Desilylation of copolymer membranes composed of poly[1-(p-trimethylsilyl)phenyl-2-(p-trimethylsilyl)phenylacetylene] for improved gas permeability.

Efficient gas-separation systems comprising gas-permeable membranes are important for energy conservation in various industrial applications. Herein, high-molecular-weight copolymers (2ab and 2ac) were synthesized in good yields by the copolymerization of 1-(p-trimethylsilyl)phenyl-2-(p-trimethylsilyl)phenylacetylene (1a) with 1-phenyl-2-(p-tert-butyl)phenylacetylene (1b) and 1-phenyl-2-(p-trimethylsilyl)phenylacetylene (1c) in various monomer feed ratios using TaCl5-n-Bu4Sn. Tough membranes were obtained by solution casting. The copolymers exhibited very high gas permeabilities (P O2 : 1700-3400 barrers). Desilylation of 2ac membranes decreased the gas permeability, but desilylation of 2ab membranes resulted in a significant increase in the gas permeability. The highest oxygen permeability coefficient obtained was 9300 barrers, which was comparable to that of poly(1-trimethylsilyl-1-propyne), a polymer known to have the highest gas permeability. This journal is © The Royal Society of Chemistry.

Introduction

Efficient gas-separation systems have been extensively investigated for various industrial applications. Gas-permeable polymer membranes have attracted significant attention as they can simplify the systems and allow energy conservation.[1,2] In recent decades, substituted polyacetylenes have been intensively examined for their practical application as gas-permeable materials.[3-6] Polyacetylenes with bulky spherical substituents show large excess free volume and high gas permeabilities because of the presence of rigid main chains comprising alternating double bonds and the steric repulsion of the bulky substituents that inhibits an effective polymer chain packing.[7,8] For instance, poly(1-trimethylsilyl-1-propyne) (PTMSP) exhibits the highest gas permeability, where the oxygen permeability coefficient (PO) is greater than 9000 barrers.[3] However, aliphatic polyacetylenes such as PTMSP are degraded by heat and oxidation.

In contrast, diarylacetylene polymers are tolerant toward pyrolysis and oxidation. Poly[1-phenyl-2-(p-trimethylsilyl)phenylacetylene] (PTMSDPA) and poly[1-phenyl-2-(p-tert-butyl)phenylacetylene] (PTBDPA) are the examples of diarylacetylene polymers, and show high PO values of up to 1000 barrers.[9-12] Therefore, these polymers are promising materials for gas separation membranes. Hu et al. synthesized polymethylated indan-containing poly(diphenylacetylene), which is the most gas-permeable polymer ever recorded to date.[13,14] Fukui et al. incorporated fluorenyl groups into poly(diarylacetylene) to improve gas permeability,[15] and the highest PO value among fluorenyl-containing poly(diarylacetylene)s is 9800 barrers.[16] The incorporation of naphthyl,[17] SiMe3,[5,6,18]tert-butyl,[18,19] halogen[20-22] into diarylacetylene is also often considered as a way to obtain high gas-permeable materials, but their gas permeability still lower than PTMSP.

In this study, the copolymerization of 1a with a tert-butyl-containing monomer (1b) and trimethylsilyl-containing monomer (1c) in various feed ratios was accomplished (Scheme 1). Free-standing membranes were fabricated from the resultant copolymers, and the desilylation of the membranes was performed using a mixture of trifluoroacetic acid/hexane. Gas permeabilities of the copolymer membranes were studied.

Scheme 1

Synthetic schemes for copolymers and desilylated copolymers.

Our research group previously reported that the copolymers of TMSDPA and TBDPA showed higher gas permeabilities than those of the homopolymers, PTMSDPA and PTBDPA.[18] Furthermore, the desilylation of the copolymer membranes enhanced the gas permeability. This suggested that some micro-scale voids were generated by the elimination of silyl groups. Therefore, the desilylation of the copolymers of 1-(p-trimethylsilyl)phenyl-2-(p-trimethylsilyl)phenylacetylene (1a), with two trimethylsilyl groups, is expected to further improve the gas permeability.

Experimental

Materials

The polymerization solvent, toluene (Fujifilm Wako Pure Chemical Co., Ltd., Osaka, Japan), was purified twice by distillation in the presence of CaH2 (Fujifilm Wako Pure Chemical Co., Ltd.). The main catalyst, TaCl5 (99.999%, Sigma-Aldrich, St. Louis, Missouri, USA), was used without further purification, while the cocatalyst, n-Bu4Sn (Fujifilm Wako Pure Chemical Co., Ltd.), was used after distillation in the presence of CaH2. Phenylacetylene (Sigma-Aldrich), 1,4-dibromobenzene, 1-bromo-4-tert-butylbenzene, triethylamine, triphenylphosphine, copper(i) iodide, dichlorobis(triphenylphosphine)palladium(ii), n-butyllithium, trifluoroacetic acid (TFA), chlorotrimethylsilane, 2-methyl-3-butyn-2-ol, sodium hydride, aqueous sodium nitrate, and other common solvents (Fujifilm Wako Pure Chemical Co., Ltd.) were used without further purification. 1-(p-Trimethylsilyl)phenyl-2-(p-trimethylsilyl)phenylacetylene (1a),[23] 1-phenyl-2-(p-tert-butyl)phenylacetylene (1b),[11] and 1-phenyl-2-(p-trimethylsilyl)phenylacetylene (1c)[10] were synthesized according to the methods reported in literature.

Measurements

The molecular weight distribution (MWD) of the copolymers was measured by gel permeation chromatography (GPC) in CHCl3 (at a 1.0 mL min−1 flow rate) at 40 °C on a Shimadzu LC-10AD chromatography system (Shimadzu Corp., Kyoto, Japan) equipped with three polystyrene gel columns (Shodex K-804L, K-805L, and K-807L; Shodex, Tokyo, Japan) and a Shimadzu RID-6A refractive index detector (Shimadzu Corp.). The weight-average molecular weight (Mw) and polydispersity ratio were calculated from the chromatograms based on the polystyrene calibration. 1H (500 MHz) and 13C (125 MHz) NMR spectra were recorded on the Jeol ECX-500 instrument (JEOL Ltd., Tokyo, Japan) in CDCl3 at room temperature. IR spectra were recorded on a Nicolet IS 5 spectrometer (Thermo Fisher Scientific, Waltham, Massachusetts, USA). The thickness of membrane was measured using a micrometer, and an average of ten points on each membrane was considered.

Gas permeability was measured using a Rikaseiki K-315-N gas permeability apparatus equipped with an MKS Baratron detector (MKS Instruments, Inc., Andover, Massachusetts, USA) at 25 °C. The downstream side of the membrane was evacuated at 0.2–1.0 Pa, while the upstream side was filled with a gas at approximately 1 atm (105 Pa). The increase in pressure in a downstream receiving vessel was measured. The permeability coefficient P expressed in barrers [1 barrer = 1 × 10−10 cm3 (STP) × cm/(cm2 × sec × cmHg)] was calculated from the slopes of the time–pressure curves in the steady state where Fick's law was obeyed. The diffusion coefficient (D) was determined by the time lag method using the following equation:where, l is the membrane thickness and θ is the time lag, which is given by the intercept of the asymptotic line of the time–pressure curve to the time axis. The solubility coefficient (S) was calculated using the equation, S= P/D.

The density of membrane was determined by hydrostatic weighing using a Mettler Toledo balance (Mettler Toledo, Columbus, Ohio, USA) and a density determination kit. In this method, a liquid with a known density (ρ0) is required, and the membrane density (ρ) is obtained by the following equation:where, MA is the weight of the membrane in air and ML is the weight of the membrane in an auxiliary liquid. Aqueous sodium nitrate was used as the auxiliary liquid.

Copolymerization

Copolymerization was carried out in a Schlenk tube equipped with a three-way stopcock at 80 °C for 24 h under dry nitrogen with the following reagent concentrations: [1a] + [1b or 1c] = 0.20 M, [TaCl5] = 20 mM, and [n-Bu4Sn] = 80 mM. A detailed procedure for copolymerization is described as follows. The monomer solution was prepared in a Schlenk tube by mixing the monomers and dry toluene. An additional Schlenk tube was charged with TaCl5, n-Bu4Sn, and dry toluene; this catalyst solution was aged at 80 °C for 10 min. Then, the monomer solution was added to the catalyst solution. After 24 h, copolymerization was quenched with a small amount of methanol. The resulting copolymer was isolated by precipitation into excess methanol. The polymer yield was determined gravimetrically.

Membrane fabrication and desilylation

The membranes (thickness: ca. 30–130 μm) of the copolymers were prepared by casting from toluene solution of the copolymers (0.15–0.50 wt%) into the Petri dishes. The dish was covered with a glass vessel to decrease the evaporation rate of the solvent (3–5 d). After the formation of the membrane, it was immersed in methanol for 24 h, followed by drying for 24 h at 25 °C. The desilylation of membranes was performed using the method described in literature.[24] The detailed procedure is described as follows. The membrane was immersed in a mixture of trifluoroacetic acid and hexane (volume ratio 1 : 1) at room temperature for 24 h. To remove the residual impurities in the polymer matrix, the membrane was immersed in acetone followed by methanol at room temperature for 24 h. Thereafter, it was dried under atmospheric pressure at room temperature for 24 h. The completion of the reaction was confirmed by comparing the IR spectra of the copolymers before and after desilylation.

Results and discussion

It is well known that TaCl5/n-Bu4Sn is the best catalyst for the metathesis polymerization of diphenylacetylene derivatives to afford high-molecular-weight polymers.[25-27] Therefore, in this study, the copolymerization of 1a with 1b and 1c was performed using TaCl5/n-Bu4Sn catalyst at 80 °C in toluene for 24 h under nitrogen. The results of copolymerization were summarized in Table 1.

Copolymerization of 1a with 1b and 1c by TaCl5–n-Bu4Sna

	Feed ratio	Copolymerb
	(1a : 1b or 1c)	Yield (%)	M w c (g mol−1)	M w/Mnc
1b	1 : 4	91	902 000	4.40
	1 : 2	84	1 260 000	4.05
	1 : 1	81	546 000	4.55
	2 : 1	73	820 000	4.03
	4 : 1	71	Insoluble
1c	1 : 4	87	1 770 000	3.57
	1 : 2	81	2 540 000	2.13
	1 : 1	88	1 350 000	2.11
	2 : 1	90	872 000	2.62
	4 : 1	80	Insoluble

In toluene at 80 °C for 24 h; [1a] + [1b or 1c] = 0.20 M, [TaCl5] = 20 mM, [n-Bu4Sn] = 40 mM.

Methanol-insoluble product.

Measured by GPC (CHCl3).

The copolymerization of 1a with 1b in the feed ratio of 1 : 4 afforded the copolymer, 2ab(1:4), whose Mw was up to 902 000. The copolymerizations of 1a with 1b in the feed ratios of 1 : 2, 1 : 1, 2 : 1, and 4 : 1 resulted in the production of copolymers, 2ab(1:2), 2ab(1:1), 2ab(2:1), and 2ab(4:1), respectively, in good yields. The molecular weights of 2ab(1:2), 2ab(1:1), and 2ab(2:1) were very high, but that of 2ab(4:1) could not be measured by GPC because 2ab(4:1) was insoluble in all solvents. The copolymerizations of 1a with 1c in the feed ratios of 1 : 4, 1 : 2, 1 : 1, and 2 : 1 under the same conditions afforded the copolymers [2ac(1:4), 2ac(1:2), 2ac(1:1), and 2ac(2:1), respectively] in high yields with high molecular weights (Mw = 872 000–2 540 000). Similar to 2ab(4:1), the copolymer 2ac(4:1) was obtained in good yield, but the solubility was poor.

Fabrication of copolymer membranes and their solubility

Tough free-standing membranes of 2ab and 2ac were fabricated by casting the copolymers from their toluene solutions, except 2ab(4:1) and 2ac(4:1), because of their insolubility. The desilylation was carried out using trifluoroacetic acid and hexane (volume ratio 1 : 1) to afford the desilylated membranes (3ab and 3ac).

To verify the completion of the desilylation of copolymer membranes, IR measurements were carried out. The IR spectra of the membranes of 2ab(1:2), 2ac(1:2), and the desilylated analogues, 3ab(1:2) and 3ac(1:2), were shown in Fig. 1. The spectra of 2ab(1:2) and 2ac(1:2) showed the peaks at 1250 cm−1, assigned to the stretching of SiC–H bond, and 1120 cm−1 corresponding to the vibration of Si–C bond. No absorptions at 1250 cm−1 and 1120 cm−1 were observed in the spectra after desilylation (Fig. 1, 3ab and 3ac). The results suggest the completion of the desilylation reaction.

Fig. 1

IR spectra of the copolymers of 2ab(1:2), 3ab(1:2), 2ac(1:2), and 3ac(1:2).

The solubilities of the copolymers were summarized in Table 2. Copolymers 2ab and 2ac showed the same solubility, and completely dissolved in relatively low polarity solvents such as toluene, chloroform, and tetrahydrofuran. However, 2ab(4:1) and 2ac(4:1) were insoluble in all solvents. Homopolymer of 1a is known to be insoluble in all solvents.[23] Copolymers 2ab(4:1) and 2ac(4:1) mainly consist of the 1a component and therefore showed a similar solubility behavior as that of the homopolymer of 1a. Similar to 2ab, the desilylated polymer 3ab was soluble in low polarity solvents. However, 3ac was not soluble in any solvent. This variation in solubility before and after desilylation should come from the substituent of tert-butyl groups. Table 2 showed the substitution ratios for the phenyl groups. When the ratio was zero or more than 90%, copolymers were insoluble. Therefore, an appropriate ratio of substituents is necessary for the solubility of poly(diphenylacetylene).

Solubility of copolymers and the desilylated analoguesa

	Toluene	CHCl3	THF	DMF	DMSO	Methanol	Substitution ratiob [%]
2ab(1:4)	+	+	+	−	−	−	60
2ab(1:2)	+	+	+	−	−	−	67
2ab(1:1)	+	+	+	−	−	−	75
2ab(2:1)	+	+	+	−	−	−	83
2ab(4:1)	−	−	−	−	−	−	90
2ac(1:4)	+	+	+	−	−	−	60
2ac(1:2)	+	+	+	−	−	−	67
2ac(1:1)	+	+	+	−	−	−	75
2ac(2:1)	+	+	+	−	−	−	83
2ac(4:1)	−	−	−	−	−	−	90
3ab(1:4)	+	+	+	−	−	−	40
3ab(1:2)	+	+	+	−	−	−	33
3ab(1:1)	±	+	+	−	−	−	25
3ab(2:1)	−	±	±	−	−	−	17
3ac(1:4)	−	−	−	−	−	−	0
3ac(1:2)	−	−	−	−	−	−	0
3ac(1:1)	−	−	−	−	−	−	0
3ac(2:1)	−	−	−	−	−	−	0

Symbols: (+) soluble, (±) partly soluble, (−) insoluble.

Ratio of the substituent (tert-butyl or SiMe3) for the phenyl groups on polymer side chain.

Gas permeability and density of copolymer membranes

The permeability coefficients (P) of the membranes of 2ab, 2ac, 3ab, and 3ac toward various gases were measured at 25 °C after conditioning the membranes by immersing in methanol for 24 h and drying to constant weight at room temperature for an additional 24 h (Table 3). The density of the copolymer membranes are shown in Table 3.

Gas permeation properties and density of the membranes of 2ab, 2ac, 3ab, and 3ac

Membrane	Ratio	P O2 a	P CO2 a	P N2 a	P O2/PN2	P CO2/PN2	Densityb
2ab	1 : 4	2100	7830	1100	1.92	7.18	0.891
	1 : 2	2900	10 000	1550	1.87	6.44	0.854
	1 : 1	2200	8080	1130	1.94	7.13	0.884
	2 : 1	1700	6900	740	2.30	9.31	0.902
2ac	1 : 4	1800	6940	870	2.07	7.98	0.900
	1 : 2	2700	9280	1470	1.83	6.31	0.866
	1 : 1	3000	10 200	1610	1.86	6.35	0.858
	2 : 1	3400	10 300	2060	1.65	5.00	0.787
3ab	1 : 4	6200	13 000	4000	1.55	3.25	0.837
	1 : 2	9300	18 670	7500	1.24	2.49	0.757
	1 : 1	6600	13 800	4800	1.38	2.88	0.812
	2 : 1	4400	10 940	2990	1.47	3.66	0.848
3ac	1 : 4	980	4000	380	2.54	10.39	0.984
	1 : 2	990	4100	390	2.54	10.51	0.972
	1 : 1	1070	4300	480	2.24	8.98	0.957
	2 : 1	1080	4500	490	2.21	9.20	0.951
Poly(TMSDPA-co-TBDPA)[18]	1 : 1	2300	8300	1300	1.8	6.4
DSpoly(TMSDPA-co-TBDPA)[18]	1 : 1	2700	8600	1600	1.7	5.4
PTMSDPA[12]		1200	4900	560	2.1	8.7	0.91
PTBDPA[11]		1100	4800	500	2.2	9.6
PTMSP[12]		9700	34 000	6300	1.5	5.4	0.75
Indan-polyacetylenes[13,14]		18 700	44 200	16 600	1.13	2.66
Naphthyl-polyacetylenes[17]		4300	13 000	2700	1.6	4.8
Flurenyl-polyacetylenes[15,16]		9800	24 000	8100	1.2	3.0
Halogen-polyacetylenes[20–22]		5400	19 000	3700	1.5	5.1	1.01
PIM-1 (ref. 28)		2270	13 600	823	2.8	16.5	1.09

Units: 1 × 10−10 cm3 (STP) × cm/(cm2 × sec × cmHg) (=1 barrer).

Determined by hydrostatic weighing; units: g cm−3.

High gas permeabilities were observed for the membranes of 2ab and 2ac. The PO values of 2ab were in the range of 1700–2900 barrers, which were higher than that of PTBDPA (PO: 1100 barrers). Copolymer 2ab(1:2) exhibited the highest gas permeability and its PO value was 2900 barrers, which was 2.6 times as that of PTBDPA. Similarly, 2ac showed a higher gas permeability than that of PTMSDPA. The PO values of 2ac were in the range of 1800–3400 barrers. The higher gas permeability of copolymers may be due to their asymmetrical structure of the repeating unit that generates micro-scale voids. It is obvious that incorporation of trimethylsilyl group or tert-butyl group into poly(diphenylacetylene)s in the way of copolymerization is effective to enhance the gas permeability.

The CO2 and N2 permeabilities of 2ab and 2ac were higher than those of PTBDPA and PTMSDPA, as well as the corresponding oxygen permeabilities. This suggests that the membranes of 2ab and 2ac have large free volumes. Accordingly, the membranes of 2ab and 2ac showed low densities of 0.787–0.902 g cm−3.

The desilylated membranes of 3ab exhibited significantly higher oxygen permeabilities than those of 2ab. The membrane density reduced upon desilylation. This indicates that the spaces occupied by the silyl groups were retained as micro-scale voids after desilylation due to the steric hindrance of tert-butyl groups. It is suggesting that the incorporation of tert-butyl groups prevents the chain packing effectively to enhance the gas permeability. Notably, 3ab(1:2) exhibited extremely high permeability and its PO value reached up to 9300 barrers, which was the highest value observed among all synthetic polymers. Contrary to 3ab, the desilylated membranes of 3ac showed lower gas permeabilities than those of the membranes of 2ac. The decrement of gas permeability through desilylation was due to the elimination of spherical bulky trimethylsilyl groups that caused the membrane to become dense. The PO values of 3ac were in the range of 980–1080 barrers. These were similar to the PO value of 900 barrers for poly(1-phenyl-2-phenylacetylene), which was formed by the desilylation of PTMSDPA.[18] The densities of the membranes increased after desilylation. These results indicate that tert-butyl groups play an important role in the generation of micro-voids through membrane desilylation.

Interestingly, the PO values of 3ab were significantly higher than those of desilylated poly(TMSDPA–TBDPA) reported in our previous work,[18] irrespective of the similarity in chemical structures (Fig. 2). For example, the PO of 3ab(1:1) was 6600 barrers, but the PO of desilylated poly(TMSDPA–TBDPA) (1 : 1) was 2700 barrers. The gas permeability increased as the amount of eliminated silyl group increased, when the desilylated polymer had tert-butyl groups.

Fig. 2

Chemical structures of 2ab, poly(TMSDPA–TBDPA), and their desilylated polymers.

The oxygen/nitrogen separation factors (PO/PN) for all the membranes before and after desilylation were in the range of 1.2–2.5, and the values tended to increase with a decrease in the PO value. These findings are consistent with the general observation that the highly gas-permeable polymers typically exhibit low gas separation abilities.

Table 3 showed the data of several gas-permeable polyacetylenes and PIM-1 [a typical polymer in polymers of intrinsic microporosity (PIMs)].[28] PTMSP[3] has been known as the most gas-permeable polymer, whose PO value is as high as 9700 barrers. Indan-based polyacetylenes[13,14] show the highest gas permeability among all the existing polymers. The other polyacetylenes containing various aromatic rings also show high gas permeability. PIM-1 (ref. 28) is known to show high gas permeability (PO = 2270 barrers). Compared to these highly gas-permeable polymers, the desilylated membranes 3ab(1:2) comparably showed ultrahigh permeability.

Diffusivity and solubility coefficients

To investigate the changes in gas permeability through desilylation in more detail, gas diffusion coefficients (D) were measured by the time lag method and the gas solubility coefficients (S) were calculated using the P and D values. Table 4 included the D and S values of 2ab, 2ac, 3ab, and 3ac.

Gas diffusion coefficients (D)a and solubility coefficients (S)b of 2ab, 2ac, 3ab, and 3ac

Membrane	Ratio	D O2 × 107	D CO2 × 107	D N2 × 107	S O2 × 103	S CO2 × 103	S N2 × 103
2ab	1 : 4	130	160	110	16.3	48	9.9
	1 : 2	160	163	135	18.1	59	11.6
	1 : 1	135	160	118	16.1	48	9.6
	2 : 1	105	120	70	15.7	54	10.0
2ac	1 : 4	83	90	68	20.9	75	12.9
	1 : 2	155	130	105	17.1	67	14.3
	1 : 1	165	140	115	17.9	73	14.3
	2 : 1	180	160	130	19.5	65	16.0
3ab	1 : 4	135	180	120	43.9	80	33.2
	1 : 2	340	355	295	27.3	60	25.2
	1 : 1	170	180	140	35.3	81	33.2
	2 : 1	110	150	91	39.5	77	33.0
3ac	1 : 4	78	68	63	12.5	59	7.0
	1 : 2	80	68	64	12.0	58	7.1
	1 : 1	87	70	65	12.3	60	7.3
	2 : 1	99	76	70	10.9	58	7.0

Determined by the “time lag” method at 25 °C; units: cm2 s−1.

Calculated using equation, S = P/D; units: cm3 (STP)/(cm3 × cmHg).

The membranes of 3ab showed higher gas diffusivity and gas solubility values than those of 2ab. The increase in diffusivity and solubility was due to the increase in the free volume of the membrane. Contrary to 3ab, the membranes of 3ac exhibited lower gas diffusivities and solubilities than those of 2ac. These behaviors were observed for all gases. If the membrane has an affinity to a specific gas, the variation in diffusivity and solubility for a specific gas is different from those for the other gases. Therefore, the changes in gas permeability through desilylation are probably due to the change in free volume rather than the affinity to a specific gas.

Conclusions

Two types of diphenylacetylene copolymers (2ab and 2ac) were synthesized in various monomer feed ratios by metathesis copolymerization. The copolymers, 2ab and 2ac, exhibited high gas permeabilities and the PO values were in the range of 1700–3400 barrers. The desilylation of 2ab led to an extremely high gas permeability. Particularly, 3ab(1:2) showed the highest gas permeability in comparison to other copolymers, and a PO value of 9300 barrers was obtained. In contrast, the desilylation of 2ac resulted in a decrease in gas permeability. The desilylation generated free volume in the membranes with poly(diphenylacetylene) possessing tert-butyl groups.

Conflicts of interest

There are no conflicts to declare.