Gas Transport Properties of Polybenzimidazole and Poly(Phenylene Oxide) Mixed Matrix Membranes Incorporated with PDA-Functionalised Titanate Nanotubes
© The Author(s). 2016
Received: 23 April 2016
Accepted: 5 September 2016
Published: 3 January 2017
Functionalised titanate nanotubes (TiNTs) were incorporated to poly(5,5-bisbenzimidazole-2,2-diyl-1,3-phenylene) (PBI) or poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) for improving the interfacial compatibility between the polymer matrix and inorganic material and for altering the gas separation performance of the neat polymer membranes. Functionalisation consisted in oxidative polymerisation of dopamine-hydrochloride on the surface of non-functionalised TiNTs. Transmission electron microscopy (TEM) confirmed that a thin polydopamine (PDA) layer was created on the surface of TiNTs. 1.5, 3, 6, and 9 wt.% of PDA-functionalised TiNTs (PDA-TiNTs) were dispersed to each type of polymer matrix to create so-called mixed matrix membranes (MMMs). Infrared spectroscopy confirmed that –OH and –NH groups exist on the surface of PDA-TiNTs and that the nanotubes interact via H-bonding with PBI but not with PPO. The distribution of PDA-TiNTs in the MMMs was to some extent uniform as scanning electron microscope (SEM) studies showed. Beyond, PDA-TiNTs exhibit positive effect on gas transport properties, resulting in increased selectivities of MMMs. The addition of nanotubes caused a decrease in permeabilities but an increase in selectivities. It is shown that 9 wt.% of PDA-TiNTs in PBI gave a rise to CO2/N2 and CO2/CH4 selectivities of 112 and 63 %, respectively. In case of PPO-PDA-TiNT MMMs, CO2/N2 and CO2/CH4 selectivity increased about 25 and 17 %, respectively. Sorption measurement showed that the presence of PDA-TiNTs in PBI caused an increase in CO2 sorption, whereas the influence on other gases is less noticeable.
KeywordsPolybenzimidazole Poly(phenylene oxide) Titanate nanotubes Polydopamine Mixed matrix membrane Gas separation Permeability Selectivity Sorption isotherms
Nanomaterials have attracted considerable interest in many applications, including the field of membrane science [1–7]. In the last decades, numerous works have been published on the use of inorganic particles in various polymeric membrane structures and their functionalities [1, 8–13]. The goal of such so-called mixed matrix membranes (MMMs) is to achieve a system with more useful structural or functional properties unattainable by any of the constituent itself which may help to overcome the efficiency-productivity trade-off of neat polymer materials .
Thus far, a wide range of nanoparticles were used in MMMs, e.g. zeolites, metal organic frameworks (MOFs), mesoporous silicas, carbon molecular sieves, or carbon nanotubes (CNTs) [9, 15–17]. The choice of nanoparticles for the desired gas separation is of greatest significance, because major variables as gas adsorption or molecular sieving abilities of the nanoparticles may seriously affect the MMM performance. Beyond, uniformly dispersed nanoparticles in the polymer matrix as well as interfacial bonding notably influence gas transport properties [9, 16, 18].
Hitherto, CNTs and their potential for MMMs have been studied in great detail and seems to be a prospective filler for overcoming the efficiency-productivity trade-off of neat polymer membranes because of their high aspect ratio [1, 19]. Several polymeric materials have been tested to prepare MMMs as shown later in the text to alter their gas separation characteristics. For example, Wang et al. has embedded multi-walled nanotubes (MWNT) into PEG-based Pebax solution to separate CO2/CH4 and CO2/N2, respectively . Another study of Pebax membranes was performed by Murali et al. . It was shown that MWNT in the Pebax matrix enhances substantially the permeability of H2, O2, CO2, and N2. Khan et al. incorporated pristine MWNTs into PIM-1, which caused as well an increase in gas permeabilities . Rajabi et al. reported on the addition of functionalised MWNT to polyvinylchloride (PVC) membranes which resulted in better gas separation performance, especially for CO2/CH4 . Studies by Kim and his group demonstrated that the permeabilities of O2, N2, and CH4 increased proportionally to the amount of open-ended CNTs in the polymer matrix . Li and his co-workers prepared MMMs from Matrimid and a combination of CNT and graphene oxide . It was found that the MMMs with CNTs and graphene oxide had better gas separation performance than those with only one of these components, showing excellent separation properties. Cong et al. added single-walled carbon nanotubes (SWNT) and MWNT to brominated poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) for CO2/N2 separation . They observed that the MMMs had an increased CO2 permeability, while the CO2/N2 selectivity remained the same. In addition, permeabilities increased with the content of CNT.
However, there are several drawbacks regarding the use of CNTs in MMMs [1, 19]: (1) extreme amount of energy consumption because of high process temperatures (1000–3700 °C); (2) expensive fabrication owing to lasers and inert atmosphere; and (3) additional purification steps necessary due to by-products. With respect to these major issues, the utilisation of new fillers in MMMs is desirable.
Hence, this work focuses on titanate nanotubes (TiNTs), a relatively new class of nanotubes made of non-carbon material. These nanotubes possess similar morphology as CNTs, but the synthesis is carried out at far lower temperatures and at low cost [26, 27]. Moreover, variations of TiNT compositions and structures may be practically unlimited and the possibility of functionalisation is a useful way of treatment that may improve adhesion to polymer matrices. To date, most research about TiNTs has been directed towards the efficient synthesis and functionalisation of this filler as well on investigation of its unique morphology and physico-chemical properties [27–29]. However, almost all underlying studies about nanotubes for use in MMMs are connected to CNTs. Therefore, this study describes membranes altered by functionalised TiNTs and continues our earlier work in which and non-modified TiNTs were added to poly(5,5-bisbenzimidazole-2,2-diyl-1,3-phenylene) (PBI) or PPO . Although various researches have been carried out on those polymers with diverse inorganic fillers, e.g. ZIF-8 , SBA-15 , silica nano particles [33, 34], there is so far no academic literature available on using functionalised TiNTs combined with PBI or PPO as membrane materials for gas separation, to the best of our knowledge. Therefore, the aim was to investigate the effect of TiNT on the gas transport properties of PPO and PBI membranes. It was found that the incorporation of non-modified TiNTs to PPO formed unselective voids, because the obtained ideal selectivities remained constant while permeabilities of all investigated gases increased. In case of PBI, the compatibility of non-modified TiNTs and the PBI matrix was enhanced because of the Ti–O groups present in TiNT which could interact with the N–H bonds present in PBI. These interactions increased the effective path of the penetrants, resulting in lower permeabilities and higher ideal selectivities.
Therefore, in continuation of this study , we attempted to functionalise the TiNTs in order to improve the adhesion between the filler and the polymer matrix, targeting to diminish the unselective voids and thus improving the membrane separation characteristics.
For the functionalisation was utilised polydopamine (PDA), because the formation of PDA layers onto nanotubes emerged as very efficient and facile [35–38]. Besides, PDA appears as a promising adhesive for subsequent surface-mediated reactions which allows tailoring properties according the used source materials [39, 40].
In the present work, the feasibility of the formation of MMMs based on PBI or PPO, respectively, and various amounts of PDA-TiNTs was studied. The as-prepared MMMs were characterised for their morphology, physico-chemical properties, and gas separation performance, aiming to investigate the effect of PDA-TiNT content on PBI or PPO membranes.
The PPO powder was purchased from Spolana Neratovice (Czech Republic). For dissolving PPO chloroform (Lachner, Czech Republic) was used as received.
Poly(5,5-bisbenzimidazole-2,2-diyl-1,3-phenylene) (PBI) was supplied by Hoechst Celanese and used as received as a 10 wt.% N,N-dimethylacetamide (DMAc) solution with a lithium chloride content of ~2 wt.%.
TiO2 powder (rutile modification), dopamine-hydrochloride, and tris(hydroxymethyl)aminomethane (TRIS) buffer (Sigma 7-9™, 99 %) were purchased from Sigma Aldrich, Germany.
Nanotubes Synthesis and Functionalisation
TiNTs were synthesised by hydrothermal treatment. The details of the synthesis method of TiNTs are described elsewhere . A thin PDA layer was created onto the surface of TiNTs by oxidative polymerisation of dopamine-hydrochloride. This procedure involves mixing of 800 ml of TiNT suspension (2 mg/ml) with 1.6 g of dopamine-hydrochloride dissolved in 5 g of TRIS buffer adjusted to pH = 8.5. The PDA coating on TiNTs was carried out for 3.5 h at 25 °C. After coating, the pH was adjusted to 6.4 using 35 wt.% HCl. The suspension was purified using dialysis tubing cellulose membrane and the resulting PDA-functionalised TiNTs (PDA-TiNTs) were isolated using freeze drying.
Preparation of Mixed Matrix Membranes
Two series of MMMs were prepared: PPO-PDA-TiNT MMMs and PBI-PDA-TiNT MMMs. For the preparation of PPO-PDA-TiNT MMMs, PPO was dissolved in chloroform to obtain a 5 wt.% casting solution and stirred for 24 h. The PDA-TiNT was then added to the polymer solution and stirred with a magnetic stirrer for 24 h. The content of PDA-TiNT in the membranes was 1.5, 3, 6, and 9 wt.%, respectively. In case of PBI-PDA-TiNT MMMs, the PBI solution was diluted with DMAc to obtain as well a 5 wt.% polymer solution. The same amounts of PDA-TiNT were added as well to the polymer solutions of PBI. The details of the membrane preparation have been described in the previous work . The resulting membranes had a thickness between 20 and 60 μm.
Characterisation and Measurements
The size and morphology of TiNTs were analysed by using a transmission electron microscope (TEM) Tecnai G Spirit (FEI, 120 kV).
The specific surface area (S BET) of the TiNT and PDA-TiNT samples was measured by a gas adsorption technique on a Gemini VII 2390 (Micromeritics Instruments Corp., Norcross, USA) with nitrogen as the sorbate. The surface area was calculated from the Brunauer-Emmett-Teller (BET) adsorption/desorption isotherm using the Gemini software. It characterises materials in the region of micropores (<2 nm) .Calculations were done with a sample density ρ = 1.3 g/ml.
The cross section and surface morphology of the membranes were observed using a Quanta 200 FEG scanning electron microscope.
The thermal behaviour of the neat polymers and MMMs were studied on DSC Perkin Elmer 8500. The heating was performed in two steps. The first step was the continuous heating at 100 °C for 1 h in order to remove the volatiles from the polymer matrix. The second step was the increase of the temperature from 0 to 500 °C at the rate of 100 °C/min in a nitrogen purge (25 cm3/min).
Wide-angle X-ray scattering (WAXS) experiments were performed using a pinhole camera (modified Molecular Metrology System, Rigaku, Japan) attached to a micro-focused X-ray beam generator (Rigaku MicroMax 003) operating at 50 kV and 0.6 mA (30 W). The camera was equipped with removable and interchangeable Imaging Plate 23 × 25 cm (Fujifilm). Experimental setup covered the momentum transfer (q) range of 0.4–3.6 Å−1. While q = (4π/λ)sinθ, where λ = 1.54 Ǻ is the wavelength and 2θ is the scattering angle. Calibrations of the centre and sample-to-detector distance were made using Si powder. Samples were measured in transmission mode for 30 min.
The FTIR spectra of membranes were obtained using Spectrum 100 spectrometer (PerkinElmer, USA) equipped with a mercury–cadmium–telluride (MCT) detector in the wavelength range from 650 to 4000 cm−1 and universal attenuated total reflectance accessory (ATR) with a diamond prism. Spectral resolution was 4 cm−1 with 16 scans taken for each spectrum. The FTIR spectra of the nanotubes (TiNT, PDA-TiNT) and PDA were recorded on a Perkin Elmer Paragon 1000PC FTIR spectrometer using the reflective ATR technique Specac MKII Golden Gate Single Reflection ATR System with a diamond crystal. All spectra were measured in the wavenumber range 450–4400 cm−1 with a resolution of 4 cm−1 and with 32 scans.
The accuracy of the measurement is given by the sum of the relative accuracies of each measured term of Eq. 1. The relative error of Δp p/Δt measured with MKS Baratron is smaller than 0.3 % plus the inaccuracy attributed to the resolution of the pressure transducer which is 1/10 of mbar. The relative standard deviation of the calibrated volume is less than 0.1 %, of the membrane area less than 0.5 %, and of the feed pressure 0.2 %. In case of the thickness of the membrane, the value can be measured as precise as 1 μm.
Sorption studies were performed on the gravimetric sorption balance IGA-002, Hiden Isochema, UK, according to the procedure described in . The sorption isotherms were measured by stepwise pressure changes (pressure increase rate 100 mbar/min) within the pressure range of 0.01–4 bar. The sorption balance consists of a large capacity microbalance (5 g) with a resolution of 0.1 μg and excellent long-term stability of ±1 μg.
Results and Discussion
Results of S BET determination
S BET (m2/g)
In the IR spectrum of aromatic PDA a broad region between 3680 and 2350 cm−1 can be distinguished which can be ascribed to N–H stretching (3500–3200 cm−1), C–H stretching (3100–3000 cm−1), and H-bonded O–H stretching (3570–3200 cm−1). Moreover, the absorption peaks in the 1650–1430 cm−1 range correspond to C=C stretching in the benzene ring; however, they are partially overlapped with the ring stretching region of heterocycles containing N–H groups (1600–1300 cm−1). Similarly, the C–N stretching bands of aromatic amines appear in the region 1360–1250 cm−1 and overlap the peak attributed to the O–H bending in phenols (1410–1310 cm−1). The peak at 1052 cm−1 might correspond to the in-plane C–H bending in aromatic ring [43, 44].
The functionalisation of titanium nanotubes by PDA polymer led to visible changes in IR spectra what might be due to H-bonding interactions between both components (Fig. 2). As a first confirmation, we can point out the broadening of the band in 3680–1810 cm−1 region. Accordingly, in the region of lower wavelengths new peaks occur (1628, 1038 cm-1) and some peaks become narrower and shifted to higher wavelengths (e.g. 1552, 1294 cm−1) compared to the spectra of PDA. It is assumed that those changes are a result of hydrogen bond interactions between N–H and O–H groups of the PDA polymer and the Ti-O groups of the nanotubes. Nevertheless, the exact interpretation of the peaks is not possible due to measurement difficulties.
In the case of PPO-PDA-TiNT MMMs (Fig. 3b) no changes of any peak has been observed, although PDA possess –OH groups which are suitable to interact with the oxygen atom present in PPO. However, the aromatic rings of PDA are very bulky, wherefore it is assumed that the interactions between the PDA-TiNT and PPO are restricted because of steric hindrance.
T g of neat polymers and MMMs
Amount of PDA-TiNT in membrane
T g (°C)
The T g for PBI was found to be 413.5 °C which is similar to published data .With addition of 6 wt% of PDA-TiNT the T g noticeably increased. It is assumed, that the aromatic rings of PDA, which are attached on the surface of TiNT, reduces the segmental mobility of PBI polymer chains. In addition, the PDA-TiNTs seem to be able to interact with neat PBI as FTIR studies showed.
PPO-PDA-TiNT MMMs were characterised by no significant change in T g compared to neat PPO. It is assumed that the slight differences arouse from changes in chain mobility, but not from interaction between the functional groups of PDA-TiNT with PPO which is in accordance with the FTIR results.
Dependent on the material used, separation properties are different. As anticipated, PBI is less permeable than PPO, because of its rigid aromatic molecular structure and its relatively high chain packing density [15, 31]. High permeability of PPO among aromatic polymeric membranes can be attributed to the ether linkages, which introduce more flexibility to the polymer chain, steric hindrance of the methyl groups and lower packing density due to the absence of polar groups [47, 48].
In both matrices, the presence of filler causes a decrease in permeability. With the increase of the PDA-TiNT amount the permeability drops. As generally known, the presence of fillers brings a sort of physical barriers in the membrane that act as obstacles in the diffusive path of a gas molecule permeating across the membrane. These obstacles increase the tortuosity for gas molecules in the present MMMs, thus decrease its permeability [5, 34].
Besides, the orientation of nanotubes inside the matrix plays also a role in the separation characteristic. It was reported in the literature that MMMs with well oriented, open-ended CNTs, which are accessible for gas molecules, exhibit increased permeability [23, 25, 49]. In the present study, as SEM results show, PDA-TiNTs are randomly orientated, sometimes interconnected or even agglomerated. Therefore, it is concluded that the accessibility of the tunnels for gas molecules is limited, and PDA-TiNTs act more as a barrier which results in permeability decrease.
Another important aspect is the influence of modifier on the membrane performance. From the obtained permeability and SEM analysis, it can be concluded that both polymers adhere well to PDA-TiNTs, although FTIR studies could detect only changes in the FTIR spectra of PBI upon addition of PDA-TiNTs. Therefore, we can conclude that the sufficient adhesion between PPO and PDA-TiNT might be caused by van der Waals forces . In case of weaker interactions, gas accessible voids would be formed at the interface of PDA-TiNT and PPO, which in turn would result in a major boost of gas permeability, whereas selectivity would remain the same or would be close to the one of neat polymer [8, 51, 52].
Even though modifications led to improved adhesion for both polymers, it is interesting to note that permeability and selectivity values of PBI-PDA-TiNT MMMs are lower in comparison to those of PBI membranes with non-modified TiNTs. This result suggests that the gas transport properties of PDA have to be taken into account. When one compares the results of PPO-TiNT MMMs from earlier work  with the results of this study, a decrease in permeability can be observed for the PPO-PDA-TiNT MMMs along with an increase in selectivity. In contrary, PPO-TiNT MMMs showed an increase in permeability while selectivity remained almost constant. The observed increase can be explained by formed voids between the non-modified TiNTs and the PPO, which have negligible resistance to the flow of gas and thus cause an increase in permeability. The voids, however, are poorly selective, wherefore the selectivity remains constant. Thus, functionalisation of TiNTs improves the adhesion between the two phases and consequently minimise undesirable voids, which results in a decline in permeability.
The dependence of ideal selectivity of selected gas pairs on the concentration of PDA-TiNT in PBI (Fig. 6c) and PPO (Fig. 6d), respectively, is presented in Fig. 6. As the plots indicate PDA-TiNTs influence mainly the gas selectivity of PBI based MMMs. Addition of 9 wt.% PDA-TiNT to PBI resulted in an increase of 112 and 63 % in the selectivity of CO2/N2 or CO2/CH4, respectively. Similarly, the selectivity of H2/N2, H2/O2, and O2/N2 increased by 57, 40, and 12.50 %, respectively. The high selectivity values for CO2/N2 and CO2/CH4 can be explained by the polar functional groups of PDA on the TiNTs, confirmed by FTIR. Those polar groups usually exhibit a stronger interaction with polar gases, such as CO2, than with nonpolar gases, e.g. N2 or CH4. Thus, the polar gas solubility can be enhanced and the gas permeability can be increased which facilitates the improvement of the total CO2/N2 or CO2/CH4 selectivity . Ideal selectivities of the other gas pairs further demonstrate that gases with a smaller kinetic diameter, e.g. H2 permeate easily through the intimately connected PBI matrix with PDA-TiNT than bigger gas molecules, e.g. N2 or O2, resulting in higher selectivity ratio.
As for PPO-PDA-TiNT MMMs, gas selectivities increase for all gas pairs besides O2/N2, which remains nearly constant, meaning that the permeabilities of O2 and N2 decreased at the same rate. Analogically, selectivity increase for the gas pairs H2/N2 and H2/O2 are similar (50 %; 49 %). The CO2 over N2 selectivity enhancement (25 %) was larger than over CH4 (17 %), even though the kinetic diameter of CH4 (3.8 Å) is greater than the one of N2 (3.64 Å). However, CH4 solubilise better in PPO than N2, wherefore CH4 preferably permeates through the MMM than N2, which is in accordance with the literature [53–55].
Comparing the results of PBI-PDA-TiNT and PPO-PDA-TiNT MMMs, one can see that the addition of PDA-TiNT influence the separation performance of PBI-PDA-TiNT MMMs more significant than of PPO-PDA-TiNT MMMs.
As can be seen from Figs. 7a and 8a, gas diffusivities of all studied gases decreased with PDA-TiNT loading. After incorporation of 9 wt.% PDA-TiNTs to PBI, diffusion coefficients for H2, O2, N2, CO2, and CH4 decreased by around 37, 32, 28, 46, and 39 %, respectively (Fig. 7c). In case of PPO, the diffusion coefficients decreased about 54, 38, 46, 52, and 48 % for H2, O2, N2, CO2, and CH4, respectively (Fig. 8c). These results suggest that there are no voids between PDA-TiNTs and polymer chains leading to higher gas diffusion resistance and therefore to decreased diffusion coefficients. If there had been some voids, gas separation performance would be affected to a great extent, because the gaps at the interface between the nanotubes and the polymer provide a less resistive route for gases; thus, gas permeability would increase. Moreover, it is assumed that the addition of PDA-TiNTs facilitates polymer chain packing and reduces the free volume between the polymer chains due to the decreased diffusion coefficients. In addition, it seems, that the channels of the added nanotubes are not readily accessible for the gas molecules, elsewise the prepared MMMs would possess higher gas permeabilities due to a more effectively transport of the gas molecules through the tunnels of the nanotubes.
Despite the decrease in diffusivity, the solubility of PBI-PDA-TiNT MMMs remained relatively unchanged for H2, N2, O2, and CH4 with the increase of nanotubes, besides for CO2 (Fig. 7b). CO2 is a more condensable gas than all the other studied gases. The increased gas condensability leads to the enhancement of the solubility of the gas in the polymer matrix, which confirms the dominancy of the solution mechanism in the permeation of CO2 through the MMMs upon addition of PDA-TiNTs. Moreover, the strong polar affinity between the functional groups of PDA on the surface of the nanotubes and CO2 results in an increased CO2 solubility. With the increase of PDA-TiNT in the matrix the CO2 solubility increases due to the larger content of CO2-facilitated transport sites in the membranes, which leads to a steady increase of CO2/N2 solubility selectivity (Fig. 7d). In contrary, the solubility values of the PPO-PDA-TiNT MMMs showed for all gases an increase in solubility coefficients (Fig. 8b). Highest increase of solubility selectivity was found for the gas pair H2/N2, which might be due to the differences in kinetic diameters of these gas molecules. If one compares the solubility values of the neat polymers, it can be found that the solubility coefficients (Fig. 8d) of all gases besides CH4 are higher for PPO than for PBI (Fig. 7d); thus, the gases can solubilise better in PPO than in PBI. The differences in the solubilities between those two polymers are caused by the different molecular structures of PBI and PPO.
From the obtained diffusivity and solubility results (Figs. 7, and 8), it can be concluded that the addition of PDA-modified TiNTs to PBI or PPO deteriorates the permeability properties by decreasing gas diffusivity of all studied gases due to enhanced chain packing density and reduced chain segment mobility. In addition, the incorporation of modified nanotubes into PBI or PPO enhances the solubility of condensable gases by increasing the number of functional groups. Moreover, it is believed that the modified TiNTs act as impermeable filler which are lowering the permeability of all gases; hence, hindering the diffusion of the gases through the MMMs. Furthermore, the MMMs have no evidence of unselective voids.
As can be seen from Fig. 9 the composition of MMMs plays an important role. Depending on the choice of material and filler, the obtained results fall close to the trade-off line. The addition of different types of nanotubes caused either an increase in permeability and decrease in selectivity or vice versa. Only in the case of Matrimid-based MMMs, an increase in both parameters could be observed. Nevertheless, additional studies on MMMs need to be performed in order to overcome this upper bound.
Technical achievable performance according to types of membranes investigated by different research groups
Table 3 reveals that PPO and PBI have been utilised in several types of research as a polymer matrix for membrane preparation. The gas transport properties of the neat membranes are in accordance with those of other works. Upon addition of any kind of filler, different effects on the separation performance can be observed. In general, the addition of silica, ZIF-8, SBA-15, and non-modified or modified TiNTs, respectively, to PPO or PBI cause an increase in gas selectivities. As far as permeabilities are concerned, they either increased or decreased. This can be explained by good adhesion of the polymer chains on the surface of the filler. Poor adhesion of the polymer onto the filler can cause voids between the interface of polymer chains and fillers, which leads to an increase in permeability, whereas selectivity remains almost the same. In conclusion, the employment of fillers can result in high selective membranes with superb permeation rates depending on the choice of polymer and filler.
At low to moderate pressures, the sorption is determined by the Langmuir term and describes the non-equilibrium region of the curve. Furthermore, it characterises the sorption on the holes or “microvoids” from the non-equilibrium nature of glassy polymers. At higher pressures, Henry’s law solubility predominates. Its corresponding Henry’s law term describes the equilibrium region and is related to the dissolution of gases into the dense equilibrium structure of rubbery polymers.
In general, the measured sorption data for the more condensable gases CO2 and CH4 can be well fitted by the dual-mode sorption model (Fig. 10). Even the lower condensable gases N2 and O2 exhibit slightly nonlinear pressure dependence. In case of H2, which does not exhibit significant gas sorption, follows the Henry’s type gas sorption model, which demonstrates a linear relationship of gas sorption with pressure.
For PBI, sorption isotherms are similar to previously reported ones . In contrary, the 6 wt.% PBI-PDA-TiNT MMM (Fig. 10) exhibits a significant decrease in gas sorption capacity of CH4, N2, O2, and H2. In previous study with non-modified TiNTs, it was found that the addition of fillers increase the sorption capacity , which is in agreement with other published data of MMMs made of nanotubes or other nanoparticles, respectively [56, 62, 63]. In the present case, functionalisation of TiNTs alters the lattice structure of TiNTs which led to a remarkable decrease in the specific area as the values of Table 1 show. This might cause a decline in the sorption capacity of the surface or inner side of TiNTs. Besides, functionalisation improves the filler-polymer interface compatibility for MMMs as can be concluded from IR results, wherefore the gas molecules cannot adsorb in the interlayer spacing between polymer and nanotubes. These major changes achieved in the material’s structure lower considerably its sorption capacity for the gases CH4, N2, O2, and H2.
In case of CO2, the sorption capacity of the 6 wt.% PBI-PDA-TiNT MMM is in the same amount as of neat PBI membrane due to the inclusion of new sorption sites throughout the addition of modified nanotubes. The abundant CO2 selective groups, on the surface of TiNTs, enlarge the CO2 adsorption capacity due to increased polar interactions between the gas molecules and the existing functionalised surface. Similar trends of increased CO2 sorption capacity were also observed in other reported MMMs [12, 63] and were demonstrated in our previous study with non-modified TiNTs . To conclude, the improvement of sorption capacity by PDA-TiNTs can be attributed to the increase of sorption sites.
MMMs were prepared using TiNTs functionalised with PDA and a PBI and PPO, respectively, as the polymer matrix. TEM and S BET analysis have confirmed the successful PDA functionalisation of TiNTs. The SEM images of the prepared MMM revealed that the functionalised TiNTs are well dispersed throughout the both matrices. Besides, it seems that the nanotubes form agglomerates in the membrane due to functionalisation by PDA. As DSC studies showed, with addition of 6 wt% of PDA-TiNT to PBI the T g noticeably increased, whereas for the PPO matrix not. According to FTIR studies, PBI and PDA-TiNT interacts possibly via hydrogen bonds. In contrary, no interactions between PPO and PDA-TiNT were observed. In any case, it seems, there is good interfacial adhesion and the absence of voids between PDA-TiNTs and both polymer matrices, as the gas permeabilities decreased for all gases with the addition of PDA-TiNTs. Lowest permeabilities were observed with the highest content of PDA-TiNT in the MMMs. It is clear that addition of SWNTs to a polymer matrix can improve certain selectivities as well as permeabilities of small molecules. On the other hand, the addition of TiNTs to a polymer matrix can improve certain gas selectivities, especially for CO2/N2 and CO2/CH4. Further, the sorption capacity of neat PBI and PBI-PDA-TiNT was studied for H2, O2, N2, CH4, and CO2. The sorption isotherms elucidated that the presence of PDA-TiNTs in PBI has a positive effect on CO2 sorption.
Attenuated total reflectance
Mixed matrix membrane
Metal organic framework
- S BET :
Specific surface area
Transmission electron microscope
Wide-angle X-ray scattering
- A :
Langmuir affinity constant
- C :
Total gas concentration
- cH :
Langmuir capacity constant
- kD :
- l :
- P :
- p :
Penetrant partial pressure
- p i :
- p p :
Ideal gas constant
- S :
- T :
- t :
- V p :
Volume of the product part
- α :
We would like to thank our colleagues of the other departments for providing the nanotubes, for the WAXS measurements, and for the help with TEM images.
This work was supported by the Ministry of Education, Youth and Sports of Czech Republic within the National Sustainability Program I (NPU I), Project LO1507 POLYMAT.
The concept and the design of the study were carried out by VG. VG prepared the membranes and studied the gas permeation and sorption properties. JK fabricated the nanotubes and performed the measurements on DSC. MP carried out the studies on FTIR. VG, MP, and JK collaborated in the acquisition, analysis, and interpretation of the data as well as in writing and reviewing the manuscript. ZP revised the manuscript critically for important intellectual content. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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- Van der Bruggen B (2012) The separation power of nanotubes in membranes: a review. ISRN Nanotechnol 2012:1–17. doi:https://doi.org/10.5402/2012/693485 View ArticleGoogle Scholar
- Boudon J, Papa A, Paris J, Millot N (2014) Titanate nanotubes as a versatile platform for nanomedicine. In: Seifalian A, De Mel A, Kalaskar DM, editors. Nanomedicine. One Central Press, pp 403–428Google Scholar
- Ahn CH, Baek Y, Lee C et al (2012) Carbon nanotube-based membranes: fabrication and application to desalination. J Ind Eng Chem 18:1551–1559. doi:https://doi.org/10.1016/j.jiec.2012.04.005 View ArticleGoogle Scholar
- Dumee LF, Sears K, Schütz J et al (2010) Characterization and evaluation of carbon nanotube Bucky-Paper membranes for direct contact membrane distillation. J Memb Sci 351:36–43. doi:https://doi.org/10.1016/j.memsci.2010.01.025 View ArticleGoogle Scholar
- Adams R, Carson C, Ward J et al (2010) Metal organic framework mixed matrix membranes for gas separations. Microporous Mesoporous Mater 131:13–20. doi:https://doi.org/10.1016/j.micromeso.2009.11.035 View ArticleGoogle Scholar
- Kusworo TD, Johari S, Widiasa IN (2012) The uses of carbon nanotubes mixed matrix membranes (MMM) for biogas purification. Int J Waste Resour 2:5–10. doi:https://doi.org/10.12777/ijwr.v2i1.22 View ArticleGoogle Scholar
- Sears K, Dumée L, Schütz J et al (2010) Recent developments in carbon nanotube membranes for water purification and gas separation. Materials (Basel) 3:127–149. doi:https://doi.org/10.3390/ma3010127 View ArticleGoogle Scholar
- Rezakazemi M, Ebadi A (2014) Progress in polymer science state-of-the-art membrane based CO2 separation using mixed matrix membranes (MMMs): an overview on current status and future directions. Prog Polym Sci 39:817–861. doi:https://doi.org/10.1016/j.progpolymsci.2014.01.003 View ArticleGoogle Scholar
- Hashemifard SA, Ismail AF, Matsuura T (2011) Mixed matrix membrane incorporated with large pore size halloysite nanotubes (HNTs) as filler for gas separation: morphological diagram. Chem Eng J 172:581–590. doi:https://doi.org/10.1016/j.cej.2011.06.031 View ArticleGoogle Scholar
- Kusworo TD, Ismail AF, Widiasa IN, Johari S (2008) CO2 removal from biogas using carbon nanotubes mixed matrix membranes. In: Proc. Int. Grad. Eng. Sci. pp 1–7Google Scholar
- Wei Y, Shen L, Wang F et al (2011) Synthesis and characterization of novel nanocomposite membrane of sodium titanate/Nafion ®. Mater Lett 65:1684–1687. doi:https://doi.org/10.1016/j.matlet.2011.02.081 View ArticleGoogle Scholar
- Khan MM, Filiz V, Bengtson G et al (2012) Functionalized carbon nanotubes mixed matrix membranes of polymers of intrinsic microporosity for gas separation. Nanoscale Res Lett 7:1–12. doi:https://doi.org/10.1186/1556-276X-7-504 View ArticleGoogle Scholar
- Li Q, Zhang H, Tu Z et al (2012) Impregnation of amine-tailored titanate nanotubes in polymer electrolyte membranes. J Memb Sci 423–424:284–292, http://dx.doi.org/10.1016/j.memsci.2012.08.025 View ArticleGoogle Scholar
- Robeson LM (2008) The upper bound revisited. J Memb Sci 320:390–400. doi:https://doi.org/10.1016/j.memsci.2008.04.030 View ArticleGoogle Scholar
- Li P, Wang Z, Qiao Z et al (2015) Recent developments in membranes for efficient hydrogen purification. J Memb Sci 495:130–168. doi:https://doi.org/10.1016/j.memsci.2015.08.010 View ArticleGoogle Scholar
- Lin R, Ge L, Hou L et al (2014) Mixed matrix membranes with strengthened MOFs/polymer interfacial interaction and improved membrane performance. Appl Mater Interfaces 6:5609–5618. doi:https://doi.org/10.1021/am500081e View ArticleGoogle Scholar
- Slobodian P, Svoboda P, Kárászová M et al (2014) Carbon nanotube- and carbon fiber-reinforcement of ethylene-octene copolymer membranes for gas and vapor separation. Membranes (Basel) 4:20–39. doi:https://doi.org/10.3390/membranes4010020 View ArticleGoogle Scholar
- Mustafa A, Kusworo TD, Busairi A, Ismail AF (2012) Increasing the performance of PES-CNTs mixed matrix membrane using carbon nanotubes (CNTs) functionalization. Int J Waste Resour 2:22–24View ArticleGoogle Scholar
- Krishnamurthy G, Agarwal S (2013) Efficient synthesis of carbon nanotubes with improved surface area by low-temperature solvothermal route from dichlorobenzene. Chem Pap 67:1396–1403. doi:https://doi.org/10.2478/s11696-013-0397-6 View ArticleGoogle Scholar
- Wang S, Liu Y, Huang S et al (2014) Pebax-PEG-MWCNT hybrid membranes with enhanced CO2 capture properties. J Memb Sci 460:62–70. doi:https://doi.org/10.1016/j.memsci.2014.02.036 View ArticleGoogle Scholar
- Surya Murali R, Sridhar S, Sankarshana T, Ravikumar YVL (2010) Gas permeation behavior of pebax-1657 nanocomposite membrane incorporated with multiwalled carbon nanotubes. Ind Eng Chem Res 49:6530–6538. doi:https://doi.org/10.1021/ie9016495 View ArticleGoogle Scholar
- Rajabi Z, Moghadassi AR, Hosseini SM, Mohammadi M (2013) Preparation and characterization of polyvinylchloride based mixed matrix membrane filled with multi walled carbon nano tubes for carbon dioxide separation. J Ind Eng Chem 19:347–352. doi:https://doi.org/10.1016/j.jiec.2012.08.023 View ArticleGoogle Scholar
- Kim S, Pechar TW, Marand E (2006) Poly(imide siloxane) and carbon nanotube mixed matrix membranes for gas separation. Desalination 192:330–339. doi:https://doi.org/10.1016/j.desal.2005.03.098 View ArticleGoogle Scholar
- Li X, Ma L, Zhang H et al (2015) Synergistic effect of combining carbon nanotubes and graphene oxide ins mixed matrix membranes for efficient CO2 separation. J Memb Sci 479:1–10, http://dx.doi.org/10.1016/j.memsci.2015.01.014 View ArticleGoogle Scholar
- Cong H, Zhang J, Radosz M, Shen Y (2007) Carbon nanotube composite membranes of brominated poly(2,6-diphenyl-1,4-phenylene oxide) for gas separation. J Memb Sci 294:178–185. doi:https://doi.org/10.1016/j.memsci.2007.02.035 View ArticleGoogle Scholar
- Liu R, Yang W, Chueng H, Ren B (2015) Preparation and application of titanate nanotubes on dye degradation from aqueous media by UV irradiation. J Spectrosc. 1–9. doi: https://doi.org/10.1155/2015/680183
- Rodrigues CM, Ferreira OP, Alves OL (2010) Interaction of sodium titanate nanotubes with organic acids and base: chemical, structural and morphological stabilities. J Braz Chem Soc 21:1341–1348View ArticleGoogle Scholar
- Nada A, Moustafa Y, Hamdy A (2014) Improvement of titanium dioxide nanotubes through study washing effect on hydrothermal. Br J Environ Sci 2:29–40Google Scholar
- Králová D, Grinevich A, Šlouf M (2009) Preparation of titanate nanotubes and their surface modification by plasma polymerization. 3:2008–2009. doi: https://doi.org/10.3217/978-3-85125-062-6-559
- Giel V, Galajdová B, Popelková D et al (2015) Gas transport properties of novel mixed matrix membranes made of titanate nanotubes and PBI or PPO. Desalin Water Treat 56:3285–3293. doi:https://doi.org/10.1080/19443994.2014.981931 Google Scholar
- Yang T, Chung T-S (2013) High performance ZIF-8/PBI nano-composite membranes for high temperature hydrogen separation consisting of carbon monoxide and water vapor. Int J Hydrogen Energy 38:229–239. doi:https://doi.org/10.1016/j.ijhydene.2012.10.045 View ArticleGoogle Scholar
- Weng TH, Tseng HH, Wey MY (2011) Effect of SBA-15 texture on the gas separation characteristics of SBA-15/polymer multilayer mixed matrix membrane. J Memb Sci 369:550–559. doi:https://doi.org/10.1016/j.memsci.2010.12.039 View ArticleGoogle Scholar
- Zhuang GL, Tseng HH, Wey MY (2014) Preparation of PPO-silica mixed matrix membranes by in-situ sol-gel method for H2/CO2 separation. Int J Hydrogen Energy 39:17178–17190. doi:https://doi.org/10.1016/j.ijhydene.2014.08.050 View ArticleGoogle Scholar
- Sadeghi M, Semsarzadeh MA, Moadel H (2009) Enhancement of the gas separation properties of polybenzimidazole (PBI) membrane by incorporation of silica nano particles. J Memb Sci 331:21–30. doi:https://doi.org/10.1016/j.memsci.2008.12.073 View ArticleGoogle Scholar
- Shi C, Deng C, Zhang X, Yang P (2013) Synthesis of highly water-dispersible polydopamine-modified multiwalled carbon nanotubes for matrix-assisted laser desorption/ionization mass spectrometry analysis. ACS Appl Mater Interfaces 5:7770–7776. doi:https://doi.org/10.1021/am4024143 View ArticleGoogle Scholar
- Kim SW, Kim T, Kim YS et al (2012) Surface modifications for the effective dispersion of carbon nanotubes in solvents and polymers. Carbon N Y 50:3–33. doi:https://doi.org/10.1016/j.carbon.2011.08.011 View ArticleGoogle Scholar
- Jiang Y, Lu Y, Zhang L et al (2012) Preparation and characterization of silver nanoparticles immobilized on multi-walled carbon nanotubes by poly(dopamine) functionalization. J Nanoparticle Res 14:1–10. doi:https://doi.org/10.1007/s11051-012-0938-x Google Scholar
- Hebbar RS, Isloor AM, Ananda K, Ismail AF (2016) Fabrication of polydopamine functionalized halloysite nanotube/polyetherimide membranes for heavy metal removal. J Mater Chem A 4:1764–1774. doi:https://doi.org/10.1039/C5TA09281G View ArticleGoogle Scholar
- Wan Q, Tian J, Liu M et al (2015) Surface modification of carbon nanotubes via combination of mussel inspired chemistry and chain transfer free radical polymerization. Appl Surf Sci 346:335–341. doi:https://doi.org/10.1016/j.apsusc.2015.04.012 View ArticleGoogle Scholar
- Hu H, Yu B, Ye Q et al (2010) Modification of carbon nanotubes with a nanothin polydopamine layer and polydimethylamino-ethyl methacrylate brushes. Carbon N Y 48:2347–2353. doi:https://doi.org/10.1016/j.carbon.2010.03.014 View ArticleGoogle Scholar
- Rouquerolt J, Avnir D, Fairbridge CW et al (1994) Recommendations for the characterization of porous solids. Pure Appl Chem 66:1739–1758. doi:https://doi.org/10.1351/pac199466081739 Google Scholar
- Williams MMR (1977) The mathematics of diffusion. Ann Nucl Energy 4:205–206. doi:https://doi.org/10.1016/0306-4549(77)90072-X View ArticleGoogle Scholar
- Stuart B (2004) Infrared spectroscopy: fundamentals and applications. doi:https://doi.org/10.1002/0470011149 View ArticleGoogle Scholar
- Coates J (2000) Interpretation of infrared spectra, a practical approach interpretation of infrared spectra, a practical approach. In: Encycl. Anal. Chem. John Wiley & Sons Ltd, Chichester, pp 10815–10837Google Scholar
- Chung TS, Guo WF, Liu Y (2006) Enhanced Matrimid membranes for pervaporation by homogenous blends with polybenzimidazole (PBI). J Memb Sci 271:221–231. doi:https://doi.org/10.1016/j.memsci.2005.07.042 View ArticleGoogle Scholar
- Paul DR (1979) Gas sorption and transport in glassy polymers. Berichte der Bunsengesellschaft für Phys Chemie 83:294–302. doi:https://doi.org/10.1002/bbpc.19790830403 View ArticleGoogle Scholar
- Chowdhury G, Kruczek B, Matsuura T (2001) Polyphenylene oxide and modified polyphenylene oxide membranes: gas. Vapor and Liquid Separation. doi:https://doi.org/10.1007/978-1-4615-1483-1 Google Scholar
- Mittal V (2015) Manufacturing of nanocomposites with engineering plastics. doi:https://doi.org/10.1016/B978-1-78242-308-9.09993-6 Google Scholar
- Sharma A, Tripathi B, Vijay YK (2010) Dramatic improvement in properties of magnetically aligned CNT/polymer nanocomposites. J Memb Sci 361:89–95. doi:https://doi.org/10.1016/j.memsci.2010.06.005 View ArticleGoogle Scholar
- Jiang LY, Huang Y, Jiang H et al (2006) A cohesive law for carbon nanotube/polymer interfaces based on the van der Waals force. J Mech Phys Solids 54:2436–2452. doi:https://doi.org/10.1016/j.jmps.2006.04.009 View ArticleGoogle Scholar
- Mahajan R, Koros WJ (2002) Mixed matrix membrane materials with glassy polymers. Part 1. Polym Eng Sci 42:1420–1431View ArticleGoogle Scholar
- Mahajan R, Koros WJ (2000) Factors controlling successful formation of mixed-matrix gas separation materials. Ind Eng Chem Res 39:2692–2696. doi:https://doi.org/10.1021/ie990799r View ArticleGoogle Scholar
- Matteucci S, Yampolskii Y, Freeman BD, Pinnau I (2006) Transport of gases and vapors in glassy and rubbery polymers. In: Mater. Sci. Membr. Gas Vap. Sep. John Wiley & Sons, Ltd, Chichester, pp 1–47Google Scholar
- Tena A, Marcos-Fernández A, Lozano AE et al (2013) Thermally segregated copolymers with PPO blocks for nitrogen removal from natural gas. Ind Eng Chem Res 52:4312–4322. doi:https://doi.org/10.1021/ie303378k View ArticleGoogle Scholar
- Ismail AF, Khulbe K, Matsuura T (2015) Gas separation membranes—polymeric and inorganic. doi:https://doi.org/10.1007/978-3-319-01095-3 Google Scholar
- Khan MM, Filiz V, Bengtson G et al (2013) Enhanced gas permeability by fabricating mixed matrix membranes of functionalized multiwalled carbon nanotubes and polymers of intrinsic microporosity (PIM). J Memb Sci 436:109–120. doi:https://doi.org/10.1016/j.memsci.2013.02.032 View ArticleGoogle Scholar
- Ge L, Zhu Z, Rudolph V (2011) Enhanced gas permeability by fabricating functionalized multi-walled carbon nanotubes and polyethersulfone nanocomposite membrane. Sep Purif Technol 78:76–82. doi:https://doi.org/10.1016/j.seppur.2011.01.024 View ArticleGoogle Scholar
- Freeman BD, Carolina N (1999) Basis of permeability/selectivity tradeoff relations in polymeric gas separation membranes. Macromolecules 32:375–380. doi:https://doi.org/10.1021/ma9814548 View ArticleGoogle Scholar
- Paul D, Koros WJ (1976) Effect of partially immobilizing sorption on permeability and the diffusion time lag. J Polym Sci Polym Phys Ed 14:675–685. doi:https://doi.org/10.1002/pol.1976.180140409 View ArticleGoogle Scholar
- Wang R, Cao C, Chung T (2002) A critical review on diffusivity and the characterization of diffusivity of 6FDA – 6FpDA polyimide membranes for gas separation. J Memb Sci 198:259–271. doi:https://doi.org/10.1016/S0376-7388(01)00665-2 View ArticleGoogle Scholar
- Koros WJ, Chan AH, Paul DR (1977) Sorption and transport of various gases in polycarbonate. J Memb Sci 2:165–190, http://dx.doi.org/10.1016/S0376-7388(00)83242-1 View ArticleGoogle Scholar
- Kim S, Chen L, Johnson JK, Marand E (2007) Polysulfone and functionalized carbon nanotube mixed matrix membranes for gas separation: theory and experiment. J Memb Sci 294:147–158. doi:https://doi.org/10.1016/j.memsci.2007.02.028 View ArticleGoogle Scholar
- Lin R, Ge L, Liu S et al (2015) Mixed-matrix membranes with metal–organic framework-decorated CNT fillers for efficient CO2 separation. ACS Appl Mater Interfaces 7:14750–14757. doi:https://doi.org/10.1021/acsami.5b02680 View ArticleGoogle Scholar