Study on the influence of reaction time on the structure and properties of the PVDF membrane modified through the method of atom transfer radical polymerization.
Poly(vinylidene fluoride) (PVDF) membrane possesses many good performances, such as excellent chemical resistance, good thermal property, mechanical properties, unique ferroelectric, piezoelectric, dielectric properties, good biocompatibility, and excellent membrane forming capability, among others , It has been widely used in microfiltration and ultrafiltration processes, but its hydrophobic nature badly limits the potential application of PVDF membranes in aqueous solution separation and it does not have stimuli-responsive properties [2-5], Therefore, the surface properties modification of PVDF membrane is of crucial importance to its widespread applications. The incorporation of desirable functionalities onto PVDF surfaces can be accomplished via several modification methods, such as ozonization, plasma treatment, electron beam (EB) exposure, click reaction , and surface-initiated controlled radical polymerizations [6-8],
Surface-initiated controlled radical polymerization does not require strict experimental conditions and could obtain graft copolymers with well-controlled structures. As a result, we can use this method to modify the surface properties of PVDF membrane [9-13] and obtain a stimuli-responsive membrane.
In this work, a thermo-responsive membrane, PVDF-g-PNIPAAm, was successfully prepared from PVDF membrane through surface-initiated atom transfer radical polymerization (ATRP) of a thermo-responsive monomer N-isopropyl acrylamide (NIPAAm). The influence of the reaction time on ATRP was studied in detail. The chemical composition and surface morphology of the PVDF flat membrane before and after modification were determined by scanning electron microscopy (SEM), Fourier transform infrared (FTIR), and X-ray photoelectron spectroscopy (XPS). The thermal stability was characterized by differential scanning calorimetry (DSC). The hydrophilic property and antifouling property of PVDF-g-PNIPAAm membranes were characterized by contact-angle measurement and protein solution permeation experiments. The thermo-responsive permeability was measured by water fluxes experiment.
PVDF was purchased from Solvey (Brussels, Belgium). NIPAAm was obtained from Tokyo Chemical Industry (Tokyo, Japan), and used after further purification by recrystallization in hexane and toluene and then drying at room temperature. Copper (I) chloride (CuCl) was synthesized from Copper (II) chloride dihydrate (Cu[Cl.sub.2] x 2[H.sub.2]O) in laboratory . Tris(2-dimethylaminoethyl) amine ([Me.sub.6]TREN) was synthesized from tris(2-aminoethyl) amine (TREN) , All other reagents were analytical-grade and used without further purification.
Preparation of Membrane
PVDF flat membrane was prepared by phase inversion from a N,N-Dimethylformamide solution containing 16 wt% polymer. The solution of PVDF was cast and strickled onto a glass plate, and then immersed into pure water bath at 25[degrees], after the coat on the glass plate had been subjected to a short time of evaporation in the air. The membranes were thoroughly rinsed with distilled water to remove the residual solvent and freeze-dried in a lyophilizer (CHRIST company, ALPHA 1-2) for 6 h at 60 degrees below zero .
Surface-Initiated ATRP from the PVDF Membranes
For the preparation of PVDF-g-PNIPAAm membrane, one piece of the PVDF flat membrane (0.084 g), NIPAAm (0.672g), CuCl (15 mg), and [Me.sub.6]TREN (69 mg) were added to 20 mL ethanol . The reaction flask was sealed under a nitrogen atmosphere and kept in an 60[degrees]C oil bath for 12, 24, or 36 h. After the reaction, the PVDF membrane with surface-grafted PNIPAAm was removed from the reaction mixture and washed thoroughly with distilled water, to remove any residual monomer and PNIPAAm homopolymer.
The chemical composition of the PVDF membrane before and after modification was determined by FTIR and XPS, respectively. FTIR spectra were recorded on a Bruker Model Tensor 37 spectrophotometer. XPS data were obtained with an EDAX electron spectrometer from VG Scientific (Waltham, MA) using 300-W AlKa radiation. The binding energies were referenced to the C1s line at 284.6 eV from adventitious carbon.
Table 1 was the element content of the PVDF and PVDF-g-PNIPAAm membranes surface determined by XPS. The grafting degree of the PNIPAAm on the surface of PVDF membrane was calculated according to the following equation:
[X.sub.PNIPAAm] = [N] - 0.42/([N] - 0.42) + (1/2)[F] x 100% (1)
where [X.sub.PNIPAAm is the grafting degree of the PNIPAAm on the surface of PVDF membrane (%), [N] is the nitrogen element content of the PVDF-g-PNIPAAm membrane surface (%), 0.42 is the nitrogen element content of the PVDF membrane surface polluted by atmosphere (%), [F] is the fluorin element content of the PVDF-g-PNIPAAm membrane surface (%), respectively.
A contact angle instrument JYS-180 was used to measure static water contact angles of the membranes at 25[degrees]C and 60% relative humidity using a sessile drop method. For each angle reported, at least five sample readings from different surface locations were averaged. The angles reported were reliable to [+ or -] 2[degrees]. The surface morphology of the membranes was studied by SEM, using a Hitachi S-4300 electron microscope. DSC was performed on PVDF and PVDF-g-PNIPAAm membranes, its derivatives using a PERKIN ELMER 200F3 Pyris 1 calorimeter.
Measurements of the Thermo-Responsive Flux
Pure water was used to study the dependence of permeation rate on temperature. Flux experiments were carried out under a pressure of 0.1 MPa. During the measurement, the cell and permeating solution were kept in a thermostat water bath allowing the exact control of temperature. Flux (J, L/[m.sup.2]h) was calculated according to the following equation:
J = V/t x m (2)
where V is the volume of liquid (L), m is effective area of the membrane ([m.sup.2]), and t is time (h), respectively.
Protein Fouling Measurement
Bovine serum albumin (BSA) permeation experiments were performed at 25[degrees] under 0.1 MPa of trans-membrane pressure. The concentration of the BSA aqueous solution was 0.5 mg/mL. Flux was calculated according to Eq. 1. BSA solution flux ([J.sub.p]) was measured with the interval of 5 min, and [J.sub.0] was the beginning flux. The flux ratio [J.sub.p]/[J.sub.0] against filtration time was plotted to investigate the fouling resistance of modified membrane to protein.
RESULTS AND DISCUSSION
Characterization of the PVDF-g-PNIPAAm Membrane
To characterize the modified PVDF membrane, FTIR and XPS spectra were investigated. As shown in Fig. 1, compared with the pristine PVDF membrane substrates, the spectra of the PVDF-g-PNIPAAm membrane revealed the appearance of the absorption band at 3435, 1650, and 1541 [cm.sup.-1], which were attributed to the stretching of amido group, the stretching of amide carbonyl group and the bending of amido group, respectively. Besides, the absorption band became stronger gradually with the increase of reaction time. The FTIR results suggested that grafting of PNIPAAm on the PVDF substrates took place and the reaction time of 36 h was in favor of surface-initiated ATRP.
Figure 2 shows the XPS spectra of the PVDF membrane and PVDF-g-PNIPAAm membranes. The C 1s core-level spectrum of PVDF membrane can be curve-fitted with three peak components, with BEs at 284.8 eV for the neutral C-H species, 286.0 eV for the C[H.sub.2] (PVDF) species, and 291.1 eV for the C[F.sub.2] species. Two emissions at 284.6 and 687.0 eV, assigned to the binding energy of C 1s and F 1s, respectively, were indeed observed in the XPS spectrum of PVDF (Fig. 2a). For the PVDF-g-PNIPAAm membranes, the presence of distinctive N 1s signal and O 1s signal in the wide scan spectra of Fig. 2c, e, and g, because of the grafted PNIPAAm, indicated that NIPAAm had been successfully grafted on the PVDF membranes in the ATRP process. Moreover, two new peak components, with BEs at 286.9 eV for the N-C species (not disjoin with BE at 286.0 eV for the C[H.sub.2]) species and 288.1 eV for the N-C=0 species, can also be assigned to the grafted PNIPAAm. In addition, O 1s signal in the wide scan spectra became stronger gradually from Fig. 2c-g. The graft concentration of the PNIPAAm on PVDF membrane can be derived from relative ratio of N-C=0 peak component to the C[F.sub.2] peak component, namely graft concentration, which was depended on the reaction time. The sequence of the reaction time giving rise of higher grafting concentration was as 36 > 24 > 12 h.
The Grafting Degree of the PNIPAAm on the Surface of PVDF Membrane
Figure 3 was the grafting degree of the PNIPAAm on the surface of PVDF membrane with the reaction time. It can be observed that [X.sub.PNIPAAm] was increased with the increase in the reaction time. When the reaction time was 36 h, the grafting degree of the PNIPAAm on the surface of PVDF membrane could reach to 33%. As we know ATRP is controllable reaction. So [X.SUB.PNIPAAm] was increased with the increase in the reaction time on ATRP.
Figure 4 shows clearly that compared to the pristine PVDF membrane whose water contact angle was changed from 95[degrees] to 87[degrees] in 600 s, the water contact angle of PVDF-g-PNIPAAm membrane descended obviously. When the reaction time was 36 h, the water contact angle decreased to 50[degrees] after 600 s. It was indicated that the hydrophilicity of the grafted membrane was improved, and with the reaction time increased , the hydrophilicity of the modified membrane improved gradually. It corresponded to the results of XPS, and the reaction time of 36 h was in favor of surface-initiated ATRP.
Structure and Surface Morphology
To determine the structure and surface morphology of PVDF membranes grafted with PNIPAAm, SEM was used. Figure 5 shows SEM images of the pristine hydrophobic PVDF membrane and the PVDF-g-PNIPAAm membranes modified by direct surface-initiated ATRP of NIPAAm with the reaction time 12, 24, and 36 h. It can be observed that the hydrophobic PVDF membrane showed relatively high porosity and big pore size , but the modified membranes have grown thicker and the pore sizes have become smaller with the increase of the reaction time.
Figure 6 summarizes the thermal stability of the pristine membrane and PVDF-g-PNIPAAm membranes investigated by DSC. The pristine membrane showed a main melting peak at 169[degrees]. The position of the melting peak of the PVDF-g-PNIPAAm membrane was almost the same as that of the PVDF membrane. It illuminated that both the PVDF membrane and the PVDF-g-PNIPAAm membranes have good thermal stability. Besides, the melting temperature became a little lower with the increase of the reaction time.
Permeability and Protein Solution Permeation
Grafting PNIPAAm chains on the PVDF membrane imparted thermo-response to the membranes. To confirm the thermo-responsive properties, the flux of water through the PVDF-g-PNIPAAm membranes was investigated at the temperature from 20[degrees]C to 44[degrees]C. The water flux curves were showed in Fig. 7. It can be observed that the water flux of the pristine hydrophobic PVDF membrane was very low and had little change with the increase in temperature. For the PVDF-g-PNIPAAm membrane, the water flux was improved greatly. With the increase in temperature, the water flux showed a sharply increase at about 32[degrees]C. Especially, when the reaction time was 36 h, the water flux of PVDF-g-PNIPAAm membrane was 159.97 L/[m.sup.2] h at 20[degrees]C, as compared with that of 206.92 L/[m.sup.2] h at 44[degrees]. The reaction time for ATRP played a vital role in affecting the water flux. With the increase of the reaction time, the water flux of the modified membranes increased because of the existence of more hydrophilic PNIPAAm on the PVDF membrane surface.
The huge variation in permeation in response to the temperature can be attributed to the change in structure of the membrane grafted with PNIPAAm chains. When the permeating temperature was below the LCST of the PNIPAAm (around 32[degrees]), the PNIPAAm chains on the pore surfaces can be assumed a highly extended conformation, leading to a reduction in the effective pore size. Therefore, the water flux was small. On the other hand, at temperatures above the LCST, the PNIPAAm can be assumed a more compact or collapsed conformation, resulting in the opening of the pores of the membrane . As a result, the water flux became higher than that at the permeating temperature below the LCST . Especially, when the permeating temperature changed in LCST, the water flux suddenly increased. This illuminated that the PVDF-g-PNIPAAm membranes exhibited good thermo-responsive permeability.
To investigate antifouling property of the PVDF and PVDF-g-PNIPAAm membranes, a BSA filtration experiment was conducted. As shown in Fig. 8, it was found that the pristine PVDF membrane exhibited a dramatic decline of flux with increasing in filtration time, and the relative flux ratio [J.sub.p]/[J.sub.o] declined to 50% for 30 min. However, to the PVDF-g-PNIPAAm membrane, with the increase of the reaction time, [J.sub.p]/[J.sub.o] increased gradually. In other words, the variation in flux for the reason of membrane fouling was improved for the increasing in the grafting of PNIPAAm. To the PVDF-g-PNIPAAm membrane (the reaction time was 36 h), the [J.sub.p]/[J.sub.o] can remained 85% after 30 min filtration time. Those results indicated that the modified membranes exhibited excellent antifouling property.
PVDF-g-PNIPAAm membrane was successfully prepared by the modification of PVDF membrane with thermo-responsive monomer NIPAAm through the method of surface-initiated ATRP. The pore sizes of the PVDF-g-PNIPAAm membrane were reduced for the grafting of PNIPAAm, and the hydrophilicity of the modified membranes was improved. The PVDF-g-PNIPAAm membranes have good thermal stability, thermo-responsive permeability, and antifouling property, and with the increase of the reaction time, the hydrophilicity, thermo-response, and antifouling property were all improved.
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Yiping Zhao, Haiyang Zhao, Kaipeng Zhou, Guifang Zhang, Li Chen, Xia Feng
State Key Laboratory of Hollow Fiber Membrane Materials and Processes, School of Material Science & Engineering, Tianjin Polytechnic University, Tianjin 300387, People's Republic of China
Correspondence to: Y. Zhao; e-mail: firstname.lastname@example.org or L. Chen; e-mail: email@example.com
contract grant sponsor: the National Science Foundation of China; contract grant number: 21074091, 21174103, and 31200719; contract grant sponsor: the project of Science and Technical Development of China contract grant number: 2007AA03Z533; contract grant sponsor: the Science Foundation of Tianjin contract grant number: 12JCYBJC11200; contract grant sponsor: the Key Grant Project of Chinese Ministry of Education contract grant number: 209005.
TABLE 1. The element content of the PVDF and PVDF-g-PNIPAAm membranes surface. Membrane Cls (%) Nls (%) Ols (%) FIs (%) PVDF 45.93 0.42 2.16 51.49 12 h 59.18 3.02 12.91 24.89 24 h 64.63 3.74 13.67 17.96 36 h 65.94 4.35 15.68 14.03
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|Author:||Zhao, Yiping; Zhao, Haiyang; Zhou, Kaipeng; Zhang, Guifang; Chen, Li; Feng, Xia|
|Publication:||Polymer Engineering and Science|
|Date:||May 1, 2014|
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