Localized Heat Generation from Magnetically Responsive Membranes

Article pubs.acs.org/IECR

Localized Heat Generation from Magnetically Responsive Membranes Xianghong Qian,*,† Qian Yang,‡ Anh Vu,‡ and S. Ranil Wickramasinghe‡ †

Department of Biomedical Engineering, ‡Ralph E Martin Department of Chemical Engineering, University of Arkansas, Fayetteville, Arkansas 72701, United States ABSTRACT: Commercial thin film composite nanofiltration NF270 and microfiltration polyethylene terephthalate (PET) track-etched membranes were modified by grafting thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) polymers via surface initiated atom transfer radical polymerization (ATRP). The polymer chain ends were conjugated with superparamagnetic (Fe3O4) nanoparticles. Each tailored superparamagnetic nanoparticle acts as a nanoheater under an external oscillating magnetic field. The localized heat generated induces the lower critical solution temperature (LCST) transition for the PNIPAM in aqueous solution. The phase transition of the polymer leads to the alteration of membrane transport properties. Localized heating affects membrane permeability differently for NF270 and PET membranes based on whether the modification occurs on the membrane surfaces or inside the membrane pores. Filtration of polystyrene latex particles through PET membranes demonstrates that it is possible to modulate the pore size by applying an external oscillating magnetic field.

1. INTRODUCTION Stimuli-responsive polymers that switch their chemical or physical properties by responding to environmental conditions have gained extensive attention recently.1−3 Different types of stimuli can be applied to induce responses from these polymers including pH, ionic strength, temperature, specific ions or molecules, light, electric or magnetic field. Membrane materials incorporating these stimuli-responsive polymers are termed stimuli-responsive membranes.4−8 Earlier studies7 show that grafting responsive polymers on the membrane surface or inside the pore structure can lead to reversible changes of membrane properties, realizing the controlled permeation of target molecules or “smart” valve or gate functionality. Membrane properties that are responsive to temperature are of particular interest due to its relative ease of application.7 One of the most investigated temperature-responsive polymers is poly(N-isopropylacrylamide) (PNIPAM) that exhibits a lower critical solution temperature (LCST) at 32 °C in aqueous solution.3,9−11 PNIPAM undergoes a dramatic conformational change from a more hydrophilic coil-like structure below its LCST to a more hydrophobic folded conformation above its LCST. Pioneering work on the temperature-responsive membrane by Yamaguchi et al. involves the purification of a nonionic surfactant from aqueous solution by grafting PNIPAM on a polypropylene microfiltration membrane surface using plasma treatment.12 Their results clearly demonstrate a reversible conformational change of grafted PNIPAM layer leading to a temperature-dependent adsorption and desorption of the surfactant on a membrane surface. At temperatures above PNIPAM’s LCST, the surfactant adsorbs onto the grafted © XXXX American Chemical Society

PNIPAM chains resulting from hydrophobic interaction. At temperatures below its LCST, PNIPAM chains become hydrated and the surfactant desorbs from the membrane surface. Earlier studies13,14 also demonstrate that the conformational switch induced by temperature change for grafted PNIPAM on nanofiltration (NF) membranes is effective to resist membrane fouling for water treatment. Recent results15 for polyethylene terephthalate (PET) track-etched membrane modified with cross-linked PNIPAM in the membrane pores leads to switchable permeability. However, all of these experiments rely on changing the temperatures of the bulk feed solutions. There are several disadvantages in altering the bulk feed solution temperature including the slow rate of temperature change and the cost associated with heating and cooling of a large quantity of solution. Therefore, it is not realistic to adopt these temperatureresponsive membranes for applications such as water treatment. Besides extensive experimental studies, significant theoretical understanding3,9−11 has been achieved on the nature of LCST transition for PNIPAM and, in particular, the effects of various salt ions on the reduction of transition temperature. Our earlier studies have demonstrated the feasibility of developing magnetically responsive membranes for potential applications in water treatment and as remote-valve control.6,7,16 In the earlier work, superparamagnetic nanoparticles were immobilized on the chain ends of polymers grafted on a Received: May 12, 2016 Revised: July 23, 2016 Accepted: August 1, 2016

A

DOI: 10.1021/acs.iecr.6b01820 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

absolute temperature, and τ0 is a constant. Since the relaxation times depend strongly on hydrodynamic and magnetic volumes of the nanoparticles, one linearly and the other exponentially, the two relaxation mechanisms will dominate at different particle sizes. The hydrodynamic volume includes volumes of both the magnetic particle and its protective coating; whereas, the magnetic volume is the volume of the magnetic part of the nanoparticle only. The effective relaxation time is τ = τBτN/(τB + τN). The power generated depends on the relaxation time and the magnetic field parameters:21,22

membrane surface to induce mixing at the liquid−membrane interface to minimize concentration polarization. In that case, a hydrophilic and flexible polymer, poly(2-hydroxyethyl methacrylate) (poly(HEMA)), acts as a linker between the membrane surface and end-capped magnetic nanoparticle. The movement of the nanoparticle together with the polymer under an external oscillating magnetic field induces mixing at the interface. To overcome the problem of heating the entire feed solutions associated with the application of thermoresponsive membranes, rapid and localized heat generation is desired. Here stimuli of both magnetic responsiveness and thermoresponsiveness are incorporated in the same membrane system by using PNIPAM as a polymer linker to conjugate the carbocyclic acid functionalized superparamagnetic nanoparticle with an amine group on the membrane surface. Application of an oscillating magnetic field and rapid and localized heat generated by the nanoparticles induces LCST transition for the PNIPAM. The conformational switching of the functionalized polymer chains leads to the removal of foulants deposited on the membrane surface. Moreover, it can also be used to achieve active control of the membrane pore sizes. Earlier work17 attached nanoparticles on the PET membrane substrate directly followed by grafting PNIPAM on the nanoparticles using UV-initiated polymerization for controlled valve applications, but it is difficult to control the molecular weight of the grafted polymers using UV initiated polymerization, and the process introduces high polydispersity. ATRP will be able to control both the chain length and chain density of the grafted polymers and thereby the density of the nanoparticles grafted and thus the amount of localized heat generated. Moreover, grafting polymers using ATRP will allow great control of the pore size of the PET membranes for controlled valve applications. Superparamagnetic particles are single domain magnetic nanoparticles that behave like paramagnets with a giant magnetic moment residing in each particle.18 The exchange coupling of the atomic magnetic moments inside the particle is strong resulting in the alignment of the moments to each other. However, the size of the nanoparticle is such that creating domain walls inside the particle is energetically unfavorable so that each particle has only a single magnetic domain. Moreover, the magnetic anisotropy energy is weaker than the thermal energy leading to the random orientation of the giant moment in space. However, these giant moments will align in the direction of an applied external magnetic field. Superparamagnetic particles exhibit no remanence once the external magnetic field is removed, as the magnetic dipoles are dispersed by Brownian and Néel relaxations,19,20 described by times τB and τN, respectively. Brownian relaxation occurs as the particles try to randomize their magnetic moments with respect to the external magnetic field by physical rotation. Néel relaxation is caused by the rotation of magnetic moments within the particles themselves without any physical movement of the particles when the field is removed. The dissipation of energy due to relaxation will induce heating. The two relaxation times depend on particle diameter differently18,21 with

3VHη kBT

(1)

τN = τ0 e KVM / kBT

(2)

τB =

P = πμ0 χ0 H2f

2πfτ 1 + (2πfτ )2

(3)

where f is the field switching frequency, χ0 is the magnetic susceptibility and is dependent on the external field as well as on the matrix, and μ0 is vacuum permeability. To control the amount of heat produced (and temperature), the switching frequency can be adjusted to operate either at or off the resonance frequency where 2πfτ ≈ 1. At the resonance frequency, a maximum amount of heat will be produced whereas the heating effect will be reduced using off-resonance frequency. Therefore, we will be able to tune the amount of localized heating by choosing a nanoparticle size which determines the relaxation time along with the strength and oscillating frequency of the external magnetic field. We choose magnetite (Fe3O4) superparamagnetic nanoparticles, as they are relatively stable compared to metal nanoparticles in air. Further, once coated, the particles will be stable in neutral and mildly acidic and basic aqueous solutions such as wastewater. Both microfiltration PET and nanofiltration NF270 membranes were investigated in this proof-of-concept work. PNIPAM was grafted to the membrane surface by atom-transfer radical polymerization (ATRP),23 which allows for independent control of polymer chain length and chain density by varying the reaction time and the initiator concentration/immobilization time. Here the density of PNIPAM chains was systematically controlled by adding a nonreactive molecule similar to the structure of the reactive initiator to the immobilization solution. After attaching superparamagnetic Fe3O4 nanoparticles to the PNIPAM chain ends, fluxes through the membrane were measured in the presence or absence of an external oscillating magnetic field at a fixed frequency of 1000 Hz. The flux variation resulting from the PNIPAM conformational change due to localized heating was observed. Evidence for pore size control comes from the filtration experiments of polystyrene latex particles through the PET membranes, which demonstrates that the cutoff size of particles that can pass through the membrane increases in the presence of the magnetic field. As the PET membrane surface and inside the pore contain both carboxylic acid and hydroxyl groups, an oxidative hydrolysis procedure is used to convert the hydroxyl groups on the membrane surface and inside the pore to carboxylic acid groups in order to increase the polymerization initiation sites. As there are abundant numbers of reactive amine sites on NF270 membranes, no prefunctionalization treatment was conducted.

2. EXPERIMENTAL SECTION 2.1. Materials. NF 270 flat-sheet thin film composite polyamide membranes were obtained from Dow Filmtec (Edina, MN, USA). PET membranes with a nominal pore

VH and VM are the hydrodynamic and magnetic volumes of the nanoparticles; η is the solution viscosity, K is the effective anisotropy constant; kB is the Boltzmann constant; T is the B


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Scheme 1. Reaction Sequences for Track-Etched PET Membranes for Achieving High (Top) and Low (Bottom) Graft Densitiesa

a

The polymer grafting chain density is controlled by the ratio between ATRP initiator 2-bromo-2-methylpropionyl bromide and the non-reactive propionyl bromide.

Scheme 2. Steps of Chemical Reaction for Grafting PNIPAM Chains on NF270 Membrane Surface and Conjugating Superparamagnetic Nanoparticles to the Polymer Chain Ends

diameter of 400 nm and a thickness of 23 μm were purchased from Oxyphen GmbH (Dresden, Germany). All membrane samples used in this study were cut from large sheets into a circular specimen with a diameter of 25 mm. Iron oxide superparamagnetic nanoparticles with a 15 nm core diameter and a 5 nm coating layer functionalized with carboxylic acid groups were purchased from Ocean Nanotech (Springdale, AR, USA). Acetonitrile was purified by refluxing with boric anhydride and was distilled before use. Copper(I) bromide (99.999%, Aldrich, St. Louis, MO) and copper(II) bromide (99+%, Acros Organics, NJ) were commercial products and used without further purification. 2-Bromo-2-methylpropionyl bromide (98%, BMPB, Alfa Aesar, Ward Hill, MA), propionyl bromide (95%, Acros Organics, NJ), triethylamine (TEA) (>99% Alfa Aesar, Ward Hill, MA), 2,2′-bipyridine (Bpy) (Aldrich, St. Louis, MO), 4-(N′,N′-dimethylamino) pyridine (DMAP) (≥99% Fluka, St. Louis, MO), N,N,N′,N″,N″-pentamethyl diethylenetriamine (99%, PMDETA, Aldrich, St. Louis, MO), hydrazine hydrate (Alfa Aesar, Ward Hill, MA), hydrochloric acid (6 M) (EMD Millipore, Billerica, MA), 1-ethyl-3-(3-(dimethylamino)propyl) carbodiimide (EDC) (Alfa Aesar, Ward Hill, MA), Nhydroxysuccinimide (NHS) (Alfa Aesar, Ward Hill, MA) and

methanol (99.8%, EMD Millipore, Billerica, MA) were used as received. Sulfuric acid was from Fisher Scientific (Schwerte, Germany), potassium permanganate (KMnO4), N,N′-diisopropylcarbodiimide (DPCI), ethanolamine, 1-hydroxybenzotriazolehydrate (HOBth) were from Sigma-Aldrich (Munich, Germany); dimethylformamide (DMF) was from EMD Millipore. Water used in all synthesis and measurements was from a Milli-Q system. 2.2. Premodification. Membranes were cut and rinsed with DI water for 2 min and then with 50:50 (v/v) ethanol/water for 2 h on a shake bed. Membranes were then dried in a vacuum oven under 40 °C overnight. 2.3. Oxidative Hydrolysis for PET Membranes. To increase the reactive site density on the PET membranes, oxidative hydrolysis reactions were conducted to increase the carboxylic acid groups on membrane substrates following Scheme 1. The previously dried membrane sheets were placed in 150 mL of 0.75 N H2SO4 solution dissolved with 7.50 g of KMnO4. The vessel was then tightly sealed. The membranes were reacted for 2.5 h under gentle shaking. The membranes were then washed twice with purified water, four times with 6 M HCl for 2 min, four times with water for 2 min, and finally twice C


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represents the area of the membrane and is 4.9 cm2 for the NF270 and PET membranes used in this study. 2.7. Gabriel Synthesis. To convert the halide atom from alkyl halide at the polymer chain end to a primary amine group, the Gabriel synthesis procedure was followed as in our earlier studies.6,7,16,24 For NF270 membranes, 10 mL of saturated potassium phthalimide solution in ethanol was placed into a small glass vial containing one membrane disk. The vial was then sealed and placed on an incubator shaker at 40 °C for 6 h. After the reaction, the membrane was rinsed with ethanol, then with water/ethanol mixture solution twice for 2 min, and finally with ethanol before being dried. The subsequent step includes dissolving 7 mL of hydrazine hydrate into a total of 25 mL of 6 M HCl solution. Then 10 mL of this solution was placed into each small glass vial containing one membrane disk. The vials were placed on an incubator shaker at 40 °C for 6 h. Upon the completion of the reaction, the membrane samples were thoroughly washed with a water/ethanol mixture solution to ensure no phthalimide precipitate remained. Membranes were then dried under vacuum at 40 °C overnight. As PET membranes are more stable in DMF which is a better solvent for dissolving potassium phthalimide, membrane discs were added to a mixture of 3 g of potassium phthalimide salt in DMF and reacted in heat-shaker at 60 °C and 120 rpm for 6 h. After reaction, membranes were washed twice with DMF, four times with ethanol, and then four times with DI water. Membranes were then added into a solution of 7 mL of hydrazine hydrate in 25 mL of 6 M HCl in a heat-shaker at 60 °C and 120 rpm for 6 h. After reaction, membranes were washed twice with 6 M HCl, four times with DI water, and then dried for overnight in a 40 °C vacuum oven. 2.8. Nanoparticle Conjugation. Superparamagnetic nanoparticles were conjugated to the membrane surface to form amide bonds between the carboxylic acid groups from the nanoparticles and the primary amines of the grafted polymers. For the conjugation, a carbodiimide activated amide formation procedure was followed. A total of 31.2 mg of EDC and 38.7 mg of NHS was added to 10 mL of Milli-Q water. The mixture was then shaken vigorously on a vortex mixer. A total of 30 μL of carboxylic acid functionalized Fe3O4 nanoparticles in buffer solution (5 g/L) was added, but not agitated. A total of 1.5 mL of this solution was then added to a glass vial containing a membrane disk. This vial was sealed and incubated in the dark for 4 h. Thereafter the membrane was removed, rinsed twice with water, and then washed in water/ethanol mixture solution. The membrane was then dried in vacuum oven overnight at 40 °C. 2.9. Surface Characterization. Scanning electron microscopy (SEM) images of functionalized PET membranes were taken using a FEI/Philips Sirion Field Emission SEM (Hillsboro, OR, USA). Samples were coated with a 10 nm gold layer before SEM analysis. X-ray photoelectron spectroscopy (XPS) measurement of the membrane surface region was conducted using a Physical Electron 5800 ultrahigh vacuum XPS-Auger spectrometer (Chanhassen, MN, USA) at a 45° takeoff angle. Twenty highresolution scans at C (282−292 eV), N (395−407 eV), O (527− 541 eV), and Fe (705−730 eV) regions were averaged to monitor the changes during the various functionalization steps. All samples were measured sequentially under the same conditions including area analyzed and the incidence angle. Atomic force microscopy (AFM) (Bruker, Santa Barbara, CA) was used to image the nanoparticles on NF270 membranes after surface modification.

with ethanol for 2 min. The membrane sheets were then dried for 3 h at 40 °C. 2.4. Prefunctionalization of PET Membranes. Before ATRP initiators were anchored, the carboxylic acid groups on the membrane surface were converted to hydroxyl groups for further reaction as shown in Scheme 1. The previously oxidative treated membrane sheets were submerged in 150 mL of DMF solution containing 2.30 g of HOBth and 0.95 g of DPCI. The membranes were reacted for 30 min with gentle shaking. Afterward, the membranes were washed twice with DMF and then immediately placed in 150 mL of DMF solution containing 4.58 g of ethanolamine for 3 h under gentle shaking. The membrane sheets were washed twice with DMF for 2 min and twice with ethanol for 2 min. The membranes were then dried for overnight in a 40 °C vacuum oven. 2.5. Initiator Immobilization for NF270 and PET Membranes. Initiator immobilization for the PET and NF270 follows the same procedure even though the membranes have different reactive groups on the surfaces. The hydroxyl groups on the prefunctionalized PET membranes and secondary amine groups on NF270 membranes are the reactive groups for the initiator immobilization. The reaction scheme for NF270 is shown in Scheme 2. A reaction solution of 10 mL of freshly dried acetonitrile containing 4-(N′,N′-dimethylamino) pyridine (DMAP, 5 mM) and triethylamine (TEA, 10 mM) was prepared. Membrane samples were put in small vials and 10 mL of the abovementioned reaction solution was added to each vial. After 100 μL of BMPB was added to each sample, and the vials were sealed. After a predetermined period time of incubation on a shaker at room temperature, membranes were taken out and rinsed with acetonitrile and a water/ethanol mixture solution (1:1, v/v), then dried in a vacuum oven at 40 °C overnight. 2.6. Surface Initiated ATRP of N-Isopropylacrylamide (NIPAM). Initiator immobilized NF270 and PET membrane samples were put in Schlenck flasks equipped with rubber stoppers. The flasks were then sealed, evacuated, and backfilled with argon three times. NIPAM (0.25 M) and N,N,N′,N″,N″pentamethyldiethylenetriamine (PMDETA) were dissolved in the mixed 1:1 (v/v) methanol/water solvent and purged with N2 for 30 min. In the meantime, copper(I) chloride and copper(II) chloride were added to the solution with vigorous stirring in the presence of the argon stream. The ratios among various components in the ATRP reaction solution are [NIPAM]: [CuCl]:[CuCl2]:[PMDETA] = 50:1:0.2:1.25. Thereafter the reaction solution was cannulated into several flasks (10 mL for each sample) and incubated at room temperature for a predetermined time. The membranes were quickly removed from the Schlenck flask and immersed in 50 mL of quenching solution consisting of 1:1 (v/v) methanol/water solution with 250 mg copper(II) bromide and 625 μL PMDETA. The quenching solution was used to stop the polymerization and to ensure that the polymer chain ends were terminated with halides. A 1:1 (v/v) water/ethanol mixed solvent was then used to clean the membranes. After the membranes were dried in vacuum oven at 40 °C overnight, the degree of grafting, DG (μg/cm2), was calculated using the following equation: DG =

W1 − W0 Am

(4)

where W0 is the mass of the unmodified membrane and W1 is the mass of the membrane after modification and drying. Am D


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Scheme 3. Filtration Setup with the Solenoids To Generate the Oscillating Magnetic Field and the Dimensions of Various Components

2.10. Membrane Permeability. For NF270 membranes, each membrane was rinsed with Milli-Q water for 30 s per side and placed in an Amicon 8010 stirred filtration cell (EMD Millipore, Billerica, MA, USA). The cell was filled with a 1:1 (v/ v) water/ethanol mixture. N2 gas at 1.4 bar for 5 min was applied to promote the permeation of the fluid through the membrane. The membrane was then removed, rinsed with water, and precompacted at 4.8 bar for 5 min with Milli-Q water. Finally, the membrane was removed again from the cell and allowed to equilibrate in Milli-Q water for 2 h. After these prefiltration steps, the Milli-Q water flux was determined. For PET membranes, each membrane was equilibrated in water for 30 min and then placed in an Amicon cell. Instead of using a gas cylinder, the trans-membrane pressure was adjusted by the height of a water reservoir above the membrane. Permeability was then calculated from dividing the flux by the trans-membrane pressure. Membrane performance under an oscillating magnetic field was studied using a custom built system.6 Scheme 3 shows the filtration setup and the dimensions of the components involved. The membrane cell was placed between two symmetrically positioned stainless-steel core solenoids. The magnetic field strength generated by the solenoids and experienced by the conjugated nanoparticles depends strongly on the position of the membrane and the distance between the solenoids and the membrane. The membrane surface is aligned to the same height as the solenoids to maximize the magnetic field experienced by the functionalized nanoparticles on membrane substrate. Ideally, the solenoids should be placed as close to the filtration cell as possible in order to reach the maximum magnetic field. However, the solenoids will generate heat during the operation. To maximize the magnetic field, yet minimize the heat transfer from the solenoids to the filtration cell, the Amicon cell and the solenoids were separated by 2.8 cm as shown in Scheme 3. Moreover, two insulating foams were inserted to further prevent any heat transfer occurring. To ensure that heat generated from the solenoids does not complicate the filtration measurements, temperatures of filtration cell wall and the permeate were monitored during the filtration experiments in the presence and absence of the external oscillating magnetic field. The temperatures of the feed solutions as well as the temperatures close to the membrane surface were measured before and after the filtration experiments. A computer-operated programmable logic

controller (PLC, Click Koya, Automation Direct, Cumming, GA, USA) controls the rate at which the two solenoids connect to the circuit by alternatively activating the two solid-state relays. This determines the frequency of the oscillating magnetic field. The solenoids are powered by a tunable (max 20 V, 5 A) power supply from Agilent Technology (Santa Clara, CA, USA). In the current study, the two solenoids were positioned on the opposite sides of the filtration cell with the magnetic field direction parallel to the membrane. The solenoids were operated with about 8 V voltage and 2 A current. The measured magnetic field is between 30 and 50 G at various positions on the membrane surface. The nominal frequency of the oscillating magnetic field was chosen at 1 kHz, which is the maximum for the current setup. To estimate the power generated from the superparamagentic magnetite particles, both Néel and Brownian relaxation times need to be determined. The magnetite nanoparticles purchased have a 7.5 nm radius of magnetic core and 5 nm thick coating layer functionalized with carboxylic acid groups. Magnetite has an anisotropy constant K of 3 × 104 J/m3. At room temperature of 293 K (25 °C), the estimated Néel relaxation time τN = 4.9 × 10−4 s. On the other hand, the estimation of the Brownian relaxation time is somewhat complicated as the nanoparticles are tethered to the PNIPAM chain. On the basis of the nanoparticle diameter of 25 nm (magnetic core + coating layer) as well as the polymer chain length about 50 nm, assuming the water viscosity η = 0.001 Pa·s, the estimated Brownian relaxation time τB = 1.64 × 10−4 s. Brownian relaxation could be significantly longer than the estimated time as these conjugated nanoparticles are not entirely free to rotate. The overall relaxation time τ = 1.23 × 10−4 s. In the event that Brownian relaxation is significantly longer, the relaxation time will be dominated by the Néel relaxation and is approximately 5 × 10−4 s. The frequency we used is only about 2−10 times lower limited by our apparatus. The overall power generated will be dependent on the number of nanoparticles grafted on the membrane substrate as well as the magnetic susceptibility χ0 which is a function of the external magnetic field and the matrix surrounding the nanoparticles.25 It will be difficult to estimate the amount of power generated directly. However, earlier studies26−28 on heat generation from magnetite nanoparticles indicate that 15 nm diameter of particle size is close to the optimal size to generate heat in an aqueous solution. The nanoparticles we purchased have about 2.5 nm uncertainty in E


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origin. Earlier studies6,16 show that the initial polymerization is rather rapid followed by a period of relatively slower linear growth. This is probably due to the relative high concentration of the monomer at the beginning of polymerization. The high chain density samples exhibit a nonlinear DG increase with ATRP time during the 4 h period. The growth is almost linear during the first 2 h and then slows down gradually. During the initial stage of the ATRP reaction, the DG value of the high-chain-density sample increases more rapidly than that of the low-chain-density sample. If the polymerization sites on the high-density-membrane are twice as many as those on the low density membrane, the slope of the initial polymer growth for the high density membrane should double that of the low density membrane. Clearly this is not the case. The DG increases from over 50 μg/cm2 to over 120 μg/cm2 during the second hour of reaction for the high chain density membrane, whereas DG increases from over 30 μg/cm2 to about 50 μg/cm2 for the low chain density membrane. There are several possible reasons. The first reason is that the actual number of polymerization sites on high density membrane is not twice as many as that of the low density membrane due to the potential limitation of the available amine groups on the membrane surface and the effectiveness of the initiation reaction. The second possible reason is due to the relative low ratio of catalyst and monomer to the available polymerization sites on the high density membrane compared to that of the low density membrane leading to a relative slow growth. The third reason is that there is a high probability of radical chain termination resulting from the close proximity of the polymer chains for the high chain density membranes. This last reason together with the possibility of burying active chain ends for longer chains is the main factor for the nonlinear growth as polymerization continues. After PNIPAM grafting, the bromide atoms at the polymer chain ends were converted to the primary amine groups by Gabriel synthesis (see Scheme 2). The converted amine groups were used to conjugate with the carboxylic acid groups on superparamagnetic nanoparticles to form amide bonds in the presence of EDC and NHS. The modification of the membrane surface was successful as confirmed by X-ray photoelectron spectroscopy (XPS) as shown in Figures 2 and 3. Figure 2 shows that the impurity peaks related to the Na+ ions disappear after grafting PNIPAM polymer chains on the membrane surface. Further, it shows that Fe peaks appear after nanoparticle attachment. Figure 3 shows the high-resolution peaks near the Br 3d and Fe 2p positions. The Br 3d XPS peak was observed after initiator immobilization as shown in the left panel of Figure 3. The Br 3d peak from the sample with high initiator density is much higher than the corresponding low density sample indicating higher Br content on the membrane surface. The right panel on Figure 3 exhibits the Fe 2p XPS peak after nanoparticle conjugation for both high and low chain densities. Similar to the Br peak, the high chain density sample exhibits a higher Fe peak than the corresponding low chain density sample. The direct observation of the nanoparticles on membrane surfaces is achieved from scanning electron microscopy (SEM) images and atomic force microscopy (AFM) images as shown in Figures 4 and 5, respectively. As can be seen from Figures 4 and 5, the superparamagnetic nanoparticles are clearly visible with SEM and AFM. In particular, the nanoparticles appear to be evenly distributed on the entire membrane surface modified with high-density polymer chains. No significant aggregation was observed. However, on membranes surfaces modified with low-density polymer chains,

diameter. This will cause inefficiency in heat generation as the relaxation times are very sensitive to the size of the nanoparticles. However, temperatures measured could provide a qualitative estimate of the heat generated in our system. To ensure sufficient heat generated for the experiments, the oscillating magnetic field was applied to the filtration cell with membrane and Milli-Q water for 30 min before each flux measurement. 2.11. Latex Particle Filtration Experiments. PNIPAMgrafted PET membranes with varying surface modification conditions were each first placed in an Amicon 8010 cell. Then 10 mL of freshly prepared latex particle suspension was added. Filtration experiments were conducted at a pressure range between 15 and 20 psig depending on the chain density and the actual DG value of each specific membrane. Once all the liquid had gone through the membrane, the particles in the retentate were redispersed by filling the cell with about 10 mL of DI water. The feed solution was filtrated through the membrane at the same condition as before. This process was repeated three times to ensure that particles smaller than the membrane pores could go through the membrane and reach the filtrate. Finally, the latex particles in the retentate were redispersed into 10 mL of DI water and sonicated for 5 min. Particle size distribution was then measured by dynamic light scattering (DSL, Delsa Nano, Beckman). For experiments conducted under an external oscillating magnetic field, the filtration cell with the membrane and particle suspension loaded was placed under the field for 30 min prior to the filtration measurement.

3. RESULTS AND DISCUSSION 3.1. NF270 Membrane Modification and Characterization. The procedure for modifying NF270 membrane surface is shown in Scheme 2. ATRP initiator was immobilized directly on the membrane surface without any chemical pretreatment. The low chain density samples were obtained by mixing the structurally similar noninitiating propionyl bromide with the BMPB initiator (1:1, v/v). PNIPAM chains were then grown by surface initiated ATRP. The DG results are shown in Figure 1. For low chain density samples, DG value increases with ATRP time linearly during the 1−4 h ATRP time indicating excellent control over the grafting process. However, it should be pointed out that the slope of the DG over time does not go through the

Figure 1. Degree of grafting (DG) values of PNIPAM polymer chains grafted from NF270 membrane surface with high (■) and low (●) initiator densities. F


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Figure 2. XPS for the unmodified NF270 membrane substrate (bottom), PNIPAM polymer chain-grafted membrane surface (middle), as well as after nanoparticle (Fe3O4) conjugation (top).

Figure 3. High resolution XPS data exhibiting Br 3d region after initiator immobilization on NF270 surface (left) and Fe 2p region after subsequent nanoparticle conjugation (right).

Figure 4. SEM images of nanoparticle immobilized NF270 membranes at high (left) and low (right) initiator densities.

nanoparticles can only be observed sporadically as seen from the SEM image on the right panel of Figure 4. 3.2. NF270 Water Flux and the Effects of Localized Heating. To investigate the effect of heat generated from the superparamagnetic nanoparticles on the conformation of grafted PNIPAM chains on NF270 membrane surfaces, DI water fluxes

through the membranes are compared in the presence and absence of an oscillating magnetic field as shown in Table 1. For the lower density membrane samples, flux increases slightly for longer chains (higher DG value) under an external field, whereas it decreases for shorter chains (lower DG value). But the change is relatively small at about 10% or less. The small change is G


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Figure 5. AFM images of functionalized NF270 membranes with Fe3O4 superparamagnetic nanoparticles at high initiator density.

This effect is likely to be insignificant as a linear growth of the polymer chains is observed for lower density initiator immobilized membranes. Second, the external magnetic field applied could induce polymer chains to move causing micromixing as shown in our earlier work.6,7,16 This effect is more pronounced for longer chains than for shorter chains as shown before.16 The stretched polymer conformation due to the magnetic force exerted on the nanoparticle tethered to the polymer chain end improves water flux.16 However, for membranes with higher polymer chain density, significant flux decrease was observed under an external magnetic field. This is probably caused by the LCST transition of the thermoresponsive PNIPAM chains due to the heat generated from the nanoparticles under an oscillating magnetic field nanoparticles. The collapsed hydrophobic higher density PNIPAM chains cause additional resistance to water permeation leading to a reduction in the flux. To ensure that it is the heat generated from the conjugated nanoparticles that cause flux modulation rather than the heat produced and transferred from the operating solenoids in the presence of the external magnetic field, temperature and DI water flux measurements were conducted for the unmodified NF270 membrane and for a modified NF270 membrane. The initial temperature and the temperature of the feed after turning on the magnetic field for 30 min prefiltration were measured. The temperatures of the cell wall and the permeate were monitored during the 30 min filtration process in the presence of the magnetic field. The temperature of the feed just above the membrane surface was measured after the filtration. This is

Table 1. Fluxes of NF270 Membranes Modified with Different Densities of Polymers/Nanoparticles and Different Lengths of Polymer Chains with and W/O Applying an Oscillating Magnetic Field water flux (L/(m2h) @ 4.1 bar) DG (μg/cm2)

ATRP time (h)

chain density

no magnetic field

magnetic field

flux change (%)

32.1 108.2 53.1 187.8

1 4 1 4

low

16.5 ± 1.5 9.6 ± 2.2 14.9 ± 2.6 6.7 ± 2.1

14.8 ± 2.9 10.4 ± 1.7 10.1 ± 1.4 3.3 ± 0.6

−10.3 8.4 −30.2 −50.7

high

probably due to the low coverage of the PNIPAM chains and the corresponding low density of end-capped nanoparticles on the surface. For the shorter chains, heat generated from the nanoparticles induces localized temperature increase leading to the contraction of the polymer chains. The collapsing of the polymer chains causes the flux to decrease slightly due to the dehydration of the polymer chains. For longer chains at lower density, an increase in flux was observed and is probably caused by several opposing factors. First, the longer polymer chains could potentially reduce the number of nanoparticles immobilized on a membrane surface since the chain ends for the longer chains could be buried in the polymer matrix reducing the nanoparticles conjugated. The polymer chains are probably less dehydrated due to the relative lower amount of heat generated.

Table 2. Temperature and Flux Measurements for the Unmodified NF270 Membrane in the Presence and Absence of an External Oscillating Magnetic Field no field time (min) feed T/°C membrane surface T/°C permeate T/°C cell wall T/°C flux (L/(m2 h)

25.6 25.6 25.6 25.6 22.5 ± 0.2

field (30 min) 25.7 ± 0.2 25.8 ± 0.3 N/A 25.9 ± 0.2 N/A

filtration with field 10 N/A N/A 25.6 ± 0.2 26.1 ± 0.2 22.5 ± 0.2

20 N/A N/A 25.6 ± 0.1 26.3 ± 0.1 22.4 ± 0.1 H

filtration without field 30 26.1 ± 0.2 26.2 ± 0.2 25.7 ± 0.1 26.5 ± 0.1 22.5 ± 0.2

40 N/A N/A 25.6 ± 0.2 26.2 ± 0.2 22.6 ± 0.2

50 N/A N/A 25.6 ± 0.1 25.8 ± 0.3 22.4 ± 0.2

60 25.5 ± 0.1 25.6 ± 0.1 25.6 ± 0.1 25.6 ± 0.1 22.5 ± 0.1


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Table 3. Temperature and Flux Measurements for the Modified NF270 Membrane (Low Chain Density, DG = 80 μg/cm2) with and without an External Oscillating Magnetic Field time (min) feed T/°C membrane surface T/°C permeate T/°C cell wall T/°C flux (L/(m2 h))

no field

field (30 min)

0 25.6 25.6 25.6 25.6 14.5 ± 0.2

0 26.0 ± 0.2 27.8 ± 0.3 N/A 26.1 ± 0.3 N/A

filtration with field 10 N/A N/A 25.9 ± 0.2 26.3 ± 0.1 13.1 ± 0.2

20 N/A N/A 26.3 ± 0.1 26.5 ± 0.2 12.5 ± 0.1

followed by another 30 min of filtration in the absence of the external magnetic field. The fluxes are also measured during the filtration process. Tables 2 and 3 show the measurements for the unmodified NF270 and membrane modified with low chain density of PNIPAM respectively. The modification was conducted with 2 h of initiator immobilization and 4 h of ATRP resulting in a grafting degree of 80 μg/cm2. Table 2 shows that the temperatures of the feed before and after 30 min holding in the presence of magnetic field are more or less the same for the unmodified membrane. There is a slight increase in the feed temperature of about 0.5 °C after an additional 30 min DI water filtration. The feed temperature returns to the room temperature after 30 more min of DI water filtration without the field. The temperature of the filtration cell wall was monitored throughout the process and it can be seen that there is less than 1 °C increase in temperature after 1 h of the filtration experiments with the field. The permeate temperature remains the same as the room temperature for all the conditions as the permeate go through some connecting tubes before measurements could be made. The measured DI water fluxes are unchanged in the presence or absence of the magnetic field. It can be seen that there is very little heat transferred from the solenoids to the filtration cell. The effects of the magnetic field on the temperature and flux during the filtration process come from the localized heating generated from the conjugated nanoparticles. Further, this effect is reversible when the field is removed. Table 3 shows the temperature and flux measurement for the superparamagnetic nanoparticle conjugated NF270 membrane at low chain grafting density at the same operating conditions as the unmodified membrane. It can be seen that the feed temperature increases by about 0.5 °C after holding in the presence of the magnetic field for 30 min. It was further increased by another 1 °C (a total of 1.5 °C) after 30 min of filtration experiment. The feed temperature reduces by more than 1 °C after another 30 min of filtration without the field. On the other hand, the temperature near the membrane surface increases by 2 °C after 30 min of holding and by 4 °C after an additional 30 min of filtration experiments in the presence of the field. The temperature returns to room temperature with 30 more min of filtration without the field. The temperature of the permeate increases slightly by 1 °C after the 30 min of filtration with the field and goes back to room temperature without the field. The monitored cell wall temperature is similar to the unmodified membrane case. More importantly, a flux decline is observed in the presence of the magnetic field even though there is an increase in the temperature. This is consistent with the flux decline observed earlier. It should be pointed out that a temperature increase to 29.5 °C measured near the membrane surface after 30 min of prefiltration holding and 30 min of filtration with magnetic field is moderate. Actually it is still below the LCST of 32 °C for PNIPAM. However, there is a couple of minutes delay after the filtration to

filtration without field 30 27.1 ± 0.2 29.5 ± 0.3 27.1 ± 0.3 26.8 ± 0.3 12.1 ± 0.3

40 N/A N/A 26.5 ± 0.1 26.2 ± 0.1 12.9 ± 0.2

50 N/A N/A 25.9 ± 0.2 25.9 ± 0.2 13.5 ± 0.1

60 25.9 ± 0.1 26.2 ± 0.2 25.6 ± 0.1 25.6 ± 0.1 14.3 ± 0.2

measure the temperature. Also the thermocouple is positioned in the feed close to the membrane surface only. The actual temperature could be somewhat higher. Moreover, for grafted PNIPAM, the hydrophilic to hydrophobic transition temperature could also be slightly lower than the bulk transition temperature. For high chain density membranes with more nanoparticles conjugated, the temperature increase should be higher. The reduction in flux clearly indicates that PNIPAM chains have collapsed on the membrane surface in the presence of the oscillating magnetic field. 3.3. PET Membrane Modification and Characterization. The DG values from the PET membranes with different chain densities are shown in Figure 6. At the same ATRP times, higher

Figure 6. DG values of PNIPAM grafted from PET membranes with high (■), medium (●), and low (▲) initiator densities; data points 1,1′ and 2,2′ are samples with similar DG but different chain densities.

DG values were observed for the higher initiator density samples indicating higher polymer chain densities grafted onto the membrane surface. For all the three chain densities, DG values exhibit almost linear increase with ATRP time indicating that the grafting of PNIPAM was well controlled. Circled data points (1 and 1′, 2 and 2′) in Figure 6 are those samples with similar DG values but different chain densities and chain lengths. For each pair, the sample with lower polymer chain density reached the similar DG value to that higher polymer chain density sample at longer ATRP time, for example, 2 h for sample 1 and 4 h for sample 1′. Therefore, it is reasonable to conclude that the length of the PNIPAM chains for samples 1′ and 2′ is longer than the chains for corresponding sample 1 and 2. Both polymer chain density and chain length appear to affect membrane permeability and will be discussed in more detail during the next section. Figure 7a shows the XPS spectra for the unmodified and PNIPAM-grafted PET membranes. As can be seen, the unmodified PET membrane exhibits C and O peaks from the I


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Figure 8. DI water fluxes for PET membranes grafted with PNIPAM polymer chains and chain-end-capped magnetic nanoparticles with different densities; data points 1, 1′ and 2, 2′ are membranes with similar DG values but different chain densities (cf. Figure 6).

are likely to form highly uneven islands of grafted polymers on the membrane surface and inside the pores, whereas the shorter polymer chains with higher density would tend to form uniform layers of grafted polymers on the membrane surface and inside the pores. It is clear that the more uniform polymer coverage will lead to higher resistance to water flow and thus larger decrease in flux compared to the uneven island-like polymer coverage. To generate localized heating, an external oscillating magnetic field at 1 kHz was applied. Fluxes were measured before and after the field was applied. Table 4 shows the fluxes in the absence and presence of an external magnetic field for low, medium, and high PNIPAM grafting densities as well as for both longer and shorter polymer chains. As can be seen, in the presence of an external magnetic field, fluxes are higher than the corresponding values without the field for all three membranes at low, medium, and high polymer chain densities as well as at different chain lengths. This is very different from what was observed for NF270 membranes. For PET membranes, flux is dominated by the size of the membrane pores. During modification, PNIPAM chains are grafted onto the surface of the membranes but also inside the pores. For polymer chains grafted at low density, localized heating under an external magnetic field increases the temperature above the LCST of PNIPAM in water at 32 °C. This causes the polymers to collapse thus increasing the size of the pores. This increase in pore size and therefore water flux is slight for polymers grafted at low density. However, for polymer chains grafted at medium density, this heating effect is more significant as there are more nanoparticles attached. Moreover, the collapse of the more dense PNIPAM layers increases the pore size more substantially than the corresponding membranes grafted with low chain density polymers. As a result, significant increase in water flux was observed. For both PET membranes grafted with low and medium density polymer chains, the effects are more pronounced for the longer chains, which is expected. However, at high grafting density, PNIPAM chains are much more entangled with each other, and the further dehydration of the polymers will only lead to slight increase in pore size as shown in Table 4. This entanglement will be expected to be more severe for longer chains as evidenced by less flux increase for the membrane modified with longer ATRP time. The increase in flux for the modified PET membranes can also be caused by the temperature increase in the presence of the field. Actually it is difficult to

Figure 7. A broad range XPS spectra of unmodified and PNIPAMgrafted PET membranes at low and high chain densities (a), and Fe XPS spectra of PET membranes immobilized with nanoparticle at low, medium, and high densities (b).

polyester backbone only. After grafting PNIPAM, the N peak also appears. Further, the peak intensity increases as the chain density increases. The core-level Fe 2p XPS spectra shown in Figure 7b demonstrate the presence of the Fe peak indicating the success of immobilizing nanoparticles on the membrane surface. Moreover, it clearly shows that the Fe peak intensity increases with the density of nanoparticles immobilized. 3.4. PET Water Flux and the Effects of Nanoparticle Heating. Since the PET membranes have cylindrical pore structures, the flux through these membranes can be correlated with the effective pore diameter.29 For all three membranes with different chain densities, flux decreases with ATRP time (i.e., chain length) in the absence of an external magnetic field (see Figure 8). When the density of polymer chains grafted on the membrane surface is low, the decrease of flux is insignificant as the polymer chain length increases as shown in Figure 8. On the other hand, comparing fluxes for the samples with similar DG values (1 with 1′ and 2 with 2′) in Figures 6 and 8, respectively, it is clear that flux decreases more for higher density chains. For example, membrane 1 and 1′ have almost the same DG value (around 90 μg/cm2); however, a much lower flux was measured for membrane 1 which has a higher polymer chain density. Likewise, membrane 2 modified with higher density polymer chains has a lower flux than membrane 2′ modified with lower density polymer chains even though the DG values of both membranes are nearly the same. Furthermore, the conformations and properties of the polymer chains on the membrane surface are quite different. The longer polymer chains with lower density J


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Table 4. Fluxes of Modified PET Membranes with Low, Medium, and High DG Values and the Corresponding Polymer Chain/ Nanoparticle Densities with and without an External Oscillating Magnetic Field water flux (L/(m2 h bar)) 2

DG (μg/cm )

ATRP time (h)

38.8 91.8 85.7 110.2 104.1 193.9

1 4 2 3 2 4

chain density low medium High

no magnetic field

magnetic field

flux change (%)

42800 ± 1900 37100 ± 1100 27600 ± 900 19500 ± 2600 14500 ± 400 7300 ± 200

43700 ± 1900 39000 ± 1700 34800 ± 600 28500 ± 800 16400 ± 900 8000 ± 500

2.1 5.2 26.3 46.1 12.9 9.9

particle size distribution appears to be similar to those at higher chain densities as shown in Figure 9d. However, the particle size distribution measured by DLS looks significantly different when the filtration was conducted in the presence of an external field. Only a small percentage of particles with sizes between 200 and 500 nm were retained with the overall size distribution shifts toward larger particles. This is probably due to the further increase of the pore size after the PNIPAM chain collapse. Modified with low density polymer chains and the 25 nm magnetic nanoparticles attached at the chain ends, the pores for the modified PET membranes appear to be uneven, thereby a small percentage of small latex particles are retained. Further due to the passage of the majority of small particles below 400 nm, the relative size distribution alters shifting toward larger sizes as shown in Figure 9d′. For the three modification conditions with different chain densities, it is clear that applying an external field increases the overall membrane pore diameter due to the collapse of the polymer chains after going through its LCST transition. During the filtration experiments, no apparent heating of the water was observed. Therefore, the heat is generated from the localized nanoparticle relaxation in the presence of an oscillating magnetic field. The amount of the heat generated appears to be sufficient to raise the local temperature above the LCST temperature of PNIPAM leading to the collapse of the polymer chains and consequently the permeabilities of both NF270 and PET membranes. The temperature induced conformational transition also leads to changes in the pore sizes for the PET membranes.

separate the effects from pore size change and temperature increase. However, temperature change is likely not significant as the high density membranes which should have the strongest heating effect have a lower flux increase compared to the medium density membranes. A flux decline was observed for the modified NF270 membranes with the field. 3.5. Latex Particle Filtration. To confirm the pore size change for PET membranes upon applying an external oscillating magnetic field, latex particle fractionation experiments were performed with and without the field. The latex particles were synthesized in-house by the method described earlier.30 The initial particle size distribution was measured by dynamic light scattering (DLS) and is shown in Figure 9a. The particles have a broad size distribution ranging from about 110 to 1000 nm. After filtrating the latex particles with PET membranes grafted with high-density PNIPAM polymer chains, the majority of the particles were retained. The particle size distribution in the retentate from DLS measurement shows that there is only a slight increase in the particle cutoff size from 110 to 130 nm as shown in Figure 9b. Particles with sizes below 130 nm are able to pass through the modified membrane. This indicates that modification of the PET membrane with high chain density polymers reduces its pore sizes significantly. The nominal pore size for the PET membrane studied is 400 nm. Moreover, particle size distribution after filtration exhibits quite different characteristics in the presence of an oscillating magnetic field as shown in Figure 9b′. The particle size cutoff increases to about 143 nm. This slight increase in pore size is likely caused by the shrinkage of the PNIPAM polymer chains upon localized heating induced by the nanoparticles in the presence of an external magnetic field. However, for PET membranes modified with PNIPAM at medium chain density, pore size increase upon the application of an external magnetic field is rather significant. Figure 9 panels c and c′ exhibit the particle size distributions in the retentate with and without an external field, respectively. The particle size cutoff for membranes modified with medium chain density polymers increases to 179 and 225 nm in the absence and presence of an external field, respectively. The increase in pore size diameter due to the application of an external field appears to be more significant for membranes modified with medium chain density polymers, a net increase of 46 nm compared to 13 nm only for membranes modified with high chain density polymers. This is reflected in the flux results. An improvement of 26% and 46% in flux in the presence of external magnetic field was observed for the shorter and longer chains, respectively, for membranes modified with medium density polymer chains. For membranes modified with low density polymer chains, the particle size distribution appears to be dramatically different in the absence and presence of an external magnetic field. Without the field, the particle size cutoff is around 292 nm, closer to the nominal pore size of 400 nm for the base PET membranes. The shape of

4.0. CONCLUSIONS In summary, thermoresponsive PNIPAM polymer chains were grafted on NF270 and PET membranes with superparamagnetic nanoparticles conjugated at the chain ends. Chain density and chain length were independently controlled by surface initiated ATRP. The success for the membrane modification is confirmed by spectroscopic and imaging techniques including XPS, AFM, and SEM. The localized heating effect due to the relaxation of the end-capped superparamagnetic nanoparticles in the presence of an oscillating magnetic field was investigated by comparing membrane water fluxes with and without the magnetic field. In the presence of an oscillating magnetic field, localized heating induces the LCST transition of the thermoresponsive PNIPAM leading to the hydrophobic collapse of the polymer chains thus altering the water flux. The localized heating effects on NF270 and PET membranes are different. For NF270 membranes, the polymer chains were grafted on the membrane surface and flux was observed to decline in the presence of an oscillating magnetic field due to the increase in the hydrophobicity of the polymer layer. On the other hand, PNIPAM grows both on the surface and inside the pores of the PET membranes, and an increase of permeability was found after PNIPAM LCST transition induced K


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Figure 9. Size distribution of the polystyrene latex particles in the feed (a) and in the retentate after passing the particles through PNIPAM/nanoparticle modified PET membranes (b and b′, high grafting density 193.9 μg/cm2; c and c′, medium grafting density 110.2 μg/cm2; d and d′, low grafting density 38.8 μg/cm2); panels b−d represent the filtration experiments without the external magnetic field, panels b′−d′ represent the experiments under an oscillating magnetic field.

by the localized heating due to nanoparticle relaxation. The increase in flux was found highest for membranes modified with medium chain density polymers. At high polymer chain density,

polymer conformational change is limited after the LCST transition leading to only a small increase in flux. Latex particle filtration experiments confirm these findings. L


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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 479-575-8401. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Partial financial support from BARD (IS-4768-14R) is gratefully acknowledged.

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REFERENCES

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Localized Heat Generation from Magnetically Responsive Membranes

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