poly(vinylidene fluoride) membranes by an ultrasound assisted phase

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Poly(vinylidene fluoride) membranes by an ultrasound assisted phase

inversion method

Mi-mi Tao, Fu Liu ⇑, Li-xin Xue ⇑

Ningbo Institute of Materials Technology & Engineering, Chinese Academy of Sciences, 519 Zhuangshi Road, Ningbo 315201, China

a r t i c l e i n f o

Article history:

Received 23 February 2012

Received in revised form 21 July 2012

Accepted 28 August 2012

Available online 5 September 2012

Keywords:

PVDF membrane

Ultrasound

Morphology

Phase inversion

Cavitation

a b s t r a c t

Poly(vinylidene fluoride) (PVDF) membranes were prepared by an ultrasound assisted phase inversion

process. The effect of ultrasonic intensity on the evolution of membrane morphology with and without

the addition of pore former LiCl during precipitation process was comprehensively investigated. Besides

the inter-diffusion between the solvent and nonsolvent, the ultrasonic cavitation was thought to have

significant influences on phase inversion and the resultant membrane morphology. The mutual diffusion

between water and solvent during the ultrasound assisted phase inversion process was measured. The

crystalline structure was detected by wide angle X-ray diffractometer (WAXD). The thermal behavior

was studied by differential scanning calorimeter (DSC). The mechanical strength, forward and reverse

water flux, rejection to bovine serum albumin (BSA) and pepsin were also investigated. By the ultrasound

assisted phase inversion method, ultra-filtration membrane was successfully prepared, which exhibited

more preferable morphology, better mechanical property and more favorable permeability without sac-

rificing the rejection and thermal stability.

Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction

Poly(vinylidene fluoride) membrane usually prepared by phase

inversion is widely used in micro-filtration and ultra-filtration pro-

cess due to its superior physical and chemical properties. Depend-

ing on the precipitation rate, PVDF membranes can be: (a)

asymmetric, with a selective skin or large voids on a spongy sub-

layer or (b) symmetric, with an almost even porosity along the

thickness. It has been investigated that many parameters influence

the precipitation process and ultimately the morphologies and the

filtration performance of the membranes, including the composi-

tion of casting solution [1–4], precursor preparation [5,6], evapora-

tion time [7], the harshness and temperature of the coagulation

bath [8–11], etc. The phase separation behavior of PVDF/solvent/

H2O system is far more complex than amorphous polymer systemdue to the semi-crystal nature. Both liquid–liquid demixing and li-

quid–solid (crystallization) demixing are usually occurred during

the phase inversion process of PVDF membrane. These two mech-

anisms are developed to different extents according to the thermo-

dynamics and mass transfer properties. Bottino [12] prepared

PVDF membranes by casting and coagulating solutions of the poly-

mer in eight solvents and found a good correlation between sol-

vent–nonsolvent diffusivity and the membrane structure.

Numerous researches also revealed that thermodynamics have less

influence on the structures and performance of the final PVDFmembranes, while the kinetic mass transfer is the key factor to

control the membrane structure and formation [13–15].

It is known that ultrasound can induce a wide range of chemical

and physical consequences. In liquids irradiated with ultrasound,

ultrasonic cavitation serves as the primary mechanism for sono-

chemical effects, where bubble collapse produces intense local

heating, high pressures, microjet, turbulence, acoustic streaming,

etc. [16–18]. Physical effects of ultrasound include enhanced mass

transfer, emulsification, bulk thermal heating and various effects

on solids. Diverse applications of ultrasound have been explored.

Nanostructured materials, nanometer colloids and protein micro-

spheres can all be prepared by this general route [19,20]. Besides,

ultrasound is evidenced as a powerful tool to polymerize [21], de-

grade polymer [22,23] and disperse nanoparticles [24,25], and it iswidely used in the bulk or surface modification of polymers [26].

With respect to membranes for separation, ultrasound is usually

utilized in membrane cleaning [27,28]. In consideration of the

facilitation of ultrasound during physical or chemical process, 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 the best of

our knowledge, the role of ultrasound in phase inversion process

of PVDF membrane has not been studied. Here, PVDF membranes

were prepared by an ultrasound assisted phase inversion tech-

nique. The effect of ultrasonic intensity on membrane morphology

was investigated. The crystalline structure, thermal stability,

1350-4177/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.ultsonch.2012.08.013

⇑ Corresponding authors. Tel.: +86 574 86685256; fax: +86 574 86685186 (F. Liu),

tel.: +86 574 86685831; fax: +86 574 86685186 (L.-x. Xue).

E-mail addresses: [email protected] (F. Liu), [email protected] (L.-x. Xue).

Ultrasonics Sonochemistry 20 (2013) 232–238

Contents lists available at SciVerse ScienceDirect

Ultrasonics Sonochemistry

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

http://dx.doi.org/10.1016/j.ultsonch.2012.08.013

mailto:[email protected]



http://www.sciencedirect.com/science/journal/13504177

http://www.elsevier.com/locate/ultson

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http://www.sciencedirect.com/science/journal/13504177







mechanical property and permeability of membranes were also re-

ported in this paper.

2. Experimental

2.1. Materials

Commercial PVDF (FR904) was purchased from 3F company(Shanghai, China) and was dried at 80 °C in the oven for 24 h prior

to use. N ,N -dimethylfoemamide (DMF,P99.0%) and bovine serum

albumin (BSA Fraction V, Mn = 67,000) were purchased from Sin-

opharm Chemical Reagent Co. Ltd., China and Sigma Aldrich,

respectively. Lithium chloride (LiCl, P99.0%) and pepsin

(Mn = 35,000) were purchased from Aladdin Reagent Co. Ltd.,

China. All materials were used as received.

2.2. Membrane preparation

Fifteen percent by weight PVDF was dissolved in DMF at 80 °C

with a constant stirring rate for 24 h to obtain a homogenous cast-

ing solution. Moreover, six percent by weight LiCl based on the

solution weight was added to the same PVDF solution and dis-solved under the same condition. The casting solutions were kept

still at 80 °C for another 24 h and went through vacuum deaeration

to remove air bubbles.

The casting solutions were uniformly spread onto a glass plate

by a casting knife with the thickness of 200lm. The glass plate

and casting knife were heated to 80 °C before casting in order to

minimize the effect of surrounding temperature on phase inver-

sion. After the nascent film was evaporated in the air for 10 s, it

was transferred into the coagulation bath composed of deionized

water in the ultrasonic instrument (40 Hz, KQ-300DB, Kunshan,

China) and irradiated by ultrasound. The temperature of coagula-

tion bath was kept constant at 28 °C. In one minute, the mem-

branes were automatically peeled off from the glass plate and

taken out from the ultrasonic bath. The solidified membrane was

finally transferred to the fresh deionized water for 24 h to remove

the residual solvents. Therefore, the thermal effect of ultrasonic

irradiation on the coagulation bath could be ignored in one minute.

The solidified membrane was finally transferred to the fresh deion-

ized water for 24 h to remove the residual solvents. Subsequently,

the membranes were dried at room temperature. The preparation

conditions for various membranes are summarized in Table 1.

2.3. Membrane characterization

Morphological structures of the prepared PVDF membranes

were examined by scanning electron microscope (SEM, S-4800,

Hitachi, Japan). The cross section samples were fractured in liquid

nitrogen. Both the surface and cross section of the samples were

sputtered gold for 2 min for observation.To determine the mutual diffusion between water and solvent

during the membrane formation process via phase inversion, the

water and DMF contents in the membranes at a fixed time after

immersion into the ultrasonic bath were measured as follows:

the nascent membrane was immersed in the coagulation bath for

5 s and then taken out to wipe off water drop adhered to the mem-

brane surface, afterwards, the wet membranes were transferred to

specific amount (mmethanolðwaterÞ) of methanol (to extract water from

the membrane) and deionized water (to extract DMF from the

membrane), respectively and then preserved under seal. The lixiv-

ium was analyzed by coulometer (KF831, Metrohm, Switzerland)and UV–VIS–NIR spectrometer (Lambda 950, Perkin Elmer, US) at

197 nm, respectively to get the concentration of water (c H2 O) and

DMF (c DMF). Then the wet membranes should be dried to obtain

dry weight (mdry) and the contents of water and DMF (M water DMFð Þ)

can be calculated using the following equation:

M water DMFð Þ ¼C H2O DMFð Þ Â mmethanolðwaterÞ

mdry

ð1Þ

Crystalline structure of the membrane was analyzed by wide

angle X-ray diffractometer (WAXD, D8 Advance, Bruker, Germany).

All samples were detected in a continuous scan mold between 5°

and 60° 2h with the scanning speed of 0.2 s/step.

The thermal behavior of the membrane was determined by dif-

ferential scanning calorimeter (DSC) (Pyris Diamond, Perkin Elmer,US). The temperature was raised from 25 to 250 °C at a rate of

10 °C/min at a nitrogen atmosphere.

Crystallinity of the membrane was calculated by melting heat

based on the fusion enthalpy of ideal PVDF crystal ðDH f 0 ¼ 105 J=gÞ:

Crystallinity ¼DH sample

DH 0 f ð2Þ

where, DH sample is the melting heat of the sample obtained by DSC

curve.

Mechanical properties of the membranes were determined

using a tensile tester (5567, Instron, US) with the stretching rate

of 5 mm/min at room temperature. Each sample was cut into

10 Â 1 cm, and the thickness of the sample was measured accord-

ing to the SEM pictures.Pure water flux of PVDF membranes were measured by a cross

flow filtration system (Saifei company, China) (the effective mem-

brane area = 24 cm2). The measuring protocol was as follows: In

the first 40 min, the membrane was compacted at 0.15 MPa to

diminish the compaction effect and obtain the stable flux, and then

the flux was measured at 0.1 MPa. Water flux reported in this work

was the final steady flux. Afterwards, pure water was changed to

1 g/L BSA or pepsin solution. The protein concentration of both

feed and permeate solutions were examined by UV–VIS–NIR spec-

trometer (Lambda 950, Perkin Elmer, US) at 280 nm (BSA) or

270 nm (pepsin). The rejection to protein (R (%)) of the membranes

was calculated using the following equation [29]:

Rð%Þ ¼ 1 À A1 A0

Â 100 ð3Þ

Where A1 is the absorption value of the permeate solution, and

A0 is the absorption value of the feed solution.

Table 1

Preparation conditions for PVDF membranes and diffusion content of water and DMF at fixed time 5s.

Code LiCl/PVDF/DMF (wt/wt/wt) Ultrasonic intensity (W) Water contenta (g/g) DMF contenta (g/g) DMF diffusion amounta,b (g/g)

M1 0/15/85 0 0.392 3.263 2.404

M2 0/15/85 180 0.673 2.515 3.152

M3 0/15/85 300 0.545 1.675 3.992

MA1 6/15/85 0 – – –

MA2 6/15/85 180 – – –

MA3 6/15/85 300 – – –

a

Water or DMF content in one gram of PVDF membrane.b Calculated based on 15 wt.% solid content: 8515 À M DMF.

M.-m. Tao et al./ Ultrasonics Sonochemistry 20 (2013) 232–238 233



3. Results and discussion

3.1. Membrane morphologies

3.1.1. Morphologies of membranes prepared without additives

Fig. 1 shows the morphologies of M1, M2 and M3. All mem-

branes exhibit spongy sublayer, which can be seen in the cross sec-

tion in Fig. 1a and b. However, the membrane cross section close tothe top surface displays different structure with the variation of

ultrasonic intensity. Membrane M1 without ultrasonic assistance

has a dense skin layer with the thickness about 8lm, the possible

reason could be that the faster outflow of DMF and slower inflow of

water result in the higher PVDF concentration on the membrane

top layer during the phase inversion process. Increasing the ultra-

sonic intensity to 180 W, membrane M2 exhibited regular tear-

drop-like voids which develop from the top surface. When the

ultrasonic intensity was further raised to 300 W, cellular pores

with a graded distribution through the whole cross section areclearly presented and the dense skin layer or teardrop-like voids

are totally eliminated in case of membrane M3.

Fig. 1. SEM images of cross section, bottom surface and top surface morphology of M1, M2 and M3: (a) the cross section with magnification 400Â; (b) the enlarged crosssection with magnification 2000Â; (c) the bottom surface with magnification 5000Â; (d) the top surface with magnification 20,000Â.

234 M.-m. Tao et al./ Ultrasonics Sonochemistry 20 (2013) 232–238



Usually, pseudo binary (solvent and nonsolvent) diffusion the-

ory is adopted to describe the early phase separation stage of

membrane formation [30], which reproduces with good agreement

the experimental data and observations. There is evidence that a

high diffusion rate of nonsolvent gives rise to membrane with large

voids beneath the top surface [15]. Based on this and the relevant

morphology variation of the membrane cross section close to the

top skin layer, it can be inferred that the water diffusion rate intothe PVDF/DMF nascent film during the incipient phase separation

moments following the order: M2 > M3 > M1. The contents of

water and DMF in the membranes at a fixed time 5 s after immer-

sion were measured to predict the mutual diffusion rate between

water and DMF. As depicted in Table 1, water content in the mem-

brane, representing the water diffuse rate from the coagulation

bath to the membrane, reaches a maximum (0.673 g/g) when the

ultrasonic intensity is 180 W corresponding to M2, thus leading

to the formation of teardrop-like voids on the top skin layer. The

diffusion amount of DMF into the coagulation bath calculated

based on the initial composition of the casting solution and the

residual quantity progressively increases with increasing the ultra-

sonic intensity. It is well known that ultrasound can promote the

molecule movement and therefore increase the diffusion rate of

DMF. However, ultrasonic irradiation with 300 W intensity does

not further accelerate water diffusion as expected (water content:

0.545 g/g). The exceptional diffusion behavior may be the result of

stronger hydrogen bonding between the water molecules and the

carbonyl group of the DMF molecules. Petersen has pointed out

that hydrogen bonding interaction between water molecules and

carbonyl oxygen are stronger than between water molecules

[31]. In case of DMF, a special amide in view of the lack of hydrogen

bonding in the pure solvent, this effect may be strengthened by the

nitrogen atom due to the resonance forms [32]. Ultrasonic irradia-

tion promotes the mass transfer of DMF from higher concentration

zone to lower concentration zone, while the diffusion of water was

not only affected by the ultrasonic intensity, but also affected by

the hydrogen bonding interaction between water and DMF. There-

fore, despite the high-intensity ultrasound (300 W), water diffu-sion rate in the early stage of M3 is still lower than that of M2,

leading to the formation of a uniform structure with cellular pores.

The morphology of the bottom surface of the membrane also

varies with the variation of ultrasonic intensity used. AsFig. 1c pre-

sents, M1 has a dense bottom surface composed of syncretic

spherulites, while M2 and M3 exhibit porous surface with massive

inter-connected pores. As we all know, the bottom surface con-

tacted with the glass plate is far from the membrane/water inter-

face, therefore, the water/DMF diffusion has less effect on

structure evolution during phase inversion. The main factor is

the ultrasonic cavitation, which produces high-energy phenome-

non and is responsible for the generation of surface damage [33].

We believe that the impingement of shockwaves and microjets

on the surface creates the localized erosion. With the increase of ultrasonic intensity, erosion becomes more severe and finally leads

to the formation of completely porous surface. All membranes

present a dense top surface as can be shown in Fig. 1d. The delayed

phase separation leads to the evaporation of solvent and higher

polymer concentration. After the casted nascent film on the glass

plate is immersed into deionized water, the fast mass transfer at

the interface between water and film results in instantaneous

solidification of top surface, therefore, a dense top surface was

formed in all cases.

3.1.2. Morphologies of membranes prepared with LiCl

The effect of ultrasonic irradiation on morphologies of mem-

branes prepared with pore former can also be identified. When

6% by weight LiCl is used, all the membranes exhibit highly inho-mogeneous structures due to the presence of large voids of differ-

ent size and shape through the cross section as shown in Fig. 2a

and b. In membrane MA1, arrestive finger-like pores and com-

pressed cavities simultaneously appear in the porous sublayer.

For MA2, the compressed cavities are evolved into isolated pores

some of which become interlinked with the finger-like pores close

to the top surface owing to the turbulence of ultrasonic irradiation

of 180 W. The images of enlarged cross section in Fig. 2b demon-

strate that the finger-like pores in MA2 are suppressed to some ex-tent compared with those in MA1, caused by the turbulence of

ultrasound during phase inversion. Unlike the membrane without

additives, the presence of LiCl in the dope solution promotes the

diffusion of water to the PVDF dope due to the good affinity be-

tween them. Moreover, the increased dope viscosity hinders the

outflow of DMF from the polymer dope. Therefore, the evolution

of finger-like pores cannot be totally eliminated. In case of MA3,

finger-like pores are seen to grow from the top surface almost to

the bottom surface. On the one hand, the high intensity ultrasonic

irradiation may exacerbate the formation of finger-like pores by

enhancing the water diffusion rate. On the other hand, with

increasing the intensity of ultrasonic irradiation, the smaller iso-

lated pores conquered the interface tension barrier and were grad-

ually connected together to form the long finger-like pores through

the cross section. The large cavities beneath finger-like pores as

shown in MA1 derive from the polymer lean phase favorable in

growth. With the assistance of ultrasound, the coalescence be-

tween these polymer lean phases and surrounding small-scale

polymer lean phase gets easier and consequently forms the fully

developed large isolated cavities in MA2. With further improving

the ultrasonic intensity, the interface barrier between nascent fin-

ger-like pores and large cavities below was eliminated due to the

turbulence of the ultrasound, resulting in the formation of long fin-

ger-like pores.

The bottom and top surface of the membranes are illustrated in

Fig. 2c and d, respectively. As expected, the ultrasonic irradiation

made thebottomsurface more porous. There arefew discrete circu-

lar pores appearing on the mainly dense surface of MA1, while MA2

presents the most porous bottom surface with uniform pore sizedistribution when the ultrasonic intensity is 180 W. Unexpectedly,

the highest ultrasonic intensity (300 W) does not necessarily im-

prove the surface porosity further. As discussed above, numerous

bubbles are formed in the viscous dope solution and then collapse

to produce microscopic jets, which produces localized erosion in

the bottom surface and finally leads to the formation of porous

morphology. However, solidification rate may also have strong im-

pact on the resulting surface morphology when ultrasound is used

during phase inversion. The abundant finger-like cavities are dis-

played across most of the cross section in MA3, which reduces

water transfer resistance and subsequently accelerates solidifica-

tion. Due to the limitation of ultrasonic energy, the bottom surface

undergoing faster solidification cannot be badly eroded. Resul-

tantly, MA3 has a less porous bottom surface than MA2.The three membranes exhibit relatively dense top surface be-

cause of the air exposure and high polymer concentration in the

membrane surface. In all cases nanoscale pores arise as LiCl is used

as a pore former, which can be clearly seen from the high resolu-

tion image inset in MA3-d in Fig. 2.

Based on the morphological observation, it is concluded that

ultrasonic irradiation may facilitate the elimination and formation

of macrovoids, which depends on the ultrasonic intensity applied

during phase inversion. By this simple way, completely uniform

structure (M3) without additives and typical asymmetric structure

with pore formers in the bulk (MA3) can be obtained. Ultrasound

contributes a lot to make the bottom surface porous regardless of

the addition of pore former. However, the top surface was little

influenced by the ultrasound due to the air exposure and fastsolidification.




3.2. Membrane properties

Fig. 3 presents the WAXD patterns of the PVDF membranes.

Membranes prepared from the same dope solution have the simi-

lar crystalline structure. The occurrence of the distinctive peaks at

18.5° and 26.7°, corresponding to the reflections of a(020) and

a(02 1), confirms the predominant presence of a phase in M1,

M2 and M3. Despite the small shoulder at 18.5° accompanying

the superimposed peaks b(200) and b(110) at 20.8°, MA1, MA2and MA3 are dominated by b phase due to the added LiCl.

Thermal stability of PVDF membranes were characterized by

DSC measurements. Despite the significant morphological differ-

ence in these membranes, the corresponding thermograms present

minute difference as can be seen in Fig. 4. Crystallinity calculated

by melting heat based on the fusion enthalpy of ideal PVDF crystal

ðDH 0 f ¼ 105 J=gÞ and melting temperature are listed in Table 2. All

membranes exhibit almost the same melting temperature around

162 °C and membranes cast from the same dope solution have sim-

ilar crystallinity. Nevertheless, the addition of LiCl to the dope solu-tion visibly promotes crystallization, for example, the crystallinity

Fig. 2. SEM images of cross section, bottom surface and top surface morphology of MA1, MA2 and MA3: (a) the cross section with magnification 400Â; (b) the enlarged cross

section with magnification 1300Â; (c) the bottom surface with magnification 5000Â; (d) the top surface with magnification 20,000Â; image inset in MA3-d: top surface of

MA3 with magnification 100,000Â.




60.7% of MA1 is higher than that of M1 51%. LiCl may act as a nucle-

ating agent besides pore former and hence is beneficial to the crys-

tallization of PVDF chains during phase inversion.

It can be seen that the effect of ultrasonic intensity on crystal-

line behavior is faintness, which can be ascribed to the concen-

trated dope solution. The rearrangement and crystallization of

PVDF are almost not influenced due to the high entanglement of

macromolecules chains. Therefore, crystalline structure and ther-mal stability of the membranes are practically unchanged with

the variation of ultrasonic intensity.

The mechanical properties were also investigated (listed in

Table 2). Among the membranes prepared without additive, M1

has the most excellent tensile strength of 7.21 MPa. The tensile

strength of M2 and M3 are 6.21 and 6.19 MPa, respectively due

to the employment of ultrasound. When LiCl is used as the pore

former, membranes exhibit much lower tensile strength than those

without LiCl. Ultrasound assisted phase inversion generates mem-

branes with a little higher tensile strength, opposite to the situa-

tion in non-additive system. Tensile strength may be related to

the morphological structure which is varying with ultrasonic

intensity used. M1 has dense skin layers and is less porous than

M2 and M3, resulting in the highest tensile strength. The addition

of LiCl remarkably promotes the formation of large voids in mem-

branes MA, which consequently conduces to the reduction of ten-

sile strength to less than 4 MPa. The elongation may mainly

depend on the crystallinity of the membranes. M1, M2 and M3have lower crystallinity and therefore have higher elongation (in

the range of 83% to 90%), while higher crystallinity of MA1, MA2

and MA3 leads to lower elongation (around 60%).

3.3. Membrane performances

Permeability of membranes was measured by pure water flux

as can be seen in Fig. 5. Water flux reflects the pore size, pore size

distribution and morphologies of the membranes. The permeate

flux of M1, M2 and M3 was too low to be measured at 0.1 MPa

due to the dense top surface of the membrane. While, the perme-

ability of MA2 and MA3 prepared with the assistance of ultrasound

is superior to that of MA1. The steady flux of MA2 and MA3 reaches

up to 115 L/m2 h and 120 L/m2 h, respectively while MA1 shows

the lower flux of 80 L/m2 h. It is because that MA2 and MA3 have

sufficient finger-like pores through the cross section and porous

bottom surface. The reverse flux of the three membranes was

tested (water flows from bottom surface to top surface). As also

10 20 30 40 50

MA3

MA2

MA1

M3

M2 I n t e n s i t y

( a . u . )

2 θ (degree)

M1

Fig. 3. X-ray diffractograms for different membranes: M1, M2, M3, MA1, MA2, and

MA3.

80 120 160 200 240

MA3

MA2

MA1

M3

M2

E x o t h e r m

Temperature (OC)

M1

Fig. 4. DSC thermograms for different membranes: M1, M2, M3, MA1, MA2, and

MA3.

Table 2

Properties of PVDF membranes.

Code Crystallinity (%) Tm (oC) Tensile strength (MPa) Elongation (%) BSA rejection (%) Pepsin rejection (%)

M1 51.0 161.99 7.17 ± 0.22 90 ± 12 – –

M2 53.6 162.85 6.21 ± 0.39 88 ± 15 – –

M3 51.8 161.98 6.19 ± 0.12 83 ± 13 – –

MA1 60.7 162.18 3.04 ± 0.37 63 ± 16 98.9 81.4

MA2 60.1 162.34 3.88 ± 0.34 66 ± 11 99.1 79.4

MA3 61.2 162.19 3.86 ± 0.24 67 ± 12 98.9 79.8

MA1 MA2 MA340

80

120

160

200

240

P u r e w a t e r f l u

x ( L / m 2 h )

Membrane

forward flux

reverse flux

Fig. 5. Permeability of PVDF membranes: forward flux (water flows from top

surface to bottom surface), reverse flux (water flows from bottom surface to top

surface).




shown in Fig. 5, in all cases, reverse flux is higher than forward flux

(water flows from top surface to bottom surface) since the bottom

surface is more porous than the top surface. MA2 exhibits the high-

est flux, nearly twice the flux of MA1. The reverse flux decreases

with the following order: MA2 > MA3 > MA1, confirming the differ-

ence of the bottom surface micro-structure. The rejection to BSA

and pepsin of membranes prepared by the ultrasound assisted

phase inversion process is listed in Table 2. All of them have almostsimilar rejection to BSA (about 99%) and pepsin (about 80%), indi-

cating the potential application of membranes in ultra-filtration

process.

4. Conclusions

Ultrasonic irradiation during phase inversion process had a

great effect on morphologies of PVDF membranes. Increasing ultra-

sonic intensity, morphology of upper layer in membranes cast from

non-additive solution changed dramatically from original dense

skin to regular large voids and then to uniform cellular pores.

When LiCl was used as the pore former, finger-like pores were sup-

pressed and then aggravated. Furthermore, the unfavorable cavi-

ties in membrane bulk were eliminated. All results revealed that

ultrasonic irradiation plays an important role in the formation of

cross section morphology during the phase inversion regardless

of the addition of pore formers. The porous structure of bottom

surface was supposed to be mainly dominated by the ultrasonic

cavitation. However, higher ultrasonic intensity (300 W) will not

further improve the porosity. Moreover, little change was observed

in the top surface of membranes. The tensile strength and perme-

ability mainly depended on the morphological variation. The ultra-

sonic irradiation was found to have slight influence on the

crystalline structure, thermal stability and tensile elongation. Of

all membranes, MA3 prepared with the assistance of 300 W ultra-

sonic irradiation showed the highest water flux of 120 L/m2 h and

the rejection of 98.9% to BSA and the rejection of 79.8% to pepsin.

Acknowledgements

We are grateful for the financial support from the National Nat-

ural Science Foundation of China (51273211), an international

cooperation project from Ministry of Science and Technology of

China (2012DFR50470), the National 863 Foundation of China

(2012AA03A605), and the welfare technology application research

project (2011C31002).

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poly(vinylidene fluoride) membranes by an ultrasound assisted phase

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