Received: 10 March 2009, Accepted: 15 May 2009, Published online in Wiley Online Library: 21 June 2009

pH-induced on–off switching ofpolycarbonate track-etched membranes byplasma-induced surface grafting

Chunyan Lia, Bing Caoa, Wencai Wanga*, Qifang Lia, Jing Zhaoa

and Liqun Zhanga

Surface functionalization of the plasma-pretreated polycarbonate (PC) track-etched membranes via plasma-inducedthermally graft copolymerization of acrylic acid (AAc) was carried out. The resulting PC membranes with grafted AAcside chains were characterized by X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR)spectroscopy, and thermogravimetric (TG) analysis. The morphology of the PC membranes was studied by scanningelectron microscopy (SEM). The results showed that the grafted PAAc polymers were formed uniformly inside thepores throughout the entire membrane thickness. With increase in the pore-filling ratio, the pore diameters ofPAAc-graftedmembranes became smaller. The PC-g-PAAcmembranes exhibit rapid and reversible response of the fluxto the environmental pH as pH is switched between 3 and 9. Between pH 3.5 and 5.5, the membranes demonstrate apH-valve function as the carboxyl group changes from neutral to charged with a corresponding variation in chainconfiguration. Copyright � 2009 John Wiley & Sons, Ltd.

Keywords: polycarbonate track-etched membranes; acrylic acid; plasma; graft copolymerization; pH-sensitive

INTRODUCTION

Porousmembranes that change their permeation/separation pro-perties in response to environmental stimuli have drawn muchattention in recent years. The permeation properties of thesemembranes can be controlled by changing pH, temperature,electric field, concentration of chemical species, or ionic strengthof their environments.[1–6] Potential applications for thesestimuli-responsive membranes include controlled drug delivery,wastewater treatment, bio-separation, chemical sensors, etc.[7–12]

A variety of methods have been developed to functionalizematerial’s surfaces to change their properties according to therequired application, and surface initiated graft copolymerization(‘‘grafting-from’’) has become a versatile tool among them.[13]

These techniques include chemical grafting,[14,15] radiatio-n-induced grafting,[16] photo-grafting,[17,18] and plasma-inducedgrafting.[19,20] The photo-grafting method used before is easy touse, but it is not possible to make precise variations of graftingdensity because that depends on the surface density of thephysically attached photo-initiator and its reactivity with the basepolymer (which will be different for different adsorption sites),and only relatively low grafting densities can be achieved. On theother hand, plasma-induced grafting polymerization has drawnmuch attention recently because it has been found to be able tograft polymers not only on the membrane surface but also in themembrane pores by introducing proper treatment conditions.Polycarbonate (PC) track-etched membranes possess excellent

morphology and many desirable physicochemical properties,such as good thermal stability, parallel cylindrical poreswith diameters ranging from 10 nm to 20mm with narrow sizedistribution.[21] The track-etched membrane with stimuli-responsive‘‘polymer brushes’’ is advantageous over the commonmembrane

in terms of easily controlled pore size and quick response toexternal stimuli. The track-etchedmembranesmade from PCwithuniform cylindrical pores at very narrow size distribution hadbeen established as a versatile candidate for sensors andcomponents for future ‘‘lab-on-chip’’ systems.[22] However, untilnow, very few papers reported on the functionalization oftrack-etched membranes by pH-responsive polymers. Moreover,among previous investigations on pH-responsive gating mem-branes, much attention has been focused on topics such as themechanism of grafting polymerization, the effect of graftingconditions on the grafting yield, and on the thermo-responsivecharacteristics of the grafted membranes. However, systematic

(wileyonlinelibrary.com) DOI: 10.1002/pat.1485

Research Article

* Correspondence to: W. C. Wang, State Key Laboratory of Chemical ResourceEngineering, and the Key Laboratory of Beijing City on Preparation andProcessing of Novel Polymer Materials, Beijing University of Chemical Tech-nology, 15 Beisanhuan East Road, Beijing 100029, P.R. China.E-mail: wangw@mail.buct.edu.cn

a C. Y. Li, B. Cao, W. C. Wang, Q. F. Li, J. Zhao, L. Q. Zhang

State Key Laboratory of Chemical Resource Engineering, and the Key

Laboratory of Beijing City on Preparation and Processing of Novel Polymer

Materials, Beijing University of Chemical Technology, Beijing 100029, P.R.

China

Contract/grant sponsor: Beijing Nova Program; contract/grant number:

2006B16.

Contract/grant sponsor: The Major Project of Science and Technology

Research from the Ministry of Education of China; contract/grant number:

308003.

Contract/grant sponsor: The Scientific Research Foundation for the Returned

Overseas Chinese Scholars (SRF).

Contract/grant sponsor: The Program for New Century Excellent Talents in

University (NCET).

Polym. Adv. Technol. 2010, 21 698–703 Copyright � 2009 John Wiley & Sons, Ltd.

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investigations of responsive polymer-grafted membranes micro-structure and of the influence of membrane pore size (inparticular, small nanopores) on the pH-responsive behavior of thegrafted polymer brush are still lacking.In the present work, we report on the preparation and

characterization of PC track-etched membranes with acrylic acid(AAc) polymer chains by using a plasma-graft pore-fillingpolymerization method. The microstructural characteristics ofthe PC-g-PAAc membranes were systematically investigated byemploying X-ray photoelectron spectroscope (XPS), scanningelectron microscope (SEM), Fourier transform infrared (FTIR)spectroscope, contact angle measurement, and filtration exper-iments, in order to obtain some valuable guidance for furtherdevelopment of stimuli-responsive gating membranes.

EXPERIMENTAL

Materials

PC track-etched membranes with a pore size of 400 nm and athickness of 47mmwere supplied byWhatman Co. AAc, providedby Alfa Aesarcv, was used after purification by vacuumdistillation.

Plasma-pretreatment of the PC membranes

Argon plasma treatment of the PC membranes was carried out ina cylindrical metal glow discharge chamber of about 1200 cm3 involume, model SY 100, manufactured by Shitai PlasmaTechnology Company, Changzhou, China. The membrane wasplaced on the interior plate electrode of 100 cm2 in area andusing the inner side-wall as an outer electrode. The glowdischarge was carried out at an applied frequency of 13.56MHz, apower of 16W, an Ar pressure of 0.9 Torr, and an Ar flow rate of 80standard cubic centimeter per min (sccm). It was subjected to theglow discharge for 60 sec. The Ar plasma-pretreated PCmembranes were then exposed to atmosphere for about30min to effect the formation of surface peroxides andhydroperoxides for the subsequent graft copolymerizationreaction.

Graft copolymerization

The functionalized PC membrane was prepared by thermallyinduced graft copolymerization of AAc. The AAc monomer wasintroduced into a three-necked round bottom flask equippedwith a thermometer, a condenser, and a gas line. The AAcmonomer concentrations were varied from 2 to 6%. The PCmembrane was immersed into AAc monomer solution. The finalvolume of each reaction mixture was adjusted to 250ml. Thesolution was saturated with purified argon for 30min understirring. The reactor flask was then placed in a thermostated

water bath at 508C to initiate the graft copolymerization reaction.A constant flow of argon wasmaintained during the thermal graftcopolymerization process. After the desired reaction time (3 hr),the reactor flask was cooled in an ice bath and the AAcgraft-copolymerized PC (PC-g-PAAc) membrane was taken outand washed by stirring for 24 hr in copious amounts of ethanol atroom temperature. The precipitation and exhaustive washingprocess ensured the complete removal of the residual AAchomopolymer. The copolymers were then dried by pumpingunder reduced pressure for subsequent characterization over-night, shown in Fig. 1.

X-ray photoelectron spectroscopy (XPS)

XPS measurements were carried out on an ESCALAB 250 ThermoElectron Corporation with an Al Ka X-ray source (1486.6 eVphotons). The core-level signals were obtained at a photoelectrontake off angle of 458 (with respect to the sample surface). The X-raysource was run at a reduced power of 150W. The PC membraneswere mounted on the standard sample studs by means ofdouble-sided adhesive tapes. The pressure in the analysischamber was maintained at 10�8 Torr or lower during eachmeasurement. To compensate for surface charging effects, allbinding energies (BEs) were referenced to the C 1s hydrocarbonpeak at 284.6 eV. In peak synthesis, the line width (full width athalf maximum or FWHM) of Gaussian peaks was maintainedconstant for all components in a particular spectrum. Surfaceelemental stoichiometries were determined from the peak arearatios and were accurate to within �5%.

Infrared spectroscopy (FTIR)

FTIR spectra of the thin copolymer films cast from acetonesolutions were obtained from a ThemroNicoletNexus670 FTIRspectrophotometer. Each spectrum was collected by cumulating30 scans at a resolution of eight wave numbers.

Scanning electron microscopy (SEM)

The surface morphology of the membranes was studied by SEM,using a Hitachi S-4700 SEM. The membranes were mounted onthe sample studs by means of double-sided electric adhesivetapes. A thin layer of gold was sputtered on the sample surfaceprior to the SEM measurement. The SEM measurements wereperformed at an accelerating voltage of 20 kV.

Thermal analysis

The thermal properties of the pristine PC membrane andPC-g-PAAcmembrane were measured by thermogravimetric (TG)analysis. The membrane samples were heated up to 6008C at aheating rate of 108C/min under a dry nitrogen atmosphere by

Figure 1. Schematic illustration of the processes of plasma-induced thermally graft copolymerization of AAc on PC membranes. This figure is available

in color online at wileyonlinelibrary.com

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STARe system, TGA/DSC1 Simultaneous Thermal Analysis (Met-tler-Toledo Company, Switzerland).

Water contact angle measurements

Static water contact angles of the pristine PC membrane, theplasma treated PC membrane, and the PC-g-PAAc membranewere measured at 258C and 50% relative humidity by the sessiledrop method, using a 3mL water droplet by the DigitalInstruments Inc. of ZhongChen, Shanghai, China. The telescopewith a magnification power of 23� was equipped with aprotractor of 18 graduation. For each sample, at least fivemeasurements on different surface locations were averaged. Theangles reported were reliable to �38.

Measurements of the pH-dependent solution flux throughthe PC membranes

The PC-g-PAAc membrane was preconditioned by immersing inan aqueous solution of a prescribed pH value prior to beingmounted on the microfiltration cell. An aqueous solution of thesame prescribed pH value and a fixed ionic strength (I¼ 0.2mol/l)was added to the cell. The ionic strength of the solution was keptconstant by the addition of acetic acid and sodium acetate. Theflux was calculated from the weight of solution permeated perunit time and per unit area of the membrane surface under afixed pressure head of 0.1MPa/cm2.

RESULTS AND DISCUSSION

The processes of argon plasma pretreatment, and graftcopolymerization of PAAc with the PC membrane are shownin Fig. 1. The details for each process are described below.The presence of grafted PAAc chains on the PC membrane

surfaces was studied by XPS analysis after the surfaces had beensubjected to vigorous washing and extraction. Figure 2 shows therespective wide scan and C 1s core-level spectra of the pristine PCmembrane surface (Figs 2(a) and 2(b)) and the PC-g-PAAcmembrane surfaces (Figs 2(c) and 2(d)). The C 1s core-level

spectrum of the pristine PC membrane can be curve-fitted withfour peak components, having BEs at 284.6 eV for the C–H andC–C species, at 286.2 eV for the C–O species, at 288.5 eV for theO––C–O species, and at 291.4 eV for the p–p� shakeup satellite,respectively. It is noted that the peak component at BE of 283.0 eVis the C–Si species introduced in the PC membrane synthesisprocess. Surface modification of PC membrane by Ar plasmatreatment, followed by air exposure, results in the formation ofcarbonyl and carboxyl groups on the surface of the membrane[23]. The presence of the carbonyl and carboxyl species on theplasma-treated PC surface is attributable to the oxidation in air ofthe active species on the PC surface induced by the argon plasmatreatment. These peroxide species can be degraded to producethe radicals which are used in the subsequent grafting reaction.As shown in Fig. 2(d), the O–C––O species of the grafted PAAcpolymer chains at about 288.5 eV are substantially increased,while the peak at 291.4 eV for the p–p� shakeup satellitedisappears. The results indicate that the densely grafted PAAcbrushes have covered the membrane completely to a thicknessexceeding the sampling depth of the XPS technique (about7.5 nm in an organic matrix). The results also indicate PAAc havegrafted on the surface of PC membrane successfully.The chemical structures of the PC and the PC-g-PAAc

copolymers were also studied by FTIR spectroscopy. As shownin Fig. 3, the absorption bands associated with the ester ring andlinkage of PC at 1765 cm�1 (C––O stretching) are present in all thecopolymer samples. Comparing the FTIR spectra of the PC-g-PAAccopolymers with that of the pristine PC, the absorption band ofthe copolymer samples at 1722 cm�1, attributable to the O–C––Ostretching vibration, must be associated with the grafted AAcchains. In good agreement with that obtained from the XPSanalysis, the result again indicates that the PAAc has beensuccessfully grafted on the PC membrane substrates by theplasma-graft pore-filling polymerization.The surface morphology of the PC membranes was studied by

SEM at a magnification of 10.0 k�. Figure 4 shows the surface andcross-sectional images of pristine PC membranes (part (a), (b)),the PC-g-PAAc membranes with a monomer concentration of 2%(part (c), (d)), 4% (part (e), (f )), and 6% (part (g), (h)), respectively. It

Figure 2. XPS wide scan and C 1s core-level spectra of (a,b) the pristine

PC membrane and (c,d) the PC-g-PAAc membrane surfaces.Figure 3. FTIR spectroscopy of (1) the pristine PC membrane and (2) the

PC-g-PAAc membrane.

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could be seen from Figs 4(a) and 4(b) that the pristine PCmembranes have uniform pore geometry and cylindrical andstraight pores. After grafting, both surface and cross-sectionalmicrographs show that the membrane pore size decreased. Fromthe surface SEM images shown in Figs 4(a), 4(c), and 4(e), it can beseen that the surface pores of PAAc-grafted membranes aresmaller and the pore outlines are more obscure compared withthose of the pristine membranes, but a dense PAAc layer is notformed on the membrane surface even at a monomerconcentration as high as 6%. From the cross-sectional SEMimages shown in Figs 4(b), 4(d), and 4(f ), it can be clearly seenthat the uniformly grafted PAAc polymers were well formed

inside the pores throughout the entire membrane thickness, andthe PAAc polymers were filled in the membrane pores graduallywith the increase of the monomer concentration. The results areconsistent with the previous investigation that was carried outwith different substrates and different analysis methods.[19,24]

However, an interesting finding is that further increase in themonomer concentration did not significantly change thethickness of the grafted PAAc layer. The above results verifiedthat, by the plasma-graft pore-filling polymerization method,functional PAAc chains could be successfully grafted on both theouter surfaces of the membrane and the inner surfaces of themembrane pores.

Figure 4. The surface (a, c, e) and cross-sectional (b, d, e) SEM images of: (a,b) the pristine PCmembrane, and the PC-g-PAAcmembranes with monomerfeed ratios of (c,d) 2%, (e,f ) 4%, and (g,h) 6%.

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The thermal properties of the graft copolymers were studiedby TG analysis. Figure 5 shows the respective TG analysis curves ofthe pristine PC (Curve (1)) and the PC-g-PAAc copolymer (Curve(2)). In comparison with the pristine PC and the PC-g-PAAccopolymer, the copolymer samples exhibit an intermediateweight loss behavior and undergo a two-step degradationprocess. The onset of the first major weight loss occurs at thetemperature which corresponds to the decomposition of thePAAc segments in the copolymers. The second major weight lossbegins at about 4128C, which coincides with the decompositiontemperature of the PC main chain. The TG analysis curves alsoindicate that the extent of weight loss of copolymers during thefirst stage of thermal decomposition is approximately equal tothe AAc polymer content in the graft copolymer. The relativesmaller weight loss of the copolymers during the first stage ofthermal decomposition is consistent with the fact that themolecular weight of the PC repeat unit is substantially higherthan that of the AAc repeat unit.The pristine PC membrane is hydrophobic, with a water

contact angle of about 748 (Table 1). A substantial decrease in thewater contact angle of the PC membrane is achieved throughgraft copolymerization with AAc. The contact angle is reduced to

about 488 for the copolymer membranes. The water contactangle of the PC-g-PAAc membranes decreases with the increasein the AAc polymer graft concentration. This phenomenon isattributable to the hydrophilic nature of the grafted AAc polymerside chains.The characteristic pH-dependent flux of aqueous solutions

through the PC membranes is the major concern in this work.Figure 6 shows the effect of pH on the flux of the pristine PC andthe PC-g-PAAc membranes. The water flux of pristine PCmembrane was always larger than that of PC-g-PAAcmembranes,because the grafted PAAc layer made the pore diameters smaller.In particular, it is unambiguous that the permeability of aqueoussolutions through the pristine PC membranes is pH-independent(Curve (d)). On the other hand, however, the flux of the aqueoussolution through the PC-g-PAAc membranes exhibits apH-dependent behavior. The permeation rate of the aqueoussolution through the PC-g-PAAc membrane decreases with theincrease in pH of the solution from 3 to 12 (Curves (a), (b), and (c)),and the most drastic change in permeation rate being observedat solution pH values between 3.5 and 5.5. Furthermore, thepH-sensitivity of the flux through membrane is enhanced bythe increase in graft concentration. The pH-dependent flux of theaqueous solutions through the membrane at pH between 3 and12 are completely reversible. These results suggest that both theextent of interaction with the aqueous environment and theconformation of the graft chains vary, reversibly, with the pH ofthe solution to control the effective pore size of the membrane. Itshould be noticed that for a PC-g-PAAc membranes with amonomer concentration of 6%, when the pH of the permeatedsolution increased to 9 or higher, the flux of the membranesdecreases almost to zero, indicating the ‘‘off’’ status of the pores.The result indicates that the pores of the membrane have beenalmost fully filled by the grafted PAAc chains.The change in permeability in response to the change in

solution pH can be attributed to the change in conformations ofthe graft chains on the membrane surface, especially on the poresurfaces and in the sub-surface region of the pores. Due to thenon-ionizablity of the polymer chains in the pristine PCmembranes, the polymer chain conformation and the membrane

Figure 5. TG analysis curves of: (1) the pristine PCmembrane and (2) the

PC-g-PAAc membranes.

Table 1. Water contact angle and [O]/[C] ratio of pristine,plasma-treated PC, and PC-g-PAAc membranes

PC samplesWater contactangle (�38)

[O]/[C]ratioa

Pristine PC 78 0.259Plasma-pretreated PC 69 0.305AAc feed ratio of 2% 60 0.314AAc feed ratio of 4% 53 0.336AAc feed ratio of 6% 45 0.367

a aDetermined from the corrected O 1s and C 1s XPS core-levelspectra area ratio of the respective sample.

Figure 6. pH-dependent permeability of aqueous solutions through

(a) the pristine PC membrane, and the PC-g-PAAc membranes withmonomer feed ratio of (b) 2%, (c) 4%, and (d) 6%.

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pore dimension will remain constant at all pH values. On theother hand, as a weak acid (pKa¼ 4.3), the carboxylic groups ofthe grafted AAc polymer side chains can be ionized ordeprotonated to become negatively charged. With the increasein pH of the casting solution, most of the carboxylic groups aretransformed into carboxylic anions. Strong electrostatic repulsionamong the carboxylic anions, together with their stronginteraction with the aqueous solution, forces the AAc polymerside chains to adopt a highly extended conformation. Theextension of the AAc polymer side chains into the pores and inthe sub-surface region of the pores reduces the effectivedimension of the pores. As a result, the permeability of theaqueous solution through the PC membrane is reduced. On theother hand, the AAc polymer chains assume a helicalconformation under the low-pH conditions. As a result, stericobstruction to the pores of the membrane is substantiallyreduced and the permeation rate increases.

CONCLUSION

PAAc copolymers were successfully grafted on the surface andinner pore sides of PC track-etched membranes throughplasma-induced thermally graft copolymerization with AAcmonomer in aqueous solutions. The flux of aqueous solutionsthrough the PC-g-PAAc membranes exhibited a rapid andreversible response on the solution pH in the pH range of 3–9.Between pH 3.5 and 5.5, the membranes demonstrate a pH-valvefunction as the carboxyl groups change from neutral to chargedwith a corresponding variation in chain configuration. With thegrafted PAAc layers on the pore walls, the effective hydrodynamicpore diameter of the membrane could be switched reversibly bychange of the solution pH. The present study has shown thatplasma-induced graft copolymerization is a relatively simple andeffective approach to the preparation of functionalized PCmembranes with well-controlled pore size, uniform surfacecomposition, and pH-responsive on–off switching properties.

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