Enhanced Pervaporation Properties of PVA-Based Membranes Modified with Polyelectrolytes. Application to IPA Dehydration

In this work, dense and supported pervaporation polyvinyl alcohol (PVA)-based membranes modified with poly(allylamine hydrochloride) (PAH) and poly(sodium 4-styrenesulfonate)(PSS)/PAH top nanolayers were synthesized. Two main points were investigated: the role of the polyelectrolyte PAH on water selectivity of the selective polymer matrix and the impact of the porous substrate based on polyacrylonitrile (PAN) and aromatic polysulfone amide (UPM-20®), used to get supported high-performance membranes. Various methods of analysis (fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), atomic force microscopy (AFM), small-angle X-ray scattering (SAXS), porosity, contact angles, ultrafiltration) were applied to study the developed membranes. Transport characteristics of the developed membranes were studied in isopropanol dehydration by pervaporation. Obtained results are discussed in the light of the structure and physicochemical characteristics of these PVA/PAH membranes and the types of porous substrate. It was shown that the PAN-supported membrane with the selective layer based on PVA/PAH modified by 10 polyelectrolyte PSS/PAH bilayers possessed ~4.5 times higher permeation flux with the same high selectivity level (99.9 wt % water in the permeate) for the dehydration of the isopropanol (20 wt % water) at 60 °C compared to the commercial analog PERVAPTM 1201.


Introduction
Currently, membrane-based separation technologies are known to be efficient for molecular separations, and they are already considered as alternative processes to traditional methods (absorption, distillation, extractive rectification) at the industrial scale, especially for the separation and purification of azeotropic liquid mixtures and for components having close boiling points. This is because membrane separation technologies offer significant energy savings and can give rise to environmentally friendly processes. Pervaporation is one of the promising classical membrane processes for the dehydration of organic substances, especially alcohols with the use of hydrophilic membranes, because of the high water selectivity of the membranes, equipment compactness, and lower energy consumption [1][2][3][4][5][6][7]. One of the most studied mixtures for dehydration by pervaporation is the isopropanol (i-PrOH)-water system. Isopropanol is an important solvent widely used for production of various chemical reagents and as a substitute for ethanol in areas such as cosmetics, perfumes, household chemicals, disinfectants,

Dense (Self-Standing) Membranes
The preparation of the dense membranes based on polymer blend PVA-PAH was carried out according to the following procedure: a 2 wt % aqueous PVA solution was prepared by dissolving in bi-distilled water with constant stirring for 5 h at a temperature of 90 • C. The required amount of poly(allylamine hydrochloride) (PAH) (0 or 4.7 wt % with respect to the weight of the polymer) was dispersed in the filtered prepared PVA solution by ultrasonic treatment for 40 min at ambient temperature. The maximum loading of poly(allylamine hydrochloride) was limited to 4.7 wt % due to its poor dispersion in solutions having higher amounts. All of the membranes were subjected to the chemical treatment according to a previously reported method [37]; maleic acid (MA) (35 wt % with respect to the weight of the polymer) was added to the PVA and polymer blend solutions as an additional cross-linking agent to improve the degree of cross-linking of polymer chains. Chemically cross-linked PVA and PVA-PAH dense membranes were prepared by a solution casting method and evaporation of the solvent in a thermostat at 40 • C for 24 h with further heat treatment at 110 • C for 120 min. The thickness of the membranes was 45 ± 3 µm.

Supported Membranes
Supported membranes were prepared by casting of 2 wt % aqueous solution of PVA with 35 wt % MA or its polymer blend solution (PVA/PAH (4.7%)/MA (35%)) onto the surface of the porous substrate based on polyacrylonitrile (PAN) or aromatic polysulfone amide  and drying at room temperature for 24 h to evaporate the solvent. A thin fabricated selective layer of these membranes was prepared according to the procedure described for dense membranes, which had a thickness of 1 ± 0.3 µm as determined by scanning electron microscopy (SEM) measurements. Its cross-linking was achieved by heating the membrane at 110 • C for 120 min. The choice of PAN substrate was due to the fact that the Sulzer Chemtech company uses a polyacrylonitrile-based substrate for membrane production, and the choice of the UPM-20 substrate was due to its good mechanical properties, as determined by earlier studies [23,38].
In Table 1, the names of the PVA-based membranes are presented in abbreviated form.

IR Spectroscopy
IR spectroscopy was performed using the attenuated total reflection (ATR) methodology in the range of 650-4000 cm −1 at 25 • C on a BRUKER-TENSOR 27 spectrometer (Bruker Optik GmbH, Ettlingen, Germany) to determine the nature of the interaction between PVA and PAH.

Scanning Electron Microscopy (SEM)
A Zeiss Merlin SEM (Carl Zeiss SMT, Oberhochen, Germany) was used to obtain SEM micrographs of the membrane cross-sections. The membranes were submerged in liquid nitrogen for 5 min and fractured perpendicular to the surface. The prepared specimens were observed using SEM at 1 kV.

Atomic Force Microscopy (AFM)
The topography of the surfaces of membranes was studied on an NT-MDT NTegra Maximus atomic force microscope (NT-MDT Spectrum Instruments, Moscow, Russia) with standard silicon cantilevers with a rigidity of 15 N·m −1 ("NT-MDT", Russia) in tapping mode.

X-Ray Diffraction
The characterization of the membrane structure was carried out using a Bruker D8 DISCOVER diffractometer (Bruker, Karlsruhe, Germany) (40 kV, CuKα radiation, step size 0.05 • , 40 mA,) at room temperature in the Bragg-Brentano geometry. The obtained small-angle X-ray scattering (SAXS) data were analyzed with the ATSAS software package [39].

Contact-Angle Determination
Contact angles of membrane surface were measured by the sessile drop method as described in Reference [40] to study the stability of PEL bilayers. The measurements were carried out only from the side of the thin selective layer to study the surface characteristics of the membranes.
2.3.6. LbL Deposition Technique PAH (10 −2 mol/L) and PSS (10 −2 mol/L) were used as polyelectrolyte solutions. The pH for the PAH solutions was adjusted to 4 because this polyelectrolyte was fully ionized at this value.
The multilayer deposition of PEL layers was carried out by ND Multi Axis Dip Coaters ND-3D 11/5 (Nadetech, Noáin (Navarra), Spain) with a wide speed immersion range (from 1 mm·min −1 to 2000 mm·min −1 ). The membrane was clamped on a special support and immersed into solutions for Polymers 2020, 12, 14 6 of 22 different times. The procedure was as follows (shown in Figure 1): firstly, the membrane was deposited into a polycation solution of PAH for 10 min. Next, it was removed and rinsed thoroughly with water for 1 s 15 times, 5 s three times, and 15 s one time, successively. Then, the membrane was immersed in PSS solution for 10 min, after which the same water rinsing process of the membrane was repeated. In this way, one PEL bilayer was created on the membrane surface. The required number of additional bilayers was deposited according to a similar scheme. When the PAH polycation was previously introduced into the PVA matrix, the PSS polyanion would be firstly deposited on the membrane surface. The optimum number of PEL bilayers applied to the surface of the membrane was 10, because the deposition of fewer or more than 10 bilayers did not lead to the improvement of membrane transport properties [23]. 2000 mm•min −1 ). The membrane was clamped on a special support and immersed into solutions for different times. The procedure was as follows (shown in Figure 1): firstly, the membrane was deposited into a polycation solution of PAH for 10 min. Next, it was removed and rinsed thoroughly with water for 1 s 15 times, 5 s three times, and 15 s one time, successively. Then, the membrane was immersed in PSS solution for 10 min, after which the same water rinsing process of the membrane was repeated. In this way, one PEL bilayer was created on the membrane surface. The required number of additional bilayers was deposited according to a similar scheme. When the PAH polycation was previously introduced into the PVA matrix, the PSS polyanion would be firstly deposited on the membrane surface. The optimum number of PEL bilayers applied to the surface of the membrane was 10, because the deposition of fewer or more than 10 bilayers did not lead to the improvement of membrane transport properties [23].

Pervaporation experiments
Transport properties were studied using a pervaporation laboratory cell in steady-state regime at different temperatures (20,30,45, 60 °C) [38]. The composition of permeate and feed was analyzed by gas chromatography using a SHIMADZU GC-2010 chromatograph (SHIMADZU, Nancy, France) equipped with an HP-PLOT/U column (Agilent J&W GC Columns, Nancy, France) and a thermal conductivity detector.
The membrane permeation flux J (kg/(m 2 h)) was determined to be the amount of liquid transported through a unit of the membrane area per hour and was calculated as follows [41]: where W (kg) is the weight of the liquids that permeated the membrane, A (m 2 ) is the effective membrane area, and t (h) is the measurement time.
The separation factor (β) was calculated as follows [42]: Each measurement was performed at least three times to ensure good accuracy of the transport parameters, and the average value was recorded for later analysis. The mean accuracies for the transport parameters were as follows: for a dense membrane, ±0.5% for water content in the

Pervaporation Experiments
Transport properties were studied using a pervaporation laboratory cell in steady-state regime at different temperatures (20,30,45, 60 • C) [38]. The composition of permeate and feed was analyzed by gas chromatography using a SHIMADZU GC-2010 chromatograph (SHIMADZU, Nancy, France) equipped with an HP-PLOT/U column (Agilent J&W GC Columns, Nancy, France) and a thermal conductivity detector.
The membrane permeation flux J (kg/(m 2 h)) was determined to be the amount of liquid transported through a unit of the membrane area per hour and was calculated as follows [41]: where W (kg) is the weight of the liquids that permeated the membrane, A (m 2 ) is the effective membrane area, and t (h) is the measurement time.
The separation factor (β) was calculated as follows [42]: where y i and y j are the weight fractions of components i and j in the permeate, and x i and x j are the weight fractions of components i and j in the feed. Each measurement was performed at least three times to ensure good accuracy of the transport parameters, and the average value was recorded for later analysis. The mean accuracies for the transport parameters were as follows: for a dense membrane, ±0.5% for water content in the permeate and ±5% for permeation flux; for supported membranes, ±1% for water content in the permeate and ±6% for permeation flux. The porosimetry of the commercial porous substrates PAN and UPM-20 was studied by the standard porosimetry method on a Porosimeter 3.1 instrument at 30 ± 0.5 • C. The samples of the membranes were prepared in the form of a "tablet" of regular shape with a diameter of up to 23 mm. n-Octane was used as the reference liquid.

Filtration Performance of Porous Substrates
Water flux measurements and rejection calculations for commercial modified porous substrates  were carried out in a custom-made stirred dead-end filtration system ( Figure 2) at 25 • C and at the average transmembrane pressure of 2 bar. Bovine serum albumin (BSA) was used as a protein foulant and prepared as 0.5 g/L in buffered solution (pH = 7).
The permeation flux (J) of water was calculated by the following equation [43]: where V (L) is the permeate volume, A (m 2 ) is the effective membrane area, and ∆t (h) is the permeation time.
The BSA content in the permeate was evaluated by spectrophotometry on a Spectrophotometer SF-102 (NPO INTERPHOTOPHYSICS, Moscow, Russia) (at a wavelength of 280 nm corresponding to the maximum value for the protein).
The BSA rejection (R, %) was calculated by the following equation: where c p and c f are the BSA contents in the permeate and feed (g/L), respectively.

The standard porosimetry method
The porosimetry of the commercial porous substrates PAN and UPM-20 was studied by the standard porosimetry method on a Porosimeter 3.1 instrument at 30 ± 0.5 °С. The samples of the membranes were prepared in the form of a "tablet" of regular shape with a diameter of up to 23 mm. n-Octane was used as the reference liquid. The permeation flux (J) of water was calculated by the following equation [43]: where V (L) is the permeate volume, A (m 2 ) is the effective membrane area, and Δt (h) is the permeation time.
The BSA content in the permeate was evaluated by spectrophotometry on a Spectrophotometer SF-102 (NPO INTERPHOTOPHYSICS, Moscow, Russia) (at a wavelength of 280 nm corresponding to the maximum value for the protein).
The BSA rejection (R, %) was calculated by the following equation: where cp and cf are the BSA contents in the permeate and feed (g/L), respectively.

PVA-PAH structure characteristics
In our previous study [23], it was shown that the PVA-based supported membranes modified by PAH possessed increased permeability, but this effect was not fully rationalized. To investigate the influence of the PAH introduction on PVA membrane properties, a set of dense membranes were prepared and studied by IR spectroscopy and scanning electron microscopy for the first time.

PVA-PAH Structure Characteristics
In our previous study [23], it was shown that the PVA-based supported membranes modified by PAH possessed increased permeability, but this effect was not fully rationalized. To investigate the influence of the PAH introduction on PVA membrane properties, a set of dense membranes were prepared and studied by IR spectroscopy and scanning electron microscopy for the first time.  The spectrum for the modified PVA-PAН membrane was significantly different from the spectrum of PAH due to its small addition. Comparing the spectra of PVA and PVA-PAH membranes, the following changes were observed: two peaks at 2924 and 2854 cm −1 in the spectrum of the PVA-PAH membrane had a higher intensity compared to the peaks of the PVA membrane (at 2921 and 2855 cm −1 ) ( Figure 3). This pair of peaks corresponded to asymmetrical and symmetrical valence vibrations of CH2, respectively. During the modification of PVA by PAH, the peaks at 1417 cm −1 (corresponding to asymmetric deformation vibrations of the CH group) and at 1223 cm −1 (corresponding to the region of deformation vibrations of CH groups) of the PVA membrane shifted to 1422 and to 1235 cm −1 for the PVA-PAH membrane. In interpreting this phenomenon, it can be concluded that the characteristics of the СН bond changed and, perhaps, there are intermolecular interactions related to the hydrophobic parts of the PVA molecule after modification by the polyelectrolyte PAH [44].
It should be noted that, for the chemically cross-linked PVA membrane, i.e., with the addition of maleic acid (MA) and subsequent heating at 110 °C for 120 min, two structures could be formed according to the previous studies [37,45]. According to the obtained spectra, it is possible to form various bonds between PVA and MA, PVA and PAH, and PAH and MA (ether and ester bonds, The IR spectra of PAH (powder), chemically cross-linked PVA and PVA-PAH dense membranes.
The spectrum for the modified PVA-PAH membrane was significantly different from the spectrum of PAH due to its small addition. Comparing the spectra of PVA and PVA-PAH membranes, the following changes were observed: two peaks at 2924 and 2854 cm −1 in the spectrum of the PVA-PAH membrane had a higher intensity compared to the peaks of the PVA membrane (at 2921 and 2855 cm −1 ) ( Figure 3). This pair of peaks corresponded to asymmetrical and symmetrical valence vibrations of CH 2 , respectively. During the modification of PVA by PAH, the peaks at 1417 cm −1 (corresponding to asymmetric deformation vibrations of the CH group) and at 1223 cm −1 (corresponding to the region of deformation vibrations of CH groups) of the PVA membrane shifted to 1422 and to 1235 cm −1 for the PVA-PAH membrane. In interpreting this phenomenon, it can be concluded that the characteristics of the CH bond changed and, perhaps, there are intermolecular interactions related to the hydrophobic parts of the PVA molecule after modification by the polyelectrolyte PAH [44].
It should be noted that, for the chemically cross-linked PVA membrane, i.e., with the addition of maleic acid (MA) and subsequent heating at 110 • C for 120 min, two structures could be formed according to the previous studies [37,45]. According to the obtained spectra, it is possible to form various bonds between PVA and MA, PVA and PAH, and PAH and MA (ether and ester bonds, hydrogen bonds, etc.). Thus, the establishment of a structure using spectral methods of analysis is significantly complicated.
SEM was applied to visualize the effect of PAH introduction into the PVA matrix on the inner morphology of membranes. The corresponding cross-sectional micrographs of PVA and PVA-PAH membranes are presented in Figure 4. hydrogen bonds, etc.). Thus, the establishment of a structure using spectral methods of analysis is significantly complicated. SEM was applied to visualize the effect of PAH introduction into the PVA matrix on the inner morphology of membranes. The corresponding cross-sectional micrographs of PVA and PVA-PAH membranes are presented in Figure 4. It was shown by the cross-sectional SEM micrographs ( Figure 4) that the degree of roughness of the dense PVA-PAH membrane increased significantly after the introduction of PAH into the PVA matrix ( Figure 4b). The PVA membrane has a relatively smooth and uniform cross-section structure, while the introduction of PAH into the PVA matrix leads to the appearance of plastic deformations on the cross-section of the modified membrane, which can provide significant changes in transport properties compared to a pristine PVA membrane.
Additionally, the structure of the obtained PVA and PVA-PAH membranes was characterized by SAXS, and the changes in the surface properties for PVA-based membranes due to the modification by PAH were evaluated by the static sessile drop method to measure the contact angles of water. The calculated results from SAXS data and the values of the obtained contact angles for the dense membranes are presented in Table 2.  Table 2, it was determined that the radius of gyration and Porod volume slightly increased when PAH was added to the initial polymer, reflecting a higher degree of cross-linking. The introduction of PAH to the PVA matrix led to the decrease in contact angle from 67° to 61°, which indicated the hydrophilization of PVA-PAH membrane surface due to the rise in quantity of polar groups on the surface of the membrane. These changes in the structure and surface of the PVA membrane during the modification by PAH could affect the transport properties of the obtained membranes.

Transport properties of modified PVA membranes
The study of chemically cross-linked dense membranes allows an accurate description of the transport characteristics of the selected PVA polymer and PVA-PAH polymer blend, since the effect of substrate and defects in the selective layer can be excluded. To study the influence of the nature of the applied substrate for the preparation of supported mixed matrix PVA membranes, a new It was shown by the cross-sectional SEM micrographs ( Figure 4) that the degree of roughness of the dense PVA-PAH membrane increased significantly after the introduction of PAH into the PVA matrix ( Figure 4b). The PVA membrane has a relatively smooth and uniform cross-section structure, while the introduction of PAH into the PVA matrix leads to the appearance of plastic deformations on the cross-section of the modified membrane, which can provide significant changes in transport properties compared to a pristine PVA membrane.
Additionally, the structure of the obtained PVA and PVA-PAH membranes was characterized by SAXS, and the changes in the surface properties for PVA-based membranes due to the modification by PAH were evaluated by the static sessile drop method to measure the contact angles of water. The calculated results from SAXS data and the values of the obtained contact angles for the dense membranes are presented in Table 2. From the data of Table 2, it was determined that the radius of gyration and Porod volume slightly increased when PAH was added to the initial polymer, reflecting a higher degree of cross-linking. The introduction of PAH to the PVA matrix led to the decrease in contact angle from 67 to 61 • , which indicated the hydrophilization of PVA-PAH membrane surface due to the rise in quantity of polar groups on the surface of the membrane. These changes in the structure and surface of the PVA membrane during the modification by PAH could affect the transport properties of the obtained membranes.

Transport Properties of Modified PVA Membranes
The study of chemically cross-linked dense membranes allows an accurate description of the transport characteristics of the selected PVA polymer and PVA-PAH polymer blend, since the effect of substrate and defects in the selective layer can be excluded. To study the influence of the nature of the applied substrate for the preparation of supported mixed matrix PVA membranes, a new commercial porous substrate based on polyacrylonitrile (PAN) was used for the deposition of a thin selective layer based on PVA or PVA-PAH polymer blend. Further surface modification by LbL assembly (the deposition of the PSS/PAH bilayers) for pervaporation dehydration of i-PrOH (20 wt % water) was carried out to compare with the supported membranes on the porous substrate from aromatic polysulfone amide (UPM-20) studied earlier [23].

Transport Properties of PVA and PVA-PAH Membranes
Two types of membranes were developed and investigated: dense membranes based on PVA and PVA-PAH polymer blend, and supported membranes with the same compositions of the thin selective layer on a commercial porous PAN substrate. Their transport properties were studied in the separation of an industrially significant isopropanol (i-PrOH) (80 wt %)-water (20 wt %) mixture by pervaporation at 20 • C. The comparison of transport properties of obtained membranes during the isopropanol dehydration is presented in Figure 5. Two types of membranes were developed and investigated: dense membranes based on PVA and PVA-PAH polymer blend, and supported membranes with the same compositions of the thin selective layer on a commercial porous PAN substrate. Their transport properties were studied in the separation of an industrially significant isopropanol (i-PrOH) (80 wt %)-water (20 wt %) mixture by pervaporation at 20 °C. The comparison of transport properties of obtained membranes during the isopropanol dehydration is presented in Figure 5. The pervaporation results of Figure 5a demonstrate that the introduction of PAH into the PVA matrix led to a ~2.2-fold increase in permeation flux at high selectivity with respect to water ( Figure  5b) (water content in the permeate not less than 97.8 wt %) for both membrane types (dense and supported) compared to the corresponding type of pristine PVA membrane. This result was likely due to changes in inner membrane structure demonstrated by SEM ( Figure 4) and the increase in hydrophilicity of the PAH-containing membrane surface (Table 2). It should be noted that the creation of the supported membrane based on the PVA-PAH (4.7%) polymer blend with PAN substrate (PVA-PAH PAN ) led to a significant increase in the permeation flux (~4-fold) compared to the dense PVA-PAH membrane with a slight decrease in water selectivity (a reduction of water content in the permeate from 99.9 to 97.8 wt %) (Figures 5a,b). The increase in permeability and decrease in selectivity compared to unmodified PVA membranes was also noted for the PVA-PAH supported membrane when the substrate based on aromatic polysulfone amide (UPM-20) was used [23] (Table 3). The pervaporation results of Figure 5a demonstrate that the introduction of PAH into the PVA matrix led to a~2.2-fold increase in permeation flux at high selectivity with respect to water (Figure 5b) (water content in the permeate not less than 97.8 wt %) for both membrane types (dense and supported) compared to the corresponding type of pristine PVA membrane. This result was likely due to changes in inner membrane structure demonstrated by SEM ( Figure 4) and the increase in hydrophilicity of the PAH-containing membrane surface (Table 2). It should be noted that the creation of the supported membrane based on the PVA-PAH (4.7%) polymer blend with PAN substrate (PVA-PAH PAN ) led to a significant increase in the permeation flux (~4-fold) compared to the dense PVA-PAH membrane with a slight decrease in water selectivity (a reduction of water content in the permeate from 99.9 to 97.8 wt %) (Figure 5a,b). The increase in permeability and decrease in selectivity compared to unmodified PVA membranes was also noted for the PVA-PAH supported membrane when the substrate based on aromatic polysulfone amide (UPM-20) was used [23] (Table 3).  [23] It was found that the application of PAN substrate for PVA-based supported membranes led to a higher water content in the permeate and lower permeation flux compared to the application of UPM-20 substrate during pervaporation of an i-PrOH (80 wt %)/water (20 wt %) mixture at 20 • C ( Table 3). The investigation of this effect is presented in Section 3.3.
To study the inner structure of supported membranes and to estimate the thickness of a selective layer, scanning electron microscopy (SEM) was used. Figure 6 demonstrates the cross-sectional micrographs of the supported membranes based on the PVA-PAH polymer blend on PAN and UPM-20 substrates.   [23] It was found that the application of PAN substrate for PVA-based supported membranes led to a higher water content in the permeate and lower permeation flux compared to the application of UPM-20 substrate during pervaporation of an i-PrOH (80 wt %)/water (20 wt %) mixture at 20 °C ( Table 3). The investigation of this effect is presented in Section 3.3.
To study the inner structure of supported membranes and to estimate the thickness of a selective layer, scanning electron microscopy (SEM) was used. Figure 6 demonstrates the cross-sectional micrographs of the supported membranes based on the PVA-PAH polymer blend on PAN and UPM-20 substrates. A uniform nonporous structure of the dense selective PVA-PAH top layer with good adhesion to spongy porous PAN and UPM-20 substrates is presented on the micrographs ( Figure 6). Figure 6 demonstrates that the thickness of the top selective layer is determined as ∼1 ± 0.3 µm. Notably, a similar SEM images were obtained for the pristine PVA membranes (not shown here).
To study the surface topography of supported membranes, atomic force microscopy (AFM) was used. Figure 7 demonstrates AFM images of the skin layer surface based on polymer blend PVA-PAH of the supported membranes on PAN and UPM-20 substrates with a scan size of 7.5 × 7.5 µm.
The SEM micrographs of cross-section for (a) PVA-PAH PAN and (b) PVA-PAH UPM membranes.
A uniform nonporous structure of the dense selective PVA-PAH top layer with good adhesion to spongy porous PAN and UPM-20 substrates is presented on the micrographs ( Figure 6). Figure 6 demonstrates that the thickness of the top selective layer is determined as ∼1 ± 0.3 µm. Notably, a similar SEM images were obtained for the pristine PVA membranes (not shown here).
To study the surface topography of supported membranes, atomic force microscopy (AFM) was used. Figure 7 demonstrates AFM images of the skin layer surface based on polymer blend PVA-PAH of the supported membranes on PAN and UPM-20 substrates with a scan size of 7.5 × 7.5 µm.   [23] It was found that the application of PAN substrate for PVA-based supported membranes led to a higher water content in the permeate and lower permeation flux compared to the application of UPM-20 substrate during pervaporation of an i-PrOH (80 wt %)/water (20 wt %) mixture at 20 °C ( Table 3). The investigation of this effect is presented in Section 3.3.
To study the inner structure of supported membranes and to estimate the thickness of a selective layer, scanning electron microscopy (SEM) was used. Figure 6 demonstrates the cross-sectional micrographs of the supported membranes based on the PVA-PAH polymer blend on PAN and UPM-20 substrates. A uniform nonporous structure of the dense selective PVA-PAH top layer with good adhesion to spongy porous PAN and UPM-20 substrates is presented on the micrographs ( Figure 6). Figure 6 demonstrates that the thickness of the top selective layer is determined as ∼1 ± 0.3 µm. Notably, a similar SEM images were obtained for the pristine PVA membranes (not shown here).
To study the surface topography of supported membranes, atomic force microscopy (AFM) was used. Figure 7 demonstrates AFM images of the skin layer surface based on polymer blend PVA-PAH of the supported membranes on PAN and UPM-20 substrates with a scan size of 7.5 × 7.5 µm. Based on AFM images, the roughness characteristics in terms of root-mean-squared surface roughness (Rq) and average roughness (Ra) for supported PVA-PAH PAN and (b) PVA-PAH UPM membranes were calculated (Table 4). These roughness characteristics can strongly affect the sorption of the feed components on the membrane surface and the membrane permeability.  Table 4). The average surface roughness (Ra) for PVA-PAH UPM membrane increased 1.4-fold compared to the average surface roughness of PVA-PAH PAN membrane (0.48 nm). The increase in surface roughness, providing a large surface area for contact with the feed, is one of the factors that leads to facilitated sorption of penetrants and promotes faster penetration of the feed components. This results in significantly improved membrane permeability, which is in agreement with the pervaporation data (Table 3).
Due to the obtained high water content in the permeate for the PVA-PAH PAN membrane, it was chosen for further surface modification by LbL deposition of polyelectrolyte PSS/PAH bilayers with the aim of further improvement of transport parameters and investigation of the influence of the use of PAN substrate on stability and transport properties of the developed membrane.

Transport properties of membranes modified by LbL assembly
One of the perspective methods to functionalize the membrane surface in order to improve the transport membrane parameters is the deposition of polyelectrolytes via the layer-by-layer (LbL) technique. The application of this method is very relevant and allows the use of different amounts of nanoscale layers of oppositely electrically charged polyelectrolytes (polycation and polyanion) to control the modification of the surface (change in hydrophilicity, creation of charged surface, etc.). The surface of the supported PVA-based membranes was coated by 10 and 20 bilayers of polyelectrolytes: poly(allylamine hydrochloride) as polycation and poly(sodium 4-styrenesulfonate) as polyanion. The bulk modification of PVA membranes by the introduction of PAH was carried out for better adhesion of PEL bilayers with the membrane surface. The transport properties of membranes modified by PEL were studied in pervaporation separation of a mixture of i-PrOH (80 wt %) and water (20 wt %) at 20 °C (Table 5). Based on AFM images, the roughness characteristics in terms of root-mean-squared surface roughness (R q ) and average roughness (R a ) for supported PVA-PAH PAN and (b) PVA-PAH UPM membranes were calculated (Table 4). These roughness characteristics can strongly affect the sorption of the feed components on the membrane surface and the membrane permeability. It was shown that the deposition of the selective layer based on PVA-PAH on the porous UPM substrate led to a more pronounced surface roughness of supported membrane compared to the deposition on PAN substrate ( Table 4). The average surface roughness (R a ) for PVA-PAH UPM membrane increased 1.4-fold compared to the average surface roughness of PVA-PAH PAN membrane (0.48 nm). The increase in surface roughness, providing a large surface area for contact with the feed, is one of the factors that leads to facilitated sorption of penetrants and promotes faster penetration of the feed components. This results in significantly improved membrane permeability, which is in agreement with the pervaporation data (Table 3).
Due to the obtained high water content in the permeate for the PVA-PAH PAN membrane, it was chosen for further surface modification by LbL deposition of polyelectrolyte PSS/PAH bilayers with the aim of further improvement of transport parameters and investigation of the influence of the use of PAN substrate on stability and transport properties of the developed membrane.

Transport Properties of Membranes Modified by LbL Assembly
One of the perspective methods to functionalize the membrane surface in order to improve the transport membrane parameters is the deposition of polyelectrolytes via the layer-by-layer (LbL) technique. The application of this method is very relevant and allows the use of different amounts of nanoscale layers of oppositely electrically charged polyelectrolytes (polycation and polyanion) to control the modification of the surface (change in hydrophilicity, creation of charged surface, etc.). The surface of the supported PVA-based membranes was coated by 10 and 20 bilayers of polyelectrolytes: poly(allylamine hydrochloride) as polycation and poly(sodium 4-styrenesulfonate) as polyanion. The bulk modification of PVA membranes by the introduction of PAH was carried out for better adhesion of PEL bilayers with the membrane surface. The transport properties of membranes modified by PEL were studied in pervaporation separation of a mixture of i-PrOH (80 wt %) and water (20 wt %) at 20 • C (Table 5). The presented data in Table 5 demonstrate that, for all PAN-supported membranes modified by LbL deposition, there is 99.9 wt % water in the permeate, which indicates very high and improved selective properties for the developed membranes compared to the pristine supported membranes (Figure 5b). Moreover, the coating of 10 PEL bilayers on the surface of supported membranes based on PVA and PVA-PAH (4.7%) increased the permeation flux~1.5-fold and~1.2-fold, respectively, compared to the supported membranes without LbL modification (Figure 5a, Table 3), because of the formation of small hydrophilic mashes, which induced penetration of the more polar component, i.e., water [36].
For further improvement of the transport properties of the supported PVA-PAH PAN membrane, it was decided to increase the number of polyelectrolyte bilayers to 20 and to identify the dependence of transport characteristics on the bilayer amount for the dehydration of i-PrOH by pervaporation, because the deposition of fewer than 10 bilayers did not lead to the improvement of transport properties of membranes. The data in Table 5 demonstrate that an increase of polyelectrolyte bilayers up to 20 for the PVA-PAH/LbL-20 PAN membrane led to the reduction in permeation flux (~1.4-fold) at the same water content in the permeate compared to the membranes with 10 PEL bilayers (PVA-PAH/LbL-10 PAN ). This effect was due to an increase in the thickness of the polyelectrolyte layer on the membrane surface, which limited the diffusion of the separated components to the selective layer based on PVA, ultimately leading to a decrease in the membrane flux [23]. Thus, 10 PEL bilayer coatings on the PVA-PAH membrane surface were chosen as the optimal condition of surface modification by LbL deposition. It should be noted that this developed PVA-PAH/LbL-10 PAN membrane possessed significantly higher selectivity properties (99.9 wt % water content in the permeate) compared to the PVA-PAH/LbL-10 UPM supported membrane on the UPM-20 substrate (Table 5).
Thus, the data of Table 3 for PVA-PAH membranes without LbL surface modification demonstrated that the permeation flux for the PAN-supported membrane (PVA-PAH PAN ) decreased~4.5-fold compared with the permeation flux of the PVA-PAH UPM membrane on the UPM-20, while the water content in the permeate for the PVA-PAH PAN membrane deposited on the PAN was much higher compared to the UPM-supported membrane. The same trend persisted after the deposition of 10 PEL bilayers on the surface of the membranes. However, it should be emphasized that the surface modification by 10 PEL bilayers led to a significant decrease in water content in the permeate (68.4 wt %) with a slight increase in permeation flux (0.261 kg/(m 2 h)) of the supported membrane on UPM-20 (PVA-PAH/LbL-10 UPM ) compared to the PVA-PAH membrane without surface modification on UPM-20 (PVA-PAH UPM ) (92.8 wt % water in the permeate, 0.224 kg/(m 2 h)). On the other hand, for the PVA-PAH supported membrane on PAN (PVA-PAH/LbL-10 PAN ), both membrane parameters were improved (99.9 wt % water in the permeate, 0.061 kg/(m 2 h)) as compared to the supported PVA-PAH PAN membrane without surface modification (97.8 wt % water in the permeate, 0.050 kg/(m 2 h)). Thus, the obtained data demonstrate that the transport characteristics of the supported membranes depend significantly on the choice of the commercial porous substrate.

Influence of the Substrate on the Mass Transport
Despite the generally accepted opinion that the porous substrate of supported membranes does not significantly affect the mass transfer of low-molecular-weight components in pervaporation, the substrate polymer type, as well as substrate porosity, can have a significant effect on the transport properties of the supported membrane [46][47][48]. In this section, to understand the mechanism of mass transfer in pervaporation, the characteristics of commercial porous substrates PAN and UPM-20 were studied by SEM, AFM, the standard porosimetry method, contact-angle measurements, and ultrafiltration experiments.
The inner structure of commercial membranes was studied by SEM for the evaluation of inner morphology. The SEM cross-sectional micrographs of industrial PAN and UPM-20 substrates are presented in Figure 8.

Influence of the substrate on the mass transport
Despite the generally accepted opinion that the porous substrate of supported membranes does not significantly affect the mass transfer of low-molecular-weight components in pervaporation, the substrate polymer type, as well as substrate porosity, can have a significant effect on the transport properties of the supported membrane [46][47][48]. In this section, to understand the mechanism of mass transfer in pervaporation, the characteristics of commercial porous substrates PAN and UPM-20 were studied by SEM, AFM, the standard porosimetry method, contact-angle measurements, and ultrafiltration experiments.
The inner structure of commercial membranes was studied by SEM for the evaluation of inner morphology. The SEM cross-sectional micrographs of industrial PAN and UPM-20 substrates are presented in Figure 8. The presented microphotographs in Figure 8 demonstrate that the bulk porosity of the PAN substrate is much smaller than for the UPM-20 substrate, which is one of key factors for permeation flux decrease and the increase in water content in the permeate with the use of a PAN membrane as the substrate compared to the UPM-20 substrate [46].
To study porosity of commercial PAN and UPM-20 substrates, the standard porosimetry method was applied. The obtained data of characteristics of size of pore distribution are presented in Table 6.

Porosity characteristics
Commercial membrane PAN UPM-20 Porosity over weight, cm 3 /g 1.10 2.60 Porosity over volume, cm 3 /cm 3 0.59 0.78 Meso-and macro-pore surface over weight, m 2 /g 4.74 309.96 Meso-and macro-pore surface over volume, m 2 /cm 3 2.51 93.40 As it can be seen from the data presented in Table 6, the porosity parameters for the UPM-20 substrateare higher than for the PAN substrate (in agreement with SEM data (Figure 8)).
The study of the rejection of bovine serum albumin by ultrafiltration was carried out in order to study the surface porous layer of commercial substrates. The ultrafiltration results are presented in Table 7. The presented microphotographs in Figure 8 demonstrate that the bulk porosity of the PAN substrate is much smaller than for the UPM-20 substrate, which is one of key factors for permeation flux decrease and the increase in water content in the permeate with the use of a PAN membrane as the substrate compared to the UPM-20 substrate [46].
To study porosity of commercial PAN and UPM-20 substrates, the standard porosimetry method was applied. The obtained data of characteristics of size of pore distribution are presented in Table 6. Table 6. Porosity characteristics of industrial PAN and UPM-20 substrates.

PAN UPM-20
Porosity over weight, cm 3 /g 1.10 2.60 Porosity over volume, cm 3 /cm 3 0.59 0.78 Meso-and macro-pore surface over weight, m 2 /g 4.74 309.96 Meso-and macro-pore surface over volume, m 2 /cm 3 2.51 93.40 As it can be seen from the data presented in Table 6, the porosity parameters for the UPM-20 substrateare higher than for the PAN substrate (in agreement with SEM data (Figure 8)).
The study of the rejection of bovine serum albumin by ultrafiltration was carried out in order to study the surface porous layer of commercial substrates. The ultrafiltration results are presented in Table 7. Via the ultrafiltration experiment, it was demonstrated that the permeation flux of water for the UPM-20 substrate significantly exceeded the water permeation flux for the PAN (Table 7) in agreement with the porosity, SEM, and pervaporation data. However, for both commercial membranes, the values of rejection (R) after the ultrafiltration of BSA were equal (~99%), which indicated that the pore size on the skin surface layer of the substrates (pores responsible for retaining the solute) was similar (in agreement with membrane passports) and they retained BSA equally well. However, it should be noted that the bulk porosity of these substrates, as measured by the standard porosimetry method, was significantly different (Table 6), which to a greater extent affected the difference in permeation fluxes of supported membranes during pervaporation.
Atomic force microscopy (AFM) and the sessile drop method for measuring the contact angle of water were used to evaluate the difference in surface morphology of the commercial membranes. The AFM images of the skin layer surface of the PAN and UPM-20 substrates with a scan size of 7.5 × 7.5 µm are presented in Figure 9.   (Table 7) in agreement with the porosity, SEM, and pervaporation data. However, for both commercial membranes, the values of rejection (R) after the ultrafiltration of BSA were equal (~99 %), which indicated that the pore size on the skin surface layer of the substrates (pores responsible for retaining the solute) was similar (in agreement with membrane passports) and they retained BSA equally well. However, it should be noted that the bulk porosity of these substrates, as measured by the standard porosimetry method, was significantly different (Table 6), which to a greater extent affected the difference in permeation fluxes of supported membranes during pervaporation.
Atomic force microscopy (AFM) and the sessile drop method for measuring the contact angle of water were used to evaluate the difference in surface morphology of the commercial membranes. The AFM images of the skin layer surface of the PAN and UPM-20 substrates with a scan size of 7.5 × 7.5 µm are presented in Figure 9. The roughness characteristics calculated based on AFM images, as well as contact angles of water measured by the sessile drop method for assessing the hydrophilic-hydrophobic surface properties, for PAN and UPM-20 substrates are presented in Table 8.  Figure 9) and roughness characteristics (Table 8), it was demonstrated that the commercial UPM-20 membrane (the average surface roughness (Ra) was equal to 4.48) had a The roughness characteristics calculated based on AFM images, as well as contact angles of water measured by the sessile drop method for assessing the hydrophilic-hydrophobic surface properties, for PAN and UPM-20 substrates are presented in Table 8. From the AFM images ( Figure 9) and roughness characteristics (Table 8), it was demonstrated that the commercial UPM-20 membrane (the average surface roughness (R a ) was equal to 4.48) had a rougher surface compared to the PAN substrate, which could affect the roughness of a thin selective layer based on the PVA-PAH polymer blend deposited on it (Table 4). Also, the UPM-20 substrate was more hydrophobic (contact angle of water 63 • ) compared to the PAN (48 • ). The lower contact angle of the PAN substrate caused better affinity of this substrate to water and, in combination with the low porosity of substrate (SEM, ultrafiltration, and porosity data), resulted in better selectivity with respect to water of PAN-supported membranes, while the more hydrophobic UPM substrate, which possessed higher porosity, was more favorable for the penetration of both separated components (isopropanol and water). Additionally, different substrate polymer types may influence the crystallinity of the selective dense layer based on PVA, which was reflected in the transport properties of the pervaporation membrane.
Thus, based on the obtained pervaporation data on the separation of the isopropanol-water mixture and data on the characterization of commercial porous substrates, it can be concluded that a supported membrane with a thin selective layer based on PVA-PAH deposited on porous PAN substrate and modified with 10 PEL bilayers via LbL deposition (PVA-PAH/LbL-10 PAN membrane) has the best selective properties with respect to water. On the other hand, the UPM-20 substrate, characterized by a much higher porosity, did not limit the mass transport of the low-molecular-weight component, which logically favored a higher permeance [46]. Nevertheless, at the same time, the water selectivity was reduced due to the coupling effect of several parameters: a more open pore structure, a rougher and more hydrophobic surface, and facilitated diffusion of iPrOH in UPM-20 (compared to PAN).

Investigation of PAN-Supported PVA and PVA-PAH Membranes at Different Temperatures
For investigation of the industrial application of a supported PVA-based membrane modified by 4.7 wt % PAH and 10 bilayers of PEL (PVA-PAH/LbL-10 PAN ) with the best transport properties for pervaporation isopropanol dehydration, it was decided to estimate the stability of this membrane for the separation of an isopropanol-water (20 wt % water) mixture at different temperatures (20,30,45, 60 • C), since most industrial processes were carried out at elevated temperatures. The transport properties of the PVA/LbL-10 PAN and PVA-PAH/LbL-10 PAN membranes are presented in Figure 10.
rougher surface compared to the PAN substrate , which could affect the roughness of a thin selective layer based on the PVA-PAH polymer blend deposited on it (Table 4). Also, the UPM-20 substrate was more hydrophobic (contact angle of water 63°) compared to the PAN (48°). The lower contact angle of the PAN substrate caused better affinity of this substrate to water and, in combination with the low porosity of substrate (SEM, ultrafiltration, and porosity data), resulted in better selectivity with respect to water of PAN-supported membranes, while the more hydrophobic UPM substrate, which possessed higher porosity, was more favorable for the penetration of both separated components (isopropanol and water). Additionally, different substrate polymer types may influence the crystallinity of the selective dense layer based on PVA, which was reflected in the transport properties of the pervaporation membrane.
Thus, based on the obtained pervaporation data on the separation of the isopropanol-water mixture and data on the characterization of commercial porous substrates, it can be concluded that a supported membrane with a thin selective layer based on PVA-PAH deposited on porous PAN substrate and modified with 10 PEL bilayers via LbL deposition (PVA-PAH/LbL-10 PAN membrane) has the best selective properties with respect to water. On the other hand, the UPM-20 substrate, characterized by a much higher porosity, did not limit the mass transport of the low-molecular-weight component, which logically favored a higher permeance [46]. Nevertheless, at the same time, the water selectivity was reduced due to the coupling effect of several parameters: a more open pore structure, a rougher and more hydrophobic surface, and facilitated diffusion of iPrOH in UPM-20 (compared to PAN).

Investigation of PAN-supported PVA and PVA-PAH membranes at different temperatures
For investigation of the industrial application of a supported PVA-based membrane modified by 4.7 wt.% PAH and 10 bilayers of PEL (PVA-PAH/LbL-10 PAN ) with the best transport properties for pervaporation isopropanol dehydration, it was decided to estimate the stability of this membrane for the separation of an isopropanol-water (20 wt.% water) mixture at different temperatures (20, 30, 45, 60 °C), since most industrial processes were carried out at elevated temperatures. The transport properties of the PVA/LbL-10 PAN and PVA-PAH/LbL-10 PAN membranes are presented in Figure 10. The data of Figure 10 demonstrate that the permeation flux of the supported membranes increases with the rise in temperature, while the water content in the permeate remaines constant even at elevated temperature (≥99.22 wt.%). The permeation flux of the modified The data of Figure 10 demonstrate that the permeation flux of the supported membranes increases with the rise in temperature, while the water content in the permeate remaines constant even at elevated temperature (≥99.22 wt %). The permeation flux of the modified PVA-PAH/LbL-10 PAN membrane was much higher (~2-fold) compared to the permeation flux of the PVA/LbL-10 PAN membrane due to the bulk modification by PAH. It should be noted that the water content in the permeate for both membranes almost did not change during the experiment at elevated temperatures (99.99-99.22 wt % water in the permeate), indicating the stability and high selectivity of the developed membrane with respect to water.
It was shown that the selective properties of the supported PVA-PAH/LbL-10 PAN membrane remained unchanged up to 60 • C ( Figure 10) for the separation of the isopropanol-water mixture (20 wt % water).

Comparison of PVA-PAH/LbL-10 PAN Membrane with PVA-Based Membranes
The performance of the PVA-PAH/LbL-10 PAN membrane was compared with mixed matrix PVA membranes described in the literature for isopropanol pervaporation dehydration and with a commercial analogue membrane "PERVAP TM 1201" (SULZER) that contained PAN as a substrate and was designed for the dehydration of water-organic mixtures (up to 80 wt % water in the feed). Comparison of the transport properties of PVA-based membranes for pervaporation of isopropanol/water system was done in terms of the separation factor and the permeation flux under conditions close to the present study and presented in Table 9. The data of Table 9 demonstrate that the developed PVA-PAH/LbL-10 PAN membrane exhibites a very high separation factor (39,969) and high permeation flux (0.713 kg/(m 2 h)) for the pervaporation of an isopropanol-water (80/20 wt %) mixture at 60 • C. These values were much greater than obtained for PVA membranes in previous studies. Only the "PERVAP TM 2510" membrane and the PVA membrane modified by graphene oxide quantum dots possessed higher permeation flux (0.857 and 1.502 kg/(m 2 h), respectively) but with significantly decreased separation factors (1128 and 117, respectively) for pervaporation of an isopropanol/water mixture compared to the PVA-PAH/LbL-10 PAN membrane. In addition, the permeation flux of the developed PVA-PAH/LbL-10 PAN membrane in this study was 4.5-fold higher than the permeation flux of the commercial analogue "PERVAP TM 1201" with the same separation factor value (39,969) for pervaporation dehydration of isopropanol (20 wt % water) at 60 • C. These results indicate the prospective industrial applications of this membrane for the dehydration of water-organic mixtures. The contact angles measured by static sessile drop method and scanning electron microscopy (SEM) were used to study and verify the stability of the polyelectrolyte multilayer on the surface of the supported PVA-PAH/LbL-10 PAN membrane.
By measuring the contact angles, it was shown that the values of the water contact angle for the membrane surface remained unchanged and were equaled 74 • ± 3 • before and after pervaporation of isopropanol (80 wt %)-water (20 wt %) at 20 • C, which indicated that the properties of the membrane surface did not change after contact with the separated mixture and proved the stability of the polyelectrolyte layers on the surface of supported membrane during the process of dehydration by pervaporation.
Cross-sectional SEM micrographs of the PVA-PAH/LbL-10 PAN membrane were taken before and after pervaporation for the confirmation of the stability of 10 PEL bilayers. These micrographs were similar. In this connection SEM micrograph of cross-section of the PVA-PAH/LbL-10 PAN membrane only after pervaporation is presented in Figure 11.

Study of the stability of top nanolayers deposited via LbL technique on the surface of PVA-PAH/LbL-10 PAN membrane
The contact angles measured by static sessile drop method and scanning electron microscopy (SEM) were used to study and verify the stability of the polyelectrolyte multilayer on the surface of the supported PVA-PAH/LbL-10 PAN membrane.
By measuring the contact angles, it was shown that the values of the water contact angle for the membrane surface remained unchanged and were equaled 74° ± 3° before and after pervaporation of isopropanol (80 wt.%)-water (20 wt.%) at 20 °C, which indicated that the properties of the membrane surface did not change after contact with the separated mixture and proved the stability of the polyelectrolyte layers on the surface of supported membrane during the process of dehydration by pervaporation.
Cross-sectional SEM micrographs of the PVA-PAH/LbL-10 PAN membrane were taken before and after pervaporation for the confirmation of the stability of 10 PEL bilayers. These micrographs were similar. In this connection SEM micrograph of cross-section of the PVA-PAH/LbL-10 PAN membrane only after pervaporation is presented in Figure 11. On the cross-sectional SEM micrograph of the PVA-PAH/LbL-10 PAN membrane (Figure 11), three areas can be observed: (a) the region of the porous PAN substrate, (b) the selective thin nonporous PVA-PAH layer, and (c) the layer of polyelectrolytes (PSS, PAH). The size of the polyelectrolyte multilayer was determined as ~50 nm by SEM. It was found that PEL layer on the membrane surface was stable and not washed away after the experiment on the separation of the i-PrOH/water mixture.
Thus, these investigations confirm that a novel green PAN-supported membrane based on a PVA-PAH (4.7%) polymer blend and modified by 10 PEL layers (PVA-PAH/LbL-10 PAN ) have improved transport characteristics compared to a membrane based on pure PVA, as well as highly selective properties with respect to water that are preserved and unchanged at elevated temperatures. Also, the stability of the composition and the invariability of the properties of the developed supported membrane after pervaporation were confirmed, which indicated the promise of using this membrane in industrial processes for the dehydration of organic substances. On the cross-sectional SEM micrograph of the PVA-PAH/LbL-10 PAN membrane (Figure 11), three areas can be observed: (a) the region of the porous PAN substrate, (b) the selective thin nonporous PVA-PAH layer, and (c) the layer of polyelectrolytes (PSS, PAH). The size of the polyelectrolyte multilayer was determined as~50 nm by SEM. It was found that PEL layer on the membrane surface was stable and not washed away after the experiment on the separation of the i-PrOH/water mixture.
Thus, these investigations confirm that a novel green PAN-supported membrane based on a PVA-PAH (4.7%) polymer blend and modified by 10 PEL layers (PVA-PAH/LbL-10 PAN ) have improved transport characteristics compared to a membrane based on pure PVA, as well as highly selective properties with respect to water that are preserved and unchanged at elevated temperatures. Also, the stability of the composition and the invariability of the properties of the developed supported membrane after pervaporation were confirmed, which indicated the promise of using this membrane in industrial processes for the dehydration of organic substances.

Conclusions
In this work, the effect of the introduction of the widely used cationic polyelectrolyte poly(allylamine hydrochloride) (PAH) into a PVA matrix for understanding the mechanism of mass transfer in pervaporation dehydration was studied. It was shown that the modification of PVA by PAH led to a significant change in the physical membrane properties; it changed the internal and surface morphology of membranes, increased the hydrophilicity of the surface, and led to a change in the structural characteristics of the membranes. These phenomena significantly affected the transport properties of the developed PVA-based membranes and caused a~2-fold increase in permeation flux while preserving the high water content in the permeate (99.9 wt %) for the dense PVA-PAH membrane compared to the pristine dense PVA membrane during the separation of an isopropanol-water (80/20 wt %) mixture by pervaporation at 20 • C.
Additionally, the influence of the nature of the used commercial porous substrates (PAN and UPM-20) for the preparation of supported PVA-based membranes with bulk (the introduction of PAH into the PVA matrix) and surface (the deposition of the PSS/PAH bilayers) modifications for pervaporation dehydration of i-PrOH (20 wt % water) was studied.
It was found that the application of PAN substrate for PVA-based membranes with or without 10 PEL bilayers led to much higher water content in the permeate and lower permeation flux (~4-fold) compared to the application of the UPM-20 substrate during pervaporation of an i-PrOH (80 wt %)/water (20 wt %) mixture at 20 • C. It should be underlined that the effect of surface modification via LbL deposition of 10 bilayers for supported PVA-PAH membranes was quite different depending on the used substrate. Specifically, the coating of 10 bilayers (PSS/PAH) on the surface of the PVA-PAH UPM membrane led to a significant decrease in water content in the permeate with a slight increase of permeation flux compared to the PVA-PAH membrane without surface modification on UPM-20 (PVA-PAH UPM ). For the PAN-supported membrane, there was another trend; both membrane transport parameters were improved after the surface modification with 10 bilayers (PSS/PAH). Thus, it was demonstrated that the transport characteristics of the supported membranes depended significantly on the choice of the commercial porous substrate (UPM-20 and PAN) because of differences in the chemical nature of the substrate material, pore size, and porosity.
Based on the obtained results, it could be concluded that a supported membrane with a thin selective layer based on a polymer blend of PVA-PAH deposited on a porous PAN substrate and modified with 10 PEL bilayers via LbL deposition (PVA-PAH/LbL-10 PAN membrane) exhibited highly selective properties with respect to water (≥99 wt % water in the permeate) at different temperatures (20,30,45, 60 • C). Moreover, this membrane also had a permeation flux~4.5-fold higher than that of the commercial analog PERVAP TM 1201 (Sulzer) with the same selectivity level for the pervaporation dehydration of i-PrOH (20 wt % water) at 60 • C. This developed membrane is promising for industrial application in the dehydration of organic solvents.