Preparation and characterization of PVDF–SiO2 composite hollow fiber UF

Journal of Membrane Science 337 (2009) 257–265

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Journal of Membrane

Science

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 /m e m s c

i

Preparation and characterization of PVDF–SiO 2composite hollow ?ber UF membrane by sol–gel method

Li-Yun Yu a ,b ,Zhen-Liang Xu a ,b ,?,Hong-Mei Shen b ,Hu Yang b

a State Key Laboratory of Chemical Engineering,East China University of Science and Technology (ECUST),130Meilong Road,Shanghai 200237,China b

Membrane Science and Engineering R&D Lab,Chemical Engineering Research Center,ECUST,130Meilong Road,Shanghai 200237,China

a r t i c l e i n f o Article history:

Received 30November 2008

Received in revised form 23March 2009Accepted 30March 2009

Available online 9 April 2009Keywords:

Organic–inorganic

Polyvinylidene ?uoride Silica

Hollow ?ber UF membrane Sol–gel process

a b s t r a c t

Organic–inorganic polyvinylidene ?uoride (PVDF)–silica (SiO 2)composite hollow ?ber ultra?ltration (UF)membranes were prepared by the combination of a tetraethoxysilane (TEOS)sol–gel process with a wet-spinning method.The membrane formation mechanisms were investigated in terms of viscosity,precipitation kinetics and morphology.Results showed the dope viscosity increased with the increment of TEOS concentration in dope.The addition of TEOS accelerated the precipitation of the dope.SEM pictures showed the cross-section morphology of PVDF composite membranes changed from ?nger-like macrovoids to sponge-like structure with increasing SiO 2content.The mechanical,thermal stabilities and permeation property of PVDF–SiO 2composite membranes were further examined.At lower TEOS con-centration,the hydrolyzed SiO 2particles,which were homogeneously dispersed in PVDF matrix,acted as the physical crosslinking points,and led to an improvement of mechanical and thermal properties.While at higher TEOS concentration,SiO 2formed network,which restricted the movement of PVDF and led to the decrease of the mechanical and thermal stabilities.Moreover,XRD and FTIR results revealed that the crystal structure of PVDF underwent a transition from ?-phase to ?-phase due to the addition of TEOS.The contact angle and UF experimental results of PVDF–SiO 2composite membranes showed an improvement of hydrophilicity and permeability.The PVDF–SiO 2membrane prepared from the dope with 3wt.%TEOS concentration had the best UF performance and antifouling property.

? 2009 Elsevier B.V. All rights reserved.

1.Introduction

Currently,polymers are still the main materials in membrane technology with the advantages of good membrane forming ability,?exibility and low cost [1–3].However,limited chemical,mechan-ical and thermal resistance restrict the application of polymer materials [4–6].As reported in literatures [7,8],ceramics mem-branes have higher thermal and chemical resistance as well as longer lifetime,but they are still expensive and brittle with poor membrane forming https://www.360docs.net/doc/c118233106.html,posite materials could combine basic properties of organic and inorganic materials and offer spe-ci?c advantages for the preparation of arti?cial membranes with excellent separation performances,good thermal and chemical stability and adaptability to the harsh environments,as well as membrane forming ability [9–12].Therefore,organic–inorganic composite materials as new membrane materials have attracted more and more attentions [13,14].

?Corresponding author at:State Key Laboratory of Chemical Engineering,East China University of Science and Technology (ECUST),130Meilong Road,Shanghai 200237,China.Tel.:+862164252989;fax:+862164252989.

E-mail address:chemxuzl@https://www.360docs.net/doc/c118233106.html, (Z.-L.Xu).Polyvinylidene ?uoride (PVDF)is one of the most extensively applied membrane materials in the industry for outstanding antiox-idation,superior thermal and hydrolytic stabilities,as well as good mechanical and membrane forming properties.However,its hydrophobic nature that often results in severe membrane fouling and decline of permeability,has been a barrier for their application in water treatment [15].Many studies have attempted to improve the hydrophilicity of PVDF membranes with various techniques,such as physical blending,chemical grafting and surface modi?ca-tion [16].Among these methods,blending with inorganic materials is interesting due to its convenient operation and mild condition [17].

Inorganic materials that could be blended with PVDF include titanium dioxide (TiO 2)[18],zirconium dioxide (ZrO 2)[19],alu-mina (Al 2O 3)[20]and some small molecule inorganic salts,such as lithium salts [21].Among the numerous inorganic materials,sil-ica (SiO 2)is the most convenient and widely used because of its mild reactivity and well-known chemical properties [22].A simple method to obtain an organic–inorganic hybrid is mixing an organic polymer with a metal alkoxide,such as tetraethoxysilane (TEOS),followed by a sol–gel process involving hydrolysis and polyconden-sation of TEOS [23].Recently,the sol–gel technique has provided new opportunities for the preparation of organic–inorganic materi-

0376-7388/$–see front matter ? 2009 Elsevier B.V. All rights reserved.doi:10.1016/j.memsci.2009.03.054

258L.-Y.Yu et al./Journal of Membrane Science337 (2009) 257–265

als,which allows the formation of inorganic framework under mild condition and incorporation of minerals into polymers,resulting in increased chemical,mechanical and thermal stabilities without obviously decreasing the properties of the polymers[24,25].Fur-thermore,the remaining hydrogen bond clusters at the surfaces of the materials after the sol–gel reaction improve the membrane hydrophilicity and enhance the stability of composite materials [26–30].

In this work,organic–inorganic PVDF–SiO2composite hol-low?ber UF membranes were prepared by sol–gel process and wet-spinning method.The objective of this work was to system-atically reveal the viscosity and precipitation kinetics,morphology, mechanical and thermal properties,crystalline structure,as well as hydrophilicity and permeability of the PVDF–SiO2composite hollow?ber UF membranes.

2.Experimental

2.1.Materials

PVDF(Solef?1015)was purchased from Solvay Advanced Polymers,L.L.C.N,N-dimethylacetamide(DMAc),N,N-dimethyl-formamide(DMF)and tetraethoxysilane(TEOS)were all obtained from Shanghai Chemical Reagent Company.Hydrochloric acid(HCl, 36–38%)was of analytical grade(from Beijing Chemical Reagent Company)and used as received.Bovine serum albumin(BSA) (M w=67,000)was purchased from Shanghai Bio Life Sci and Tech Co.Ltd.

2.2.Preparation of hollow?ber membrane and module

13.1mL TEOS and1.8mL deionized water were added to18.1mL DMF with vigorous stirring for2h at25?C.After mixing uniformly, stable and transparent SiO2sol was obtained with pH3.0adjusted by hydrochloric acid.

SiO2sol with different concentration(0,1,2,3,4and5wt.%TEOS in dopes,which were labeled as MTEOS-0,MTEOS-1,MTEOS-2, MTEOS-3,MTEOS-4and MTEOS-5,respectively)was added drop-wise to the dope of DMAc containing18wt.%PVDF with constant stirring at25?C for24h to get a homogenous PVDF–SiO2dope for spinning.

PVDF–SiO2hollow?ber UF membranes were spun by wet-spinning method at25?C,described elsewhere[13,31].The bore ?uid solution and coagulant were40wt.%ethanol aqueous solution and pure water,respectively.The fabricated hollow?ber mem-branes were kept in the water bath for24h to remove the residual solvents,and then immersed in a tank containing50wt.%glyc-erol aqueous solution for24h to prevent the collapse of porous structures[32].And the membranes were dried in air at room tem-perature for making test modules.

Membrane modules were prepared to test the hollow?ber sep-aration performances in terms of permeation?ux and rejection quantitatively.The hollow?ber UF membrane modules were self-prepared(outside feeding),and the external diameter and inner diameter of the pipeline were0.8and0.6cm,respectively.Four hol-low?bers with an effective length of22.5cm were composed into a module.The shell sides of the two ends of the bundles were glued onto two stainless steel tees using a normal-setting epoxy resin. These modules were left overnight for curing before test.To elimi-nate the effect of the residual glycerol on module performance,each module was immersed in water for24h and run in the test system for1h under a pressure of0.1MPa before any sample collection. The schematic outline of the sample preparation was illustrated in Fig.1.

2.3.Membrane characterization

2.3.1.Viscosity and light transmittance measurements

The viscosity of dopes with different TEOS concentrations was investigated by a DV-II+PRO Digital Viscometer(Brook?eld,USA)at 25?C controlled by water bath.

Light transmittance measurement studying precipitation kinet-ics was a self-made device.The experiments were described by Li et al.[33].The light source of laser was directly exposed to a dope, about20cm above it.The dope was immersed into the water bath used as coagulation?uid.The transmitted light was detected by an optical detector.The accepted signal was then A/D converted, ampli?ed and recorded by a computer.The intensity of the trans-mitted light through the dope was measured as a function of time.

2.3.2.Morphology observation

The morphology of membranes was examined by a scanning electron microscopy(SEM)(JEOL Model JSM-6360LV,Japan).The ?bers were?rstly immersed into liquid nitrogen for a few min-utes,then broken and deposited on a copper holder.All samples were coated with gold under vacuum before test.For the same sam-ples of SEM,the map scan spectrum of energy dispersion of X-ray (EDX)(EDAX Falcon,USA)was applied to detect the particle dis-tribution pro?le on the external surface of the PVDF–SiO2hollow ?ber membrane.

2.3.3.Mechanical properties and thermal stability analyses

Mechanical properties of membranes were measured by a mate-rial test machine(SHIMADZU,Japan)at a loading velocity of 50mm/min.The report values were measured3times for each sam-ple and then averaged.The thermal stability of membranes was evaluated by thermal gravitational analysis(TGA,TA SDT-Q600, USA).The TGA measurements were carried out under nitrogen atmosphere at a heating rate of10?C/min from50to800?C.

2.3.4.X-ray diffraction(XRD)and FTIR analyses

The membrane XRD patterns were recorded on a D/max-rB diffractometer(Rigaku,Japan)equipped with graphite monochro-mated Cu K?radiation( =0.15405nm)operated at50mA and 50kV from10?to80?.The FTIR spectra of the membranes were measured using a FTIR-ATR spectrometer(Thermo

Electron Fig.1.Preparation method for PVDF–SiO2composite hollow?ber membranes.

L.-Y.Yu et al./Journal of Membrane Science 337 (2009) 257–265259

Corp.,Nicolet 5700,USA)operating in the wavenumber range 4000–400cm ?1.

2.3.5.Hydrophilicity,porosity and pore size measurements

The contact angle (?)between water and the external surface of hollow ?ber membrane was measured to evaluate the membrane hydrophilicity using a JC2000A Contact Angle Meter produced by Shanghai Zhongcheng Digital Equipment Co.,Ltd.(China).To mini-mize experimental error,the contact angles were measured 5times for each sample and then averaged.The membrane porosity ε(%)was de?ned as the volume of the pores divided by the total volume of the porous membrane.It could usually be determined by gravi-metric method,determining the weight of liquid (here pure water)contained in the membrane pores [33].ε=

(w 1?w 2)/d w

(w 1?w 2)/d w +w 2/d p

×100%

(1)

where w 1was the weight of the wet membrane (g),w 2was the weight of the dry membrane (g),d w was the pure water density (0.998g cm ?3)and d p was the polymer density (as the inorganic content in the membrane matrix was small and d p was approximate to d PVDF ,namely 1.765g cm ?3).

Mean pore radius r m (?m)was determined by ?ltration velocity method.According to Guerout–Elford–Ferry equation,r m could be calculated [34]:

r m =

(2.9?1.75ε)×8álQ

ε×A × P

(2)

where áwas the water viscosity (8.9×10?4Pa s),l was the mem-brane thickness (m),Q was the volume of the permeate water per unit time (m 3s ?1),A was the effective area of the membrane (m 2)and P was the operational pressure (0.1MPa).

Maximum pore size r max (?m)could be obtained by bubble point method.According to Laplace’s equation,maximum pore size could be calculated [35]:r max =

2 cos ?

P

(3)

where was the surface tension of water (71.96×10?3N m ?1),?was the contact angle of water to membrane (?)and P was the minimum bubble point pressure (Pa).

2.3.6.Permeation property measurements

The permeation ?ux and rejection of the membranes were mea-sured by UF experimental equipment,as described by Lang et al.[15].The rejection test was carried out with an aqueous solution of BSA (300mg L ?1).All experiments were conducted at 25?C and under the feed pressure of 0.1MPa.The newly prepared hollow ?ber membranes were pre-pressured at 0.1MPa using the pure water for 1h before measurement,then the pure water permeation (J w )was measured,?nally the permeation ?ux (J BSA )and rejection (R )for the BSA solution were measured.The concentrations of BSA in the permeate and feed were determined by an UV-spectrophotometer (Shimadzu UV-3000,Japan).The permeation ?ux (J )and rejection (R )were de?ned as formulae (4)and (5),respectively [15].J =Q A ×T

(4)

R =

1?

C P C F

×100%(5)

where J was the permeation ?ux of membrane for pure water or BSA solution (L h ?1m ?2),Q was the volume of the permeate pure water or BSA solution (L),A was the effective area of the membrane (m 2)and T was the permeation time (h).R was the rejection to BSA (%),

C P and C F were the permeate and feed concentration,respectively (wt.%).

The membrane surface morphology,in terms of the mean sur-face roughness (R a ),was measured by an atomic-force microscopy (BioScope TM ,USA)using the tapping mode.The mean roughness was de?ned as the average value of the surface relative to the cen-ter plane for which the volumes enclosed by the images above and below the plane were equal.This parameter was calculated using the following equation [16]:R a =

1

L x L y

L x

L y

f (x,y ) d x d y

(6)

here f (x ,y )was the surface relative to the central plane,and L x and L y were the dimensions of the surface.3.Results and discussion

3.1.Viscosity and precipitation kinetics of dopes

The viscosity of dope in?uenced the exchanging rate of sol-vent and non-solvent through the dope,furthermore changed the hydrolysis and condensation rate of TEOS during phase inver-sion.The demixing rate played an important role in the formation of microstructures and performances of the membranes [36].In Fig.2,the viscosity of PVDF dopes increased with the increment of TEOS concentration.The addition of SiO 2sol in the PVDF solution increased the concentration of the dope,and consequently inten-si?ed the interaction force among PVDF macromolecules [36].At the same time,because the hydroxyl groups of SiO 2sol might have a stronger adsorption on polymeric chains [24]and SiO 2sol had a higher viscosity [36],the dope viscosity increased at higher TEOS concentration.

Light transmittance experiment was used for the investigation of the precipitation kinetics of dopes.The pure water was used as the coagulation bath.In Fig.3,the addition of TEOS accelerated the demixing process,in particular,the precipitation rate reached the highest at 3wt.%TEOS in dope.

As shown in Fig.3,the precipitation rate ?rstly increased and then decreased with increasing TEOS concentration.This phe-nomenon was related to the counterpart effects of TEOS.Generally the addition of hydrophilic substance in the dope led to an acceler-ation of solvent and non-solvent exchange and was advantageous to form a porous structure [15].Due to the hydrolysis of TEOS,

the

Fig.2.Effect of TEOS concentration on the viscosity of PVDF dopes.

260L.-Y.Yu et al./Journal of Membrane Science

337 (2009) 257–265

Fig.3.Effect of TEOS concentration in PVDF dopes on precipitation kinetics(using pure water as coagulation?uid at room temperature).

resulting hydrophilic SiO2absorbed water quickly than hydropho-bic PVDF,which promoted the demixing process of the system.On the other side,precipitation rate was also related to the dope vis-cosity[36].A higher viscosity would decrease the precipitation rate of the dopes.The addition of SiO2sol increased the dope viscosity at higher concentration as shown in Fig.2.That was not favorable for the diffusion of water and solvent,resulted in a decrease of the precipitation rate.The?nal behavior of PVDF–SiO2dope depended on the comprehensive result of two effects.Therefore,the precip-itation rate decreased when the TEOS concentration in dope was higher than3wt.%(MTEOS-4and MTEOS-5).

3.2.Morphologies of membranes

The cross-section morphologies of PVDF and PVDF–SiO2mem-branes were shown in Fig.4.With the increase of TEOS concentration,a typical transition from asymmetric structure to microcellular structure could be observed in membrane cross-section.The topical asymmetric structure of membrane cross-section(MTEOS-0,MTEOS-1and MTEOS-2)became faint and the structure of sublayer underwent a transition from?nger-like to sponge-like structure(MTEOS-3,MTEOS-4and MTEOS-5).More-over,no aggregation of SiO2particles was observed,which meant that SiO2was homogeneously dispersed in PVDF matrix.The reason could be the formation of a crosslinking structure of SiO2particles with polymeric chains after the sol–gel process,which restrict-ing the growth of the?nger-like cavities.Moreover,the increase in the dope viscosity due to the addition of SiO2sol(as seen in Fig.2)hindered the demixing process,resulting in the suppression of macrovoids[8].

The morphologies for internal and external surfaces of the PVDF membranes with and without SiO2were shown in Figs.5and6. The lower amount of added TEOS(≤3wt.%)resulted in the increase of surface pores compared with PVDF membrane(MTEOS-0).This phenomenon could be interpreted as follows:the precipitation rate increased with increasing TEOS concentration from MTEOS-0to MTEOS-3(as shown in Fig.3),which favored the formation of a porous structure.However,higher TEOS concentration increased the dope viscosity(as seen in Fig.2),thereby slowing down the precipitation rate and leading to a denser surface(such as MTEOS-4 and MTEOS-5in Figs.5and6).

EDX map scanning spectra in Fig.7showed the disper-sion–aggregation phenomenon of silica on the external surface of PVDF–SiO2hollow?ber membrane made from MTEOS-3.The dif-ferent color spots over the dark background indicated the location of the relative elements on the external surface of composite mem-brane.Red,green and blue spots in Fig.7corresponded to carbon (C),?uorin(F)and silicon(Si),respectively.The EDX spectra showed almost uniform distribution of silicon,which indicated better dis-persion of silica in PVDF–SiO2hollow?ber membrane made from MTEOS-3.

3.3.Mechanical properties and thermal stability of membranes

The results of mechanical strength test including break strength and Young’s modulus were listed in Table1.It was clear that the mechanical strength of membranes enhanced with the increase of inorganic content,especially,at3wt.%TEOS concentration, the break strength and Young’s modulus reached the peak value, respectively,and then declined with the further increase of TEOS concentration.At higher TEOS concentration,the formed SiO2net-work increased the rigidness of membrane[37]and con?ned the crystallization of PVDF(which would be discussed later),this

?nally Fig.4.Cross-section morphologies of MTEOS-0to MTEOS-5(magni?cation300×).

L.-Y.Yu et al./Journal of Membrane Science 337 (2009) 257–265

261

Fig.5.Internal surface morphologies of MTEOS-0to MTEOS-5(magni?cation 20,000×

).

Fig.6.External surface morphologies of MTEOS-0to MTEOS-5(magni?cation 20,000×).

led to the decrease of the mechanical properties,such as elongation at break.

Fig.8shows the magni?cation of sublayer membranes with dif-ferent TEOS concentrations.All the membranes had a sponge-like cross-section but also presented some differences in compact mor-phologies.For the pure PVDF membrane,cellular pores formed in the cross-section.When the TEOS addition amount was 3wt.%,the cross-section became a little denser.While with the TEOS concen-tration increasing to 5wt.%,there were some massive aggregates in

Table 1

Mechanical properties of PVDF–SiO 2hollow ?ber membranes.Membrane no.Break strength (MPa)Young’s modulus (MPa)Elongation at break (%)MTEOS-0 1.61 5.75257MTEOS-1 1.908.50189MTEOS-2 2.2110.4181MTEOS-3 2.3911.3160MTEOS-4 2.109.4386.8MTEOS-5

1.73

7.33

73.9

polymeric matrix and led to looser structure in cross-section.The SEM images indicated that the addition of TEOS greatly in?uenced the membrane structures,thereby altering the mechanical prop-erties of composite membranes.There were interactions between SiO 2and PVDF.SiO 2could act as a crosslinking point in composite membranes to link the polymeric chains [22]and increase the rigid-ity of polymeric chains.So more energy was needed to break down the bond between SiO 2and PVDF,and the mechanical strength of composite membranes was improved.

As shown in Fig.9,the higher thermal decomposition temper-ature T d (de?ned as the temperature at 3%weight loss)for the PVDF–SiO 2composite membranes,compared with that of the pure PVDF membrane,indicated a better thermal stability of the com-posite membranes,probably because of the interactions between PVDF chains and SiO 2surface groups.3.4.Crystalline structure of membranes

PVDF could crystallize in four different polymorphs (?,?,?,and ?),and each crystal structure had different polymorphs [38].

262L.-Y.Yu et al./Journal of Membrane Science

337 (2009) 257–265

Fig.7.EDX map scanning spectra for the external surface of

MTEOS-3.

Fig.8.Sublayer morphologies of MTEOS-0,MTEOS-3and MTEOS-5(magni?cation 10,000×).

Here crystalline phases of PVDF membranes were investigated by XRD and FTIR spectroscopy.Fig.10shows the XRD patterns of PVDF membranes with and without SiO 2.The diffraction peaks at 2?=18.4?,20.0?and 26.6?assigned to ?(010),?(110),and ?(021),respectively,were the characteristic of the ?-phase crystal struc-ture [38].This meant that pure PVDF membrane contained

mainly

Fig.9.TGA curves of PVDF–SiO 2membranes:MTEOS-0,T d 428.12?C;MTEOS-1,T d 446.41?C;MTEOS-2,T d 450.99?C;MTEOS-3,T d 458.46?C;MTEOS-4,T d 455.73?C;MTEOS-5T d 448.82?C.

?-crystal.With the addition of TEOS,a new peak at 2?=20.78?appeared,which was assigned to ?(110)and ?(200)planes [39].In particular,the strongest diffraction peak at 2?=20.78?occurred in MTEOS-3,which indicated the maximum percent ?PVDF crys-tallinity.The generated SiO 2changed the PVDF crystal formation during the phase inversion process,from the ?-crystal structure to the ?one.The proportion of the latter went through a maximum

at

Fig.10.XRD patterns of PVDF and PVDF–SiO 2membranes with different TEOS con-centrations.

L.-Y.Yu et al./Journal of Membrane Science337 (2009) 257–265

263

Fig.11.FTIR spectra of(a)SiO2prepared by sol–gel method,(b)PVDF membrane and(c)PVDF–SiO2membrane made from MTEOS-3.

3wt.%of TEOS.This change was attributed to the PVDF chain con-?nement by the SiO2network.As reported in literature[40],only a small amount of SiO2networks led to the increase of polymer crys-tallinity in crystalline polymer blends or composites;while beyond a certain amount of SiO2,the crystallization of PVDF was con?ned, similar to poly(ethylene oxide)–SiO2hybrids[40].

Fig.11shows the FTIR spectra of SiO2,PVDF membrane and PVDF–SiO2composite membrane made from MTEOS-3.And the wavenumbers of the FTIR peaks assigned to different groups and crystallites were exhibited in Table2,from which the main conclu-sions would be:(i)con?rmation of the crystalline structures given by XRD,and(ii)the increase in formed–OH groups with the TEOS content.

3.5.Hydrophilicity,porosity and pore size of membranes

The surface hydrophilicity of membranes could affect the?ux and antifouling ability of membranes.In general,the hydrophilic-ity was evaluated by water contact angle,and higher hydrophilicity smaller contact angle[17].The contact angle data of PVDF–SiO2 composite membranes were shown in Table3.The surface hydrophilicity was improved with the SiO2amount in membrane. As revealed by FTIR spectra,large amount of–OH groups on the SiO2 particles generated by TEOS hydrolysis were responsible for the hydrophilicity increasing[8].However,the contact angle of com-posite membranes increased when TEOS concentration was higher than3wt.%.The reason could be the formation of large scale SiO2 network at higher TEOS content,which consumed–OH groups due to the condensation reaction between Si–OH.While at lower TEOS concentration,SiO2could be dispersed as smaller particles in PVDF matrix and increased the amount of–OH groups.

Table2

FTIR peak assignments for(a)SiO2prepared by sol–gel method,(b)PVDF membrane and(c)PVDF–SiO2membrane made from MTEOS-3.

Wavenumbers(cm?1)Functional groups and crystallites

3500–3000–OH stretching

2960C–H stretching

1629H–O–H bending

1378C–H deformation

1230,1155C–F stretching

1220,1080Asymmetric Si–O–Si stretching 957Si–OH stretching

800Symmetric Si–O–Si stretching 976,855,766,612,531?-Crystal of PVDF

840,470?-Crystal of PVDF Table3

Contact angle and pore structure parameters of PVDF–SiO2membranes with differ-ent TEOS concentrations.

Membrane

no.

TEOS concentration

(wt.%)

Contact

angle?(?)

Porosity

ε(%)

Pore size

r m(?m)r max(?m) MTEOS-0082.952.50.0740.18 MTEOS-1178.570.20.0890.24 MTEOS-2264.476.50.0960.44 MTEOS-3353.484.20.0990.54 MTEOS-4467.766.50.0950.42 MTEOS-5576.361.40.0820.31

The porosity and pore size information of the prepared mem-branes were listed in Table3.It was shown that the porosity(ε), mean pore size(r m)and maximum pore size(r max)increased for PVDF–SiO2membranes with lower SiO2amount.When the TEOS concentration in dope at higher?lling amount(>3wt.%),the result-ing PVDF–SiO2membranes got smallerε,r m and r max.If PVDF and SiO2were miscible,a stronger repulse force existed which led to a stronger interfacial stress between polymer and SiO2parti-cles,because PVDF was hydrophobic and SiO2was hydrophilic,and ?nally formed interfacial pores due to the shrinkage of organic phase during the demixing process[17].However,if TEOS pro-duced a network structure of SiO2which con?ned the movement of PVDF chains[22],and at higher TEOS loading the condensation reaction induced the formation of denser structure,consequently suppressed defects in the membranes.

3.6.Permeation and antifouling properties of membranes

The in?uences of TEOS concentration on permeability and rejection were investigated through UF experiments.In Fig.12,the PVDF–SiO2composite membrane permeability increased?rstly and then decreased with the increase of SiO2amount,a maximum value301L h?1m?2of pure water and255L h?1m?2of BSA solu-tion presented for membrane prepared from the dope with3wt.% TEOS concentration.The increase of the membrane hydrophilicity and pore size with lower TEOS concentration

(≤3wt.%)in Table3 could attract water molecules inside the membrane matrix and promote them to pass through the membrane and accordingly enhance the permeability.However higher TEOS concentration (>3wt.%)formed a highly viscous dope,which slowed down the formation process of PVDF–SiO2composite membranes.The for-mation of SiO2network compressed the movement of PVDF chains,

Fig.12.Curves of J w and J BSA,rejection(R)and the ratio(J BSA/J w)of PVDF–SiO2 membranes as a function of TEOS concentration.

264L.-Y.Yu et al./Journal of Membrane Science

337 (2009) 257–265

Fig.13.AFM three-dimensional external surface images of membranes:(a)MTEOS-0and (b)MTEOS-3.

hindered the formation of macrovoid structure and decreased the pose size (as shown in Figs.4–6).Moreover,the condensation of SiO 2into network consumed its surface –OH and decreased the hydrophilicity,resulting in the decrease of the permeability.Furthermore,the rejection changed slightly with the increase of TEOS concentration and had a minimum at 3wt.%TEOS.This result could be explained by the fact that the increase of pore size had a negative effect on rejection when TEOS concentration under 3wt.%.However,at higher concentration (>3wt.%),the formation of denser structure resulted in the increase of rejection.

Besides,the antifouling properties of PVDF–SiO 2composite hol-low ?ber UF membranes could be evaluated by the ratio of BSA solution ?ux (J BSA )and pure water ?ux (J w ).For the higher antifoul-ing UF membrane,the addition of BSA in the feed solution would cause a little ?ux loss and the ratio (J BSA /J w )would be higher [15].As shown in Fig.12,the ratio (J BSA /J w )increased ?rstly and then decreased slightly for PVDF–SiO 2composite membrane prepared from the dope with the increase of TEOS concentration.

Fig.13displayed three-dimensional AFM images of the mem-brane external surfaces.The surface roughness of PVDF–SiO 2composite membrane made from MTEOS-3was apparently higher than that of the PVDF membrane.In the range of the scan area 10?m ×10?m,the R a of the MTEOS-0and MTEOS-3membranes were 19.2and 30.1nm,respectively.Higher roughness might relate to the higher porosity of the external surface of membrane as seen in Fig.6,which could lead to two changes in the compos-ite membranes:one was an increase of ef?cient ?ltration area and another was a decrease of the antifouling performance [16].Cer-tain amount of hydroxyl groups on the hydrophilic SiO 2particles [41]was bene?cial to the membrane permeation.The enlarged ef?cient membrane area would increase the membrane ?ux.The membrane-fouling trend increased with roughness owing to con-taminants accumulating in the “valleys”of the rough membrane surfaces [16].Since the introduction of SiO 2particles increased the hydrophilicity of the membrane surface and pore walls,the adsorbed foulants on the PVDF–SiO 2composite membranes could be more readily dislodged by shear force than those on the pure PVDF membrane,and thus improving the antifouling performance of membrane.4.Conclusions

Organic–inorganic PVDF–SiO 2composite hollow ?ber UF mem-branes were synthesized using the sol–gel and wet-spinning process.The microstructure,mechanical property and thermal sta-bility,hydrophilicity,permeation and antifouling performance of composite membranes were improved apparently by an appropri-ate choice of TEOS concentration.The main conclusions were listed as follows:

(1)The dope viscosity increased with the increase of TEOS concen-tration.The addition of TEOS accelerated the demixing process,in particular,the precipitation rate reached the highest at 3wt.%TEOS in dope.

(2)Macrovoids were restricted or eliminated with the formation of

a sponge-like cross-section.The membranes with different sur-face morphology were produced due to the effect of inorganic particles on the dope viscosity and the precipitation rate.The SiO 2particles distributed uniformly in the membrane matrix at 3wt.%TEOS.

(3)Mechanical property,thermal stability,XRD and FTIR analy-ses indicated that there was a crosslinking structure between inorganic network and polymeric chains,which led to the improvement of mechanical and thermal properties.Crys-talline analysis results showed that the composite membranes possessed predominantly PVDF ?-phase while the polymeric membrane presented mainly PVDF ?-phase,especially the maximum percent crystallinity of PVDF occurred at the content of 3wt.%TEOS.

(4)The presence of SiO 2particles generated by TEOS hydrolysis

containing an amount of hydroxyl groups,responsible for the hydrophilicity increase of the composite membranes.At the same time,the improvement of membrane hydrophilicity also enhanced both the permeability and antifouling performance.Particularly,the composite membrane with the maximum ?ux and highest pollution resistance was obtained when TEOS was 3wt.%,which was attributed to the maximum pore size of sur-face as well as the increase of the membrane’s porosity and hydrophilicity at 3wt.%TEOS concentration.Acknowledgement

The authors acknowledge the National Key Fundamental Research Development Plan (“973”Plan,No.2003CB615705)for giving ?nancial supports in this project.References

[1]J.Xu,Z.L.Xu,Poly(vinyl chloride)(PVC)hollow ?ber ultra?ltration membranes

prepared from PVC/additives/solvent,J.Membr.Sci.208(2002)203–212.

[2]Z.L.Xu,F.A.Qusay,Polyethersulfone (PES)hollow ?ber ultra?ltration mem-branes prepared by PES/non-solvent/NMP solution,J.Membr.Sci.233(2004)101–111.

[3]T.S.Chung,Z.L.Xu,W.H.Lin,Fundamental understanding of the effect of air-gap

distance on the fabrication of hollow ?ber membranes,J.Appl.Polym.Sci.72(1999)379–395.

[4]A.V.R.Reddy,D.J.Mohan,A.Bhattacharya,et al.,Surface modi?cation of ultra-?ltration membranes by preadsorption of a negatively charged polymer.I.Permeation of water soluble polymers and inorganic salt solutions and fouling resistance properties,J.Membr.Sci.214(2003)211–221.

[5]Z.L.Xu,T.S.Chung,K.C.Loh,et al.,Polymeric asymmetric membranes

made from polyetherimide/polybenzimidazole/poly(ethylene glycol)

L.-Y.Yu et al./Journal of Membrane Science337 (2009) 257–265265

(PEI/PBI/PEG)for oil–surfactant–water separation,J.Membr.Sci.158(1999) 41–53.

[6]Z.L.Xu,T.S.Chung,Y.Huang,Effect of polyvinylpyrrolidone molecular weights

on morphology,oil/water separation,mechanical and thermal properties of polyetherimide/polyvinylpyrrolidone hollow?ber membranes,J.Appl.Polym.

Sci.74(1999)2220–2233.

[7]G.Clarizia,C.Algieri,E.Drioli,Filler-polymer combination:a route to modify gas

transport properties of a polymeric membrane,Polymer45(2004)5671–5681.

[8]Y.N.Yang,P.Wang,Preparation and characterizations of a new PS/TiO2hybrid

membranes by sol–gel process,Polymer47(2006)2683–2688.

[9]C.Guizard,A.Bac,M.Barboiu,Hybrid organic–inorganic membranes with spe-

ci?c transport properties:applications in separation and sensors technologies, Sep.Purif.Technol.25(2001)167–180.

[10]Z.H.Lu,G.J.Liu,S.Duncan,Poly(2-hydroxyethyl acrylate-co-methyl

acrylate)/SiO2/TiO2hybrid membranes,J.Membr.Sci.221(2003)113–122. [11]P.C.Chiang,W.T.Whang,M.H.Tsai,et al.,Physical and mechanical properties of

polyimide/titania hybrid?lms,Thin Solid Films447–448(2004)359–364. [12]A.Taniguchi,M.Cakmak,The suppression of strain induced crystallization

in PET through sub micron TiO2particle incorporation,Polymer45(2004) 6647–6654.

[13]T.S.Chung,Z.L.Xu,Asymmetric hollow?ber membranes prepared from mis-

cible polybenzimidazole and polyetherimide blends,J.Membr.Sci.147(1998) 35–47.

[14]Z.L.Xu,F.A.Qusay,Effect of polyethylene glycol molecular weights and concen-

trations on polyethersulfone hollow?ber ultra?ltration membranes,J.Appl.

Polym.Sci.91(2004)3398–3407.

[15]https://www.360docs.net/doc/c118233106.html,ng,Z.L.Xu,H.Yang,et al.,Preparation and characterization of PVDF–PFSA

blend hollow?ber membrane,J.Membr.Sci.288(2007)123–131.

[16]Y.Lu,S.L.Yu,B.X.Chai,et al.,Effect of nano-sized Al2O3-particle addition on

PVDF ultra?ltration membrane performance,J.Membr.Sci.276(2006)162–167.

[17]Y.N.Yang,H.X.Zhang,P.Wang,et al.,The in?uence of nano-sized TiO2?llers

on the morphologies and properties of PSF UF membrane,J.Membr.Sci.288 (2007)231–238.

[18]X.C.Cao,J.Ma,X.H.Shi,et al.,Effect of TiO2nanoparticle size on the performance

of PVDF membrane,Appl.Surf.Sci.253(2006)2003–2010.

[19]A.Bottino,G.Capannelli,https://www.360docs.net/doc/c118233106.html,ite,Preparation and characterization of novel

porous PVDF–ZrO2composite membranes,Desalination146(2002)35–40. [20]Y.Lu,S.L.Yu,B.X.Chai,Preparation of poly(vinylidene?uoride)(pvdf)ultra?l-

tration membrane modi?ed by nano-sized alumina(Al2O3)and its antifouling research,Polymer46(2005)7701–7706.

[21]D.J.Lin,C.L.Chang,L.P.Cheng,et al.,Effect of salt additive on the formation

of microporous poly(vinylidene?uoride)membranes by phase inversion from LiClO4/water/DMF/PVDF system,Polymer44(2003)413–422.

[22]T.Ogoshi,Y.Chujo,Synthesis of poly(vinylidene?uoride)(PVdF)/silica hybrids

having interpenetrating polymer network structure by using crystallization between PVdF chains,J.Polym.Sci.A:Polym.Chem.43(2005)3543–3550. [23]J.W.Park,Y.A.Seo, C.S.Ha,Investigating the crystalline structure of

poly(vinylidene?uoride)(PVDF)in PVDF/silica binary and PVDF/poly(methyl methacrylate)/silica ternary hybrid composites using FTIR and solid-state19F MAS NMR spectroscopy,Macromolecules37(2004)429–436.[24]J.W.Cho,K.I.Sul,Characterization and properties of hybrid composites prepared

from poly(vinylidene?uoride-tetra?uoroethylene)and SiO2,Polymer42(2001) 727–736.

[25]R.A.Zoppi,C.G.A.Soares,Hybrids of poly(ethylene oxide-b-amide-6)and ZrO2

sol–gel:preparation,characterization,and application in processes of mem-brane separation,Adv.Polym.Technol.21(2002)2–16.

[26]J.H.Kim,Y.M.Lee,Gas permeation properties of poly(amide-6-b-ethylene

oxide)-silica hybrid membranes,J.Membr.Sci.193(2001)209–225.

[27]S.H.Zhong,C.F.Li,X.F.Xiao,Preparation and characterization of polyimide–silica

hybrid membranes on kieselguhr-mullite supports,J.Membr.Sci.199(2002) 53–58.

[28]C.M.Wu,T.W.Xu,W.H.Yang,Fundamental studies of a new hybrid

(inorganic–organic)positively charged membrane:membrane preparation and characterizations,J.Membr.Sci.216(2003)269–278.

[29]D.S.Kim,H.B.Park,Y.M.Lee,Preparation and characterization of crosslinked

PVA/SiO2hybrid membranes containing sulfonic acid groups for direct methanol fuel cell applications,J.Membr.Sci.240(2004)37–48.

[30]R.K.Nagarale,V.K.Shahi,R.Rangarajan,Preparation of polyvinyl alcohol–silica

hybrid heterogeneous anion-exchange membranes by sol–gel method and their characterization,J.Membr.Sci.248(2005)37–44.

[31]S.P.Deshmukh,K.Li,Effect of ethanol composition in water coagulation bath

on morphology of PVDF hollow?bre membranes,J.Membr.Sci.150(1998) 75–85.

[32]Y.M.Wei,Z.L.Xu,F.A.Qusay,et al.,Polyvinyl alcohol/polysulfone(PVA/PSF)hol-

low?ber composite membranes for pervaporation separation of ethanol/water solution,J.Appl.Polym.Sci.98(2005)247–254.

[33]J.F.Li,Z.L.Xu,H.Yang,Microporous polyethersulfone membranes prepared

under the combined precipitation conditions with non-solvent additives, Polym.Adv.Technol.19(2008)251–257.

[34]C.S.Feng,B.L.Shi,Y.L.Wu,et al.,Preparation and properties of microporous

membrane from poly(vinylidene?uoride-co-tetra?uoroethylene)(F2.4)for membrane distillation,J.Membr.Sci.237(2004)15–24.

[35]J.F.Li,Z.L.Xu,H.Yang,et al.,Hydrophilic microporous PES membranes prepared

by PES/PEG/DMAc casting solutions,J.Appl.Polym.Sci.107(2008)4100–4108.

[36]Y.N.Yang,J.Wu,X.S.Chen,et al.,The research of rheology and thermodynam-

ics of organic–inorganic hybrid membrane during the membrane formation,J.

Membr.Sci.311(2008)200–207.

[37]J.F.Li,Z.L.Xu,H.Yang,et al.,Effect of TiO2nanoparticles on the surface mor-

phology and performance of microporous PES membrane,Appl.Surf.Sci.255 (2009)4725–4732.

[38]J.W.Kim,W.J.Cho,C.S.Ha,Morphology,crystalline structure,and properties of

poly(vinylidene?uoride)/silica hybrid composites,J.Polym.Sci.B:Polym.Phys.

40(2002)19–30.

[39]J.Buckley,P.Cebe,D.Cherdack,Nanocomposites of poly(vinylidene?uoride)

with organically modi?ed silicate,Polymer47(2006)2411–2422.

[40]J.W.Cho,K.I.Sul,Crystallization of poly(vinylidene?uoride)–SiO2hybrid com-

posites prepared by a sol–gel process,Fibers Polym.2(2001)135–140.

[41]P.Xu,H.T.Wang,Q.G.Du,et al.,Preparation and morphology of SiO2/PMMA

nanohybrids by microemulsion polymerization,Colloid Polym.Sci.284(2006) 755–762.