Membrane separation is one of the most effective separation technologies in terms of chemical engineering. Transmembrane difference inconcentration or chemical potential drives the separation, and this method basically does not require additional energies, such as heating/cooling. As represented by reverse osmosis membranes for water purification, various membrane separation technologies have been established and commercialized.
In gas separation by membranes, no phase change in the separation results in lower energy consumption, which can be the biggest advantage in comparison to other technologies, such as cryogenic separation processes. In addition, smaller foot print, instant operation, and lower maintenance cost due to no moving parts are beneficial for industrial applications. Membranes for air separation to enrich N2, H2 removal from NH3 or hydrocarbons, vapor recovery, and acid gas removal from natural gas have been currently industrialized. Especially, cellulose acetate membranes has been used for three decades for the natural gas sweetening, in which CO2 is separated over CH4 .
CO2 separation membranes are current topic to capture CO2 at the mass emission sources such as coal-fired plants and steel works in CO2 Capture and Storage (CCS) . CCS was approved by one of the effective options to mitigate carbon emission by the Paris Agreement in 2015. Cost reduction is the key for implementation of CCS, and the capture dominates major part of the cost. Liquid amine scrubbing has been studied to capture CO2  and used in the CCS demonstration projects. After CO2 in the source gas is separated by aqueous amine in an absorber, it is regenerated by heating in a desorber. The heating process results in large energy consumption even at thermal power stations. As described above, membrane separation could reduce the required energy to capture CO2 and hold potential to be an effective alternate capture technology.
A number of CO2 separation membranes have been developed [4, 5]. Among them, polymeric membranes would be suitable for the CO2 capture due to synthetic and module preparation feasibility, large-scale production, and much lower cost in comparison to inorganic membranes. For membrane separation, both of high flux/permeability and selectivity are required, although they show a trade-off relation . For example, thermally-rearranged polymers were recently developed and exhibited excellent CO2 separation performance over N2 and CH4 . The CO2 permeability exceeded over 1,000 barrer (1 barrer: 10−10cm3(STP)cm/(cm2⋅s⋅cmHg)). However, the CO2 selectivity was moderate or lower than 50. According to the report, the selectivity should be greater than 50 for post-combustion CO2 capture by membranes . In this research group, hyper-branched polymers with dense POE comb graft have been developed, and the polymeric membranes showed high CO2 separation properties over N2 [9, 10]. While the polymers formed a self-standing membrane with thickness above 80 μm by a solution casting method, they required a support when reducing the thickness below 80 μm to increase gas transport properties. The polymer thin film layer was thus formed on a commercial PDMS support (thickness: 60 μm) by spray-coating, the resulting TFC membranes showed CO2 permeability coefficient of 1,470 barrer with the selectivity of 25, where the thickness of the CO2-selective polymer layer was 15 μm . As well as the other CO2 separation membranes, the low selectivity limits the use for CO2 capture. Thus, improving the CO2 selectivity has been a challenge as well as elevating CO2 permeability.
For polymeric or dense membranes, gas permeability can be explained by multiplication of solubility and diffusivity, while the selectivity is given by the ratio of permeabilities. To improve CO2 solubility to the membranes, amines have been used [11, 12, 13, 14, 15]. Here, amines act as a CO2 carrier to facilitate the transportation through the membrane. In this research, poly(amidoamine) (PAMAM) was introduced to improve CO2 selectivity of the polymeric membranes from hyper-branched polymers. Incorporation of the amine to the polymeric membranes has been extensively investigated, and the resulting membranes displayed excellent CO2 separation performance especially over H2 . The preferential CO2 permeation was elucidated and explained by a CO2-selective molecular gate mechanism . Dissolved CO2 in the membranes interacted to the primary amines of PAMAM with the formation of quasi-crosslinks of PAMAM, which suppressed the permeation of other gaseous species, and as a result, the PAMAM containing membranes displayed quite high CO2 selectivity. Effect of PAMAM introduction on the CO2 separation performance was studied, and interplay between the membrane morphology and the gas transport properties were also investigated in this paper.
Monomers, methyl methacrylate (MMA), 4-chloromethyl styrene (Cl-St), POE methacrylate (POEM, Mn: 500, average oxyethylene unit: 9), ethyl 2-bromoisobutyrate (EBI), and PAMAM (20 wt% solution in methanol) were obtained from Sigma-Aldrich (MO, USA). A radical scavenger, dibutylhydroxytoluene, in the monomers was removed by passing through a basic alumina column (MP Biomedicals, Eschwege, Germany). Copper(I) chloride (CuCl), 2,2′-azobisisobutyronitrile (AIBN), and 4,4′-dimethyl-2,2′-dipyridyl (bpy) were purchased from Wako (Tokyo, Japan). AIBN was purified by recrystallization from methanol to obtain needle-like crystals before use. Other organic and inorganic chemicals were reagent grade and used without further purifications. A PDMS sheet with 60 μm thickness was available from As One (Cat.# 3-345-095, Osaka, Japan) and used as a high gas permeable support for TFC membranes.
2.2 Synthesis of macroinitiator
The hyper-branched polymers were developed using the same procedures as previously reported [9, 10]. PMMA backbone as a macroinitiator was first prepared by free radical polymerization of MMA and Cl-St. In brief, 40 mL of MMA (380 mmol) and 0.54 mL of Cl-St (3.8 mmol) were dissolved in 200 mL of toluene, and the solution was applied onto a basic alumina column to remove the radical inhibitor. A 342 mg of AIBN (2.1 mmol) was added to the solution, where the monomer to initiator ratio was 90. After N2 bubbling for 30 min at room temperature, the reaction vessel was tightly sealed and kept at 70 ° C for 1 h under stirring. The obtained macroinitiator was recovered by reprecipitation in methanol. The chemical structure and Cl-St content were determined by 1H NMR in CDCl3 on a JEOL JNM-EX270 FT (Tokyo, Japan). The yield and Cl-St content were 74.7 % and 0.78 mol%, respectively. Cl-St was statistically distributed along the polymer chain, when the monomer inclusion was less than 2 mol% . Molecular weight and the polydispersity of the macroinitiators were measured by size exclusion chromatography (SEC). The SEC system consisted of a Hitachi L-6000 pump (Hitachi, Tokyo, Japan) and a RI-2031 plus intelligent RI detector (JASCO, Tokyo, Japan). AKD-805 (Shodex, Tokyo, Japan) was used in a 860-CO column oven (JASCO). Tetrahydrofuran was used as an eluent at a flow rate of 0.8 mL/min and at 40° C, and the sample concentration was ca. 10 mg/mL. PMMA standards (Sigma-Aldrich) were used for molecular weight calibration. With the polymerization conditions, the macroinitiators had the weight-average molecular weight (Mw) of > 50 kDa with polydispersity of 1.7-2.3 . δH (270 MHz; CDCl3): 7.4-6.6 (Ar), 4.54 (ClCH2), 3.60 (OCH3), 2.1-0.8 ppm (CCH3 and polymer backbone)
2.3 Synthesis of hyper-branched polymer
Dense POE comb was propagated from Cl moiety of the polymer backbone by atom transfer radical polymerization (ATRP) of POE methacrylate (POEM). ATRP is one of facile living polymerization techniques of various allyl monomers to obtain polymers with low molecular weight dispersity and can control chain length of polymers or degree of polymerization of monomers . Hyper-branched polymers with well-defined chemical structure were thus obtained. For example, POEM (12.3 mL, 27.1 mmol) was dissolved in 30 mL of N-methylpyrrolidone (NMP) and the solution was passed through the basic alumina column. The macroinitiator above synthesized (3.0 g, 0.31 meq. of Cl-moiety), CuCl (0.31 mmol), and bpy (0.62 mmol) were added to the POEM solution. The obtained reaction suspension was kept at 90° C under stirring after N2 bubbling for 30 min at room temperature to remove O2. After 48 h, polymeric fraction was collected by reprecipitation in petroleum ether/ethanol solution (5/1 by vol.) with trace HCl, and then re-dissolved in toluene. The transition metal on the active chain end was removed by silica gel chromatography. Finally, the hyper-branched polymer, PMMA-g-PPOEM, was obtained. The chemical structure and POEM introduction were confirmed by 1H NMR in CDCl3. The yield and POEM fraction were 61.9 % and 69.9 wt%, respectively.
δH (600 MHz; CDCl3): 7.3-6.9 (Ar), 4.08 (COOCH2CH2), 3.8-3.5 (COOCH3 and CH2CH2O), 3.38 (CH2OCH3), 2.1-0.6 ppm (Ar-CH2CH2, CCH3 and polymer backbone)
2.4 Preparation of TFC membrane
TFC membranes were fabricated by spray-coating of hyper-branched polymer and PAMAM solution onto a commercial PDMS support to form a CO2-selective layer by the previously reported manner . A 0.3 g of hyper-branched polymer (POEM: 69.9 wt%) was dissolved in acetone (7.0 mL) under ultrasonic irradiation in a water bath at 100 W and 37 Hz for 10 min (Sharp UT106, Osaka, Japan). A 1.5 mL of PAMAM solution (240 mg) was added to the resulting polymer solution, where the PAMAM fraction was 44 wt% in the solutes. The obtained solution was filtered with a membrane filter (nominal pore size: 0.2 μm) to remove dust particles and sprayed several times with an air gun-spray on the support. The obtained TFC membranes were dried under vacuum. The TFC membranes with various PAMAM contents were obtained, and the thickness of the active layer was 10-60 μm measured on a Mitutoyo digimatic micrometer (Tokyo, Japan).
2.5 Gas permeability measurement
Gas permeation properties of the composite membranes were examined by a vacuum time-lag method on a Tsukuba Rika Seiki K-315N-02 (Tokyo, Japan) at 1 atm and 35 ° C as shown in Figure 1, where the effective membrane area A was 18.86 cm2, the volume of reservoirs V0 (upper) and V1 (lower) were 22.87 and 22.93 cm3, respectively. After evacuating the membranes in a cell at 35° C for at least 25 min to remove residual gaseous species, the target gas was charged in the upper reservoir at 1 atm and 35° C, and pressure increase dp2/dt(cmHg/s) in the lower reservoir was then monitored as a function of time. The gas permeability P(cm3(STP)cm/(cm2 s cmHg) is calculated from the following equation (Equation 1), and expressed in barrer. Here, T1 and T2 were 35° C, the thickness of the TFC membranes are l cm (l = lcom), and the dp2(blank)/dt was obtained from a leak test prior to gas permeation experiments. (1)
A linear relationship between p2 and t can be found at a steady state, and the gas permeability coefficient P is determined from the slope. The permeability coefficient of the support alone was experimentally determined and 2,870 barrer for CO2 and 284 barrer for N2 under the operation conditions. The permeability coefficient of the selective layer was then calculated by using a series resistance model as follows (Equation 2) [18, 19]. (2)
where Pcomp, PPDMS, and Psel, are permeability coefficients of the composite membranes, the PDMS support, and the CO2-selective layer, respectively. Accordingly, lcomp, lPDMS, and lsel are the thicknesses of the composite membrane, PDMS, and selective layer, respectively.
In this vacuum time-lag method, the apparent diffusion coefficient Dapp will be determined from the time-lag θ, which is the period of time to reach the steady state and the intercept in x-axis of the plot p vs. t, with the following equation (Equation 3) . However, the difference in time-lag θ could not be determined precisely due to the thinner active layer thickness on the gas permeation system used. (3)
Gas permeance Q is defined as P divided by l (Equation 4) and expressed in GPU (1 GPU = 10−6 cm3(STP)/(cm2⋅s⋅cmHg). The ideal separation factor of a membrane for gas A over gas B α(A/B) is given by the ratio of the permeability coefficients (or permeances) as shown in Equation 5. (4) (5)
2.6 Membrane characterizations
Thermal properties of the polymeric materials were studied by differential scanning calorimetry (DSC) on a Netzsch DSC 204 F1 Phoenix (Kanagawa, Japan). The samples were first cooled down to −100° C and equilibrated, and then the DSC spectra were recorded with a heating rate of 10° C/min from −100 to 100°C under N2 atmosphere. The surface observation of polymeric films was carried out on an Olympus OLS4000 3D laser measuring microscope.
3 Results and discussion
3.1 Preparation of hyper-branched polymers
Hyper-branched polymers of PMMA-g-PPOEM (PPOEM: polyPOEM) were prepared by two-step polymerization as shown in Figure 2. PMMA macroinitiators were developed by free radical polymerization of MMA and Cl-St. Cl-St was statistically introduced into PMMA when the content was less than 2 mol% . This means that average distance between Cl moieties is controlled, and the results were listed in Table 1. The average Cl moiety introduction was between 1.02 to 1.42 % of the monomer unit, and the yield was 61-75 % after twice purification process by reprecipitation.
Then, the POEM propagation was initiated from the Cl moiety on the PMMA macroinitiator to form dense POE comb structure. POEs with molecular weight greater than 2,000 show crystallinity, which often suppress gas permeation . Herein, the average oxyethylene unit of POEM was 9, and the POE is not considered crystalline. The degree of polymerization of POEM or the POE comb length was also readily controlled by ATRP. These synthetic procedures allowed preparing hyper-branched polymers with well-defined chemical structures. The results of PMMA-g-PPOEM synthesis were summarized in Table 2. PMMA-g-PPOEMs with three different POEM contents were successfully synthesized. The POEM fraction was from 34.4 to 69.9, where the average degree of polymerization DP of POEM was 8.1 to 45.6. The obtained hyper-branched polymers were collected by reprecipitation in petroleum ether/ethanol solution (5/1 by vol.). Although the mixture was the best poor solvent among examined for the hyper-branched polymers, POEM was not completely precipitated in the solution. Thus, the yield decreased from 78 to 62 % with increasing POEM fraction introduced in the product.
3.2 Membrane formulation and morphology
The hyper-branched polymers bearing dense POE comb show preferential CO2 permeation over N2 [9, 10], and thus would use for post-combustion CO2 capture. However, the CO2 permeability has to be improved for the use. One of the plausible approaches is reducing the membrane thickness. Although a self-standing membrane of the hyper-branched polymers with the thickness of > 80 μm was readily available by casting the polymer solution, they require a support to be handled when reducing the membrane below the thickness . The polymer solutions were spray-coated on a PDMS support to form a thin CO2-selective layer, and the thickness of the active layer of the resulting TFC membranes was 10-60 μm.
With PAMAM introduced membranes, incompatibility between POE and PAMAM resulted in macrophase separation [22, 23], and such membrane morphology would be a factor to characterize the gas transport properties. Herein, the same amount of the hyper-branched polymer was used at each sample. The phase separation was investigated by DSC and laser microscope. Figure 3 displays the DSC thermograms of PMMA-g-PPOEM (POE: 60 wt%) and mixture of the polymer and PAMAM. PAMAM-g-PPOEM(PAMAM: 0 wt%, a) showed slightthermal transition at -60 ° C, which was corresponded to the glass transition of POE. On the other hand, PAMAM containing polymers (PAMAM: 50 wt%, c) exhibited much larger transition at around -45 ° C, which would indicate melting of the amine. This result suggested the presence of PAMAM-rich phase upon macrophase separation between the polymer matrix and PAMAM. On the contrary, the polymeric membrane with lower PAMAM inclusion (PAMAM: 40 wt%, b) did not show such apparent thermal transition, which implied that some portion of PAMAM became miscible to the polymer at the lower PAMAM inclusion. A faint transition at -25 ° C may result from phase separation, although it has not been clarified.
Surface morphology of the polymeric membranes was studied by laser microscope to see the phase separationon the membrane surface as depicted in Figure 4. The surface of the lower PAMAM introduction (40 wt%) membrane was smoother than that of the higher amine loading membrane (50 wt%). With the higher amine loading membrane, bright and dark regions were more clearly confirmed on the membrane surface. The image in Figure 4b indicated the presence of PAMAM-rich domain upon macrophase separation, which was induced by the 10 wt% difference in the amine fraction. This was well consistent with that determined by DSC.
3.3 CO2 separation properties
The gas permeation performance of the TFC membranes was determined by a vacuum time-lag method. This strategy is often used to examine gas diffusivity from the time-lag until gas permeation reaches a steady state, in which pressure increase per unit time is constant . However, in this research, the thickness of the CO2-selective layer of the TFC membranes was too small to detect the time-lag precisely, and apparent diffusivity Dapp could not be determined by Equation 3. The gas transport properties were thus expressed only with permeability coefficient P. The gas permeation properties of various TFC membranes were summarized in Table 3, and effect of membrane thickness on the CO2 separation performance was drawn in Figure 5.
The CO2 permeability of both the TFC membranes and the active layers was depended on the corresponding thicknesses, and thinner membranes seemed to give higher gas transport properties as shown in Figure 5. With higher POEM fraction (43.0 and 69.9 wt%), the CO2 permeability coefficient was greater than that of the membrane with lower POEM fraction (34.4 wt%). POEM is CO2-philic, and a discontinuous increase in CO2 permeability was confirmed at the POEM fraction of 40 wt% in our previous work [9, 10]. Above the POEM fraction, POE-rich phase was formed upon microphase separation between POE and the rest of the polymeric fraction, which would enhance the CO2 solubility to provide the higher gas permeability.
In the case of the hyper-branched polymers with POEM fraction of 69.9 wt%, the TFC membrane with the thickness lcomp of 86 μm and PAMAM content of 20.0 wt% exhibited the highest CO2 permeability Pcomp (CO2) of 454 barrer, and the CO2 selectivity over N2 αcomp(CO2/N2) was 69.1. On the other hand, the highest CO2 permeability coefficient of the active layer Psel(CO2) was 176 barrer (thickness: 37 μm and PAMAM content: 13.3 wt%) with 80.4 in the selectivity αsel(CO2/N2). The obtained results were higher than those of the polymeric membranes without PAMAM incorporation.
Effect of PAMAM inclusion on the gas transport properties was thendisplayed in Figure 6. With both of the TFC membranes and the CO2-selective layer, PAMAM introduction successfully contributed to enhance the CO2 selectivity over N2, in which the amine worked as a CO2 carrier in facilitated transportation manner [24, 25]. However, the amine loading most likely suppressed the gas permeation in these experimental conditions. The increased CO2 selectivity was resulted from slight decrease in the CO2 permeability relative to large drop in the N2 permeability with increase of the amine content in the CO2-selective layer. Similar results of PAMAM containing membranes have been reported in the mixed gas experiment for CO2 separation over H2 [16, 17]. When the amine was immobilized in a crosslinked PEG network, the CO2 permeability of the resulting polymeric membranes did not change much with increase of the amine loading. On the contrary, the H2 permeability significantly dropped at higher amine loading (> 40 wt%) to give quite high CO2 selectivity over H2 greater than 1,000 at 5 kPa of CO2 partial pressure and 80 % relative humidity. As well as H2, N2 is non-polar and would suffer from salting out effect by the polar amine at the higher amine loading, and as a result the solubility of N2 would decrease sharply in comparison to that of CO2 with increase of PAMAM content. Mechanism of preferential CO2 permeation was elucidated in the CO2 separation. PAMAM containing membranes got swollen under humidity, and a part of CO2 turned to bicarbonate in the membrane with drastic enhancement of the CO2 permeability. Bicarbonate formed under humidity was thus the major migrating species through the membrane . However, the gas permeation was studied under dry conditions in this report, and CO2 would move as a carbamate form from feed to permeate side in the dry conditions. Although the CO2 solubility was improved by the addition of PAMAM, migration of carbamate through PAMAM network was much lower than that of bicarbonate formed under humidity, and thus the CO2 permeability was not increased much as expected. However, the CO2 selectivity over N2 could be enhanced by the PAMAM loading, and some membranes exhibited the selectivity over 350 with higher CO2 permeability than those without the amine.
Here, kinetic diameters of CO2 and N2 are 3.3 and 3.6 Å, respectively, thus the diffusion selectivity D(CO2)/D(N2) becomes greater than 1. In addition, POE of the hyper-branched polymers gives rise to CO2 solubility, which contributes the solution selectivity S(CO2)/S(N2) > 1. As a result, the TFC membranes showed preferential CO2 permeation over N2. To enhance the CO2 permeation selectivity, PAMAM was incorporated into the CO2-selective layer. As a result, the selectivity was successfully elevated due to further increase of the solution selectivity S(CO2)/S(N2).
A polymeric membrane from PMMA-g-PPOEM hyper-branched polymers showed preferential CO2 permeation over N2, and the CO2 permeability was increased by elevating the POE fraction and reducing the membrane thickness. In comparison to the high CO2 permeability, the CO2 selectivity over N2 was moderate (< 50) and not high enough for effective CO2 capture, such as at thermal power stations. The CO2 selectivity was improved by the addition of PAMAM. The amine loading contributed to increase the CO2 solubility to the polymeric membranes. The improved CO2 separation performance of the TFC membranes and/or the CO2-selective layer was strongly depended on the amine content. When the content exceeded 50 wt%, the significant increase of the CO2 selectivity was found and reached 350 at the operation conditions. At the higher loading, low compatibility between the amine and the hyper-branched polymers induced phase separation to provide PAMAM-rich phase, in which CO2 migrated preferentially, determined by DSC and laser microscope. In other words, formation of the amine-rich phase would be a key for elevating the CO2 selectivity. The CO2 permeability was not enhanced by the amine introduction, but would be done by reducing the thickness of the active layer as reported . The obtained results would be beneficial to develop CO2 separation materials.
The authors acknowledge Prof. Anne M. Mayes of Massachusetts Institute of Technology for invaluable support to develop the hyper-branched polymers. This research was partially supported by Grant-in-Aid for Scientific Research (C) Grant Number JP17899334 and the Advanced Carbon Technology Research and Development Program from Japan Science and Technology Agency. The International Institute for Carbon-Neutral Energy Research (WPI-I2CNER) is supported by the World Premier International Research Center Initiative (WPI), MEXT, Japan.
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About the article
Published Online: 2017-11-10
Citation Information: Open Physics, Volume 15, Issue 1, Pages 662–670, ISSN (Online) 2391-5471, DOI: https://doi.org/10.1515/phys-2017-0077.
© 2017 Ikuo Taniguchi et al.. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 License. BY-NC-ND 4.0