Circulating-infiltrating preparation of hydrophilic nylon 6 membrane to hydrophobic MWCNT@nylon composite membrane

The ion adsorption capacity has been added to the nylon 6 microfiltration (MF) membrane by grafting the fibers in its structure with carboxylated multi-walled carbon nanotubes (MWCNT-COOH). Through a direct three-step functionalization reaction with hexamethylenediamine (HMDA) and MWCNT-COOH, the multi-walled carbon nanotubes grafted nylon (MWCNT@nylon) fibers are made up of original nylon 6 and intermediate amino-enriched (nylon-NH2) membranes. Chemical structure analysis shows that HMDA and MWCNTs were grafted to the nylon 6 fiber side wall, without causing damage to either the nylon 6 backbone or the pore size of the original membrane. The SEM images confirm this and further confirm that the plexus of MWCNT-COOH has a pore size of approximately 50 nm, covering the nylon 6 fibers. The sidewall of nylon-NH2 and MWCNT@nylon fibers contains hydrophilic groups (amino and carboxylic), allowing the as-prepared membranes to reduce the wetting angle from about 70.3° to about 108.1°. This special structure opens new possibilities for polyamide membranes as well as mass production by the proposed simple method.


Introduction
In food technology, membrane technology keeps an important part of ingredient separation from a liquid stream of the manufacturing process [1][2][3][4][5].The commonly used membranes are non-woven porous polymer films or thin film composite (TFC) based on them [1][2][3][4][5][6][7][8] with different pore sizes.Fluid flows in production processes, which can include organic and/or inorganic solution, washing water and wastewater, are pumped through membranes with high pressure [5][6][7][8].According to wettability and absorption of polymer surface, the components pass through membrane are divide into two groups including soluble and insoluble ingredients [3,5,6].The soluble components are solvents (such as water or alcohols) and the organic or inorganic substances.The insoluble or slightly soluble ingredients, including lipids, proteins, suspensions often precipitate on the surface of the membrane or trapped by pores, leading to membrane fouling and reducing the flow rate across membranes or and increase the surface pressure.Such fouling problems can result in the loss of efficiency and lifetime of membranes.Recently, the thin-film nanocomposites (TFNs) are proposed to solve different membrane fouling causes and as well as increase the adsorb ability for ionic or organic substances in fluid flows.Some multilayered TFN membranes are often cast from nanocomposite onto a polymeric substrate, while others can be coated in a polymerization reaction.In both methods, nanosized materials can be easily dispersed in coating layers to enhance the original features or adapt new abilities for TFNs such as hydrophilicity, electrostatic interaction, adsorption capacity, specific surface area, etc [3,6,[9][10][11][12][13][14][15][16][17][18][19][20].Therefore, multi-layered TFN membranes have many applications in separation processes with longer lifetime than normal polymeric of TFC membranes.
To improve mechanical strength and separation efficiency for membranes, nanocarbon materials are considered potential sources due to their high porosity and diversified functionalities.Carbon nanotubes (CNTs), which consist of many carbon atoms arranged in hexagonal rings, are interesting as potential materials for surface and pore modification commercial membranes due to their high porosity and mechanical strength [21].CNTs have a perfect tubular structure of one or more rolled graphene sheets, sizing from a few to several tens of nanometers in diameter and up to several centimeters in length.Accordingly, the large porosity of the matrix made of hollow CNTs is suitable for membrane applications.The bonds between the carbon atoms on the surface of the CNTs can also be functionalized into organic groups [22][23][24][25], making the CNTs network more stable or having the ability to capture specific organic compounds, and ions in fluids [25,26].The vertical structure with high order and flexible length of CNTs helps to control the distance between material layers, allowing to control of permeability without sacrificing the strength of the membrane.In addition, CNTs have a high mechanical strength to help improve the durability of modified polymer membranes.
The combination of PA-TFC and some special functionalized CNTs has resulted in some membranes with outstanding properties.Chan et al [16] deposited the zwitterionic single-walled carbon nanotubes (Z-SWNTs) on PES substrate by vacuum filtration, then followed with interfacial polymerization of PA and Z-SWNTs matrix in a mixture of meta-phenylenediamine (mPD) and trimesoyl chloride (TMC).The resulting TFC with 20 wt% of Z-SWNTs can remove 98.6% NaCl for a set flux of 23.8 gfd from 1,000 ppm Na+ solution under the pressure of 530 psi, while PA thin film can only remove 97,6% for a set flux of 6.8 gfd [16].Zhao et al [27] used MWCNT-COOH to incorporate PA membranes via interfacial polymerization with mPD and TMC.The water flux of PA/MWNTs membranes was stable and higher, as well as showed better antifouling performance in both organic and inorganic foulant tests [27].Similarly, raw and oxidized MWCNTs embedded PA membranes were synthesized by Farahbakhsh et al [28].Both resulting membranes showed better saline solution flux and antifouling performance than bare PA membranes.Some other CNTs, which are functionalized by special groups such as hyperbranched poly(urea-urethane)s [29], aromatic amine [30], tannic acid-FeIII [31]K were also applied to enhance properties of PA-TFC.The common mechanism of such membranes is that the TFC was formed by condensation between carboxylic groups on the sidewall of CNTs and the amine groups of precursors or their interchanged positions [16,27,28,[32][33][34][35][36].The composite PA-based composite layer was formed during the condensation polymerization [16,27,28,[32][33][34][35].Hence, the porosity of such synthesized composites is distributed in the high tolerance of pore diameter and difficult to control by a matrix of nanocarbon materials.In contrast, the porosity of commercial PA membranes was controlled exactly in special pore size with low tolerance.Instead of developing a method to synthesize new PA-TFC, the functionalization of commercial PA membranes will result in layered TFC with uniform porosity and easy control.Nadizadeh and Madhavi [37] proposed the surface-initiated reversible addition-fragmentation chain transfer (SI-RAFT) polymerization to graft the zwitterion polymer on PA membrane to decrease the irreversible adsorption amount of protein to more than 96%.The nitrogen-doped graphene oxide quantum dots (N-GOQDs) grafted PA membrane was prepared by Yi et al [38] through a series of two reactions including hydrolysis of PA membrane and amidation with N-GOQDs subsequently.The partial hydrolysis of amide bonds on surface of polyamide fibers and then regenerating them with amino-containing organic compounds are believed to create strong chemical bonds between polyamide matrix and functionalized carbon-based nanomaterials [37][38][39][40][41]. Al-Gamal and Saleh [42] proposed that amide bonds generated by ethylenediamine crosslinked can reinforce the structure of the carbon nanotubes matrix.Instead of just having chemical bonds on the surface, they need to be formed in every angle of the porous structure of PA membrane to allow the carbon-based nanomaterials to infiltrate deeply and reinforce the entire internal structure of the membrane.Therefore, a unique circulating-infiltrating reaction (CIR) are proposed to prepare a high-performance nanocomposite membrane based on PA and MWCNT-COOH as below.
In this study, a novel nanocomposite membrane based on multi-walled carbon nanotubes (MWCNTs) and polyamide fibers is proposed aiming to enhance the permeability of finished membranes directly.In the structure of a PA-based microfiltration membrane, certain defects are created on the perfect side wall of polyamide fibers by hydrolysis of amide bonds to reveal amino and carboxylic groups.The hydrolysis reaction is controlled to occur partially on the sidewall of PA fibers, without breaking its backbone.To prevent reamidation of generated amino and carboxylic groups, diamine molecules are added in reaction media to take part in amidation with carboxylic groups.Then, diamine molecules with a free amino end become branches on the PA backbone.Such free amino ends can easily capture the carboxylic groups on the MWCNT-COOH, allowing them to graft on amino-enriched PA fibers.Such nanostructure is believed to increase the strength of the nylon 6 backbone as well as the permeability of the membrane due to the presence of retaining carboxylic groups on the sidewall of MWCNT-COOH.Such reactions are carried out in circulation media to infill the precursors deeper into the porous structure of the membrane compared to the immersion methods (figure 1).In common interface reactions, contact between MWCNT-COOH and nylon membranes occurs only on the outer surface of the membrane, resulting in only a small amount of MWCNT-COOH binding to the membrane.Despite the success, the TFN membranes made by this method have low ion adsorption capacity.The MWCNT-COOH layer formed on the outer surface of the membrane is also easily destroyed when applied under high pressure because small amount of amide bonds is formed.With the proposed CIR method, MWCNT-COOH is brought to the surface of the pores in the entire membrane, making it possible for MWCNT-COOH to completely cover the surface of those nylon fibers, thereby increasing the adsorbing capacity significantly.The effect of the CIR is also thought to remove residues absorbed to the sidewall of nylon 6 fibers, keeping the fiber sidewall in an active state and easily capturing highly active precursors.Furthermore, this method allows it to be mass production without affecting the inherent structure as well as the pore size of original polyamide membranes.

Synthesis of MWCNT-COOH
The MWCNT-COOH powder was produced by fluidized-bed chemical vapor deposition (FBCVD) system, which was manufactured by VNU-HCM Key Laboratory for Material Technologies (VNU-HCM MTLab) [43,44].The production process of MWCNT-COOH powder is described as follows: (i) The Mo-Fe/Al 2 O 3 catalyst was prepared by dispersing 3 mg of ammonium molybdate tetrahydrate, 40 mg of iron (III) nitrate nonahydrate and 30 mg of aluminium oxide in 30 ml ethanol.The mixture dispersion was carried out by sonication at room temperature for 30 min.Then, such dispersion was added into 1000 ml of toluene by sonication at room temperature for 30 min to obtain the liquid precursor for FBCVD reaction.
(ii) The liquid precursor was added to the reaction chamber of the FBCVD system by misting with argon air flow.The temperature of the reaction chamber was kept stable at 950 °C.The MWCNTs powder was obtained from the outlet after 30-40 min.
(iii) The MWCNTs powder was heated in air at 460 °C for 24 h, then treated in HNO 3 /HCl (volume ratio of 3:1 v/v) solution at 60 °C, to produce the purified MWCNT with a diameter of 20-30 nm.
(iv) The purified MWCNTs was treated by HNO 3 /H 2 SO 4 (volume ratio of 3:1 v/v) solution at 65 °C for 8 h to generate MWCNT-COOH dispersion.Then, such dispersion was carried out by centrifugation, vacuum filtration and drying to obtain dry MWCNT-COOH powder.

Structural treatment of nylon membrane
The media for hydrolysis and re-amidation (solution A1)was prepared by dissolving 50 mmol of NaOH and 10 mmol of HMDA in 50 ml of distilled water at 55 °C.The temperature is kept constant even until the solution was used for the reaction.
The dispersion B for chemical grafting of MWCNT-COOH on amino-enriched nylon membranes was prepared by sonicating a mixture of 1 mg of MWCNT-COOH powder into 100 ml of ethanol at 65 °C for 30 min.The temperature was kept constant until dispersion B was used for the reaction.
Nylon 6 membrane treatment is carried out in a continuous flow of precursor solution as shown in figure 1.The original membrane was washed with distilled water, then dried in an oven at 28 ± 2 °C for 3 h.The clean membrane was clamped between two glass funnels during 3 reaction steps described as follows (scheme 1): • Step 1. Solution A1 (55 °C) was pumped and circulated across the nylon 6 membrane for 30 min.In this step, the two reactions of hydrolysis and regeneration of amide bonds reversibly occurred on sidewall of nylon fibers.The amide bonds are thought to partially regenerate by coupling between the hydrolyzed carboxylic groups and amino groups from nylon itself and/or from HMDA molecules to form amino-enriched nylon (nylon-NH 2 ) fibers.
• Step 2. Hot distilled water (80 °C) was pumped and circulated across the nylon-NH 2 membrane for 30 min to remove unreacted residues.
• Step 3. Dispersion B (65 °C) was pumped and circulating-infiltrated across the nylon-NH 2 membrane for 30 min (nylon-NH 2 membrane was kept in the tunnel after step 2).In this step, the amide bonds are thought to generate by coupling between carboxylic groups of MWCNT-COOH and amino groups on sidewall of nylon-NH 2 fibers to produce nanocomposite fibers of MWCNT@nylon.The MWCNT@nylon membrane was removed from the glass funnels and cleaned by sonication in distilled water for 15 min.The retained MWCNT-COOH without amide bonding to the nylon-NH 2 surface, which were trapped in the pore of the membrane, could be removed in the last sonication step.
To evaluate the role of hydrolysis and regeneration of amide bonds in step 1, two other experiments were carried out with the replacement of reaction media in step 1.Such media were solution A2 (10 mmol of HMDA in 50 ml of distilled water) and A3 (50 ml of distilled water).The labels of as-prepared membranes were denoted as shown in table 1.

Characterizations
The chemical structures of nylon-NH 2 and MWCNT@nylon membranes were characterized using Fourier transform infrared (FTIR) and Raman spectroscopies using Spectrum Two FT-IR Spectrometer (PerkinElmer) and XploRA ONE (Horiba Scientific), respectively.Surface morphologies of membranes were analysed by field emission scanning electron microscopic (FESEM) using the JSM-7401F (JEOL).The contact angle of membranes was evaluated using Phoenix 300 (Surface & Electro Optics).

Results and discussions
The surface appearance of the original nylon 6, nylon-NH 2 , and MWCNT@nylon membranes are significantly different (figure 2).Compared with the original membrane (figure 2(a)), the appearance of nylon-NH 2 membranes (figures 2(b), (c)) was unchanged.In contrast, the MWCNT@nylon membrane (figures 2(f), (g)) was Scheme 1. Treatment of nylon 6 into nylon-NH 2 and MWCNTs@nylon via CIR.almost uniform black during sonication or after drying.It shows the good bonding between MWCNT-COOH and nylon-NH 2 alk membrane.The nylon and nylon-NH 2 w membranes could not capture MWCNT-COOH due to the absence of the −NH 2 groups formed in step 1 when pristine nylon membranes were treated with neutral solutions A3 and A2.It shows that the alkaline environment of the A1 solution creates favorable conditions for hydrolysis and regeneration of amide bonds on the surface of nylon fibers.Therefore, MWCNT-COOH graft on nylon-NH 2 alk surface through formation of amide bonds with amino groups on both HMDA and hydrolyzed points of nylon 6 fibers.
In figure 3(a), the spectrum of MWCNT-COOH clearly shows the presence of carboxylic groups through the appearance of wide bands of C=O (at 1705 cm −1 ), -COO (at around 1500 cm −1 ) and C-O-C (at 1100 cm −1 ) bonds [45,46].Spectra of original nylon 6, nylon-NH 2 alk , and MWCNT@nylon alk membranes (figures 3(b), (c), (d)) have similar backgrounds and show characteristic peaks for the chemical structure of nylon 6.In the window of 2800 to 3500 cm −1 , the characteristic bands, which attributes to stretching vibration of the N-H bond (at around 3300 cm −1 ), methylene stretching (at around 2860 and 2930 cm −1 ), and C-N bond (at around 1270 cm −1 ) have constant positions [47][48][49][50][51][52].The amide bond is determined by pair of bands that characterize the vibration of C=O (at 1632 cm −1 ) and N-H (at 1537 cm −1 ) groups [47][48][49][50][51][52].It shows that the hydrolysis reaction in the first step does not completely degrade the structure of the original nylon 6 fibers.In particular, the characteristic vibration band of C=O (1734 cm −1 ) in the carboxylic group appears clearly on the spectrum of MWCNT@nylon alk as well as a characteristic band of N-H bond has a slight shift from 683 cm −1 in spectra of nylon 6 and nylon-NH 2 to 669 cm −1 in spectrum of MWCNT@nylon alk .It shows that MWCNT-COOH has successfully grafted on nylon-NH 2 alk fibers and an amount of residual carboxylic groups are still present on the side wall of MWCNTs.Such carboxylic groups can increase the hydrophilicity of nanocomposite MWCNT@nylon alk fibers.The coverage of MWCNT on amino groups, which occurred in the hydrolysis of nylon 66, has created an endocytic bond that leads to the shift of the N-H band at 683 cm −1 .
The spectrum of MWCNT-COOH (figure 4(a)) has clear appearance of characteristic D-, G-, and 2D-band, centered at 1341, 1576, and 2685 cm −1 , respectively.The 2D peak with low intensity is believed to be attribute to the graphene layers, which were stripped from the side wall of MWCNTs during acidic treatment to generate −COOH groups.The appearance of D-, G-, and 2D-band in the spectrum of nylon-NH 2 (figure 4(c)) indicates the presence of MWCNT in the structure of MWCNT@nylon alk fibers (figure 4(d)).The spectrum of the nylon-NH 2 membrane retains the characteristics compared to that of the nylon 66 membrane while the spectrum of the MWCNT@nylon alk membrane has the characteristics of MWCNT, showing that a large amount of MWCNTs was grafted on nylon-NH 2 fiber during the last step of the reaction.In addition, the ratio of D-and G-band intensities in the MWCNT@nylon alk spectrum is lower than that of MWCNT-COOH, which also shows that MWCNTs with high defect density, corresponding to high -COOH group density, were preferentially grafted on nylon-NH 2 alk fiber by amidation reaction.The Raman results are consistent with FTIR results, helping to confirm that the amidation occurs selectively in the continuous flow of precursor solutions.Residual substances including MWCNT-COOH with low -COOH group density were washed out of the membrane by the flow, ensuring that they did not foul the pores of the membrane during the reaction.
The SEM image (figure 5) shows different surface morphology of as-prepared membranes.Compared with the uniformity of the original nylon (figures 5(a), (b)), the nylon-NH 2 fibers (figures 5(c), (d)) had more surface defects, indicating that hydrolysis has removed part of nylon 6 surface.However, this reaction was carried out for a short time, resulting in the undamaged backbone of fibers, keeping them together.Compared with the original membrane, the nylon-NH 2 alk membrane has the same pore size.Figures 5(e), (f) show the structure of MWCNT@nylon fibers.The MWCNTs create a protective layer for nylon 66 fibers, helping them avoid direct contact with foreign molecules.The plexus MWCNT coating has a pore size of about 50 nm, allowing the MWCNT@nylon alk fibers to adsorb only small molecules or ions while ignoring the large molecules.In addition, the morphologies of the Ny-MWCNT membrane show the pores between the fibers are similar in size and distribution to that of the nylon-NH 2 alk membrane.This indicates that the residues are removed from the porous structure of the MWCNT@nylon alk membrane by the CIR. Figure 6 shows the wetting angles of the membranes.After the functionalization steps, the wetting angle decreases significantly from 70.3°of the original nylon 6 membrane increase to 77.7°and 108.1°of nylon-NH 2 and MWCNT@nylon alk membranes, respectively.After the first step, the nylon-NH 2 alk fiber has a rougher surface than nylon 6 fiber, due to the hydrolysis reaction and the addition of -(CH 2 ) 6 -NH 2 branches.The sharp increase in surface roughness should have caused the hydrophobicity to increase significantly.However, the sidewall of the nylon-NH 2 is enriched with hydrophilic amino groups, which reduces the hydrophobicity, causing the wetting angle of the film to increase only a small value of about 7.4°.When MWCNT-COOH was grafted to nylon-NH 2 alk fibers, the carboxylic groups replaced the amino groups to encounter water, resulting almost constant wetting angle of MWCNT@nylon alk .The sharp increase in the wetting angle of MWCNT@nylon alk membrane can be explained that the carboxylic groups of MWCNT-COOH only preferentially bound to the amino end of HMDA or nylon 6, in this case, should be that of HMDA, and covered the remaining amino end of nylon 6.As a result, decrease in the amount of hydrophilic carboxylic on the side   wall of MWCNT@nylon alk fibers.The SEM and wetting angle results strengthen the arguments from FTIR spectra.

Conclusion
Summarizing, the chemical structural analysis and morphologies confirm the novel nanostructured membranes of MWCNT@nylon can be easily fabricated by the functionalization of nylon 66 membranes via continuous fluidic reactions.The involvement of diamines and MWCNT-COOH in the nanocomposite MWCNT@nylon fibers has significantly increased the hydrophilicity of the finished membrane.The wetting angle of such membrane is increased to 108.1°.The nanostructure of MWCNT@nylon fibers completely explains the property change of the membrane.Such special structures are produced via the simple three-step method, without changing the porosity of the original membrane.The porosity of the membrane is preserved because of the continuous flow that has swept the residues out of the membrane during the reaction.Importantly, the MWCNT@nylon fibers and the CIR method show potential applications in mass production for new nanocomposite membranes.

Table 1 .
Recipe of amino enriching treatment process in step 1 and label of as-prepared membranes.