3.1. Boehmite Characterization
3.1.1. XRD Analysis
Assisted with the XRD technique, we characterized the phase, purity, and crystallinity of the as-prepared boehmite samples synthesized by the hydrothermal method. The X-ray diffraction patterns of the boehmite sample at different times and temperatures of hydrothermal reaction are presented in Fig. 1.The boehmite crystal phase in the XRD analysis shows that all the diffraction peaks of the synthesized sample are orthorhombic and are in complete agreement with diffraction card no γ -AlOOH (JCPDS 21-1307). No peaks for other phases such as Al(OH)3 or Al2O3 were observed, indicating its high purity and crystallinity. The main peaks of γ-AlOOH in the XRD pattern (020, 120, 031, 131, 051, 220, 151, 080, 231, 171, and 251)[16] are narrowed implying that the γ-AlOOH nanostructures are well crystallized.
According to Fig. 1, it is clear that with increasing reaction time and temperature, samples tend to crystallite. More specifically, it can be observed that the intensity of the peak corresponding to the (020) crystal plane is extraordinarily strong as compared to other peaks. Therefore, it can be concluded that increasing the temperature to 200°C as well as high duration time at this temperature (24 hours) results in nanoparticles with high purity and high crystallinity.
3.1.2. FESEM and TEM analysis
Figures 2–5 show the FESEM images of the synthesized bohemian samples at different conditions mentioned in Table 1 with different magnifications (1µm, 500nm, and 300nm). The effect of time and temperature of hydrothermal reaction on the morphology of synthesized boehmite was also shown.
Figure 2 represents a synthesized boehmite at a temperature of 110 ° C for 24 hours. The reason for the synthesis of boehmite particles at this temperature is the reduction of initial production costs and ultimately the production cost. According to the images, it can be observed that the obtained sample is in the form of nanorod particles that have a very high aggregation and adhesion. High magnification images show that the synthesized samples have collapsed and become clumpy. The bright spots, which are seen as light bulbs, indicate the presence of impurities in the sample. According to the image with a magnification of 300 nm, it can be seen that the depression between the particles is such that the nanoparticles stick to each other and form a non-uniform plate.
Figure 3 shows the boehmite nanoparticles synthesized at 160oC for 18 hours. According to the images, it can be seen that the boehmite sample synthesized at this temperature still retained the rod-shaped structure, but the particle size became smaller and the nanorods did not accumulate as in Fig. 3, indicating an increase in efficiency at 160oC compared to 110 oC. Images at 300 nm magnification also show that the nanorods are stacked next to each other.
Figure 4 shows the FSTEM boehmite images synthesized at 200oC and 24 hours. According to the images, it can be seen that in this sample, nanoparticles are also in the form of rods. High magnification images show that the synthesized nanostructures are partly separate. The main reason is the appropriate temperature and time of the hydrothermal reaction. The 300 nm magnification image shows that the nanostructures in this sample have a lower length and diameter than the previously synthesized samples.
Figure 5 illustrated the FSTEM boehmite images synthesized at 200oC and 18 hours. When the time of hydrothermal reaction was decreased from 24 h to 18 h at a temperature of 200°C, the particle size becomes larger and the adhesion and aggregation of the particles increase. Therefore, according to XRD analysis and SEM images, it can be concluded that a temperature of 200 degrees and a time of 24 hours are the most suitable conditions for boehmite synthesis with high crystallinity and high purity.
Figure 6 demonstrated the TEM images of the samples synthesized at 200oC and 24 hours. The images at different magnifications show that the synthesized boehmite nanorods appear to separate well under the reported conditions.
3.2. Membrane Characterization
3.2.1. FTIR Analysis
FTIR spectra were used for characterizing the structure of the pure PUs, and PU-boehmite nanocomposite membranes. The FTIR results are shown in Figs. 7. As can be seen, the strong peak at 555 cm− 1 and 855cm− 1 which belongs to boehmite, is observed in all PU-boehmite membranes, confirming the presence of nanoparticles in hybrid membranes[17].
The absence of the NCO peak at 2270cm− 1 indicates the completion of the reaction. N–H stretching of urethanes and C = O peaks appears at around 3300cm− 1 and 1730–1670cm− 1, respectively. The peak of urethane ether linkage is at 1117–1100cm− 1.[9]. According to Figs. 7, by comparing the spectra of pure PU and PU-boehmite nanocomposite membranes, the effects of boehmite nanoparticles on the phase separation of hard and soft segments of prepared membranes, and possible interactions between boehmite and polyurethane membrane since changes in the hydrogen bonding character of polyurethane matrix by the incorporation of boehmite nanoparticles can be easily detected.
For this purpose, we focus on specific regions in Fig. 7, where strong absorptions were observed by the urethane group (specifically N-H and C = O absorption bands), and the ether group. Peak shifts and shape changes especially at the hydroxyl and amine, carbonyl, and ether regions, for the hard and soft segments are indications of an interaction between the boehmite and the PU membrane [6].
As shown in Fig. 7, No dramatic change in the peak shape or position of the absorption band at the 3300 cm− 1 (N-H) for PU-boehmite nanocomposite membrane containing 10% by weight of boehmite is seen. On the other hand, the width of the peak corresponding to N-H groups increases with an increasing amount of boehmite up to 20% by weight in the polymer. The OH groups on the boehmite surface may form a hydrogen bond with carbonyl groups or/ and N–H groups of urethane in the hard segment or ether groups in soft segments [6]. The broadening of the peaks is attributed to the overlap of N-H and OH frequencies and/or also due to the distribution of hydrogen bonds with urethane N–H groups by incorporation of a part of boehmite. [9, 18]. For a more accurate investigation of hydrogen bonding between hard and soft segments in these PU synthesized membranes, in the FT-IR spectra peaks of carbonyl groups were assessed. As seen in Fig. 7, the FTIR spectrum of pure and hybrid membranes shows a peak with a maximum at 1683 cm− 1 with a shoulder at 1726 cm− 1, which were attributed to the H-bonded and free urethane carbonyl bands [3]. As seen in Fig. 7, for PU the intensity of these two bonds is approximately equal. Moreover, with increasing boehmite nanoparticles to 10wt% the intensity of the hydrogen-bonded carbonyl bond increases dramatically. Finally, at 20 wt% the intensity of these two bonds does not much change compared to 10% by weight.
This increase in the peak intensity of bonded carbonyl group may be related to the hydrogen bond between boehmite particles and ether groups in soft segments of PU. As OH groups in boehmite interact with ether groups in the soft segments of polyol in polyurethane, the number of available ether groups of soft segments to create hydrogen bond with the N–H groups reduces, and hence urethane N–H groups interact more with carbonyl groups of hard segments of urethane. In this situation reduction in hydrogen bonding between ether groups of soft segments and urethane groups of hard segments result in intensification of bonded carbonyl peak and increasing microphase separation[4, 6, 7, 10].
The FTIR spectrum in the ether region is also shown in Fig. 7. As mentioned above, the band observed at 1117 cm− 1 in FTIR spectra of PU was attributed to the hydrogen bonding interaction between N-H and C-O-C groups [6]. As shown in Fig. 7, when the ether region of the FTIR spectra is investigated, the peaks at the ether region of the spectrum shifted to lower wavenumbers as a function of the amount of boehmite incorporation. This observation indicates with an increasing amount of boehmite nanoparticles in the PU, hydrogen bonding between the hydroxyl groups on the boehmite surface and the oxygen atoms in the ether linkages of the PTMG due to an increase in the surface contact area, and several hydroxyl groups exist formed [6].
It is well known that in PU, (N-H) groups form hydrogen bonds with both the carbonyl (C = O) of the hard segments and the oxygen (-O -) in the polyether soft segment[6]. Considering the FTIR results, it can be concluded that by increasing boehmite nanoparticles to 10wt%, nanoparticles tend to be more dispersed in the soft segments compared to hard segments of the polymer. Result in the replacement of hydrogen bonding between N-H groups and ether groups with the hydrogen bond between the boehmite’s OH groups and the ether groups of polyol in a soft segment of PU. Therefore, the amount of available ethereal pieces of soft segments groups to create hydrogen bond with the N–H groups reduces, and hence urethane N–H groups interact more with carbonyl groups of hard segments of urethane which lead to the intensification of peaks at region 1683 cm− 1 (bonded C = O) and shift peak to right at ethereal region(1107 cm− 1( (C-O-C)) in the FTIR spectra.
FTIR results were also indicated that with increasing the content of boehmite nanoparticles to 20wt%, a portion of the boehmite nanoparticles is distributed in soft segments of the polymer by the interaction of OH groups in boehmite with ether groups of polyol which lead to shifted the peak of the ethereal region to lower wavenumbers as a function of the amount of boehmite incorporation (1100 cm− 1). On the other hand, the other portion of the boehmite may interact with N-H groups of the hard segment of PU and disrupt the hydrogen bonding between urethane carbonyl groups and the urethane N–H groups in the hard segments resulting in the broadening of the N-H peak by adding 20wt% boehmite to PU.
3.2.2. DSC Analysis
Thermal analyses of the polyurethane membranes were performed by DSC measurements. The DSC thermograms of pure PU and PU-boehmite nanocomposite membranes are depicted in Fig. 8. The glass transition temperature (Tg) of PU is between − 79.81–76.49 ◦C which is related to PTMG, the soft segment in PU structure (Fig. 8). Higher Tg of PUs compared to pure PTMG (around − 88 ◦C) is ascribed to the phase mixing between the soft and hard segments[4].
The glass transition temperature is one of the most important criteria for comparing the chain mobility of the polymers. According to Fig .8, the Tg of hybrid membranes shows a slight increase with the increase of boehmite content. In comparison with PU, the Tg of PU + 10wt% boehmite nanoparticles, and PU + 20wt% boehmite nanoparticles shift from − 79.81 ◦C to -78.45 ◦C and − 77.49 ◦C. This should be attributed to the hydrogen bonding between boehmite nanoparticles and polyol chains; a decrease in the mobility of chains was expected. These interactions enhance the rigidity of the soft segments [19] and limit the movement or motions of the soft segments. Therefore, the introduction of nano-particles results in a slight increase in glass transition temperatures of the soft segments.
Figure 8 also shows an endothermic peak at the temperature of 10 − 35 ◦C that may be attributed to the crystals formed in the soft segments. Due to the higher molecular mobility of the soft segments, this Tm appeared at lower temperatures. The results indicated that by increasing the boehmite concentration, the crystalline peak of the soft segments appears at a higher temperature compared to the PU. This behavior is related to strong interactions and hydrogen bonding between the hydroxyl groups on the surface of particles and PU chains which may be restricting the mobility of PU chains. Therefore, it can be concluded that most boehmite nanoparticles are distributed in soft segments [18].
3.2.3. FESEM Analysis
Figure 9 shows the FESEM photographs of the PU and PU-boehmite composite membranes. To investigate the dispersion and compatibility between the nanoparticles and the polymer matrix cross-sectional micrographs of the polyurethane-boehmite hybrid membranes with 0, 10, and 20 wt% boehmite nanoparticles were used and presented in Fig. 9. As can be seen, the membranes are quite dense and the presence and distribution of boehmite nanoparticles in prepared membranes are evident. The approximate average size of boehmite nanoparticles in the membrane is also estimated at 50 nm. As demonstrated in the FESEM images, there are two types of dispersion of boehmite nanoparticles in the polymers. There are some particles with no aggregation which indicates effective nanoscale mixing and homogeneous dispersion into the polymer matrix. Some of the particles, however, aggregated together to form larger particles within the polymer. As shown in the FESEM images most of the aggregated particles are smaller than 180 nm in size. Moreover, aggregation and roughness increase with the increasing percentage of boehmite nanoparticles in PU membranes.
3.3. Gas permeation performance
The permeability of nitrogen, oxygen, methane, and carbon dioxide gases and the ideal selectivity of gas pairs in PU and PU-boehmite nanocomposite membranes were measured at a pressure of 10 bar and a temperature of 30 ° C, the results of which are shown in Table 2.
Table 2
O2, N2, CO2, and CH4 permeability and ideal selectivity of membranes at 8 bar and 30 oC.
Sample | Permeability (barrer) | Selectivity |
CO2 | O2 | CH4 | N2 | O2/N2 | CO2/N2 | CO2/CH4 |
PU | 130.9 | 13.3 | 21.8 | 7.96 | 1.67 | 16.45 | 6.005 |
PU + 5wt% boehmite | 127 | 12.03 | 19.84 | 6.99 | 1.72 | 18.17 | 6.40 |
PU + 10wt% boehmite | 124 | 8.94 | 16.65 | 5 | 1.78 | 24.8 | 7.45 |
PU + 15wt% boehmite | 114.9 | 8.91 | 14 | 4.45 | 2.002 | 25.82 | 8.21 |
PU + 20wt% boehmite | 102 | 8.1 | 10.93 | 3.75 | 2.16 | 27.2 | 9.33 |
The order of gas permeability in all samples is as follows: P(CO2) > > P(CH4) > P(O2) > P(N2). The high permeation rate of carbon dioxide in comparison with other noted gases in polyurethanes is due to the high solubility of carbon dioxide in polyurethane [2, 8]. In polyurethanes, due to the presence of C-O-C polar ethereal groups in the soft segment, they will provide suitable sites for the adsorption of polar permeate CO2 molecules in the PU and increase its permeability in the PU membrane[7].
In addition, carbon dioxide has a smaller kinetic diameter and higher condensability than the other studied gases, which facilitate the penetration of this gas into the membrane. Table 3 shows the condensability and kinetic diameter of pure gases [20]. As shown in Table 3, the kinetic diameter of methane is greater than nitrogen and oxygen molecules [10], but the permeability reported in Table 2 indicates more permeation of methane in comparison with oxygen and nitrogen. This difference can be explained by that the solution mechanism is predominant in the permeability of gases in polyurethane membranes [20].
Table 3
Physical properties of studied gases [10]
Gas | Kinetic Diameter(oA) | Condensability (K) |
CO2 | 3.3 | 195 |
O2 | 3.46 | 107 |
CH4 | 3.8 | 149 |
N2 | 3.64 | 71 |
These synthesized PU nanocomposite membranes in these studies exhibit typical rubbery polymer properties. Solubility is the dominant mechanism in the permeation of gases through a rubbery polymer matrix; consequently, the permeability of these polymers is solubility controlled. The solubility is dependent on the condensability of the permeate gases within the membrane, and the condensability itself is related to the critical temperature of the gases [21]. In other words, solubility coefficients of gases well correlate with their critical temperature. As predicted by the critical temperature of oxygen, nitrogen, and methane (TC, CH4 = 190.6 K, TC, N2 = 126.2 K, and TC, O2 = 154.6 K), PU and PU- boehmite are always more permeable to methane than to oxygen and nitrogen. Therefore, methane with the highest condensability would be expected to have the highest permeability, and N2 with the lowest condensability would be expected to have the lowest permeability.
In PU membranes, soft segment domains are formed as a result of microphase separation. Soft and flexible polyol segments of PU alone have the necessary flexibility and movement to create space for the movement of gas molecules and are permeable to gases, whereas the hard segment domains act as an impermeable barrier [22]. In the case of PU-boehmite nanocomposite membranes, there are two hard and soft segment regions for the distribution of boehmite nanoparticles. From an entropic point of view, it appears that boehmite nanoparticles prefer to distribute in the hard segment of PU [23]. However, FTIR and DSC results indicated that the particles were mostly located in the soft regions. The results of gas permeation tests (Table 2) also show that the gas permeability of composite membranes decreases as the amount of boehmite nanoparticles increases. Based on the above-mentioned results, it can be concluded that the greatest number of boehmite nanoparticles are distributed in the soft phase. So, gas permeability reduction could be attributed to two factors: the presence of additional dense particles such as boehmite nanoparticles decrease the free volume, restrain the polymer chain movements, and makes the diffusion path a tortuous one for gas molecules to pass through the membrane. The second is the decrease in free volume, which reduces the sorption and diffusion of penetrants in the polymer matrix [24]. The gas permeabilities of carbon dioxide, methane, oxygen, and nitrogen in the pure polymer decrease from 130.9, 21.8, 13.3, and 7.96 barrer to the amount of 102, 10.93, 8.1, and 3.75 barrer in the PU-boehmite (20wt%) membrane, respectively. Calculated reduction values of gas permeability of prepared nanocomposite membranes concerning pure PU are in the following order:
CO2 (22.08%) < O2 (39.1%) < CH4 (49.8%) < N2 (52.8%)
As mentioned earlier, as the free volume in a polymer decreases, larger gas molecules will be more restricted than smaller ones from passing through the polymer thickness, resulting in a further reduction in their permeability [22]. Therefore, the higher reduction of N2 permeability in comparison with CO2 and O2 is due to its large molecular size, and because of the presence of boehmite nanoparticles, is acceptable. On contrary, less reduction in permeability of CH4 with a large molecular size than N2 was observed which may be attributed to the higher condensability of CH4 in the membranes.
The presence of boehmite nanoparticles in the polymer interface due to its non-organic
nature would create some special spaces in the interface of organic polymer and non-organic particles such as voids, or a new phase with different morphology, which could offer suitable locations for the dissolution of condensable gases.
Also, FTIR spectra of polyurethane–boehmite samples showed that in addition to the introduction of boehmite nanoparticles, some OH groups will enter the membrane. The presences of these groups in polymer structure provide appropriate positions for the dissolution of gases. Thus, by increasing the content of boehmite in polymer, the solution of condensable methane gas in the polymer will increase, and despite its larger molecular size than N2, its permeability decreases slower.
Regarding CO2, its lower reduction of permeability with increasing boehmite content compared to other gases results from both its small molecular size and its greater solubility in the membrane, because of the increased polar groups in the polymer with increasing boehmite content. Good interaction between polar CO2 and polar OH groups in boehmite increases the dissolution of CO2 in the polymer.
Table 2 also shows the CO2/N2, CO2/ CH4, and O2/N2 ideal gas selectivity of PU-boehmite nanocomposite membranes. According to Table 2, increasing the number of boehmite nanoparticles in membranes up to 20wt% increases the selectivity of CO2/N2, CO2/CH4, and O2/N2 gases from 16.45 to 27.2, 6.005 to 9.33, and 1.67 to 2.16, respectively. The selectivities of all pair gases improve by increasing the number of boehmite nanoparticles in the membranes.
The order of increment in gas selectivity of the mentioned nanocomposite membranes by addition of boehmite nanoparticles is as follows:
CO2/N2 (39.53%) > CO2/CH4 (35.64%) > O2/N2 (22.64%)
Considering the solution-diffusion mechanism, the selectivity of gases in polymers is specified by diffusivity and solubility selectivity which enables the polymer chains to separate small molecules from large ones, condensable molecules from non-condensable ones, and polar molecules from non-polar ones [25].
By the addition of the boehmite nanoparticles into the membranes, as mentioned before, most of the particles may disperse in the soft segments. Therefore, the presence of the particles should affect the mobility of the polymer chains in soft segments and also may reduce the free spaces trapped in the soft segments. These two events would increase the molecular sieving property of the composite membrane. Result in, the permeation of larger molecular size gases is more restricted than that of small ones, except when good interactions between the nanoparticles and the polymer chains would be available. The –OH groups on the boehmite surface make good interactions with the polymer chains, so they offer molecular screening properties for the prepared nanocomposite membranes by increasing their boehmite content. In the case of O2/N2 gases, because the condensability of these two gases is very low and not very different also, they have no special interaction with the polymer matrix, the molecular size difference plays an important role. Therefore, the improvement of diffusivity-selectivity due to better molecular sieve property of nanocomposite membranes, cause the selectivity increment, and finally, the total selectivity increment is not very high.
Since the pair gases of CO2/N2 and CO2/CH4 experience high diffusivity selectivity, the high affinity of CO2 with ether groups of the soft segments and the OH groups on the boehmite enhances the solubility of this gas in the membranes; thus, the solubility selectivity will significantly improve. Finally, the increase of CO2/N2 selectivity in comparison with CO2/CH4 is attributed to the very low condensability of nitrogen in comparison with methane. As mentioned, the suitable sorption sites on polymer–boehmite interface offer more solubility of condensable gases and solution of methane due to its condensable nature which is significantly more than that of nitrogen. Therefore, despite the higher molecular size of the methane which could offer higher diffusivity selectivity of CO2/CH4 in comparison with CO2/N2, domination of the solution mechanism causes the high solution of methane and lowers the solubility selectivity.
Even though membrane separation is in general a very attractive technology, it shows some limitations for the gas separation application. It has to be considered that an efficient separation process needs polymers with higher permeability to lead to higher productivity and lowers capital cost due to minimum membrane area, and higher selectivity affords more efficient separations in a lesser number of stages, higher purity of end product, and lower power costs.
Unfortunately, the inherent trade-off between permeability and selectivity demonstrated by Robeson in 1991 and 2008 remains to be the biggest challenge in the development of polymeric membranes. Polymers with high permeability will exhibit low selectivity and vice versa. In Fig. 10, the results obtained from prepared PU-boehmite membranes were compared. Robeson’s upper bound line [26]. As shown in this Fig. 10 the prepared membrane series investigated shows good separation ability, high permeation, and selectivity for CO2/N2. All of the synthesized membranes lie close to the upper bond line. The good separation ability of these membranes indicates the high potential of these membranes for industrial applications.