Journal of Membrane Science 623 (2021) 119099

Available online 22 January 20210376-7388/© 2021 Elsevier B.V. All rights reserved.

Molecular tailoring of polystyrene-block-poly (acrylic acid) block copolymer toward additive-free asymmetric isoporous membranes via SNIPS

Kamran Foroutani a,b, Seyed Morteza Ghasemi a,c,*, Behzad Pourabbas a,b

a Faculty of Polymer Engineering, Sahand University of Technology, Sahand New Town, Tabriz, 5331817634, Iran b Department of Polymer Engineering, Nanostructured Materials Research Center, Sahand University of Technology, Tabriz, 5331817634, Iran c Institute of Polymeric Materials, Sahand University of Technology, Sahand New Town, Tabriz, 5331817634, Iran

A R T I C L E I N F O

Keywords: PS-b-PAA membranes SNIPS Molecular parameters Self-assembly Rheological characteristics

A B S T R A C T

Polystyrene-block-poly (acrylic acid) (PS-b-PAA) diblock copolymers with different compositions were system-atically synthesized by the reversible addition-fragmentation chain transfer (RAFT) polymerization and used to prepare asymmetric isoporous membranes directly through self-assembly and nonsolvent induced phase sepa-ration (SNIPS) process, without using foreign additives. A simple and efficient way, for the first time, to generate additive-free asymmetric isoporous PS-b-PAA membranes was enabled just by choosing the right blocks composition. Dynamic light scattering (DLS) measurements at dilute concentrations and solution small angle X- ray scattering (solution SAXS) analysis plus rheological measurements in concentrated regime as well as field emission scanning electron microscopy (FE-SEM) were employed to investigate the relationship between prop-erties of different block copolymer casting solutions and final membrane structures. DLS measurements and FE- SEM micrographs revealed that only micellar solutions with adequate micelle dimensions were able to produce membranes with more uniform surface pores. Micelles with short PS- lengths led to the formation of meso-structures with adequate structural characteristics in the casting solution, whereas rheological characteristics were the worst. A combination of both appropriate structural and rheological characteristics for the casting solution was necessary to achieve perfect membrane structures with ordered, high density and mono size- dispersed surface pores that all were dependent on PS/PAA blocks ratio. In addition to surface pores, mem-brane substructures were also influenced by the PS- block length. Sublayers with mostly finger-like structure along with higher permeability were obtained for the block copolymer with adequate PS- block length. Although dispersity (Ð) value of the synthesized block copolymers was acceptable (≤1.2) to form well-defined ordered isoporous structures, a slight difference in Ð had a decisive role on micellization, casting solution characteristics and final membrane structures. The most appropriate membrane structure showed a ≈150 nm thin selective- layer with ≈14 nm mean pore diameter and relatively high pore density of about 4 × 1014 pores/m2 at top surface, supported by the asymmetric substructure composed of a finger-like upper layer (66% of total thickness) and interconnected open pores at underneath. Considering its free-additive nature and high performances, these PS-b-PAA membranes are promising candidates for a wide range of size-based separation applications including biomedical and pharmaceutical industries.

1. Introduction

Self-assembly and nonsolvent induced phase separation (SNIPS) process is considered as the most demanded approach and most appli-cable strategy in the fabrication of isoporous block copolymer mem-branes with unique structures as well as high performances [1–4]. This

fast, one-step and scalable technique involves a three-step procedure; (1) preparing an appropriate block copolymer solution (2) doctor-blading the prepared copolymer solution as a film, and then, solvent evapora-tion for a given time, and (3) immersion the solution-cast film into a nonsolvent bath (mostly water) [2,4]. Hence, the final membrane structure is determined by several parameters that should be carefully

* Corresponding author. Faculty of Polymer Engineering, Sahand University of Technology, Sahand New Town, Tabriz, 5331817634, Iran. E-mail address: ghasemi@sut.ac.ir (S.M. Ghasemi).

Contents lists available at ScienceDirect

Journal of Membrane Science

journal homepage: http://www.elsevier.com/locate/memsci

https://doi.org/10.1016/j.memsci.2021.119099 Received 27 October 2020; Received in revised form 29 December 2020; Accepted 15 January 2021

Journal of Membrane Science 623 (2021) 119099

2

tuned. From this aspect, the first key factor is the identification of most convenient block copolymers, in terms of the total molecular weight, block lengths, block ratios or composition and dispersity (Ð, which is defined as the weight-average molecular weight (Mw) to the number-average molecular weight (Mn) ratio, Ð = Mw/Mn as suggested by IUPAC in 2009 [5], to characterize molecular weight distribution or MWD) [6–8]. Afterward, it is essential to choose and optimize the casting solution parameters including the concentration of block copolymer, solvent-mixtures and its composition, solvent(s) character-istics (e.g. selectivity, polarity, miscibility and etc.), solution viscosity, temperature and foreign additives in special cases [9–19]. Then, the process conditions should be carefully adjusted before casting the so-lution. Accordingly, the evaporation time before precipitation, the rate of solvent evaporation, the initial thickness of membrane precursor, ambient temperature, environmental humidity and even the rate of doctor-blading are determinant factors [20–23]. The last but not the least, is the nature of the nonsolvent and precipitation bath conditions such as type of the nonsolvent, miscibility with solvent(s) and the bath temperature [23,24]. Therefore, optimization of the all parameter-s/conditions in a relatively narrow range makes the process too complicated, especially when a new block copolymer or conditions is examined.

In general, SNIPS process is entirely based on the state of micelli-zation (i.e. self-assembly of block copolymers in solution), micelles as-sembly (i.e. type of the lattice geometry or the ordered packing of block copolymer micelles) and maintaining the stability of the organized lat-tice, that all influenced by the block copolymer molecular parameters, casting solution characteristics and processing conditions. Indeed, a successful SNIPS of block copolymers is achieved while a block copol-ymer properly self-assembles in the form of adequate nanoscopic structures including nanospheres, rods or a mixture of both, following the formation of well-ordered assemblies at the interface of air/solution, and then kinetically trapping (i.e. freezing) the self-assembled structures upon immersion into the nonsolvent bath.

Self-assembly in the bulk state depends on molecular parameters of the block copolymer. Therefore, a careful selection of block copolymer molecular parameters, i.e. blocks composition (ƒ), the total degree of polymerization (N) and the Flory-Huggins interaction parameter (χAB), is required to reach desired morphologies [25–28]. Accordingly, although self-assembly in SNIPS process takes place in the solution, however bulk data would be useful to predict the morphology of a block copolymer in solution. For instance, mostly high molecular weight polymers (over 100 kDa) are needed for the formation of well-ordered isoporous block copolymer membranes via SNIPS process [2,14]. It is from the fact that Flory-Huggins interaction parameter (χAB) related to block copolymers which have been used in the SNIPS process so far, is mostly lower than 1 [23,29]. Therefore, polymers with high molecular weight would be required to achieve a strong segregation tendency (χABN) between two polymer blocks [28].

In the SNIPS method, selfassembly process could be occurred due to different reasons during two first steps and eventually self-assembled nanoscale morphologies will be kinetically trapped by immersing in a nonsolvent bath in the last step. In the first step, micellization happens because of the unfavorable interactions between solvent and one of the blocks in a selective-solvent(s) and above the critical micelle concen-tration (CMC) [14,30]. In the second step, unfavorable interactions be-tween the two polymer blocks can lead to the micellization even in a nonselective solvent due to the solvent evaporation, mostly at the solution-air interface [31,32]. Consequently, changes in solution con-centration and also solvent selectivity due to the variation of polymer - solvent interactions during partial solvent evaporation, set the initial stage of the emerging structural features and control the structure of the micelles. In addition to micellization, packing of block copolymer mi-celles in solution also has a considerable impact on the final membrane structure [17]. In fact, the lattice geometry of micelles in solution re-sembles the surface pores network of the final membrane, changing by

copolymer concentration and the solvent - blocks and block - block in-teractions [14,16]. Thus, various lattice geometries such as disordered and/or partially ordered spherical micelles, face-centered cubic (FCC), body-centered cubic (BCC) lattice of spheres, hexagonal close-packing of spheres or cylinders (HCP), tow-dimensional hexagonal (2DH), simple cubic (SC) lattices of spherical micelles and other ordered structures can be formed by changing in the copolymer concentration and solvent composition/quality, that both are varied during solvent evaporation [13,15]. Moreover, the lattice geometry of micelles in solution can be also defined by the structure of micelles (i.e. star-like, crew-cut and reverse) which depends on the polymer-solvent interactions. Micelles of asymmetric amphiphilic block copolymer systems in a more selective solvent(s) to the hydrophilic block, are considered as “star-like/hairy” micelles or “soft spheres” in the case of large corona/core diameter ra-tios, while lower ratios are defined as “crew-cut” micelles or “hard spheres” [14,33]. However, reverse micelles are developed and assem-bled in a solvent that is more selective for the hydrophobic block [34, 35]. It is demonstrated that in the semi-diluted concentration regime, hairy/star-like micelles (soft micelles) with long-range interactions favor BCC, on the other hand, crew cut micelles (like as hard spheres) with short-range interactions favor FCC morphology [3,7,12].

Eventually, the order stabilization of micelles assembly until the last stage of membrane preparation (i.e. phase inversion step) can be retained by utilizing the right additives in the casting solution and/or the right processing conditions for the nonsolvent bath [9,10,23,36].

In general, factors including block copolymer molecular parameters, solvent(s) quality/selectivity and composition, solution viscosity and concentration, evaporation time before precipitation and the structure of micelles (i.e. star-like, crew-cut and reverse) affect micellization, the lattice geometry as well as the onset of the transition from disordered to a partially ordered and further to the desired lattice structure. All of these factors are changed during solvent evaporation and has to be trapped in an appropriate time by immersion in water. Thus, the structure formation via self-assembly of block copolymers in the SNIPS process can be considered as an interplay between thermodynamic and kinetic factors.

In this study, PS-b-PAA diblock copolymers with different composi-tions were systematically synthesized by the RAFT polymerization technique and used to study the possibility of additive-free asymmetric isoporous membranes formation directly through self-assembly and water-induced phase separation process, by simply using a single solvent (1,4-dioxane) and without any foreign additives. The challenge is the formation of membranes with suitable additive-free structures via SNIPS process, due to the high hydrophilicity and ionization degree of PAA blocks. Block copolymer composition is considered as the major effec-tive factor. Accordingly, by choosing the right blocks composition, other parameters/conditions can be tuned based on that.

2. Experimental

2.1. Material, reagents and block copolymer polymerization

Polystyrene-block-poly (acrylic acid) (PS-b-PAA) copolymers were synthesized by the reversible addition-fragmentation chain transfer (RAFT) polymerization. RAFT polymerization was chosen since it yields predictable and well-defined polymer products with relatively high purity using conventional free radical polymerization conditions (tem-perature, solvent and etc.). Synthesis parameters were optimized to obtain PS-b-PAA block copolymers having different total molecular weights and PS to PAA block ratios, with PS majority fractions. To obtain desired polymers, S-1-dodecyl-S′-(α,α′-dimethyl-α′ ′-acetic acid) trithio-carbonate (DDMAT) as a monofunctional RAFT agent was synthesized and used to prepared PAA-DDMAT macro RAFT agent with a constant length (LPAA ≈ 155). Then, it was extended with styrene. Scheme 1 represents the synthesis route of the PS-b-PAA block copolymers.

In general, by increasing the PS block length while holding the PAA

K. Foroutani et al.

Journal of Membrane Science 623 (2021) 119099

3

block length constant, the PS/PAA ratio of the polymers were increased. Synthesis conditions of the obtained block copolymers were precisely described in Supporting Information and results of polymers charac-terization were also summarized in Table S1. 1,4-dioxane (DOX, 99%, Daejung company) as solvent in membrane preparation and Deionized water (Neutron pharmachemical Co., ≤0.005 mS/cm, pH = 5–7) as nonsolvent, were directly used as received without further purification. Characteristics of the synthesized block copolymers used in the mem-brane preparation process were also given in Table 1.

2.2. Membrane preparation

Polymer solutions were prepared by dissolving PS-b-PAAs in DOX. Polymer solutions were gently stirred over a period of 24 h at room temperature. Afterward, casting solutions were kept standing for 2 h to completely release air bubbles. Then, solutions were casted onto a clean glass plate using a casting knife with the gap height of 200 ± 10 μm and immersed into a non-solvent bath (de-ionized water) for 4 h with a constant temperature of about 2 ± 0.04 ◦C (measured by ST-8811 IR thermometer), followed by drying at ambient conditions. Immersion in water was done after a certain evaporation time (ranging from 5 to 40s), in a fume hood. During solvent evaporation, variables including the ambient temperature (25 ± 0.5 ◦C), humidity (40% RH) and average air velocity (0.6 ± 0.03 m/s) remained constant, which all measured by GM816A-Benetech digital anemometer. The obtained membrane films were cut into the desired dimensions for relevant characterization.

2.3. Membrane characterization

Field emission scanning electron microscopy (FESEM) micrographs were taken by a Hitachi S-4160 at a voltage of 20 kV. The samples were mounted on flat or 90◦ aluminum stubs using double-sided, conducting carbon tape and then Au coated by a DSR1 desk sputter-coater, before imaging. For the cross-sectional membrane characterization, dried samples were fractured in liquid nitrogen. Characteristics of the mem-branes were determined using the analysis (Image J) software on the FE- SEM micrographs. The pore size distribution of the membranes was also determined by the Brunauer-Emmett-Teller (BET) method (BELsorp Mini II apparatus, Japan). Nitrogen was used as a filling gas and the temperature was 30 ◦C. Dynamic light scattering (DLS) measurements were taken with a Zetasaizer Nano ZS (Malvern Instrument Ltd, UK) at room temperature. For this purpose, a Solution of 0.5 wt % PS-b-PAA in DOX was prepared. Accordingly, the solvent was first filtered through hydrophobic PTFE filters with average pore diameters of 0.22 μm to

remove any insoluble particles or impurities. The block copolymer was then dissolved over a period of 24 h at 25 ◦C while stirring. The refractive index and viscosity of DOX at 25 ◦C are 1.420 and 1.18 cP, respectively. DLS measurements were carried out using a standard laser equipped with a He–Ne laser (operating at a wavelength (λ0) of 633 nm) and a correlator (25 ns–8000 s, max 4000 channels). Intensity auto-correlation functions g2 (q,t) were measured at scattering angles of 13◦

+ 173◦ at 25 ◦C. The electric field correlation functions g1 (q,t) were analyzed by MATLAB software using the cumulant fit up to the second order for monomodal distribution and double exponential fit for the solutions showing two distinct decay modes.

ln ⋅g1(q, t)

= ln A − Γ t +

(μ2/Γ

)t2 (1)

In Equation (1), A is amplitude, Γ is the mean decay rate, t is time, μ2 is the second moment, and μ2/Γ shows a measure of the relative width of the size distribution. The translational diffusion coefficient D was calculated from Γ = Dq2 with Equation (2):

q= 4π sin(θ

2

)/

λ (2)

where q is the absolute value of the scattering vector and θ is the scat-tering angle. The hydrodynamic radius was determined from the Sto-kes− Einstein equation.

Rh = kβT/6Dηπ (3)

In Equation (3), kβ is the Boltzmann constant (≈1.38 × 10− 23 kg m2/ K s2), T is the temperature, and η is the solvent viscosity, respectively. Small angle X-ray scattering (SAXS) measurements were performed on a Hecus X-ray systems (model S3-MICROpix, Austria) at working mode of 1D-PSD with voltage, current and exposure time of 50 kV, 1 mA and 10800 s, respectively. The block copolymer solutions (mesophase sam-ples) were loaded into 2 mm capillaries via syringe and then the capil-laries were sealed using epoxy glue to prevent solvent evaporation. The sample-to-detector distance was approximately 261 mm and the X-ray wavelength, λ (◦A), was 1.542. The scattering vector, q, is defined as q (1/◦A) = 4π sin θ/λ(◦A), where θ is half of the scattering angle. Rheo-logical measurements were carried out on a MCR 301 rheometer (Anton Paar, Austria). A 50-mm cone-and-plate geometry with d = 0.05 mm was used for the measurements. The temperature during the tests was controlled at 25 ◦C by a Peltier controlled system. The isothermal fre-quency sweep experiments for the 15 wt % copolymer solutions were done at a fixed strain of 7.5%. Pure water fluxes of the PS-b-PAA membranes were measured by a vacuum filtration device at a trans-membrane pressure (ΔP) of 0.8 bar and at ambient temperature of about 25 ◦C. The effective membrane area was 13.2 cm2. The normalized permeance, normalizing the flux by the transmembrane pressure, was calculated as following:

Permeability=ΔV/AΔtΔP = J/ΔP (4)

ΔV; the volume of water (L), Δt; the certain period of time (h), A; the surface area of the membrane (m2), ΔP; the pressure drop (bar) across the membrane and J; the flux of the permeate (L.m− 2. h− 1). All mea-surements were conducted employing demineralized water with an electrical conductivity of ≤0.005 mS/cm. Retention measurements were

Scheme 1. Synthesis route of the PS-b-PAA block copolymers.

Table 1 Molecular weight and fraction of the synthesized PS-b-PAAs used in the mem-brane preparation process.

Block copolymer ♠

Mn (gmol− 1) $

Ð ♀ Mn

(gmol− 1)‡wt % PS†

wt % PAA◆

PS88-b-PAA1294 93900 1.15 93000 88 12

PS84-b-PAA1671 70800 1.08 69600 84 16

PS72-b-PAA2840 40400 1.20 39900 72 28

♠The numbers at the subscript correspond to the amount of the respective block in wt%, and upper case number indicates the total molecular weight in kg/mol, $

& ♀ determined by GPC, ‡, † & ◆ determined by 1H NMR.

K. Foroutani et al.

Journal of Membrane Science 623 (2021) 119099

4

carried out for bovine serum albumin (BSA) solution (0.05 wt % in deionized and demineralized water, Neutron pharmachemical Co., ≤0.005 mS/cm, pH = 5–7 at 25 ◦C) as feed solution, at 0.8 bar. The concentration of BSA in solution is proportional to peak area. Therefore, the retention percentage is calculated from [(1 - (permeate area)/(feed area)) × 100], where the protein concentration of both, permeate and feed, measured with an UV–visible spectrophotometer at 280 nm.

3. Results and discussion

In this study, we aimed to determine the optimized PS-length (i.e. the optimized PS to PAA block ratio) in order to obtain highly ordered isoporous PS-b-PAA membrane surfaces with high pore density, without the aid of foreign additives. To attain this purpose, the influence of following parameters on the final membrane morphology have been investigated; PS-b-PAA micellization in dilute solutions, micelles as-sembly and structural characteristics at concentrated regimes and rheological properties of the casting solutions as well. The effects of dispersity index on the block copolymer micellization, casting solution characteristics as well as final membrane structure is evaluated. In addition, membrane substructure morphologies along with membrane performances were compared for PS-b-PAA block copolymers with different PS-lengths.

3.1. Overall path to obtain the final isoporous structures

Thus far, foreign additives have been necessarily utilized in the SNIPS of PS-b-PAA. Besides, various solvents have been used, mostly as solvent mixtures, for the preparation of isoporous membranes by the SNIPS process. However, for the membranes prepared in this study, the casting solution contained only polymer (11.5–15 wt %) and a single solvent (1,4-dioxane), without the aid of additives. Top and selective layer-cross section images of PS-b-PAA block copolymer membranes are shown in Fig. 1. It has been reported that the PAA blocks are not suitable for the phase inversion process [37]. The high hydrophilicity of the PAA

block makes the precipitation in water difficult/impossible [36,38]. Moreover, intermicellar electrostatic repulsion due to the high ioniza-tion (i.e. dissociation) degree of carboxylic acid units of the PAA blocks in polar solvent(s) or in an aqueous medium, decreases the stability of the micelles assembly in casting solution [39–41]. Recently Cheng et al. [36] reported for the first time the preparation of asymmetric PS-b-PAA membranes by combining self-assembly of metal-block copolymer complexes and non-solvent induced phase separation. They claimed that it is not possible to prepare isoporous PS-b-PAA membrane structures without using additives. Moreover, in most recent publication [18], SNIPS of PS-b-PAA was conducted through blending with poly (ethylene glycol) (PEG) as additives, in order to tailor the pore size and perme-ability of the obtained membranes. Herein, it is shown that by choosing the right PS-block length, isoporous structures can be obtained without using any additives and directly through precipitation in water (Fig. 1), showing comparable performance relative to previous works, in parallel with lower polymer consumption. Obviously, utilizing water as the nonsolvent is essential for the procedure simplicity and cost-reduction in an industrial membrane production.

DOX has been chosen as solvent. It has low vapor pressure (4.95 kPa at 25 ◦C [42]), thus slowly evaporates in the second stage of membrane preparation. Accordingly, a gently external airflow was induced in a fume hood (with average air velocity of 0.6 ± 0.03 m/s), in order to develop an adequate solvent concentration gradient, perpendicular to the film surface and also to adjust the evaporation time window that led to a desired structure (≈40 s), see Fig. 2. In addition, DOX is a water miscible solvent with a very low dielectric constant [42]. More impor-tantly, DOX is a better solvent for the PS block than for PAA (Table 2). Therefore, reverse micelles with PAA cores can be formed in PS-b-PAA solutions (in DOX). The micelle assembly in solution leads to the hex-agonal close-packing (HCP) of spheres, when the evaporation time before precipitation is up to 40 s (as will be discussed in section 3.3). In the final stage of membrane preparation (i.e. phase inversion process), the high freezing point of DOX (11.8 ◦C [42,43]) would be resulted in retaining the uniformity and regularity of micelles assembly by

Fig. 1. (a) Photograph showing the flexibility of the PS84-b-PAA1671 membrane, cast from 15 wt % polymer concentration in DOX with solvent evaporation time of 40

s. (b) FESEM images of PS84-b-PAA1671 membrane top surface at different magnifications. (c) Cross-section FESEM images showing the thin top layer and the mac-

rovoids in sublayer at different magnifications.

K. Foroutani et al.

Journal of Membrane Science 623 (2021) 119099

5

decreasing their mobility, as the solution cast film immersed in the water bath with low temperature of about 2 ± 0.04 ◦C (Fig. 2). It is worth noting that additionally to precipitation in a water bath at room tem-perature, other efforts to retain the lattice geometry arrangement, such as replacing the nonsolvent with a non/less polar nonsolvent like diethyl ether, was also unsuccessful (data are not shown in the present work).

3.2. Influence of block ratios on the micellization and final membrane structure

In this section, to study the influence of solvent in dilute regime, first, the best solvent medium for the isoporous PS-b-PAA membrane forma-tion is theoretically determined. Following this, the influence of different block ratios on micellization in dilute solutions as well as final membrane structure are investigated.

It has been demonstrated that the nature of the solvent system (i.e. the solubility parameter and the dielectric constant) plays a decisive role in the formation of various self-assembled block copolymer aggregates (especially in the case of PS-b-PAA, due to the ionizable nature of PAA blocks), such as spheres, short/long rods, large compound micelles (LCMs) and etc. [25]. Indeed, the nature of the solvent system directly affects the polymer chain dimensions, consequently controlling the aggregate morphology. On the other hand, PS-b-PAA has varied be-haviors in different mediums (i.e. aqueous, organic or a mixture of both). For example, it is classified as block ionomers in organic solvents and formed “reverse micelles”, in which the ionic head groups form the core, surrounded by a corona of hydrocarbon chains [34,35]. In addi-tion, owing to the high ionization degree of carboxylic acid units of the PAA blocks in aqueous media, that leads to the intermicellar electro-static repulsion and unstable micelles assembly, solvent medium needs to be selected more carefully [39]. Accordingly, it seems that the best medium candidate for the purpose of isoporous PS-b-PAA membrane formation is the organic one, which could be resulted in the formation of reverse micelles with a controllable PAA ionization in an appropriate level. So far, various organic solvent systems have been used to prepare isoporous membranes directly through SNIPS process that are mainly being solvent-mixtures rather than a single solvent. Since solvent and polymer interaction is roughly correlated to their solubility parameters,

equation (5) is used to evaluate the most selective organic solvent to PS block, besides proposing a minimum polarity (i.e. low dielectric constant).

χ12 =[(δD1 − δD2)

2+ 0.25(δP1 − δP2)

2+ 0.25(δH1 − δH2)

2]V/

RT (5)

Where χ12 is the Flory-Huggins interaction parameter between the polymer block (1) and the solvent (2), V is the molar volume of the solvent; R is the gas constant (8.314 Jmol− 1K− 1); T is the absolute temperature (298 K); and D, P and H are related to the energy densities from dispersion, dipolar intermolecular force (polarity) and hydrogen bonds between molecules, respectively. Accordingly, the selectivity of the solvents was estimated by comparing the Flory-Huggins interaction parameters (χ) of each block/solvent combination and results were lis-ted in Table 2. If the difference is a positive number, the solvent is PS- selective; and vice versa, a negative difference, defines a PAA-selective one.

As can be found out from Table 2, the most PS-selective solvent is DOX which has the lowest dielectric constant, as well. Therefore, among common solvents that are used for the preparation of isoporous mem-branes via SNIPS, DOX chose for further experimental investigations, in both dilute and concentrated regimes, concerning effects of PS/PAA block ratios on the membrane structure formation.

The influence of different block ratios on PS-b-PAAs micellization behavior at concentration of 0.5 wt % in DOX is investigated by dynamic light scattering (DLS) technique. The results of the DLS measurements are listed in Table 3.

On the other hand, the radius of gyration (Rg) of PS-b-PAAs with different block ratios in theta solvent, random coil, can be theoretically calculated through equation (6):

R2g = r2

/6 = nl2C∞

/6 = iNl2C∞

/6 (6)

r; the end-to-end distance for a random coil in dilute solution, C∞; the polymer characteristic ratio, n; the number of main chain bonds in the backbone (iN), i; the number of backbone bonds per monomer unit (segmental length), N; the degree of polymerization, and l; the length of a backbone bond [47,48]. In addition, for random coils under theta condition (i.e. linear chains with an ideal conformation), hydrodynamic radius (Rh) is proportional to Rg as Rh/Rg = 0.665, whereas for real chain conformation (good solvent), Rh/Rg = 0.640 [48]. Based on equation (6) and under theta conditions, for PS-b-PAAs with i = 2, l = 0.154 nm, C∞

Fig. 2. FESEM micrographs of PS88-b-PAA1294 membrane surfaces, cast from 11.5 wt % block copolymer concentration in DOX with the evaporation time of 40s, (a)

solvent evaporating under natural convection, (b) immersing the solution cast film in a water bath at room temperature, and (c) solvent evaporating under forced convection in a fume hood and then immersing in a low temperature water bath.

Table 2 Dielectric constant (ε) of solvents and calculated χ parameters for the copolymer blocks and the solvents, by equation (2) [15,42,44–46].

DOX THF DMF Ac DMAc NMP MeCN

PAA 2.12 1.28 0.43 1.13 0.67 1.28 1.09 PS 0.25 0.32 1.24 0.65 1.06 0.78 1.24 Δχa 1.87 0.96 - 0.81 0.47 - 0.39 0.5 - 0.15 ε 2.25 7.6 37 21 37.8 32.3 37

a In equation (5), δ for the copolymer blocks and the solvents is calculated based on the Hansen’s solubility parameters [44], as δ = (δD

2 + δP2 + δH

2 )1/2, except for PAA which is cited from Ref. [46].

Table 3 Hydrodynamic diameter (Dh = 2Rh) of PS-b-PAAs with different block ratios determined by DLS at a concentration of 0.5 wt % in DOX.

Block copolymer Hydrodynamic diametera (nm)/Relative amount (%)

PS88-b-PAA1294 149/85% 16/15%

PS84-b-PAA1671 31/100%

PS72-b-PAA2840 15/84% 61/16%

a Averaged from the intensity method.

K. Foroutani et al.

Journal of Membrane Science 623 (2021) 119099

6

= 9.85 (for both PS and PAA segments) and total degree of polymeri-zation of about 941, 716 and 431, hydrodynamic diameter (Dh = 2Rh) is calculated around 11.4, 9.9 and 7.7 nm, respectively.

DLS analysis can be used to detect individual polymer chains and also their aggregates, like micelles or superstructures (agglomerates). By comparing DLS measurements with theoretically calculated Dh of PS-b- PAAs, it is revealed that species with large hydrodynamic diameters are formed in all solutions, indicating the selectivity of the respective sol-vent system. In both PS84-b-PAA16

71 & PS72-b-PAA2840 solutions, micelles

are mostly formed, whereas larger species such as agglomerates present in the PS88-b-PAA12

94 solution. In fact, PS88-b-PAA1294 in DOX shows the

largest Dh, with the highest amount, indicating a strong agglomeration of the block copolymer along with PS-block swelling. These super-structures coexist with a rather small amount of micelles and/or indi-vidual block copolymer chains. Since DOX is a PS-selective solvent, it is expected that the PS blocks with the largest length (e.g. PS88-b-PAA12

94) will adopt a more extended/swelling form, creating superstructures or large compounds.

Accordingly, PS/PAA block ratio have influence on the size of ag-gregates produced in a PS-selective solvent, like DOX. The larger species are created by increasing of the PS-block length. However, only micelle- like aggregates (i.e. smaller species) with adequate dimensions in comparison with superstructures or large compound micelles, would form ordered isoporous membrane surfaces with high pore densities, as shown in Fig. 3b and c versus Fig. 3a. Results also show that it is possible to exceed the onset of micelle formation even at very low concentrations (Table 3), that will be suitable for the fabrication of final membrane structures with appropriate and low enough casting solution concen-trations (11.5–15 wt %) by choosing the right PS-block length and sol-vent medium, as clearly seen in Fig. 3. The structural development to induce isoporous PS88-b-PAA12

94 membranes by varying polymer con-centration in casting solution and solvent evaporation time, in parallel, is illustrated in Fig. S3.

The casting solution conditions for each sample plus the average pore diameter, pore density (number of pores per m2) and surface porosity (total areas of pores on surface) of the PS-b-PAA membranes illustrated in Fig. 3, were analyzed by SEM-images analyzer (Image J) and data were summarized in Table 4.

Influence of the presence of a bad-solvent (water) relative to the PS block in the casting solution on both micellization behavior and mem-brane formation was investigated for the PS88-b-PAA12

94 block copolymer

with the highest PS-block length. Therefore, in the case of PS88-b-PAA1294

solution (in 99 wt % DOX and 0.5 wt % H2O), DLS measurements at concentration of 0.5 wt % were shown that 90% micelles with z-average hydrodynamic diameter of 20 nm coexist with a rather small amount of superstructures (≈10%), in comparison with z-average hydrodynamic diameter of 149 nm (with relative amount of 85%) in the absence of water. In addition, the influence of water in improving of the surface porosity, pores density & uniformity can be clearly seen from FE-SEM micrographs in Fig. 3d and e. Consequently, the water content in the PS88-b-PAA12

94 solution decreased the size of aggregates and also pro-posed more uniform surface pores. The low affinity of water to the majority PS block, most probably leads to the micellization in the form of smaller aggregates. It can be also supposed that in the absence of water, PS-block will swell more in a more PS-selective solvent, like DOX. It should be noted that using higher amount of water in the solution, i.e. 2 wt %, led to the macro-phase separation and adequate membrane structures were not obtained. Moreover, the attempts to fabricate large scale membranes from PS72-b-PAA28

40 block copolymer, even by using high polymer concentrations (e.g. 25–30 wt %) were unsuccessful. In fact, PS72-b-PAA28

40 membranes with ≈100 cm2 surface area were split off upon immersion into the water bath, and therefor couldn’t maintain their integrity. The obtained membranes can be seen in supporting

Fig. 3. FE-SEM micrographs of the top surface of membranes cast from different block copolymer solutions at evaporation time of 40 s; (a) 15 wt % PS88-b-PAA1294, (b)

15 wt % PS84-b-PAA1671, (c) 15 wt % PS72-b-PAA28

40, and (d) 11.5 wt % PS88-b-PAA1294, all in DOX, and (e) 11.5 wt % PS88-b-PAA12

94 in 88/0.5 wt % DOX/H2O.

Table 4 Features and characteristics of the surface pores related to different PS-b-PAA membranes, presented in Fig. 3.

Block copolymer

Polymer concentration (wt %)

Solvent (s)

Dp♠ ±

STD♀

(nm)

Pore density ( ×1014

pores/m2)

Surface porosity (%)

PS88-b- PAA12

94 15 DOX 19 ± 5 2.05 5.8

PS84-b- PAA16

71 15 DOX 14 ± 4 4.02 6.2

PS72-b- PAA28

40 15 DOX 21 ± 7 2.87 7.7

PS88-b- PAA12

94 11.5 DOX 19 ± 6 1.23 2.6

PS88-b- PAA12

94 11.5 DOX/

H2O 18 ± 5 2.21 4.5

♠ Mean pore diameter, ♀ The pores size distribution (standard deviation of pore diameter, STD).

K. Foroutani et al.

Journal of Membrane Science 623 (2021) 119099

7

information, Figs. S4a and S4b.

3.3. Influence of PS-block length on casting solution characteristics in concentrated regime

As already shown in Fig. 3, membranes from PS-b-PAA solutions with 3 different PS-block lengths were prepared and presented various qualities for surface pores. Block copolymer with the largest PS-block length (PS88-b-PAA12

94) led to an imperfect order. For the PS84-b-PAA1671,

regularly ordered pores were obtained. Partially ordered pores were formed in the case of PS72-b-PAA28

40, however large scale film formation was not practicable. Properties and characteristics of a casting solution have a direct and essential impact on the final membrane structure. Accordingly, solution SAXS and rheological tests (frequency sweep ex-periments) were used to investigate the structural and rheological characteristics of casting solutions in concentrated regime.

Self-assembly of PS-b-PAAs in the presence of a selective solvent like 1,4-dioxane, even at dilute solutions, led to the micellization as demonstrated by DLS measurements in preceding section. In concen-trated regimes, e.g. 15 wt %, micelles assembly could form different lattice geometries and mesophase structures. Thus, solution SAXS was used as an efficient tool to evaluate quantitatively the structural char-acteristics of casting solutions, Fig. 4.

The increased scattering intensity of the curves in Fig. 4 indicates that at the same copolymer concentrations (e.g. 15 wt %), micro-phase separation in different casting solutions is increased [13,49], followed the order of PS72-b-PAA28

40 > PS84-b-PAA1671 > PS88-b-PAA12

94. Moreover, SAXS profiles illustrated in Fig. 4, revealed that a partially ordered structure has been formed in all 15 wt % PS-b-PAA solutions in DOX. Indeed, a second weak and broad peak besides the primary peak (the principal peak, q*) in a SAXS profile indicates the existence of a partially ordered structure [13,15,50]. The exact mesostructure of samples was also determined from SAXS profiles. Therefore, relative peak positions (q/q*) for different PS-b-PAA solutions, specified by dash markings in Fig. 4, are shown in Table 5.

At 15 wt % polymer concentrations, PS88-b-PAA1294 & PS84-b-PAA16

71

show body-centered cubic (BCC) structure with 1:1.41:1.73:2.23

relative peak positions (missing peak at q/q* = 2), while PS72-b-PAA2840

has face-centered cubic (FCC) structure with q/q* =

1:1.15:1.63:1.91:2.31 [16,51]. It is worth noting that PS84-b-PAA1671 so-

lution with 17.5 wt % concentration (SAXS curve is not depicted here), mainly had a hexagonally close-packed (HCP structure [51]. Since the solvent evaporation step cannot be omitted from the membrane prepa-ration procedure, the optimized casting solution concentration is 15 wt %. In fact, upon solvent evaporation, polymer concentration at the sol-ution/air interface is increased, leading to a change in lattice geometry from a partially ordered structure to mainly HCP lattice [17].

More investigations were conducted by studying the structural characteristics of different casting solutions in DOX at 15 wt % con-centration and results were summarized in Table 6.

From the first peak in the scattering pattern (i.e. q*), some charac-teristics including the micelle-to-micelle distance or domain spacing (i.e. d = 2π/q*, using Bragg’s law [16]), the full-width at half-maximum (FWHM) of the primary scattering peak [16], and the grain size of mesophases (i.e. grain size = 5.56/FWHM, using the Scherrer relation [52,53]) are calculated for all samples, Table 6. Since the primary peak (q*) is relatively shifted to lower values, the self-assembly of PS84-b-PAA16

71 in DOX is relatively better induced in comparison with other solutions [49]. The obtained domain spacing for three different block copolymers in DOX is large, which could be mainly resulted from the strong segregation tendency between the PS/PAA blocks. The full width at half maximum (FWHM) of the first reflection in the PS88-b-PAA12

94 profile was larger compared to the PS84-b-PAA1671 and

PS72-b-PAA2840, which indicates better long-range order of the micelles

assembly in latter cases [16]. Additionally, based on the Scherrer for-mula, grain size relates inversely to the FWHM. Grain boundaries are also considered as defects, and thus larger grain size means less defects are present in a structure [52]. Accordingly, PS88-b-PAA12

94 micelles would form a lattice with more defects (i.e. smaller grain sizes) in solution.

Although the PS72-b-PAA2840 solution showed better structural char-

acteristics in concentrated regime, as mentioned earlier, large scale membrane formation was failed. Generally, in addition to the structural characteristics such as grain size, long-range order, microphase sepa-ration and microphase-separated structures, some rheological properties ant its effective parameters including flow behavior, solution viscosity, interfacial properties, intermicellar interactions and entanglement of block copolymer chains play a critical role in the successful membrane formation via SNIPS process. Accordingly, the rheological characteris-tics of different PS-b-PAA solutions at 15 wt % concentration were studied. First, the viscoelastic properties and flow behavior of the samples (mesostructures) were investigated, Fig. 5.

It has been demonstrated that in the rheological measurements when storage modulus (G′) is higher than loss modulus (G′′) in whole fre-quency range, signifying the solid like behavior [52]. As seen in Fig. 5, the storage and loss moduli of PS-b-PAA solutions exhibit the same and gradually increased, over the whole frequency range. Besides, all PS-b-PAA solutions show the solid like behavior. Generally, a combi-nation of microscopic- and mesoscopic-scale parameters causes to show the viscoelasticity with a solid-like behavior in mesostructures, in a relatively broad range of frequency. More precisely, the solid like rheological behavior of mesostructures is mainly influenced by different parameters including the entanglement of block copolymer chains, microphase-separated domains and dynamic interfacial properties (in the microscopic scale) plus domain size and intermicellar interactions (related to the mesoscopic scale) [54,55]. As clearly seen in Fig. 5, both G′ and G′′ for PS72-b-PAA28

40 have the lowest values among samples, whereas PS84-b-PAA16

71 presents better properties in comparison with the others. In following and after presenting viscosity results of casting so-lutions, an interpretation of the rheological characteristics for each sample will be provided.

Another important factor in the membrane formation via SNIPS is the casting solution viscosity. Solution viscosity has influence on the

Fig. 4. SAXS curves of 15 wt % solutions in DOX, for PS-b-PAAs with different PS-block length. Dash markings in PS88-b-PAA12

94 & PS84-b-PAA1671 correspond to

expected peak positions for a BCC lattice, while in the case of PS72-b-PAA2840

corresponds to expected peak positions for a FCC lattice.

K. Foroutani et al.

Journal of Membrane Science 623 (2021) 119099

8

mobility of the swollen block copolymer chains. It limits chain mobility appropriately to ensure structure formation during the phase trans-formation and inversion, in SNIPS process [6,12]. Accordingly, complex viscosity of different PS-b-PAA solutions at concentration of 15 wt % was also examined and compared in Fig. 6.

The complex viscosity over the whole frequency range followed the order of PS84-b-PAA16

71> PS88-b-PAA1294> PS72-b-PAA28

40, and decreased following the increase of angular frequency (Fig. 6). Though PS88-b-

PAA1294 has higher molecular weight, PS84-b-PAA16

71 shows the highest viscosity. Here again, PS72-b-PAA28

40 represents the lowest rheological properties.

The influence of PS-block length on the quality of PS-b-PAA mem-brane formation as well as structural characteristics and rheological properties of the respective casting solutions would be discussed in more detail, based on obtained results from FE-SEM micrographs (Fig. 3) along with data in dilute (Table 3) and concentrated regimes (Figs. 4–6). According to Fig. 3, the PS84-b-PAA16

71 block copolymer membrane had the most perfect order with regular pore size and high porosity. There-fore, PS-b-PAAs with larger and smaller PS-lengths were compared in following, to evaluate their membrane preparation capability and also to investigate more precisely about the reasons of a successful PS-b-PAA membrane formation in effect of varying the PS- block length.

In the case of PS88-b-PAA1294, the block copolymer has a higher mo-

lecular weight along with longer PS-length with the same PAA block length. The PS88-b-PAA12

94 formed a membrane with low surface porosity (and low surface-pores density) having an imperfect order, Fig. 3a. The PS88-b-PAA12

94 dilute solution (0.5 wt %) mainly contained very large species (superstructures or agglomerates), Table 3. Additionally, in the concentrated regime (15 wt %), although a partially ordered structure was detected, relatively weakened micro-phase separation occurred in the solution. The structural characteristics of the PS88-b-PAA12

94 also revealed that the obtained mesostructures in solution had the smallest grain boundaries, in other words, leading to create much more defects in the structure (Fig. 4). It seems that the state of micellization affect the rheological behavior of mesostructures (Figs. 5 and 6). Accordingly complex viscosity, storage and loss modulus are lowered, in comparison with the rheological properties of the PS84-b-PAA16

71 solution, most probably due to the large size of aggregates plus inadequate micro-phase separation. Generally, longer corona (PS) segments increase the inter-micellar entanglement and interaction, dynamic interfacial properties and consequently, enhancing the solution viscosity as well as the sta-bility of micelles assembly. However, too large aggregates cause less micelle formation in solution and also the micelles with longer corona (PS) length are more deformable, leading to an imperfect order.

In the case of PS72-b-PAA2840, a block copolymer having similar PAA-

block lengths but smaller PS- block lengths comparing with PS84-b- PAA16

71, large scale membrane fabrication via SNIPS was failed even by using high polymer concentration (e.g. 30 wt %) in casting solutions (Fig. S4). DLS measurement (Table 3) and solution SAXS experiment (Fig. 4) were demonstrated that micellization and micro-phase separa-tion in both dilute and concentrated regimes proceeded properly, and suitable structural characteristics were almost achieved. However, its rheological properties were not able to fulfill the requirements of membrane formation via SNIPS (Figs. 5 and 6). It could be due to the smaller corona (PS) length of micelles which caused to a decrease in the intermicellar entanglements between PS chains in addition to the reduction in viscosity as well as the stability of micelles assembly.

To conclude, a combination of both appropriate structural and rheological characteristics is necessary to prepare a perfect membrane and only specific block lengths can form such structures.

Table 5 Relative peak positions (q/q*) and defined structures for different PS-b-PAA solutions.

Block copolymer Polymer concentration (wt %) q/q* Structure

PS88-b-PAA1294 15 1 1.41 1.73 2.23 BCC$ with missing peak at 2

PS84-b-PAA1671 15 1 1.41 1.73 2.23 BCC with missing peak at 2

PS84-b-PAA1671 17.5 1 1.73 2 2.64 HCP♀ with weakened peaks at 1.41 & 2.44

PS72-b-PAA2840 15 1 1.15 1.63 1.91 2.31 FCC♠

$Body-centered cubic lattice, and.♀ Hexagonally close-packed spheres [51], ♠Face-centered cubic lattice [16].

Table 6 The structural characteristics of samples illustrated in Fig. 4.

Block copolymer

q* (nm− 1) ♠ d (nm) $ FWHM (nm− 1)†

Grain size (nm) ♀

PS88-b-PAA1294 0.148 42 0.0246 226

PS84-b-PAA1671 0.147 43 0.0183 303

PS72-b-PAA2840 0.156 40 0.0123 452

♠ The primary peak in each curve; $ The micelle-to-micelle distance, d = 2π/q* [16]; † The full width at half maximum [16], ♀The grain size of mesophases, Grain size = 5.56/FWHM [52].

Fig. 5. Storage and loss modulus as a function of angular frequency for different PS-b-PAA solutions with 15 wt % polymer concentrations.

Fig. 6. Complex viscosity as a function of angular frequency for PS-b-PAA so-lutions at concentration of 15 wt %.

K. Foroutani et al.

Journal of Membrane Science 623 (2021) 119099

9

3.4. Influence of dispersity (Ð) on the micellization, casting solution characteristics and final membrane structures

As it was mentioned in the introduction section, SNIPS process is fundamentally based on the micellization and micelles assembly in so-lution as well as maintenance of the ordered packing of micelles during phase inversion. Generally, in the SNIPS process, block copolymer mo-lecular parameters including molecular weight, blocks ratio (i.e. composition) and dispersity (Ð = Mw/Mn) considerably influence on micellization and lattice geometry formed in the casting solution. Accordingly, final membrane structure is also significantly dependent on block copolymer molecular parameters. In the case of dispersity, it is reported that a value for Ð ≤ 1.2 is generally acceptable/recommended for a block copolymer to form well-defined ordered isoporous structures [8,56–58]. Increasing the Ð from 1.25 to higher values would change the morphology of self-assembled block copolymers in solution [3,8,58–63]. Although block copolymers with acceptable and recommended Ð value (≤1.2) were used in present study to prepare isoporous membranes, however, micellization and casting solution characteristics in addition to final membrane structures were almost affected, even by slight dif-ferences in Ð values. It should be also noted that such a small difference between Ð values for the synthesized block copolymers with various molecular weights is inevitable (1.2, 1.08 and 1.15 for PS72-b-PAA28

40, PS84-b-PAA16

71 and PS88-b-PAA1294, respectively) [63]. In following, the

effects of dispersity on the block copolymer micellization, casting so-lution characteristics (from structural and rheological standpoints) and consequently on the final membrane structure is evaluated.

As it was determined by DLS measurements (Table 3), micellization of PS84-b-PAA16

71 led to the formation of quite (100%) similar aggregates in size. However, in the case of PS72-b-PAA28

40 and PS88-b-PAA1294 block

copolymers, small amount, ≈ 15–16%, of aggregates (and/or individual polymer chains) with different sizes were detected. It could be due to the Ð differences, leading to the size distribution of micelles in respective solutions (even with small relative amount).

In the concentrated regime and based on SAXS data (Fig. 4), the self- assembly of PS84-b-PAA16

71 in DOX is relatively better induced in com-parison with other casting solutions. It is reported that self-assembly of block copolymers in the presence of solvents forms ordered mesophase structures, especially the inversed cubic or hexagonal phases, as it was seen in Fig. 4. The “vacancy” and “dislocation” as kinds of defects in mesophases which influenced on the quality of induced block copolymer self-assembly could be dependent on various factors, including the state of micellization (e.g. micelle size distribution) [52,54,64]. Therefore, the influence of varying Ð (according to preceding paragraph) on the structural characteristics of block copolymer casting solutions is iden-tified. Moreover, the higher scattering intensity in SAXS diagrams generally means a stronger microphase separation for block copolymers [18]. However, PS72-b-PAA28

40 block copolymer with the lowest MW (or segregation strength, χN) showed the strongest scattering among the three copolymers, Fig. 4. On the other hand, PS72-b-PAA28

40 also has the largest dispersity value (1.2) compared to other samples (1.08 and 1.15 for PS84-b-PAA16

71 and PS88-b-PAA1294, respectively). It is revealed that

scattering intensity is affected by dispersity. Accordingly, as the Ð rises, the scattering intensity increases [65]. In fact, an increase in block copolymer dispersity will bring the disordered system closer to its order-disorder transition (ODT). In other words, order-disorder transi-tion for a disordered polydisperse diblock copolymer occurs more readily than its monodisperse counterpart [66].

As seen in Fig. 6, the complex viscosity over the whole frequency range followed the order of PS84-b-PAA16

71> PS88-b-PAA1294> PS72-b-

PAA2840. It seems that dispersity (Ð) affects casting solution viscosity by

influencing on the state of micellization. Therefore, additionally to the micelles size distribution, the shape of micelles is dependent on polymer dispersity. Accordingly, partially formation of semi-bald micelles due to the polydispersed chains [63], most probably caused lower intermicellar entanglements between PS chains. As a consequence, solution viscosity

decreased for samples by the order of PS84-b-PAA1671> PS88-b-PAA12

94>

PS72-b-PAA2840, having Ð values of 1.08, 1.15 and 1.2, respectively.

Moreover, in the case of samples with broader polymer dispersity, smaller aggregates (owing to micelles size distribution) and/or indi-vidual chains with lower molar mass could lead to show weaker rheo-logical properties from the aspect of solution viscosity (Fig. 6) as well as viscoelastic properties and flow behavior (Fig. 5).

Eventually, the final membrane structure could be also affected by dispersity. Based on theoretical studies (within the frame work of self- consistent mean-field theory, SCMF theory), destabilization of an or-dered state structure for asymmetric diblock copolymers upon increases in the polymer dispersity (Ð) is predicted [67]. On the other hand, it has been demonstrated that the ordered structure of micelles assembly in the solution resembles the surface structure and pore geometry of the final isoporous membrane. Moreover, the formation mechanism of isoporous membranes via SNIPS is based on the self-assembled block copolymer aggregates in solution [14,17]. Results of DLS measurements, SAXS analysis and rheological tests showed that the type of micelles packing (lattice geometry) in casting solution is directly dependent on the state of micellization. Since micellization is mainly affected by block copol-ymer molecular parameters, therefore the final membrane structure, in addition to casting/process conditions, is also dependent on blocks ratio, Ð and molecular weight of a block copolymer.

3.5. Membrane substructure morphology and performance

The increased permeability and higher fluxes have an essential role in the industrial membrane processes. In general, some membrane characteristics including pore size, porosity, pore density at top surface besides selective-layer thickness and the structure of internal-layers have major influences on the membrane permeability. Among mentioned key factors, the support structure (i.e. finger-like and/or spongy) can considerably change the permeability of a membrane [68]. Thus far, various strategies and actions have been employed to intensify macrovoids formation in the membrane substructures prepared via NIPS method. All the effective factors could be divided into two main cate-gories; (1) increasing the non-solvent tendency to penetrate into the solution-cast film, and (2) decreasing the viscosity of the casting solution [69]. One of the best applicable, effective and commercially efficient approaches is the reduction of the polymer concentration in the casting solution. Utilizing blends or additives in casting solutions could decrease the polymer concentration, leading to more open substructures and/or thinner membranes [18,68]. For example Yang et al. [18], have been recently used poly (ethylene glycol) as an additive to tailor the pore size and permeability of PS-b-PAA membranes prepared by the SNIPS method. However, in the case of using metal ions, membrane prepara-tion led to a sponge-like substructures though metal complexation was effective for isoporous membrane formation [36]. In this study, mac-rovoids were formed in the membrane substructures by choosing the right blocks ratio without the aid of any additive, as depicted in Fig. 7.

As can be found out from Fig. 7, all three membranes have a tendency for the macrovoid formation in substructures. However, as discussed before, large surface-area membrane fabrication via SNIPS was failed by using PS72-b-PAA28

40 block copolymer. Between PS84-b-PAA1671 and PS88-b-

PAA1294, macrovoids are more similar to the finger-like structures in the

former. Indeed, the PS88-b-PAA1294 tends to form a substructure inter-

mediate between sponge- and finger-like with smaller voids, most probably due to the higher molecular weight that decreases the chain mobility during the phase inversion. The pore size and pore size distri-bution of membranes were also measured via Brunauer–Emmett–Teller (BET) analysis. The BET results were obtained from nitrogen adsorption isotherms using the Barrett–Joyner–Halenda (BJH) method (Fig. 7d-f). The appeared peak in the lower size range is attributed to the sharp cutoff of the selective layer of membrane, whereas in the larger size range, broader distributions corresponded to sub-layers with increasing pore sizes formed by macro-phase separation, as can be seen in Fig. 7

K. Foroutani et al.

Journal of Membrane Science 623 (2021) 119099

10

[36,68,70]. Results showed that the surface pore diameter of the membranes prepared using PS72-b-PAA28

40 was 16–27 nm, PS84-b-PAA1671

was 9–16 nm and PS88-b-PAA1294 was 13–23 nm.

In the case of PS84-b-PAA1671, membrane shows the asymmetric sub-

structure composed of a finger-like upper layer supported by a spongy- layer. The length of finger-like pores to the membrane thickness was determined around 66%. Accordingly, a membrane (i.e. PS84-b-PAA16

71) with similar substructure to the best obtained membrane in Ref. [18] is produced, whereas lower polymer concentrations (15 wt % rather than 24 wt %) are consumed, without using foreign additives as well. In addition, because of the often toxic nature of additives (e.g. some of the transition metal salts), they are not usually considered as an appropriate choice, regarding the aimed applications of these membranes [49].

Furthermore, water flux measurements were carried out using a vacuum filtration device at 0.8 bar transmembrane pressure (ΔP) and room temperature (≈24 ◦C). The water flux results of samples were listed in Table 7. All the membranes had an appropriate water flux, ranging between 250 upto 350 L/m2 h bar.

The water flux correlates highly with the membrane structure at the top surface and sublayers as well. In the case of PS88-b-PAA12

94, the (15/ 40) membrane, i.e. numbers in bracket indicate the polymer concen-tration in casting solution (wt %) and the solvent evaporation time (s) respectively, shows better surface-pores characteristics. However, higher pure water fluxes were obtained for the (11.5/40) membrane. Most probably due to the lower polymer concentration, the resulting substructure of the membrane contains more open pores and macrovoids.

In the case of PS84-b-PAA1671, higher water flux was determined for the

(15/40) membrane in comparison with the (15/120). It seems that

solvent evaporation at second step of the SNIPS process has more impact on the surface pores than sublayer, leading to the formation of a denser membrane at the surface (after 120 s evaporation). Therefore, it can be concluded that both surface-pores characteristics and internal structures have influences on the membrane permeability and performance. Moreover, the retention of bovine albumin (BSA, molecular weight 66 000 g/mol, hydrodynamic diameter of ≈7.6 nm [23]) was measured for the (15/120) membrane, by filtering the protein solution and analyzing UV absorption. It was able to retain 81% of BSA, representing high enough selectivity while still keeping high flux of about 250 L/m2 h bar. It is worth noting that all membranes had suitable resistance against compaction and/or swelling, since water fluxes over 8 times measure-ments showed little to no change (carrying out in 7 h’ period with 1-h break between each test).

4. Conclusions

Three series of PS-b-PAAs including PS88-b-PAA1294, PS84-b-PAA16

71 and PS72-b-PAA28

40 with different PS to PAA block ratios, having similar PAA lengths, were systematically synthesized to prepare additive-free asymmetric isoporous membranes directly through SNIPS process. The influence of PS- block length on essential processes in the SNIPS method including micellization, micelles assembly and its stability was studied to prepare additive-free structures. Theoretical calculations on commonly used organic solvents in the SNIPS process so far, predicted that 1,4-dioxane (DOX) has the most selectivity to PS segments than PAA, besides, its low dielectric constant prevents from PAA swelling due to the carboxylic groups ionization. DLS measurements revealed that large-size species were developed in all three PS-b-PAAs solutions in DOX and hydrodynamic diameters of samples followed the order of PS88-b-PAA12

94 (mainly 149) > PS84-b-PAA1671 (totally 31 nm) > PS72-b-

PAA2840 (mainly 15 nm). Membranes with ordered surface pores along

with high density were exclusively produced in the case of PS-b-PAAs that formed relatively small micelles in dilute solutions, namely PS84- b-PAA16

71 and PS72-b-PAA2840. The created superstructures from PS88-b-

PAA1294 in DOX, became smaller in size (Dh) from 149 to 20 nm in the

presence of only 0.5 wt % water (i.e. a bad solvent to PS segments), resulting in membranes with better surface-pores characteristics. The structural characteristics of mesostructures in casting solutions including micro-phase separation intensity, long-range order of micelles

Fig. 7. Cross-sectional morphologies of different PS-b-PAA membranes; (a) PS72-b-PAA2840, (b) PS84-b-PAA16

71 and (c) PS88-b-PAA1294. SNIPS conditions: 15 wt %

polymer concentration and 40 s evaporation time. Pore size distribution calculated from the N2 adsorption isotherm using the Barrett-Joyner-Halenda (BJH) method for membranes; (d) PS72-b-PAA28

40, (e) PS84-b-PAA1671 and (f) PS88-b-PAA12

94.

Table 7 Water flux of PS-b-PAA membranes prepared at different SNIPS conditions.

Block copolymer SNIPS conditions (wt %/sec) ♠ Water flux (L/m2.h.bar)

PS88-b-PAA1294 (11.5/40) 304

(15/40) 265 PS84-b-PAA16

71 (15/40) 357 (15/120) 250

PS72-b-PAA2840 - -

♠ (the polymer concentration in the casting solution/the evaporation time). 1,4- dioxane was used as solvent in all casting solutions to prepare membranes.

K. Foroutani et al.

Journal of Membrane Science 623 (2021) 119099

11

assembly, grain sizes (i.e. defects), investigated by solution SAXS anal-ysis, were better for the block copolymer with smaller PS- block length. However, large-area membrane formation from PS72-b-PAA28

40 casting solutions was unsuccessful (even by trying high polymer concentrations, up to 30 wt %), due to the poor rheological properties. The rheological characteristics of casting solutions such as solution viscosity, viscoelastic properties and flow behavior, followed the order of PS84-b-PAA16

71 >

PS88-b-PAA1294 > PS72-b-PAA28

40. Reverse micelles of the PS72-b-PAA2840 in

DOX, with small corona (PS)- length, led to the formation of FCC lattice, whereas the obtained micelles from PS84-b-PAA16

71 and PS88-b-PAA1294 act

similar to the “soft-micelles” and thus favor BCC lattice geometry, as characterized by SAXS profiles. Results from DLS, SAXS and rheometry experiments indicated that the formation of membranes with appro-priate structures achieved by a combination of structural and rheolog-ical properties. It was revealed that dispersity (Đ) has a decisive influence on micellization, casting solution characteristics and therefore final membrane structures, even though all three block copolymers had an acceptable and recommended value (≤1.2) along with slight differ-ences. The best structure and performance of membranes were attained for PS84-b-PAA16

71 block copolymer. Sublayers with mainly finger-like macrovoids were also created by choosing the right blocks composi-tion, without using any foreign additives. The selective-layer surface pores and sublayer morphology were both dependent on PS/PAA blocks ratio, influencing on the overall performance of membranes. These re-sults indicate the potential of prepared membranes as a promising candidate in highly sensitive applications such as biomedical or phar-maceutical separations, owing to the intrinsic biocompatibility and antibacterial nature of PS-b-PAA block copolymers, impurity/additive free structure of membranes along with high level of reproducibility and strict quality controllability (in large scale) of the approach.

CRediT authorship contribution statement

Kamran Foroutani: Methodology, Visualization, Investigation, Formal analysis, Writing - original draft. Seyed Morteza Ghasemi: Supervision, Conceptualization, Resources, Data curation, Validation, Writing - review & editing, Project administration. Behzad Pourabbas: Supervision, Resources.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors gratefully acknowledge Mrs. Leila Tolami from Iran Polymer and Petrochemical Institute (IPPI) for her help with the SAXS experiments, Mrs. Salehi and Mr. Ehsan Ansari from University of Teh-ran (UT) for FESEM analysis. We also thank Dr. Fatemeh Ghorbani from Tabriz University of Medical Science for her kindly support in providing the BSA protein.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi. org/10.1016/j.memsci.2021.119099.

References

[1] S.P. Nunes, Block copolymer membranes, in: Sustainable Nanoscale Engineering, Elsevier, 2020, pp. 297–316.

[2] M. Radjabian, V. Abetz, Advanced porous polymer membranes from self- assembling block copolymers, Prog. Polym. Sci. 102 (2020) 101219.

[3] S.P. Nunes, Block copolymer membranes for aqueous solution applications, Macromolecules 49 (2016) 2905–2916.

[4] K.-V. Peinemann, V. Abetz, P.F. Simon, Asymmetric superstructure formed in a block copolymer via phase separation, Nat. Mater. 6 (2007) 992–996.

[5] R.F. Stepto, Dispersity in polymer science (IUPAC Recommendations 2009), Pure Appl. Chem. 81 (2009) 351–353.

[6] S. Rangou, K. Buhr, V. Filiz, J.I. Clodt, B. Lademann, J. Hahn, A. Jung, V. Abetz, Self-organized isoporous membranes with tailored pore sizes, J. Membr. Sci. 451 (2014) 266–275.

[7] M. Karunakaran, S.P. Nunes, X. Qiu, H. Yu, K.-V. Peinemann, Isoporous PS-b-PEO ultrafiltration membranes via self-assembly and water-induced phase separation, J. Membr. Sci. 453 (2014) 471–477.

[8] D. Wu, F. Xu, B. Sun, R. Fu, H. He, K. Matyjaszewski, Design and preparation of porous polymers, Chem. Rev. 112 (2012) 3959–4015.

[9] S.P. Nunes, R. Sougrat, B. Hooghan, D.H. Anjum, A.R. Behzad, L. Zhao, N. Pradeep, I. Pinnau, U. Vainio, K.-V. Peinemann, Ultraporous films with uniform nanochannels by block copolymer micelles assembly, Macromolecules 43 (2010) 8079–8085.

[10] S.P. Nunes, A.R. Behzad, B. Hooghan, R. Sougrat, M. Karunakaran, N. Pradeep, U. Vainio, K.-V. Peinemann, Switchable pH-responsive polymeric membranes prepared via block copolymer micelle assembly, ACS Nano 5 (2011) 3516–3522.

[11] P. Madhavan, K.-V. Peinemann, S.P. Nunes, Complexation-tailored morphology of asymmetric block copolymer membranes, ACS Appl. Mater. Interfaces 5 (2013) 7152–7159.

[12] S.P. Nunes, M. Karunakaran, N. Pradeep, A.R. Behzad, B. Hooghan, R. Sougrat, H. He, K.-V. Peinemann, From micelle supramolecular assemblies in selective solvents to isoporous membranes, Langmuir 27 (2011) 10184–10190.

[13] M. Radjabian, C. Abetz, B. Fischer, A. Meyer, V. Abetz, Influence of solvent on the structure of an amphiphilic block copolymer in solution and in formation of an integral asymmetric membrane, ACS Appl. Mater. Interfaces 9 (2017) 31224–31234.

[14] L. Oss-Ronen, J. Schmidt, V. Abetz, A. Radulescu, Y. Cohen, Y. Talmon, Characterization of block copolymer self-assembly: from solution to nanoporous membranes, Macromolecules 45 (2012) 9631–9642.

[15] S. Dami, C. Abetz, B. Fischer, M. Radjabian, P. Georgopanos, V. Abetz, A correlation between structural features of an amphiphilic diblock copolymer in solution and the structure of the porous surface in an integral asymmetric membrane, Polymer 126 (2017) 376–385.

[16] D.S. Marques, U. Vainio, N.M. Chaparro, V.M. Calo, A.R. Bezahd, J.W. Pitera, K.- V. Peinemann, S.P. Nunes, Self-assembly in casting solutions of block copolymer membranes, Soft Matter 9 (2013) 5557–5564.

[17] R.M. Dorin, D.b.S. Marques, H. Sai, U. Vainio, W.A. Phillip, K.-V. Peinemann, S. P. Nunes, U. Wiesner, Solution small-angle X-ray scattering as a screening and predictive tool in the fabrication of asymmetric block copolymer membranes, ACS Macro Lett. 1 (2012) 614–617.

[18] C.-y. Yang, G.-d. Zhu, Z. Yi, Y.-y. Qiu, L.-f. Liu, C.-j. Gao, Tailoring the pore size and permeability of isoporous membranes through blending with poly (ethylene glycol): toward the balance of macro-and microphase separation, J. Membr. Sci. 598 (2020) 117755.

[19] G.-D. Zhu, Y.-R. Yin, Z.-N. Li, Y.-Y. Qiu, Z. Yi, C.-J. Gao, Organic acids interacting with block copolymers have broadened the window that retains isoporous structures, J. Membr. Sci. 582 (2019) 391–401.

[20] D.S. Marques, R.M. Dorin, U. Wiesner, D.-M. Smilgies, A.R. Behzad, U. Vainio, K.- V. Peinemann, S.P. Nunes, Time-resolved GISAXS and cryo-microscopy characterization of block copolymer membrane formation, Polymer 55 (2014) 1327–1332.

[21] Y.M. Li, Q. Zhang, J.R. Alvarez-Palacio, I.F. Hakem, Y. Gu, M.R. Bockstaller, U. Wiesner, Effect of humidity on surface structure and permeation of triblock terpolymer derived SNIPS membranes, Polymer 126 (2017) 368–375.

[22] J. Wang, M.M. Rahman, C. Abetz, S. Rangou, Z. Zhang, V. Abetz, Novel post- treatment approaches to tailor the pore size of PS-b-PHEMA isoporous membranes, Macromol. Rapid Commun. 39 (2018) 1800435.

[23] A. Jung, V. Filiz, S. Rangou, K. Buhr, P. Merten, J. Hahn, J. Clodt, C. Abetz, V. Abetz, Formation of integral asymmetric membranes of AB diblock and ABC triblock copolymers by phase inversion, Macromol. Rapid Commun. 34 (2013) 610–615.

[24] M. Mocan, H. Wahdat, H.M. van der Kooij, W.M. de Vos, M. Kamperman, Systematic variation of membrane casting parameters to control the structure of thermo-responsive isoporous membranes, J. Membr. Sci. 548 (2018) 502–509.

[25] Y. Mai, A. Eisenberg, Self-assembly of block copolymers, Chem. Soc. Rev. 41 (2012) 5969–5985.

[26] H. Feng, X. Lu, W. Wang, N.-G. Kang, J.W. Mays, Block copolymers: synthesis, self- assembly, and applications, Polymers 9 (2017) 494.

[27] S. Forster, T. Plantenberg, From self-organizing polymers to nanohybrid and biomaterials, Angew. Chem. Int. Ed. 41 (2002) 688–714.

[28] M.W. Matsen, M. Schick, Self-assembly of block copolymers, Curr. Opin. Colloid Interface Sci. 1 (1996) 329–336.

[29] G.-D. Zhu, C.-Y. Yang, Y.-R. Yin, Z. Yi, X.-H. Chen, L.-F. Liu, C.-J. Gao, Preparation of isoporous membranes from low χ block copolymers via co-assembly with H- bond interacting homopolymers, J. Membr. Sci. 589 (2019) 117255.

[30] M. Karayianni, S. Pispas, Self-assembly of amphiphilic block copolymers in selective solvents, in: Fluorescence Studies of Polymer Containing Systems, Springer, 2016, pp. 27–63.

[31] W.A. Phillip, M.A. Hillmyer, E. Cussler, Cylinder orientation mechanism in block copolymer thin films upon solvent evaporation, Macromolecules 43 (2010) 7763–7770.

K. Foroutani et al.

Journal of Membrane Science 623 (2021) 119099

12

[32] W.A. Phillip, B. O’Neill, M. Rodwogin, M.A. Hillmyer, E. Cussler, Self-assembled block copolymer thin films as water filtration membranes, ACS Appl. Mater. Interfaces 2 (2010) 847–853.

[33] J. Buitenhuis, S. Forster, Block copolymer micelles: viscoelasticity and interaction potential of soft spheres, J. Chem. Phys. 107 (1997) 262–272.

[34] P. Alexandridis, B. Lindman, Amphiphilic Block Copolymers: Self-Assembly and Applications, Elsevier, 2000.

[35] D. Voulgaris, C. Tsitsilianis, Aggregation behavior of polystyrene/poly (acrylic acid) heteroarm star copolymers in 1, 4-dioxane and aqueous media, Macromol. Chem. Phys. 202 (2001) 3284–3292.

[36] J. Cheng, M. Xu, P. Cheng, W. Zhang, N. Li, Y. Wang, J. Yang, K. Liang, P. Li, H. Yu, Metal ions ‘sewing’isoporous membranes with polystyrene-block-poly (acrylic acid) block copolymer, J. Membr. Sci. 587 (2019) 117086.

[37] H. Yu, X. Qiu, A.R. Behzad, V. Musteata, D.-M. Smilgies, S.P. Nunes, K.- V. Peinemann, Asymmetric block copolymer membranes with ultrahigh porosity and hierarchical pore structure by plain solvent evaporation, Chem. Commun. 52 (2016) 12064–12067.

[38] W. Zhang, P. Cheng, J. Cheng, N. Li, Y. Wang, J. Yang, X. Qiu, H. Tan, H. Yu, Ultrathin nanoporous membrane fabrication based on block copolymer micelles, J. Membr. Sci. 570 (2019) 427–435.

[39] L. Zhang, A. Eisenberg, Formation of crew-cut aggregates of various morphologies from amphiphilic block copolymers in solution, Polym. Adv. Technol. 9 (1998) 677–699.

[40] C. Xu, X. Fu, M. Fryd, S. Xu, B.B. Wayland, K.I. Winey, R.J. Composto, Reversible stimuli-responsive nanostructures assembled from amphiphilic block copolymers, Nano Lett. 6 (2006) 282–287.

[41] C. Xu, B.B. Wayland, M. Fryd, K.I. Winey, R.J. Composto, pH-responsive nanostructures assembled from amphiphilic block copolymers, Macromolecules 39 (2006) 6063–6070.

[42] W.M. Haynes, CRC Handbook of Chemistry and Physics, CRC press, 2014. [43] L. Li, J. Long, L. Li, H. Cao, T. Tang, X. Xi, L. Qin, Y. Lai, X. Wang, Quantitative

determination of residual 1, 4-dioxane in three-dimensional printed bone scaffold, Journal of orthopaedic translation 13 (2018) 58–67.

[44] C.M. Hansen, Hansen Solubility Parameters: a User’s Handbook, CRC press, 2007. [45] A.F. Barton, Handbook of Polymer-Liquid Interaction Parameters and Solubility

Parameters, CRC press, 1990. [46] Y. Xie, B. Sutisna, S.P. Nunes, Membranes prepared by self-assembly and chelation

assisted phase inversion, Chem. Commun. 53 (2017) 6609–6612. [47] L.H. Sperling, Introduction to Physical Polymer Science, John Wiley & Sons, 2005. [48] I. Teraoka, Polymer Solutions, John Wiley & Sons, Inc, 2002. [49] K. Sankhala, D.F. Wieland, J. Koll, M. Radjabian, C. Abetz, V. Abetz, Self-assembly

of block copolymers during hollow fiber spinning: an in situ small-angle X-ray scattering study, Nanoscale 11 (2019) 7634–7647.

[50] J. Lee, J. Park, J. Oh, S. Lee, S.Y. Kim, M. Seo, Nanoporous poly (ether sulfone) from polylactide-b-poly (ether sulfone)-b-polylactide precursor, Polymer 180 (2019) 121704.

[51] M.J. Park, J. Bang, T. Harada, K. Char, T.P. Lodge, Epitaxial transitions among FCC, HCP, BCC, and cylinder phases in a block copolymer solution, Macromolecules 37 (2004) 9064–9075.

[52] S. Qavi, R. Foudazi, Rheological characteristics of mesophases of block copolymer solutions, Rheol. Acta 58 (2019) 483–498.

[53] P. Scherrer, Bestimmung der inneren Struktur und der Große von Kolloidteilchen mittels Rontgenstrahlen, in: Kolloidchemie Ein Lehrbuch, Springer, 1912, pp. 387–409.

[54] M. Doi, J.L. Harden, T. Ohta, Anomalous rheological behavior of ordered phases of block copolymers. 2, Macromolecules 26 (1993) 4935–4944.

[55] T. Ohta, Y. Enomoto, J.L. Harden, M. Doi, Anomalous rheological behavior of ordered phases of block copolymers. 1, Macromolecules 26 (1993) 4928–4934.

[56] P.R. Malenfant, J. Wan, S.T. Taylor, M. Manoharan, Self-assembly of an organic–inorganic block copolymer for nano-ordered ceramics, Nat. Nanotechnol. 2 (2007) 43–46.

[57] F.S. Bates, G. Fredrickson, Block copolymers-designer soft materials, Phys. Today (2000) 52.

[58] G. Ertl, H.-J. Freund, Catalysis and surface science, Phys. Today 52 (1999) 32–38. [59] O. Terreau, C. Bartels, A. Eisenberg, Effect of poly (acrylic acid) block length

distribution on polystyrene-b-poly (acrylic acid) block copolymer aggregates in solution. 2. A partial phase diagram, Langmuir 20 (2004) 637–645.

[60] D.J. Adams, M.F. Butler, A.C. Weaver, Effect of block length, polydispersity, and salt concentration on PEO− PDEAMA block copolymer structures in dilute solution, Langmuir 22 (2006) 4534–4540.

[61] K.I. Seno, S. Kanaoka, S. Aoshima, Thermosensitive diblock copolymers with designed molecular weight distribution: synthesis by continuous living cationic polymerization and micellization behavior, J. Polym. Sci. Polym. Chem. 46 (2008) 2212–2221.

[62] J. Guo, Y. Zhou, L. Qiu, C. Yuan, F. Yan, Self-assembly of amphiphilic random co- poly (ionic liquid) s: the effect of anions, molecular weight, and molecular weight distribution, Polym. Chem. 4 (2013) 4004–4009.

[63] S. Cui, L. Chen, L. Yu, J. Ding, Synergism among polydispersed amphiphilic block copolymers leading to spontaneous physical hydrogelation upon heating, Macromolecules 53 (2020) 7726–7739.

[64] W.F. Brinkman, P.E. Cladis, Defects in liquid crystals, PhT 35 (1982) 48–56. [65] N.A. Lynd, A.J. Meuler, M.A. Hillmyer, Polydispersity and block copolymer self-

assembly, Prog. Polym. Sci. 33 (2008) 875–893. [66] K. Hong, J. Noolandi, The effect of polydispersity on the microphase separation of

a blook copolymer system, Polym. Commun. 25 (1984) 265–268. [67] M.W. Matsen, Polydispersity-induced macrophase separation in diblock copolymer

melts, Phys. Rev. Lett. 99 (2007) 148304. [68] J.I. Clodt, B. Bajer, K. Buhr, J. Hahn, V. Filiz, V. Abetz, Performance study of

isoporous membranes with tailored pore sizes, J. Membr. Sci. 495 (2015) 334–340. [69] M.A. Frommer, R.M. Messalem, Mechanism of membrane formation. VI.

Convective flows and large void formation during membrane precipitation, Ind. Eng. Chem. Prod. Res. Dev. 12 (1973) 328–333.

[70] H. Yu, X. Qiu, S.P. Nunes, K.V. Peinemann, Self-assembled isoporous block copolymer membranes with tuned pore sizes, Angew. Chem. 126 (2014) 10236–10240.

K. Foroutani et al.