Effect of Temperature Exposition of Casting Solution on Properties Of

fibers

Article Eﬀect of Temperature Exposition of Casting Solution on Properties of Polysulfone Hollow Fiber Membranes

Ilya Borisov , Vladimir Vasilevsky, Dmitry Matveev, Anna Ovcharova , Alexey Volkov and Vladimir Volkov *

A.V. Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, Moscow 119991, Russia; [email protected] (I.B.); [email protected] (V.V.); [email protected] (D.M.); ovcharoﬀ@ips.ac.ru (A.O.); [email protected] (A.V.) * Correspondence: [email protected]; Tel.: +7-495-647-59-27

Received: 18 November 2019; Accepted: 11 December 2019; Published: 14 December 2019

Abstract: It was shown for the first time that the conditions of thermal treatment of the casting solution significantly affect the morphology and transport properties of porous, flat, and hollow fiber polysulfone (PSf) membranes. It is ascertained that the main solution components that are subjected to thermo-oxidative destruction are the pore-forming agent polyethylene glycol (PEG) and solvent N-methyl-2-pyrrolidone (NMP). It is proved that hydroxyl groups of PEG actively react in the process of the casting solution thermo-oxidative destruction. It is shown that despite the chemical conversion taking place in the casting solution, their stability towards coagulation virtually does not change. The differences in the membrane morphology associated with the increase of thermal treatment time at 120 ◦C are not connected to the thermodynamic properties of the casting solutions, but with the kinetics of the phase separation. It is revealed that the change of morphology and transport properties of membranes is connected with the increase of the casting solution viscosity. The rise of solution viscosity resulted in the slowdown of the phase separation and formation of a more densely packed membrane structure with less pronounced macropores. It is determined experimentally that with the increase of casting solution thermal treatment time, the membrane selective layer thickness increases. This leads to the decrease of gas permeance and the rise of the He/CO2 selectivity for flat and hollow fiber membranes. In the case of hollow fibers, the fall of gas permeance is also connected with the appearance of the sponge-like layer at the outer surface of membranes. The increase of selectivity and decline of permeance indicates the reduction of selective layer pore size and its densification, which agrees well with the calculation results of the average membrane density. The results obtained are relevant to any polymeric casting solution containing NMP and/or PEG and treated at temperatures above 60 ◦C.

Keywords: asymmetric membranes; thermal regime; casting solution; polysulfone; ﬂat membranes; hollow ﬁber membranes; morphology; transport properties

1. Introduction Polysulfone (PSf) and polyethersulfone (PES) are among the most widely used commercial bulk polymers in the fabrication of different kinds of separation membranes [1–4]. Their good mechanical properties, high chemical resistance, and easy-to-process make these membrane materials attractive for the spinning of hollow fiber membranes suitable for a broad range of applications such as micro-, ultra-, and nanofiltration, gas separation, membrane contactors [5–8]. The general approach for the fabrication of integrally skinned asymmetric hollow fiber membranes is the

Fibers 2019, 7, 110; doi:10.3390/fib7120110 www.mdpi.com/journal/fibers Fibers 2019, 7, 110 2 of 16 non-solvent induced phase separation (NIPS) method when the polymer is coagulated from its solution by the contact with “poor” solvent (e.g., water). The solvents traditionally used for polysulfone are N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMFA), dimethylacetamide (DMAc), or dimethylsulfoxide (DMSO), which are low-volatile liquids with the boiling point above 150 ◦C. Meanwhile, the recent advances have been also made on the replacement of these harmful solvents by more environmentally friendly ones (methyl-5-(dimethylamino)-2-methyl-5-oxopentanoate (Rhodiasolv PolarClean®), 1,3-dimethylimidazolium dimethyl phosphate, etc.) [9–13]. The porous structure of the membrane is determined by the diffusion rate of the solvent from the casting solution into the coagulation bath and non-solvent in the opposite direction. This process is controlled by the different parameters including composition and viscosity of casting solution, the conditions of membrane fabrication (air gap height, polymers solution, and internal coagulant flow rates, etc.), the interaction between components of casting solution and coagulation bath. For instance, it was shown [14] that the presence of components (1,4-dioxane, diethylene glycol or dimethyl ether, acetone) with lower miscibility with coagulant (water) rather than solvent (NMP) resulted in a more packed and dense skin layer of the asymmetric PSf membrane; while the membrane with a more open pore structure was obtained in the case of addition of γ-butyrolactone, possessing greater miscibility with water than NMP. Higher porosity of the drainage layer and hence, permeance of resulted asymmetric membranes, can be achieved by the introduction of pore-forming components in the casting solution like polyethyleneglycol (PEG) and its derivatives [15–17], polyvinylpyrrolidone (PVP) [18–20], and surfactants [21,22]. PEG and its derivatives are more often employed as pore-forming components in the formation of asymmetric membranes. However, the casting solutions have already a high viscosity at room temperature (about 103-105 cP [17,23]), and their viscosity further increases several times with the addition of PEG [17]. To ensure the effective mixing, filtration, and degassing, the casting solution is typically prepared at an elevated temperature. Table1 presents some examples of the protocol of preparation of casting solutions where PEG was used as the pore-forming agent. It can be noted that the casting solution was often prepared at temperature 60–130 ◦C in the presence of atmospheric oxygen since those polymers are thermally stable. For instance, PSf is stable for thermo-oxidative destruction for an extended period (more than a year) even at temperature 140 ◦C. Whereas, other components of casting solution such as NMP and PEG possess lower stability towards degradation at elevated temperatures. It was shown that NMP reacts in the presence of atmospheric oxygen at the temperature of 60–80 ◦C with the formation of peroxides [24], upon the destruction of which N-methylsuccinimide and atomic oxygen are formed. Thereby, NMP does not only undergo thermal degradation but can also act as a promoter of further oxidation of other components in the common solution [24]. Furthermore, PEG can also be oxidized in the presence of atmospheric oxygen forming formic acid, while PEG end groups can transform from the hydroxyl to the carbonyl, carboxyl, or ester groups [25–27]. The degradation process of PEG is associated with the reduction of its molecular weight. Any variation in the chemical composition of the casting solution might cause certain changes in the thermodynamics and kinetics of the phase separation process during the formation of asymmetric membranes, and finally, their porous structure and separation performance. Bearing this in mind, the goal of this study was the evaluation of the influence of thermal treatment during the preparation of casting solutions on the structure and transport properties of the flat-sheet and hollow fiber membranes prepared by the non-solvent induced phase separation (NIPS) method. Polysulfone was selected as membrane material due to its good mechanical and film-forming properties, thermal and chemical stability [28]. NMP and PEG were used as a solvent and pore-forming component, respectively, due to their wide applications in the fabrication of different types of membranes. Fibers 2019, 7, 110 3 of 16

Table 1. Conditions of fabrication of asymmetric hollow ﬁber membranes by non-solvent induced phase separation (NIPS) method with polyethylene glycol (PEG) as the pore-forming component.

Preparation Pore-Forming Preparation Polymer Solvent Temperature, Ref. Component Time, h ◦C PES PEG-200, –400, –600 DMFA 80 - [29] PVDF PEG-600 DMAA 60 24 [30] PES PEG-10000 NMP 60 48 [31] NMP/water PSf PEG-200 25–27 - [32] (95/5) PEG-600, -6000, -20000, PSf NMP 70 24 [33] -35000, -150000 PES PEG-10000 NMP 45 - [34] PVDF + HFP PEG-200, -600, -6000 NMP 60 72 [35] PSf + HBPE PEG-400 NMP 25 24 [36] PEG-200, -600, -6000, PES NMP 25 - [37] -10000 PES DEG NMP 25 - [38] PEG-200, -400, -600, PSf DMFA 60 12 [39] -4000, -20000, -35000 Polylactide PEG-6000 DMSO, NMP 130/90 3/-[40] PES DEG NMP 80 24 [41] PSf PEG-400 DMAA 120 4 [42] PEG-400, -2000, -6000, PPSU NMP 120 4 [43] -20000, -35000, -40000

2. Experimental Part

2.1. Materials The materials used to prepare spinning solutions were PSf pellets, Ultrason®S 6010 (from BASF, Ludwigshafen, Germany) and N-methyl-2-pyrrolidone (NMP 99% extra pure) supplied by Acros Organics, used as the base polymer and solvent, respectively, with no supplementary puriﬁcation. The pore-forming additive used in the polymer solution was polyethylene glycol having an average molecular weight of 400 g/mol (PEG-400) supplied by Acros Organics (Geel, Belgium).

2.2. Preparation of Spinning Solution For spinning solutions preparation, the mass ratio PSf to PEG-400 was 1:1.25. PSf concentration was 23.9 wt%. PSf and PEG-400 were placed into the temperature-controlled reservoir and stirred under 70 ◦C temperature and mixing rate 150 rpm; after that, NMP solvent was added into the solution and stirring continued under 120 ◦C and 500 rpm for 6–24 h. After preparation, the spinning solution was cooled to 23 0.1 C and its dynamic viscosity was measured using Brookfield viscometer DV2T-RV ± ◦ and RV-07 spindle (rotation speed 100 rpm). Hollow fiber membranes spinning was preceded by casting solution heating to 120 ◦C to reduce its viscosity. Then, the polymer solution was filtered through the metal mesh (cutoff rating 4–5 µm) under gas pressure 0.18–0.20 MPa. After filtration, the polymer solution was cooled to room temperature and degassed under vacuum.

2.3. Flat Membranes Preparation Flat membranes were prepared according to [44]. The polymer solution was cast onto a glass plate using a casting blade; the formed thin ﬁlm was subsequently immersed into distilled water. The time interval between the application of solution on the glass and immersion in water did not exceed 10 s. This time was the same for all ﬁlms. The temperature of the coagulation bath (distilled water) was kept 25 ◦C. The resulting membranes were rinsed to remove the residual solvent and stored in Fibers 2019, 7, 110 4 of 16 distilled water at least for 24 h. Then the membrane was sequentially washed with ethanol and hexane (each time for an hour) and dried at room temperature in air.

2.4. Fabrication of Hollow Fiber Membranes Hollow fiber membranes were prepared via a dry–wet phase inversion technique in the free fall spinning mode in the air when bore fluid was brought into liquid polymer solution orifice, resulting in the selective layer appearance on the fibers lumen side. The air gap height was 0.8 m, the pressure above the polymer solution was 200 kPa, and the pressure above the internal coagulant was 40 kPa. After passing the air gap, the spun fiber gets into the take-up bath with water by gravity and coils of its own accord. The spinneret ring sectional area was 1.77 mm2. Hollow fibers prepared upon contact with an aqueous medium (internal non-solvent and take-up bath with water) were subsequently treated and dried to remove residual water from pore volume. For this purpose, membranes were washed in a polar solvent (ethanol), then in nonpolar solvent (hexane) and, finally, dried under ambient conditions. The method was used to prevent capillary mesopores contraction [45], which may be present when water is drastically removed by conventional drying.

2.5. Gas Permeance Measurements Helium and carbon dioxide were used as test gases, as their molecular mass difference provides a reliable way to determine Knudsen gas flow using the ideal selectivity value (permeability coefficients of individual gases ratio). The pure gas permeances through flat and hollow fiber membranes were measured using a volumetric membrane apparatus. The hollow fiber membrane was set inside the measuring cell. The gas flow was introduced to the internal volume of the sample. The volumetric gas flow passing through the membrane was measured using a dry gas meter (SHINAGAWA, Japan). Gas permeance measurements were carried out at room temperature (23 2 C) under transmembrane ± ◦ pressure from 0.5 to 2 bar while permeate gas pressure was kept constant at 1 bar. Gas permeance was calculated using the equation: " # P Q m3 = , (1) l p S m2 h atm · · · where Q—gas volumetric flow rate through the membrane, m3/h; p—transmembrane pressure, atm; S—membrane surface, m2. The gas volumetric flow rate was calculated using the equation: " # V m3 Q = , (2) τ h where V—the volume of gas passed through the membrane, m3; τ—time for the specified gas volume transfer, h. The membrane area was calculated using the equation: h i S = π d l m2 , (3) · · where π—constant equal to 3.14; d—hollow fiber internal diameter, m; l—fiber length, m. Ideal selectivity was calculated using the equation:

(P1/l) P α = = 1 , (4) (P2/l) P2 where (P /l)—He permeance, m3/(m2 h atm); (P /l)—CO permeance, m3/(m2 h atm). 1 · · 2 2 · · 2.6. Spectrophotometric Analysis The optical properties of casting solutions studied in this work were examined by using the spectrophotometer PE-5400UF («Ekroshim» Co. Ltd., Kaliningrad, Russia). Measurements were conducted in the regime of optical density (A) detection. The optical spectrum of the initial NMP Fibers 2019, 7, 110 5 of 16 and NMP/PEG mixture were determined with respect to air. The change in optical density of PEG, PEG/NMP mixture, and NMP mixtures with ethylene glycol (EG) and tetraethylene glycol dimethyl ether (tetraglyme) after the thermal treatment was characterized with respect to the initial substances (untreated thermally).

2.7. Coagulation Value The coagulation value was used as a measure of the casting solution thermodynamic stability [16]. It is equal to the weight of the H2O/NMP mixture, added to 1 g of the casting solution at the temperature of 25 ◦C, when the coagulation was observed visually, and the solution does not become homogeneous in 24 h. The H2O/NMP ratio in the coagulant was 25/75 wt%, respectively.

2.8. Scanning Electron Microscopy Cross-sections of the hollow ﬁber membranes were analyzed by scanning electron microscopy (SEM) on a Hitachi Table top Microscope TM 3030 Plus with proprietary highly sensitive low-vacuum secondary electron detector (Hitachi High Technologies America Inc., Greenville, SC, USA), accelerating voltage was equal to 15 kV.

3. Results and Discussion

3.1. Effect of Heat Treatment on Spinning Solution Properties As discussed above, the spinning solutions are typically viscous due to the high concentration of polymer and the presence of different additives such as pore-forming components (e.g., PEG). However, the defect-free hollow fibers with the desired, reproducible parameters can be yielded in the case of good homogeneity of the casting solution that makes important appropriate processing of all prior steps, including mixing, filtering, and degassing. To overcome the problem of high viscosity and to reduce the time, the spinning solution is usually processed at elevated temperatures, and in this study, the temperature of 120 ◦C was selected to conform to the maximal temperature used in literature to prepare casting solutions with PEG and NMP. The exposure time of PSf-based spinning solution at 120 ◦C was 1, 2, 6, 11, 16, and 24 h, respectively. Figure1 shows the spinning solutions after 6, 16, and 24 h of stirring at 120 ◦C and 9 h at 140 ◦C. It can be seen that the color of the polymer solution changed from light yellow to brown with respect to exposure time and temperature, and the more intense color was found in the case of 140 ◦C. Figure2 shows the optical spectra of PEG, exposed to air at 120 ◦C for 2, 6, and 16 h, obtained with respect to initial PEG. All spectra have a maximum of absorption at the wavelength of 250 nm, which conforms to the UV region. Since no maximums of absorbance were observed in the visible region, it can be concluded from the literature [25–27] that PEG and its degradation products are not the cause of the change of the polymer solution color. On the other hand, the increase of absorption in the UV-range with longer exposure time at 120 ◦C can be attributed to the degradation of PEG molecules. Fibers 2019, 7, 110 6 of 16 Fibers 2019, 7, x FOR PEER REVIEW 6 of 15 Fibers 2019, 7, x FOR PEER REVIEW 6 of 15

Figure 1. Photographs of casting solutions obtained at different times and temperatures of mixing. Figure 1. PhotographsPhotographs of of casting solutions obtained at different diﬀerent times and temperatures of mixing.

6 6

5 2 h 5 2 h 6 h 4 6 h 4 16 h 16 h

3 3

Optical density Optical 2

1 1

0 0 0 200 400 600 800 1000 1200 0 200 400Wavelength, 600 800nm 1000 1200 Wavelength, nm

Figure 2. The polyethylene glycol (PEG) optical spectra after the thermal treatment at 120 ◦C with respectFigure to 2. the The initial polyethylene PEG. glycol (PEG) optical spectra after the thermal treatment at 120 °С with Figure 2. The polyethylene glycol (PEG) optical spectra after the thermal treatment at 120 °С with respect to the initial PEG. Torespect get insightto the initial into PEG. the degradation process, the samples of pure NMP and the mixture of NMP with PEGTo get (75 insight/25 wt%) into were the alsodegradation annealed process, at the temperature the samples of of 120 pure◦C. NMP It should and bethe pointed mixture out of NMP that To get insight into the degradation process, the samples of pure NMP and the mixture of NMP bothwith spectra PEG (75/25 for untreated wt%) were NMP also and annealed NMP/PEG at the samples temperature were nearly of 120 the °C. same It should and they be pointed are transparent out that with PEG (75/25 wt%) were also annealed at the temperature of 120 °C. It should be pointed out that inboth the visiblespectra range. for untreated However, NMP the pronounced and NMP/PEG diﬀerence samples in their were behavior nearly wasthe same found and during they heat are both spectra for untreated NMP and NMP/PEG samples were nearly the same and they are treatmenttransparent as canin the be visible seen from range. the However, optical spectra the pr inonounced Figure3 .difference In both spectra in their of behavior thermally was treated found transparent in the visible range. However, the pronounced difference in their behavior was found samples,during heat a shoulder treatment appears as can with be seen the maximal from the optical optica densityl spectra in in the Figure blue-violet 3. In both region spectra of the of spectrum, thermally during heat treatment as can be seen from the optical spectra in Figure 3. In both spectra of thermally whichtreated decreases samples, along a shoulder the long-wave appears with region the of maxima spectra.l optical This explains density the in the yellow-orange blue-violet region color of of the the treated samples, a shoulder appears with the maximal optical density in the blue-violet region of the thermallyspectrum, treated which liquids decreases since along blue isthe an long-wave additional colorregion to of yellow spectra. and This orange. explains It is known the yellow-orange [46,47] that spectrum, which decreases along the long-wave region of spectra. This explains the yellow-orange NMPcolor has of the a tendency thermally for treated oxidation liquids in the since presence blue ofis oxygenan additional from the color air atto anyellow elevated and temperature orange. It is color of the thermally treated liquids since blue is an additional color to yellow and orange. It is withknown the formation[46,47] that of NMP yellow-brown has a tendency colored for products oxidation of in its the oxidation presence and of followedoxygen from polymerization the air at an known [46,47] that NMP has a tendency for oxidation in the presence of oxygen from the air at an (gumming).elevated temperature The increase with of optical the formation density with of yello the timew-brown of thermal colored treatment products at 120of its◦C oxidation indicates theand elevated temperature with the formation of yellow-brown colored products of its oxidation and followed polymerization (gumming). The increase of optical density with the time of thermal followed polymerization (gumming). The increase of optical density with the time of thermal treatment at 120 °C indicates the accumulation of NMP degradation products. To get the insight of treatment at 120 °C indicates the accumulation of NMP degradation products. To get the insight of

Fibers 2019, 7, 110 7 of 16 Fibers 2019, 7, x FOR PEER REVIEW 7 of 15 theaccumulation degradation of NMPprocess, degradation the samples products. of pure To NM getP the and insight the mixture of the degradation of NMP with process, PEG (75/25 the samples wt%) wereof pure also NMP annealed and the at mixture the temperature of NMP with of PEG120 (75°C./ 25It wt%)should were be alsopointed annealed out that at the both temperature spectra for of 120untreated◦C. It shouldNMP and be pointedNMP/PEG out samples that both were spectra nearly for th untreatede same and NMP they and are NMPtransparent/PEG samples in the visible were range.nearly theHowever, same and thethey pronounced are transparent difference in the in visibletheir behavior range. However, was found the during pronounced heat treatment diﬀerence as in cantheir be behavior seen from was the found optical during spectra heat treatmentFigure 3. asIn can both be seenspectra from of thethermally optical spectratreated Figuresamples,3. In a shoulderboth spectra appears of thermally with the treatedmaximal samples, optical adensity shoulder in the appears blue-violet with theregion maximal of theoptical spectrum, density which in decreasesthe blue-violet along region the long-wave of the spectrum, region which of spectra. decreases This along explains the long-wave the yellow-orange region of spectra.color of Thisthe thermallyexplains the treated yellow-orange liquids since color blue of theis an thermally additional treated color liquidsto yellow since and blue orange. is an It additional is known color [46,47] to thatyellow NMP and has orange. a tendency It is known for oxidation [46,47] that in NMPthe pres hasence a tendency of oxygen for from oxidation the air in theat an presence elevated of temperatureoxygen from thewith air the at anformation elevated of temperature yellow-brown with colored the formation products of yellow-brownof its oxidation colored and followed products polymerizationof its oxidation and(gumming). followed The polymerization increase of optical (gumming). density The with increase the time of optical of thermal density treatment with the at time 120 °Cof thermalindicates treatment the accumulation at 120 ◦C of indicates NMP degradation the accumulation products. of NMP degradation products.

NMP NMP/PEG 6 6 1 h 5 5 1 h 2 h 2 h 6 h 4 4 6 h 24 h 24 h 3 3 Optical density 2 Optical density Optical 2

1 1

0 100 200 300 400 500 600 700 800 900 0 Wavelength, nm 100 200 300 400 500 600 700 800 900 Wavelength, nm

(a) (b)

Figure 3. Figure 3. TheThe N-methyl-2-pyrrolidone N-methyl-2-pyrrolidone (NMP) and NMP/PEG NMP/PEG (75/25) (75/25) optical spectra after the thermal treatment at 120 C with respect to the initial NMP (a) and NMP/PEG (b). treatment at 120 ◦°С with respect to the initial NMP (a) and NMP/PEG (b). In the first 1–6 h of thermal treatment, the intensity of NMP and NMP/PEG light absorption builds In the first 1–6 h of thermal treatment, the intensity of NMP and NMP/PEG light absorption up slowly. However, after 24 h of thermal treatment, the intensity of coloration and optical density of builds up slowly. However, after 24 h of thermal treatment, the intensity of coloration and optical NMP becomes markedly higher compared to the NMP/PEG mixture. It seems that the addition of PEG density of NMP becomes markedly higher compared to the NMP/PEG mixture. It seems that the in NMP led to a less pronounced degradation process in contrast to pure NMP. Bearing in mind the fact addition of PEG in NMP led to a less pronounced degradation process in contrast to pure NMP. that PEG molecules might also interact with the dissolved oxygen or other components presented in a Bearing in mind the fact that PEG molecules might also interact with the dissolved oxygen or other common solution, the effect of hydroxyl groups on NMP degradation was evaluated. To this end, NMP components presented in a common solution, the effect of hydroxyl groups on NMP degradation solvent was also heated for 24 h at 120 C in the presence of 5 wt% of ethylene glycol (EG) or 25 wt% was evaluated. To this end, NMP solvent◦ was also heated for 24 h at 120 °C in the presence of 5 wt% tetraethylene glycol dimethyl ether (tetraglyme). Tetraglyme, being analog of PEG, does not contain of ethylene glycol (EG) or 25 wt% tetraethylene glycol dimethyl ether (tetraglyme). Tetraglyme, ending hydroxyl groups since they are replaced with the dimethyl ether group; while ethylene glycol being analog of PEG, does not contain ending hydroxyl groups since they are replaced with the is a diol. It is interesting to notice from Figure4 that both systems of pure NMP and NMP /tetraglyme dimethyl ether group; while ethylene glycol is a diol. It is interesting to notice from Figure 4 that did not reveal a noticeable difference in the optical spectra that might indicate a certain similarity in both systems of pure NMP and NMP/tetraglyme did not reveal a noticeable difference in the optical the degradation process; meanwhile, the presence of ethylene glycol and PEG allowed to reduce the spectra that might indicate a certain similarity in the degradation process; meanwhile, the presence coloration of the NMP-based solution. Thus, it allows to conclude that the hydroxy groups presented of ethylene glycol and PEG allowed to reduce the coloration of the NMP-based solution. Thus, it in PEG or ethylene glycol shall play a noticeable role in the process of thermo-oxidative degradation of allows to conclude that the hydroxy groups presented in PEG or ethylene glycol shall play a the NMP solvent and spinning solution, respectively. noticeable role in the process of thermo-oxidative degradation of the NMP solvent and spinning solution, respectively.

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6 NMP/PEG

5 NMP/EG NMP/tetraglyme 4 NMP

3 Opticaldensity 2

0 100 200 300 400 500 600 700 800 900 Wavelength, nm

Figure 4. The optical spectra of NMP, NMP/PEG, NMP/ethylene glycol (EG), and NMP /tetraglyme

afterFigure the 4. thermal The optical treatment spectra at 120 of NMP,◦C for NMP/PEG, 24 h with respect NMP/ toethylene the initial glycol samples. (EG), and NMP/tetraglyme after the thermal treatment at 120 °С for 24 h with respect to the initial samples. The analysis of casting solutions is complicated due to their high viscosity and tendency to phase separationThe analysis at the contact of casting with watersolutions vapor is complicated from the air. due Therefore, to their the high same viscosity amount and of each tendency casting to solutionphase separation was coagulated at the incontact the fixed with amount water ofvapor water, from then the the air. coagulation Therefore, bath the same was filtered amount through of each filtercasting paper. solution The same was procedurecoagulated was in the also fixed done amount for the referenceof water, spinningthen the solutioncoagulation containing bath was 23.9 filtered wt% PSfthrough in NMP filter without paper. addition The same of PEG-400. procedure It iswas important also done to notice for the that reference all filtered spinning solutions solution were visuallycontaining colorless, 23.9 wt% while PSf the in residuals, NMP without looked addition like resin of and PEG-400. remained It is on important filtered paper, to notice had athat light all brownfiltered color, solutions and were were insoluble visually in colorless, water. The while analysis the ofresiduals, the optical looked density like of theresin filtered and remained coagulation on bathfiltered used paper, for precipitation had a light of brown the PSf color,/NMP and/PEG were spinning insoluble solution in water. revealed The the analysis peak at of 270–280 the optical nm, anddensity the intensityof the filtered of this coagulation peak was increasedbath used withfor precipitation a longer time of the of thermal PSf/NMP/PEG exposure spinning (see Figure solution5). Furthermore,revealed the itpeak can at be 270–280 noted that nm, exposition and the intensity time has of more this epeakffect was on the increased intensity with of absorption a longer time than of thethermal treatment exposure temperature (see Figure because 5). Furthermore, the higher peak it can was be observed noted that in the exposition case of 16 time h treatment has more at effect 120 ◦ Con comparedthe intensity with of 11 absorption h at 140 ◦C. than Since the no treatment such a peak temp waserature found because in the casethe higher of the referencepeak was PSf observed solution in (withoutthe case additionof 16 h treatment of PEG), itat can 120 be °C concluded compared that with the 11 water-soluble hours at 140 components°C. Since no ofsuch the a degraded peak was solutionfound in have the case PEG-based of the reference origin, and PSf PEG solution reacts (without in the spinning addition solution of PEG), at ita can high betemperature. concluded that the water-soluble components of the degraded solution have PEG-based origin, and PEG reacts in the spinning solution at a high temperature.

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140 С, 11 h 5 120 С (without PEG), 24 h

4 120 С, 16 h

120 С, 24 h 3 Optical density Optical 2

0 200 250 300 350 400 Wavelength, nm

Figure 5. The optical spectra of the water phase after polysulfone (PSf) precipitation with respect to the Figure 5. The optical spectra of the water phase after polysulfone (PSf) precipitation with respect to water phase without PEG. the water phase without PEG. 3.2. Flat Membranes 3.2. Flat Membranes It is important to emphasize that the chemical interactions of the components of the casting solutionIt is duringimportant the to heat emphasize treatment that process the chemical have a significant interactions eff ectof the on thecomponents viscosity ofof thethe solutioncasting solutionresulted during in a change the heat of the treatment membrane process morphology have a and significant their transport effect on characteristics. the viscosity Inof the the beginning, solution resultedthe effect in of a thermal change treatment of the membrane on the membrane morphology characteristics and their was transport evaluated characteristics. by using the flat-sheet In the beginning,configuration the ofeffect membranes. of thermal As treatment can be seen on from the Tablemembrane2, an increase characteristics in the heat was treatment evaluated time by fromusing 6 theto 24flat-sheet h leads toconfiguration an increase in of the me viscositymbranes. of theAs castingcan be solutionseen from by aTa factorble 2, of an 1.5. increase The structure in the ofheat the treatmentmembranes time was from visualized 6 to 24 andh leads examined to an increase using SEM in the (Figure viscosity6). It of was the found castin thatg solution the membranes by a factor have of 1.5.an asymmetricThe structure structure of the withmembranes the thin was selective visualiz layered andand the examined porous supportusing SEM pierced (Figure with 6). elongated It was foundmacrovoids, that the and membranes the flat-sheet have membranesan asymmetric with structure a similar with structure the thin were selective obtained layer in and studies the porous [43,48]. supportThe thickness pierced of with the fine-porous elongated macrovoids, selective layer and was the estimated flat-sheet by membranes analysis of with SEM a cross-sectionssimilar structure and werethe obtained obtained values in studies are listed [43,48]. in TableThe 2thickness. A substantial of the increasefine-porous in the selective selective layer layer was thickness estimated from by 3 analysisto 15 µm of was SEM observed cross-sections with the and rise the of obtained the thermal values treatment are listed time, in Table which 2. might A substantial be attributed increase to thein theincrease selective of the layer casting thickness solution from viscosity. 3 to 15 However, μm was theobserved overall with membrane the rise thickness of the thermal remains treatment nearly the time,same, which at around might 100 beµ attributedm. This is to connected the increase to the of the fact casting that the solution casting viscosity. solution was However, applied the on overall a glass membraneplate using thickness the casting remains blade with near aly 200 theµ msame, size at of thearound gap, 100 and μ them. polymerThis is connected concentration to the is supposedfact that theto becasting the same solution during was the applied thermal on treatment. a glass plate using the casting blade with a 200 μm size of the gap, and the polymer concentration is supposed to be the same during the thermal treatment. A changeTable 2.in Geometricalthe thickness and of gas the transport selective characteristics layer naturally of flat polysulfoneaffects the (PSf) transport membranes. properties of resulted membranes. For instance, a five time increase of the selective layer thickness from 3 to 15 Selective Membrane μm resultedTime of in about five times the decline of thLayere gas permeanceP/l (He), (see TableP /2)l (CO since), the fine-porous Viscosity, cP Thickness, 2 α (He/CO ) selectiveTreatment, layer h introduces the major contributionThickness, in them 3mass-transport/(m2 h atm) m 3resistance/(m2 h atm) of the whole2 µm · · · · membrane. Sponge-like and macroporous layersµm act as support in the asymmetric membrane. The rise of 6the ideal selectivity 31,200 α (He/CO 1022) with the 3 increase of 41.1the selective layer 15.9 thickness is 2.6 worth 11 37,000 103 10 22.2 7.9 2.8 noting. The maximal ideal selectivity α (He/CO2) = 3.0 is observed for the selective layer thickness of 16 39,900 99 11 19.3 6.6 3.0 15 μm.24 This value 46,900is close to the 102value of selectivity 15 α (He/CO 8.72) = 3.3, which 2.9 is typical 3.0for the Knudsen flow. Thus, it may be concluded that in the case of flat asymmetric membranes, the change in the chemical composition of low-molecular components of the casting solution and its viscosity during the thermal treatment leads to the increase of the selective layer thickness and its densification. Similar dependencies were observed in the work [49], where the effect of PSf concentration in the casting solution on the gas transport properties of flat-sheet membranes was

Fibers 2019, 7, x FOR PEER REVIEW 10 of 15

Fibersstudied.2019 , With7, 110 the increase of polysulfone concentration, the casting solution viscosity increased,10 of 16 which, in turn, explained the decline of gas permeance and the rise of membrane selectivity.

(a) (b)

Figure 6. Scanning electron microscopy (SEM) micrographs of flat PSf membranes obtained from solutionsFigure 6. withScanning different electron time ofmicroscopy thermal treatment (SEM) micrographs at 120 ◦C(a) of 6 h,flat (b )PSf 11 h,membranes (c) 16 h, (d )obtained 24 h. from solutions with different time of thermal treatment at 120 °С (a) 6 h, (b) 11 h, (c) 16 h, (d) 24 h. A change in the thickness of the selective layer naturally affects the transport properties of resulted membranes. For instance, a five time increase of the selective layer thickness from 3 to 15 µm resulted Table 2. Geometrical and gas transport characteristics of flat polysulfone (PSf) membranes. in about five times the decline of the gas permeance (see Table2) since the fine-porous selective layer introduces the major contribution in the mass-transportSelective resistance of the whole membrane. Sponge-like Time of Membrane P/l (He), P/l (CO2), Viscosity Layer α Treatment,and macroporous layers actThickness, as support in the asymmetric membrane.m3/(m2∙h∙at The risem3 of/(m the2∙h ideal∙at selectivity α , cP Thickness, (He/CO2) (Heh/ CO2) with the increase ofμm the selective layer thickness ism) worth noting.m) The maximal ideal selectivity α (He/CO2) = 3.0 is observed for the selectiveμm layer thickness of 15 µm. This value is close to the6 value of selectivity31,200 α (He102/CO 2) = 3.3, which 3 is typical for 41.1 the Knudsen flow. 15.9 Thus, it may 2.6 be concluded11 that37,000 in the case of flat 103 asymmetric membranes, 10 the change 22.2 in the chemical 7.9 composition 2.8 of low-molecular16 components39,900 of the 99 casting solution 11 and its viscosity 19.3 during the thermal 6.6 treatment 3.0 leads to the24 increase of46,900 the selective layer 102 thickness and its 15 densification. Similar8.7 dependencies 2.9 were observed 3.0 in the work [49], where the effect of PSf concentration in the casting solution on the gas transport properties3.3. Hollow ofFiber flat-sheet Membranes membranes was studied. With the increase of polysulfone concentration, the casting solution viscosity increased, which, in turn, explained the decline of gas permeance and the rise ofFigure membrane 7 presents selectivity. SEM images of the cross-section (up) and external surface (down) of hollow fiber membranes, obtained from the spinning solutions treated with a different time at 120 °C. Evidently, the membranes have an asymmetric structure with the thin selective layer and the porous support pierced with finger-like macrovoids. Meanwhile, for the first sample (6 h of thermal

Fibers 2019, 7, 110 11 of 16

3.3. Hollow Fiber Membranes Figure7 presents SEM images of the cross-section (up) and external surface (down) of hollow fiber membranes, obtained from the spinning solutions treated with a different time at 120 ◦C. Evidently, the membranes have an asymmetric structure with the thin selective layer and the porous support pierced with finger-like macrovoids. Meanwhile, for the first sample (6 h of thermal treatment), macropores Fibers 2019, 7, x FOR PEER REVIEW 11 of 15 revealed on the outer side of the hollow fiber membrane. The increase of thermal treatment time (11, thermal16, and 24treatment h) resulted time in (11, the 16, formation and 24 h) of resulted the sponge-like in the formation structure of the sponge-like undersurface structure layer from of the the undersurfaceouter side of thelayer membrane, from the and outer in theside case of the of 24 membrane, h of treatment, and thein the thickness case of of 24 this h of layer treatment, was around the thickness50 µm. of this layer was around 50 μm.

Figure 7. SEM images of the cross-sections (upper row) and the outer surface (lower row) of the hollow Figure 7. SEM images of the cross-sections (upper row) and the outer surface (lower row) of the fiber membranes obtained from solutions with different time of thermal treatment at 120 ◦C: (a) 6 h, (b) hollow11 h, (c )fiber 16 h, membranes (d) 24 h. obtained from solutions with different time of thermal treatment at 120 °С: (a) 6 h, (b) 11 h, (c) 16 h, (d) 24 h. By analysis of the SEM images, the geometrical parameters of the fiber, such as average outer and By analysis of the SEM images, the geometrical parameters of the fiber, such as average outer inner diameters (dout иdin) and fiber wall thickness were estimated. Besides, the average weight of and inner diameters (dout и din) and fiber wall thickness were estimated. Besides, the average weight 1 meter of fiber (ml) was also determined. Given the outer and inner diameters of the fiber, and the of 1 meter of fiber (ml) was also determined. Given the outer and inner diameters of the fiber, and the weight of its unit length, the average density was calculated by using the following equation: weight of its unit length, the average density was calculated by using the following equation:

4ml 3 = 4𝑚 , g/cm . (5) ρ𝜌= ,g/cm . π d2 d2 (5) 𝜋(𝑑out −−𝑑in)

TheThe diameters diameters dd andand average average weight of fiber ﬁber ml werewere given given in cm and g/cm, g/cm, respectively. ItIt isis well-knownwell-known that that the morphologythe morphology and transport and transport properties properties of the phase-inversion of the phase-inversion membranes membranessubstantially substantially depend on the depend thermodynamic on the thermodynamic stability of casting stability solutions of andcasting the kineticssolutions of theand phase the kineticsdecay [16 of, 50the]. phase It can decay be seen [16,50]. from TableIt can3 bethat seen all from membrane Table 3 samples that all havemembrane close coagulationsamples have values. close coagulationTherefore, despite values.the Therefore, chemical despite transformations the chemic takingal transformations place in the castingtaking solution,place in thethe stabilitycasting solution,of corresponded the stability polymeric of corresponded solutions towardspolymeric the solutions coagulation towards did the not coagulation change. Consequently, did not change. the Consequently,diﬀerences in the the membrane differences morphology in the membrane with the mo increaserphology of thermal with the treatment increase time of thermal shall be treatment connected timewith shall the coagulation be connected kinetics. with the coagulation kinetics.

Table 3. Properties of the casting solutions, density, and gas permeability of the hollow fiber membranes.

Treatment Viscosity, Coag. Density, P/l (He), P/l (CO2), α Time, h cP Value, g/g g/cm3 m3/( m2∙h∙atm) m3/( m2∙h∙atm) (He/CO2) 6 31,100 0.182 0.253 580 240 2.4 11 37,300 0.183 0.276 54 18 3.0 16 40,100 0.183 0.284 42 13 3.3 24 46,800 0.185 0.313 30 10 3.3

With increasing heat treatment time, the viscosity of the spinning solution increased significantly. The rate of phase separation is determined by the diffusion of the non-solvent (water) into the polymeric solution. With an increase in the viscosity of the solution, the diffusion flux of the

Fibers 2019, 7, 110 12 of 16

Table 3. Properties of the casting solutions, density, and gas permeability of the hollow ﬁber membranes.

Treatment Coag. Value, Density, P/l (He), P/l (CO ), Viscosity, cP 2 α (He/CO ) Time, h g/g g/cm3 m3/(m2 h atm) m3/(m2 h atm) 2 · · · · 6 31,100 0.182 0.253 580 240 2.4 11 37,300 0.183 0.276 54 18 3.0 16 40,100 0.183 0.284 42 13 3.3 24 46,800 0.185 0.313 30 10 3.3

With increasing heat treatment time, the viscosity of the spinning solution increased significantly. The rate of phase separation is determined by the diffusion of the non-solvent (water) into the polymeric solution. With an increase in the viscosity of the solution, the diffusion flux of the non-solvent into the PSf solution decreases, and the relaxation time of polymer chains in the solution also increases [16,50]. This leads to a slowdown in phase decomposition and the formation of a denser membrane structure (increase in membrane density from 0.253 to 0.313 g/cm3, Table2) with less pronounced macropores (Figure7). In the case of flat-sheet membranes preparation, the solution was placed on the glass surface. Thereby, the coagulant (water) contacted with only one side of the membrane. However, the hollow fiber membranes were obtained in the free fall spinning mode, i.e., at the outlet of the casting solution from the spinneret, the inner surface was brought into contact with the internal coagulant (water). After passing the air gap, the fiber fell into the take-up bath with water. Thereby, from that moment, the outer surface of the membrane began to contact the coagulant. The lowest solution viscosity (31,100 cP, 6 h) allows forming an asymmetric membrane structure through the whole wall thickness of the hollow fiber during the time of the air gap passage. Therefore, only in this case, macropores outcrop on the external surface of the hollow fiber (Figure7). The increase of the solution viscosity results in the asymmetric membrane structure not having enough time to fully form along the whole thickness of the hollow fiber and at the outer surface of the membrane spongy structure is formed. For the solution with viscosity 46,800 cP (24 h), its thickness is around 50 µm (Figure7). The analysis of gas permeance and ideal He/CO2 selectivity revealed that the increase of thermal treatment time and the solution viscosity resulted in the decline of gas permeance and an increase of selectivity. At the exposure time at 120 ◦C of 16 h or greater, He/CO2 selectivity was equal to 3.3, which conforms to the Knudsen flow. The increase of selectivity indicates the decrease of the selective layer pore size due to its densification, while the decrease of gas permeance can be associated with the densification of the selective layer and the formation of the sponge-like layer at the outer side of hollow fibers at the time of 11 h or greater. Similar correlations were obtained in work [51], where the effect of spinning temperature on the structure and transport properties of ultrafiltration asymmetric hollow fiber membranes made of PSf was studied. With the increase of the temperature, the solution viscosity was reduced significantly, which resulted in the slowdown of the polymer coagulation process, a decrease of the dense selective membrane layer thickness, the rise of the water flux through the membrane and reduction of the nominal molecular weight cut-off in the process of dextrane water solution separation [51].

4. Conclusions The thermal treatment of the casting solution substantially affects the morphology and transport properties of porous flat-sheet and hollow fiber membranes made of polysulfone. It was found that with increasing of exposure time at a temperature of 120 ◦C, the color intensity of the casting solution was increased. Spectrophotometric studies showed that the coloration appearance is associated with the NMP thermo-oxidative destruction with the formation of yellow-brown products. Meanwhile, hydroxyl groups of PEG actively react in the thermo-oxidative destruction of the casting solution. Fibers 2019, 7, 110 13 of 16

The results obtained are relevant to any polymeric casting solution containing NMP and/or PEG and treated at temperatures above 60 ◦C. It is found that the casting solution viscosity monotonically rises with the increase of the thermal treatment time. It was concluded that despite the chemical transitions occurring in the casting solutions, their stability towards coagulation does not virtually change. The difference in the membrane morphology with the growth of the thermal treatment time is associated not with the thermodynamic properties of the polymer solutions, but with the phase decay kinetics. All obtained flat and hollow fiber membranes have an asymmetric structure with the selective layer and the porous support pierced with finger-like macrovoids. In the case of flat membranes, the rise of thermal treatment time leads to the growth of the selective layer thickness, which results in the decline of the gas permeance. Meanwhile, the increase of the ideal He/CO2 selectivity allows concluding that the selective layer compacts with the rise of the thermal treatment time. The hollow fiber selective layer was formed at the inner side of the membrane. Meantime, only for the hollow fiber sample, with 6 h of thermal treatment macrovoids outcrop on the external surface of the membrane. The increase of thermal treatment time (11, 16, and 24 h) leads to the formation of a sponge-like layer at the outermost surface of the membrane, therewith, in case of 24 h treatment, the thickness of this layer is around 50 µm. After more than 16 h of the solution thermal treatment, the He/CO2 selectivity is equal to 3.3, which conforms to the Knudsen gas flow. The increase of selectivity suggests the decrease of the selective layer pore size due to densification. The decline of gas permeance with the rise of thermal treatment time is associated both with the selective layer densification and with the formation of a sponge-like layer at the outermost surface of the membrane.

Author Contributions: Conceptualization, I.B., A.V. and V.V. (Vladimir Volkov); methodology, V.V. (Vladimir Vasilevsky), I.B.; investigation, D.M., A.O.; data analysis, I.B., V.V. (Vladimir Vasilevsky), D.M., A.V. and V.V. (Vladimir Volkov); writing—original draft preparation, I.B.; writing—review and editing, V.V. (Vladimir Volkov), D.M. and A.V.; supervision, V.V. (Vladimir Volkov). Funding: This work was carried out within the State Program of the A.V.Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences (TIPS RAS). Acknowledgments: The authors thank D.S. Bakhtin for SEM analysis, and Kirill Kutuzov for his assistance in experimental work. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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