polymers Article Phase Separation within a Thin Layer of Polymer Solution as Prompt Technique to Predict Membrane Morphology and Transport Properties Tatiana Anokhina 1, Ilya Borisov 1,*, Alexey Yushkin 1, Gleb Vaganov 2, Andrey Didenko 2 and Alexey Volkov 1 1 A.V. Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, Leninsky pr., 29, 119991 Moscow, Russia; [email protected] (T.A.); [email protected] (A.Y.); [email protected] (A.V.) 2 Institute of Macromolecular Compounds, Russian Academy of Sciences, Bolshoy pr. V.O., 31, 199004 St. Petersburg, Russia; [email protected] (G.V.); [email protected] (A.D.) * Correspondence: [email protected]; Tel.: +7-(495)-647-59-27 (ext. 2-02); Fax: +7-(495)-633-85-20 Received: 2 November 2020; Accepted: 23 November 2020; Published: 25 November 2020 Abstract: In this work, the precipitation of a thin layer of a polymer solution was proposed to imitate the process of asymmetric membrane formation by a non-solvent induced phase separation (NIPS) technique. The phase inversion within the thin (<500 µm) and bulk (~2 cm) layer of polyamic-acid (PAA) in N-methyl-2-pyrrolidone (NMP) by using water as non-solvent was considered. It was shown that polymer films formed within the “limited” layer of polymer solution showed a good agreement with the morphology of corresponded asymmetric flat-sheet membranes even in the case of three-component casting solution (PAA/NMP/EtOH). At the same time, the polymer films formed on the interface of two bulk phases (“infinite” regime) did not fully correspond to the membrane structure. It was shown that up to 50% of NMP solvent in PAA solution can be replaced by ethanol, which can have a renewable origin. By changing the ethanol content in the casting solution, the average size of transport pores can be varied in the range of 12–80 nm, and the liquid permeance from 16.6 up to 207 kg/m2 h bar. To summarize, the precipitation of polymer solution within the thin layer can · · be considered a prompt technique and a powerful tool for fast screening and optimization of the complex composition of casting solutions using its small quantity. Furthermore, the prediction of membrane morphology can be done without casting the membrane, further post-treatment procedures, and scanning electron microscopy (SEM) analysis. Keywords: polyimides; polyamic acid; flat-sheet membrane; non-solvent induced phase separation; casting solution; kinetics of solution precipitation 1. Introduction Aromatic polyimides (PI) are materials with high mechanical strength and chemical stability due to the rigid structure of the polymer backbone and strong intramolecular and intermolecular interactions [1–4]. However, their insolubility in organic solvents complicates their treatment and the process of preparation of membranes based on them [5,6]. For aromatic polyimides insoluble in organic solvents, Schumann and Strathmann first proposed and patented the process which includes the formation of ultrafiltration and reverse osmosis flat membranes from solutions of a PAA prepolymer with its subsequent thermal or chemical imidization to obtain PI membranes [7]. The difficulty of PAA-based membrane formation by NIPS is often associated with high viscosity of the polymer solution due to the presence of a large number of carboxyl groups that form intermolecular hydrogen bonds. This in turn leads to a low deposition rate in the polymer solution. Therefore, the process of Polymers 2020, 12, 2785; doi:10.3390/polym12122785 www.mdpi.com/journal/polymers Polymers 2020, 12, 2785 2 of 14 choosing the optimal composition of the casting solution for the PAA-based membranes is very challenging, since it includes the determination of the morphology, porosity, transport, and mechanical properties of already formed membranes. To optimize the parameters of the membrane formation by the NIPS method, one resorts to the study of the solution itself: determination of the viscosity, construction of ternary phase diagrams, and study of the kinetics of the solution precipitation [8]. The latter is investigated using the method of a polymer deposition in a layer of an infinite thickness (“drop” method), carried out between a microscope slide and a cover glass. This approach allows visualizing the structure of the polymer layer and determining the deposition rate as the velocity of the deposition front [8–10]; it also makes it possible to determine the mechanism of transport of the precipitant into the volume of the polymer solution and to qualitatively evaluate the effect of the type of the polymer, its concentration, the selected solvent, and precipitant on the kinetics of the solution precipitation. Unfortunately, this method cannot adequately imitate the process of membrane formation, since it does not provide an understanding of the actual time of a polymeric membrane formation with a given thickness (<500 µm) and its morphology. The concentration profiles of the precipitant and solvent are also blurred and do not correspond to the concentration profiles in the thin film of the polymer solution. Therefore, the “drop” method allows only the comparison of the relative deposition rates of two solutions and does not allow the determination of the deposition time of a polymer film with a given thickness. Moreover, the structure of a real membrane can differ significantly from the structure formed during the deposition of a polymer layer of infinite thickness. These differences arise due to the dilution of the precipitant with a solvent and differences in the concentration profiles of the precipitant in the volume of the polymer solution [9]. The goal of this work was the development of a prompt technique to evaluate the kinetics of the deposition of polymers in a thin layer (<500 µm) for the prediction of the porous structure of thin membranes. As a model polymer we chose polyamic acid, which is the product of the reaction between resorcinol dianhydride R (1,3-bis-(3,30,4,40-dicarboxyphenoxy) benzene) and four nuclear diamine BAPB (4,40-bis(4”-aminophenoxy) biphenyl). The thermoplastic polyimide R-BAPB prepared during the imidization of PAA (see Figure1) demonstrates superior mechanical characteristics and is capable of controlled crystallization and recrystallization, as was previously exemplified by films, coatings, and binders for composite materials [5,11–13]. The combination of physicochemical and operational properties of PI R-BAPB, comparable to or exceeding the properties of commercial PI Kapton, Matrimid 5218, P84, ULTEM, makes it a promising material for membranes that are resistant to aggressive solutions, that can perform at elevated temperatures, and can be used in various areas, including organic solvents filtration [14]. Polymers 2020, 12, x FOR PEER REVIEW 3 of 15 FigureFigure 1. 1.Synthesis Synthesis of ofPI R-BAPB. PI R-BAPB. 2. Materials and Methods 2.1. List of Symbols and Acronyms. LLDP—liquid–liquid displacement porometry; NIPS—non–solvent induced phase separation; NMP—N–methyl–2–pyrrolidone; PAA—polyamic acid; PI—polyimide; PSD—pore size distribution; SEM—scanning electron microscopy; d—total thickness of the polymer layer, μm; t—time, s; v—deposition rate in limited layer, μm/s; D—diameter, nm; γ—interfacial tension, 10−3 N/m; θ—contact angle, °; J1—flux of the displacing liquid in the presence of the wetting liquid, l/m2 h; J2—flux the displacing liquid only, l/m2 h; Δp—differential pressure, bar; η—viscosity, mPa∙s; P—permeance, kg/m2 h bar; C—concentration, wt. %. 2.2. Materials The monomers used were 1,3-bis-(3,3′,4,4′-dicarboxyphenoxy) benzene dianhydride (dianhydride R), melting point Tm = 163–165 °С (OOO TekhKhimProm, Republic of Tatarstan, Russia), dried at 140 °С in vacuo; 4,4′-bis(4”-aminophenoxy) biphenyl (BAPB), Tm = 198–199 °С, (VWR Polymers 2020, 12, 2785 3 of 14 2. Materials and Methods 2.1. List of Symbols and Acronyms LLDP—liquid–liquid displacement porometry; NIPS—non–solvent induced phase separation; NMP—N–methyl–2–pyrrolidone; PAA—polyamic acid; PI—polyimide; PSD—pore size distribution; SEM—scanning electron microscopy; d—total thickness of the polymer layer, µm; t—time, s; v—deposition rate in limited layer, µm/s; D—diameter, nm; 3 γ—interfacial tension, 10− N/m; θ—contact angle, ◦; 2 J1—flux of the displacing liquid in the presence of the wetting liquid, L/m h; 2 J2—flux the displacing liquid only, L/m h; Dp—differential pressure, bar; η—viscosity, mPa s; · P—permeance, kg/m2 h bar; C—concentration, wt.%. 2.2. Materials The monomers used were 1,3-bis-(3,30,4,40-dicarboxyphenoxy) benzene dianhydride (dianhydride R), melting point Tm = 163–165 ◦C (OOO TekhKhimProm, Republic of Tatarstan, Russia), dried at 140 ◦C in vacuo; 4,40-bis(4”-aminophenoxy) biphenyl (BAPB), Tm = 198–199 ◦C, (VWR International, Atlanta, GA, USA), dried at 140 ◦C in vacuo. Phthalic anhydride, Tm = 131–134 ◦C (Sigma-Aldrich Co. LLC, St. Louis, MO, USA) was chosen to limit chain growth during polycondensation. N-methyl-2-pyrrolidone (Sigma-Aldrich Co. LLC, St. Louis, MO, USA), used as a solvent, was specially prepared by drying 20 3 20 (over CaH2) and distilling; boiling point Tb = 202 ◦C, density d4 1.024 g cm− , refractive index nD 1.4684. Absolute ethanol was used as a non-solvent introduced into the casting solution, and distilled water was used as a precipitant. 2.3. PAA Synthesis The preparation of a PAA solution for membrane formation was carried out per the procedure described in [13]. PAA was synthesized by polycondensation of 1,3-bis-(3,30,4,40-dicarboxyphenoxy) benzene dianhydride and 4,40-bis(4”-aminophenoxy) biphenyl in N-methyl-2-pyrrolidone (NMP). The equimolar ratio of the reagents was strictly observed during the synthesis. PAA was stirred for 4 h in an inert argon atmosphere at room temperature.