TIPS behavior for IPP/nano-SiO2blend membrane formation and its contribution to membrane morphology and performance☆

Zhensheng Yang*,Zheng SunDongsheng CuiPingli Li,Zhiying Wang

1School of Chemical Engineering and Technology,Hebei University of Technology,Tianjin 300130,China

2School of Chemical Engineering and Technology,Tianjin University,Tianjin 300072,China

Keywords:Isotactic polypropylene Nanoparticles Blend Membrane Thermally induced phase separation

A B S T R A C T In the present work,the TIPS behavior of isotactic polypropylene(iPP)/di-n-butyl phthalate(DBP)/dioctyl phthalate(DOP)/nano-SiO2system and the competition relation between liquid-liquid phase separation and polymer crystallizationaresuccessfullyadjustedbyaddingnano-SiO2.Theliquid-liquidphaseseparationtemperatureofthesystem increases with increasing nano-SiO2content.Besides,iPP crystallization temperature is also changed after adding nano-SiO2.IPP/nano-SiO2blend hollow fiber microporous membrane is prepared via TIPS method.SEM photos show that the membrane exhibits mixed morphology combining cellular structure relating to liquid-liquid phase separation and branch structure originating from polymer crystallization.The relative weight of cellular structurefirst decreases and then increases with the increase of nano-SiO2content.Furthermore,porosity,connectivity among pores and pure water flux of the membrane first increase and then decrease with increasing nano-SiO2content.However,mechanicalperformanceofthemembraneisimprovedatalltimeswithincreasingnano-SiO2content.

1.Introduction

Isotactic polypropylene(iPP)is an outstanding membrane material because of its low cost,good mechanical properties,high thermal stability and excellent resistance to acids,alkalis and organic solvents.However,due to the lack of solvent for iPP at room temperature,iPP porous membranes are difficult to prepare by non-solvent induced phase separation(NIPS)process[1].An alternative way is thermally induced phase separation(TIPS)method,where a homogeneous iPP solution is formed by dissolving iPP in diluents at about the melting temperature of pure iPP,and then phase separation is induced by cooling the iPP solution[2].In addition,TIPS process has many unique advantages such as low tendency to form finger-like macrovoids,high porosity,and the ability to form interesting microstructures with narrow pore size distribution[3,4].

However,drawbacks,such as severe fouling,enclosed pore structure,hamperthewideapplicationofiPPporousmembranes[1].Doping hydrophilic inorganic particles in the casting solution is an ideal way to overcome these drawbacks.Although some efforts have been made to obtain polymer/inorganic particles blend porous membrane by NIPS method,a limited amount of researches have been conducted on the preparation of porous membrane with polymer/diluents/inorganic particles system via TIPS method[5-13].

Shi et al.[5]prepared PVDF/IL-TiO2(poly(vinylidene fluoride)denoted as PVDF,ionic liquid modified nano-TiO2denoted as IL-TiO2)hybrid membranes via TIPS method,using dimethyl phthalate(DMP)as diluent and IL-TiO2as additive.The addition of IL-TiO2particles to the PVDF/DMP mixture had a strong effect on crystallization temperature and crystallization formation in the TIPS process.Pure water flux and porosity first increased and then decreased with the increase of IL-TiO2content,the antifouling property of hybrid membrane was improved by the addition of IL-TiO2.Liu et al.[7]investigated the effects of modified nanoSiO2agents on the morphology and performance of ultra-high-molecular weight polyethylene(UHMWPE)membranes via TIPS.The results showed that the morphology of the UHMWPE membrane could be disturbed by modified nanoSiO2.Porosity and the rejection ofBovineserum albumin(BSA)increased withincreasingmodified nanoSiO2content.Shi and Liu investigated the effect of inorganic particles on the crystallization process of the system,which,however,did not include the competition relationship between liquid-liquid phase separation and polymer crystallization.Cui et al.[10]studied the effect of SiO2particles on performance of PVDF blend membranes prepared viaTIPSprocess.Theresultsshowedthatwaterfluxandtensilestrength increased with the increase of micro-size SiO2content and attained its maximum at a certain dosage.The morphology of PVDF membranes and the formation of the spherulites were disturbed by the SiO2addition.Li et al.[12]investigated the effects of CaCO3nanoparticles on the morphology and strength of a PVDF flat membrane prepared via the TIPS process.The results showed that the spherulitic morphology of membranes had obviously changed with the different CaCO3dosages,and the existence of CaCO3had a negative effect on the mechanical strength of the membranes.However,Li did not explain the formation mechanism of the morphologies in the phase-separation process in detail,and further investigation on the membrane properties,such as porosity,pore size distribution,and water flux,was lacking.

Based on the above descriptions,we can conclude that the existing studies only simply investigated the effects of inorganic particle content onmembranemorphologyandperformance,andthestudiesoftheinternalmechanism were onlylimited tothe impactof inorganic particlecontent on the crystallization(solid-liquid phase separation)process of the system.We all know that phase separation process can take place in the form of solid-liquid,liquid-liquid ortheircombination,and the competitive relation betweensolid-liquid phase separation and liquid-liquid phase separation is crucial for the final membrane morphology and performance.Thus,it is necessary to explore the action mechanism of inorganic particles on the morphology and performance of the blend membrane prepared via TIPS process from the viewpoint of phase separation.Unfortunately,which is hard to find in the existing reports.

Inourpreviousstudies,theTIPSbehaviourofiPP/di-n-butylphthalate(DBP)/dioctyl phthalate(DOP)system was studied by pseudo-binary approach[14].Furthermore,iPP hollow fiber microporous membrane was prepared via TIPS with the mixed diluents of DBP and DOP[1,15].In the present study,nano-SiO2was added into the above iPP/DBP/DOP system,and iPP/nano-SiO2blend hollow fiber microporous membrane was prepared via TIPS method.The aim of this work is to investigate the effects of nano-SiO2on both TIPS behavior of iPP/DBP/DOP system and the formation process of iPP/nano-SiO2blend membrane.Furthermore,the relationship between phase separation and the morphology andperformanceoftheblendmembranewasdiscussed.Theinformation gained from this study will be useful for the fundamental knowledge necessary to prepare crystalline polymer/inorganic particles blend porous membrane by TIPS method.

2.Experimental

2.1.Materials

IPP(T30S,MFR=3.0 g/10 min)was purchased from Daqing Petrochemical Co.(China).Di-n-butyl phthalate(DBP),dioctyl phthalate(DOP),isopropyl alcohol and ethyl acetate were analysis grade and were purchased from Tianjin Bodi Chemical Reagents Co.(China).Nano-SiO2particles with average particle size from 20 nm to 50 nm were purchased from Zhejiang Hongsheng Material Technology Co.(China).All materials were used without further purification.

2.2.Measurement of TIPS behavior of iPP/DBP/DOP/nano-SiO2system

The iPP mass fraction in casting solution(α)was 0.30,and DBP mass fraction in the mixed diluents(β)was 0.30.The adding amount of nano-SiO2(γ)varied from 0%to 5%based on iPP mass.Firstly,a certain amount of nano-SiO2was uniformly dispersed in mixture of DBP and DOP by an ultrasonic device.Then,the nano-SiO2/DBP/DOP system was fed into a flask with iPP together in proportion,heated at 180°C under a nitrogen gas atmosphere,and stirred for about 3 h until a homogeneous melted solution was obtained.The melted solution was poured on a copper plate and cooled at room temperature,the loss of diluents was below 0.4 wt%based on initial amount of mixed diluent.

An inverted microscope(Ningbo ShunYu Instrument Co.,XD30,China)withahotstage(Linkam,THMS600,UK)andatemperaturecontroller(Linkam,TMS92,UK)was used to measure liquid-liquid phase separation temperature of resulting blend specimens.A specimen of approximately 400 μm in thickness was placed in a small culture dish,then the top edge of culture dish was coated with vacuum grease,and pressed tightly coverslip to prevent diluents loss by evaporation.Liquid-liquid phase separation equilibrium temperature was measured according to Kim's method[16].The sample was heated rapidly to 170°C,annealed at a constant temperature in the vicinity of the expected phase boundary for 10 min.The temperature at which the optical image first started to change during annealing was taken as the liquid-liquid phase separation temperature.With the annealing temperature rising,the induction time would be longer.

A Perkin-Elmer Diamond DSC was used to determine dynamic crystallization temperature of resulting blend specimens.The sealed aluminum DSC pan containing 3-5 mg specimen was heated to 170°C and maintained at this temperature for 10 min.It was then cooled down at the rate of 20 °C·min?1.The onset of the exothermic peak duringthecoolingwastakenasthedynamiccrystallization temperature.

2.3.Membrane preparation

Fig.1.Schematic of TIPS spinning process for preparation of iPP hollow-fiber microporous membrane.

The preparation equipment of iPP hollow fiber microporous membrane is shown in Fig.1,the spinneret consists of outer and inner tubes.The inner diameter of the outer tube,the outer diameter and the inner diameter of the inner tube,are 5.4 mm,3.3 mm and 2.0 mm,respectively.

Based on initial experiment,in the case of α =0.3 and β =0.3,the resultingmembranes showedbetter spinnability,permeability and mechanical strength.Here,as a variable,the nano-SiO2content γ changed from 0%to 5%based on iPP mass.The homogeneous melted solution of nano-SiO2/iPP/DBP/DOP system was obtained,similar to that described in Section 2.2.Subsequently,the melted solution was cooled to 160°C and vacuum degassed.

In the spinning process,the air gap distance was 100 mm in all experiments.The temperature of spinneret was 140°C,theflow of casting solutionwas(12.2±0.2)ml·min?1,theborefluidwasDBPthatdidnot require to preheat,the flow of DBP was 3.8 ml·min?1,the coolant was DBP and its temperature was 80°C.The nascent membranes were wound on a take-up device at a winding rate of(22±2)m·min?1.

The diluents in the nascent membranes were extracted by immersing inethylacetatefor48h,andthentheethylacetatewasreplaced by isopropanol.The final membranes were dried at room temperature.

2.4.Characterization of iPP/nano-SiO2blend membrane

2.4.1.Membrane morphology

The iPP hollow fiber microporous membranes were freeze-dried with a freeze dryer.The dry membranes were fractured in liquid nitrogen and treated with Au/Pd sputtering.The inner surface,outer surface and cross section of the membranes were observed by a Philips XL30 scanning electron microscope(SEM)under an accelerating voltage of 20 kV.

2.4.2.Water contact angle

The static contact angles of iPP hollow fiber microporous membranes were measured using a contact angle goniometer(Dataphysics OCA15).Droplets of distilled water(2 μl)were dropped at different places of the membranes and at least 5 readings were taken to determine average values.

2.4.3.Porosity

The sample with a length of about 100 mm was dried in a vacuum oven for 2 h.The weight of the dry sample was measured and recorded as md(g).The dry sample was immersed in liquid paraffin for 12 h,it wasweighedassoonastheliquidparaffinonoutersurfacewasremoved with filter paper and the liquid paraffin in the fiber tube was blown away,themasswasrecordedasmw(g);Thewetsamplewasplacedvertically in a 1 ml graduated glass tube containing liquid paraffin,and the risingvalueofliquidlevelwasthemembranevolumeVt(ml).Theporosityofresultingmembranesample(ε)wasdefinedasfollowingequation:

where ρ (g·ml?1)is the density of liquid paraffin.At least three samples were tested for each experimental result.

2.4.4.Pore size distribution and connectivity among pores

Poresizedistributionandconnectivityamongporesweretestedbya method reported by Piatkiewicz et al.[17].An immersion membrane module was made with six dry fiber membranes which each had an effective length of 200 mm.Nitrogen gas was forced to permeate from the inside to the outside of the hollow fibers.

Forthedrymembranemodule,thenitrogengasflowunderdifferent pressures was measured.Then,the membrane module was immersed in anhydrous ethanol for 12 h to ensure that all pores of the hollowfibers were filled with anhydrous ethanol.For the wet membrane module,firstly,the operating pressure slowly increased to bubble point pressure,at this time,the corresponding pore size was the maximum.Then the pressure increased by a certain step size,when the gas flow of the wet membrane module was equal to that of the dry membrane module,the corresponding pore size was the minimum.The mean pore size(dm)was corresponded to the transmembrane pressure drop at 50%of the maximum flow.

For iPP hollow fiber membrane,when gas and liquid are nitrogen and ethanol,respectively,Josefiak et al.[18]described the relationship between pore size and transmembrane pressure drop by Eq.(2).

where d(m)and p(Pa)represent pore size and trans-membrane pressure drop,respectively.

The temperature is corrected based on the Yang-Laplace equation.The surface tensions of anhydrous ethanol at 25°C and experimental temperature are 2.363 × 10?2N·m?1and σ N·m?1,respectively,and the relationship between d(m)and p(Pa)can be expressed by Eq.(3)[19].

Based on the Hagen-Poiseuille equation,the pore size distribution function based on the straight through cylindrical pore volume(V)can be expressed by Eq.(4),where it is assumed that the fluid channel consists of straight through cylindrical hole of different pore size[1].

where Ji(m·s?1),di(m)and pi(Pa)represent the total nitrogen flux based on inner surface area of the hollow fiber,pore size and transmembrane pressure drop,respectively,￣di(m)is the mean pore size between pi-1and pi,and it is calculated by Eq.(5).

The porosity of the straight through cylindrical pore is calculated on the basis of the unit membrane wall volume by Eq.(6):

WhereD(m)andδ(m)representtheinnerdiameterandwallthickness of the fiber membrane,η (Pa·s)is the viscosity of nitrogen gas under operating conditions.

Tortuosity factor(τ)is the multiple of the average pore channel length to the membrane thickness[20].A pore model used to estimate the filtration rate of microfiltration membrane is approximated,and get the Eq.(7)[21].

The degree of the pore channel twists and turns,membrane section asymmetry,porosity,etc.will change τ.The closer the τ is to 1,the closer the pore is to the through pore,the better the connectivity among pores is.

2.4.5.Pure water flux

The pure water flux of single fiber was measured at 0.1 MPa,similar to that described by Matsuyama et al.[22].Firstly,the membrane was immersed in isopropanol for 12 h to ensure that all pores of the sample were filled with isopropanol,otherwise the pure water flux was zero.Then,pure water was forced to permeate from the inside to the outside ofthehollowfiber.Timebegantorecord after30minrunning.Thepure water flux was calculated on the basis of the inner surface area of the hollow fiber.For comparison,thepure water flux was linearly corrected to a value at a membrane thickness of 130 μm.At least three samples were tested for each experimental result.

Fig.2.Optical microscope photos of iPP/DBP/DOP/nano-SiO2system,γ =1.0%,the annealing temperature is 130 °C.

2.4.6.Mechanical properties

Nitrogen gas was forced to permeate from the inside to the outside of the hollow fiber with a length of about 100 mm.The pressure that made the hollow fiber rupture was the bursting pressure of the hollowfiber.At least three samples were tested for each experimental result.

The tensile strength and breaking elongation of a single fiber were measured by HD021NS(Nantong Hongda experimental equipment co,China).The fiber membrane was subjected to axial stretching at a stretching rate of 100 mm·min?1.When the sample was broken,the tensilestrengthandbreakingelongationofthefibermembranewereobtained.At least three samples were tested for each experimental result.

3.Results and Discussion

3.1.TIPS behavior of iPP/DBP/DOP/nano-SiO2system

When γ =1.0%and the annealing temperature is 130 °C,the induction time for the occurrence of the optical image is 430 s(see Fig.2).However,when the annealing temperature rises to 131°C,the inductiontimeexceeds40 min.Thus,theliquid-liquid phaseseparation temperature(tcloud)of the system is determined to be 130°C,which is slightly lower than the liquid-liquid phase separation equilibrium temperature[16].The tcloudunder different γ are listed in Table 1.

Table 1 TIPS results of iPP/DBP/DOP/nano-SiO2system

The test results of the DSC are shown in Fig.3,and the DSC data are summarized in Table 1.The onset temperature of crystallization(ton)is takenasthenucleationtemperatureoftheiPP,whichalsoindicatesthat the simple liquid-liquid phase separation stage of the system is terminated and enters crystallization stage of iPP.The full width at half maximumofthecrystalpeak(Δt1)representsthetotalcrystallizationrateof iPP,the larger the Δt1is,the smaller the total crystallization rate is[23].The melting peak temperature(tm)is the dynamic melting point of iPP.

Table1showsthattcloudincreaseswiththeincreasingγ.IPPis anonpolar polymer material,DOP is a good solvent,on the contrary,DBP is a poor solvent[14].However,nano-SiO2is an inorganic material.Therefore,nano-SiO2and the above organic material are quite different in structure.The increase of γ means that the thermodynamic instability of the system will increase and the liquid-liquid phase separation will occur at higher temperature.In the case of γ≥2%,nano-SiO2will form agglomerate particles(see Fig.4(d4-e4)),so tcloudwill not show an obvious increase.

As it can be seen from Table 1,tonfirst increases and then decreases with the increasing γ.This is because nano-SiO2,as a nucleating agent,reduces the interface free energy for crystallization nucleation and increases the nucleation density significantly as well[7,24].However,when γ is relatively high,nano-SiO2will tend to be saturated for nucleation,that is,nano-SiO2will form agglomerate particles in the case of γ≥2%,which will have a gradually stronger rigid hindrance on the nucleation and crystallization process of iPP,so the tonwill decline again[5].TheΔt1isrelatedtothecrystallizationtemperatureandnucleation density,which is consistentwith thechange trendof the crystallization temperature and contrary to that of nucleation density[25].The result that Δt1first decreases and then increases is determined by the combined effect of the two factors,in which the nucleation density plays a key role.The impact of γ on tmis negligible,which means that the degree of crystal completion of each sample is similar.

Fig.3.DSC curves of iPP/DBP/DOP/nano-SiO2systems,(a)cooling curves;(b)heating curves.

Fig.4.SEM images of iPP hollow fiber membranes.(a1)-(e1),inner surface 1000×;(a2)-(e2),outer surface 1000×;(a3)-(e3),cross section 500×;(a4)-(e4),enlarged cross section 10000×.

When the interaction between the crystalline polymer and the diluent is weak,the TIPS process of the polymer solution consists of simple liquid-liquid phase separation stage and subsequent polymer crystallization stage.The competitive relationship between liquidliquid phase separation and polymer crystallization is indicated by Δt(Δt=tcloud? ton)[14,26].When Δt is considerably high,it means that liquid-liquid phase separation is dominant.When γ ≤ 1%,Δt decreases slightly compared with the system without adding nano-SiO2,which means that nano-SiO2can slightly promote the process of polymer crystallization.However,Δt significantly increases with the increase of γ in the case of γ ≥ 2%,namely nano-SiO2is conducive to liquid-liquid phase separation at this time.Therefore,the TIPS behavior of iPP/DBP/DOP system can be successfully adjusted by systematically changing γ.

3.2.Membrane morphology

Fig.4(a-e1)showsthattheinnersurfaceofthemembranesarefilled with narrow cellular pores.This is because the bore fluid DBP dilutes the inner surface layer,which reduces the mass fraction of iPP(α)on the inner surface layer.Furthermore,the double direction diffusion between DBP and DOP increases the mass fraction of DBP(β)on the inner surface layer.As a result,tcloudincreases,the contribution of liquid-liquid phase separation to the membranes morphology is enhanced,and the membranes show a typically cellular morphology[1,15].The axial stretching applied to the membranes deforms the cellular pores into a narrow shape.With the increase of γ,tcloudincreases,that promotes the growth of cellular pores.On the other hand,the viscosity of the casting solution containing nano-particles increases rapidly with the increasing γ,which hinders the double direction diffusion between DBP and DOP,and increases the growth resistance of cellular pores[7].As a result,the membrane morphology is the most loose and the cellular pores are the most obvious in the case of γ=1%.

Fig.4(a2-e2)shows that the cellular pores are absent on the outer surface of the membranes.In the case of γ≤1%,the outer surface of the membranes show rough spherical morphology,and the particle size decreases with the increasing γ.Due to the presence of air gap distance,the evaporation of diluents,especially DBP,results in a decrease in β and an increase in α on outer surface,which restrains the occurrence of liquid-liquid phase separation.As a result,the membranes show rough spherical morphology.Where nano-SiO2acts as a nucleating agent,the increase of γ means that the nucleation density increases,which results in a decrease of spherulite size.In the case of γ≥2%,the outer surface of the membranes is dense morphology.As can be seen from Table 1,Δt(Δt=tcloud? ton)is considerably high at this time,which restrains the occurrence of iPP crystallization.As a result,the liquid-liquid phase separation still can occur even if the diluents evaporate.However,the quenching effect of the cooling bath,the rapid gelation of the outer surface and the higher α can restrain the appearance of cellular pores.Therefore,the outer surface of the membranes is dense morphology.

Fig.4(a3-e4)showsthatthemembranesamplesexhibitmixedmorphology combining cellular structure relating to liquid-liquid phase separation and branch structure originating from polymer crystallization.Inthecaseofγ=0%,therelativeweightofcellularstructureissignificantly greater than that of branch structure,which is the normal morphology of liquid-liquid phase separation and subsequent polymer crystallization.Forγ =0.5%and γ =1.0%,therelative weightof branch structure increases and the dimension of branch structure becomes smaller,with the increase of γ.As we all know,the total crystallization rate of polymer is determined by both the nucleation rate and the growth rate[27].The viscosity of the casting solution increases rapidly with the increasing γ,which reduces the growth rate of the crystal.In fact,however,the total crystallization rate is increased(Δt1is decreased).Thus,we can conclude that the increase of total crystallization rate is due to the heterogeneous nucleation of nano-SiO2[27,28].As a result,thenucleationdensityincreases,andthecrystallization iscarried outata largenumber ofnucleationsites,whichmakes thedimensionof branch structure smaller.Furthermore,the increase of total crystallization rate leads to the increase of relative weight of thebranch structure.In the case of γ≥2%,the relative weight of branch structure decreases with the increasing γ.The reason is that tcloudand Δt increase signi ficantly with the increasing γ,in the competition between liquid-liquid phase separation and iPP crystallization,the former is more dominant,so the branch structure originating from polymer crystallization is restrained.Meanwhile,the increase of tcloudprolongs the growth time of cellular pores,as a result,the cellular pore size will become larger.

3.3.Water contact angle

As can be seen from Fig.5,the water contact angles decrease with the increasing γ.In order to investigate the effect of nano-SiO2on water contact angle,the surface composition and cross section composition(casting solution composition)of microporous membranes were analyzed by XPS(Perkin Elmer,PHI-1600).The area of sample used to analyze was 0.8 mm2and 5 samples were taken for each membrane.

Fig.6 is the XPS spectrum of iPP/nano-SiO2blend membrane in the case of γ=1%.The characteristic peak of Si(2p)shows that the membrane surfaceexists SiO2.TheC/Si atomic ratioof themembrane surface is calculatedbythepeakarea,whichis smallerthanthat of castingsolution(see Table 2).

Fig.5.Effect of nano-SiO2addition on water contact angle of hollow fiber membrane.

Fig.6.XPS spectrum for outer surface of the hollow fiber membrane,γ=1%.

Table 2 The C/Si atomic ratio of the membrane surface and the casting solution

Minimization of interface free energy is the main driving force for surface segregation.The interfacial energy between hydrophilic nano-SiO2and water is lower than that between hydrophobic iPP and water.In order to reduce the interface free energy,hydrophilic nano-SiO2will spontaneously migrate to the membrane surface[29,30].In addition,the membrane formation process is synchronized with the surface segregation one,which will hinder the further progress of surface segregation.As a result,a part of nano-SiO2transfers to the membrane surface from the membrane body,which is beneficial to the hydrophilicity of membrane surface.

3.4.Porosity

Fig.7 shows that with the increase of γ,the porosity first increases andthendecreases.Inthecaseof γ=1%,theporosity ofthe membrane is the highest.

In general,the porosity of the porous membrane is lower than the volume fraction of the diluent in the casting solution.The reason is that the diluent is extracted from the membrane,the membrane pores will collapse,and the collapse degree of membrane pores is closely related to the membrane structure.In this paper,the relative weight of cellularstructurefirstdecreasesandthenincreases,andthecollapsedegree of cellular structure is much larger than that of branch structure.Thus,the porosity of the resulting blend membranes first increases and then decreases with the increasing γ.

Fig.7.Effect of nano-SiO2addition on porosity of hollow fiber membrane.

3.5.Pore size distribution and connectivity among pores

Fig.8 shows the dry-wet N2flux curve and pore size distribution curve at γ=0%.The bubble-point pressure and the pressure at which the flux of dry membrane is equal to that of the wet membrane(intersection point pressure)are 0.14 MPa and 0.39 MPa,respectively.Accordingly,the maximumpore size and theminimum oneare 0.45 μm and0.16μm,respectively.Furthermore,thepressureatwhichthefluxof thewet membraneis equalto 50%of the maximum flux is 0.33 MPa,accordingly,the mean pore size is 0.19 μm.Finally,by measuring other samples,we draw the conclusion that all samples possess a narrow pore size distribution.The important data are summarized in Table 3.

In order to investigate the effect of nano-SiO2on the connectivity among pores,the εopenand τ are obtained by dry-wet nitrogen gasflux curves,the results are listed in Table 3.With the increase of γ,εopenfirst increases and then decreases,τ decreases at first and then increases.When γ =1%,εopenis the highest,τ is the lowest,and the connectivity among pores is the best.

Obviously,thedecrease of volumefraction of diluent incastingsolution is disadvantageous to connectivity among pores.With the increase of γ,the volume fraction of diluent decreases,τ should increase at all times,however,which is contradictory to Table 3.Thus,the effect of γ onconnectivityamongporesshouldbeinvestigatedfromtheviewpoint of membrane structure.

AsshowninFig.4,whenγ=0%,therelativeweightofcellularstructure is significantly greater than that of branch structure,and most of cellular pores are isolated due to the fact that the concentration of iPP in the casting solution is comparatively high,so the connectivity among pores is poor.In the case of γ =0.5%or γ =1.0%,the relative weight of branch structure increases with the increasing γ,the fluid channel is mainly composed of branch pores and the voids between the particles,so the connectivity among pores is better.When γ≥2%,therelativeweightofcellularstructureincreasesagainwiththeincreasing γ,and plenty of cellular pores are plugged with SiO2agglomerate particles,which is disadvantageous to the connectivity among pores.Furthermore,tcloudand Δt increase significantly as γ increases,the further growth of cellular pores will make the pore walls more dense,which is also disadvantageous to the connectivity among pores.As a result,the connectivity among pores decreases.

3.6.Pure water flux

According to Fig.9,with the increase of γ,the pure water flux of the membrane first increases and then decreases.When γ=1%,the pure water flux is the highest—408.2 L·m?2·h?1,which increases by 51.1%compared with that of γ=0%.Firstly,the water contact angle of the membranes surface significantly decreases after adding nano-SiO2,which can increase the water flux of the resulting membrane.In addition,porosity,membrane structure,trans-membrane pressure drop and operating temperature can also affect the water flux,which are investigated by the pore model of microporous membrane(Eq.8)[20,31,32].

Fig.8.Dry-wet N2flux and pore size distribution at γ=0.(a)Dry-wet N2flux;(b)pore size distribution.

Table 3 Performance parameters of resulting hollow fiber membrane

Fig.9.Effect of nano-SiO2addition on pure water flux of hollow fiber membrane.

where J/p represents the pure water flux under unit trans-membrane pressure drop,dmand δ represent mean pore size andmembrane thickness,respectively,and μ is the viscosity of fluid.

According to the above model,when the membrane samples are corrected to the same thickness(130 μm),and the trans-membrane pressure drop and operating temperature are the same,J/p is only a function associated with εopendm2that is a combination of membrane pore parameters.

The relative weight of branch structure first increases and then decreaseswiththeincreasingγ,correspondingly,εopendm2willfirstincrease and then decrease,which is consistent with Fig.9.

3.7.Mechanical properties

According to Figs.10 and 11,the bursting pressure,tensile strength andbreakingelongationofthemembranesamplesincreasewiththeincreasing γ.When γ =1%,a bursting pressure of 0.63 MPa,a tensile strength of 5.98 MPa and a breaking elongation of 37.69%are obtained,which increase by 8.6%,68.5%and 79.5%,respectively,compared with the membrane sample of γ=0%.

Fig.10.Effect of nano-SiO2on bursting pressure of hollow fiber membrane.

Fig.11.Effect of nano-SiO2on tensile performance of hollow fiber membrane.

The mechanical properties of porous membranes depend heavily on polymer aggregation structures[33].For crystalline polymers,there are two aggregation types:fringed micelle and spherulitic[21].When the membrane morphology is a typical cellular structure,the pore walls of the membrane are a continuous polymer-rich matrix phase in which amorphous chains wander from crystallite to crystallite and hold them together by primary bonds[34].Consequently,the mechanical properties of the membrane with a typical cellular structure are excellent.In contrast,when membrane morphology is a branch structure,the voids between the aggregated particles formed a continuous and interconnected porous network.The linkage points are relatively limited only on the interface between crystalline particles,which reduces the mechanical properties of the membranes[33].In the current work,the resultingmembranesexhibitmixedstructurescombiningcellularstructure and branch structure,and the relative weight of branch structurefirst increases and then decreases with the increasing γ.Thus,the mechanical properties of the resulting membranes should first decrease and then increase with the increasing γ.In fact,paradoxically,the mechanicalpropertieshavebeenincreasingwiththeincreaseofγ.Thereason is that the effect of nano-SiO2on the polymeric matrix is crucial to the mechanical properties of the resulting membranes.

Thenanoparticlesdisperseinthepolymericmatrix.Whenthemembrane is subjected to external forces,the stress concentration effect will occur due to the presence of rigid inorganic particles,which is easy to drive the surrounding polymeric matrix to produce micro-cracks(orcrazing),therebyabsorbingacertaindeformationwork.Meanwhile,the polymeric matrix between the particles can yield and deform in a ductile manner,which can absorb impact energy.In addition,the presence of rigid inorganic particles will hinder and blunt the propagation of the crazing,which will improve its toughness[35,36].Thus,the mechanicalpropertiesoftheresultingmembranesshouldincreasewith increasing γ.

In summary,the mechanical properties of iPP/nano-SiO2blend hollow fiber microporous membrane are determined by both the change ofmembranestructureandtheeffectofnano-particlesonthepolymeric matrix.

4.Conclusions

Nano-SiO2particles had a significant impact on the thermally induced phase separation(TIPS)behavior of the isotactic polypropylene(iPP)/di-n-butyl phthalate(DBP)/dioctyl phthalate(DOP)system.The liquid-liquid phase separation temperature tcloudincreased with the increasing nano-SiO2content(γ),and that,as a nucleating agent,nano-SiO2changed the crystallization temperature and total crystallization rate of iPP.

The membrane samples exhibited mixed morphology combining cellular structure relating to liquid-liquid phase separation and branch structure originating from polymer crystallization.The competition relation between liquid-liquid phase separation and polymer crystallization was adjusted successfully by adding nano-SiO2.As a result,the relative weight of cellular structure first decreased and then increased with the increasing γ.

XPS data indicated that nano-SiO2particles can transfer to the surface from the body of the membrane.Consequently,the hydrophilicity of the membrane surface was increased.Furthermore,all samples obtained possessed a narrow pore size distribution.In addition,the porosity,the connectivity among pores and the pure water flux first increased and then decreased with increasing γ,in contrast to this,the mechanical properties of the membrane increased at all times.Finally,itwassuggestedthatthecomprehensiveperformanceofthemembrane obtained was the best in the case of γ=1.0%.