Influence of Ultrasonication Conditions on the Structure and

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Influence of Ultrasonication Conditions on the Structure and Performance of Poly(vinylidene fluoride) Membranes Prepared by the Phase Inversion Method Ting Qu,† Kai Pan,*,† Li Li,‡ Bin Liang,† Lei Wang,† and Bing Cao*,† †

Key Laboratory of Carbon Fiber and Functional Polymers, Ministry of Education, Beijing University of Chemical Technology, Beijing, 100029, China ‡ Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14850, United States ABSTRACT: The morphology and crystallization properties of poly(vinylidene fluoride) ultrafiltration membranes prepared by the phase inversion method were investigated under ultrasound-assisted conditions in a coagulation bath. All the membranes showed improved performance compared with that of the pristine membrane, including preferable morphology, higher porosity, and higher flux for water and a Rose Bengal (RB) aqueous solution. With an increase in the ultrasonic intensity, the flux of membranes for both water and RB aqueous solution increased gradually, while the rejection by all membranes was nearly 80%. The porosity was measured by the percentage water content. Phase separation was accelerated and macrovoids developed with an increase in ultrasonic intensity, resulting in an improvement in membrane porosity. The crystalline structure was assessed by wide-angle X-ray diffractometry and attenuated total reflectance Fourier transform infrared spectroscopy. It was also shown that the β crystalline phase was partly converted to the α crystalline phase in the ultrasound-assisted phase inversion process. Membranes prepared by ultrasound-assisted phase inversion exhibited elongation greater than that of the pristine membrane. be obtained. However, the introduction of an ultrasonic field in the coagulation bath to assist phase inversion for membrane preparation has been rarely reported. Poly(vinylidene fluoride) (PVDF) is one of the most widely used and thoroughly researched membrane materials. PVDF membranes with excellent physicochemical properties are urgently needed. Several studies on the crystalline structure of porous PVDF membranes prepared by the phase inversion method have been reported.23−26 The crystal structure of membranes determines their properties and applications in different fields. Understanding the factors for the control of crystallization evolution is therefore a great necessity. The β phase of PVDF membranes is reported to be piezoelectric27 and has the potential to adsorb proteins on the membrane surface, which can cause the fouling of membranes. In addition, the density of the α phase (1.92 g/cm3) is lower than that of the β phase (1.97 g/cm3). A high percentage of the α phase may lead to loosely arranged polymer chains in the PVDF membrane and thus higher water flux. These two phases can be converted into each other by appropriate thermal, mechanical, or electric treatments.28−31 As the precipitation temperature is elevated, the membrane demonstrates reduced total crystallinity and a much lower β/α phase ratio in the surface layers.32 To the best of our knowledge, the role of different ultrasonic frequencies and powers in the phase inversion process of PVDF membrane preparation has not been studied. In this paper, a series of PVDF membranes were prepared in an ultrasoundassisted coagulation bath under various ultrasonication

1. INTRODUCTION Membrane separation technique plays an important role in wastewater treatment. Nowadays, the most common method of preparing membranes is nonsolvent phase inversion. The factors affecting membrane performance have been studied extensively, such as the composition of the casting solution,1−7 precursor preparation,8,9 evaporation time,1,10 the harshness and temperature of the coagulation bath,11−14 and outfield assistance. Recently, outfield assistance has been used as a new way to control the phase inversion process, in which a specific field (such as a magnetic field, electric field, ultrasonic field,15 etc.) is introduced in the coagulation bath. It is well-known that ultrasound can induce a wide range of chemical and physical consequences. Energy is transmitted by the vibration of the molecules in the environment where the wave is spread.16 In ultrasonic waves, cavitation is the phenomena responsible for the formation, growth, and collapse of microbubbles or cavities occurring in extremely small time interval (milliseconds) in a liquid.17 Ultrasonic cavitation serves as the primary mechanism for sonochemical effects, where bubble collapse produces intense local heating (3000−5000 K), high pressures (500−10,000 atm), microjets, turbulence, and acoustic streaming.18−20 The influence of ultrasound includes enhanced mass transfer, emulsification, bulk thermal heating, and various effects on solids. Diverse applications of ultrasound have been explored. Ultrasound is usually used in separation membrane cleaning,21,22 and it has the potential to assist the mass transfer between the solvent and nonsolvent, crystallization growth, and structure evolution during phase separation of polymeric membranes to optimize their physicochemical properties. The thermodynamic and kinetic rates of the system can be controlled during the nonsolvent phase inversion process so that the desired asymmetric membrane structure can © 2014 American Chemical Society

Received: Revised: Accepted: Published: 8228

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conditions. The influence of ultrasonic intensity on the morphology, crystal structure, and permeability of PVDF membranes was analyzed.

Table 1. Processing Conditions of PVDF Membranes

2. EXPERIMENTAL SECTION 2.1. Materials. Commercial PVDF (Solef 1015; Mn = 238,000; Mw = 573,000; Tm=173 °C) was purchased from Solvay Solexis Chemical Reagent and dried at room temperature in an oven at 25 °C for 12 h prior to use. Reagent grades of N-methyl-2-pyrrolidone (NMP), isobutanol, and Rose Bengal (RB; Mn = 1060) were purchased from Sinopharm Chemical Reagent Beijing Co. Ltd. and Sigma-Aldrich. Molecular sieves (5 Å, Sinopharm Chemical Reagent Beijing Co. Ltd.) were dried at 300 °C in a muffle furnace for 4 h prior to use. All these compounds were purified and dried before use. 2.2. Preparation of Membrane. The casting solution precursors were prepared by dissolving 15 wt % PVDF in NMP at 50 °C with a stirring rate of 10 rpm for 12 h. The obtained solution was suction filtered and then stored in a clean Erlenmeyer flask for 24 h to remove air bubbles. The glass plate and casting knife were thoroughly cleaned and kept in an oven at 80 °C for 20 min to minimize the effect of temperature differences on phase inversion before use. The PVDF/NMP solution was cast at room temperature on the surface of a glass plate with the formation of a film 150 μm in thickness. The glass plate was then immediately immersed in an ultrasonic bath (45/80/100 kHz; 120−300 W; KQ-300VDE, Shumei, Kunshan, China) for 20 min for the phase inversion process. After being processed, the membranes were washed with deionized water three times and then vacuum-dried for 12 h at room temperature before testing. The ultrasound-assisted phase inversion process is shown in Figure 1, and the corresponding processing conditions for membranes are summarized in Table 1. In the power group, the ultrasonic powers were 120, 180, 240, and 300 W for A-1, A-2, A-3, and A-4, respectively, with a fixed ultrasonic field frequency (45 kHz). In the frequency group, the ultrasonic frequencies were 45, 80, and 100 kHz for

serial number

frequency (kHz)

power (W)

time (min)

A-0 A-1 A-2 A-3 A-4 A-5 A-6

0 45 45 45 45 80 100

0 120 180 240 300 300 300

0 1 1 1 1 1 1

A-4, A-5, and A-6, respectively, with a fixed ultrasonic field power (300 W). 2.3. Characterization of Membrane. The cross-sectional morphology of the membranes after freeze-fracturing in liquid nitrogen was examined with a Hitachi 4700 scanning electron microscopy (SEM) instrument at an accelerating voltage of 20 kV. A thin layer of palladium was sputtered on the membrane surface to avoid charging before measurement. The crystalline structure of the membranes was assessed using a Bruker D8 advanced wide-angle X-ray diffractometry (WAXD) with Cu Kα monochromatic radiation (λ = 0.15406 nm). The analysis was performed at 40 kV and 40 mA over a 2θ range of 5−60° with a step size of 0.02° and a count time of 4 s per step. The infrared spectra of the materials were obtained by attenuated total reflection Fourier transform infrared (ATRFTIR) spectroscopy on a PerkinElmer Spectrum RX-I instrument with 4 cm−1 resolution. Each sample was scanned 20 times. The penetration depth d can be estimated as follows.32 1 d= 2 2πncσ(sin θ − nx 2)1/2 (1) where θ is the incident angle of light on the ATR element (zinc selenide crystal) surface, σ the wavenumber, nc the refractive index of ATR element, and nx the ratio of the refractive index of samples to that of ATR element. θ was selected to be 45°.33 The refractive indices of PVDF and zinc selenide crystal were assumed to be 1.5 and 2.4, respectively. The PVDF membranes were immersed in isobutanol for 24 h and then weighed immediately after removing isobutanol from the surface to measure their porosity.34 The porosity was calculated by the following equation: Ak =

(W2 − W1)ρ1 ρ1W2 + (ρ2 − ρ1)W1

(2)

where W1 (g) is the initial membrane weight, W2 (g) the immersed membrane weight, ρ1 (kg m−3) the density of PVDF, and ρ2 (kg m−3) the density of isobutanol. The maximum tensile strength and the elongation degree to break PVDF membranes were measured using a tensile test machine (LR30, Lloyd Instruments, UK) at a stretching rate of 20 mm min−1 with a preload of 0.5 N. Each sample was cut into a specific dumbbell shape (5 cm long, 0.8 cm wide at the ends, and 0.4 cm wide in the middle). The thickness of membrane samples was also measured using a thickness measuring instrument (DRK 203A, Drick, China) before examination. 2.4. Filtration Experiments of Membrane. Rejection and flux experiments were carried out with an HP4750 cell (Sterlitech, U.S.). The solution in the nitrogen gas pressurized chamber (5 bar) was stirred, and all experiments were carried out at 25 °C. The initial concentration of the feed dye solution

Figure 1. Flowchart of the membrane prepared by ultrasound-assisted phase inversion process. 8229

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Figure 2. SEM images of cross sections of the membranes under different ultrasonication conditions.

Figure 3. Effect of different ultrasonication conditions on the porosities of PVDF (15 wt %) membrane: (a) different ultrasonic power and (b) different ultrasonic frequency.

was 10 mg L−1, and the concentrations of the feed and permeate solution were measured using an ultraviolet−visible spectrophotometer (TU-1810, Pgeneral, China). The absorbance of RB aqueous solution at 548 nm was measured. The rejection rate of the dye by the membranes was calculated by the following equation: R=

Cf − Cp Cf

× 100%

The pseudobinary diffusion theory is generally used to explain the initial phase separation process for membrane formation.36 It has been reported that the high intensity of ultrasonic irradiation may cause a high diffusion rate of the nonsolvent phase.37 With increasing intensity of the ultrasonic irradiation, the coalescence between polymer phases is easier and, consequently, fully developed large isolated cavities are formed. By further increasing the ultrasonic intensity, the interface barrier between nascent fingerlike pores and large cavities is eliminated because of the turbulence of ultrasound, resulting in the formation of long fingerlike pores beneath the upper surface.15 Increasing the frequency leads to smaller amplitudes of cavitation bubbles when the ultrasonic power was fixed at 300 W, indicating that the ultrasonic cavitation intensity was also correspondingly reduced. Therefore, the penetration rate of water molecules into the cast solution decreased. Membrane A6 (100 kHz) had a cross-sectional morphology similar to that of membrane A-0. Although the isolated large holes were still seen, the number of irregular macropores was decreased. When the ultrasonic frequency was reduced to 80 kHz, those independent large holes were connected to each other and no particular macrovoids disappeared in the A-5 membrane. The membrane structure was a mixture of fingerlike holes and a more sponge-like structure. As the ultrasonic frequency was further reduced to 45 kHz (A-4), the fingerlike holes structure extended to the bottom of the membrane. On the basis of the results shown above, the structure of a PVDF/NMP membrane can be controlled by adjusting the power and frequency of ultrasound. High-power, low-frequency ultrasound is required to obtain a high percentage of fingerlike holes that have good permeability.

(3)

where Cp is the dye concentration of the permeate and Cf is the initial feed concentration. All the membranes were tested by the rejection of RB aqueous solution (10 mg L−1).

3. RESULTS AND DISCUSSION 3.1. Membrane Morphology. Figure 2 shows the crosssectional morphology of membranes prepared under different ultrasonication conditions. The membrane prepared without ultrasonic assistance (A-0) showed an irregular macroporous structure. The presence of irregular macropores in the structure may lead to greater resistance to water mass transfer and thus lower permeability. When the phase inversion process is assisted by ultrasound, the transport of small molecules is increased with increasing ultrasonic intensity in a liquid solution by convection in an otherwise stagnant or relatively slow-moving fluid.35 With an increase in ultrasonic cavitation intensity, the penetration rate of water molecules into the cast solution is increased and the liquid convection is also improved. Membranes A-1 to A-4 showed a fingerlike pore structure. Increasing the ultrasonic power led to the formation of more uniform fingerlike holes. Homogeneous long and straight fingerlike structures were found almost throughout the entire cross-section for sample A4. 8230

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Figure 4. WAXD spectra of PVDF membranes prepared under different ultrasonication conditions: (a) different ultrasonic power and (b) different ultrasonic frequency.

Figure 5. ATR-FTIR spectra of PVDF membranes under different ultrasonication conditions: (a) different ultrasonic power and (b) different ultrasonic frequency.

3.2. Porosity of PVDF Membranes under Different Ultrasonic Fields. Figure 3 shows the porosities of PVDF membranes prepared under different ultrasonication conditions. The results show that porosity Ak was also strongly dependent on the ultrasonic power and frequency: A0 < A1 < A2 < A3 < A4 and A0 < A6 < A5 < A4. All ultrasonicated membranes showed porosity higher than that of the nonultrasonicated membrane, which was attributed to the acceleration of mass transfer between the solvent and nonsolvent phases during the

ultrasound-assisted phase inversion process. The coalescence between these polymer lean phases and the surrounding smallscale polymer lean phase was easier. The interface barrier between nascent fingerlike pores and large cavities was eliminated because of the turbulence of ultrasound, resulting in the formation of long fingerlike pores.15 In addition, the skin of the membrane was formed quickly in a few seconds, and there are no obvious influence on the conformation of 8231

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membrane skins by ultrasound-assisted phase inversion. So the porosities of all ultrasonicated membranes improved. 3.3. Crystalline Structure of the PVDF Membrane Bulk by WAXD and ATR-FTIR. 3.3.1. WAXD Study of the PVDF Membrane. WAXD was used to study the polymorphic behavior of the PVDF membrane. As shown in Figure 4, the α phase and β phases were observed for all membranes, which was confirmed by the presence of the peak at 18.6°, related to the α (020) plane, and the peak at 26.7°, related to the α (021) plane. The simultaneous presence of α and β will produce an overlap around 2θ = 20°, related to the (110) plane. It can also be observed that the intensity of the peak at 18.6° for the nonultrasonicated membrane (A-0) was weaker than that for the ultrasonicated membranes, while the peak at 20° was sharper. As shown in Figure 4a, the intensity of the peak at 26.7° (α plane) increased with an increase in ultrasonic power. Meanwhile, in Figure 4b, increasing the ultrasonic frequency led to a decrease in the intensity of the peak at 26.7° (α plane). The phase transformation from the β to α phase occurs at temperatures above 140 °C.38 Ultrasonic cavitation bubble collapse produces intense local heating (3000−5000 K) and high pressures (500−10,000 atm),18−20 providing the conditions for phase transformation. 3.3.2. Crystalline Structure of the PVDF Membrane Upper Surface by ATR-FTIR. ATR-FTIR was used to obtain the crystalline phase of the top surface of PVDF skin layers; the results are shown in Figure 5. The peaks at 840 and 1275 cm−1 are related to the β phase, while the other peaks refer to the α phase. Both phases are indicated in the figure. Ultrasonicated membranes displayed obvious α characteristic peaks at 1423, 1383, 1210, 1175, 1070, 975, 875, 796, 762, and 612 cm−1, but weak β peaks. Membrane A-0 showed week α peaks but obvious β peaks. The IR absorption at 840 cm−1 (β phase) and 762 cm−1 (α phase) were roughly estimated using the Lambert−Beer law,32,39 as shown in eqs 4 and 5 Aβ840 = log

Aα762 = log

I β0 Iβ

= K β840CXβ d t

Iα0 = Kα762CXαd t Iα

Table 2. Analysis of Membrane Crystallization by the Lambert−Beer Law log(I0α/Iα)

log(I0β/Iβ)

762 A840 β /1.26Aα

A-0 A-1 A-2 A-3 A-4 A-5 A-6

0.103 0.137 0.161 0.231 0.207 0.178 0.150

0.169 0.197 0.225 0.195 0.171 0.218 0.221

1.302 1.141 1.109 0.670 0.656 0.972 1.169

ultrasonic frequency led to an increase in the β/α phase ratio from A-4 (45 kHz) to A-5 (80 kHz) and A-6 (100 kHz) with 300 W of ultrasonic power. The highest D-value of the β/α phase ratio between membrane A-0 and all the ultrasonicated membranes was 0.646. The difference in the β/α phase ratio of PVDF membranes can be attributed to two main factors: (1) the dipolar interaction at the interface between the PVDF nucleus and solvent molecules and (2) the crystallization rate. The crystallization rate is considered to be the primary factor leading to the difference in the phase ratio because the influence of the dipolar interaction may be negligible for the same type of membrane. However, it has been confirmed that the energy of trans− gauche polymer chains related to the α phase are lower than that related to the β phase.40 For polymer chains with desirable mobility in a well-dissolved solution, the alteration of the conformation can occur instantaneously. The Hansen solubility parameter is used to characterize the distance between the polymer and the solvent in a three-dimensional δd, δp, and δh Hansen space. The lower the δt, P−S value, the better PVDF is dissolved in the corresponding solvent.41 NMP with the low δt, P−S value is a good solvent used to dissolve PVDF. When ultrasonic cavitation occurs during the nonsolvent phase inversion process, the instantaneous crystal transition process can be controlled by ultrasonic intense local heating and high pressure. The low β/α phase ratio at an increased ultrasonic intensity might be explained by the low degree of entanglement and the high mobility of PVDF chains, which facilitate the formation of the stable α phase. 3.4. Study on the Mechanical Property of PVDF Membranes in the Ultrasonic Bath. Table 3 shows the

(4)

(5)

where I0 and I are the incident and reflected radiation intensities, respectively; K and dt are the absorption coefficient and the thickness of the membrane, respectively; X is the crystallinity of each phase, and C is the average total monomer 4 2 840 concentration. The values of K762 α and Kβ were 6.1 × 10 cm mol−1 and 7.7 × 104 cm2 mol−1, respectively.32 The absorptions of those two characteristic peaks were used to calculate the phase ratio between β and α: Xβ Aβ840 Aβ840 F (β ) = = = F (α ) Xα (K β840/Kα762)Aα762 1.26Aα762

sample

Table 3. Effect of Different Ultrasonication Conditions on the Mechanical Behavior of PVDF (15 wt %) Membranes

(6)

The detailed results are shown in Table 2. The final results show the β phase was present in a higher percentage in A-0 than in the other membranes. In addition, the results also show that the phase ratio was strongly dependent on the ultrasonic power and frequency. It can be observed that the β/α phase ratios decreased with an increase in ultrasonic power for A-1 (120 W) to A-2 (180 W), A-3 (240 W), and A-4 (300 W) at a fixed ultrasonic frequency (45 kHz). Meanwhile, increasing the

serial number

tensile strength (MPa)

elongation (%)

A-0 A-1 A-2 A-3 A-4 A-5 A-6

3.190 3.100 3.225 4.025 3.730 4.363 3.750

54.207 89.798 94.140 111.818 124.41 99.178 99.073

thickness (μm) 40 39 38 39 39 37 37

± ± ± ± ± ± ±

1 2 2 1 2 2 1

influence of ultrasonic cavitation on the tensile strength and elongation of PVDF membranes. Macrovoids tended to form in the ultrasonic bath, which indicate that ultrasonic cavitation resulted in a loss of mechanical properties. It has been reported that membranes with high porosity and macro-sized pores have higher pure water flux and RB aqueous solution flux, whereas 8232

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Figure 6. Permeability and rejection of PVDF membranes under different ultrasonic intensity: (a) different ultrasonic power and (b) different ultrasonic frequency; forward flux (water flows from top surface to bottom surface); the rejection is to RB.

ultrasonic power and frequency. The highest flux was 129.8 L m−2 h−1 for water, or 84.5 L m−2 h−1 for RB aqueous solution, while keeping the rejection of RB at a high level of 87.0%. The PVDF membrane structure could be regulated by changing the ultrasonic power and frequency. Both α and β phases were observed for all membranes. Moreover, part of the β phase was converted to the α phase in the ultrasound-assisted phase inversion process. The PVDF membrane with the best performance was prepared with ultrasonication conditions of 45 kHz/300 W/1 min.

numerous macrovoids in the membrane may cause some losses in the mechanical properties.42 In addition, membranes prepared by ultrasound-assisted phase inversion exhibited elongation greater than that of the pristine membrane. Greater elongation indicated that the ultrasonicated membranes had lower β phase crystallinity. The polymer chains of the ultrasonicated membranes were arranged loosely, which created a relatively higher water flux and RB aqueous solution flux. 3.5. Filtration Performance of Membrane. The principle for the removal of multivalent ions from aqueous solutions by ultrafiltration membranes may be attributed to the size exclusion effect. Water flux is directly related to the pore size distribution and morphology of the membrane. Rejection performance is mainly controlled by the top layer of the membrane. The permeability of the membranes was measured by water flux and RB aqueous solution flux; the results are shown in Figure 6. Ultrasonicated membranes showed water and RB aqueous solution fluxes higher than those of the nonultrasonicated membrane. With an increase in ultrasonic power for A-1 (120 W), to A-2 (180 W), A-3 (240 W), and A-4 (300 W) at a fixed ultrasonic frequency (45 kHz), the fluxes increased in this order: A-0 < A-1 < A-2 < A-3 < A-4. Meanwhile, increasing the ultrasonic frequency led to a decrease in flux from A-4 (45 kHz) to A-5 (80 kHz) and A-6 (100 kHz) with 300 W of ultrasonic power. It is known that the threshold of ultrasonic cavitation becomes higher with an increase in ultrasonic frequency. The number of cavitation bubbles decreased in the coagulation bath. Therefore, the ultrasonic intensity increased with decreased frequency. The result is in accordance with the porosity results described above. The ultrasonicated membranes had more fingerlike pores through the cross section and greater porosity, but RB rejection was consistently about 80%. This indicates the potential application of the membranes in ultrafiltration processes.

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: 86-10-64413857. Fax: 86-10-64436876. E-mail: pankai@ mail.buct.edu.cn. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The project was supported by the National Natural Science Foundation of China (51373014) and the Opening Project of the State Key Laboratory of Chemical Resource Engineering (CRE-2012-C-206).

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REFERENCES

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4. CONCLUSIONS In the current study, the influence of the ultrasonication conditions was studied on PVDF membranes prepared by the ultrasound-assisted phase inversion method. High ultrasonic power and low ultrasonic frequency promoted the formation of fingerlike holes in the membranes, which led to an increase in porosity and elongation. Flux was strongly dependent on the 8233

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dx.doi.org/10.1021/ie5012905 | Ind. Eng. Chem. Res. 2014, 53, 8228−8234