Interfacially polymerized polyamide thin film composite membranes: Preparation, characterization and performance evaluation

Interfacially polymerized polyamide thin film composite membranes: Preparation, characterization and performance evaluation

Desalination 287 (2012) 310–316 Contents lists available at ScienceDirect Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m ...

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Desalination 287 (2012) 310–316

Contents lists available at ScienceDirect

Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l

Interfacially polymerized polyamide thin film composite membranes: Preparation, characterization and performance evaluation Adel Soroush a, Jalal Barzin a,⁎, Mahdi Barikani a, Mahdi Fathizadeh b a b

Iran Polymer and Petrochemical Institute, P.O. Box 14965/115, Tehran, Iran Department of Chemical Engineering, Amirkabir University of Technology (Tehran Polytechnic), Tehran, Iran

a r t i c l e

i n f o

Article history: Received 25 May 2011 Received in revised form 5 July 2011 Accepted 20 July 2011 Available online 17 August 2011 Keywords: Thin film composite membrane Interfacial polymerization Membrane characterization Performance evaluation

a b s t r a c t The formation of polyamide thin film composite (TFC) membranes via interfacial polymerization (IP) of mphenylenediamine (MPD) in water with trimesoyl chloride (TMC) in hexane was studied. Parametric studies were conducted by varying reaction time, curing time and curing temperature. It is evident from the results that increasing the polymerization time results in decreasing the membrane surface roughness and increasing solid–liquid interfacial energy. Also with increasing the polymerization time, surface morphology changes from “nodular” and “leaf like” morphology to “hill and valley”. The other involving parameters were the thin film thickness, which the results indicated that PA thin film thickness increased with polymerization time and moreover the acidic feature of PA film varied during polymerization process. Increasing polymerization time led to decreasing membrane water flux and increasing salt rejection since the PA layer became thicker and the extent of cross-linking increased. Also, it was shown that the curing conditions affect on membrane performance and with increasing curing time and temperature, salt rejection was increased and flux was decreased. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Several methods can be carried out in desalination and industrial water treatment. Reverse osmosis (RO) is one of these methods which are simple to operate in comparison with other separation processes are widely used in chemical, environmental and food industries [1–6]. Most of the commercially successful RO and nanofiltration (NF) membranes are thin film composite (TFC) membranes which consist of a barrier active dense layer and a porous sublayer. This active layer is usually formed via interfacial polymerization (IP) technique which can provide good selectivity and high water permeation rate. One of the successful TFC membranes is polyamide (PA) composite membranes. PA active layer polymerized interfacially on top of polysulfone (PS) and polyethersulfone (PES) porous support membrane [7–11]. Between a wide range of reactive monomers used in IP process to produce PA layer, m-phenylene diamine (MPD) and trimesoyl chloride (TMC) have special importance since they provide aromatic cross-linked PA layer and hence provide high water flux. In this IP process, the immiscible aqueous and organic phases brought together and a thin film immediately formed between two phases at the organic

⁎ Corresponding author at: Biomaterials Department, P.O. Box 14965/115, Iran Polymer and Petrochemical Institute, Tehran, Iran. Tel.: +98 21 44580050; fax: + 98 21 44580161. E-mail address: [email protected] (J. Barzin). 0011-9164/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2011.07.048

side of the interface due to low solubility of acid chloride in water and fairly good solubility of amines in organic solvents [12,13]. The interfacial polymerization time is very fast and many factors such as reaction time, reaction temperature, curing conditions and monomer concentration can affect PA structure. Also, the produced film is extremely thin so its characterization by common methods used in the bulk polymer characterization is not possible and therefore use of surface analysis methods is necessary. TFC membranes preparation has been studied by different groups of researchers who focused on several fields such as effect of PA molecular structure on membrane properties and performance [14], mechanical properties of thin film layer [15], surface feature [16], transport modeling and mathematical modeling of interfacial polymerization [17,18] and use of nanozeolite particle in thin film nanocomposites [19]. In this study, PA TFC membranes were prepared by interfacial polymerization of MPD and TMC as two important and widely used monomers in IP process on top of the PES porous support membrane which prepared by non-solvent induced phase separation. PES has been studied thoroughly and is well known for its proper properties such as favorable mechanical strength, thermal and chemical stability and excellent biocompatibility [20–24]. Several TFC were prepared under different polymerization conditions such as polymerization time, curing time and curing temperature. The effect of polymerization time as one of the important kinetic parameters on active layer thickness, morphology, structure, hydrophilicity, roughness and thermal properties were investigated by scanning electron microscopy (SEM), attenuated total

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reflectance infrared spectroscopy (ATR-IR), contact angle measurement (CA), atomic force microscopy (AFM) and differential scanning calorimetry (DSC). Finally, the performance of the TFC membranes was characterized.

311

θ and γL are the average contact angle and the liquid surface tension (for pure water at 25 °C is 72.8 mJ/m 2), respectively [10]. DSC measurement of dried samples was carried out under nitrogen atmosphere using NETZCH, 200 F3. The flow rate of nitrogen gas was 20 ml/min and the heating rate was 10 °C/min from 25 °C to 300 °C.

2. Experimental 2.3. Membrane performance evaluation 2.1. Preparation of TFC membranes Polymeric solution was prepared by dissolving 15 wt.% PES (Ultrason E6020P, BASF) and 2.5 wt.% polyvinylpyrrolidone (PVP, Mw = 40,000 g/mol, Aldrich) as a pore former polymeric additive in N, N-Dimethylacetamide (DMAc, Merck) as a solvent at 25 °C with continuous stirring. The well mixed bubble free solution was cast on a glass plate followed by immersion and coagulation in distilled water as non-solvent bath. After coagulation, membranes were moved to the second water bath for 24 h until morphology became set and most of solvent and water-soluble polymer were removed. Microporous PES support membranes was taped onto an Al plate and immersed in aqueous solution containing 2 wt.% MPD (Aldrich) and soaked for 2 min. Then the membrane surface was rolled out with a soft rubber roller and followed by hanging the soaked membrane to eliminate remaining bubble and excess solution. Next, the membrane was dipped into organic solution containing 0.1 wt.% TMC (Aldrich) in n-hexane (Merck) for 15, 45, 90 and 180 s. The resulting TFC membranes were dried in a controlled temperature during specific period of time and temperature. The TFC membrane preparation conditions are presented in Table 1. 2.2. Characterization of TFC membranes The chemical characterization and thickness measurement of PA layer of TFC membranes were accomplished by ATR-IR spectroscopy instrument (EQUINOX55, LSI) using multiple internal reflectance apparatus and ZnSe (n = 2.43) prism as an internal reflection element was fixed at a 45° angle of incidence [25]. Top surface morphology of TFC membranes was evaluated by SEM (Philips XL 30). For SEM evaluations, the membranes were coated by a thin layer of gold under vacuum condition. Surface roughness (RMS) and relative surface area (Δ) were characterized for a 10 × 10 μm quadrangle sample by AFM DualScope C-21(DME, Denmark) in contact mode in the air. The water contact angle (CA) of the TFC membranes surface was measured by sessile drop method (OCA-20, DataPhysics Instruments GmbH, Germany) at room temperature. De-ionized water was used as a probing liquid for several different points of membrane surface and the average of measured value was reported as a CA of membrane surface. Also, the solid–liquid interfacial free energy which is the better determination of surface hydrophilicity than CA [10,26], was evaluated using Eq. (1): −ΔGSL = γL ½1 + cosθ = Δ

The performance test of TFC NF membranes were conducted with a cross-flow cell (Fig. 1) using solution of pure sodium chloride with 1000 ppm concentration in distilled water under constant pressure (6 bar) and temperature (25 °C). Quadrangular membrane samples (4× 5 cm2) were placed in cell with the active layer facing the incoming feed. The flux and the solute rejection were determined by measuring the volumetric permeate flow directly (L/m2h) and ion conductivity (Hanna) of permeation by using conductivity–concentration curve, respectively. Solute rejection calculated by following equation:   Rejectionð% Þ=100× Cf –Cp Cf

Where Cf and Cp are the ion-conductivity of feed solution and permeate, respectively. 3. Results and discussion 3.1. Surface properties Effect of polymerization time as one of the important kinetic parameters on surface feature of TFC membrane which play critical role in membrane performance, was studied by using CA, SEM and AFM analysis. Water contact angle of membrane surface can evaluate surface hydrophilicity and free energy. Results showed that adding PA layer on top of the PES microporous support layer CA decreased from 69° for PES support membrane due to the presence of amide, carboxylic and hydroxyl functional groups which increased surface hydrophilicity and surface free energy (Table 2). But polymerization time didn't influence much on water contact angle of TFC membranes because TFC membranes have rough surface and this roughness can produce some errors on CA measurements whereas surface free energy provide better understanding about surface hydrophilicity. TFC membrane surface CA changed between 57° and 68° and surface free energy increased with increase of polymerization time. Top surface morphology of TFC membranes was investigated by SEM method. As shown in Fig. 2, surface morphology is a function of polymerization time. During the first stage of polymerization process (15 s), “nodular” and “leaf like” morphology were observed. However, as polymerization time increased, surface morphology changed to “hill

ð1Þ

Table 1 TFC membrane preparation conditions. Sample number Reaction time (s) Curing time (min) Curing temperature (°C) 1 2 3 4 5 6 7 8 9

15 45 90 180 45 45 45 45 45

7 7 7 7 7 7 3 15 30

55 55 55 55 35 85 55 55 55

ð2Þ

Fig. 1. The cross flow system for water treatment process.

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Table 2 TFC membranes properties prepared in different polymerization time. Sample

Contact angles (°)

δfilm (μm)

Tg (°C)

Td (°C)

1 2 3 4

57 57 63 68

0.14 0.19 0.23 0.28

150 153 157 165

218 222 230 232

and valley” and at the final stage of polymerization, surface became more coarse but more homogenous. Then the AFM method was used to obtain quantitative investigation of surface roughness changes. 3-D topographic AFM images of TFC membranes top surface are shown in Fig. 3 and surface area and its roughness presented in Table 2. As it is obvious from the results, forming PA layer by IP process increased surface roughness of PES support layer from 10.41 nm to 97.8 nm. During the first stage of polymerization, surface was too rough and increasing polymerization time led to decreasing surface roughness.

Fig. 2. Micrographs of the surfaces of PES sublayer (a) and polyamide TFC membranes prepared at (b):15 s, (c):45 s, (d):90 s and (e):180 s.

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Fig. 3. 3D AFM images of PES sublayer (a) and polyamide TFC membranes prepared at (b):15 s, (c):45 s, (d):90 s and (e):180 s.

During the first stage of polymerization process, reaction took place so fast and the interface between the two immiscible phases is diffusional. As the polymerization time increased, PA layer formed and became denser and thicker so MPD diffusion toward TMC became slower and harder. This phenomenon is called “self-limiting” and mentioned by other authors [27]. It can change chemical structure and surface free energy and as a result, the surface roughness and morphology can change too. To measure the PA layer thickness and its chemical structure, ATR-IR method was used. ATR-IR spectroscopy provides easy and convenient method to grasp the chemical structure and thickness of the skin layer of the TFC membranes [27–29]. The ATR-IR spectra of TFC membranes

prepared at various reaction times are presented in Fig. 4. In the spectra, the Amide bands of the active layer were appeared at 1660, 1609, 1547 and 786 cm- 1 for C=O stretch, C-N stretch, polyamide aromatic ring breathing and amide, respectively. The 1488 cm- 1 is a characteristic of CH3-C-CH3 stretching and 1241 cm- 1 peak is due to C-O-C stretching of sublayer PES. Also, the peaks in 900–1200 cm- 1 can be attributed to the skeletal aliphatic C-C aromatic hydrogen bending and aliphatic C-H rocking of PA. The PA thin film thickness can accurately be estimated by using ATRFTIR spectroscopy from the characteristic IR bands of PA group's intensities. The depth penetration of the reflected IR beam in this method is typically below 1 μm and the thickness of PA layer is far below

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Symmetric stretching of O=S=O C-O stretching Amide (polyamide)

Aromatic ether

C=C aromatic ring stretching CH3-C-CH3 stretching

Amide I (C=O)

Pendant carboxylic group (COOH)

Fig. 5. Acid content in the polyamide film as a function of the polymerization time (prepared at 25 °C). Fig. 4. Infrared spectra of the polyamide films prepared at different polymerization times, 15, 45, 90 and 180 s.

1 μm. Therefore, this technique can be considered as a suitable method for analyzing the active layer of TFC membranes. The thickness of PA layers of membranes at different polymerization time was shown in Table 2. It is evident from the results that PA film thickness increased as polymerization time increased. However, this film thickness growth was not linear due to the “self-limiting” phenomenon. Other researchers also mentioned that the thickness of PA film prepared by interfacial polymerization is nearly proportional to root square of polymerization time in short polymerization process [30].

more degree of crosslinking, the higher degradation temperature can be achieved. To obtain the relation between degree of crosslinking and polymerization time, PES support layer was completely solved in dichloromethane solution and the un-dissolved polyamide layer was extracted from the solution via filtration. DSC thermo-grams of PA layer of TFC membranes and their glass transition and degradation temperature is reported in Fig. 6 and Table 2, respectively. Increasing the polymerization time resulted in developing the crosslinking and therefore, the degradation temperature increased. Since the interfacial polymerization on TFC membrane was taken place very fast and formed very thin film, thermal analysis is one of the best and new methods for studying the degree of crosslinking in this process.

3.2. Thin film layer structure 3.3. Performance of membranes IR analysis is based on the fact that each chemical group in a sample absorbs IR radiation at some characteristic frequencies. The use of the Beer–Lambert law allows the concentration of an absorbing species in the reaction medium to be determined. Since TMC is multifunctional, the reaction between TMC and MPD can lead to two types of PA structure in the same time. One of them is totally cross-linked and another is linear structure with pendant carboxylic acid from the hydrolysis of remain acyl chloride groups that give PA film acidic character. To evaluate PA structures, the 1730 cm- 1 band characteristic of carboxylic acid was employed as a quantitative indication of acid content and 1660 cm- 1 as amide groups. The ratio of acid content in the film by the 1730 cm- 1 band to the amide band 1660 cm- 1 as a function of polymerization time was shown in Fig. 5. Results showed that at the first stage of reaction between MPD and TMC, MPD molecules diffused to organic phase and could react easily with TMC, So, the ratio of 1730/1660 decreased because there was enough amount of MPD to react with TMC and produced crosslinked PA. With increasing time, as was mentioned before, diffusion of MPD became slower and fewer, so excess amounts of TMC remained in reaction medium which brought more linear amide unit with acid pendant groups and therefore, the ratio of 1730/1660 increased. Jin et.al reported the same results in long polymerization time period [27]. In this study an earlier stages of IP reaction between MPD and TMC were covered, but the results had good adaption with Jin et.al's work. Differential scanning calorimetry (DSC) was used to estimate the effect of polymerization time on the degree of polyamide crosslinking, indirectly. In this technique, the amount of necessary energy for chemical and physical transition in polymers such as crystallization, glass transition and degradation process was calculated. Degradation process depends on the factors related to physical state of polymers. One of these factors which plays an important role in degradation process in a crosslinked polymer is the degree of crosslinking. The

To find a rational relation between pervious results and TFC membrane performance, the performance characteristics of prepared TFC membranes were investigated using cross flow membrane test cell and results are depicted in Fig. 7. It should be mentioned that the polyester non-woven fabric support was not used in the study and therefore the operating pressure was low (6 bar) and the results were lower than commercially RO processes which work on high operating pressure and were same the NF process. It can be seen that with increasing polymerization time, salt rejection was increased from approximately 60% to 75%, while flux showed a drastic decrease to the half value of the first point. These results were predictable due to the increment of the reaction time, thickness of the active layer and the

180 sec

90 sec

45 sec

15 sec

Fig. 6. DSC thermo-grams of PA layers of TFC membranes prepared in 15, 45, 90 and 180 s.

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315

Fig. 9. Effect of curing temperature on membrane performance for membranes prepared at 45 s and 7 min of reaction time and curing time, respectively. Fig. 7. Effect of polymerization time on performance, surface roughness and surface free energy of TFC membranes.

degree of crosslinking increased. On the other hand, surface roughness and surface area were decreased. Heat curing is often used to remove the residual organic solvent from polyamide thin film and as a result promote crosslinking by additional reaction of unreacted amines and carboxyl groups. Also, at high temperature there is a chance of pore shrinkage in the support membrane so it can cause an increase in salt rejection [10]. In addition, as curing time increased, the membrane subjected to temperature prolonged time which results in increased densification similar as increasing curing temperature. The effect of curing condition of PA active layer on TFC membrane performance was shown in Figs. 8 and 9. With increasing the curing temperature from 35 to 85 °C, flux decreased and salt rejection increased. At higher curing temperatures, for example above the 80 °C (higher than n-hexane boiling point (69 °C)), removal of organic solvent and consequently crosslinking increased and PES support membrane shrinked. So, water flux decreased and salt rejection increased cooperatively. This pattern was observed by Hoek et al [10] and Meihong et al. in preparation of TFC membrane from piperazine (PIP) and TMC [11]. 4. Conclusion The relationship between the kinetic and process factors which affect interfacial polymerization and PA features was investigated. The results confirmed self-limiting phenomenon in interfacial polymerization of TMC and MPD which was characterized by several methods such as ATR-IR and salt rejection test. Reaction time had great effect on

Fig. 8. Effect of curing time on membrane performance for membranes prepared at 45 s and 55 °C of reaction time and curing temperature, respectively.

properties and structure of active layer, which with increasing reaction time, film became denser and more crosslinked, film thickness increased and surface morphology changed from “nodular” and “leaf like” to “hill and valley”. Surface roughness decreased when reaction time increased. Furthermore, process conditions such as curing temperature and curing time had some effects on TFC membrane structure and properties. When curing temperature increased, water flux decreased and salt rejection increased. Curing time like curing temperature had similar effects on membrane properties and performance. Finally, with comparison with our previous work, cross-linked polyamide which was prepared by IP reaction of MPD and TMC, had great improvement than non cross-linked polyamide even at low pressures.

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