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Gas separation properties and morphology of asymmetric hollow fiber membranes made from cardo polyamide
Journal of Membrane Science 243 (2004) 59–68
Gas separation properties and morphology of asymmetric hollow fiber membranes made from cardo polyamide Shingo Kazama a,∗ , Masao Sakashita b a
Research Institute of Innovative Technology for the Earth, 9-2 Kizugawadai, Kizu-cho, Soraku-gun, Kyoto 619-0292, Japan b Japan Technical Information Service, Kojimachi 1-Chome, Chiyoda-ku, Tokyo 102, Japan Received 12 February 2004; received in revised form 4 June 2004; accepted 10 June 2004 Available online 7 August 2004
Abstract Asymmetric hollow fiber membranes were prepared from a cardo polyamide by using a wet phase inversion process. Polymer dope contained the cardo polyamide (20 wt. parts), LiCl (5 wt. parts) and NMP (100 wt. parts), and coagulant was water both for bore fluid and a coagulation bath. The membrane showed the selectivity of oxygen over nitrogen of 6.0, which was similar to that of a dense film, and the oxygen permeation rate of 12 × 10−6 cm3 (STP)/(cm2 s cmHg) (12 GPU, and 9.0 × 10−11 m3 /(m2 s Pa)) at 25 ◦ C. The membrane was stable up to 240 ◦ C with an O2 /N2 selectivity of 2.3. Morphology of the membrane was investigated by scanning electron microscopy (SEM) and an ultrathin sectioning method with transmission electron microscopy (TEM). It was observed that the cardo polyamide hollow fiber membrane had the skin layer in the inside surface, whereas porosity on the outside surface. The inner skin layer was constructed from nodular substances, and classified three parts, that is, a topmost layer, an underlying transition region, and a porous substrate, because of the difference in morphology. The topmost layer consisted of rounded cylindrical nodules elongated along spinning flow, whose average dimensions were 6.5 ± 1.5 nm in diameter and 15 ± 4 nm in length. The orientation of the nodules along spinning flow was observed. The nodules were tiered in double to form the dense, defect-free topmost layer, of which the thickness was about 11 nm on the average. In the underlying transition region, even though the nodule diameter was similar, there observed many micro voids, and its thickness was about 80 nm. And, in the porous substrate, nodules became larger as receding from the surface. On the other hand, the outer surface of the hollow fiber membrane is porous. Gas permeation data estimated a defect-free skin thickness of about 100 nm, and indicated that the topmost layer and the underlying transition region might be a skin layer for gas permeation, even though micro voids existed in the underlying transition region in TEM observation. © 2004 Elsevier B.V. All rights reserved. Keywords: Asymmetric hollow fiber membrane; Cardo polyamide; Gas separation; Transmission electron microscopy; Morphology
1. Introduction Membrane separation is worth noting as an energy-saving alternative to the cryogenic or pressure swing adsorption processes for gas separation applications. In the last decade the membrane separation processes have gained wide acceptance for removing CO2 from natural gas or producing N2 from air. To expand the applications of membrane based gas separation, it is necessary to develop new membrane materials which have both excellent permeability and permselectivity [1,2]. The processibility of the membrane material is another important requirement for the application of ∗ Corresponding author. Tel.: +81-774-75-2300; fax: +81-774-75-2319. E-mail address: [email protected] (S. Kazama).
0376-7388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2004.06.012
membrane based gas separation, because an ultra thin active layer of the polymeric materials can yield a high gas flux. To obtain the ultra thin active layer, a phase inversion process is popular for preparing a commercial gas separation membrane, and the process requires polymer solubility in organic solvent. The phase inversion process was developed by Loeb and Sourirajan to produce an integrally skinned asymmetric cellulose acetate membrane [3]. Various kinds of asymmetric membranes have been prepared by the phase inversion process [4–10]. In dry–wet phase inversion process, the outlet of the spinneret or casting knife that transfers the polymer solution from a closed reservoir is submerged directly in a liquid coagulation medium that is a non solvent for the polymer. During the time between extrusion of the nascent membrane and its immersion, evaporation of solvent may occur,
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good solubility enables the preparation of an ultrathin active layer as asymmetric membrane with a phase inversion process. In this study, we made the integrally skinned asymmetric hollow fiber membrane from bis(phenyl)fluorene-based cardo polyamide, and investigated gas permeation properties of the membrane. And, the fine structures of the skin layer were visualized by an ultrathin sectioning method for TEM and the morphology was discussed.
2. Experimental
Fig. 1. Chemical structure of bis(phenyl)fluorene-based cardo polyamide.
thereby producing a more concentrated polymer region near the air–solution interface to promote the formation of a skin layer [11]. It was reported that an integrally skinned asymmetric membrane was prepared by the wet phase inversion process without evaporation of solvent [12,13]. The transmission electron microscopy (TEM), scanning electron microscopy (SEM) and atomic force microscopy (AFM) have been applied to characterize the morphology of asymmetric membrane [14–18]. The microscopic observation revealed that the skin layer contained polymeric nodules. Typical asymmetric membranes have extremely complex physical structures. For simplicity, the membrane cross-section is usually expressed as having mainly two separate regions, the skin layer and the porous underlying substructure, typically a finger-like support and a sponge-like support. Furthermore, the skin layer is classified into two regions, the selective layer and the transition layer [19]. “Cardo” means “hinge” or “loop” in Latin; therefore, polymers which contain loop shaped moieties in main chains have been called “cardo polymers” by Korshak et al. [20]. A series of the polymers containing bis(phenyl)fluorenes or bis(phenyl)phtarides as cardo moieties have been synthesized and their physical properties reported [21,22]. As shown in Fig. 1, the bis(phenyl)fluorene-based cardo polymer has a structure in which a bulky fluorene unit protrudes vertically from the polymer main chain. This chemical structure of four phenyl rings connected to a quaternary carbon leads to severe rotational hindrance of the phenyl groups. Thus, the stiff, bulky cardo moiety must hinder the packing and reduce the rotational mobility of main chains. Therefore, the cardo moiety is supposed to have the potential of improving the gas transport properties. In fact, the improved gas transport properties of several cardo polymers were reported [23–26]. In addition, the phenylfluorene-based cardo polymers show high thermal stability, high solubility, high transparency, high refraction index and so on. This
Fig. 1 shows the chemical structure of a cardo polyamide. The polyamide was synthesized via solution polycondensation reaction from 9,9-bis(4-aminophenyl)-fluorene and terephthaloyl chloride by using N,N-dimethylacetamide (DMAc) as a solvent and triethylamine as an acid acceptor [27]. The polymer end was stabilized by benzoyl chloride. The filtrate of the reaction product was poured slowly into methanol to precipitate the polymer. The polyamide was purified twice by reprecipitation using DMAc and methanol. Inherent viscosity of the polyamides was 0.65 dl/g, measured with 0.5 g/dl DMAc solution at 30 ◦ C. The polyamide obtained was soluble in some polar organic solvents, e.g. DMAc and N-methyl-2-pyrrolidinone (NMP), in concentration of 25 wt.% or more. A dense, solution-cast film with thickness of ca. 30 m was prepared as described in the previous paper [26]. Fig. 2 shows a schematic diagram of a hollow fiber preparation apparatus. Polymer dope was prepared from 20 wt. parts of the polyamide, 5 wt. parts of LiCl and 100 wt. parts of DMAc in a glass vessel at room temperature under N2 atmosphere, and filtrated with 10 m sintered metal filter. LiCl has an effect of increasing solubility of the polyamide by breaking the hydrogen bonding. The viscosity of the dope was measured with rotational viscometer (Toki RE-80L) at 25 ◦ C. The polymer dope was extruded through the annular hole of a tube-in-orifice spinneret and inner coagulant (bore fluid), which was degassed, deionized water, extruded through the tube, simultaneously. Dimensions of the spinneret were shown in the same figure. After the passage of an air gap, the nascent fiber was immersed into a coagulation bath, which was tap water, followed by collected with a take-up drum. Even though an air gap exists, the bore side of the nascent fiber would be kept in a wet state. This spinning procedure was performed at 25 ◦ C. After detached from the drum, the fibers were rinsed in tap water at least 24 h. The fibers were then dried under ambient atmosphere, followed at 140 ◦ C for 1 h. Fifty hollow fibers of 15 cm long were fabricated into a test module. Test gas, Ar, CO2 , He, N2 or O2 , was fed into bore of the hollow fibers with a pressure of 2.0 kgf/cm2 (1.96 × 105 Pa). Permeate was maintained at atmospheric pressure. Gas permeation rates were measured with a bubble flow meter. A mixture of 21% O2 and 79% N2 was also
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Fig. 2. Schematic representation of the spinning set and spinneret dimensions.
tested. A feed rate of the mixture was maintained 100 times larger or more than that of the permeate in order to avoid concentration gradient. Composition of the permeate was determined by gas chromatography, and permeation rate of each gas was calculated. Permeation rate was expressed in a gas permeation unit, GPU, where GPU = 1 × 10−6 cm3 (STP)/(cm2 s cmHg) (7.5 × 10−12 N m3 /(m2 s Pa)). Permeability of the dense film was measured by a high vacuum time lag apparatus (Rikaseiki Co.) at a constant temperature of 25 ◦ C under 76 cmHg (1.0 × 105 Pa) pressure difference. NBS standard reference material 1470, polyester plastic film for gas transmission, was used for calibration of the apparatus. Permeability coefficients are expressed in Barrer, where Barrer = 1 × 10−10 cm3 (STP)cm/(cm2 s cmHg) (7.5 × 10−18 N m3 /(m2 s Pa)). Specimens for electron microscopic observation were prepared as follows [28]. For scanning electron microscopy, the dry hollow fiber membranes were immersed in ethanol for several minutes, followed by encapsulated with ethanol. The gelatin capsule was frozen with liquid N2 and fractured, which gave a fine section of hollow fibers. Pt–Pd was sputtered on the specimen in about 10 nm thickness for a conductive coating. For transmission electron microscopy, osmium tetroxide was used to enhance image contrast. Wet membrane was kept in 1 wt.% tannic acid–1.5 wt.% glutaric dialdehyde aqueous solution for 10 min and rinsed in tap water. The specimen was immersed and shaken slowly in a 2 wt.% osmium tetroxide aqueous solution for 12 h at room temperature, followed by rinsed in tap water at least for 1 h. This procedure was carried out twice. The water in micro voids of the membrane was replaced with embeding media as follows; placed in aqueous ethanol for 20 min, ethanol for 20 min, a mixture of 50 wt.% of epoxy resin and propylene oxide for 12 h, and finally in epoxy resin itself for 12 h. Epon 812 (Shell Company) was used for the embedding epoxy resin [29]. Next, the specimen was encapsulated in a mold, followed by curing at 60 ◦ C
for 15 h. After trimming the cured block, an ultra thin section, about 50 nm thick, of the specimen was obtained with an ultra microtome MICROM HM335E (Microm GmbH). A piece of the section was directly put on a copper mesh holder. The fine structures of the asymmetric hollow fiber membranes were observed using Hitachi S-700 scanning electron microscope and H-700 transmission electron microscope. Accelerating voltage was 15 kV for SEM, and 100 kV for TEM observation.
3. Results and discussion Spinning conditions for hollow fibers of the cardo polyamide were listed in Table 1. In this study, the cardo polyamide of an inherent viscosity of 0.65 dl/g was used. Table 1 Spinning conditions Polymer dope Composition Polyamide (wt. parts) Inherent viscosity (dl/g) LiCl (wt. parts) DMAc (wt. parts) Viscosity (Pa s)
20 0.65 5 100 4.0
Coagulant Bore fluid (inner coagulant) Coagulation bath
Deionized water Tap water
Spinning parameters Dope flow rate (ml/min) Bore flow rate (ml/min) Take-up rate (m/min) Temperature Spinneret (◦ C) Internal coagulant (◦ C) Coagulation bath (◦ C) Air gap (cm)
0.64 0.52 7.2 ca. 25 ca. 25 ca. 25 30
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Table 2 Gas transport properties of dense homogeneous film and asymmetric hollow fiber membrane made from cardo polyamide hollow fiber membrane Gas
Kinetic diameter (nm)
Pure gas O2 CO2 He Ar N2 Mixed gas O2 N2 a b
0.346 0.33 0.26 0.34 0.364
Experimental temperature (◦ C)
25 25 25 25 25 25 25
Dense homogeneous film
Asymmetric hollow fiber
Permeability coefficient P (Barrer)a
Selectivity over N2 PX/PN2
Permeation rate Q (GPU)b
Selectivity over N2 QX/QN2
Decrease from dense film (%)
1.26 7.60 12.8 – 0.204
6.2 37 63 – –
12 68 108 4.33 2.0
6.0 34 54 2.1 –
3.2 8.1 14
– –
12 2.0
6.0 –
– –
Barrer: 1 × 10−10 cm3 (STP)cm/(cm2 s cmHg) = 7.5 × 10−18 N m3 m/(cm2 s Pa). GPU: 1 × 10−6 cm3 (STP)/(cm2 s cmHg) = 7.5 × 10−12 N m3 /(cm2 s Pa).
The dimensions of the cardo polyamide hollow fiber membrane were 300 m in the outer diameter and 225 m in the inner diameter. Table 2 shows gas permeation properties of a dense homogeneous film and the asymmetric hollow fiber membrane of the cardo polyamide. The data of kinetic diameter was taken from a literature [30]. A decrease in the selectivity of the asymmetric membrane from the dense film was 3.2% for O2 /N2 selectivity, 8.1% for CO2 /N2 , and 14% for He/N2 . And the order of the decrease in the selectivity over N2 was well correlated to that of the kinetic diameters of 0.346, 0.33 and 0.26 nm for O2 , CO2 and He, respectively. That is, the larger decrease in the selectivity was, the smaller kinetic diameter was. The decreasing selectivity and the kinetic diameter was correlated to substructure resistance [31]. The selectivity of the hollow fiber membrane was slightly smaller, however comparable to that of the dense film. The similar selectivity should be caused by a defect-free skin layer which was formed with the wet phase inversion process in this study. A thickness of the skin layer was estimated as about 100 nm, by dividing the material N2 permeability coefficient by the N2 permeation rate of the hollow fiber. The skin thickness was comparable to other asymmetric membranes [32]. Compared between pure and mixed gas permeation, O2 permeation rate and O2 /N2 selectivity were completely same at 25 ◦ C. In a polymeric material, a gas permeation model is described as solution-diffusion theory. The coincidence may imply that O2 and N2 would dissolve in and diffuse through the cardo polyamide independently, not interfered with each other. Fig. 3 shows a temperature dependence of the permeation rates of O2 and N2 , and O2 /N2 selectivity of the cardo polyamide hollow fiber membrane, where the measurement was carried out from 20 to 240 ◦ C using a mixture of O2 and N2 . In the figure, the values were plotted as Arrhenius’s equation. The upper limit of the experimental temperature was determined from a thermal stability of glue used for fabricating a module. As shown in Fig. 3, both O2 and N2
permeation rate increased with the increase of temperature linearly, while O2 /N2 selectivity decreased. The permeation rate of O2 at 240 ◦ C is about 20 times larger than that at 20 ◦ C, whereas O2 /N2 selectivity at 240 ◦ C had a factor of 2.3. The excellent thermal stability of the membrane up to 240 ◦ C was caused by the high thermal stability of the cardo polyamide, where a glass transition temperature (Tg ) of 419 ◦ C and a 10% weight loss temperature of 480 ◦ C [22]. The excellent thermal stability was derived from the bulky fluorene unit which significantly increases the barriers to main chain motion, and consequently increases the Tg of the cardo polyamide.
Fig. 3. Temperature dependence of O2 , N2 permeation rate and O2 /N2 selectivity.
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The activation energy of O2 permeation for the asymmetric hollow fiber membrane was 14.9 kJ/mol. While, the activation energy of O2 permeation was 17.5 kJ/mol for the dense homogeneous film. As mentioned above, the permselectivity of the asymmetric membrane was slightly smaller than that of the dense film. And furthermore, the extent of the decrease in the gas selectivity over N2 was large for a small kinetic diameter of the gas molecule. In the asymmetric membrane, the cardo polyamide might take a loose morphology compared to the dense film. On the other hand, larger activation energy for asymmetric membrane compared to dense film was obtained in polysulfone, polycarbonate, and poly(ester carbonate) [33]. The activation energy of permeation for O2 would be influenced by the conditions of membrane preparation. Fig. 4 shows SEM micrographs of a cross section of the hollow fiber membrane. In the figure, macropores grow from the vicinity of the inner to the outer surface of the membrane. In this study, due to an air gap of 30 cm, there exists a time before immersion into external coagulation bath. During this period of time, the precipitation by the bore fluid of water could possibly reach the opposite outer surface of the nascent hollow fiber membrane. Fig. 5 shows magnified images of the vicinity of the outer and inner surface. From the figure, it was observed that the outer surface was rough and porous. On the other hand, an integrally skinned layer was observed in the inner surface vicinity. The SEM observation indicated an inner skin layer was active for gas separation. From SEM observations, we recognized the inner skin layer of the cardo polyamide hollow fiber membrane. However, a fine structure of the skin was not clear in the SEM micrographs. To visualize the fine structure, TEM observation was carried out. Fig. 6 shows a TEM image of a cross sec-
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tional membrane wall of the cardo polyamide hollow fiber membrane. The image coincided with that obtained from SEM observation. In the TEM image, macropores occupied almost the entire wall of the hollow fiber, and the macropores spread from the substructure of inner skin layer to almost the outer surface, and some of macropores penetrated through the outer surface to produce the openings. And furthermore, in the vicinity of the outer surface, many open-cell structures were observed, whereas relatively dense structure was observed in the inner surface region. The results support that the cardo polyamide hollow fiber membrane has an inner skin layer. Fig. 7 shows a magnified TEM micrograph of a cross section of the inner skin layer in the cardo polyamide hollow fiber membrane, where the OsO4 staining process was performed. In the figure, a white represents the polymeric materials, and a black the OsO4 , which represents micro voids. Black accumulations on the surface might be an artifact during the staining process. From the figure, the inner skin layer was constructed from nodular substances, and classified three parts, that is, a topmost layer, an underlying transition region, and a porous substrate, because of the difference in morphology. In the topmost layer, nodules of 6.5 ± 1.5 nm in diameter were aggregated closely to produce a defect-free layer, and the nodules were tiered alternately in double. As the result, the thickness of the topmost layer was 11 ± 2 nm. In the underlying transition region, even though the nodule dimensions were similar to those in the topmost layer, there observed many micro voids. And, in the porous substrate, nodule became larger as receding from the surface. The border between the underlying transition region and the porous substrate was not clear; however, in the part within 90 nm from the surface, the nodule size looks like similar. Therefore, the underly-
Fig. 4. SEM images of cross section of cardo polyamide hollow fiber membrane.
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Fig. 5. SEM images of cross section near inside and outside surface of cardo polyamide hollow fiber membrane.
ing transition region was estimated as about 80 nm in the thickness. The same specimen stained with OsO4 was observed with SEM, however, the topmost layer and the underlying transition region were not distinguished and recognized as the selective layer, because of the limited resolution of SEM. From the TEM observation, it was revealed that so called selective skin layer observed with SEM has the further fine structure of the topmost layer and the underlying transition region in the cardo polyamide hollow fiber membrane. And, the porous substrate in the TEM observation also corresponded to a transition layer in SEM observation. TEM observation method in this study was useful to analyze a skin layer of the asymmetric membrane. A nodular skin layer was observed in many asymmetric membranes, like polyamide-hydrazide membranes, cellulose acetate membranes and so on [34,35]. However, the nodule diameter of 6.5 ± 1.5 nm in the cardo polyamide membrane is small compared to those of other membranes, for example 40–80 nm of polyamide-hydrazide membranes and 18.8 nm of cellulose acetate membrane. In the OsO4 staining method, wet hollow fibers were examined, because dry hollow fibers had a difficulty in the staining. Therefore, the TEM image in Fig. 7 may not represent a dry membrane. In other words, nature of a dry skin layer might be different from a TEM observation in this study. Actually, wet hollow fibers of the cardo polyamide
shrunk about 10% in length to produce the dry gas separation membrane. However, even though considered 10% shrinkage, the morphology observed with TEM would be reflected in nature of the dry membrane, because the topmost layer and underlying region had distinct different morphology. Therefore, the topmost layer would play an important role in gas separation of the dry cardo polyamide membrane. A parallel section of the inner skin layer to a spinning flow was also investigated with TEM. Fig. 8 shows the TEM image of the inner skin layer parallel to a spinning flow. In the figure, the morphology was not clear as to that of the cross sectional image in Fig. 7, because of the difficulty in specimen preparation. However, the topmost layer and the underlying region were observed as well as in the cross section, and both parts consist of nodular materials. In the topmost layer and the vicinity of it, the nodules were elongated along the direction of a spinning flow to form oval nodules. In the topmost layer, the oval nodules seem to be 15 ± 4 nm long in the direction of the flow. TEM observation revealed the fine structure of the inner skin layer of the cardo polyamide hollow fiber membrane. In Fig. 9, a schematic diagram of the inner skin layer was outlined as the result of several TEM observations. The nodules have complicated forms, such as rounded cylindrical, rugby ball like, tangled, and so on. However, the nodules are represented by rounded cylindrical forms for simplicity in the figure. The nodules are densely aggregated and tiered in
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Fig. 6. TEM images of cross section of cardo polyamide hollow fiber membrane.
double to form a defect-free topmost layer with thickness of 11 ± 2 nm, and orientated along the spinning flow. The orientation of nodules was also observed in another asymmetric hollow fiber membrane [36]. And, loosely assembled nodules form an underlying transition region of thickness about 80 nm, where micro voids exist. The nodules of the topmost layer and the underlying region have almost the same size. Further from the surface, a porous substrate exists. In the porous substrate, polymeric nodules become larger as they exist further from the surface. Similar morphology of skin layer was suggested by Kesting [35]. The similar nodule size in the topmost layer and the underlying region, of which total thickness was about 90 nm, was observed. The nodule size in two regions might represent the status of the cardo polyamide molecular chains in the polymer dope. Hypothetically, if a nodule has the same density as a dense homogeneous film, d = 1.231 g/cm3 , one cylindrical nodule of 6.5 nm in diameter and 15 nm in length may contain about 770 repeating units of cardo polyamide.
The 770 repeating units correspond to a molecular weight of about 370,000, because a molecular weight of a unit of the cardo polyamide is 479. While, a weight-average molecular weight (Mw ) measured by gel permeation chromatography was 200,000 for an inherent viscosity of 0.65 dl/g. Therefore, it is estimated that one nodule may consist of two cardo polyamide macromolecules on the average. It likely occurs that two polymer chains could be entangled in a concentrated solution. When a polymer dope and a coagulant contact just after extrusion from a spinneret, the quick precipitation might occur in the region within about 90 nm from the interface between the polymer dope and the coagulant. The quick precipitation would induce the similar nodule size in the topmost layer and the underlying transition region, of which size corresponding to a status in the dope. In other words, in the region of the topmost layer and the underlying transition region, precipitation might be too fast to allow that polyamide molecules would aggregate and grow into large nodules [37–39]. On the other hand, in the region of the
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Fig. 7. TEM image of cross section of inner surface vicinity of cardo polyamide hollow fiber membrane.
porous substrate, the aggregation of macromolecules could occur because of slower precipitation. The thicknesses of the topmost layer and the underlying transition region were about 11 and 80 nm, respectively, therefore the total thickness of two parts was about 91 nm. The thickness of 91 nm was close to that calculated with a permeation experiment, 100 nm. The coincidence in the thickness may imply that the topmost layer and the underlying transition region determine the gas permeation in the cardo polyamide membrane. From TEM observation, permselectivity seems to be mainly attributed to the topmost layer, and the underlying transition region would be a barrier to
gas permeation. The resistance towards gas transport of the sublayer (underlying transition region) of asymmetric membrane was observed [31,40]. The microscopic observation indicated that the inner skin was active for gas separation of the cardo polyamide hollow fiber membrane. To confirm the result, the influence of the additive in an inner coagulant or a coagulation bath was examined. The addition of a polymer solvent, DMAc may affect the permselectivity of the hollow fiber membranes. When DMAc was added to the inner coagulant, O2 /N2 selectivity decreased with increasing DMAc concentration, and at 10% DMAc concentration, O2 /N2 selectivity vanished.
Fig. 8. TEM image of parallel section of inner surface vicinity of cardo polyamide hollow fiber membrane.
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4. Conclusions
Fig. 9. Schematic diagram of the inner skin layer of cardo polyamide hollow fiber membrane.
On the other hand, DMAc added to the coagulation bath did not affect the permselectivity. These results also support that the inner skin layer is active for gas separation of the cardo polyamide hollow fiber membrane. The defect-free inner skin layer was fabricated in the cardo polyamide hollow fiber membrane, by using the wet phase inversion method in this study. In the process, polymer dope contacts internal coagulant just after the extrusion from a spinneret, which induces simultaneous precipitation. In other words, no evaporation of polymer solvent occurred. It is generally accepted that higher polymer concentration is required to obtain the dense, defect-free skin layer. To get the condition, evaporation of a volatile solvent from a nascent membrane was positively induced [11,41]. And so, a way to control the outer surface form is the hot topics of making a good selectivity of asymmetric hollow fiber membranes. However, this work demonstrated that some polymeric materials like the cardo polyamide could yield the dense, defect-free skin without evaporation of the solvent. The model of forming the dense, defect-free topmost layer without the evaporation of the solvent is an interesting matter. As mentioned above, some papers presented the way to produce a skin layer without positive solvent evaporation [12,13]. However, the model of forming the inner skin layer of the cardo polyamide asymmetric hollow fiber membrane is not clear enough. Hypothetically, phenomena of an increased polymer concentration might have occurred in the liquid–solid interface between a spin dope and a spinneret, because of minimizing the liquid–solid interfacial energy. Actually, it is reported that an increased concentration of polydimethysiloxane was observed in the interface, though it was the interface between air and the toluene solution [42]. Anyway, further investigation is necessary for demonstrating the mechanism of an inner skin formation of the cardo polyamide.
Inner skinned asymmetric hollow fiber membrane is prepared from a cardo polyamide by using a wet phase inversion process. In the process, polymer dope of the cardo polyamide, LiCl and NMP contacts with an internal coagulant of water just after the extrusion from a spinneret, and immediate precipitation produce an active skin layer for gas separation in the inner surface of the hollow fiber membrane. That is, no evaporation of polymer solvent is required to produce the active skin layer for gas separation. The inner skin layer was constructed from nodular substances, and classified three parts, that is, a topmost layer, an underlying transition region, and a porous substrate, because of the difference in morphology. In the topmost layer, nodules of 6.5 ± 1.5 nm in diameter were aggregated closely to produce a defect-free layer, and the nodules were tiered alternately in double. As the result, the thickness of the topmost layer was 11 ± 2 nm. In the underlying transition region of about 80 nm thick, even though the nodule dimensions were similar to those in the topmost layer, there observed many micro voids. And, in the porous substrate, nodules became larger as receding from the surface. On the other hand, the outer surface of the hollow fiber membrane is porous. Gas permeation data estimated a defect-free skin thickness of about 100 nm, and would indicate that the topmost layer and the underlying transition region might form a skin layer for gas permeation, even though micro voids existed in the underlying transition region in TEM observation. The membrane shows an O2 /N2 selectivity of 6.0 and an O2 permeation rate of 12 GPU (9.0 × 10−11 m/(m2 s Pa). The permselectivity is similar to that of dense homogeneous film, which means the skin layer of the membranes is dense and defect-free. The membrane was thermally stable up to 240 ◦ C with an O2 /N2 selectivity of 2.3.
Acknowledgements This work was supported by Ministry of Economy, Trade and Industry of Japan. The authors would like to thank Dr. T. Teramoto, Mr. K. Harada, and Mr. H. Ando of Nippon Steel Chemical Co. Ltd. for their useful suggestions on synthesis. References [1] B.D. Freeman, I. Pinnau, Polymer membrane for gas and vapor separation, American Chemical Society, Washington, DC, 1999. [2] W.J. Koros, G.K. Fleming, Membrane-based gas separations, J. Membr. Sci. 83 (1993) 1. [3] S. Loeb, S. Sourirajan, Sea water demineralization by means of an osmotic membrane, Adv. Chem. Ser. 38 (1962) 117. [4] R. Kesting, Synthetic Polymeric Membranes: A Structural Perspective, second ed., John Wiley & Sons, New York, 1985. [5] I. Pinnau, M.W. Hellums, W.J. Koros, Gas transport through homogeneous and asymmetric polyestercarbonate membranes, Polymer 32 (1991) 2612.
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Gas separation properties and morphology of asymmetric hollow fiber membranes made from cardo polyamide Shingo Kazama a,∗ , Masao Sakashita b a
Research Institute of Innovative Technology for the Earth, 9-2 Kizugawadai, Kizu-cho, Soraku-gun, Kyoto 619-0292, Japan b Japan Technical Information Service, Kojimachi 1-Chome, Chiyoda-ku, Tokyo 102, Japan Received 12 February 2004; received in revised form 4 June 2004; accepted 10 June 2004 Available online 7 August 2004
Abstract Asymmetric hollow fiber membranes were prepared from a cardo polyamide by using a wet phase inversion process. Polymer dope contained the cardo polyamide (20 wt. parts), LiCl (5 wt. parts) and NMP (100 wt. parts), and coagulant was water both for bore fluid and a coagulation bath. The membrane showed the selectivity of oxygen over nitrogen of 6.0, which was similar to that of a dense film, and the oxygen permeation rate of 12 × 10−6 cm3 (STP)/(cm2 s cmHg) (12 GPU, and 9.0 × 10−11 m3 /(m2 s Pa)) at 25 ◦ C. The membrane was stable up to 240 ◦ C with an O2 /N2 selectivity of 2.3. Morphology of the membrane was investigated by scanning electron microscopy (SEM) and an ultrathin sectioning method with transmission electron microscopy (TEM). It was observed that the cardo polyamide hollow fiber membrane had the skin layer in the inside surface, whereas porosity on the outside surface. The inner skin layer was constructed from nodular substances, and classified three parts, that is, a topmost layer, an underlying transition region, and a porous substrate, because of the difference in morphology. The topmost layer consisted of rounded cylindrical nodules elongated along spinning flow, whose average dimensions were 6.5 ± 1.5 nm in diameter and 15 ± 4 nm in length. The orientation of the nodules along spinning flow was observed. The nodules were tiered in double to form the dense, defect-free topmost layer, of which the thickness was about 11 nm on the average. In the underlying transition region, even though the nodule diameter was similar, there observed many micro voids, and its thickness was about 80 nm. And, in the porous substrate, nodules became larger as receding from the surface. On the other hand, the outer surface of the hollow fiber membrane is porous. Gas permeation data estimated a defect-free skin thickness of about 100 nm, and indicated that the topmost layer and the underlying transition region might be a skin layer for gas permeation, even though micro voids existed in the underlying transition region in TEM observation. © 2004 Elsevier B.V. All rights reserved. Keywords: Asymmetric hollow fiber membrane; Cardo polyamide; Gas separation; Transmission electron microscopy; Morphology
1. Introduction Membrane separation is worth noting as an energy-saving alternative to the cryogenic or pressure swing adsorption processes for gas separation applications. In the last decade the membrane separation processes have gained wide acceptance for removing CO2 from natural gas or producing N2 from air. To expand the applications of membrane based gas separation, it is necessary to develop new membrane materials which have both excellent permeability and permselectivity [1,2]. The processibility of the membrane material is another important requirement for the application of ∗ Corresponding author. Tel.: +81-774-75-2300; fax: +81-774-75-2319. E-mail address: [email protected] (S. Kazama).
0376-7388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2004.06.012
membrane based gas separation, because an ultra thin active layer of the polymeric materials can yield a high gas flux. To obtain the ultra thin active layer, a phase inversion process is popular for preparing a commercial gas separation membrane, and the process requires polymer solubility in organic solvent. The phase inversion process was developed by Loeb and Sourirajan to produce an integrally skinned asymmetric cellulose acetate membrane [3]. Various kinds of asymmetric membranes have been prepared by the phase inversion process [4–10]. In dry–wet phase inversion process, the outlet of the spinneret or casting knife that transfers the polymer solution from a closed reservoir is submerged directly in a liquid coagulation medium that is a non solvent for the polymer. During the time between extrusion of the nascent membrane and its immersion, evaporation of solvent may occur,
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good solubility enables the preparation of an ultrathin active layer as asymmetric membrane with a phase inversion process. In this study, we made the integrally skinned asymmetric hollow fiber membrane from bis(phenyl)fluorene-based cardo polyamide, and investigated gas permeation properties of the membrane. And, the fine structures of the skin layer were visualized by an ultrathin sectioning method for TEM and the morphology was discussed.
2. Experimental
Fig. 1. Chemical structure of bis(phenyl)fluorene-based cardo polyamide.
thereby producing a more concentrated polymer region near the air–solution interface to promote the formation of a skin layer [11]. It was reported that an integrally skinned asymmetric membrane was prepared by the wet phase inversion process without evaporation of solvent [12,13]. The transmission electron microscopy (TEM), scanning electron microscopy (SEM) and atomic force microscopy (AFM) have been applied to characterize the morphology of asymmetric membrane [14–18]. The microscopic observation revealed that the skin layer contained polymeric nodules. Typical asymmetric membranes have extremely complex physical structures. For simplicity, the membrane cross-section is usually expressed as having mainly two separate regions, the skin layer and the porous underlying substructure, typically a finger-like support and a sponge-like support. Furthermore, the skin layer is classified into two regions, the selective layer and the transition layer [19]. “Cardo” means “hinge” or “loop” in Latin; therefore, polymers which contain loop shaped moieties in main chains have been called “cardo polymers” by Korshak et al. [20]. A series of the polymers containing bis(phenyl)fluorenes or bis(phenyl)phtarides as cardo moieties have been synthesized and their physical properties reported [21,22]. As shown in Fig. 1, the bis(phenyl)fluorene-based cardo polymer has a structure in which a bulky fluorene unit protrudes vertically from the polymer main chain. This chemical structure of four phenyl rings connected to a quaternary carbon leads to severe rotational hindrance of the phenyl groups. Thus, the stiff, bulky cardo moiety must hinder the packing and reduce the rotational mobility of main chains. Therefore, the cardo moiety is supposed to have the potential of improving the gas transport properties. In fact, the improved gas transport properties of several cardo polymers were reported [23–26]. In addition, the phenylfluorene-based cardo polymers show high thermal stability, high solubility, high transparency, high refraction index and so on. This
Fig. 1 shows the chemical structure of a cardo polyamide. The polyamide was synthesized via solution polycondensation reaction from 9,9-bis(4-aminophenyl)-fluorene and terephthaloyl chloride by using N,N-dimethylacetamide (DMAc) as a solvent and triethylamine as an acid acceptor [27]. The polymer end was stabilized by benzoyl chloride. The filtrate of the reaction product was poured slowly into methanol to precipitate the polymer. The polyamide was purified twice by reprecipitation using DMAc and methanol. Inherent viscosity of the polyamides was 0.65 dl/g, measured with 0.5 g/dl DMAc solution at 30 ◦ C. The polyamide obtained was soluble in some polar organic solvents, e.g. DMAc and N-methyl-2-pyrrolidinone (NMP), in concentration of 25 wt.% or more. A dense, solution-cast film with thickness of ca. 30 m was prepared as described in the previous paper [26]. Fig. 2 shows a schematic diagram of a hollow fiber preparation apparatus. Polymer dope was prepared from 20 wt. parts of the polyamide, 5 wt. parts of LiCl and 100 wt. parts of DMAc in a glass vessel at room temperature under N2 atmosphere, and filtrated with 10 m sintered metal filter. LiCl has an effect of increasing solubility of the polyamide by breaking the hydrogen bonding. The viscosity of the dope was measured with rotational viscometer (Toki RE-80L) at 25 ◦ C. The polymer dope was extruded through the annular hole of a tube-in-orifice spinneret and inner coagulant (bore fluid), which was degassed, deionized water, extruded through the tube, simultaneously. Dimensions of the spinneret were shown in the same figure. After the passage of an air gap, the nascent fiber was immersed into a coagulation bath, which was tap water, followed by collected with a take-up drum. Even though an air gap exists, the bore side of the nascent fiber would be kept in a wet state. This spinning procedure was performed at 25 ◦ C. After detached from the drum, the fibers were rinsed in tap water at least 24 h. The fibers were then dried under ambient atmosphere, followed at 140 ◦ C for 1 h. Fifty hollow fibers of 15 cm long were fabricated into a test module. Test gas, Ar, CO2 , He, N2 or O2 , was fed into bore of the hollow fibers with a pressure of 2.0 kgf/cm2 (1.96 × 105 Pa). Permeate was maintained at atmospheric pressure. Gas permeation rates were measured with a bubble flow meter. A mixture of 21% O2 and 79% N2 was also
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Fig. 2. Schematic representation of the spinning set and spinneret dimensions.
tested. A feed rate of the mixture was maintained 100 times larger or more than that of the permeate in order to avoid concentration gradient. Composition of the permeate was determined by gas chromatography, and permeation rate of each gas was calculated. Permeation rate was expressed in a gas permeation unit, GPU, where GPU = 1 × 10−6 cm3 (STP)/(cm2 s cmHg) (7.5 × 10−12 N m3 /(m2 s Pa)). Permeability of the dense film was measured by a high vacuum time lag apparatus (Rikaseiki Co.) at a constant temperature of 25 ◦ C under 76 cmHg (1.0 × 105 Pa) pressure difference. NBS standard reference material 1470, polyester plastic film for gas transmission, was used for calibration of the apparatus. Permeability coefficients are expressed in Barrer, where Barrer = 1 × 10−10 cm3 (STP)cm/(cm2 s cmHg) (7.5 × 10−18 N m3 /(m2 s Pa)). Specimens for electron microscopic observation were prepared as follows [28]. For scanning electron microscopy, the dry hollow fiber membranes were immersed in ethanol for several minutes, followed by encapsulated with ethanol. The gelatin capsule was frozen with liquid N2 and fractured, which gave a fine section of hollow fibers. Pt–Pd was sputtered on the specimen in about 10 nm thickness for a conductive coating. For transmission electron microscopy, osmium tetroxide was used to enhance image contrast. Wet membrane was kept in 1 wt.% tannic acid–1.5 wt.% glutaric dialdehyde aqueous solution for 10 min and rinsed in tap water. The specimen was immersed and shaken slowly in a 2 wt.% osmium tetroxide aqueous solution for 12 h at room temperature, followed by rinsed in tap water at least for 1 h. This procedure was carried out twice. The water in micro voids of the membrane was replaced with embeding media as follows; placed in aqueous ethanol for 20 min, ethanol for 20 min, a mixture of 50 wt.% of epoxy resin and propylene oxide for 12 h, and finally in epoxy resin itself for 12 h. Epon 812 (Shell Company) was used for the embedding epoxy resin [29]. Next, the specimen was encapsulated in a mold, followed by curing at 60 ◦ C
for 15 h. After trimming the cured block, an ultra thin section, about 50 nm thick, of the specimen was obtained with an ultra microtome MICROM HM335E (Microm GmbH). A piece of the section was directly put on a copper mesh holder. The fine structures of the asymmetric hollow fiber membranes were observed using Hitachi S-700 scanning electron microscope and H-700 transmission electron microscope. Accelerating voltage was 15 kV for SEM, and 100 kV for TEM observation.
3. Results and discussion Spinning conditions for hollow fibers of the cardo polyamide were listed in Table 1. In this study, the cardo polyamide of an inherent viscosity of 0.65 dl/g was used. Table 1 Spinning conditions Polymer dope Composition Polyamide (wt. parts) Inherent viscosity (dl/g) LiCl (wt. parts) DMAc (wt. parts) Viscosity (Pa s)
20 0.65 5 100 4.0
Coagulant Bore fluid (inner coagulant) Coagulation bath
Deionized water Tap water
Spinning parameters Dope flow rate (ml/min) Bore flow rate (ml/min) Take-up rate (m/min) Temperature Spinneret (◦ C) Internal coagulant (◦ C) Coagulation bath (◦ C) Air gap (cm)
0.64 0.52 7.2 ca. 25 ca. 25 ca. 25 30
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Table 2 Gas transport properties of dense homogeneous film and asymmetric hollow fiber membrane made from cardo polyamide hollow fiber membrane Gas
Kinetic diameter (nm)
Pure gas O2 CO2 He Ar N2 Mixed gas O2 N2 a b
0.346 0.33 0.26 0.34 0.364
Experimental temperature (◦ C)
25 25 25 25 25 25 25
Dense homogeneous film
Asymmetric hollow fiber
Permeability coefficient P (Barrer)a
Selectivity over N2 PX/PN2
Permeation rate Q (GPU)b
Selectivity over N2 QX/QN2
Decrease from dense film (%)
1.26 7.60 12.8 – 0.204
6.2 37 63 – –
12 68 108 4.33 2.0
6.0 34 54 2.1 –
3.2 8.1 14
– –
12 2.0
6.0 –
– –
Barrer: 1 × 10−10 cm3 (STP)cm/(cm2 s cmHg) = 7.5 × 10−18 N m3 m/(cm2 s Pa). GPU: 1 × 10−6 cm3 (STP)/(cm2 s cmHg) = 7.5 × 10−12 N m3 /(cm2 s Pa).
The dimensions of the cardo polyamide hollow fiber membrane were 300 m in the outer diameter and 225 m in the inner diameter. Table 2 shows gas permeation properties of a dense homogeneous film and the asymmetric hollow fiber membrane of the cardo polyamide. The data of kinetic diameter was taken from a literature [30]. A decrease in the selectivity of the asymmetric membrane from the dense film was 3.2% for O2 /N2 selectivity, 8.1% for CO2 /N2 , and 14% for He/N2 . And the order of the decrease in the selectivity over N2 was well correlated to that of the kinetic diameters of 0.346, 0.33 and 0.26 nm for O2 , CO2 and He, respectively. That is, the larger decrease in the selectivity was, the smaller kinetic diameter was. The decreasing selectivity and the kinetic diameter was correlated to substructure resistance [31]. The selectivity of the hollow fiber membrane was slightly smaller, however comparable to that of the dense film. The similar selectivity should be caused by a defect-free skin layer which was formed with the wet phase inversion process in this study. A thickness of the skin layer was estimated as about 100 nm, by dividing the material N2 permeability coefficient by the N2 permeation rate of the hollow fiber. The skin thickness was comparable to other asymmetric membranes [32]. Compared between pure and mixed gas permeation, O2 permeation rate and O2 /N2 selectivity were completely same at 25 ◦ C. In a polymeric material, a gas permeation model is described as solution-diffusion theory. The coincidence may imply that O2 and N2 would dissolve in and diffuse through the cardo polyamide independently, not interfered with each other. Fig. 3 shows a temperature dependence of the permeation rates of O2 and N2 , and O2 /N2 selectivity of the cardo polyamide hollow fiber membrane, where the measurement was carried out from 20 to 240 ◦ C using a mixture of O2 and N2 . In the figure, the values were plotted as Arrhenius’s equation. The upper limit of the experimental temperature was determined from a thermal stability of glue used for fabricating a module. As shown in Fig. 3, both O2 and N2
permeation rate increased with the increase of temperature linearly, while O2 /N2 selectivity decreased. The permeation rate of O2 at 240 ◦ C is about 20 times larger than that at 20 ◦ C, whereas O2 /N2 selectivity at 240 ◦ C had a factor of 2.3. The excellent thermal stability of the membrane up to 240 ◦ C was caused by the high thermal stability of the cardo polyamide, where a glass transition temperature (Tg ) of 419 ◦ C and a 10% weight loss temperature of 480 ◦ C [22]. The excellent thermal stability was derived from the bulky fluorene unit which significantly increases the barriers to main chain motion, and consequently increases the Tg of the cardo polyamide.
Fig. 3. Temperature dependence of O2 , N2 permeation rate and O2 /N2 selectivity.
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The activation energy of O2 permeation for the asymmetric hollow fiber membrane was 14.9 kJ/mol. While, the activation energy of O2 permeation was 17.5 kJ/mol for the dense homogeneous film. As mentioned above, the permselectivity of the asymmetric membrane was slightly smaller than that of the dense film. And furthermore, the extent of the decrease in the gas selectivity over N2 was large for a small kinetic diameter of the gas molecule. In the asymmetric membrane, the cardo polyamide might take a loose morphology compared to the dense film. On the other hand, larger activation energy for asymmetric membrane compared to dense film was obtained in polysulfone, polycarbonate, and poly(ester carbonate) [33]. The activation energy of permeation for O2 would be influenced by the conditions of membrane preparation. Fig. 4 shows SEM micrographs of a cross section of the hollow fiber membrane. In the figure, macropores grow from the vicinity of the inner to the outer surface of the membrane. In this study, due to an air gap of 30 cm, there exists a time before immersion into external coagulation bath. During this period of time, the precipitation by the bore fluid of water could possibly reach the opposite outer surface of the nascent hollow fiber membrane. Fig. 5 shows magnified images of the vicinity of the outer and inner surface. From the figure, it was observed that the outer surface was rough and porous. On the other hand, an integrally skinned layer was observed in the inner surface vicinity. The SEM observation indicated an inner skin layer was active for gas separation. From SEM observations, we recognized the inner skin layer of the cardo polyamide hollow fiber membrane. However, a fine structure of the skin was not clear in the SEM micrographs. To visualize the fine structure, TEM observation was carried out. Fig. 6 shows a TEM image of a cross sec-
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tional membrane wall of the cardo polyamide hollow fiber membrane. The image coincided with that obtained from SEM observation. In the TEM image, macropores occupied almost the entire wall of the hollow fiber, and the macropores spread from the substructure of inner skin layer to almost the outer surface, and some of macropores penetrated through the outer surface to produce the openings. And furthermore, in the vicinity of the outer surface, many open-cell structures were observed, whereas relatively dense structure was observed in the inner surface region. The results support that the cardo polyamide hollow fiber membrane has an inner skin layer. Fig. 7 shows a magnified TEM micrograph of a cross section of the inner skin layer in the cardo polyamide hollow fiber membrane, where the OsO4 staining process was performed. In the figure, a white represents the polymeric materials, and a black the OsO4 , which represents micro voids. Black accumulations on the surface might be an artifact during the staining process. From the figure, the inner skin layer was constructed from nodular substances, and classified three parts, that is, a topmost layer, an underlying transition region, and a porous substrate, because of the difference in morphology. In the topmost layer, nodules of 6.5 ± 1.5 nm in diameter were aggregated closely to produce a defect-free layer, and the nodules were tiered alternately in double. As the result, the thickness of the topmost layer was 11 ± 2 nm. In the underlying transition region, even though the nodule dimensions were similar to those in the topmost layer, there observed many micro voids. And, in the porous substrate, nodule became larger as receding from the surface. The border between the underlying transition region and the porous substrate was not clear; however, in the part within 90 nm from the surface, the nodule size looks like similar. Therefore, the underly-
Fig. 4. SEM images of cross section of cardo polyamide hollow fiber membrane.
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Fig. 5. SEM images of cross section near inside and outside surface of cardo polyamide hollow fiber membrane.
ing transition region was estimated as about 80 nm in the thickness. The same specimen stained with OsO4 was observed with SEM, however, the topmost layer and the underlying transition region were not distinguished and recognized as the selective layer, because of the limited resolution of SEM. From the TEM observation, it was revealed that so called selective skin layer observed with SEM has the further fine structure of the topmost layer and the underlying transition region in the cardo polyamide hollow fiber membrane. And, the porous substrate in the TEM observation also corresponded to a transition layer in SEM observation. TEM observation method in this study was useful to analyze a skin layer of the asymmetric membrane. A nodular skin layer was observed in many asymmetric membranes, like polyamide-hydrazide membranes, cellulose acetate membranes and so on [34,35]. However, the nodule diameter of 6.5 ± 1.5 nm in the cardo polyamide membrane is small compared to those of other membranes, for example 40–80 nm of polyamide-hydrazide membranes and 18.8 nm of cellulose acetate membrane. In the OsO4 staining method, wet hollow fibers were examined, because dry hollow fibers had a difficulty in the staining. Therefore, the TEM image in Fig. 7 may not represent a dry membrane. In other words, nature of a dry skin layer might be different from a TEM observation in this study. Actually, wet hollow fibers of the cardo polyamide
shrunk about 10% in length to produce the dry gas separation membrane. However, even though considered 10% shrinkage, the morphology observed with TEM would be reflected in nature of the dry membrane, because the topmost layer and underlying region had distinct different morphology. Therefore, the topmost layer would play an important role in gas separation of the dry cardo polyamide membrane. A parallel section of the inner skin layer to a spinning flow was also investigated with TEM. Fig. 8 shows the TEM image of the inner skin layer parallel to a spinning flow. In the figure, the morphology was not clear as to that of the cross sectional image in Fig. 7, because of the difficulty in specimen preparation. However, the topmost layer and the underlying region were observed as well as in the cross section, and both parts consist of nodular materials. In the topmost layer and the vicinity of it, the nodules were elongated along the direction of a spinning flow to form oval nodules. In the topmost layer, the oval nodules seem to be 15 ± 4 nm long in the direction of the flow. TEM observation revealed the fine structure of the inner skin layer of the cardo polyamide hollow fiber membrane. In Fig. 9, a schematic diagram of the inner skin layer was outlined as the result of several TEM observations. The nodules have complicated forms, such as rounded cylindrical, rugby ball like, tangled, and so on. However, the nodules are represented by rounded cylindrical forms for simplicity in the figure. The nodules are densely aggregated and tiered in
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Fig. 6. TEM images of cross section of cardo polyamide hollow fiber membrane.
double to form a defect-free topmost layer with thickness of 11 ± 2 nm, and orientated along the spinning flow. The orientation of nodules was also observed in another asymmetric hollow fiber membrane [36]. And, loosely assembled nodules form an underlying transition region of thickness about 80 nm, where micro voids exist. The nodules of the topmost layer and the underlying region have almost the same size. Further from the surface, a porous substrate exists. In the porous substrate, polymeric nodules become larger as they exist further from the surface. Similar morphology of skin layer was suggested by Kesting [35]. The similar nodule size in the topmost layer and the underlying region, of which total thickness was about 90 nm, was observed. The nodule size in two regions might represent the status of the cardo polyamide molecular chains in the polymer dope. Hypothetically, if a nodule has the same density as a dense homogeneous film, d = 1.231 g/cm3 , one cylindrical nodule of 6.5 nm in diameter and 15 nm in length may contain about 770 repeating units of cardo polyamide.
The 770 repeating units correspond to a molecular weight of about 370,000, because a molecular weight of a unit of the cardo polyamide is 479. While, a weight-average molecular weight (Mw ) measured by gel permeation chromatography was 200,000 for an inherent viscosity of 0.65 dl/g. Therefore, it is estimated that one nodule may consist of two cardo polyamide macromolecules on the average. It likely occurs that two polymer chains could be entangled in a concentrated solution. When a polymer dope and a coagulant contact just after extrusion from a spinneret, the quick precipitation might occur in the region within about 90 nm from the interface between the polymer dope and the coagulant. The quick precipitation would induce the similar nodule size in the topmost layer and the underlying transition region, of which size corresponding to a status in the dope. In other words, in the region of the topmost layer and the underlying transition region, precipitation might be too fast to allow that polyamide molecules would aggregate and grow into large nodules [37–39]. On the other hand, in the region of the
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Fig. 7. TEM image of cross section of inner surface vicinity of cardo polyamide hollow fiber membrane.
porous substrate, the aggregation of macromolecules could occur because of slower precipitation. The thicknesses of the topmost layer and the underlying transition region were about 11 and 80 nm, respectively, therefore the total thickness of two parts was about 91 nm. The thickness of 91 nm was close to that calculated with a permeation experiment, 100 nm. The coincidence in the thickness may imply that the topmost layer and the underlying transition region determine the gas permeation in the cardo polyamide membrane. From TEM observation, permselectivity seems to be mainly attributed to the topmost layer, and the underlying transition region would be a barrier to
gas permeation. The resistance towards gas transport of the sublayer (underlying transition region) of asymmetric membrane was observed [31,40]. The microscopic observation indicated that the inner skin was active for gas separation of the cardo polyamide hollow fiber membrane. To confirm the result, the influence of the additive in an inner coagulant or a coagulation bath was examined. The addition of a polymer solvent, DMAc may affect the permselectivity of the hollow fiber membranes. When DMAc was added to the inner coagulant, O2 /N2 selectivity decreased with increasing DMAc concentration, and at 10% DMAc concentration, O2 /N2 selectivity vanished.
Fig. 8. TEM image of parallel section of inner surface vicinity of cardo polyamide hollow fiber membrane.
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4. Conclusions
Fig. 9. Schematic diagram of the inner skin layer of cardo polyamide hollow fiber membrane.
On the other hand, DMAc added to the coagulation bath did not affect the permselectivity. These results also support that the inner skin layer is active for gas separation of the cardo polyamide hollow fiber membrane. The defect-free inner skin layer was fabricated in the cardo polyamide hollow fiber membrane, by using the wet phase inversion method in this study. In the process, polymer dope contacts internal coagulant just after the extrusion from a spinneret, which induces simultaneous precipitation. In other words, no evaporation of polymer solvent occurred. It is generally accepted that higher polymer concentration is required to obtain the dense, defect-free skin layer. To get the condition, evaporation of a volatile solvent from a nascent membrane was positively induced [11,41]. And so, a way to control the outer surface form is the hot topics of making a good selectivity of asymmetric hollow fiber membranes. However, this work demonstrated that some polymeric materials like the cardo polyamide could yield the dense, defect-free skin without evaporation of the solvent. The model of forming the dense, defect-free topmost layer without the evaporation of the solvent is an interesting matter. As mentioned above, some papers presented the way to produce a skin layer without positive solvent evaporation [12,13]. However, the model of forming the inner skin layer of the cardo polyamide asymmetric hollow fiber membrane is not clear enough. Hypothetically, phenomena of an increased polymer concentration might have occurred in the liquid–solid interface between a spin dope and a spinneret, because of minimizing the liquid–solid interfacial energy. Actually, it is reported that an increased concentration of polydimethysiloxane was observed in the interface, though it was the interface between air and the toluene solution [42]. Anyway, further investigation is necessary for demonstrating the mechanism of an inner skin formation of the cardo polyamide.
Inner skinned asymmetric hollow fiber membrane is prepared from a cardo polyamide by using a wet phase inversion process. In the process, polymer dope of the cardo polyamide, LiCl and NMP contacts with an internal coagulant of water just after the extrusion from a spinneret, and immediate precipitation produce an active skin layer for gas separation in the inner surface of the hollow fiber membrane. That is, no evaporation of polymer solvent is required to produce the active skin layer for gas separation. The inner skin layer was constructed from nodular substances, and classified three parts, that is, a topmost layer, an underlying transition region, and a porous substrate, because of the difference in morphology. In the topmost layer, nodules of 6.5 ± 1.5 nm in diameter were aggregated closely to produce a defect-free layer, and the nodules were tiered alternately in double. As the result, the thickness of the topmost layer was 11 ± 2 nm. In the underlying transition region of about 80 nm thick, even though the nodule dimensions were similar to those in the topmost layer, there observed many micro voids. And, in the porous substrate, nodules became larger as receding from the surface. On the other hand, the outer surface of the hollow fiber membrane is porous. Gas permeation data estimated a defect-free skin thickness of about 100 nm, and would indicate that the topmost layer and the underlying transition region might form a skin layer for gas permeation, even though micro voids existed in the underlying transition region in TEM observation. The membrane shows an O2 /N2 selectivity of 6.0 and an O2 permeation rate of 12 GPU (9.0 × 10−11 m/(m2 s Pa). The permselectivity is similar to that of dense homogeneous film, which means the skin layer of the membranes is dense and defect-free. The membrane was thermally stable up to 240 ◦ C with an O2 /N2 selectivity of 2.3.
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