Introduction

Gas separation using polymeric membranes has markedly attracted attention during the past several decades [1,2,3]. Compared to conventional separation processes such as pressure-swing adsorption, chemical absorption, and cryogenic distillation, membrane-based gas separation is advantageous because of its low capital and operating costs, high energy efficiency, and ease of operation [4,5,6]. In this regard, a large number of polymeric membranes have been developed to achieve high gas permeation and separation abilities. Polyimides (PIs) [7,8,9,10]. and polybenzoxazoles (PBOs) [11,12,13]. classified as rigid-rod aromatic polymers are promising materials for high-performance gas separation membranes because of their high gas permselectivities and high thermal and mechanical properties. Recently, Pinnau and coworkers have shown that intrinsically microporous, triptycene-based PIs with a large fractional free volume and a well-defined free volume architecture have great potential as high-performance gas separation membranes [14,15,16]. On the other hand, it has been reported that thermally rearranged PBOs, named TR-PBOs, show extraordinarily high gas permselectivities owing to their high fractional free volume and the intrinsic size and distribution of their free volume holes [6,17,18,19,20,21].

Another approach, hybridization with inorganic compounds, has also been emphasized as a method for modifying PIs and PBOs to improve their gas transport properties [4‚22,23,24]. In our previous study, it has been found PI–silica hybrid membranes prepared via a sol–gel reaction with several alkoxysilane species possess controlled gas permselectivity, suggesting that free volume holes created around the polymer/silica interfacial area can be tuned to achieve a desired gas permselectivity [25]. Additionally, PBO–silica hybrid membranes prepared with tetraethoxysilane (TEOS) as the selected alkoxysilane have shown improved CO2/CH4 selectivity [26]. To date, however, there are no experimental reports on the gas transport properties of PBO–silica hybrid membranes prepared with different kinds of alkoxysilanes.

In the present work, PBO–silica hybrid membranes were prepared with different alkoxysilane species via a sol–gel process, and their gas transport properties were investigated. During membrane preparation, several thermal treatment temperatures were examined. It has been revealed that increasing the treatment temperature results in PBOs with improved gas permeability owing to the increased inter-chain distance and fractional free volume [11, 27]. With the simultaneous application of hybridization and thermal treatment techniques, the preparation of PBO-based high-performance gas separation membranes with attractive gas permeability and/or selectivity would be expected.

Experimental procedures

Materials

2,2-Bis(3-amino-4-hydroxyphenyl)hexafluoropropane (6FAHP), 4,4’-oxybis(benzoic acid chloride) (OBC), TEOS, and phenyltrimethoxysilane (PhTMS) were purchased from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan). N,O-Bis(trimethylsilyl)acetamide (BSA), an amide-type silylation agent, was obtained from Sigma-Aldrich Japan (Tokyo, Japan). 3-(Triethoxysilyl)propylsuccinic anhydride (TEOSPSA), a coupling agent, and N,N-dimethylacetamide (DMAc), a solvent, were supplied by AZmax Co. (Tokyo, Japan) and Nacalai Tesque, Inc. (Kyoto, Japan), respectively. These regents and solvent were used as received. Chemical structures of the monomers are shown in Fig. 1.

Fig. 1
figure 1

Chemical structures of the monomers

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Polymerization

As a precursor, poly(o-hydroxy amide) (PHA) was synthesized by the in situ silylation method described as follows [28, 29]: 5 mmol of 6FAHP was dissolved in 10 ml of DMAc in a 50 ml three-neck flask under N2 flow. Next, 20 mmol of BSA was added to this solution with stirring at room temperature, and the solution was continually stirred for 1 h. Then, the solution was cooled in an ice-ethanol bath at 0–5 °C, and 4.85 mmol of OBC was added with stirring. The mixture was kept stirring at 0–5 °C for 1 h and then at room temperature for 3 h under N2 flow. The resulting polymer solution was poured into distilled water. The precipitated polymer was collected and washed thoroughly with distilled water, followed by vacuum drying at 90 °C for 4 h. Finally, amine-terminated 6FAHP-OBC poly(o-hydroxy amide), PHA(6FAHP-OBC) was obtained as a solid polymer. A schematic representation of the synthesis of PHA(6FAHP-OBC) is shown in Fig. 2.

Fig. 2
figure 2

Schematic representation of the synthesis of PHA(6FAHP-OBC)

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Membrane formation

PHA(6FAHP-OBC) (0.5 g) was dissolved in 5 ml of DMAc, and 0.050 g of TEOSPSA was added with stirring. To this solution, appropriate amounts of a selected alkoxysilane, TEOS or PhTMS, and distilled water and a catalytic amount of diluted hydrochloric acid were added, and the reaction mixture was stirred overnight. The preparation conditions of the corresponding reaction mixtures are summarized in Table 1. Next, the mixture was cast on a PET sheet and dried at 85 °C for 1 h in a heating oven to form a thin membrane. The prepared membrane was peeled off, fixed between two window-opened metal frames, and then dehydrocyclized and hybridized by subsequent stepwise heating in a heating oven under N2 flow: (a) 100 °C for 1 h, (b) 200 °C for 1 h, and (c) a selected temperature of 400, 420, or 450 °C for 1 h. Finally, 6FAHP-OBC polybenzoxazole (PBO(6FAHP-OBC))–silica (SiO2) hybrid membranes were obtained. Because our previous work demonstrated that the thermal decomposition of pristine PBO(6FAHP-OBC) initiated at ~470 °C[26]. the upper limit of the thermal treatment temperature was set to 450 °C.

Table 1 Preparation condition of the PBO(6FAHP-OBC)– silica hybrid membranes
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Here, the hybrid membranes prepared with TEOS or PhTMS are denoted PBO(6FAHP-OBC)–SiO2 (TEOS) and PBO(6FAHP-OBC)–SiO2 (PhTMS), respectively.

Measurements

Attenuated total reflection Fourier transform infrared (ATR FT-IR) spectra were recorded by an FT/IR-4100 equipped with an ATR PRO ONE (ZnSe prism) (JASCO Corp., Japan) at a wavenumber range of 550–4000 per cm and a resolution of 2 per cm. Scanning electron microscopy (SEM) images were acquired using an S-3400N variable pressure SEM (Hitachi High-Technologies Corp., Japan) at an accelerating voltage of 10 kV. Samples for the SEM analyses were coated using an E-1010 ion sputter coater (Hitachi High-Technologies Corp., Japan) with a platinum target. Differential scanning calorimetry (DSC) measurements were carried out with a DSC-60 (Shimadzu Corp., Japan) at a heating rate of 10 °C/min and a temperature range of 25–400 °C under N2 flow. Thermogravimetric–differential thermal analysis (TG–DTA) experiments were performed with a DTG-60 (Shimadzu Corp., Japan) at a heating rate of 10 °C/min and a temperature range of 25–800 °C under air flow. CO2, O2, N2, and CH4 permeation measurements were carried out with a constant volume/variable pressure apparatus at 76 cmHg and 25 °C. The permeability coefficient, P (barrer, 1 barrer = 1 × 10−10 cm 3(STP) cm/cm2s cmHg), was determined by the following equation [30]:

$$P = \left[ {\frac{{273}}{T} \cdot \frac{V}{A} \cdot L \cdot \frac{1}{p} \cdot \frac{1}{{76}} \cdot \frac{{{\rm{d}}p}}{{{\rm{d}}t}}} \right] \times 10^{10}$$
(1)

where T is the absolute temperature (K), V is the downstream volume (cm3), A is the membrane area (cm2), L is the membrane thickness (cm), p is the upstream pressure (cmHg), and dp/dt is the permeation rate (cmHg/s). The gas permeability coefficient can be explained on the basis of the solution-diffusion mechanism, which is represented by the following equation [31, 32]:

$$P = D \times S$$
(2)

where D (cm2/s) is the diffusion coefficient and S (cm3(STP)/cm3polym. cmHg) is the solubility coefficient. The diffusion coefficient was calculated by the time-lag method represented by the following equation [33]:

$$D = \frac{{L^2}}{{6t}}$$
(3)

where t (s) is the time-lag. Wide-angle X-ray scattering (WAXS) patterns were recorded by an MX-Labo (Mac Science Co., Ltd.) using Cu-Kα radiation with a wavelength of λ = 1.54 Å. The scan range was from 10 to 25° under a voltage of 40 kV and a current of 18 mA. The average d-spacing, d(Å), was determined based on Bragg’s law [27]:

$$n\lambda = 2d\,{\mathrm{sin}}\,\theta$$
(4)

where n is an integer, λ denotes the X-ray wavelength, and θ indicates the diffraction angle. A floating method was used for measuring the densities of pristine PBO(6FAHP-OBC) membranes prepared by different thermal protocols with NaBr aqueous solution at 25 °C. According to the group contribution method, the fractional free volume (FFV) of a polymer can be estimated by the following equations [34, 35]:

$${\rm{FFV}} = \frac{{V_{{\rm{sp}}} - 1.3V_{\rm w}}}{{V_{{\rm sp}}}}$$
(5)
$$V_{{\rm sp}} = \frac{M}{\rho }$$
(6)

where V sp (cm3/mol) is the specific molar volume, Vw (cm3/mol) is the van der Waals volume of the repeating unit, M (g/mol) is the molecular weight of the repeating unit, and ρ (g/cm3) is the experimental density.

Results and discussion

Membrane characterization

Figure 3 shows ATR FT-IR spectra of PHA(6FAHP-OBC), PBO(6FAHP-OBC), and silica hybrid membranes (SiO2 content: 30 wt%). For PHA(6FAHP-OBC), the bands assigned to the amide linkage are found at ~1540 (N–H bending) and 1650 per cm (C=O stretching) [27, 36]. On the other hand, for PBO(6FAHP-OBC), the bands attributed to the amide linkage mentioned above disappeared, and new absorption bands appeared at 1052 (phenyl–O–C stretching), 1480 (benzoxazole ring), and 1612 per cm (C=N stretching), which are characteristic bands of PBO [12, 18, 27, 37, 38]. This finding indicates that PHA(6FAHP-OBC) is successfully converted into PBO(6FAHP-OBC) by the established thermal treatment protocol. For the hybrid membranes PBO(6FAHP-OBC)–SiO2 (TEOS) and SiO2 (PhTMS) (SiO2 content: 30 wt%), a strong absorption band at ~1000–1200 per cm assigned to Si–O–Si asymmetric stretching [39, 40]. is also observed. Additionally, for PBO(6FAHP-OBC)–SiO2 (PhTMS), the absorption band at 697 per cm (out-of-plane deformation of phenyl groups) is observed, together with bands at 1135 and 1430 per cm related to Si–phenyl deformation [39, 41]. These results confirm that the sol–gel reaction occurs during the membrane preparation process and that the expected Si–O–Si network structures are formed in the PBO(6FAHP-OBC)–SiO2 (TEOS) and SiO2 (PhTMS) hybrid membranes.

Fig. 3
figure 3

ATR FT-IR spectra of a PHA(6FAHP-OBC), b PBO(6FAHP-OBC), c PBO(6FAHP-OBC)–SiO2 (TEOS) (SiO2 content: 30 wt%), and d PBO(6FAHP-OBC)–SiO2 (PhTMS) (SiO2 content: 30 wt%); b–d are prepared at 400 °C

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Figure 4 presents cross-sectional SEM images of PBO(6FAHP-OBC)–SiO2 (TEOS) and SiO2 (PhTMS) (SiO2 content: 30 wt%). PBO(6FAHP-OBC)–SiO2 (TEOS) has a rugged cross-sectional morphology because of the approximately 1–3 μm of aggregated silica domains (Fig. 4a). On the other hand, PBO(6FAHP-OBC)–SiO2 (PhTMS) in Fig. 4b shows smaller and more dispersed silica domains than PBO(6FAHP-OBC)–SiO2 (TEOS). The small and well-dispersed silica domains in PBO(6FAHP-OBC)–SiO2 (PhTMS) might result from the phenyl substituent of the PhTMS moiety. The phenyl substituent might function as an inhibitor of the formation of the Si–O–Si network during the sol–gel process, resulting in suppressed aggregation of the silica domains. In addition, the phenyl substituent would also improve the affinity for the PBO matrix, leading to well-dispersed silica domains.

Fig. 4
figure 4

Cross-sectional SEM images of a PBO(6FAHP-OBC)–SiO2 (TEOS) (SiO2 content: 30 wt%) and b PBO(6FAHP-OBC)–SiO2 (PhTMS) (SiO2 content: 30 wt%) prepared at 400 °C

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The thermal properties of the hybrid membranes were investigated by DSC and TG–DTA measurements. Glass transition temperatures (Tgs) and 5% weight-loss temperatures (Td5s) of the hybrid membranes are listed in Table 2, in addition to the silica contents determined from the residual mass at 800 °C. As expected, all the hybrid membranes are confirmed to contain appropriate amounts of silica. The Td5s of the hybrid membranes increase with increasing silica content. This result indicates that hybridization with silica increases the thermal stability of the PBO matrix. The PBO(6FAHP-OBC)–SiO2 (TEOS) hybrid membranes show higher Tgs than pristine PBO(6FAHP-OBC), suggesting the formation of a robust three-dimensional Si–O–Si network. On the other hand, the PBO(6FAHP-OBC)–SiO2 (PhTMS) hybrid membranes show lower Tgs than PBO(6FAHP-OBC). This result suggests that phenyl substituent of the PhTMS moiety leads to insufficient formation of the Si–O–Si network and effective disruption of molecular chain packing[25, 42], which decreases the constraint of molecular chains in the hybrid membranes.

Table 2 Thermal properties of the PBO(6FAHP-OBC)–silica hybrid membranes prepared at 400 °C
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Gas transport properties

The gas permeability, diffusion, and solubility coefficients of the PBO(6FAHP-OBC)–silica hybrid membranes prepared with different alkoxysilanes and thermal treatment protocols are summarized in Table 3. Unfortunately, we could not carry out gas permeation measurements for the 450 °C-treated hybrid membranes containing 30 wt% of silica due to shrinkage-induced breakage of the membranes during the cooling process of membrane formation. First, from the perspective of the effect of hybridization with silica, it should be noted that the gas permeability of the hybrid membranes increases with increasing silica content, mainly in conjunction with increased gas diffusivity. Similar enhancements of gas permeability and/or diffusivity have been reported by Merkel et al. for high free volume, glassy polymer-nanosized silica composites, and they have concluded the silica particles yield a polymer/particle interfacial area and provide disruption of polymer chain packing, that affects the gas transport behavior [43, 44]. In another study on positron annihilation lifetime spectroscopy analyses of PI-based mixed matrix membranes (MMMs), it has been determined that the incorporation of inorganic nanoparticles accelerates the formation of additional free volume holes, which leads to increased gas diffusivity and, as a consequence, increased gas permeability of the MMMs [45]. Therefore, the increased gas permeability attributed to the increased gas diffusivity of the PBO(6FAHP-OBC)–silica membranes might be caused by the formation of free volume holes around the polymer/silica interfacial area. Notably, the gas permeability and diffusivity of PBO(6FAHP-OBC)–SiO2 (PhTMS) are higher than those of PBO(6FAHP-OBC)–SiO2 (TEOS). The more enhanced permeability and diffusivity of PBO(6FAHP-OBC)–SiO2 (PhTMS) are considered to be due to formation of free volume holes around the polymer/silica interfacial area facilitated by the phenyl substituent of the PhTMS moiety. It has also been revealed that adding surface-modified silica nanoparticles into a polymer matrix facilitates the formation of nanogaps between nanoparticles and polymer chain segments [46, 47]. As presented in Fig. 4b, compared to PBO(6FAHP-OBC)–SiO2 (TEOS), PBO(6FAHP-OBC)–SiO2 (PhTMS) features small and well-dispersed silica domains, which in turn can increase the polymer/silica interfacial area and thus would also contribute to the higher gas permeability and diffusivity of PBO(6FAHP-OBC)–SiO2 (PhTMS).

Table 3 Gas transport properties, d-spacing, and FFV of the PBO(6FAHP-OBC)–silica hybrid membranes at 25 °C
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The thermal treatment temperature also affects to gas transport properties of pristine PBO(6FAHP-OBC) and its silica hybrid membranes. As shown in Table 3, the gas permeability and diffusivity of PBO(6FAHP-OBC) and its silica hybrid membranes increase with increasing thermal treatment temperature. Figure 5 presents WAXS patterns of pristine PBO(6FAHP-OBC) membranes prepared with different thermal treatment protocols, and d-spacing values calculated from Eq. (4) are listed in Table 3 together with FFV values obtained from Eqs. (5) and (6). Both the d-spacing attributed to intermolecular chain distance and FFV increase with increasing thermal treatment temperature. Wang et al. have reported that for a PBO membrane, increased treatment temperature causes increases in inter-chain distance and free volume radius, which lead to enhanced gas permeabilities [11, 27]. The improved gas permeability and diffusivity of PBO(6FAHP-OBC) and its silica hybrid membranes by increased treatment temperature are therefore attributed to the increased intermolecular chain distance and FFV. Especially for the hybrid membranes, the gas permeability and diffusivity are efficiently improved by the effects of both hybridization with silica and increased treatment temperature.

Fig. 5
figure 5

WAXS patterns of pristine PBO(6FAHP-OBC)s prepared at a 400 °C, b 420 °C, and c 450 °C

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The ideal gas selectivity for the combination of gases A and B (α(A/B)) is defined by the following equation [48]:

$$\alpha {{(\rm A/B)}} = \frac{P{(\rm A)}}{P{(\rm B)}} = \frac{D{(\rm A)}}{D{(\rm B)}} \times \frac{S{(\rm A)}}{S(\rm B)} = {\alpha }^D {{(\rm A/B)}} \times {\alpha }^S {{(\rm A/B)}}$$
(7)

where αD(A/B) is the diffusivity selectivity and αS(A/B) is the solubility selectivity. The O2/N2 and CO2/CH4 selectivities of the PBO(6FAHP-OBC)–silica hybrid membranes are listed in Table 4. It is found that the ideal selectivity (α(A/B)) for a given gas pair essentially depends on the diffusivity selectivity (αD(A/B)) rather than the solubility selectivity (αS(A/B)), which is consistent with a general understanding of the gas separation behavior of glassy polymers [48]. The obtained α(O2/N2) and α(CO2/CH4) values are plotted against the O2 and CO2 permeability coefficients in Figs. 6 and 7, respectively. In Fig. 6, the α(O2/N2) values slightly decrease with increasing O2 permeability, along with the upper bound trade-off line for O2/N2 separation demonstrated by Robeson in 1991 [49]. This behavior is consistent with the general trend that glassy polymers that are more permeable are less selective, and vice versa [48]. Nevertheless, it can be seen that the PBO(6FAHP-OBC)–silica hybrid membranes have a favorable O2/N2 separation ability because they show relatively high α(O2/N2) values just below the upper bound presented in 1991.

Table 4 O2/N2 and CO2/CH4 selectivities of the PBO(6FAHP-OBC)–silica hybrid membranes at 25 °C
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Fig. 6
figure 6

Ideal O2/N2 selectivity of PBO(6FAHP-OBC)–SiO2 (TEOS) (filled symbols) and PBO(6FAHP-OBC)–SiO2 (PhTMS) (unfilled symbols) plotted against the O2 permeability coefficient measured at 25 °C; the attached number represents the SiO2 content (wt%) in the membrane

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Fig. 7
figure 7

Ideal CO2/CH4 selectivity of PBO(6FAHP-OBC)–SiO2 (TEOS) (filled symbols) and PBO(6FAHP-OBC)–SiO2 (PhTMS) (unfilled symbols) plotted against the CO2 permeability coefficient measured at 25 °C; the attached number represents the SiO2 content (wt%) in the membrane

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For CO2/CH4 separation, interesting behaviors are observed. As shown in Fig. 7, the α(CO2/CH4) of the PBO(6FAHP-OBC)–SiO2 (TEOS) increases with increasing CO2 permeability with a tendency to cross the upper bound for CO2/CH4 separation [49, 50]. Considering the difference in kinetic diameter between CO2 (3.3 Å) and CH4 (3.87 Å)[48], the improved CO2/CH4 separation ability of PBO(6FAHP-OBC)–SiO2 (TEOS) might be due to characteristic free volume holes around the polymer/silica interfacial area that improve size-selective CO2/CH4 separation. On the other hand, the α(CO2/CH4) of the PBO(6FAHP-OBC)–SiO2 (PhTMS) monotonically decreases, in conjunction with decreased αD(CO2/CH4) (Table 4), with increasing CO2 permeability along with the upper bound. The distinctive difference between PBO(6FAHP-OBC)–SiO2 (TEOS) and PBO(6FAHP-OBC)–SiO2 (PhTMS) may be due to the differences in the mean size and total amount of free volume holes created around the polymer/silica interfacial area. The size-selective CO2/CH4 separation ability of PBO(6FAHP-OBC)–SiO2 (PhTMS) is sacrificed by excess increases in the mean size and total amount of free volume holes caused by the phenyl substituent of the PhTMS moiety, whereas the overall gas permeability is markedly increased owing to increased gas diffusivity.

It should be mentioned that PBO(6FAHP-OBC) and its silica hybrid membranes maintain fundamental CO2/CH4 separation behavior, whereas their CO2 permeability is highly accelerated with increasing treatment temperature. In particular, 450 °C-treated PBO(6FAHP-OBC)–SiO2 (TEOS) possesses an excellent CO2/CH4 separation ability that exceeds the upper bound updated in 2008. This prominent CO2/CH4 separation ability might be attributable to the simultaneous effects of (1) characteristic free volume holes created around the polymer/silica interfacial area caused by the hybridization with TEOS-induced silica and (2) enhanced intermolecular chain distance and free volume holes due to the thermal treatment, which improves size-selective CO2/CH4 separation. Even though 450 °C-treated PBO(6FAHP-OBC)–SiO2 (TEOS) cannot exceed the advanced trade-off line of CO2/CH4 separation for existing TR-polymers, including TR-PBOs (Fig. 7)[17, 50, 51]. the PBO(6FAHP-OBC)–silica hybrid membranes have an attractive CO2/CH4 separation ability that can be tailored to specific requirements by altering the alkoxysilane species and the thermal treatment temperature.

Conclusions

The gas transport properties of PBO(6FAHP-OBC)–silica hybrid membranes prepared by applying different alkoxysilane species and thermal treatment protocols were investigated. The thermal decomposition temperatures of the PBO(6FAHP-OBC)–SiO2 (TEOS) and SiO2 (PhTMS) hybrid membranes increase with increasing silica content, indicating that hybridization with silica improves the thermal stability. The gas permeability of the PBO(6FAHP-OBC)–silica hybrid membranes also increases with increasing silica content, suggesting the formation of free volume holes around the polymer/silica interfacial area. For PBO(6FAHP-OBC)–SiO2 (PhTMS), a more enhanced permeability is observed, which might be attributed to the formation of free volume holes induced by the phenyl substituent of the PhTMS moiety. The gas permeability of the PBO(6FAHP-OBC)–silica hybrid membranes is further increased with increasing thermal treatment temperature owing to the increased intermolecular chain distance and FFV. In particular, 450 characteristic free volume holes created around the polymer/silica interfacial area caused by the hybridization with TEOS-induced silica and [2]. enhanced intermolecular chain distance and free volume holes due to the thermal treatment, which improves size-selective CO2/CH4 separation. With the simultaneous application of hybridization and thermal treatment techniques, the development of PBO-based high-performance gas separation membranes with attractive gas permeability and/or selectivity would be expected.