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Self-Assembly Directed Aerogel and Membrane Formation from a Magnetic Composite: An Approach to Develop Multifunctional Materials Balachandran Vivek, and edamana prasad ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15765 • Publication Date (Web): 06 Feb 2017 Downloaded from http://pubs.acs.org on February 11, 2017

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ACS Applied Materials & Interfaces

Self-Assembly Directed Aerogel and Membrane Formation from a Magnetic Composite: An Approach to Develop Multifunctional Materials Balachandran Vivek and Edamana Prasad* Department of Chemistry, Indian Institute of Technology Madras (IIT M), Chennai 600 036 Abstract Herein, we report the preparation of an aerogel and a membrane from a magnetic composite material by tuning the self-assembly at the molecular level. The gel exhibits excellent oil absorption property and the membrane shows remarkable autonomous self-healing property. The composite is formed from organosilicon modified poly(amido amine) {PAMAM} dendrimer, which are linked with iron oxide nanoparticles and poly vinyl alcohol (PVA). Upon addition of a cross-linker (formaldehyde), the system undergoes a fast self-assembly and gelation process. The aerogel, obtained after drying the hydrogel, was modified with 1bromohexadecane at room temperature and utilized for removing oil from water with 22.9 g/g absorption capacity. Intriguingly, the same system forms a membrane with 97 % autonomous self-healing ability, in the absence of the cross linker. The membrane was used to remove salt content from water with an efficiency of 85 %. The control experiments suggest that the presence of the magnetic material (iron oxide) play a key role in the formation of both aerogel and membrane.

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Keywords: Aerogel, membrane, water desalination, Oil removal, self-healing, Poly(amido amine) organosilicon dendrimer. Introduction Smart materials1-6 are capable of exhibiting properties such as self-healing,7-10 shape memory,11,12 and stimuli responsiveness.13-18 In the recent past, smart materials containing magnetic compounds19-21 have been catching special recognition due to their role in developing materials with tunable magnetic properties, and relatively high surface to volume ratio. Smart magnetic nanomaterials have also applications in areas such as data storage, device fabrication, biomedical and materials research.22-24 Very recently, self-assembly and gelation based on smart magnetic nano-systems are reported and they have important applications in medicinal chemistry, biomedical imaging and chemical sensing.22,25-28 For example, magnetic ferrofluid based ferrogels have been used for developing artificial muscles, drug delivery and hyperthermia treatment.29,30 Another recent report describes the use of magnetic gel based smart materials towards developing self-healing materials.31 Among magnetic gel systems, magnetic aerogels are a special category which has not been explored widely. Aerogels32-36 are gels with highly porous structure which are capable of maintaining their shape even in the absence of solvents. Aerogels have excellent absorption properties and they have been used in the field of gas adsorption and oil removal.37,38 While aerogels with physisorbed magnetic nanoparticles are known,39-41 aerogels with chemically attached magnetic nanoparticles are very rarely reported in the literature.42,43 The present study describes the development of a novel smart magnetic aerogel, where the magnetic nanoparticles are covalently connected with the organic moiety in a polymer matrix in presence of an additive (cross linker). Interestingly, the same material also forms a membrane with autonomous self-healing ability in the absence of the additive. The composite


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used to prepare the aerogel contains magnetic nanoparticles (iron oxides), G-4 dendrimer [poly (amido amine) {PAMAM}] functionalized with organosilicon, poly vinyl alcohol and formaldehyde. We hypothesize that the aerogels can act as excellent absorbents due to the mesoporous structure of the aerogel. Further, the magnetic composite gel makes potential candidate for absorbing oils from water and can be used to clean up large area oil-spills. In a similar line, the membrane from the material can be utilized for water purification by desalination due to the presence of magnetic nanoparticles and the ability of the membrane to self-heal is expected to provide increased lifetime to the material. The current approach initiates the use of tuning the self-assembly of the components in a composite to develop multifunctional materials. Experimental Section Preparation

of

PAMAMOS:

Poly(amido

amine)-(3-acryloyloxy)

trimethoxysilane

(PAMAMOS) was synthesized by the following procedure.44 PAMAM (generation four)(0.06g, 0.004 mol) was dissolved in 3 mL dry methanol and taken in a two necked round bottom flask with a condenser, under nitrogen atmosphere. (3-Acryloyloxy) trimethoxysilane (1.05g, 0.004 mol) was added in to the flask and the mixture was stirred at room temperature under nitrogen atmosphere for 3 days. The reaction mixture was transferred to a dialysis bag (LA395) and the bag was kept in methanol-water mixture (1:1) under stirring conditions. The product formation was identified by 1H NMR data. 1H NMR spectra (DMSO):δ 1.02 (m;CH2-Si); 1.75 (m; -COO-CH2-CH2-CH2-);

2-4 (PAMAM protons); 3.40 (s; Si-OCH3);

13

C

NMR (Figure S1) (DMSO; δ 10.1 (-CH2-Si-); 21.2,32.4,33.6,38.1,42.8,43.5 (PAMAM peaks); 50.1 (-Si-OCH3). The PAMAMOS formation was further confirmed by MALDI, (Figure S2), where the single peak at M/Z= 45,407 corresponds to [M+Na]+ ion. The other two peaks correspond to PAMAM moieties with partially modified amine groups. For



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example, the M/Z ratio of 22,000 and 30,000, corresponds to the 71% and 58% of peripheral amine modified PAMAM. Preparation of Fe2O3-PAMAMOS: Modification of Fe2O3 with PAMAMOS was carried out by refluxing Fe2O3 (0.005g, 3.1×10-5 mol) and PAMAMOS (2.46g, 9.3×10-5 mol) in toluene/methanol mixture (4:1) for 3 days. The mixture was filtered and washed with methanol and dried under vacuum. The composite formation was confirmed by FT-IR spectra, SEM, TEM , powder XRD and thermal analysis (vide infra). Preparation of aerogel and membrane: Fe2O3-PAMAMOS (0.2g) was added in to an aqueous solution (1.5 mL) of poly(vinyl alcohol) (0.2g) and 1 mL of HCl was added to the system. The mixture was stirred at room temperature for 1day and then was transferred to a glass vial. To this mixture, conc. H2SO4 (400µl) and HCHO (37-41% w/v, 400µl) were added drop wise and it was kept for10 minutes to form the gel. Keeping the system for five minutes, the solvent was found to be separated out from the gel, as an upper layer on the top of the gel. After removing the solvent, the gel was washed with doubly distilled water and dried at room temperature. In order to prepare the membrane, the thick solution obtained before addition of formaldehyde was transferred in to a petri dish, spread uniformly and dried. The membrane was peeled off and washed with distilled water and dried at room temperature. The thickness of the membrane was 200µm. Hydrophobic modification of aerogel: The hydrophobic modification has been carried out according to a reported procedure.43 The aerogel (50 mg) was immersed in to 10 ml of 1bromohexadecane and acetonitrile mixture for 48 hr and dried under vacuum at 60 oC for 3 days. The hydrophobic modification of the aerogel was confirmed by FT-IR spectra (vide infra).


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Characterization: FT-IR spectra of the compounds were recorded by Jasco 4100. Thermal analysis of the sample was taken by TGA7 Perkin Elmer TGA7, Q500 Hi-Res TGA. Powder XRD of the gel, membrane and nanoparticles was recorded by Bruker D8 Advance X-ray diffractometer using Cu-Kα radiation (λ = 1.54 Å). Morphological analysis of the samples was carried out by FEI Quanta FEG 400 High Resolution Scanning Electron Microscope and JEOL 3010 transmission electron microscope with a UHR pole piece operates at an accelerating voltage 300 kV. Magnetic properties of the samples were obtained by Quantum Design SQUID magnetometer. Rheological measurements were carried out using an Anton Paar rheometer MCR 502. H1 NMR spectra of the PAMAMOS were recorded by Bruker 500 MHz NMR Spectrometer. Gas adsorption experiments were carried out by Chemisorb 2750 from Micromeritics and ASAP2020 from Micromeritics. Mass spectra were recorded by Voyager-DE PRO MALDI/TOF mass spectrometer with 2,5-dihydroxy benzoic acid as the matrix. The salt content in water was monitored by Perkin Elmer Optima 5300 DV ICP-OES analysis. Results and Discussions Initially, PAMAM dendrimer (fourth generation) was modified by (3-acryloyloxy) trimethoxysilane in methanol under nitrogen atmosphere.44 The as-synthesized poly (amido amine organosilicon) {PAMAMOS} was very sensitive to air and was kept under nitrogen atmosphere. The structure of the peripherally modified PAMAM was confirmed by 1H NMR {the peak at δ 1.75 (m; -COO-CH2-CH2-CH2-)} and mass spectrometry (M/Z= 45,407) (Figures S1 and S2). The NMR and MALDI results indicate that 80% peripheral amine groups of PAMAM dendrimer was modified during the reaction. The commercially available Fe2O3 nanoparticles were modified with PAMAMOS by stirring the nanoparticles in toluenemethanol mixture (1:4 ratio) under nitrogen atmosphere. The PAMAMOS modified Fe2O3 was characterized by FT-IR, powder XRD, TGA, SEM, and TEM. Figure S3 (supporting 5 ACS Paragon Plus Environment


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information) shows the FT-IR spectrum of Fe2O3 nanoparticle and Fe2O3-PAMAMOS. In Figure S3, the broad band at 3400 cm-1 corresponds to -N-H

stretching vibrations of

PAMAMOS in Fe2O3-PAMAMOS. The bands at 1606 and 1321cm-1 correspond to the characteristic vibrational peaks of carbonyl groups in PAMAMOS. The stretching vibration of Fe-O bond, which appears at 1048 cm-1, is shifted to 1129 cm-1 upon addition of PAMAMOS due to the Fe-O-Si bond formation, which confirms the modification Fe2O3 with PAMAMOS. Fe2O3-PAMAMOS was stirred with poly (vinyl alcohol) (PVA) at room temperature in presence of HCl. The hydrolysis reaction between Si-OCH3 and hydroxyl group of PVA is known, which links PAMAMOS to PVA. To the above mixture containing Fe2O3PAMAMOS linked PVA, 400µL of conc. H2SO4 was added drop wise, followed by 400µl of HCHO. The mixture was kept for ten minutes. It was observed that the solvent was slowly expelled from the self-assembly and formed a layer above the solid material. Removal of solvent from the vial resulted in the gel as shown in Figure 1 a. The minimum amount of formaldehyde required for gelation was found to be 400µL. In order to prepare the aerogel, the gel was dried over 24 h at room temperature to completely remove the solvent. Figures 1 b shows photograph of the aerogel placed on a leaf, indicating its light weight nature. The elasticity and the shape memory of the gel were examined by providing a mechanical stress over the gel. It was observed that the gel gets the shape back immediately after removing the mechanical stress [Figure 1 (d-f)], indicating that the aerogel has excellent shape memory property. Photograph of the dried aerogel and the photograph confirming the light weight nature of the aerogel are given in the supporting information (Figure S4 and S5). It is interesting to note that gelation was not resulted until formaldehyde was added. Addition of formaldehyde might impart extra cross linking to the system, through the cross-linking the –OH group in PVA molecular chain with Si-OCH3 in the PAMAMOS to produce –OHCHO6 ACS Paragon Plus Environment

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linkage. FT-IR experiment results (vide infra) also support the hypothesis. As a result, the self-assembled molecular networks might expand and consequently, holding the solvents in the gel matrix becomes difficult. This leads to the expulsion of the solvent medium and subsequent formation of the aerogel. The N2 adsorption isotherm (Figure 1g) exhibits an IUPAC type curve, which indicates the presence of a mesoporous surface for the aerogel. The BET (Brunauer Emmet Teller) surface area was found to be 8 m2/g and the maximum volume of gas that can be adsorbed in to the aerogel is found to be 12.8 cm3/g. The BET analysis showed that the obtained aerogel has surface area of 28 m2/g with a pore diameter of 188 nm.

Figure 1: (a) Shows the photograph of aerogel in inverted vial and (inset) free standing aerogel; (b) Photograph shows typical light weight property of aerogel; (c) Photograph of membrane; (d-f) Photographs showing shape memory effect of two pieces of aerogel with 10 mg of weight; and (g) Inert gas adsorption desorption isotherms of aerogel. Interestingly, the system, which appears as a viscous solution in the absence of formaldehyde, turned to a robust membrane upon transferring it to a petri dish and keeping for overnight. The membrane can be peeled off from the petri dish and the thickness of the membrane was found to be 200µm. Figure 1c shows the photograph of the membrane.



Figure 2a represents the FT-IR spectra of the aerogel and the membrane. The bands at 1067 cm-1 correspond to the stretching vibration of Si-O-C in the membrane which confirms the presence of linkage between PAMAMOS periphery and PVA. The bands at 1110 cm-1and 1189 cm-1, in aerogel, represent the stretching vibration of Si-O-C and C-O-C, which corresponds to the formation of PVA linking and formaldehyde crosslinking. Figure 2b represents the powder XRD patterns of membrane and aerogel. It can be seen that the characteristic peaks at 2ɵ =30.5o, 35.7o, 43.6o, 57.7o and 62.9o correspond to diffractions coming from (111), (200), (311), (222), (400), (422) and (511) planes of Fe2O3. The XRD of Fe2O3 has shown in the supporting information (Figure S6). The presence of PVA is reflected in the powder XRD of the membrane as a broad peak at 2ɵ=20.3o. However, the same is absent in the case of the aerogel, presumably due to the formaldehyde crosslinking which provides high crystallinity for the aerogel.

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c

60

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40 20 0 0

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Figure 2: (a) FT-IR spectra of magnetic membrane and magnetic aerogel; (b) Powder XRD pattern of magnetic aerogel and membrane; and (c) TGA analysis of magnetic membrane and aerogel.


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Thermal properties of the gel and membrane were monitored by thermo gravimetric analysis (TGA). The thermal stability parameters of the membrane and aerogel were shown in Figure 2c. Figure S7 and S8 show the TGA analysis of Fe2O3 nanoparticles and Fe2O3- PAMAM. In the case of Fe2O3- PAMAM, there is a 30% weight loss at 250 oC and 380 9oC, which is due to the degradation of PAMAM and 14% weight loss at 473 oC, which is due to the extra water content in the Fe2O3- PAMAM. Figure 2c (red) shows the thermal decomposition of membrane, where the weight loss corresponds to 17% at 238 oC is due to the decomposition of PVA polymer. In this case, the degradation of PAMAM was observed at higher temperature (400 and 600 oC) compared to that required for PAMAMOS, which indicates that the presence crosslinking between PAMAMOS and PVA. Figure 2c (blue) shows the thermal decomposition of the aerogel and as it is evident from the Figure, the thermal stability of aerogel is higher than that of the membrane, presumably due to extra cross linking. The morphology of the magnetic nanoparticles, Fe2O3-PAMAMOS, aerogel and the membrane was analysed using scanning electron microscope (SEM). Figure 3a shows the sphere shaped magnetic nanoparticles (Fe2O3) with size in the range of 30-40 nm. These magnetic spheres were agglomerated upon binding with PAMAMOS due to the increased interactions between the nanoparticles as a result of the surface modification (Figure 3b). Figure 3c shows the morphology of the magnetic aerogel, where the interconnected network is highly porous in nature with a pore size of 300-400 nm. As inferred from the SEM image, the nanoparticles were uniformly embedded in to the polymer networks. The SEM image of the membrane is given in Figure 3d. The comparison between Figure 3c and Figure 3d suggests that there is a huge reduction of pore size during the membrane formation, resulting in a wrinkled surface.



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Figure 3: Scanning electron microscopic image of (a) Fe2O3 nanoparticles; (b) Fe2O3PAMAMOS nanoparticles;(c) aerogel and (d) membrane In order to get better insight in to the morphological properties of the gel and membrane, TEM analysis was carried out and the results are consistent with the SEM analysis. The TEM images of Fe2O3 nanoparticle are given in Figure S9a, where 20-40 nm sized nanoparticles are visible. Modification of the nanoparticles with PAMAM derivative leads to more agglomeration, which is evident from the TEM image given in Figure S9b. Figures S9c and S9d represent the HRTEM images of aerogel and membrane. Fine structures of entangled fibre with diameter of 80-100 nm can be seen in Figure S9c. TEM image of membrane (Figure S9d) revealed that there is a homogeneous distribution of Fe2O3-PAMAMOS in the PVA matrix. A control experiment was carried out where PVA and formaldehyde are allowed to react in presence of HCl and H2SO4 without Fe2O3-PAMAMOS (Figure S10a).The resulting mixture shows no gelation properties, indicating that Fe2O3-PAMAMOS nanoparticles are acting as cross-linker for the formation of aerogel and the membrane. Another reaction was carried out where PVA and formaldehyde are reacted with PAMAMOS alone without Fe2O3 (Figure S10b).The resulting mixture also shows no gelation properties.


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The results taken together indicate that aerogel was formed by extra cross-linking of formaldehyde. The FTIR study suggests that formaldehyde is connected to PVA through CO-C linkage. In addition, there is a possibility of the formation of formaldehyde linkage between Fe2O3-PAMAMOS in the networks. On the other hand, the membrane was formed by covalent bond formation between Si-OCH3 groups of Fe2O3-PAMAMOS and hydroxyl group of PVA, resulting in a Si-O-C linkage. The Fe2O3-PAMAMOS particles act as a crosslinker between PVA long chains. Schematic representation of the formation of the aerogel and membrane is shown in Scheme 1.

Scheme 1: Schematic representation of aerogel and membrane formation. Mechanical Behaviour Next, we have carried out the rheological behaviour of both the samples. In strain sweep experiment, G’ value is higher than G’’ value in both aerogels and membrane, which is 11 ACS Paragon Plus Environment


indicative of successful gelation. The G’ value of the aerogel was found to be 14681 Pa higher than that of the membrane (Figure 4a). This corroborates the hypothesis that extra crosslinking through HCHO imparts high strength to the aerogel system. In frequency sweep experiment (Figure 5b), G’ and G’’ values for aerogel and membrane are invariant which represent their characteristic elastic nature. For further investigation of mechanical properties of aerogel and membrane, a stress versus strain experiment was carried out. Figures 4c and 4d show the stress –strain plots of magnetic aerogel and membrane, respectively. The measured stress is increased with strain in the case of aerogel and membrane, which shows the elastic nature of the gel and membrane. In Figure 4c, the yield point is seen at 82% strain with a yield stress of 4.8 KPa. There is no yield point observed in the aerogel, up to a fixed strain value (90%)( Figure 4d). This is due to the high mechanical strength of aerogel as result of formaldehyde cross-linking. 100000

b

a 100000

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G' of aerogel G' of membrane G'' of aerogel G'' of membrane

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Figure 4: (a and b) The amplitude sweep curve and frequency sweep curve of magnetic aerogel and membrane; (c and d) Stress versus strain curve of magnetic aerogel and membrane.


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Magnetic properties The magnetic properties of the membrane and aerogel were measured through magnetic field profile (magnetic moment versus magnetic field) and temperature profile studies (magnetic moment versus temperature). In magnetic field profile curve (Figure 5a, 5b, 5d and 5e), membrane and aerogel shows high magnetic response to the external magnet. The magnetic properties of the samples were characterized using SQUID magnetometer at two different temperatures (5K &300K). The zero field cooled (ZFC) and field cooled (FC) magnetization curves of all the materials were also carried out by SQUID magnetometer. Nanoparticles (Fe2O3) and Fe2O3-PAMAMOS (modified nanoparticles) show distinct transition at 50K and 215K in temperature profile diagram and show hysteresis curve in magnetic field profile diagram (Figures S11, S12, S13, S14, S15 and S16). The slight ferrimagnetic behaviour shown by the aerogel and membrane at 5K is further diminished at 300K (Figure 5a-e). From figure 5b and 5e, it is clear that both membrane and aerogel shows superparamagnetic behaviour at 300 K. The temperature profile curves (Figure 5c and 5f) of membrane and aerogel show transition temperature at 25K which is attributed to the super paramagnetic behaviour with coercive force of 20.2 Oe (membrane) and 35.7Oe (aerogel). Generally, magnetic moment decreases with increasing the temperature which is a common observation in the case of Fe2O3. Surprisingly, the magnetic moments of membrane and aerogel increases as the temperature increases, in both ZFC and FC conditions. This might be due to the fact that when the temperature increases, the particles lose the restricted arrangement which causes increased surface spin and higher dipolar interaction with neighbouring particles in the gel medium, in presence of permanent magnetic field. The higher dipolar interaction between the nanoparticles in the soft network of a gel might result in a controlled aggregation and increased magnetic moment.45



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Figure 5: Magnetic moment versus magnetic field curve of membrane at 5K and 300K (a and b) with photograph showing the magnetic nature of the membrane; (c) Temperature profile curve (Magnetic moment versus temperature at constant magnetic field 500 Oe of membrane at field cool condition (top curve) and at zero field cooled condition (bottom curve) ; (d and e) Magnetic moment versus magnetic field curve of aerogel at 5K and 300K with photograph; (f) Temperature profile curve (Magnetic moment versus temperature at constant magnetic field 500 Oe of aerogel at field cool condition (top curve) and at zero field cooled condition (bottom curve). Removal of oil from water The obtained aerogel were hydrophobically modified by stirring the aerogel (50 mg) in 1bromohexadecane (10 ml) for 48 hr. The functionalization is due to the feasible reaction between the free hydroxyl group of poly (vinyl alcohol) and bromide in 1- bromohexadecane at room temperature.

43

The alkylation of aerogel will increase the oleophilicity as well as

hydrophobicity of the aerogel which drives the oil absorption ability. The water contact angle of aerogel is found to be 91.34 o which indicates hydrophobic nature of the aerogel (Figure S17). Figure 6 (a-f) represent the capacity of the aerogel to absorb engine oil. The photographs are taken in presence of UV light. As expected, the interconnected 3D pores on 14 ACS Paragon Plus Environment

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the surface of the composite material have excellent oil absorption capacity. The oil was kept in petri dish with water and the aerogel was moved over the oil water mixture using an external magnet. The oil was completely absorbed in to the aerogel. Figure 6 (a-f) shows the photograph of oil absorbed aerogel. Figure 6g represents the rate of oil absorption with time. The maximum absorption capacity (22.9g/g) was reached within 10 min. The oil can be recovered by simply squeezing the materials and the reusability of the material was checked. The results indicate that the material provides identical output, up to eight cycles (Figure 6h).

Figure 6: The photograph showing the oil removal; (a) oil in water; (b and c) absorbing oil from water using aerogel driven by external magnet; (d and e) oil has completely absorbed by aerogel; (f) oil absorbed aerogel. Quantitative analysis of the rate absorption capacity (g); (h) Recycling ability of aerogel, absorbed oil can be removed by squeezing the aerogel. The average difference in the absorption capacity for eight cycles was about 1.67g/g (Figure 6h). The reason for oil absorption was mainly due to the easy functionalization of aerogel by 1- bromohexadecane and superhydrophobicity induced by the 1- bromohexadecane. The high 15 ACS Paragon Plus Environment


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surface to volume ratio of the nano composite helps for absorption of oil in large quantity. An added advantage in the case is that the system can be driven by an external magnet so that stirring, filtration etc. can be avoided. Permeability Measurements Membrane permeability at different pH was measured by changing the feeding water pH from 1 to 11. The detailed procedure for permeability measurements is explained in the supporting information (S2-S3). Figure 7a represents a plot of permeability of the membrane versus pH. It is clear from Figure 7a that membrane permeability increases as the pH of the medium becomes basic. This may be due to the increase in the pore size at higher pH values. At basic pH, all the hydroxyl groups from PVA are converted in to oxonium ions (RO-) and the electrostatic repulsion between these ions could result in increased pore size. Figure 7b shows the dependence of permeate flux of membrane with transmembrane pressure. As expected, the water flux was increasing with increasing the transmembrane pressure, due to pressure induced enlarged pore size. The swelling studies of the membrane were carried out using a reported procedure and the swelling degree was measured with respect to the amount of Fe2O3-PAMAMOS in the membrane (see supporting information S2). The swelling degree was slightly decreased from 1.27(g/g) to 1(g/g) as the Fe2O3-PAMAMOS weight is increased from 0.2g to 0.6g (Figure S18). The higher amount of Fe2O3 leads to more crosslinking to the system which will decrease the swelling ability. Next, the stability of the membrane was examined in different solvents, and the results indicate that the membrane was stable in a series of polar and non-polar solvents. Table S1 in the supporting information summarizes the stability study of the membrane in different solvents.


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Figure 7: (a) Permeate flux of magnetic membrane at different feeding water pH (the data points are the average of three runs); (b) permeate flux of magnetic membrane with different transmembrane pressure and the photograph of magnetic membrane. Self-healing properties Self-healing is the ability of materials to repair themselves to the damages caused by chemically or mechanically. For visualizing the self-healing properties, the membrane was deposited on to glass slide. A mechanical damage was imparted on the membrane using a sharp object. The membrane was allowed to keep for 48 hours on the glass plate at room temperature. After 48 h, the mechanically damaged part was recombined to get a healed membrane. As it is evident from Figure 8 a-e, the damage in the membrane was completely disappeared. To understand self-healing property in details, stressstrain studies were carried out to measure the stress regaining ability of the membrane (Figure 8f). Initially, the stressstrain nature of the membrane prior to the damage was recorded. A mechanical damage was imparted to the sample and the sample was allowed to rest. The stress-strain properties were then measured in definite intervals of time. The efficiency of self-healing by the membrane is found to be 97 % after 48h (black line in Figure 8f), which is calculated by the ratio between the strength of healed membrane to the strength of the original membrane. The autonomous 17 ACS Paragon Plus Environment


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self-healing ability in the present system can be explained based on the interactions between ferric ion and the hydroxyl ion of poly (vinyl) alcohol (organic-nanocomposite interactions46). As a result of this interaction, the iron nanoparticles present in the cut surface diffuse through the polymer networks, which results in the autonomous healing process in the system. In addition to this, hydrogen bonding between PAMAMOS and poly vinyl alcohol also play a major role in assisting the self-healing process.

Figure 8: (a-e) Photographs of autonomous self-healing nature of membrane casted on glass plate; (f) Stress-strain curve of self-healing membrane at different time interval. Water Desalination The membrane was used for desalination purposes based on the morphological analysis and combination. As shown in Figure 9a, a set up was created in such a way that the membrane (200µm thickness) was kept above the membrane holder (which is represented by arrow mark) in between glass funnel and conical flask. The whole set up was connected to vacuum pump for filtration. The saline water (33g/L) was kept in the glass funnel and allowed to filter 18 ACS Paragon Plus Environment

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using pump. At definite time interval (10 min), the sample water was taken out and the salt concentration was measured using ICP-OES techniques. Figure 9b shows the salt adsorption yield with respect to time. The adsorption efficiency was increasing with increasing the concentration of Fe2O3-PAMAMOS nanoparticles (Figure 9c). The water desalination is mainly due to the trapping of salt ions in to the PAMAM moiety and also depends on the high surface to volume ratio of magnetic nanoparticles.

Figure 9: (a) Filtration set up, where a magnetic membrane is used to filter saline water. The membrane is kept in the filter holder and the saline water is fed from the upper flask. The whole system is connected to a vacuum set up to adjust the filtration speed. The saline water will pass through the membrane and pure water is collected at the bottom flask. ; (b and c) rate of salt adsorption with time and amount of Fe2O3 nanoparticles. Conclusion The present work introduces the concept of generating multi-functional materials from the same composite by tuning the self-assembly of the components at molecular level. We have developed an easy-to-make and low cost synthetic procedure to obtain aerogel and a 19 ACS Paragon Plus Environment


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membrane, from the same nanocomposite. In present study, Fe2O3 nanoparticles were modified by peripherally silicon substituted PAMAM dendrimer in PVA matrix. The composite material forms an aerogel and membrane by tuning the self-assembly in presence and absence of a cross linker. The hydrophobically modified aerogel has been used to remove oil from water with recycling ability and efficiency of 80-85%. The membrane from the composite has been used for desalination purposes. We believe that, our magnetic aerogel can be driven by external magnets for large scale oil spill recovery and enable the development of low cost technology for oil spill recovery. Additionally, the self-healing property of the membrane is expected to provide durability and long-term utility, which are important aspects of membranes from an industry perspective. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Phone: 044-2257-4232 Fax: 044-2257-4202 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This work was done under the financial support of department of science and technology (DST), Government of India {SR/NM/NS-12/2011 (G)}. We thank Prof Abhijit Deshpande, Department of Chemical Engineering, IIT madras for the SEM and rheological studies, DST unit of nanoscience IIT madras for TEM facility.


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Supporting Information Experimental procedure for the preparation of membrane permeability test, Chemical stability and swelling dynamics. Experimental details for the preparation of saline water and oil absorption studies. 1HNMR spectra of PAMAMOS, MALDI spectra of PAMAMOS, Photograph of aerogel, swelling kinetics, table showing the chemical stability of membrane, FT-IR spectra, adsorption isotherm, powder XRD and the magnetic properties.

This

information is available free of charge via the internet at http://pubs.acs.org

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