows: A solution of melamine (A 3 ) (305mg, 2.43mmol) and isophthalaldehyde (B 2 ) (500mg, 3.73 mmol) (A3/ B2 = 0.65) in 1,3-dimethyl-2-imidazolidinone was stirred for 2h at 65℃. Then, the mixture was poured into a 100mL beaker containing 50.0mL ethyl acetate. After precipitation, the liquid was filtered. The crud solid product was purified by vacuum drying to give a white solid.
Other polymerizations of melamine and isophthalaldehyde were carried out similarly. The results are shown in Table 1.

Synthesis of HBP-PAZ-SIO n
A typical procedure for synthesis of HBP-PAZ-SiO 9 was as follows: To a mixture of HBP-PAZ (5.00mg) and THF (1.20mL), oligosiloxane end capping reagent 3 (n = 9) (175mg, 0.200mmol) was added and refluxed for 48h. The mixture was filtered, solvent was removed by evaporation. The crude product was purified by vacuum drying to give HBP-PAZ-SiO 9 as a brown viscous liquid.

Preparation of HBP-PAZ-SIO n /EC and HBP-PAZ-SIO n /PS blend membranes
HBP-PAZ-SiO n /EC and HBP-PAZ-SiO n /PS blend membranes, were fabricated as follows: a solution of HBP-PAZ-SiO n in THF (1.5 mg/mL) and a solution of the substrate EC in ethanol (30 mg/mL) (for PS, in CH 2 Cl 2 ) were blended together, and then the resulting blend solution was cast on a Teflon sheet. After evaporating the solvent for 24h at room temperature, the membrane was detached from the sheet and dried in vacuo for 24 h. And then the carbon dioxide and nitrogen permeability were measured by a gas chromatographic method by using YANACO GTR-11 MH [35,36]. The active permeation area was 1.77cm 2 and the thickness of the membranes were 120-310μm.

Synthesis of oligosiloxane end-capping reagents
Three kinds of oligosiloxane end-capping reagents SiO n (n = 9, 18 and 39) were synthesized by living polymerization in the different feed ratio of hexamethylcyclotrisiloxane/lithium trimethylsilanolate with yield of 52.1, 34.7 and 34.4%, respectively. The n of SiO n were 9, 18 and 39 which were confirmed by the integral ratio of OSi((CH 3 ) 2 ) to the terminal SiH in the 1 H NMR spectra of 2. The observed values were consistent with the theoretical calculation values.

Synthesis of hyperbranched polyazomethine
Hyperbranched polyazomethines (HPB-PAZ) were successfully synthesized by condensation polymerization of melamine (A 3 ) and isophthalaldehyde (B 2 ) with yields higher than 56.6%. The chemical structures were confirmed by FT-IR spectra ( Figure 1) because they had insoluble part. The -C = N-stretching vibration band around 1543cm −1 , the NH 2 and -HC = O stretching vibration band of end groups around 3469-3133cm −1 and 1671cm −1 were observed. It indicates the condensation polymerization between amine and aldehyde was achieved to form HPB-PAZ. Many amine and aldehyde end groups remain in HPB-PAZ.
By changing the A 3 /B 2 feed ratio from 0.50 to 0.65, the solubility and molecular weight (THF soluble part) were changed, the results are shown in Table 1. When the A 3 /B 2 feed ratio reaches to 0.65, the HPB-PAZ shows the highest solubility of 19.5% in THF and 20.9% in DMSO. The M n and M w values of the soluble part were also the highest among the four HPB-PAZs (M n = 3,500, M w = 3,600, DP≈15).

3.3.
Synthesis of soluble oligosiloxane-end-capped hyperbranched polyazomethine HBP-PAZ-SIO n Soluble oligosiloxane-end-capped hyperbranched polyazomethines (HBP-PAZ-SiO n ) were synthesized by reaction of the aldehyde end groups of HBP-PAZ with the amine group of the oligosiloxane end capping reagent SiO n (n = 9, 18, 39). The yields of the resulting HBP-PAZ-SiO n were higher than 94%. From the FT-IR spectra of HBP-PAZ-SiO 39 (Figure 1), the new Si-C and Si-O stretching vibration bands around 1263 and 1091cm −1 are found and the C = O stretching vibration band of the aldehyde end group of HBP-PAZ decreased. It indicates that the end capping reaction has been achieved. Judging from these data, an example of chemical structure is shown in Figure 2. 3.4. The CO 2 /N 2 separation of HBP-PAZ-SIO n / EC and HBP-PAZ-SIO n /PS blend membranes A 5wt% of soluble oligosiloxane-end-capped hyperbranched polyazomethine (HBP-PAZ-SiO n ) was blended with EC and PS to give a self-standing membrane. The CO 2 /N 2 permeation experiments were carried out and the results are shown in Table 2. By adding HBP-PAZ-SiO 9 , the permeability (P CO2 ) of EC and PS were enhanced and reached more than 9 times higher values without any drops of the pemselectivity (P CO2 /P N2 ) ( Table 2, Nos. 2 and 6). In addition, with increasing the length of oligosiloxane (n) of HBP-PAZ-SiO n from 9 to 39, the P CO2 increased and reached values about 15-16 times higher than those for EC and PS substrates (Table 2, Nos. 4 and 8).
The extremely high enhancements of P CO2 were caused by the enhancements of diffusivity by the introduction of the SiO n chains. Although these enhancements of P CO2 were usually reported, simultaneous large decreases in P CO2 / P N2 were usually observed. In this study, surprisingly no decreases were observed. This may be caused by the interaction of amino groups in HBP-PAZ-SiO n with CO 2 , a week acid. The interaction avoided decreasing the selectivity.

Conclusions
Three soluble oligosiloxane-end-capped hyperbranched polyazomethines HBP-PAZ-SiO n (n = 9, 18 and 39) were synthesized by reaction of aldehyde end group of HBP-PAZ with the amine group of oligosiloxane end capping reagent SiO n (n = 9, 18, 39). The permeability of EC and PS substrate membranes were enhanced more than 15 times by using the HBP-PAZ-SiO 39 as modifiers without any drop on pemselectivity. In the three HBP-PAZ-SiO n , HBP-PAZ-SiO 39 showed the highest performance due to the longer oligosiloxane chain end. This unusual improvement has been achieved by both enhancement of diffusivity for carbon dioxide and nitrogen by the oligosiloxane groups and enhancement of affinity of the amino groups with carbon dioxide at the end groups of HBP-PAZ-SiO n .

Disclosure statement
No potential conflict of interest was reported by the authors.