Microporous polypropylene hollow fiber membrane

Journal of Membrane Science 196 (2002) 221–229

Microporous polypropylene hollow fiber membrane Part I. Surface modification by the graft polymerization of acrylic acid Zhikang Xu∗ , Jianli Wang, Liqiang Shen, Dongfeng Men, Youyi Xu Institute of Polymer Science, Zhejiang University, Hangzhou 310027, PR China Received 1 February 2001; received in revised form 16 July 2001; accepted 16 July 2001

Abstract Surface modification of microporous polypropylene hollow fiber membranes was performed by graft polymerization of acrylic acid. The effects of temperature, polymerization medium, monomer concentration and multifunctional cross-linker on the graft polymerization were studied. The grafting degree on the polypropylene membrane can be greatly increased upon the addition of divinylbenzene (DVB). Scanning electron microscopy (SEM) pictures demonstrated that PAA grafted mainly on the surface of the membrane. Mercury porosimetry analysis revealed that the grafting also took place within the pores. Contact angle and absorbed water measurements showed excellent wetting properties with water for the grafted membranes. The grafted membrane did not show high flux for ultrapure water because the grafting of AA plugs the pores of the membrane. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Surface modification; Graft polymerization; Hydrophilicity; Polypropylene hollow fiber membrane

1. Introduction The importance of surface chemistry in determining the separation characteristics of a membrane is well-recognized [1–4]. In recent years, there has been much interest in developing surface treatments to alter the chemical and physical properties of membrane surfaces. For microfiltration and ultrafiltration processes, it is well known that the major drawback in the extensive use of membranes is membrane fouling, which results in flux reduction during operation. In the case of fouling caused by the adsorption of proteins on the membrane, the hydrophobic interaction between membrane surfaces and proteins has been considered ∗ Corresponding author. Fax: +86-571-7951773. E-mail address: [email protected] (Z. Xu).

a dominating factor. With respect to the prevention of membrane fouling, hydrophilic membranes are normally favored. Hydrophilic polymer membranes, however, are susceptible to chemical and thermal impacts in their applications because such materials readily adsorbed water and the membrane characters can be changed dramatically by the adsorbed water. Different methods such as hydrophilizing agents coating, plasma modification, corona discharge and flame treatment [5] can be applied to modify membrane surfaces, e.g. to increase or to reduce hydrophilicity, adsorption of molecules, ionic change, biocompatibility, etc. without affecting bulk properties. One of the simplest modification methods is to treat the membranes with hydrophilizing agents such as alcohols, surfactants, polyelectrolyte complexes, or a coating with hydrophilic compounds. However, it is difficult

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to sustain the initial hydrophilicity. The micropores of the alcohol wetted membranes maintain hydrophilicity during immersion in water, but once the membrane is dried, the hydrophilicity is instantly lost. In the case of surfactant impregnated membranes, the hydrophilicity decreases with time as the surfactant tends to leak out from the pores. Although these techniques are extremely simple, they do not impart permanent hydrophilicity to the membranes because the surfactants adsorb only physically and do not bind chemically to the surface of the membrane. In the past 10 years, more complex and advanced technologies, such as surface grafting [6–14], pore filled grafting [15,16], coating/interfacial polymerization [17] and in situ polymerization [18], are being used to modify the surface properties of membranes. Among these methods, the graft polymerization is one of the most efficient methods to control the polar monomer onto the membrane surface and to modify durably the surface properties of membranes. In graft polymerization, active sites were first introduced to the membrane surface by glow-discharge [6] ozone treatment [7], ␥-radiation [8], UV irradiation [9], plasma treatment [10,12,13], free radicals [11] or electron beam radiation [14,15], and then initiate monomer for polymerization. Microporous polyolefin membranes have many desirable properties including very high void volumes, well-controlled porosity, and chemical inertness, where the base polyolefin is a bulk polymer such polypropylene or polyethylene, these membranes are also potentially of low cost. In our previous papers [19,20], the microporous PP hollow fiber membranes were prepared by a melt-spinning/cold-stretching (MSCS) technique, and these hydrophobic membranes were successfully applied for the separation of NH3 from wastewater and CO2 from N2 /CO2 gas mixture. However, because PP membranes are relatively hydrophobic and non-wettable with water, the potential application in aqueous fluid is limited. Therefore, there is much interest for us in the modification of the membranes to improve hydrophilicity and to functionalize surfaces. The primary objective of this project is to modify the surface of PP hollow fiber membranes by graft polymerization, resulting in a more hydrophilic interface with aqueous or a more compatible interface with biological fluids. Acrylic acid, N-carboxyanhydride (NCA) of ␣-amino acid, vinyl and epoxypropyl monomer with sugar group are

selected as grafting monomers for this purpose. In this paper, the graft polymerization of acrylic acid, initiated by free radical, on the surface of PP hollow fiber membranes was investigated. Acrylic acid was used as a hydrophilic monomer for the advantages of its ionic character and its gelling capacity as a polymer.

2. Experimental 2.1. Material Microporous polypropylene hollow fiber membranes were prepared with melt-extruded/cold-stretched (MECS) method in our lab. The inner and outer diameters of this hollow fiber were 240 and 290 ␮m, respectively, with porosity of 50% and an average pore diameter of 0.070 ␮m. Acrylic acid and benzoyl peroxide (BOP) were both purchased from Aldrich and used for grafting without purification. Divinylbenzene (DVB) was used as co-monomer to form partial cross-link in some of the reactions. Toluene, dichloroethane, and ethanol (all analytical reagents) were commercial products and used without further purification. Ultrapure water with a conductivity of 18 S/cm was used as polymerization medium with ethanol and for water permeability measurements. 2.2. Procedures of graft polymerization To remove any chemicals adsorbed on the membrane surface, the nascent PP hollow fiber membranes were washed with acetone, two washes 3 min each, and dried in a vacuum oven at room temperature for 24 h before initial mass was determined. Monomer solution of acrylic acid with various concentration (from 10 to 70 wt.%) and benzoyl peroxide (0.5 mol% of acrylic acid) in 50 ml toluene were mixed together after first dissolving the initiator in a small amount of toluene. The mixture solution was purged by nitrogen gas for 5 min and dropped into one glass flask in nitrogen atmosphere, in which the membrane was put on the bottom. Then, the flask was placed in an oil bath at a temperature of 60–75◦ C and was shaken for 4 h with a shaking speed of 150 rpm. Then, the AA-grafted hollow fiber was removed from the reaction flask and washed in 5% NaOH water/ethanol (50/50, v/v) solution and extracted in a Soxhlet

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extractor using methanol as solvent for 24 h to remove any residual monomer and polymer. The hollow fiber was then washed with large amount of water and dried under vacuum and weighed. The degree of AA grafting (Dg ) was defined as Dg (%) =

(W1 − W0 ) × 100 W0

where W1 is the weight of grafted, extracted and dried membrane and W0 the weight of membrane before grafting. For grafting polymerization at 70◦ C in toluene, DVB was utilized as a cross-linking agent. The procedure replicated the one described previously with the addition of DVB. The amount of DVB in the monomer solution was 5, 10 and 20 wt.% of acrylic acid. 2.3. Structure analysis and properties measurement The density of each hollow fiber membrane was measured at 25◦ C using a density gradient column. The surfaces of the nascent and the grafted PP membrane were analyzed by a Bruck 22 FT-IR. The measurements were performed by using ZnSe crystal as an attenuated total reflection accessory. The outer surface of the hollow fiber membranes were also examined using a scanning electron microscope (Hitachi S-570). For this examination, membrane specimens were dried at low vacuum for 10–12 h before sputter coating with gold (15–20 nm thick). The porosity and size of the pores were measured using a mercury porosimeter. A weighed amount of hollow fiber membrane was introduced into the chamber filled with mercury. Then the pressure was increased to fill the micropores of the membrane with mercury. By monitoring the volume change of mercury and the corresponding pressure, the pore size and porosity of the membrane were obtained. The contact angle of the PP membrane was measured to quantify the change in its hydrophilicity with Chengde JY-82. The membrane sample was placed on a trestle, a drop of ultrapure water was introduced on the membrane at ambient temperature (25◦ C) and humidity. The contact angles reported here were measured when the water drop moved over the dry surface, the so-called advancing contact angle. The hollow fiber membrane about 20 cm long was positioned in a U-configuration. Ultrapure water was

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forced to permeate through the pores radial outwards at a constant permeation pressure of 0.02 MPa. The flow rate of the water penetrating the outside surface of the hollow fiber was measured, and the pure water flux (PWF) was calculated by dividing the flow rate by the inside surface area of the hollow fiber (Q, l/m2 h). The measurements were performed at 25◦ C. PAA-grafted membranes were immersed in ultrapure water at 25◦ C for 24 h. After the treatment, excess water on the membrane outer surface was wiped by filter paper and then the weight of the treated membrane was measured. Absorbed water of the grafted membrane was defined as follows: absorbed water(%) =

(W2 − W1 ) × 100 W1

where W2 and W1 are the weights of the starting and treated membrane, respectively.

3. Results and discussion 3.1. Graft polymerization It is well known that homopolymerization always competes with graft polymerization. Therefore, how to control the reaction conditions is the key to get membranes with suitable surface modification. Factors that influence graft polymerization were studied, respectively. The effect of temperature on the graft polymerization was studied at the concentration of 20, 40 and 60 wt.% AA in toluene. As can be seen from Fig. 1, with the increase polymerization temperature from 60 to 70◦ C, the amount of grafted PAA on the membrane surface increased gradually. The increase of grafting degree at the concentration of 60 wt.% AA is more remarkable than those of 20 and 40 wt.% are. However, the grafting degree seemed to reach a maximum near a temperature of 70◦ C. By raising the temperature further from 70 to 75◦ C, the grafting degree decreased slightly. This behavior is expected because raising temperature increase the number of free radicals generated by the thermo-decomposition of the initiator, which results in more reactive sites on the membrane surface. Another effect is that high temperature enhances the diffusion of the monomer to the active site for polymerization. On the other hand, for the polymerization with high monomer

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Fig. 1. Effect of temperature on the graft polymerization of AA in toluene.

Fig. 2. Relationship between the grafting degree and AA concentration in different medium at 70◦ C.

concentration, the viscosity of solution was increased rapidly because the gel effect might be generated. For the inhomogeneous reaction system, high viscosity hinders the diffusion of monomer from bulk solution to membrane surface, which resulted in the decrease of grafting yield. However, the exact reason for the substantial decrease of grafting degree at 75◦ C is unknown at this time and more detail work needs to be done to interpret this point. Fig. 2 illustrate the effect of solvents and monomer concentration on the grafting degree of the hollow fiber membranes at 70◦ C. The increase of monomer concentration increased the grafting degree. This figure shows that the grafting degree of polymerization in toluene and dichloroethane is higher than that of polymerization in ethanol. This is thought to be related to the different solubility of poly(acrylic acid) (solubility reference δ > 10 cal1/2 /cm3/2 ) in toluene (δ = 8.9 cal1/2 /cm3/2 ), dichloroethane (δ = 9.1 cal1/2 /cm3/2 ) and ethanol (δ = 12.7 cal1/2 /cm3/2 ) [21]. It could be presumed that PAA would be relatively easily dissolved by ethanol compared with more hydrophobic solvent toluene and dichloroethane. It was expected that the viscosity of former solution is

higher than that of late solution. It would be faster for the diffusion of monomer from bulk solution to the interface of membrane when the viscosity of the solution is low. Consequently, the polymerization rate, which related with the yield of grafting PAA on the membrane, will be higher when toluene or dichloroethane was used as solvent. From the comparison of the results mentioned above and the economic or environmental arguments, toluene seems to be the best one for the graft polymerization of acrylic acid in this study. A certain amount of a multifunctional cross-linker added to the reaction was found to be more preferable in graft polymerization. Table 1 and Fig. 3 show the influence of DVB on the degree of graft polymerization. The grafting degree on the polypropylene membrane can be greatly increased upon the addition of DVB, especially for the polymerization at high AA concentration. For example, the grafting degree of polymerization at the concentration of 70% AA and 5% DVB is 67.9%, meanwhile that of polymerization at 70% AA only is 44.1%. The former is 1.5 times of the later. This result can be ascribed to the co-polymerization of AA and DVB. In our cases, the total monomer

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Table 1 Typical results for the modification of PP hollow fiber membranes by the graft polymerization of acrylic acid DVB (%)

Real density (g/cm3 )

Sample no.

Monomer concentration (%)

Dg

I-1 I-2 I-3 I-4 I-5 I-6 I-7

10 20 30 40 50 60 70

0 0 0 0 0 0 0

2.90 7.00 11.0 17.5 22.6 36.8 44.1

± ± ± ± ± ± ±

0.03 0.05 0.02 0.1 0.8 0.3 0.9

0.7789 0.7800 0.7843 0.7900 0.8218 0.8616 0.9218

± ± ± ± ± ± ±

8 3 5 6 6 9 5

× × × × × × ×

10−4 10−4 10−4 10−4 10−4 10−4 10−4

39 38 35 27 24 – –

II-1 II-2 II-3 II-4 II-5 II-6 II-7

10 20 30 40 50 60 70

5 5 5 5 5 5 5

3.10 5.85 12.7 21.3 31.3 42.3 67.9

± ± ± ± ± ± ±

0.02 0.04 0.7 0.5 0.3 0.8 0.7

0.7842 0.7883 0.8318 0.8927 0.9453 0.9645 1.0721

± ± ± ± ± ± ±

9 8 8 7 3 7 6

× × × × × × ×

10−4 10−4 10−4 10−4 10−4 10−4 10−4

37 ± 2 29 ± 1 21 ± 1 – – – –

0.036 ± 0.008 0.034 ± 0.004 0.024 ± 0.002 – – – –

III-1 III-2 III-3 III-4 III-5 III-6 III-7

10 20 30 40 50 60 70

10 10 10 10 10 10 10

3.35 11.2 16.7 24.3 40.0 57.8 80.7

± ± ± ± ± ± ±

0.02 0.4 0.3 0.1 0.4 0.8 0.3

0.7863 0.8074 0.8568 0.9093 0.9480 1.3186 1.3778

± ± ± ± ± ± ±

9 7 7 9 4 2 7

× × × × × × ×

10−4 10−4 10−4 10−4 10−4 10−4 10−4

36 ± 4 23 ± 3 – – – – –

0.037 ± 0.005 0.026 ± 0.003 – – – – –

IV-1 IV-2 IV-3 IV-4 IV-5 IV-6 IV-7

10 20 30 40 50 60 70

20 20 20 20 20 20 20

3.67 13.4 26.5 33.7 45.4 63.0 87.1

± ± ± ± ± ± ±

0.04 0.2 0.3 0.3 0.4 0.7 0.9

0.7928 0.8406 0.9233 0.9561 1.1273 1.3564 1.4012

± ± ± ± ± ± ±

9 7 9 5 3 2 5

× × × × × × ×

10−4 10−4 10−4 10−4 10−4 10−4 10−4

33 ± 2 15 ± 1 – – – – –

0.039 ± 0.005 0.023 ± 0.007 – – – – –

concentration for the co-polymerization is higher than the AA homopolymerization. 3.2. Characterization and properties After graft polymerization, a thin grafting layer of PAA was generated on the surface of the PP hollow fiber membrane, which can be confirmed by the FT-IR–ATR spectroscopy analysis. As can be seen from Fig. 4, there is no absorption peak around 1710 cm−1 in the spectrum of the nascent membrane. One strong vibration peak can be seen at 1710 cm−1 for the spectrum of the grafted membrane, which can be attributed to the stretch of the C=O groups. Due to the stretch of the hydrogen bonded with OH groups, a distinct absorption peak can also be seen at 3100 cm−1 for the modified membranes.

Average porosity (%) ± ± ± ± ±

4 3 3 2 2

Average pore size (␮m) 0.038 0.037 0.036 0.034 0.026 – –

± ± ± ± ±

0.005 0.003 0.005 0.006 0.003

SEM was employed to examine the surface structure of certain modified membranes. The SEM pictures showed that the membrane surface became smoother by the graft polymerization (Fig. 5). It also looked like that the surface porosity was somewhat reduced because poly(acrylic acid) chains covered the original surfaces of the PP membrane. The membrane modified with 4.4 wt.% PAA is shown in Fig. 5(b). Comparing with the micrograph of the unmodified membrane (Fig. 5(a)), it is obviously that some pores become smaller and more round in shape. The micrograph of the membrane modified with 36 wt.% acrylic acid (Fig. 5(c)) indicates that there is an extensive covering of the surface with grafted polymer. Due to the grafting of the membranes with PAA, as can be seen from Table 1, the density of the modified membranes increases with the grafting degree.

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Fig. 3. Effect of divinylbenzene on the grafting degree in toluene at 70◦ C.

Fig. 4. ATR FT-IR spectra of (a) nascent PP membrane and (b) grafted membrane with Dg of 4.4 wt.% PAA.

Fig. 5. Scanning electron microscopy pictures of outer surface for the PP hollow fiber membrane: (a) original membrane; (b) grafted membrane at 60◦ C with Dg of 4.4 wt.% PAA; (c) grafted membrane at 60◦ C with Dg of 36.9 wt.% PAA.

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Fig. 7. Pore size distribution measured by mercury porosimetry.

Fig. 6. Effects of grafting degree on the properties of the grafted membranes: (a) average pore size (Rav ) calculated from the pore diameter determined by mercury porosimetry; (b) the cosine value of contact angles between membrane surface and water at 25◦ C; (c) absorbed water at 25◦ C.

On the other hand, the porosity as well as the average pores diameter, which were obtained from mercury porosimetry measurements, decreases sharply and then gradually with the increase of grafting degree (Fig. 6). The density of modified membranes with DVB/AA mixed monomer is higher than that modified with only AA, because of higher grafting degree at the same monomer concentration. The porosity and the average pore size equal zero when the grafting degree is above 25%. A mercury porosimetry comparison study was plotted in Fig. 7 for the virgin membrane and a modified sample with 4.4 wt.% PAA. It is worth noting that the pore size is bimodal distributed for the modified PP membrane in the range of 0.01–0.50 and 0.50–0.11 ␮m. The virgin membrane contains a single

distribution around 0.075 ␮m. Therefore, the grafting membrane has pores that are smaller than those of the virgin sample does, which is in agreement with the SEM results. The measurement of the contact angle between ultrapure water and a membrane surface is one of the easiest ways to characterize the hydrophilicity of a membrane surface. When water is applied to the surface, the outermost surface layers interact with the water. A hydrophobic surface with low free energy gives a high contact angle with water, whereas a wet high-energy surface allows the drop to spread, i.e. gives a low contact angle. The value obtained for the grafted membranes are graphically represented in Fig. 6(b). The measurements were carried out immediately after the drying of the modified membranes. It was expected that the water contact angle for the grafted membranes would be smaller, due to high surface energy by the introduction of highly polar carboxy groups onto the membrane surface. As can be seen from Fig. 6(b), the contact angle of the grafted membranes decreased substantially compared to the virgin membrane. The grafted membranes that had been grafted for more than 60% PAA were wetted completely, supporting the fact that the membrane surface became highly hydrophilic with grafting PAA. The hydrophilicity of membrane surface can also be

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Table 2 Water flux at 25◦ C and 0.2 atm Sample

Water flux (l/m2 h)

Unmodified Treated with ethanol Modified with 2.95% of AA Modified with 5.85% of AA/DVB Modified with 11.0% of AA

0.00 48.88 4.32 1.60 0.652

± ± ± ±

0.06 0.02 0.03 0.007

characterized by the determination of absorbed water. The membranes grafted with a higher percentage of AA had excellent wettability and appeared translucent when immersed in water. The absorbed water increase of the membranes after graft polymerization is shown in Fig. 6(c). The absorbed water of the membrane with grafting was increased by 10–30% with increasing the grafting degree from 4.4 to 22.5 wt.% (Table 2). The measurements of pure water permeability were carried out for several modified membranes. This process was repeated 4 times for each membrane module. Unmodified membrane has not the ability to let water flow across when it was not treated with alcohol. Flux experiment indicates that membrane modified with AA is stable enough. However, the water flux was considerably low. This could be explained on the basis that the grafting of AA plugs the pores of the membrane, and hence, reduces the effective free cross-section for the water flow. PAA grafted into the pores will also swell within the pores and subsequently reduce the free flow of the water expected, by reducing the effective area of the porous volume of the membrane.

4. Conclusions In this paper, microporous polypropylene hollow fiber membranes were grafted with acrylic acid for surface modification. Factors that influence graft polymerization were studied, respectively. It was found that the grafting degree reached a maximum near a temperature of 70◦ C. Toluene seems to be the best medium for the graft polymerization of acrylic acid in this study. The increase of monomer concentration increased the grafting degree. A certain amount of a multifunctional cross-linker added to the reaction was found to more preferable in graft polymerization. The grafting degree on the polypropylene membrane can

be greatly increased upon the addition of DVB. SEM pictures and mercury porosimetry analysis demonstrated that the PAA could be grafted mainly on the surface of the membrane. Contact angle and absorbed water measurements showed excellent wetting properties with water for the grafted membranes. However, the grafted membrane did not show high water flux because the grafting of AA plugs the pores of the membrane.

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