Temperature- and pH-sensitive nylon membranes prepared via consecutive surface-initiated atom transfer radical graft polymerizations

Journal of Membrane Science 342 (2009) 300–306

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Temperature- and pH-sensitive nylon membranes prepared via consecutive surface-initiated atom transfer radical graft polymerizations Z.B. Zhang a,b , X.L. Zhu b , F.J. Xu a,c , K.G. Neoh a , E.T. Kang a,∗ a b c

Department of Chemical and Biomolecular Engineering, National University of Singapore, Kent Ridge, Singapore 119260, Singapore School of Chemistry & Chemical Engineering, Soochow University, Suzhou 215006, PR China College of Materials Science & Engineering, Beijing University of Chemical Technology, Beijing 100029, PR China

a r t i c l e

i n f o

Article history: Received 7 February 2009 Received in revised form 24 June 2009 Accepted 3 July 2009 Available online 14 July 2009 Keywords: Stimuli-responsive brushes Nylon membrane ATRP PNIPAAm PDMAEMA

a b s t r a c t Thermo- and pH-responsive nylon membranes were prepared via consecutive surface-initiated atom transfer radical polymerizations (ATRPs) of N-isopropylacrylamide (NIPAAm) and N,N -dimethyl aminoethyl methacrylate (DMAEMA). The amide groups of the membrane and pore surfaces were first activated for the immobilizing of alkyl halide ATRP initiator. The kinetics study revealed that the chain growth from the membranes was consistent with a “controlled” process. The dormant chain ends of the grafted NIPAAm polymer (PNIPAAm) or DMAEMA polymer (PDMAEMA) on the nylon membranes could be reactivated for the consecutive surface-initiated ATRP to produce the corresponding nylon membranes functionalized with PNIPAAm-b-PDMAEMA or PDMAEMA-b-PNIPAAm diblock copolymer brushes. The permeation of aqueous solution through the functional membranes showed an abrupt change with temperature between 30 and 35 ◦ C, and pH between 6 and 8. The temperature- and pH-responsive permeation through the functional nylon membranes was found to be reversible and irrespective of the sequence of the graft segments. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Stimuli-responsive materials have been of great interests because of their useful applications in various areas [1–6]. Stimuliresponsive materials can be designed to sense and respond to various changes in environmental conditions, such as temperature [7–10], pH [11,12], magnetic or electric fields [13–15], ionic strength, and light [16,17]. The stimuli-responsive materials can be used in the form of hydrogel [2], micelles [12], particles [18], and membranes [19–22]. Among them, the stimuli-responsive membranes have promising applications in areas, such as in controlled drug delivery, liquid mixture separation and water treatment, because the flux or permeation through the membranes can be controlled or triggered by external stimuli [20,21,23]. Nylon membrane is one of the most versatile membranes because of its outstanding mechanical strength, flexibility, toughness and abrasion resistance, as well as its good resistance to solvents, oils, and biological fluids [24–26]. Nylon membranes are also widely used as matrices or substrates in biochemistry and life sciences [27–29], separation and purification [30–33], and hemodialysis [34]. However, more and more practical applications require the use of functionalized nylon membranes. “Smart” nylon

∗ Corresponding author. Tel.: +65 6874 2189; fax: +65 6779 1936. E-mail address: [email protected] (E.T. Kang). 0376-7388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2009.07.004

membranes capable of responses to multiple environmental stimuli are of both scientific and technological interest. The incorporation of desirable functionalities onto nylon surfaces can be accomplished via several methods, such as plasma treatment [35,36], UV irradiation [37,38], N-alkylation [25,39], potassium peroxydisulfate oxidation [26,40], and surface-initiated atom transfer radical polymerization (ATRP) [41]. ATRP is a recently developed “living” or “controlled” radical polymerization method [42,43]. It is possible to prepare well-defined polymer brushes on various substrates via surface-initiated ATRP [44–47], provided that the ATRP initiators can be immobilized on the substrate surfaces a priori. In the present work, functional nylon membranes with thermoand pH-sensitivity are designed and synthesized via consecutive surface-initiated ATRP. Poly(N-isopropylacrylamide) (PNIPAAm) and poly(N,N -dimethyl aminoethyl methacrylate) (PDMAEMA) are selected as the functional polymer brushes, because they are wellknown, respectively, as the thermo- [22,48] and pH-responsive [24,49] polymers. The covalent immobilization of ATRP initiators on the nylon membrane and its pore surfaces can be accomplished in a simple two-step process developed earlier (Scheme 1) [41]. Functional polymer brushes of PNIPAAm and PDMAEMA, as well as their diblock copolymer brushes of different block lengths and spatial block sequence, are then grafted from the membrane and pore surfaces via surface-initiated ATRP (Scheme 1). The surface composition and morphology of the modified nylon membranes are characterized by X-ray photoelectron spectroscopy (XPS) and

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Scheme 1. Diagram illustrating the activation of the amide groups of the nylon membrane by formaldehyde to produce the N-methylol-terminated Nylon-OH membrane, reaction of the hydroxyl groups of the Nylon-OH membrane with 2-bromoisobutyrate bromide to produce the alkyl halide-terminated Nylon-Br membrane, and surfaceinitiated ATRPs from the Nylon-Br membrane.

scanning electron microscopy (SEM), respectively. Temperatureand pH-responses of the surface-functionalized nylon membrane are evaluated from the permeation rate of aqueous solutions as a function of temperature and pH.

per(I) bromide (CuBr, 99%), and copper(II) bromide (CuBr2 , >98%) were also obtained from Sigma–Aldrich Chemical Co.

2.2. Immobilization of atom-transfer radical polymerization (ATRP) initiators on the nylon membranes 2. Experimental 2.1. Materials Nylon microfiltration membranes (GE nylon membrane, R12SP02500) with a diameter of about 25 mm and a thickness of about 65 ␮m were obtained from GE Osmonics Labstore of Minnesota, MN. The membrane with an average pore size of about 5 ␮m was reinforced by polyester fibers. The surface area (SA) and specific surface area (SSA) were about 4.9 cm2 and 4587 cm2 /g, respectively [41]. Formaldehyde (>36.5% w/w), 2-bromoisobutyryl bromide (BIBB, 98%), and the monomers, N-isopropylacrylamide (NIPAAm, >99%) and 2(N,N -dimethylamino)ethyl methacrylate (DMAEMA, >98%), were obtained from Sigma–Aldrich Chemical Co. of Milwaukee, WI. The monomers were used after removal of the inhibitors in a readyto-use disposable inhibitor-removal column (Sigma–Aldrich). 1,1,4,7,10,10-Hexamethyl triethylene tetramine (HMTETA, 99%), N,N,N ,N ,N -pentamethyldiethylenetriamine (PMDETA, 99%), cop-

The immobilization of the ATRP initiators on the nylon membrane was carried out in two steps (Scheme 1): (1) activation of the surface amide groups by formaldehyde to produce the N-methylol polyamide membrane (Nylon-OH membrane) and (2) reaction of the hydroxyl groups of the Nylon-OH membrane with 2-bromoisobutyryl bromide (BIBB) to produce the 2-bromoisobutyryl-immobilized membrane (nylon-Br membrane) for the subsequent surface-initiated ATRP. Briefly, 50 mL of formaldehyde solution and 1 mL of 85 wt% phosphoric acid were introduced into a 100 mL flask containing 30 membrane discs. The reaction mixture was kept at 60 ◦ C for 12 h to produce the NylonOH membranes. After the reaction, the Nylon-OH membranes were washed and extracted exhaustively with copious amounts of water and then dried by pumping under reduced pressure. For the reaction of hydroxyl groups of the Nylon-OH membrane with BIBB, 50 mL of chloroform and 4 mL of triethylamine were introduced into a 100 mL flask containing 26 Nylon-OH membrane discs, followed by the addition of 0.8 mL of BIBB. The reaction mixture was gently

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stirred for 5 min at 0 ◦ C and then for 15 min at room temperature to produce the Nylon-Br membranes. The Nylon-Br membranes were washed exhaustively with copious amounts of acetone and then with a methanol/water (1:1 v/v) mixture prior to being dried under reduced pressure. 2.3. Surface-initiated ATRP For surface-initiated ATRP of DMAEMA from the Nylon-Br membranes, the reaction was carried out using a [DMAEMA (1 mL)]:[CuBr]:[CuBr2 ]:[HMTETA] molar feed ratio of 100:1:0.4:1 in 6 mL of a water–methanol mixture (H2 O:CH3 OH = 1:1 (v/v)) at room temperature in a Pyrex tube containing five NylonBr membrane discs. As for the graft polymerization of NIPAAm from the Nylon-Br substrates, the reaction was carried out using a [NIPAAm (1 g)]:[CuBr]:[CuBr2 ]:[PMDETA] molar feed ratio of 100:1:0.2:1 in 6 mL of deionized water at room temperature in a Pyrex tube containing five Nylon-Br membrane discs. The graft polymerization was allowed to proceed for a predetermined period of time. The resulting nylon membranes with surface-grafted DMAEMA polymer (PDMAEMA) and NIPAAm polymer (PNIPAAm) are referred to as the Nylon-g-PDMAEMA and Nylon-g-PNIPAAm membranes, respectively. The resulting Nylong-PDMAEMA and Nylon-g-PNIPAAm membranes were washed and extracted thoroughly with ethanol and then doubly distilled water. The membranes were immersed subsequently in a large volume of ethanol for about 48 h to ensure the complete removal of the adhered and physically adsorbed reactants, prior to being dried under reduced pressure. The monomer conversion was determined gravimetrically according to the equation: conversion (%) = (Wa − Wb )/Wm , where Wa and Wb represent the weights of the dry membrane after and before graft polymerization, respectively, and Wm is the weight of the monomer added. The dormant alkyl halide end groups of the grafted chains can be used as the macro-ATRP initiators for the subsequent block copolymerization. Nylon-g-PDMAEMA-b-PNIPAAm and Nylon-gPNIPAAm-b-PDMAEMA membrane were prepared by reactivating the dormant chain ends on the corresponding Nylon-g-PDMAEMA and Nylon-g-PNIPAAm membranes. The second round of surfaceinitiated ATRP was carried out under the same reaction conditions as those described above for the preparation of the initial block. 2.4. Measurements of the temperature- and pH-dependent flux through the graft-modified nylon membranes The flux of aqueous solutions through the membranes was carried out under an argon pressure of 0.2 kg/cm2 . The pristine nylon membrane, the Nylon-g-PDMAEMA-b-PNIPAAm membrane, or the Nylon-g-PNIPAAm-b-PDMAEMA membrane was mounted on the microfiltration cell (Toyo Roshi UHP-25, Tokyo, Japan) which was immersed in a thermostated water bath (LAUDA, Nr. U16005, Germany) at a predetermined temperature. An aqueous solution of prescribed pH value was introduced into the microfiltration cell, which was equilibrated in the thermostated water bath for 30 min prior to the initiation of the flux. The flux was calculated from the permeate volume as a function of time. The pH value of the solution was determined by a Mettler Toledo Delta 320 pH meter. 2.5. Membrane characterization The surface chemical composition and morphology of the modified nylon membranes were characterized by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). XPS measurements were performed on a Kratos AXIS HSi spectrometer using a monochromatized Al K␣ X-ray source (1486.6 eV photons). Surface elemental stoichiometries were determined from the sen-

sitivity factor-corrected spectral area ratios and were reliable to within ±5%. The surface morphologies of the membrane discs were imaged using a scanning electron microscope (JEOL SEM, model 5600LV). Prior to the SEM measurements, the membranes were fixed on the metal holders and sputtered with a thin Pt layer. 3. Results and discussion The immobilization of atom transfer radical polymerization (ATRP) initiators on the nylon membrane is the precondition for the surface-initiated ATRP. A simple two-step method for the effective immobilization of ATRP initiators on the nylon membrane was developed earlier [41]. Initially, the amide groups on the nylon membrane and pore surfaces were activated by formaldehyde to produce the N-methylol polyamide membrane (Nylon-OH). Then, the hydroxyl groups on the Nylon-OH membrane were reacted with 2-bromoisobutyryl bromide (BIBB) to produce the alkyl halide-functionalized nylon membrane (Nylon-Br). The surface and internal morphologies of the nylon membrane remain practically unchanged after the modification and immobilization of the ATRP initiators [41]. The subsequent surface-initiated ATRP was carried out from the Nylon-Br membrane. Results of the surface functionalization processes are discussed below. 3.1. Surface-initiated ATRP from the Nylon-Br membrane In this work, (2-dimethyl amino)ethyl methacrylate (DMAEMA) and N-isopropylacrylamide (NIPAAm) were selected as the model monomers for the preparation of functional polymer brushes from the Nylon-Br membranes via surface-initiated ATRP. Poly(N-isopropylacrylamide) (PNIPAAm) is a well-known thermoresponsive polymer, which exhibits a lower critical solution temperature (LCST) of about 32 ◦ C in an aqueous medium. The reversible phase transition of PNIPAAm has been widely used in the synthesis of thermo-responsive materials. Poly[2-(N,N -dimethyl aminoethyl methacrylate)] (PDMAEMA) is a pH-sensitive polymer arising from the pendant amine groups. The nylon membrane with its surface (including the pore surfaces) grafted with PDMAEMA and PNIPAAm is expected to be sensitive to changes in both temperature and pH, and may have practical uses in stimuli-responsive and controlled permeation. CuBr2 acting as the deactivator in the ATRP process was added to the polymerization system in order to maintain a well-controlled surface-initiated ATRP from the early stage of the polymerization [25,47]. The presence of grafted PNIPAAm and PDMAEMA brushes on the Nylon-Br membranes was confirmed by XPS analysis after the surfaces had been subjected to vigorous washing and extraction. Fig. 1 shows the respective wide-scan and C 1s core-level spectra of Nylon-g-PDMAEMA2, Nylon-g-PNIPAAm2, Nylon-g-PDMAEMA2b-PNIPAAm and Nylon-g-PNIPAAm2-b-PDMAEMA membranes. The conditions for the surface functionalization of the respective membranes are given in Table 1. The C 1s core-level spectrum of the Nylon-g-PDMAEMA2 membrane surface (Fig. 1b) can be curve-fitted into four peak components with BEs at about 284.6, 285.7, 286.2, and 288.7 eV, attributable to the C–H, C–N, C–O and O C–O species, respectively [50]. The O C–N species at the BE of 287.4 eV from the pristine nylon membrane [41] has disappeared completely. The [C–N]:[C–O]:[O C–O] peak component area ratio of about 3.1:1.0:1.1 is in fairly good agreement with the theoretical molar ratio of 3:1:1 for the PDMAEMA homopolymer. These XPS results indicate that the membrane surface has been completely covered by the PDMAEMA brushes to a thickness exceeding the sampling depth of the XPS technique (about 7.5 nm in an organic matrix [51]). Fig. 1c and d shows the respective wide scan and C 1s core-level spectra of the Nylon-g-PNIPAAm2 membrane. The C 1s

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spectrum of the Nylon-g-PDMAEMA2-b-PNIPAAm (Fig. 1f) can be curve-fitted into four peak components with BEs at about 284.6, 285.7, 286.2, and 288.7 eV, attributable to the C–H, C–N, C–O and O C–O species, respectively. The peak components at the BEs of about 285.7 and 287.4 eV can be attributed, respectively, to the C–N and O C–N species associated with PNIPAAm. The C 1s peak components at the BEs of 286.2 and 288.7 eV, associated with the C–O and O C–O species from the underlying PDMAEMA segment have disappeared almost completely. With the alkyl halide on the Nylong-PNIPAAm2 membrane as the macro-initiator for the consecutive ATRP of DMAEMA, the corresponding C 1s core-level spectrum of the Nylon-g-PNIPAAm2-b-PDMAEMA membrane (Fig. 1h) can be curve-fitted into five peak components with BEs at about 284.6, 285.7, 286.2, 287.4 and 288.7 eV, attributable to the C–H, C–N, C–O, O C–N and O C–O species, respectively. Two new peak components (compared to those of Nylon-g-PNIPAAm2 surface of Fig. 1d) at the BEs of 286.2 and 288.7 eV, attributable to the C–O and O C–O species, respectively, of the grafted DMAEMA polymer chains have appeared. Thus, the block copolymerization results have confirmed that the dormant alkyl halide groups at the grafted PDMAEMA and PNIPAAm chain ends have allowed reactivation during the subsequent surface-initiated block copolymerization process, resulting in the formation of diblock copolymer brushes on the nylon membranes. 3.2. Morphology of the surface-functionalized membranes

Fig. 1. Wide-scan and C 1s core-level spectra of the Nylon-g-P(PDMAEMA)2, Nylon-g-PNIPAAM2, Nylon-g-P(PDMAEMA)2-b-PNIPAAm and Nylon-g-PNIPAAM2b-PDMAEMA membrane surfaces. (The preparation conditions for the membranes are given in Table 1).

core-level spectrum of the Nylon-g-PNIPAAm2 membrane surface (Fig. 1d) can be curve-fitted into three peak components with BEs at about 284.6, 285.7 and 287.4 eV, attributable to the C–H, C–N and O C–N species, respectively [50]. The [C–H]:[C–N]:[O C–N] peak component ratio of about 4.2:0.9:1.0 is in good agreement with the theoretical molar ratio of 4:1:1 for the PNIPAAm homopolymer, indicating that the membrane surface has been completely covered by the PNIPAAm brushes. The dormant alkyl halide groups of Nylon-g-PDMAEMA2 can be reactivated for the preparation of diblock copolymer brushes in the subsequent surface-initiated ATRP of NIPAAm. The C 1s core-level

Fig. 2 shows the respective SEM images (top views) of the (a) pristine nylon, (b) Nylon-Br, (c) Nylon-g-PDMAEMA2, (d) Nylon-g-PNIPAAM2, (e) Nylon-g-PDMAEMA2-b-PNIPAAm and (f) Nylon-g-PNIPAAm2-b-PDMAEMA membranes. After the graft polymerization, the membrane surface became densely covered by polymer chains, and the effective dimension of pore channels was reduced from that of the starting nylon (Fig. 2a) and Nylon-Br (Fig. 2b) membranes. Thus, the SEM results are consistent with the presence of grafted homopolymers and block copolymers of DMAEMA and NIPAAm. The kinetics of graft polymerizations of DMAEMA and NIPAAm from nylon membranes via surface-initiated ATRP was investigated. The results are shown in Fig. 3. DMAEMA and NIPAAm can be readily graft polymerized by ATRP from the modified nylon membrane (Nylon-Br) at room temperature. Furthermore, an approximate linear increase in ln([M]0 /[M]), where [M]0 and [M] were the initial and instantaneous monomer concentrations, respectively, with polymerization time was observed. These results suggested that the chain growth of PDMAEMA and PNIPAAm from the Nylon-Br membranes via surface-initiated ATRP was well-controlled [43]. The graft yield based on the membrane surface area (SA), denoted as GYSA , and on the specific surface area (SSA), denoted as GYSSA , can

Table 1 Reaction conditions and graft yield (GY) of the graft-modified nylon membranes from surface-initiated ATRP. Samplea

Nylon-g-PDMAEMA1 Nylon-g-PDMAEMA2 Nylon-g-PNIPAAm1 Nylon-g-PNIPAAm2 Nylon-g-PDMAEMA2-b-PNIPAAmc Nylon-g-PNIPAAm2-b-PDMAEMAc

Reaction time (min)

15 30 5 10 10 45

Graft yield (GY)b GYSA (mg/cm2 )

GYSSA (␮g/cm2 )

6.0 9.1 6.1 16.4 9.1 + 15.7 16.4 + 12.9

6.4 9.7 6.5 17.5 9.7 + 16.8 17.5 + 13.8

a Reaction conditions: DMAEMA as monomer – [DMAEMA (1 mL)]:[CuBr]:[CuBr2 ]:[HMTETA] molar feed ratio = 100:1:0.4:1 in 6 mL of a water–methanol mixture (H2 O:CH3 OH = 1:1 (v/v)) at room temperature; NIPAAm as the monomer – [NIPAAm (1 g)]:[CuBr]:[CuBr2 ]:[PMDETA] molar feed ratio = 100:1:0.2:1 in 6 mL of deionized water at room temperature. b Graft yield (GY) is defined as GYSA (or GYSSA ) = (Wa − Wb )/SA (or (Wa − Wb )/SSA), where Wa and Wb represent the weights of the dry membrane after and before grafting, respectively, and SA and SSA are the respective surface area and specific surface area of the membrane. c Surface-initiated ATRP from the corresponding Nylon-g-PDMAEMA2 or Nylon-g-PNIPAAm2 membrane surfaces.

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Fig. 2. SEM images (top view) of the (a) pristine nylon, (b) Nylon-Br, (c) Nylon-g-PDMAEMA2, (d) Nylon-g-PNIPAAm2, (e) Nylon-g-PDMAEMA2-b-PNIPAAm and (f) Nylon-gPNIPAAm2-b-(DMAEMA) membranes. (The preparation conditions for the membranes are given in Table 1).

be evaluated (Table 1). GYSA and GYSSA were calculated according to the equations: GYSA = (Wa − Wb )/SA and GYSSA = (Wa − Wb )/SSA, respectively. The GYSA and GYSSA values are about 9.1 mg/cm2 and 9.7 ␮g/cm2 for the Nylon-g-PDMAEMA2 membrane, respectively, and about 16.4 mg/cm2 and 17.5 ␮g/cm2 for the Nylon-g-PNIPAAm2 membrane. Furthermore, grafted block copolymers, PDMAEMA-bPNIPAAm and PNIPAAm-b-PDMAEMA, were also obtained using the respective Nylon-g-PDMAEMA and Nylon-g-PNIPAAm membrane surface as the macro-ATRP initiators. The respective GYSSA values confirm with that the pore surfaces of the nylon membranes have also been densely grafted with homo- and block copolymers (Table 1).

Fig. 3. Kinetics of graft polymerization of DMAEMA and NIPAAm from the surface of nylon membrane via surface-initiated ATRP. ([M]0 and [M] are the initial and instantaneous monomer concentrations, respectively).

3.3. Thermo- and pH-responsive flux of aqueous medium through the membranes Since the pore channels of the nylon membrane have been successfully grafted with the PNIPAAm and PDMAEMA block copolymers via consecutive surface-initiated ATRP, the flux through the membrane is expected to show responses to changes in temperature and pH of the medium or environment. The flux of aqueous solutions through the Nylon-g-PDMAEMA2-b-PNIPAAm and Nylon-g-PNIPAAm2-b-PDMAEMA membranes was investigated as functions of both temperature (in the temperature range of 25–45 ◦ C at pH = 7.0) and pH (in the pH range of 4.8–12 at 25 and 35 ◦ C) of the permeate. The results are summarized in Figs. 4 and 5, respectively. In Fig. 4, the flux of the aqueous solution through the membranes was almost unchanged when the temperature was below 30 ◦ C. A dramatic increase in permeation rate was observed at temperatures between 30 and 35 ◦ C. When the permeate temperature was increased to 35 ◦ C, the flux again exhibited a weak dependence on temperature. The flux at 40 ◦ C was about 2.4 times that at 20 ◦ C for the Nylon-gPNIPAAm2-b-PDMAEMA membrane (Fig. 4a), and 2.7 times for the Nylon-g-PDMAEMA2-b-PNIPAAm membrane (Fig. 4b). The similarity in temperature-dependent flux behavior for the Nylong-PNIPAAm2-b-PDMAEMA and Nylon-g-PDMAEMA2-b-PNIPAAm membranes indicate that the temperature-response is independent of the sequence of the stimuli-responsive block in the diblock polymer chain. In a control experiment using pristine nylon membranes, no obvious dependence of flux on permeate temperature was found. The flux through the pristine nylon membrane was about 3.1 mL/(min cm2 ) in the temperature range of 25–45 ◦ C, which was about 55 times that of the Nylon-g-PNIPAAm2-b-PDMAEMA membrane (Fig. 4a) at 25 ◦ C. All these results indicate that the thermo-responsive properties of the graft-modified nylon membranes are regulated by the PNIPAAm segments. The lower critical

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Fig. 4. Temperature-dependent permeability of aqueous solutions in the temperature range of 25–45 ◦ C through (a) the Nylon-g-PNIPAAm2-b-PDMAEMA and pristine nylon membrane, and (b) the Nylon-g-PDMAEMA2-b-PNIPAAM membrane. The pH of the solution was 7.0 and the imposed pressure for flow was 0.2 kg/cm2 .

solution temperature (LCST) of the NIPAAM polymer is about 32 ◦ C. At a permeate temperature below the LCST, the grafted PNIPAAM segments are hydrophilic and assume an extended (hydrated) conformation on the membrane and pore surfaces, reducing the effective pore size and thus, the permeation rate of the aqueous solution. At permeate temperatures above the LCST, the PNIPAAm chains associate hydrophobically on the membrane and pore surfaces, resulting in the opening up of pores in the membrane and hence the observed increase in flow rate. Meanwhile, when the temperature was lowed from 45 to 25 ◦ C, the flux also showed an abrupt decrease between 35 and 30 ◦ C, indicating that the temperature-response was completely reversible. Again, the Nylong-PDMAEMA2-b-PNIPAAm and Nylon-g-PNIPAAm2-b-PDMAEMA membranes showed the same temperature responses, regardless of the block sequence, in repeated temperature cycling. The flow rate of aqueous solution through the Nylong-PNIPAAm2-b-PDMAEMA and Nylon-g-PDMAEMA2-b-PNIPAAm membranes was also measured as a function of permeate pH at a constant temperature of 25 ◦ C (Fig. 5a) and 35 ◦ C (Fig. 5b). At both temperatures, the flux exhibits a marked increase when pH of the medium is increased from 6 to 8. The flux through pristine nylon membrane (Fig. 5a) does not exhibit any dependence on the pH of permeate. The pH-sensitive flux through the surfacefunctionalized nylon membranes can be attributed to the presence of PDMAEMA segments on the grafted nylon membrane. PDMAEMA has amine pendant groups. Under acidic conditions (pH < 6.0), the amine groups are partially protonated and assume a more extended conformation, resulting in a decrease in effective pore size of the graft-modified nylon membrane. Under basic conditions (pH > 7.5), PDMAEMA becomes deprotonated and more hydrophobic, and can assume a more compact conformation [1]. The flux thus

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Fig. 5. pH-dependent permeability of aqueous solutions through the Nylong-PDMAEMA2-b-PNIPAAM and Nylon-g-PNIPAAm2-b-PDMAEMA membranes at temperatures of (a) 25 ◦ C, and (b) 35 ◦ C. The imposed pressure for flow was 0.2 kg/cm2 . The thickness of the pristine nylon membranes was about 65 ␮m, while the thicknesses of Nylon-g-PDMAEMA2-b-PNIPAAM and Nylon-g-PNIPAAm2b-PDMAEMA membranes were about 80 and 85 ␮m, respectively.

increases as a result of the increase in effective pore size. For the Nylon-g-PDMAEMA2-b-PNIPAAm membrane, the flux at pH = 12 and temperature = 35 ◦ C was 0.24 mL/(min cm2 ), which was 2.5 times that at pH = 4.8 (Fig. 5b). Similar pH-dependent results were obtained for the Nylon-g-PNIPAAm2-b-PDMAEMA membrane. The sequence of the polymer chains again does not exert a significant effect on the pH-response of the graft-modified membranes. 4. Conclusions Nylon membrane and its pore surfaces were successfully functionalized via consecutive atom transfer radical graft polymerization of DMAEMA and NIPAAm. Kinetics studies revealed the features of “living”/controlled radical polymerization. XPS and SEM results confirmed the successfully grafting of the stimuli-responsive polymers and block copolymers from the nylon membranes. The flux of aqueous solution through the surfacefunctionalized nylon membranes exhibited a transition in the temperature range between 30 and 35 ◦ C, and in the pH range between 6 and 8. The sequence of PNIPAAm and PDMAEMA blocks in the grafted copolymer brushes did not have an obvious effect on the observed thermo- and pH-response of the membranes. The functionalized nylon membranes are thus “smart” membranes suitable for applications in water treatment, aqueous separation and controlled flux of aqueous media. References [1] E.S. Gil, S.M. Hudson, Stimuli-responsive polymers and their bioconjugates, Prog. Polym. Sci. 29 (2004) 1173.

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