Thermally On−Off Switching Membranes Prepared by Pore-Filling Poly

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Thermally On-Off Switching Membranes Prepared by Pore-Filling Poly(N-isopropylacrylamide) Hydrogels Wencai Wang,* Xiaodong Tian, Yiping Feng, Bing Cao, Wantai Yang, and Liqun Zhang Key Laboratory of Carbon Fiber and Functional Polymers, Ministry of Education, and Key Laboratory of Beijing City on Preparation and Processing of NoVel Polymer Materials, Beijing UniVersity of Chemical Technology, Beijing 100029, China

Pore-filling N-isopropylacrylamide (NIPAAM) polymer hydrogels were successfully grafted onto track-etched polycarbonate (PC) membranes by plasma-induced graft copolymerization. The microstructure and morphology of the PC-g-PNIPAAM membranes were investigated by XPS, SEM, ATR-FTIR, TGA, and water flux experiments. The effective pore sizes were regulated by the volume change of the cross-linked PNIPAAM hydrogels in the temperature range around its lower critical solution temperature (LCST). The PC-g-PNIPAAM membranes demonstrated a fast and reversible valve switching mechanism in a small temperature range. The on-off water flux ratio became more significant with the increase of the monomer concentration. Contact angle results showed that the thermal-responsive gating characteristics of PC-g-PNIPAAM membranes were mainly dependent on the pore size change. 1. Introduction Recently, much attention has been drawn to environmental stimuli-responsive gating membranes, which exhibit permeability changes in response to external stimuli such as temperature, pH, ionic strength, electric field, and substance species.1-7 Such smart membranes are considered to have great potential in a wide variety of applications, including controlled drug delivery, chemical separation, water treatment, chemical sensing, bioreactors, etc. Among the environmental stimuli, temperature is the most easily designed and controlled, so thermo-responsive membranes have received special attention. Environmental stimuli-sensitive membranes can be prepared by grafting of functional polymers or graft copolymerization of functional monomers directly onto the existing porous membranes.2-6 The porous substrate provides mechanical strength and dimensional stability, while the conformational changes of the grafted functional polymers result in environment-responsive characteristics. The permeability of these membranes can be controlled or adjusted by the grafted polymer according to the external chemical and/or physical environment. Thermo-sensitive polymers, such as poly(N-isopropylacrylamide) (PNIPAAM), are known to exhibit phase transitions in water at the lower critical solution temperature (LCST).8 In a narrow temperature range, hydration-dehydration changes in aqueous solutions occur. The macroscopic phase separations of these polymer solutions are accompanied by large conformational changes in the polymer chains.9,10 Because of the dramatic thermo-sensitive properties, PNIPAAM hydrogels have attracted great interest for a wide variety of applications. Microporous membrane grafted PNIPAAM hydrogels can serve as a valve regulating the permeation base on temperature changes, as shown in Figure 1. In comparison to planar surfaces, the controlled functionalization of porous materials is more complicated. Important issues to be investigated are the accessibility of and the mass transfer in pores during grafting reactions and the consequences for grafted layer structure and functionality.11 The conformation of functional polymers grafted on the membrane substrates is * To whom correspondence should be addressed. Tel.: +86-1064456158. Fax: +86-10-64433964. E-mail: [email protected].

very important to the stimuli-responsive permeability of gating membranes. The parameters such as the grafting yield and the morphological location of grafted polymeric gates, the molecular size of permeate, and the initial membrane pore size have been verified to have significant influences on the stimuli-responsive characteristics of the gating membranes.12 For smart membranes, stable gating characteristics under different operation pressures are very important and essential.13 Most stimuli-sensitive membranes have been prepared by grafting linear stimuliresponsive polymer chains onto the substrate membranes.14 However, the linear grafted “smart” membranes are usually considered to be weak for enduring high operation pressure because the linear grafted gates might be easily collapsed at high operation pressures. This paper aims to develop a functional membrane consisting of a thermo-sensitive polymer gel, PNIPAAM, on the surface and inside the pores of track-etched polycarbonate (PC) membranes, which have straight cylindrical pores with a sharp pore-size distribution. A plasma-induced grafting polymerization method was used to prepare the PC-g-PNIPAAM membranes. The microstructures and chemical compositions of the prepared membranes were characterized by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The thermo-dependent hydrophilic/hydrophobic transition of the membrane surface around the response temperature of grafted membranes was investigated by measuring the contact angle. The effect of temperature on solute permeability was determined on the PNIPAAM grafted PC membranes.

Figure 1. Schematic illustration of the on-off switching mechanism of PNIAAM hydrogels grafted onto polycarbonate (PC) track-etched membranes.

10.1021/ie9008666  2010 American Chemical Society Published on Web 12/30/2009

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2. Experimental Section 2.1. Materials. Polycarbonate (PC) membranes (Nuclepore) with pore sizes 0.8 µm and 47 mm in diameter were purchased from Millipore (Bedford, MA). N,N-Methylenebisacrylamide (NMBA), which was used as cross-linker, was obtained from Sigma-Aldrich (St. Louis, MO). N-Isopropylacrylamide (NIPAAM) was obtained from Sigma-Aldrich (St. Louis, MO). The sodium dodecyl sulfate (SDS), which was used as surfactant, was obtained from Beijing Chemical Reagent Company. All solvents and other chemicals were of analytical grade and were used as received. Deionized water (18.2 MΩ, 25 °C) was collected from a Milli-Q Plus water purification system (Millipore). 2.2. Grafting of PNIPAAM Hydrogels via PlasmaInduced Polymerization. Argon plasma treatment of the PC membranes was carried out in a cylindrical metal glow discharge chamber of about 1200 cm3 volume, Model SY 100, manufactured by Shitai Plasma Technology Co., Changzhou, China. The PC membrane was placed on the interior plate electrode of 100 cm2 area and using the inner side wall as the outer electrode. The glow discharge was carried out at an applied frequency of 13.56 MHz, a power of 100 W, an Ar pressure of 0.9 Torr, and an Ar flow rate of 80 standard cubic centimeters per min (sccm). It was subjected to the glow discharge for 60 s. The Ar plasma pretreated PC membranes were then exposed to the atmosphere for about 30 min to effect the formation of surface peroxides and hydroperoxides for the subsequent graft copolymerization reaction with NIPAAM.15,16 The grafting of PNIPAAM hydrogels on the surface of PC membranes was prepared by thermally induced graft copolymerization of NIPAAM with the plasma-pretreated PC membrane in deionized water solution at 60 °C for 18 h and under an argon atmosphere.14 The plasma-pretreated PC membrane was immersed into a three-neck round-bottom flask equipped with a thermometer, a condenser, and a gas line. The NIPAAM monomer, 0.03 mmol of SDS, and 1.6 mmol of NMBA were introduced into the flask. The NIPAAM monomer concentrations were varied from 0.03 to 0.09 g/mL. The final volume of each reaction mixture was adjusted to 50 mL. The solution was saturated with purified argon for 30 min under stirring. The reactor flask was then placed in a thermostated water bath at 60 °C to initiate the graft copolymerization reaction. A constant flow of argon was maintained during the thermal graft copolymerization process. As the argon plasma treatment made the free radicals both on the surface and inside the pore surface of the PC membranes, during the thermal graft copolymerization process, NIPAAM was grafted both on the surface and in the pore. After the desired reaction time, the reactor flask was cooled in an ice bath and the NIPAAM graft copolymerized PC membrane was taken out. The grafted membrane was rinsed in copious deionized water under stirring in a constant-temperature bath for 24 h to remove the residual unreacted monomer and PNIPAAM homopolymer, and then dried by pumping under reduced pressure for subsequent characterization. 2.3. Infrared Spectroscopy (ATR-FTIR). The chemical structure was performed by Fourier transform infrared spectrometry (FTIR) from a Themro Nicolet Nexus670 FTIR spectrophotometer. Each spectrum was collected by cumulating 30 scans at a resolution of 8 wavenumbers. 2.4. X-ray Photoelectron Spectroscopy (XPS). XPS measurements were carried out on an ESCALAB 250 made by Thermo Electron Corp. with a Al KR X-ray source (1486.6 eV) at a constant retard ratio of 40. The core-level signals were obtained at a photoelectron takeoff angle of 75° (with respect to the sample surface). The X-ray source was run at a reduced

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power of 150 W. The PC membranes were mounted on the standard sample studs by means of double-sided adhesive tape. The pressure in the analysis chamber was maintained at 10-8 Torr or lower during each measurement. The survey spectra were scanned from 0 to 1350 eV binding energy in 1 eV steps. To compensate for surface charging effects, all binding energies (BEs) were referenced to the C 1s hydrocarbon peak at 284.6 eV. In the synthesis the line width (full width at half-maximum or fwhm) of Gaussian peaks was maintained constant for all components in a particular spectrum. Surface elemental stoichiometries were determined from the peak area ratios and were accurate to within (10%. 2.5. Scanning Electron Microscopy (SEM). The surface morphology of the PC membranes was studied by scanning electron microscopy (SEM), using a HITACHI S-4700 scanning electron microscope. The membranes were mounted on the sample studs by means of double-sided adhesive tape. A thin layer of gold was sputtered on the sample surface prior to SEM measurement. The SEM measurements were performed at an accelerating voltage of 20 kV. 2.6. Water Contact Angle Measurements. Static water contact angles of the PC membranes were measured at 25 °C and 50% relative humidity by the sessile drop method, using a 3 µL water droplet in a telescopic goniometer (JC 2000C contact angle instrument, manufactured by Digital Instruments Inc. of ZhongChen, Shanghai, China.). The telescope with a magnification power of 23× was equipped with a protractor of 1° graduation. For each sample, at least five measurements on different surface locations were averaged. According to the equipment deviation, the angles reported were reliable to (3°. 2.7. Thermogravimetric Analysis. The thermal properties of the PC membranes were measured by thermogravimetric (TG) analysis. For the TG analysis, the polymer samples were heated to 500 °C at a heating rate of 10 °C/min under a dry air atmosphere in a TG analyzer (Netzsch, Germany). 2.8. Thermo-Responsive Filtration Experiments. To examine the pore “on-off” switching of the PC-g-PNIPAAM membranes regulated by the hydrogel response to temperature change, the water permeability at various temperatures was determined. The water flux experiments of pristine and grafted membranes were performed by using a microfiltration cell. The membrane was fixed on a cell under a constant transmembrane pressure of 0.1 MPa. The diameter of the effective membrane area for filtration was 39 mm. The temperatures of the membranes and the feed water were controlled by a thermostatic water bath. The temperature range was from 20 to 40 °C. The water permeability through the pristine and PC-g-PNIPAAM membranes at different temperatures was studied by measuring the water flux. The water flux data were used to calculate effective pore sizes (D*) of the membranes at various temperatures using HagenPoiseuille’s law:17 J)

NAπ∆P D* 8ηL 2

( )

4

(1)

where J is the water flux (mL cm-2 s-1) of the membrane, N is the pore density (cm-2), A is the effective membrane area (cm2, 0.5 cm2 in this study), ∆P is the pressure (0.1 MPa in this study), η is the water viscosity at the corresponding temperature (ranging from 1.005 × 10-3 to 0.656 × 10-3 Pa for temperatures 20-40 °C), and L is the thickness of the membrane (cm). D* is the membrane pore mean diameter (cm).

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Figure 2. XPS wide-scan and C 1s core-level spectra of (a, b) pristine PC membranes and PNIPAAM grafted PC membranes with NIPAAM concentrations of (c, d) 3% and (e, f) 8%, respectively.

The pore-filling ratio (F) is defined to show the grafting degree. d0 and dg stand for the average pore diameters before and after grafting, respectively (µm). F)1-

() dg d0

2

(2)

3. Results and Discussion The PC membranes were pretreated by argon plasma pretreatment to introduce peroxide and hydroperoxide groups on the surface of the membranes, followed by thermally induced graft copolymerization of PNIPAAM hydrogels on the surface as well as inside the pores of the PC membranes. 3.1. XPS Analysis of the PC-g-PNIPAAM Membranes. The presence of grafted PNIPAAM gels on the PC membrane surfaces was studied by XPS analysis after the membranes had been subjected to vigorous washing and extraction. Figure 2 shows the respective wide-scan and C 1s core-level spectra of the pristine PC membrane surface (Figure 2a,b), the PC-gPNIPAAM membrane with NIPAAM feed ratio of 3% (Figure 2c,d), and the PC-g-PNIPAAM membrane with NIPAAM feed ratio of 8% (Figure 2e,f), respectively. As shown in Figure 2a, for the pristine PC membranes, the wide-scan spectra show the peak components of C 1s and O 1s. The presence of the surface grafted PNIPAAM can be deduced from the appearance of the N 1s signal at the binding energy (BE) of about 398.5 eV in the wide-scan spectra, shown in Figure 2c,e. The C 1s corelevel spectrum of the pristine PC membrane can be curve fitted with five peak components, having binding energies (BEs) at 284.6 eV for the C-H and C-C species, at 286.2 eV for the C-O species, at 288.5 eV for the OdCsO species, and at 291.0 eV for the π-π* shakeup satellite, respectively.18 An additional peak component is noted at a BE of 283.0 eV, which is the C-Si species introduced in the PC membrane synthesis process. Surface modification of PC membrane by Ar plasma treatment, followed by air exposure, results in the formation of carbonyl and carboxyl groups on the surface of the membrane. These peroxide species can be degraded to produce the radicals which are used in the subsequent grafting reaction. The C-N species

Figure 3. ATR-FTIR spectra of the pristine PC membrane and PC-gPNIPAAM membranes with monomer concentrations of 5% and 8%, respectively.

of the grafted PNIPAAM polymer chains have a C 1s peak component BE at about 285.6 eV. The BEs of the N-CdO species of the grafted PNIPAAM polymer chains and the carboxyl OsCdO species of the PC cannot be resolved unambiguously. The two species are represented by a single peak component at a BE of about 288.5 eV. The increase in NIPAAM polymer graft concentration with the NIPAAM to PC molar feed ratio is readily indicated by the steady increase in the intensity ratio of the C-N to the C-O species. It should also be noted that the peak at 291.4 eV for the π-π* shakeup satellite became smaller. The results indicate that the densely grafted PNIPAAM brushes have covered the membrane to a thickness below the sampling depth of the XPS technique (about 7.5 nm in an organic matrix).19 All the above results indicate that PNIPAAM has successfully grafted on the surface of PC membrane. 3.2. FTIR Spectroscopy of the PC-g-PNIPAAM Copolymers. The chemical structures of the pristine PC and the PCg-PNIPAAM membranes were also studied by ATR-FTIR spectroscopy. As shown in Figure 3, the absorption bands at 1765 cm-1 (CdO stretching) which are due to the ester group of the polycarbonate structure are present in all the PC membrane samples. However, comparing the spectra of the PCg-PNIPAAM membranes with that of the pristine PC, the absorption bands of the copolymer samples at 1722, 1650, and 1550 cm1 were ascribed to the amide bond of PNIPAAM. The former was for the carbonyl (C-O) stretching vibration and the latter were from the combined sorption of both C-N stretching and N-H bending vibrations.20 The composite membrane obtained from higher monomer concentration exhibited higher sorption intensity at these amide characteristic peaks, confirming the higher grafting yield of the PNIPAAM with higher monomer concentration. In good agreement with that obtained from the XPS analysis, the result indicates again that the PNIPAAM has been successfully grafted on the PC membrane substrates by plasma graft pore-filling polymerization. 3.3. Thermogravimetric Analyses of the PC-g-PNIPAAM Copolymers. The thermal properties of the graft copolymers were studied by thermogravimetric (TG) analysis. Figure 4 shows the respective TG analysis curves of the pristine PC and

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Figure 4. TGA curves of pristine PC and PC-g-PNIPAAM (5%) membranes.

the PC-g-PNIPAAM copolymer. In comparison with the pristine PC and the PC-g-PNIPAAM copolymer, the copolymer samples exhibit an intermediate weight loss behavior and undergo a twostep degradation process. The onset of the first major weight loss occurs at a temperature of about 100 °C, which corresponds to the lost of the H2O in the PNIPAAM hydrogels in the copolymers, followed by the decomposition of the PNIPAAM chains. The second major weight loss begins at about 350 °C, which coincides with the decomposition temperature of the PC main chain. 3.4. Surface Morphology of the PC-g-PNIPAAM Membranes. The surface morphology of the PC membranes was studied by SEM. Figure 5 shows the surface and cross-sectional images of pristine PC membranes (Figure 5a,b) and PC-gPNIPAAM membranes with monomer concentrations of 5% (Figure 5c,d), 8% (Figure 5e,f), and 8.4% (Figure 5g,h), respectively. It could be seen from Figure 5a,b that the pristine PC membranes revealed cylindrical and straight pores, having uniform pore geometry with a pore size of 0.7-0.9 µm, which was consistent with the reported 0.8 µm nominal pore size. After graft copolymerization, the entire membrane surface was covered by a thin polymer layer, indicating that the membrane surface had been evenly modified. From the SEM images of the membrane surfaces, as shown in Figure 5c-h, both pore narrowing and pore blocking were observed as consequences of the surface grafting with PNIPAAM gels. From the cross-sectional SEM images shown in Figure 5b,d,f,h, it can be obviously seen that the uniformly grafted PNIPAAM polymers were well formed inside the pores throughout the entire membrane thickness, and the PNIPAAM hydrogels filled in the membrane pores gradually with the increase of the monomer concentration. As the grafting level increased at higher NIPAAM concentration, more thermo-sensitive polymer chains attached themselves onto the pores and the surface of PC film. By adjusting the initial monomer concentration, one could control the morphology of the composite membranes and further regulate their on-off switching behavior. The microstructure gave strong evidence for the occurrence of the polymerization of NIPAAM onto the PC membrane. The results are in good agreement with the previous investigation carried out with different substrates and different analysis methods.21 However, an interesting finding is that further increase of the monomer concentration did not significantly change the thickness of the grafted PNIPAAM layer, as the surfaces of the majority of pores were almost completely blocked (see Figure

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5e,g). Nevertheless, the above results verified that functional PNIPAAM chains could be successfully grafted on both the outer surfaces of the membrane and the inner surfaces of the membrane pores by the plasma graft pore-filling polymerization method, and by adjusting the initial monomer concentration, one could control the morphology of the composite membranes and further regulate their on-off switching behavior. It should be kept in mind that all of the SEM images were obtained under dry conditions. Therefore, the actual surface morphology during ultrafiltration would be even more different from that of the pristine PC membranes as a result of the pronounced hydrogel character of the grafted PNIPAAM polymers. 3.5. Water Contact Angles of the PC-g-PNIPAAM Membranes. Figure 6 shows the water contact angle (θ) of pristine PC membrane and the PC-g-PNIPAAM membranes at 25 and 40 °C. The contact angle of the pristine PC membrane decreased from 75° at 25 °C to 66° at 40 °C, which was due to the liquid surface tension between liquid and air decreasing by increasing the temperature. On the other hand, however, the water contact angle of the PC-g-PNIPAAM membranes shows a reverse tendency, as shown in Figure 6: the contact angle of the PC-g-PNIPAAM membrane with a monomer concentration of 8% increases from 65° at 25 °C to 92° at 40 °C. This result agrees well with a previous report by Xie.14 The water contact angle of the modified membrane is affected by the membrane matrix (e.g., the surface porosity), the degree of surface coverage (related to the degree of grafting), and the structure of the grafted polymer (e.g., the chain length). If the membrane surface has been completely covered by the grafted polymer, then the water contact angle will mainly depend on the hydrophilic/hydrophobic balance of this polymer. The hydrophilicity/hydrophobicity of grafted PNIPAAM played a leading role with the temperature around the LCST of PNIPAAM. The PC-gPNIPAAM membranes have lower water contact angles at 25 °C, which is attributable to the hydrophilic nature of the grafted NIPAAm polymer side chains below its LCST. As shown in Figure 6a, the water contact angle of the PC-g-PNIPAAM membrane decreases with the increase in NIPAAM monomer concentration. With the temperature increase to above PNIPAAM’s LCST, the grafted PNIPAAM polymer chains change to shrinkage configuration; as a result, the water contact angle increases obviously. It can also be seen from Figure 6b that, for the same PC-g-PNIPAAM membrane, the ∆θ is increased with the increase of the monomer concentration. In short, the hydrophilic/hydrophobic transition of the grafted PNIPAAM layer on the PC-g-PNIPAAM membrane surface made the PC-g-PNIPAAM hydrophobic when the temperature increased above the LCST. A hydrophilic membrane surface and a hydrophilic pore surface should be helpful to increase the water flux of membrane rather than a hydrophobic membrane surface and a hydrophobic pore surface. 3.6. Temperature-Dependent Flux of Aqueous Solution through the PC-g-PNIPAAM Membranes. To investigate the pore “on-off” switching controlled by the PNIPAAM hydrogels with response to the change of the environment temperature, water fluxes from the pristine and PNIPAAM grafted PC membranes at temperatures ranging from 25 to 40 °C were carried out. Figure 7a shows the water fluxes of the pristine and PNIPAAM grafted PC membranes with monomer concentrations from 3% to 8%. The effect of temperature change on the effective diameter and the pore filling ratio (F), calculated from eqs 1 and 2, are shown in Figure 7b and 7c, respectively. For the pristine membrane, the water flux is slightly increased with the increase of the temperature; this is due to the decrease of water viscosity, which results in the higher water flux. On the other hand, however, the PNIPAAM grafted PC membranes show obvious thermo-

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Figure 5. Top surface and cross-sectional SEM images of (a, b) pristine PC membrane and PC-g-PNIPAAM membranes with monomer concentrations of (c, d) 5%, (e, f) 8%, and (g, h) 8.4%, respectively.

responsive characteristics. The water flux was much lower at temperatures below 31 °C than at temperatures above 33 °C. At the temperature increases to the LCST of PNIPAAM (32 °C), the water flux increases substantially. The valve mechanism was achieved by the transition between the polymer expansion below the PNIPAAM’s LCST and the shrinkage (deswelling) above the LCST. When the temperature is below the LCST, the grafted PNIPAAM polymer on the surface and inner pore is in a loose coil figuration, and the pores were at the “off” state blocked by the swelling PNIPAAM gels. Whereas when the temperature increases to above the LCST, the grafted PNIPAAM changes to a shrinkage configuration. As a result, the pores were changed to the “on” state, and the water flux was then increased obviously. The on-off gating characteristics are more obvious when the pores are grafted with higher PNIPAAM gels (e.g., 8%), as shown in

Figure 7. According to Hagen-Poiseuille’s law, the mean diameter of membrane pores can be evaluated by eq 1. As seen in Figure 7b, the pore diameters of pristine membrane are constant when the temperature is increased from 25 to 40 °C. In contrast, the pore diameter of the PC-g-PNIPAAM membranes became larger with the increase of the temperature above its LCST. Figure 8 shows the effect of the pore-filling ratio on the water flux and effective pore diameter of the PC-g-PNIPAAM membranes at 25 and 40 °C. The water flux of the pristine PC membrane was always larger than that of grafted PC membranes at both 25 and 40 °C, because the grafted PNIPAAM hydrogel inside the pores reduced the pore size of the PC membrane. With the pore-filling ratio increase, the water flux at both 25 and 40 °C became lower. That is because water at different temperatures has changing viscosity, to eliminate the effect of

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Figure 6. Temperature-dependent variations of (a) contact angles (θ) of pristine PC membrane and PC-g-PNIPAAM membranes with different monomer concentrations, and (b) the difference in water contact angle (∆θ) for 40 and 25 °C.

water viscosity on the water flux of the membrane. A coefficient called the pore diameter ratio (Rd,T) is introduced here; it is defined as the ratio of the pore mean diameter of the grafted membrane to that of the pristine membrane at the same temperature. The pore diameter ratio (Rd,T) can be calculated according to the following equation: Rd,T

dg ) d0

(3)

For the PC-g-PNIPAAM membranes, the values of both Rd,40 and Rd,25 decreased with the pore-filling ratios increase. According to Xie’s research,14 the larger the difference between Rd,40 and Rd,25 is, the more significant the thermo-responsive characteristics of the PC-g-PNIPAAM membranes. Similarly, the on-off ratio also calculated from the ratio of permeability at 40 °C to that at 25 °C, and the result is shown in Figure 8c. The increase in monomer concentration would increase the membrane on-off effect. However, with further increase of the monomer concentration to higher than 10%, the on-off effect

Figure 7. Temperature-dependent characteristics of (a) water flux, (b) pore size, and (c) pore-filling ratios of pristine membrane and PC-g-PNIPAAM membranes with different monomer concentrations.

gradually disappeared. This result is in good agreement with the previous report by Lue et al.22 The experimental results in Figures 7 and 8 showed that the water flux of the PC-gPNIPAAM membrane is mainly dependent on the change of pore size, which is further proof that the surface inside the pores was successfully grafted with NIPAAM. 4. Conclusion Cross-linked PNIPAAM hydrogel polymer grafted onto PC membranes by plasma graft pore-filling polymerization exhibited rapid and reversible thermo-sensitive characteristics. Several analyses supported the valve regulation mechanism: XPS spectra expounded PNIAAM grafted to the PC successfully; SEM

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Excellent Talents in University (NCET) by the Ministry of Education of China. Literature Cited

Figure 8. Effect of pore-filling ratios on (a) water flux, (b) pore diameter, and (c) water flux ratio of 40 °C/25 °C of PC-g-PNIPAAM membranes.

visualized the pore morphology at a swelling state, carried out at room temperature lower than the LCST. The PC-g-NIPAAM membranes demonstrated fast and reversible swelling changes in a small temperature range. The results showed that the thermo-responsive gating characteristics of the water flux of PCg-PNIPAAM membranes were mainly dependent on the pore size change rather than the variation of membrane/pore surface hydrophilicity. Acknowledgment The authors are grateful for the financial support from the Beijing Nova Program (Grant 2006B16), the Major Project of Science and Technology Research from the Ministry of Education of China (308003), and the Program for New Century

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ReceiVed for reView May 26, 2009 ReVised manuscript receiVed November 16, 2009 Accepted December 7, 2009 IE9008666