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Modification of cellulose acetate membrane using monosodium glutamate additives prepared by microwave heating
Journal of Industrial and Engineering Chemistry 18 (2012) 2115–2123
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Modification of cellulose acetate membrane using monosodium glutamate additives prepared by microwave heating Chan Mieow Kee, Ani Idris * Department of Bioprocess Engineering, Faculty of Chemical Engineering, Universiti Teknologi Malaysia, 81300 UTM, Skudai, Johor, Malaysia
A R T I C L E I N F O
Article history: Received 13 March 2012 Accepted 13 June 2012 Available online 19 June 2012 Keywords: Cellulose acetate Formic acid Monosodium glutamate Microwave Membrane
A B S T R A C T
Cellulose acetate polymeric solutions with different concentrations of monosodium glutamate in formic acid were prepared using both traditional and microwave heating methods. The membrane separation performance and its physical properties were evaluated. Results indicated that the membrane consisting of 6 wt% additive prepared using microwave heating exhibited better solute permeability. Microwave heating was also found to enhance the thermal stability and the surface roughness of the membrane. The use of microwave for membrane fabrication not only saves time and cost, but also produces membranes with good properties. This paper therefore discusses the plausibility of manufacturing membranes using microwave heating techniques. ß 2012 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction Low energy consumption of using membrane technology in a broad range of application triggers the membranologists to fabricate low cost membranes with high separation efficiency. Jang and Lee [1] improved the oxygen barrier property of biaxially orientad polypropylene film by introducing polyvinyl alcohol into the casting solution. Atmospheric pressure plasma treatment was adopted by Park et al. [2] to determine the ideal treatment condition to produce low cost electron-spun polyamideimide film. Another approach was attempted by Song et al. [3], where polyrhodanine was immobilized onto the anodic aluminium oxide membrane. The resultant membrane exhibited excellent separation performance in remove ion Hg (II). In the past few years, polyethersulfone ultrafiltration membranes with high rejection rates and good fluxes were successfully fabricated by Idris et al. [4] using microwave (MW) heating. The merit of using this irradiation heating in processing materials is not only limited to cost and time savings [5] but also contributes to uniform heating, initiates some desired chemical reactions and accelerates the reaction rate. All the mentioned benefits are closely related to how the energy is being transferred to the materials. In conventional heating, heat is transferred through convection or conduction from the surface of the materials inwards, which is due to the thermal gradients within the materials [6]. A different
* Corresponding author. Tel.: +60 7 5535603; fax: +60 7 5581463. E-mail addresses: [email protected], [email protected] (A. Idris).
phenomenon exists in MW heating; where the electromagnetic energy is converted into thermal energy by interacting with the molecules of the materials. As microwaves can penetrate into the materials, heat is generated volumetrically within the material rather than from the external source, unlike the traditional heating process [7]. In MW heating, the dielectric property is a very important factor as it determines which materials in the mixture couple well with the electromagnetic field and absorb the MW energy. Material with too high dielectric loss factor such as steel causes reflection while thermoplastic, which has low dielectric loss factor is transparent to MW effect. Dielectric loss factor of water, for instance, falls between this range, and hence makes it a MW receptive material [6]. Cellulose acetate (CA) is a popular polymer that is suitable for microfiltration, ultrafiltration and reverse osmosis. Its excellent properties such as good flux and high salt rejection, cheap, biodegradable, hydrophilic and non-toxic make it an attractive membrane forming material [8]. The use of MW in processing cellulose material was reported by many researchers recently [9,10]. Dogan and Hilmioglu [9] investigated the effect of MW heating on cellulose dissolution. The resultant membrane fabricated from this cellulose solution showed no differences in the crystallinity as well as the peak in FTIR spectra. However, no permeability experiments were performed in the mentioned studies. Thus, the effect of MW heating on the separation performance of the resultant membrane remained unknown. Several researchers [10] claimed that the time for cellulose esterification which normally took 2 days were reduced to several minutes and a higher degree of esterification occurred within 10 min.
1226-086X/$ – see front matter ß 2012 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jiec.2012.06.005
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Based on literature reviews, PVP and PEG are the most favorable additives in membrane fabrication due to the hydrophilic characteristic [11–13]. Thus, there is need to explore on the possibilities of other types of additives which are more environmental friendly. Monosodium glutamate (MSG), a well known food additive which is highly hydrophilic due to the hydroxyl group in the structure is employed as an additive in this study. It is easily available and cheap. It is also a MW active additive [14], where 5 wt% solution of MSG Exhibits 0.311 dielectric loss tangent (2450 MHz) [15]. Besides the very recent publication [16] on the use of MSG for hemodialysis membrane, no other similar work has been published. Additionally, the influence of MW on the CA–MSG membrane has not been explored and therefore is the subject of this investigation.
temperature profile of the solution during heating process. Water was used as the testing solution so as to obtain an approximate relation between heating mode and heating period. After determining the suitable combination, all the CA solutions were prepared under low heating mode for 8 min.
2. Experimental
The dialysis testing system used in this work was similar to that described by Idris and Lee [11]. Dialysis cell with 30 cm2 effective membrane area was used in this study. Solute reservoir was filled with a mixture of 1 mg/ml of urea, 0.1 mg/ml of creatinine and 1 mg/ml of bovine serum albumin. The flow rate for the solute and dialysate reservoirs were set at 50 ml/min and 100 ml/min respectively and samples were collected at both reservoirs for every 0.5 h. This experiment was carried out at 37 8C for 3.5 h. Urea Nitrogen (Diacetyl) Reagent [18], Creatinine (Direct) Reagent [19] and Biuret Reagent [20] were used to measure the urea, creatinine and bovine serum albumin concentrations respectively of the collected samples. The permeability experiment was repeated at least three times for each membrane formulation so as to ensure the reproducibility of data points. The diffusive permeability, clearance and overall mass transfer coefficient were calculated using Eqs. (1)–(3) respectively. Diffusive permeability, Pm is given by the following equation [21]:
2.1. Materials Food grade formic acid with 98% purity supplied by AppliChem was used as the solvent for both CA and MSG. CA (acetylation degree: 39.8%) with molecular weight of 100 kDa was purchased from Acros while MSG was obtained from commercial sources (Ajinomoto). Probe solute, such as bovine serum albumin (Sigma), creatinine (Kanto) and urea (Quantum) were used in the dialysis experiment. The concentrations of urea and creatinine in the samples were determined using Urea Nitrogen (Diacetyl) Reagent and Creatinine (Direct) Reagent, respectively. Both reagents were purchased from Eagle Diagnostics. Bovine serum albumin concentration was measured using self prepared Biuret reagent. 2.2. Membrane preparation In the traditional method (TM), polymeric dope solutions with 20 wt% CA and different concentrations of MSG in formic acid were prepared at 70 8C with continuous agitation for 4 h. In the MW method, the same dope solution formulation was prepared and heated inside a MW (Panasonic) for 8 min at low mode [17]. Table 1 shows the composition of the various dope solutions prepared. The mode and heating time were chosen based on the temperature calibration and would be explained in the following Section 2.2.1. The abbreviation TM or MW in front of CA indicated the type of heating process used for dope preparation while the number behind CA indicated the weight percent of MSG used. For example, MWCA2 depicted that the CA membrane was produced with 2 wt% of MSG using MW heating. The bottled CA solution was placed in an ultrasonic bath to remove the bubbles. Sufficient amount of the CA solution was spread over a dust-free glass plate using a casting knife with 200 mm gap and immediately immersed into distilled water coagulation bath at room temperature. In order to remove the residual solvent, the nascent membrane was post treated in hot water (90 8C) and methanol. 2.2.1. Determination of MW heating mode and mixing time In order to determine the level of heating mode and desired duration for dissolving the CA in formic acid, thermocouple data logger (Pico Technology Limited) was used to record the
2.3. Membrane thickness After the phase inversion process, the thickness of the membrane was measured using digital caliper with accuracy 0.01 mm (Mitutoyo) at 5 different positions on the membrane. 2.4. Permeability study
Pm ¼
ln½DCðt 1 Þ=DCðt 2 Þ s½ð1=V a Þ þ ð1=V b Þ½t 2 t 1
Clearance, K is described accordingly as follows [22]: K¼
aV Q B ½1 expðaDt Þ expðaDt Þ
Abb.
CA (wt%)
Formic acid (wt%)
MSG (wt%)
CA0 CA2 CA4 CA6 CA8
20 20 20 20 20
80 78 76 74 72
0 2 4 6 8
(2)
Overall resistance to solute transport, 1/Ko is described as [23] QB 1 ðK=Q D Þ ln Ko ¼ (3) 1 ðK=Q B Þ Sð1 ðQ B =Q D ÞÞ where Co and Ct are the initial and periodic concentrations, respectively; DC(t) is the solute concentration difference between the solute and dialysate reservoirs at the sampling time, t1 and t2; V is the reservoir volume; S is the surface area of the membrane; t is the time; a is the slope of the plot of ln(Ct/Co) versus time; Dt* is the ratio of the total circuit volume to the solute flow rate, and QB and QD are the flow rates of the solute and dialysate reservoirs, respectively. 2.5. Ultrafiltration rate (UFR) The UFR was obtained from Eq. (4): UFR ¼
Table 1 Membrane formulations.
(1)
V SP
(4)
where S is the effective membrane surface area 30 cm2, V is the water flux at 37 8C and P represents the operating pressure, which is 250 mmHg. 2.6. Porosity measurement Gravimetric method was adopted to evaluate membrane porosity. Membrane was cut to the size of 1 cm by 2 cm. The
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dry (wb) and wet (wa) membranes were weighed. The wet membrane was prepared by dipping the dried membrane into distilled water for 24 h. Membrane porosity (e) was determined according to the following equation [24]
e¼
ðwa wb Þ=rH2 O ½ðwa wb Þ=rH2 O þ ½wb =rmembrane
(5)
In this case, rH2 O is the density of water (0.998 g/cm3) and rmembrane is the density of membrane. In order to determine the membrane density, the empty pycnometer (wo) was first weighed. One third of the pycnometer was filled with sample and weighed (w1), then filled with distilled water and weighed (w2) again. Finally, the pycnometer was emptied, filled with distilled water only and weighed (w3). Density of membrane was calculated according to Eq. (6).
rmembrane ¼
wp Vs
(6)
where wp refers to the weight of sample (w1 wo) while Vs = volume of the sample (V V 0H2 O ); V 0H2 O ¼ ðw2 w1 Þ=rH2 O . V represents volume of the distilled water that fills the empty pycnometer. 2.7. FTIR study
2.11. Atomic force microscope (AFM) The surface morphology and roughness of the membranes was analyzed using atomic force microscope (SII Nanotechnology SPMSPI3800N). The scanning area of the membrane was approximately 5 mm 5 mm. Roughness parameters, mean surface roughness (Ra), root mean square roughness (RMS) and peak to valley (P–V) were calculated by the software. 2.12. Scanning electron microscope (SEM) The microstructure of the membranes was observed using scanning electron microscope (Model SUPRA 35VP). Membrane was snapped in liquid nitrogen to obtain a clean cut. The sample was then mounted onto brass plates using double-sided cellophane tapes in a lateral position and sputter-coated with platinum. The sample was then ready for imaging. 2.13. Viscosity measurement The viscosity of the CA solution was measured using Plate and Cone Viscometer (Brookfield DVII) at room temperature. At least three measurements were obtained for each solution. 2.14. Pore size determination
FTIR spectrum of the sufficiently thin film membrane at room temperature was recorded by Perkin Elmer Spectrum One FTIR Spectrometer. Each spectrum was recorded in the mid IR region between 370 cm1 and 4000 cm1. 2.8. Contact angle measurement (CAM) Contact angle measurement (KSV, CAM 101) instrument was employed to evaluate the wetting characteristics of the membrane surface. A drop of distilled water was allowed to adhere on the membrane surface. Goniometer was aligned and focused on the membrane–water interface. Each membrane surface was measured for at least 10 times at different positions so as to ensure reproducible results. 2.9. Differential scanning calorimetry (DSC) Approximately 10 mg of sample membrane was crimped in an aluminium sample pan and heated under nitrogen atmosphere from 30 8C to 300 8C at a rate of 20 8C/min in the differential scanning calorimetry (Perkin Elmer DSC7). Glass transition temperature (Tg) and melting temperature (Tm) were determined from the DSC diagram. 2.10. Thermo gravimetric analysis (TGA) The thermal stability of the membrane was investigated using thermal analysis instrument (Mettler Toledo TGA/SDTA 1600 8C). 4–5 mg of sample was initially dried at 100 8C to remove moisture. This was followed by a heating process from 30 8C to 600 8C in nitrogen atmosphere at a constant heating rate of 20 8C/min. Based on the TGA data the activation energy was calculated according to Eq. (7) [25]. 2 1 Ea RZTm þ ln ln ¼ y RT Ea b
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(7)
Eq. (7) shows a linear relationship, where activation energy (Ea) can be obtained from slope of the plots of ln(ln y1) versus (1/T), y is the fraction of the initial molecules not yet decomposed, T is the temperature (K) and R refers to gas constant (8.314 J/mol K).
The mean pore size of the membrane was determined using Guerout–Elford–Ferry equation [26]: rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð2:90 1:75eÞ8vmd (8) rp ¼ ePS where rp is the pore radius, d is the membrane thickness, n is the water flux, P is the operating pressure (250 mmHg), S is the effective membrane area (30 cm2), m is the water viscosity and e is the membrane porosity. 3. Results and discussion 3.1. Temperature calibration and control for MW irradiation As reported by Idris and Lee [11] and Bakir [27], the desired temperature to dissolve CA in carboxylic acid was approximately 70 8C. In this study the CA solutions were also prepared at 70 8C when TM heating was employed. An attempt was made to obtain the same temperature range for MW heating by using water as the media for calibration. Dielectric loss factor is a measurement on the ease of heating material under MW irradiation [28]. According to Horikoshi et al. [29] formic acid possessed 42.2 while water exhibited 8.52 dielectric loss factors. This indicated that faster heating can be achieved in formic acid than water. Table 2 shows the MW oven calibration at low heating mode for different durations. The initial calibration using water as the testing solution was performed up to approximately 60 8C only because the formic Table 2 MW oven calibration at low heating mode for different durations. Duration (min)
Temperature (8C)
1 2 3 4 5 6 7 8 9
30.58 0.75 33.32 0.34 37.31 0.37 41.65 0.59 43.79 0.38 46.99 0.31 49.29 0.23 53.60 0.69 57.63 0.37
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Table 3 Temperature of the MWCA solutions under low heating mode MW irradiation for 8 min. Temperature (8C)
MWCA0 MWCA2 MWCA4 MWCA6 MWCA8
TMCA
40000
Viscosity (mPas)
CA solution
50000
68.16 68.82 73.52 74.10 76.62
MWCA 30000 20000 10000
acid will be heated faster compared to water. By using the trial and error method, the duration time was found to be approximately 8 min for the temperature to reach approximately 70 10 8C. Table 3 depicts the temperature of the MWCA solutions under low heating mode MW irradiation for 8 min. The results demonstrated that the dissolution temperature for all the MWCA series membranes was kept in the desired range (68.16–76.62 8C). During this period all the CA was completely dissolved in formic acid. The low heating mode was chosen because the temperature required for dissolution was low which was in the region of 70 8C. If the medium heating mode was chosen, the temperature would exceed 80 8C within 2 min and CA could not be dissolved completely in the formic acid within this short duration. Moreover, the formic acid evaporates at this temperature. It should be noted that the temperature of the MWCA solution was increased with the presence of MSG, and this increment was dependent on the concentration of MSG used. This proves that MSG is a MW active material [14] and its dielectric loss factor property contributes to the increment in temperature. 3.2. Separation performance Table 4 illustrates the influence of heating techniques and MSG on the performance of CA membranes in terms of urea, creatinine, and bovine serum albumin permeabilities. As the concentration of MSG increased from 2 to 6%, the solute permeabilities increased. However, further increase of MSG concentration in the dope solution beyond this point, did not contribute to an improvement on the membrane separation performance. Meanwhile Table 5 depicts the porosity data for both TMCA and MWCA series membranes. It was observed that MSG behaved as the pore former, where the membranes became more porous and thus facilitated the solute permeability. Highest porosity was achieved when concentration of MSG in the membrane was 6%, thus explained for the highest solute permeability presented in Table 4.
0 CA0
CA2
CA4
CA6
CA8
Polymer solution Fig. 1. Viscosity data for MWCA and TMCA series membranes.
It is interesting to note that the solute permeabilities of the MWCA series membranes are in fact better than TMCA series membranes and this can be observed from Table 4 (except for membrane without MSG). The viscosity of the solution seemed to influence the membrane’s permeability. Viscosity data in Fig. 1 indicate that MWCA series polymer solutions exhibit lower viscosity compared to TMCA series except for the membranes without MSG (MWCA0). Low viscosity in MWCA series polymer solution can be explained by the chain scission phenomena. The presence of MSG seemed to induce this chain scission reaction. Since MSG is a MW active ingredient, it is able to generate heat upon exposure to microwave irradiation. There is the possibility that the heat generated during MW irradiation caused this chain scission to occur. The results seemed to be in agreement with the study on polyethersulfone solutions [30] where it was reported that chain scission occurred under MW radiation producing solutions with lower viscosity. During the phase inversion process, the low viscosity solution resulted in a rapid movement of the solvent into the non-solvent droplet thus explaining for the less dense structure of MWCA series compared to TMCA series membranes as illustrated in Fig. 2. When analyzing the data in Table 4, it is observed that the urea and creatinine permeabilities are 80.03 2.61 104 cm/min and 50.18 13.2 104 cm/min respectively for TMCA0 while MWCA0 exhibits approximately 53% lower permeability for both solute. It is observed that for membranes without MSG, the microwave prepared membranes (MWCA0) has a higher viscosity (10,589 MPa s) compared to TMCA0 (5020 MPa s). CA is known as a thermoplastic and hence its dielectric loss factor is very low or maybe transparent
Table 4 Solute permeability (104 cm/min) and UFR (ml/h m2 mmHg) of TMCA and MWCA series membranes. Membrane ID
CA0 CA2 CA4 CA6 CA8
TMCA
MWCA
Urea
Creatinine
BSA
UFR
Urea
Creatinine
BSA
UFR
80.03 2.61 33.91 4.71 130.13 6.76 156.67 12.55 66.78 2.67
50.18 13.20 11.34 2.83 87.23 4.13 84.15 5.67 43.96 8.15
8.61 8.28 5.27 1.45 4.69 3.24 7.95 3.21 7.25 1.03
1.25 0.14 1.61 0.21 5.67 0.14 32.92 0.12 55.69 2.84
38.64 3.64 90.12 23.46 143.31 14.85 160.13 8.13 108.46 9.40
22.96 8.27 23.06 9.62 90.90 15.00 100.71 7.71 71.04 8.25
3.73 2.67 4.86 1.90 7.85 3.49 12.45 1.77 10.09 2.14
0.59 0.17 2.62 0.13 10.86 1.54 45.41 0.05 52.94 1.63
Table 5 Thickness (mm), CAM (8), porosity and mean pore size (nm) of TMCA and MWCA series membranes. Membrane ID
CA0 CA2 CA4 CA6 CA8
TMCA
MWCA
Thickness
CAM
Porosity
Mean pore size
Thickness
CAM (8)
Porosity
Mean pore size
0.25 0.03 0.54 0.04 0.56 0.03 0.66 0.04 0.59 0.05
59.96 2.54 62.46 1.72 65.30 1.82 61.83 2.27 53.99 1.10
0.35 0.05 0.20 0.02 0.36 0.02 0.56 0.05 0.48 0.04
9.64 2.41 24.25 1.13 40.80 6.73 71.63 2.69 86.37 4.90
0.36 0.06 0.37 0.06 0.41 0.02 0.46 0.03 0.48 0.08
54.55 2.12 58.63 2.47 56.87 3.02 68.37 1.93 65.84 2.26
0.49 0.05 0.53 0.08 0.57 0.03 0.61 0.37 0.51 0.05
9.34 0.63 19.53 4.79 28.48 2.67 57.39 1.54 74.38 4.86
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Fig. 2. SEM images of (a) MWCA0, (b) MWCA2, (c) MWCA4, (d) MWCA6, (e) MWCA8, (f) TMCA0, (g) TMCA2, (h) TMCA4, (i) TMCA6 and (j) TMCA8 membranes.
to the MW [6]. When MSG was absent, only the dielectric property of formic acid was involved during the MW heating. The heat generated by formic acid may not be sufficient to cause chain scission of CA chain, instead the CA may aggregate among themselves during the mixing process thus explaining for the higher viscosity in MWCA0. This high viscosity in MWCA0 polymer solutions (see Fig. 1) caused the formation of thick and dense structure, as depicted in Fig. 2a. This is further proven by the overall resistance to diffusion solute transport as shown in Fig. 3, where MWCA0 presented higher resistance to solute compared to TMCA0. In terms of structure Fig. 2a, d and e looks almost alike; however in terms of performance; membranes in Fig. 2d and e are far better due to the thin membranes
thickness which is clearly shown in Fig. 2d and e. MSG was a MW reactive material and hence promoted the chain scission effect under microwave irradiation thus indirectly influences the phase inversion process. Apparently during the phase separation process the swelling phenomenon is reduced and densificaton phenomenon occurred, thus produce membranes which are much thinner compared to the membranes without MSG. According to Morti et al. [31], the overall resistance to diffusive solute transport is the sum of the resistances in blood, membrane and dialysate. By assuming the resistances in solute and distilled water constant, the increment of overall resistance to diffusive solute transport was mainly contributed by membrane resistance.
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Overall resistances to diffusive solute transport (x 104 min/m)
2120
30 Urea Creatinine BSA
25 20 15 10 5 0 CA0
CA2
CA4
CA6
CA8
Membrane Fig. 3. Overall resistances to diffusive solute transport on different heating process of CA membrane (solid line: MWCA series, dashed line: TMCA series).
In this study the high molecular weight CA (100 kDa) was used and thus shrinkage tend to occur in the transverse direction. As a result of the high shrinkage, macrovoids were sometimes formed so as to relieve the stress, as demonstrated in some of the SEM images in Fig. 2. Similar observation was found by Kesting [32], where the nascent membrane tend to shrink, particularly in the transverse direction, when high molecular weight polymer was employed as the membrane forming material. In general, the thick skin layer and dense substructure morphology of the traditional heated membranes TMCA as shown in Fig. 2f–j also contributes to the sudden decrease of solute permeabilities. Table 5 shows the porosity data of MWCA and TMCA series membrane. In general, the MWCA are more porous than the TMCA thus explains for the better solute permeabilities as shown in Table 4. This could be attributed to the micro-porous structures observed in MWCA series membranes instead of the dense substructures in TMCA series membranes. Higher porosity of MWCA series membranes was also responsible for the improved permeability property. Based on the permeabilities performance in Table 4, MWCA6 is the best membrane with high solute permeability (162.13 8.13 104 cm/min for urea and 100.71 7.71 104 cm/min for creatinine) and bovine serum albumin retention. This permeability performance was very much better than commercialized regenerated cellulose membrane, which exhibited 100 104 cm/min urea permeability [21]. It was worthwhile to mention that MSG seemed to induce vertical swelling in TMCA series membranes. As illustrated in Fig. 2f–j, the presence of MSG tends to swell the membrane vertically and thus the thickness increases from TMCA0 to TMCA6 as depicted in the thickness data in Table 5. Such phenomenon was very much reduced in the MWCA series membrane. As reported by Shukla and Anantheswaran [14], MSG was a MW reactive material and hence promoted the chain scission effect under microwave irradiation. This would indirectly influence the phase inversion process during the membrane formation due to the low viscosity solution formed. The microwave irradiation seemed to reduce the
swelling and densification phenomenon probably caused by the reduced viscosity of the microwave solution. UFR is a particularly important parameter used when classifying whether the membrane produced is in the high or low flux group. In this study, high flux CA membrane was successfully fabricated by adding appropriate amount of MSG (6 wt%) into the CA solution. Both TM and MW heating membranes showed similar trend where the CA0 to CA4 were classified as low flux membranes, while CA6 and CA8 as high flux membranes. Table 4 exhibits the UFR increases proportionally when the concentration of the MSG increases. This concluded that MSG played a crucial role in enhancing the membrane porosity (Table 5) and the membrane performance was further improved when microwave irradiation technique was adopted. The CAM results in Table 5 indicate that all the CA membranes are hydrophilic as the contact angles are lower than 908. Generally the contact angles for the MWCA series membranes were lower than the TMCA series membranes except for membrane MWCA6 and MWCA8. Although the porosities of MWCA6 and MWCA8 were higher than TMCA6 and TMCA8; but the mean pores sizes were smaller. Contact angle of a membrane is governed by its porosity and pore size distribution [33]. Thus, it is reasonable to deduce that the higher contact angle of these two membranes compared to TMCA6 and TMCA8 membranes were due to the small pore size; as indicated in Table 5. Further study on the membrane surface property was performed using AFM to investigate the surface roughness. Fig. 4a–d shows the AFM images of TMCA0, TMCA6, MWCA0, and MWCA6, respectively. Upon comparing Fig. 4a and c, it is observed that MWCA0 membrane had a rougher surface compared to TMCA0 membrane. High polymer aggregation among the CA chains in MWCA0 solution could influence the phase inversion process and causing the formation of nodules on the membrane. According to Kesting [32], membrane fabricated with higher molecular weight polymer grade resulted in better skin integrity. When MSG was present, membrane surface became rougher than the bare membrane regardless of the type of the heating process applied (Fig. 4). This may due to the chain scission phenomenon where the skin integrity of the membrane was affected. However, when comparing MWCA6 and TMCA6, it appeared that MWCA6 was much smoother than TMCA6. The MWCA6 prepared by using microwave heating produced low viscosity solutions which eased the casting process thus produced membrane with smoother surface (see Fig. 1). 3.3. Physical properties of membrane Table 6 illustrates the Tm values which were interpreted from DSC diagram in Fig. 5 for all the TMCA series membranes. These values obtained were in accordance with the cited literatures [34,35], which was approximately 230 8C. It was observed that the melting point for the TMCA series membranes depend solely on CA and not MSG (although MSG was present in the dope solution and has Tm of 232 8C). The results of the Tm values obtained are in line with the FTIR spectra in Fig. 6 which indicates the absence of N–H
Table 6 TGA and DSC result of TMCA and MWCA series membranes. Membrane ID
CA0 CA2 CA4 CA6 CA8
TM CA series
MW CA series
Tg (8C)
Tm (8C)
Tdeg (8C)
Activation energy (kJ/mol)
Tg (8C)
Tm (8C)
Tdeg (8C)
Activation energy (kJ/mol)
116.22 114.17 125.90 116.90 113.43
230.80 229.53 231.13 230.00 229.74
304.16 239.55 250.45 285.79 285.67
2.65 1.48 1.45 2.24 1.60
110.80 118.17 109.85 111.22 111.86
227.80 – – – –
267.66 279.64 285.57 291.61 286.74
1.69 1.96 1.87 2.11 2.00
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Fig. 4. AFM images of the membrane (a) TMCA0, (b) TMCA6, (c) MWCA0 and (d) MWCA6.
(2500–3100 cm1) and C–N (1350–1000 cm1) (which is usually present in MSG). MSG behaved only as a pore former and was washed away during phase inversion thus explained for its absence in the FTIR spectra. 90
Heat Flow Endo up (mW)
80 70 60 50 40 30
TM CA0 TM CA2 TM CA4 TM CA6 TM CA8
20 10 0 0
50
100
150
200
Tempeture (oC) Fig. 5. DSC diagram of TMCA series membranes.
250
300
FTIR spectra in Fig. 6 show the CA classical chemical bonds. Wave number between 3400 and 3700 cm1 represents the O–H stretching. Bands appear at 1739 and 1370 cm1 are assigned to C5 5O stretching and C–H symmetric deformation vibration of acetate group. Both 1232 and 1048 cm1 are attributed to C–O stretching, where the former is contributed by the acetate group while the latter is contributed by cellulose group [36]. The FTIR spectra analysis suggested that the MSG and MW irradiation technique did not cause any major effect on the CA chemical bonds. The connection between Tm and crystalline structure was well established in the literature [37]. In Table 6 (interpreted from DSC diagram in Fig. 7), there was a slight decrease in Tm value for the MWCA0 compared to TMCA0. This was probably attributed to the MW heating, where the irradiation probably disrupted the CA crystallinity up to a certain degree. The presence of MSG in the CA solution probably disturbed and disrupted the well arranged crystal structure in CA during the phase inversion. This effect might not occur in the TMCA series membranes because under normal heating the MSG was not able to disrupt the entanglement of long molecular weight of CA thus the crystalline structure remained intact and thereby the Tm was retained at 230 8C. In the case of MWCA2 to MWCA8 membranes, both MSG and MW irradiation disrupted the crystalline structure in CA and this was proven by the
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Fig. 6. FTIR spectra of TMCA0, MWCA0, TMCA6 and MWCA6 membranes.
absence of Tm in all the membranes. As observed in Table 6, MWCA2 to MWCA8 membranes has no Tm. The results of Tg for both TM and MW series membranes are qualitatively generalized in Table 6. Generally, TMCA series membranes exhibited higher Tg compared to MWCA series membrane except for TMCA2. The lower Tg values in MWCA series membranes again suggested the occurrence of chain scission which resulted in the shortened CA chains due to MW irradiation. The results seemed to be in agreement with the research revealed by Singh et al. [38] where they reported higher Tg was found for higher molecular weight polymer film. The abnormal point at TMCA2 may due to the thick skin layer, as described in Fig. 2g, which may reduce the free volume to a certain level as the value of Tg is always related to the free volume in the membrane [39,40]. Figs. 8 and 9 show the cellulose degradation process in the temperature region between 30 8C and 330 8C for the MWCA and TMCA series membranes respectively. The volatilization of the volatile compounds, main thermal degradation of cellulose acetate chains occurred at 330 8C to 450 8C and after this point,
120
carbonization of the products to ash started. In the TGA studies mentioned earlier it was found that the Tdeg of MWCA0 was much lower than TMCA0. Classical cellulose degradation steps, discussed by Chatterjee [41] were observed in both the TGA diagrams. As discussed in previous section, MW caused chain scission and thus shortened CA chains. The results seemed to be in agreement with the work done by Calahorra et al. [42] where higher molecular weight cellulose promised a better thermal stability property. When MSG was present, the thermal stability of MWCA series membranes was improved, but the additive contributed a negative impact on TMCA series membranes. According to Zulkifli et al. [43], thermal stability of a sample was very much dependent on its activation energy, where sample with high activation energy resulted in high thermal stability property. Thus, the results in Table 6 reveals that, the activation energy of the MWCA series membranes is improved by the presence of MSG but this effect seems to be suppressed in TMCA series membranes The activation energy of MWCA0 was 1.69 kJ/mol (the lowest in the MWCA series) while MWCA6 was 2.11 kJ/mol. For TMCA series membranes, TMCA0 exhibited the highest activation energy and it was 0.41 kJ/mol higher than TMCA6. Thus the presence of MSG and
MWCA0
MWCA4
80
MWCA0 MWCA2 MWCA4 MWCA6 MWCA8
100
MWCA6
Degradation (%)
Heat Flow Endo up (mW)
120
MWCA2
100
MWCA8
60
40
20
80 60 40 20
0 0
50
100
150
Temperature
200
250
(oC)
Fig. 7. DSC diagram of MWCA series membranes.
300
0
0
100
200
300
400
500
Temperature (oC) Fig. 8. TGA diagram MWCA series membranes.
600
700
C.M. Kee, A. Idris / Journal of Industrial and Engineering Chemistry 18 (2012) 2115–2123 [2] [3] [4] [5]
120 TM CA0
100
TM CA2
[6]
Degradation (%)
TM CA4 TM CA6
80
[7]
TM CA8
[8]
60 [9] [10]
40 [11] [12] [13]
20
[14]
0 0
100
200
300
400
500
600
700
Temperature (oC) Fig. 9. TGA diagram TMCA series membranes.
microwave irradiation method improved the thermal stability of the membranes and this was proven by the high activation energy of the membranes. 4. Conclusion
[15] [16] [17] [18] [19] [20] [21] [22] [23]
The influence of different heating methods on CA membrane separation performances and physical properties were systematically revealed. The use of MW irradiation method can reduce the dissolution time in CA membrane fabrication and thus saves energy and cost. Membranes produced by the MW irradiation exhibited better solute separation, higher UFR and smoother membrane surface, especially in the presence of microwave active material such as MSG. In addition, the concentration of MSG must be kept to 6% since further increment beyond this value did not seem to improve the uremic toxins separation. Since MW heating offers numerous advantages on the resultant membrane, especially when a MW active additive is present; the use of this MW apparatus to prepare polymeric solution in membrane industries will be a trend in the next few decades. Acknowledgement Financial support from the Ministry of Science, Technology and Environment through the FRGS funding (Vot no: 78673) is gratefully acknowledged. References [1] J. Jang, D.K. Lee, Polymers 45 (2004) 1599.
[24] [25] [26] [27]
[28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43]
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G.Y. Heo, S.J. Park, Carbohydrate Polymers 88 (2012) 562. J. Song, H. Oh, H. Kong, J. Jang, Journal of Hazardous Materials 187 (2011) 311. A. Idris, I. Ahmed, M. Misran, Desalination 249 (2009) 541. Y.A. Cengel, M.A. Boles, Thermodynamics an Engineering Approach, seventh ed., McGraw Hill, New York, 2011. E.T. Thostenson, T.W. Chou, Composites Part A: Applied Science and Manufacturing 30 (1999) 1055. T.J. Appleton, R.I. Colder, S.W. Kingman, I.S. Lowndes, A.G. Read, Applied Energy 81 (2005) 85. R. Malaisamy, R. Mahendran, D. Mohan, M. Rajendran, V. Mohan, Journal of Applied Polymer Science 86 (2002) 1749. H. Dogan, N.D. Hilmioglu, Carbohydrate Polymers 75 (2009) 90. C. Satge, B. Verneuil, P. Branland, R. Granet, P. Krausz, J. Rozier, C. Petit, Carbohydrate Polymers 49 (2002) 373. A. Idris, K.Y. Lee, Journal of Membrane Science 280 (2006) 920. J.C. An, Journal of Industrial and Engineering Chemistry 15 (2009) 148. Y.W. Chang, G. Shin, Journal of Industrial and Engineering Chemistry 17 (2011) 730. T.P. Shukla, R.C. Anantheswaran, in: A.K. Datta, R.C. Anantheswaran (Eds.), Handbook of Microwave Technology for Food Applications, Marcel Dekker, Inc., New York, 2001, p. 355. J.G. Lyng, L. Zhang, N.P. Brunton, Meat Science 69 (2005) 589. M.K. Chan, A. Idris, Separation and Purification Technology 75 (2010) 102. I. Ahmed, High performance ultrafiltration membrane using novel technique, Ph. D. Thesis, Universiti Teknologi Malaysia, Malaysia, 2008. W.H. Marsh, B. Fingerhut, H. Miller, Clinical Chemistry 11 (1965) 624. D. Heinegard, G. Tiderstrom, Clinica Chimica Acta 43 (1973) 305. A.G. Gornall, C.J. Bardawill, M.M. David, Journal of Biological Chemistry 177 (1949) 751. Y. Seita, A. Mochizuki, M. Nakagawa, K. Takanashi, S. Yamashita, Journal of Applied Polymer Science 65 (1997) 1703. S.M. Morti, A.L. Zydney, American Society for Artificial Internal Organs Journal 44 (1998) 319. A.S. Michaels, Transactions – American Society for Artificial Internal Organs 12 (1966) 387. Y. Abe, A. Mochizuki, Journal of Applied Polymer Science 89 (2003) 333. A. Broido, Journal of Polymer Science 7 (1969) 1761. L. Ye, Z. Fan, J. Zhou, Membrane Science and Technology (1981) 24. M.F. Bakir, Production of flat sheet dialysis membrane and its performance in formic acid solvent, Bachelor Thesis, University Teknologi Malaysia, Malaysia, 2006. C.A. Harper, E.M. Petrie, Plastic Materials, Processes – A Concise Encyclopedia, first ed., John Wiley and Sons Inc., New Jersey, 2003. S. Horikoshi, M. Abe, N. Serpone, Applied Catalysis B: Environmental 89 (2009) 284. A. Idris, I. Ahmed, Journal of Applied Polymer Science 108 (2008) 302. S. Morti, J. Shao, A.L. Zydney, Journal of Membrane Science 224 (2003) 39. R.E. Kesting, Synthetic Polymeric Membranes – A Structural Perspective, second ed., Wiley Interscience, Canada, 1985. Y.H. Zhao, B.K. Zhu, X.T. Ma, Y.Y. Xu, Journal of Membrane Science 290 (2007) 222. K. Scott, Handbook of Industrial Membranes, second ed., Elsevier Science and Technology Books, UK, 1999. A.J.M. Valente, A.Y. Polishchuk, H.D. Burrows, V.M.M. Lobo, European Polymer Journal 41 (2005) 275. G. Socrates, Infrared and Raman Characteristic Group Frequencies – Tables and Charts, third ed., John Wiley & Sons Ltd., England, 2001. G. Farrow, Polymers 4 (1963) 191. L. Singh, P.J. Ludovice, C.L. Henderson, Thin Solid Films 449 (2004) 231. J.J. Shieh, T.S. Chung, Journal of Membrane Science 140 (1998) 67. A.K. Fritzsche, C.A. Cruse, R.E. Kesting, M.K. Murphy, Journal of Applied Polymer Science 39 (1990) 1949. P.K. Chatterjee, Journal of Polymer Science 6 (1968) 3217. M.E. Calahorra, M. Cortazar, J.I. Eguizzabal, G.M. Guzman, Journal of Applied Polymer Science 37 (1989) 3305. A.F.M. Zulkifli, S. Zakaria, K. Badri, Solid State Science and Technology 18 (2010) 53.
Contents lists available at SciVerse ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Modification of cellulose acetate membrane using monosodium glutamate additives prepared by microwave heating Chan Mieow Kee, Ani Idris * Department of Bioprocess Engineering, Faculty of Chemical Engineering, Universiti Teknologi Malaysia, 81300 UTM, Skudai, Johor, Malaysia
A R T I C L E I N F O
Article history: Received 13 March 2012 Accepted 13 June 2012 Available online 19 June 2012 Keywords: Cellulose acetate Formic acid Monosodium glutamate Microwave Membrane
A B S T R A C T
Cellulose acetate polymeric solutions with different concentrations of monosodium glutamate in formic acid were prepared using both traditional and microwave heating methods. The membrane separation performance and its physical properties were evaluated. Results indicated that the membrane consisting of 6 wt% additive prepared using microwave heating exhibited better solute permeability. Microwave heating was also found to enhance the thermal stability and the surface roughness of the membrane. The use of microwave for membrane fabrication not only saves time and cost, but also produces membranes with good properties. This paper therefore discusses the plausibility of manufacturing membranes using microwave heating techniques. ß 2012 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction Low energy consumption of using membrane technology in a broad range of application triggers the membranologists to fabricate low cost membranes with high separation efficiency. Jang and Lee [1] improved the oxygen barrier property of biaxially orientad polypropylene film by introducing polyvinyl alcohol into the casting solution. Atmospheric pressure plasma treatment was adopted by Park et al. [2] to determine the ideal treatment condition to produce low cost electron-spun polyamideimide film. Another approach was attempted by Song et al. [3], where polyrhodanine was immobilized onto the anodic aluminium oxide membrane. The resultant membrane exhibited excellent separation performance in remove ion Hg (II). In the past few years, polyethersulfone ultrafiltration membranes with high rejection rates and good fluxes were successfully fabricated by Idris et al. [4] using microwave (MW) heating. The merit of using this irradiation heating in processing materials is not only limited to cost and time savings [5] but also contributes to uniform heating, initiates some desired chemical reactions and accelerates the reaction rate. All the mentioned benefits are closely related to how the energy is being transferred to the materials. In conventional heating, heat is transferred through convection or conduction from the surface of the materials inwards, which is due to the thermal gradients within the materials [6]. A different
* Corresponding author. Tel.: +60 7 5535603; fax: +60 7 5581463. E-mail addresses: [email protected], [email protected] (A. Idris).
phenomenon exists in MW heating; where the electromagnetic energy is converted into thermal energy by interacting with the molecules of the materials. As microwaves can penetrate into the materials, heat is generated volumetrically within the material rather than from the external source, unlike the traditional heating process [7]. In MW heating, the dielectric property is a very important factor as it determines which materials in the mixture couple well with the electromagnetic field and absorb the MW energy. Material with too high dielectric loss factor such as steel causes reflection while thermoplastic, which has low dielectric loss factor is transparent to MW effect. Dielectric loss factor of water, for instance, falls between this range, and hence makes it a MW receptive material [6]. Cellulose acetate (CA) is a popular polymer that is suitable for microfiltration, ultrafiltration and reverse osmosis. Its excellent properties such as good flux and high salt rejection, cheap, biodegradable, hydrophilic and non-toxic make it an attractive membrane forming material [8]. The use of MW in processing cellulose material was reported by many researchers recently [9,10]. Dogan and Hilmioglu [9] investigated the effect of MW heating on cellulose dissolution. The resultant membrane fabricated from this cellulose solution showed no differences in the crystallinity as well as the peak in FTIR spectra. However, no permeability experiments were performed in the mentioned studies. Thus, the effect of MW heating on the separation performance of the resultant membrane remained unknown. Several researchers [10] claimed that the time for cellulose esterification which normally took 2 days were reduced to several minutes and a higher degree of esterification occurred within 10 min.
1226-086X/$ – see front matter ß 2012 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jiec.2012.06.005
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Based on literature reviews, PVP and PEG are the most favorable additives in membrane fabrication due to the hydrophilic characteristic [11–13]. Thus, there is need to explore on the possibilities of other types of additives which are more environmental friendly. Monosodium glutamate (MSG), a well known food additive which is highly hydrophilic due to the hydroxyl group in the structure is employed as an additive in this study. It is easily available and cheap. It is also a MW active additive [14], where 5 wt% solution of MSG Exhibits 0.311 dielectric loss tangent (2450 MHz) [15]. Besides the very recent publication [16] on the use of MSG for hemodialysis membrane, no other similar work has been published. Additionally, the influence of MW on the CA–MSG membrane has not been explored and therefore is the subject of this investigation.
temperature profile of the solution during heating process. Water was used as the testing solution so as to obtain an approximate relation between heating mode and heating period. After determining the suitable combination, all the CA solutions were prepared under low heating mode for 8 min.
2. Experimental
The dialysis testing system used in this work was similar to that described by Idris and Lee [11]. Dialysis cell with 30 cm2 effective membrane area was used in this study. Solute reservoir was filled with a mixture of 1 mg/ml of urea, 0.1 mg/ml of creatinine and 1 mg/ml of bovine serum albumin. The flow rate for the solute and dialysate reservoirs were set at 50 ml/min and 100 ml/min respectively and samples were collected at both reservoirs for every 0.5 h. This experiment was carried out at 37 8C for 3.5 h. Urea Nitrogen (Diacetyl) Reagent [18], Creatinine (Direct) Reagent [19] and Biuret Reagent [20] were used to measure the urea, creatinine and bovine serum albumin concentrations respectively of the collected samples. The permeability experiment was repeated at least three times for each membrane formulation so as to ensure the reproducibility of data points. The diffusive permeability, clearance and overall mass transfer coefficient were calculated using Eqs. (1)–(3) respectively. Diffusive permeability, Pm is given by the following equation [21]:
2.1. Materials Food grade formic acid with 98% purity supplied by AppliChem was used as the solvent for both CA and MSG. CA (acetylation degree: 39.8%) with molecular weight of 100 kDa was purchased from Acros while MSG was obtained from commercial sources (Ajinomoto). Probe solute, such as bovine serum albumin (Sigma), creatinine (Kanto) and urea (Quantum) were used in the dialysis experiment. The concentrations of urea and creatinine in the samples were determined using Urea Nitrogen (Diacetyl) Reagent and Creatinine (Direct) Reagent, respectively. Both reagents were purchased from Eagle Diagnostics. Bovine serum albumin concentration was measured using self prepared Biuret reagent. 2.2. Membrane preparation In the traditional method (TM), polymeric dope solutions with 20 wt% CA and different concentrations of MSG in formic acid were prepared at 70 8C with continuous agitation for 4 h. In the MW method, the same dope solution formulation was prepared and heated inside a MW (Panasonic) for 8 min at low mode [17]. Table 1 shows the composition of the various dope solutions prepared. The mode and heating time were chosen based on the temperature calibration and would be explained in the following Section 2.2.1. The abbreviation TM or MW in front of CA indicated the type of heating process used for dope preparation while the number behind CA indicated the weight percent of MSG used. For example, MWCA2 depicted that the CA membrane was produced with 2 wt% of MSG using MW heating. The bottled CA solution was placed in an ultrasonic bath to remove the bubbles. Sufficient amount of the CA solution was spread over a dust-free glass plate using a casting knife with 200 mm gap and immediately immersed into distilled water coagulation bath at room temperature. In order to remove the residual solvent, the nascent membrane was post treated in hot water (90 8C) and methanol. 2.2.1. Determination of MW heating mode and mixing time In order to determine the level of heating mode and desired duration for dissolving the CA in formic acid, thermocouple data logger (Pico Technology Limited) was used to record the
2.3. Membrane thickness After the phase inversion process, the thickness of the membrane was measured using digital caliper with accuracy 0.01 mm (Mitutoyo) at 5 different positions on the membrane. 2.4. Permeability study
Pm ¼
ln½DCðt 1 Þ=DCðt 2 Þ s½ð1=V a Þ þ ð1=V b Þ½t 2 t 1
Clearance, K is described accordingly as follows [22]: K¼
aV Q B ½1 expðaDt Þ expðaDt Þ
Abb.
CA (wt%)
Formic acid (wt%)
MSG (wt%)
CA0 CA2 CA4 CA6 CA8
20 20 20 20 20
80 78 76 74 72
0 2 4 6 8
(2)
Overall resistance to solute transport, 1/Ko is described as [23] QB 1 ðK=Q D Þ ln Ko ¼ (3) 1 ðK=Q B Þ Sð1 ðQ B =Q D ÞÞ where Co and Ct are the initial and periodic concentrations, respectively; DC(t) is the solute concentration difference between the solute and dialysate reservoirs at the sampling time, t1 and t2; V is the reservoir volume; S is the surface area of the membrane; t is the time; a is the slope of the plot of ln(Ct/Co) versus time; Dt* is the ratio of the total circuit volume to the solute flow rate, and QB and QD are the flow rates of the solute and dialysate reservoirs, respectively. 2.5. Ultrafiltration rate (UFR) The UFR was obtained from Eq. (4): UFR ¼
Table 1 Membrane formulations.
(1)
V SP
(4)
where S is the effective membrane surface area 30 cm2, V is the water flux at 37 8C and P represents the operating pressure, which is 250 mmHg. 2.6. Porosity measurement Gravimetric method was adopted to evaluate membrane porosity. Membrane was cut to the size of 1 cm by 2 cm. The
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dry (wb) and wet (wa) membranes were weighed. The wet membrane was prepared by dipping the dried membrane into distilled water for 24 h. Membrane porosity (e) was determined according to the following equation [24]
e¼
ðwa wb Þ=rH2 O ½ðwa wb Þ=rH2 O þ ½wb =rmembrane
(5)
In this case, rH2 O is the density of water (0.998 g/cm3) and rmembrane is the density of membrane. In order to determine the membrane density, the empty pycnometer (wo) was first weighed. One third of the pycnometer was filled with sample and weighed (w1), then filled with distilled water and weighed (w2) again. Finally, the pycnometer was emptied, filled with distilled water only and weighed (w3). Density of membrane was calculated according to Eq. (6).
rmembrane ¼
wp Vs
(6)
where wp refers to the weight of sample (w1 wo) while Vs = volume of the sample (V V 0H2 O ); V 0H2 O ¼ ðw2 w1 Þ=rH2 O . V represents volume of the distilled water that fills the empty pycnometer. 2.7. FTIR study
2.11. Atomic force microscope (AFM) The surface morphology and roughness of the membranes was analyzed using atomic force microscope (SII Nanotechnology SPMSPI3800N). The scanning area of the membrane was approximately 5 mm 5 mm. Roughness parameters, mean surface roughness (Ra), root mean square roughness (RMS) and peak to valley (P–V) were calculated by the software. 2.12. Scanning electron microscope (SEM) The microstructure of the membranes was observed using scanning electron microscope (Model SUPRA 35VP). Membrane was snapped in liquid nitrogen to obtain a clean cut. The sample was then mounted onto brass plates using double-sided cellophane tapes in a lateral position and sputter-coated with platinum. The sample was then ready for imaging. 2.13. Viscosity measurement The viscosity of the CA solution was measured using Plate and Cone Viscometer (Brookfield DVII) at room temperature. At least three measurements were obtained for each solution. 2.14. Pore size determination
FTIR spectrum of the sufficiently thin film membrane at room temperature was recorded by Perkin Elmer Spectrum One FTIR Spectrometer. Each spectrum was recorded in the mid IR region between 370 cm1 and 4000 cm1. 2.8. Contact angle measurement (CAM) Contact angle measurement (KSV, CAM 101) instrument was employed to evaluate the wetting characteristics of the membrane surface. A drop of distilled water was allowed to adhere on the membrane surface. Goniometer was aligned and focused on the membrane–water interface. Each membrane surface was measured for at least 10 times at different positions so as to ensure reproducible results. 2.9. Differential scanning calorimetry (DSC) Approximately 10 mg of sample membrane was crimped in an aluminium sample pan and heated under nitrogen atmosphere from 30 8C to 300 8C at a rate of 20 8C/min in the differential scanning calorimetry (Perkin Elmer DSC7). Glass transition temperature (Tg) and melting temperature (Tm) were determined from the DSC diagram. 2.10. Thermo gravimetric analysis (TGA) The thermal stability of the membrane was investigated using thermal analysis instrument (Mettler Toledo TGA/SDTA 1600 8C). 4–5 mg of sample was initially dried at 100 8C to remove moisture. This was followed by a heating process from 30 8C to 600 8C in nitrogen atmosphere at a constant heating rate of 20 8C/min. Based on the TGA data the activation energy was calculated according to Eq. (7) [25]. 2 1 Ea RZTm þ ln ln ¼ y RT Ea b
2117
(7)
Eq. (7) shows a linear relationship, where activation energy (Ea) can be obtained from slope of the plots of ln(ln y1) versus (1/T), y is the fraction of the initial molecules not yet decomposed, T is the temperature (K) and R refers to gas constant (8.314 J/mol K).
The mean pore size of the membrane was determined using Guerout–Elford–Ferry equation [26]: rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð2:90 1:75eÞ8vmd (8) rp ¼ ePS where rp is the pore radius, d is the membrane thickness, n is the water flux, P is the operating pressure (250 mmHg), S is the effective membrane area (30 cm2), m is the water viscosity and e is the membrane porosity. 3. Results and discussion 3.1. Temperature calibration and control for MW irradiation As reported by Idris and Lee [11] and Bakir [27], the desired temperature to dissolve CA in carboxylic acid was approximately 70 8C. In this study the CA solutions were also prepared at 70 8C when TM heating was employed. An attempt was made to obtain the same temperature range for MW heating by using water as the media for calibration. Dielectric loss factor is a measurement on the ease of heating material under MW irradiation [28]. According to Horikoshi et al. [29] formic acid possessed 42.2 while water exhibited 8.52 dielectric loss factors. This indicated that faster heating can be achieved in formic acid than water. Table 2 shows the MW oven calibration at low heating mode for different durations. The initial calibration using water as the testing solution was performed up to approximately 60 8C only because the formic Table 2 MW oven calibration at low heating mode for different durations. Duration (min)
Temperature (8C)
1 2 3 4 5 6 7 8 9
30.58 0.75 33.32 0.34 37.31 0.37 41.65 0.59 43.79 0.38 46.99 0.31 49.29 0.23 53.60 0.69 57.63 0.37
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Table 3 Temperature of the MWCA solutions under low heating mode MW irradiation for 8 min. Temperature (8C)
MWCA0 MWCA2 MWCA4 MWCA6 MWCA8
TMCA
40000
Viscosity (mPas)
CA solution
50000
68.16 68.82 73.52 74.10 76.62
MWCA 30000 20000 10000
acid will be heated faster compared to water. By using the trial and error method, the duration time was found to be approximately 8 min for the temperature to reach approximately 70 10 8C. Table 3 depicts the temperature of the MWCA solutions under low heating mode MW irradiation for 8 min. The results demonstrated that the dissolution temperature for all the MWCA series membranes was kept in the desired range (68.16–76.62 8C). During this period all the CA was completely dissolved in formic acid. The low heating mode was chosen because the temperature required for dissolution was low which was in the region of 70 8C. If the medium heating mode was chosen, the temperature would exceed 80 8C within 2 min and CA could not be dissolved completely in the formic acid within this short duration. Moreover, the formic acid evaporates at this temperature. It should be noted that the temperature of the MWCA solution was increased with the presence of MSG, and this increment was dependent on the concentration of MSG used. This proves that MSG is a MW active material [14] and its dielectric loss factor property contributes to the increment in temperature. 3.2. Separation performance Table 4 illustrates the influence of heating techniques and MSG on the performance of CA membranes in terms of urea, creatinine, and bovine serum albumin permeabilities. As the concentration of MSG increased from 2 to 6%, the solute permeabilities increased. However, further increase of MSG concentration in the dope solution beyond this point, did not contribute to an improvement on the membrane separation performance. Meanwhile Table 5 depicts the porosity data for both TMCA and MWCA series membranes. It was observed that MSG behaved as the pore former, where the membranes became more porous and thus facilitated the solute permeability. Highest porosity was achieved when concentration of MSG in the membrane was 6%, thus explained for the highest solute permeability presented in Table 4.
0 CA0
CA2
CA4
CA6
CA8
Polymer solution Fig. 1. Viscosity data for MWCA and TMCA series membranes.
It is interesting to note that the solute permeabilities of the MWCA series membranes are in fact better than TMCA series membranes and this can be observed from Table 4 (except for membrane without MSG). The viscosity of the solution seemed to influence the membrane’s permeability. Viscosity data in Fig. 1 indicate that MWCA series polymer solutions exhibit lower viscosity compared to TMCA series except for the membranes without MSG (MWCA0). Low viscosity in MWCA series polymer solution can be explained by the chain scission phenomena. The presence of MSG seemed to induce this chain scission reaction. Since MSG is a MW active ingredient, it is able to generate heat upon exposure to microwave irradiation. There is the possibility that the heat generated during MW irradiation caused this chain scission to occur. The results seemed to be in agreement with the study on polyethersulfone solutions [30] where it was reported that chain scission occurred under MW radiation producing solutions with lower viscosity. During the phase inversion process, the low viscosity solution resulted in a rapid movement of the solvent into the non-solvent droplet thus explaining for the less dense structure of MWCA series compared to TMCA series membranes as illustrated in Fig. 2. When analyzing the data in Table 4, it is observed that the urea and creatinine permeabilities are 80.03 2.61 104 cm/min and 50.18 13.2 104 cm/min respectively for TMCA0 while MWCA0 exhibits approximately 53% lower permeability for both solute. It is observed that for membranes without MSG, the microwave prepared membranes (MWCA0) has a higher viscosity (10,589 MPa s) compared to TMCA0 (5020 MPa s). CA is known as a thermoplastic and hence its dielectric loss factor is very low or maybe transparent
Table 4 Solute permeability (104 cm/min) and UFR (ml/h m2 mmHg) of TMCA and MWCA series membranes. Membrane ID
CA0 CA2 CA4 CA6 CA8
TMCA
MWCA
Urea
Creatinine
BSA
UFR
Urea
Creatinine
BSA
UFR
80.03 2.61 33.91 4.71 130.13 6.76 156.67 12.55 66.78 2.67
50.18 13.20 11.34 2.83 87.23 4.13 84.15 5.67 43.96 8.15
8.61 8.28 5.27 1.45 4.69 3.24 7.95 3.21 7.25 1.03
1.25 0.14 1.61 0.21 5.67 0.14 32.92 0.12 55.69 2.84
38.64 3.64 90.12 23.46 143.31 14.85 160.13 8.13 108.46 9.40
22.96 8.27 23.06 9.62 90.90 15.00 100.71 7.71 71.04 8.25
3.73 2.67 4.86 1.90 7.85 3.49 12.45 1.77 10.09 2.14
0.59 0.17 2.62 0.13 10.86 1.54 45.41 0.05 52.94 1.63
Table 5 Thickness (mm), CAM (8), porosity and mean pore size (nm) of TMCA and MWCA series membranes. Membrane ID
CA0 CA2 CA4 CA6 CA8
TMCA
MWCA
Thickness
CAM
Porosity
Mean pore size
Thickness
CAM (8)
Porosity
Mean pore size
0.25 0.03 0.54 0.04 0.56 0.03 0.66 0.04 0.59 0.05
59.96 2.54 62.46 1.72 65.30 1.82 61.83 2.27 53.99 1.10
0.35 0.05 0.20 0.02 0.36 0.02 0.56 0.05 0.48 0.04
9.64 2.41 24.25 1.13 40.80 6.73 71.63 2.69 86.37 4.90
0.36 0.06 0.37 0.06 0.41 0.02 0.46 0.03 0.48 0.08
54.55 2.12 58.63 2.47 56.87 3.02 68.37 1.93 65.84 2.26
0.49 0.05 0.53 0.08 0.57 0.03 0.61 0.37 0.51 0.05
9.34 0.63 19.53 4.79 28.48 2.67 57.39 1.54 74.38 4.86
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Fig. 2. SEM images of (a) MWCA0, (b) MWCA2, (c) MWCA4, (d) MWCA6, (e) MWCA8, (f) TMCA0, (g) TMCA2, (h) TMCA4, (i) TMCA6 and (j) TMCA8 membranes.
to the MW [6]. When MSG was absent, only the dielectric property of formic acid was involved during the MW heating. The heat generated by formic acid may not be sufficient to cause chain scission of CA chain, instead the CA may aggregate among themselves during the mixing process thus explaining for the higher viscosity in MWCA0. This high viscosity in MWCA0 polymer solutions (see Fig. 1) caused the formation of thick and dense structure, as depicted in Fig. 2a. This is further proven by the overall resistance to diffusion solute transport as shown in Fig. 3, where MWCA0 presented higher resistance to solute compared to TMCA0. In terms of structure Fig. 2a, d and e looks almost alike; however in terms of performance; membranes in Fig. 2d and e are far better due to the thin membranes
thickness which is clearly shown in Fig. 2d and e. MSG was a MW reactive material and hence promoted the chain scission effect under microwave irradiation thus indirectly influences the phase inversion process. Apparently during the phase separation process the swelling phenomenon is reduced and densificaton phenomenon occurred, thus produce membranes which are much thinner compared to the membranes without MSG. According to Morti et al. [31], the overall resistance to diffusive solute transport is the sum of the resistances in blood, membrane and dialysate. By assuming the resistances in solute and distilled water constant, the increment of overall resistance to diffusive solute transport was mainly contributed by membrane resistance.
C.M. Kee, A. Idris / Journal of Industrial and Engineering Chemistry 18 (2012) 2115–2123
Overall resistances to diffusive solute transport (x 104 min/m)
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30 Urea Creatinine BSA
25 20 15 10 5 0 CA0
CA2
CA4
CA6
CA8
Membrane Fig. 3. Overall resistances to diffusive solute transport on different heating process of CA membrane (solid line: MWCA series, dashed line: TMCA series).
In this study the high molecular weight CA (100 kDa) was used and thus shrinkage tend to occur in the transverse direction. As a result of the high shrinkage, macrovoids were sometimes formed so as to relieve the stress, as demonstrated in some of the SEM images in Fig. 2. Similar observation was found by Kesting [32], where the nascent membrane tend to shrink, particularly in the transverse direction, when high molecular weight polymer was employed as the membrane forming material. In general, the thick skin layer and dense substructure morphology of the traditional heated membranes TMCA as shown in Fig. 2f–j also contributes to the sudden decrease of solute permeabilities. Table 5 shows the porosity data of MWCA and TMCA series membrane. In general, the MWCA are more porous than the TMCA thus explains for the better solute permeabilities as shown in Table 4. This could be attributed to the micro-porous structures observed in MWCA series membranes instead of the dense substructures in TMCA series membranes. Higher porosity of MWCA series membranes was also responsible for the improved permeability property. Based on the permeabilities performance in Table 4, MWCA6 is the best membrane with high solute permeability (162.13 8.13 104 cm/min for urea and 100.71 7.71 104 cm/min for creatinine) and bovine serum albumin retention. This permeability performance was very much better than commercialized regenerated cellulose membrane, which exhibited 100 104 cm/min urea permeability [21]. It was worthwhile to mention that MSG seemed to induce vertical swelling in TMCA series membranes. As illustrated in Fig. 2f–j, the presence of MSG tends to swell the membrane vertically and thus the thickness increases from TMCA0 to TMCA6 as depicted in the thickness data in Table 5. Such phenomenon was very much reduced in the MWCA series membrane. As reported by Shukla and Anantheswaran [14], MSG was a MW reactive material and hence promoted the chain scission effect under microwave irradiation. This would indirectly influence the phase inversion process during the membrane formation due to the low viscosity solution formed. The microwave irradiation seemed to reduce the
swelling and densification phenomenon probably caused by the reduced viscosity of the microwave solution. UFR is a particularly important parameter used when classifying whether the membrane produced is in the high or low flux group. In this study, high flux CA membrane was successfully fabricated by adding appropriate amount of MSG (6 wt%) into the CA solution. Both TM and MW heating membranes showed similar trend where the CA0 to CA4 were classified as low flux membranes, while CA6 and CA8 as high flux membranes. Table 4 exhibits the UFR increases proportionally when the concentration of the MSG increases. This concluded that MSG played a crucial role in enhancing the membrane porosity (Table 5) and the membrane performance was further improved when microwave irradiation technique was adopted. The CAM results in Table 5 indicate that all the CA membranes are hydrophilic as the contact angles are lower than 908. Generally the contact angles for the MWCA series membranes were lower than the TMCA series membranes except for membrane MWCA6 and MWCA8. Although the porosities of MWCA6 and MWCA8 were higher than TMCA6 and TMCA8; but the mean pores sizes were smaller. Contact angle of a membrane is governed by its porosity and pore size distribution [33]. Thus, it is reasonable to deduce that the higher contact angle of these two membranes compared to TMCA6 and TMCA8 membranes were due to the small pore size; as indicated in Table 5. Further study on the membrane surface property was performed using AFM to investigate the surface roughness. Fig. 4a–d shows the AFM images of TMCA0, TMCA6, MWCA0, and MWCA6, respectively. Upon comparing Fig. 4a and c, it is observed that MWCA0 membrane had a rougher surface compared to TMCA0 membrane. High polymer aggregation among the CA chains in MWCA0 solution could influence the phase inversion process and causing the formation of nodules on the membrane. According to Kesting [32], membrane fabricated with higher molecular weight polymer grade resulted in better skin integrity. When MSG was present, membrane surface became rougher than the bare membrane regardless of the type of the heating process applied (Fig. 4). This may due to the chain scission phenomenon where the skin integrity of the membrane was affected. However, when comparing MWCA6 and TMCA6, it appeared that MWCA6 was much smoother than TMCA6. The MWCA6 prepared by using microwave heating produced low viscosity solutions which eased the casting process thus produced membrane with smoother surface (see Fig. 1). 3.3. Physical properties of membrane Table 6 illustrates the Tm values which were interpreted from DSC diagram in Fig. 5 for all the TMCA series membranes. These values obtained were in accordance with the cited literatures [34,35], which was approximately 230 8C. It was observed that the melting point for the TMCA series membranes depend solely on CA and not MSG (although MSG was present in the dope solution and has Tm of 232 8C). The results of the Tm values obtained are in line with the FTIR spectra in Fig. 6 which indicates the absence of N–H
Table 6 TGA and DSC result of TMCA and MWCA series membranes. Membrane ID
CA0 CA2 CA4 CA6 CA8
TM CA series
MW CA series
Tg (8C)
Tm (8C)
Tdeg (8C)
Activation energy (kJ/mol)
Tg (8C)
Tm (8C)
Tdeg (8C)
Activation energy (kJ/mol)
116.22 114.17 125.90 116.90 113.43
230.80 229.53 231.13 230.00 229.74
304.16 239.55 250.45 285.79 285.67
2.65 1.48 1.45 2.24 1.60
110.80 118.17 109.85 111.22 111.86
227.80 – – – –
267.66 279.64 285.57 291.61 286.74
1.69 1.96 1.87 2.11 2.00
C.M. Kee, A. Idris / Journal of Industrial and Engineering Chemistry 18 (2012) 2115–2123
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Fig. 4. AFM images of the membrane (a) TMCA0, (b) TMCA6, (c) MWCA0 and (d) MWCA6.
(2500–3100 cm1) and C–N (1350–1000 cm1) (which is usually present in MSG). MSG behaved only as a pore former and was washed away during phase inversion thus explained for its absence in the FTIR spectra. 90
Heat Flow Endo up (mW)
80 70 60 50 40 30
TM CA0 TM CA2 TM CA4 TM CA6 TM CA8
20 10 0 0
50
100
150
200
Tempeture (oC) Fig. 5. DSC diagram of TMCA series membranes.
250
300
FTIR spectra in Fig. 6 show the CA classical chemical bonds. Wave number between 3400 and 3700 cm1 represents the O–H stretching. Bands appear at 1739 and 1370 cm1 are assigned to C5 5O stretching and C–H symmetric deformation vibration of acetate group. Both 1232 and 1048 cm1 are attributed to C–O stretching, where the former is contributed by the acetate group while the latter is contributed by cellulose group [36]. The FTIR spectra analysis suggested that the MSG and MW irradiation technique did not cause any major effect on the CA chemical bonds. The connection between Tm and crystalline structure was well established in the literature [37]. In Table 6 (interpreted from DSC diagram in Fig. 7), there was a slight decrease in Tm value for the MWCA0 compared to TMCA0. This was probably attributed to the MW heating, where the irradiation probably disrupted the CA crystallinity up to a certain degree. The presence of MSG in the CA solution probably disturbed and disrupted the well arranged crystal structure in CA during the phase inversion. This effect might not occur in the TMCA series membranes because under normal heating the MSG was not able to disrupt the entanglement of long molecular weight of CA thus the crystalline structure remained intact and thereby the Tm was retained at 230 8C. In the case of MWCA2 to MWCA8 membranes, both MSG and MW irradiation disrupted the crystalline structure in CA and this was proven by the
C.M. Kee, A. Idris / Journal of Industrial and Engineering Chemistry 18 (2012) 2115–2123
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Fig. 6. FTIR spectra of TMCA0, MWCA0, TMCA6 and MWCA6 membranes.
absence of Tm in all the membranes. As observed in Table 6, MWCA2 to MWCA8 membranes has no Tm. The results of Tg for both TM and MW series membranes are qualitatively generalized in Table 6. Generally, TMCA series membranes exhibited higher Tg compared to MWCA series membrane except for TMCA2. The lower Tg values in MWCA series membranes again suggested the occurrence of chain scission which resulted in the shortened CA chains due to MW irradiation. The results seemed to be in agreement with the research revealed by Singh et al. [38] where they reported higher Tg was found for higher molecular weight polymer film. The abnormal point at TMCA2 may due to the thick skin layer, as described in Fig. 2g, which may reduce the free volume to a certain level as the value of Tg is always related to the free volume in the membrane [39,40]. Figs. 8 and 9 show the cellulose degradation process in the temperature region between 30 8C and 330 8C for the MWCA and TMCA series membranes respectively. The volatilization of the volatile compounds, main thermal degradation of cellulose acetate chains occurred at 330 8C to 450 8C and after this point,
120
carbonization of the products to ash started. In the TGA studies mentioned earlier it was found that the Tdeg of MWCA0 was much lower than TMCA0. Classical cellulose degradation steps, discussed by Chatterjee [41] were observed in both the TGA diagrams. As discussed in previous section, MW caused chain scission and thus shortened CA chains. The results seemed to be in agreement with the work done by Calahorra et al. [42] where higher molecular weight cellulose promised a better thermal stability property. When MSG was present, the thermal stability of MWCA series membranes was improved, but the additive contributed a negative impact on TMCA series membranes. According to Zulkifli et al. [43], thermal stability of a sample was very much dependent on its activation energy, where sample with high activation energy resulted in high thermal stability property. Thus, the results in Table 6 reveals that, the activation energy of the MWCA series membranes is improved by the presence of MSG but this effect seems to be suppressed in TMCA series membranes The activation energy of MWCA0 was 1.69 kJ/mol (the lowest in the MWCA series) while MWCA6 was 2.11 kJ/mol. For TMCA series membranes, TMCA0 exhibited the highest activation energy and it was 0.41 kJ/mol higher than TMCA6. Thus the presence of MSG and
MWCA0
MWCA4
80
MWCA0 MWCA2 MWCA4 MWCA6 MWCA8
100
MWCA6
Degradation (%)
Heat Flow Endo up (mW)
120
MWCA2
100
MWCA8
60
40
20
80 60 40 20
0 0
50
100
150
Temperature
200
250
(oC)
Fig. 7. DSC diagram of MWCA series membranes.
300
0
0
100
200
300
400
500
Temperature (oC) Fig. 8. TGA diagram MWCA series membranes.
600
700
C.M. Kee, A. Idris / Journal of Industrial and Engineering Chemistry 18 (2012) 2115–2123 [2] [3] [4] [5]
120 TM CA0
100
TM CA2
[6]
Degradation (%)
TM CA4 TM CA6
80
[7]
TM CA8
[8]
60 [9] [10]
40 [11] [12] [13]
20
[14]
0 0
100
200
300
400
500
600
700
Temperature (oC) Fig. 9. TGA diagram TMCA series membranes.
microwave irradiation method improved the thermal stability of the membranes and this was proven by the high activation energy of the membranes. 4. Conclusion
[15] [16] [17] [18] [19] [20] [21] [22] [23]
The influence of different heating methods on CA membrane separation performances and physical properties were systematically revealed. The use of MW irradiation method can reduce the dissolution time in CA membrane fabrication and thus saves energy and cost. Membranes produced by the MW irradiation exhibited better solute separation, higher UFR and smoother membrane surface, especially in the presence of microwave active material such as MSG. In addition, the concentration of MSG must be kept to 6% since further increment beyond this value did not seem to improve the uremic toxins separation. Since MW heating offers numerous advantages on the resultant membrane, especially when a MW active additive is present; the use of this MW apparatus to prepare polymeric solution in membrane industries will be a trend in the next few decades. Acknowledgement Financial support from the Ministry of Science, Technology and Environment through the FRGS funding (Vot no: 78673) is gratefully acknowledged. References [1] J. Jang, D.K. Lee, Polymers 45 (2004) 1599.
[24] [25] [26] [27]
[28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43]
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