Induction and Tunability of Self-Healing Property of Dendron Based

May 18, 2016 - The present work describes the achievement of remarkable tunability of self-healing property for a low molecular weight hybrid gel, bas...
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Induction and Tunability of Self-Healing Property of Dendron Based Hydrogel Using Clay Nanocomposite Balachandran Vivek, Prashant Kumar, and Edamana Prasad* Department of Chemistry, Indian Institute of Technology Madras (IIT M), Chennai 600 036, India S Supporting Information *

ABSTRACT: Low molecular weight gels have relatively poor self-healing capacity compared to that of polymeric gels. Induction and tuning of the healing capacity of low molecular weight gels to achieve desired applications are thus challenging tasks. The present work describes the achievement of remarkable tunability of self-healing property for a low molecular weight hybrid gel, based on poly(aryl ether) dendron derivative (PAD). The hybrid gel has been synthesized using PAD and poly(amido amine) {PAMAM} dendrimer derivative (QPD), which are intercalated in the montmorillonite clay (MMT) layers. The self-healing of the hybrid gel (QPD-MMTPAD) was demonstrated through experiments where the distorted gel regained the initial value of storage modulus (G′) within a few minutes. Further, the propensity of self-healing of the gel has been tuned as a function of QPD concentration. The mechanically stable QPD-MMT-PAD hybrid gel has been utilized for the adsorption of ppm level concentration of polycyclic aromatic hydrocarbons (PAHs) such as βnaphthol, pyrene, and phenenathrene from water with excellent efficiency (80−98%).

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INTRODUCTION Gels based on smart materials1−4 exhibit useful properties such as self-healing,5,6 high mechanical strength,7,8 and stimulus responsive nature,9,10 which make them suitable candidates for a number of applications in biomedical areas and materials science. Among the various properties of a smart material, selfhealing (i.e., the ability of material to repair themselves so that they can improve their lifetime and the safety of the product) has gained much attraction.11−13 Such materials can potentially bring improved technologies in gel based motion sensors, organic electronics, electrically conductive coatings,14−17 tissue engineering, and cell therapy.18−21 There are many kinds of materials such as polymers, ceramics, concrete, etc. that can show exceptional self-healing nature. Self-healing gels based on organic polymeric materials are well-known, and noncovalent interactions such as hydrogen bonding,22−25 π−π interactions,26,27 and host−guest interactions28−30 can act as the driving forces for bringing self-healing in organic self-assembled systems. Also, thermoreversible covalent networks or crosslinked dynamic covalent bonds act as driving force for selfhealing.31,32 The self-healing can also be triggered by light,33 sound,34 temperature,32and pH.35 Among the several reports based on polymer based self-healing materials,36−39 hydrogels based on polymers have gained great attention during the past few years for their interesting self-healing properties.40−42 In the recent past, however, low molecular weight gels43,44(LMWGs) have gained considerable attention over their polymeric analogues for preparing self-healing systems. There are several advantages for using LMWG due to (i) their reversible formation and biodegradable nature, (ii) better homogeneity compared to polymeric systems, and (iii) superior © XXXX American Chemical Society

tunable properties in the LMWGs compared to their polymeric counterparts. Among the reported works on LMWGs, the following types exhibit self-healing properties: gels based on organic salts, low molecular weight organic compounds with metal ions,45,46 amino acids,47,48 peptides,49 and glucosamine.50 Recently, self-healing properties of a hybrid LMWG have been reported where the gel was prepared by incorporating graphene into an amino acid based low molecular weight system and graphene.47 There are a few reports on self-healing properties of hybrid gel systems.51,52 While LMWGs with self-healing properties are available, induction and regulation self-healing properties of LMWGs by judicious choice of additives have not been reported. Herein, we report the induction and “tunability” of self-healing in low molecular weight supramolecular hydrogels based on poly(aryl ether) dendrons and quaternized PAMAM {poly(amido amine)} incorporated montmorillonite clay nanocomposites. The structural analysis of the hybrid gel was carried out by FTIR, PXRD, TGA, DSC, rheology, SEM, and TEM. The tunability of self-healing was confirmed by rheology and optical microscope.The tuning of self-healing property as well as mechanical strength of the material makes them an excellent candidate for removing polycyclic aromatic hydrocarbons from water through adsorption. Received: January 28, 2016 Revised: April 22, 2016

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DOI: 10.1021/acs.jpcb.6b00935 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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Scheme 1. Schematic Representation of the Intercalation of Quaternized PAMAM Dendrimer into Clay To Form QPD-MMTa

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The material was characterized by FT-IR, PXRD, TGA, and DSC.

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EXPERIMENTAL SECTION Poly(amido amine) generation 4 was purchased from SigmaAldrich Chemical Co. (USA). Poly(amido amine) dendrimer generations 0 and 1 were synthesized according to the normal procedure reported in the literature. Montmorillonite clay (MMT) was obtained from Sigma-Aldrich Chemical Co., USA. Epibromohydrin, N,N-dimethyl-n-octylamine, and rhodamine B were purchased from Spectrochem Pvt. Ltd. (India). Pyrene, βnaphthol, and phenanthrene were purchased from SigmaAldrich Chemical Co.(USA). Methyl-3,4,5-trihydroxybenzoate, benzyl chloride, and tetrabutylammonium iodide were purchased from Alfa Aesar (USA). Hydrazine monohydrate and glucose potassium carbonate were obtained from Spectrochem Pvt. Ltd. Preparation of Hydrophobically Modified Quaternized PAMAM Intercalated Montmorillonite Nanocomposites (QPD-MMT). Quaternization of PAMAM dendrimer (QPD) using N,N-dimethyl-n-octylamine was carried out according to the procedure reported in the literature. QPD (0.06 g, 4.5 × 10−5 mol) is dissolved in 15 mL of DMF. pH of the solution was maintained between 2 and 3 using HCl. The above solution was added dropwise to 100 mL of doubly distilled water, where the swelled Na-MMT (1 g) was dispersed properly. The mixture was stirred at 60 °C for 3 days. The slightly yellow colored precipitate was washed three times with methanol and doubly distilled water, filtered, and dried under vacuum at 50 °C for 24 h. Preparation of QPD-MMT−Glucose-Cored Poly(aryl ether) Dendron Hybrid Gel (QPD-MMT-PAD). Glucosecored poly(aryl ether) dendron was synthesized based on the reported procedure. Poly(aryl ether) dendron (0.01 g, 1.6 × 10−5 mol) was dissolved in 1 mL of DMSO, heated for 5 min at 70 °C, and sonicated for 5 min. QPD-MMT (0.02 g) was added into 1 mL of doubly distilled water and sonicated for 15 min to disperse properly. Both mixtures were heated at 70 °C for 10 min, and QPD-MMT solution was added dropwise to the other solution containing poly(aryl ether). The mixture was sonicated for 15 min to form the hybrid gel. Characterization Methods. FT-IR spectra of the hydrogel were taken by Jasco 4100. SEM images of the gel and nanocomposites were obtained by FEI Quanta FEG 400 high

resolution scanning electron microscope. TEM analyses were carried out in JEOL 3010 with a UHR pole piece operating at an accelerating voltage of 300 kV. Rheological measurements were done using an Anton Paar rheometer. Uptake of polycyclic aromatic hydrocarbons (PAH) was monitored by Horiba Jobin Yvon FluoroMax-4 spectrofluorometer. TGA and DSC were recorded by TGA7 PerkinElmer TGA7, Q500 HiRes TGA, and DSC7 PerkinElmer Q200 MDSC. Powder XRD measurements were recorded by Bruker D8 Advance X-ray diffractometer using Cu−Kα radiation (λ = 1.54 Å). Optical microscopic images were taken by Nikon Eclipse LV100POL.

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RESULTS AND DISCUSSION Preparation of poly(aryl ether) dendron (PAD) was carried out according to the reported method.53The critical gel concentration was found to be 0.1% (W/V). PAD forms stable gels in DMSO−water system (1:1), and the gel morphology was found to be fibrous (Figure S3). However, no self-healing property has been observed for this low molecular weight gel. We hypothesized that self-healing can be induced via increasing the probability of hydrogen bonding and hydrophobic interactions. In order to achieve increased hydrogen bonding network, PAMAM dendrimer has been introduced into PAD. Nonetheless, gelation was not achieved by mixing PAD and PAMAM in various molar ratios. In order to achieve gelation in the hybrid system, sodium montmorillonite (MMT) clay was added since presence of MMT is known to assist swelling and gelation.54 The PAMAM dendrimers (both first and fourth generations) were quaternized by long alkyl chains,55,56 prior to the addition of MMT, so that the dendrimer derivatives have better interaction with the clay layers. The intercalation by quaternized PAMAM dendrimer in clay was achieved in DMF−water mixture by precipitation method57 (Scheme 1), where quaternized dendrimer is bound to the negatively charged clay surface. The process was carried out by slow addition of QPD solution to the clay mixture at 60 °C and at pH 2−3. The precipitate was washed three times with methanol and doubly distilled water and dried under vacuum at 50 °C for 24 h. The dried nanocomposite QPD-MMT was used to prepare the trihybrid gel QPD-MMT-PAD. QPD-MMT (0.02g) was B

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QPD-MMT. This indicates that the amide groups in QPDMMT interact with PAD through hydrogen bonding. Thermal properties of the QPD-MMT were studied by TGA and DSC, and the results are shown in Figure 2a,b. The TGA curve shows three distinct peaks at 220, 370, and 540 °C. While the first two peaks correspond to the two step degradation of PAMAM as well as loss of water from clay nanocomposites, the third peak represents the decomposition of clay nanocomposites. The peaks at 220 and 370 °C correspond to the presence of QPD in MMT layers. DSC data of QPD-MMT (Figure 2b) was compared with that of PAMAM dendrimer. The glass transition temperature (Tg) value of PAMAM dendrimer (4th generation)58 is −28 °C, and the value is increased to −10 °C upon incorporating QPD in MMT. The change in Tg value indicates that there is considerable interaction between the QPD and MMT, and the QPDMMT nanocomposite requires more temperature to overcome the molecular interactions to cross over from the amorphous phase to the crystalline phase. The TGA and DSC of the QPDMMT (generation 1) are shown in Figure S2. Figure 3a shows the PXRD pattern of QPD-MMT with different PAMAM dendrimer generations (G1 and G4), along with MMT. As shown in Figure 3a, Na-MMT has a peak at 8.89° (2θ), and the corresponding interlayer distance (d) is 0.99 nm. As a result of the intercalation of the dendrimer, there is an interlayer expansion in G1-QPD-MMT and G4-QPDMMT nanocomposites. For G1-QPD-MMT, the peaks come at 2θ = 5.99°, which corresponds to a d value of 1.4 nm. Thus, the interlayer expansion (Δd) is 0.41 nm. In the case of G4-QPDMMT, the 2θ value appears at 5.4° and the corresponding d value is 1.6 nm. The Δd in this case is, thus, 0.61 nm. Figure 3b shows the PXRD pattern of MMT, PAD, and QPD-MMTPAD. The powder XRD of both PAD and QPD-MMT-PAD systems shows the characteristic broad peak at 2θ = 21.8°. This suggests that the QPD-MMT is uniformly distributed in the gel system. A sharp peak at 2θ = 7.9 in the QPD-MMT-PAD with d = 1.2 nm corresponds to the characteristic peak of MMT. Since there is no expansion for the clay layers in the presence of the dendron network, it can be assumed that the poly(aryl ether) network does not intercalate in to the clay. The peak at 2θ = 24.5° in QPD-MMT-PAD is attributed to the π−π interaction in the gel system, and the less intense peak at 29.2° (d = 3.05) indicates hydrophobic interaction between the hydrophobically modified PAMAM in the clay and the poly(aryl ether) network. The peak at 19.1° (2θ) with d value 0.46 nm suggests the hydrogen bonding interaction

dispersed well in doubly distilled water by sonicating, and poly(aryl ether) dendron derivative was dissolved in DMSO by heating and sonicating. The dispersion and the solution were heated for 10 min and mixed by slow addition. The mixed solution was sonicated again for 15 min to form a robust gel. The FT-IR spectra of QPD-MMT-PAD, QPD-MMT, and MMT were analyzed (Figure 1) to explain the structure as well

Figure 1. FTIR spectra of QPD-MMT-PAD, QPD-MMT, and MMT. The molecular interactions lead to change in vibrational frequency of carbonyl and amide −NH peaks.

as interactions between the molecular components. The characteristic bands at 915, 1111, and 1041 cm−1 shown in Figure 1 represent the stretching vibrations (Al−OH, Si−OH) and bending vibrations (Si−OH) of hydroxyl groups of MMT. Broad peaks at 3460 and 3634 cm−1 also represent the O−H stretching vibrations in MMT. The peaks at 3237 and 2958 cm−1 in QPD-MMT represent the asymmetric and symmetric stretching of −N−H bonds in PAMAM dendrimer. The sharp peaks at 2846 cm−1 correspond to −C−H stretching frequency of the long alkyl chain attached to PAMAM dendrimer. Bands at 3404 and 1598 cm−1 represent the hydrogen bonded N−H and amide groups in poly(aryl ether) dendron (Figure S1). In QPD-MMT, most of the prominent peaks were broadened as well as shifted due to the interaction of clay with the gel. There is a shift observed from1665 cm−1 (QPD) to 1645 cm−1 in QPD-MMT, which is due to the interaction between QPD and MMT surface. The amide band vibration in QPD-MMT-PAD appears as a broad band at 1591 cm−1, which is shifted to lower frequency (1651 cm−1) compared to that of

Figure 2. (a) Thermogravimetric (TGA) and (b) differential scanning calorimetric (DSC) analysis of QPD-MMT. C

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Figure 3. (a) PXRD data of MMT and QPD-MMT (both G1 and G4); (b) PXRD of QPD-MMT-PAD, PAD, and MMT.

structure of the clay (Figure 4a) is perturbed by the interaction with QPD (Figure 4b). The granular structure in Figure 4b also indicates agglomeration of the clay particle due to the QPD intercalation. Figure 4c shows the SEM image of PAD, where the entangled fibers of the PAD gel are visible. Upon addition of PAD to QPD-MMT, at xerogel condition, the clay networks are seen to be grafted into thick gel fibers with porous network of nonuniform pore size (∼100−150 nm) (Figure 4d). This also reveals that the fiber structure of the gel has been maintained in the QPD-MMT-PAD hybrid system and the QPD-MMT is completely immersed in the fiber network due to the hydrophobic interaction between QPD and the poly(aryl ether) dendron. Additional SEM images have been given in the Supporting Information to support this argument (Figure S3). In order to understand the interactions between MMT, QPD, and PAD, TEM images have been taken. Figures 5a and 5b) show the TEM images of MMT and QPD intercalated MMT. In the TEM image of MMT (Figure 5a), a few thick closely packed stacks of planes are observed. The clay platelets tend to separate from the stack in intercalated regions, which are evident from the lower contrast areas of Figure 5b. TEM image of PAD shows thin fibrous networks (Figure 5c), which become thicker upon addition of QPD-MMT. The width of the fibers is in the range of 250−500 nm. As observed in Figure 5d,

between the MMT and the glucose moiety in the poly(aryl ether) dendron.47,59,60 Figures 4a−4d show the microrange structure of MMT, QPD-MMT, QPD-MMT-PAD, and PAD. The layered

Figure 4. (a) SEM images of montmorillonite clay (MMT); (b) QPDMMT; (c) poly(aryl ether) gel (PAD); (d) and QPD-MMT-PAD (scale bar = 5 μm).

Figure 5. TEM images of (a) MMT; (b) QPD-MMT (scale bar = 50 nm); (c) PAD; and (d) QPD-MMT-PAD (scale bar = 1 μm). D

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Figure 6. Rheological analysis of QPD-MMT-PAD and PAD alone: (a) strain sweep curve of QPD-MMT-PAD and PAD; (b) frequency sweep curve of QPD-MMT-PAD and PAD. Incorporation of QPD-MMT has improved the mechanical strength of PAD.

Scheme 2. Photographs of Hybrid Gel (QPD-MMT-PAD) Formation and Its Schematic Representation

Figure 7. (a) Photographs of QPD-MMT-PAD with proper shape, (b) self-healing of the gel, and (c) self-healed gel. (d−j) Optical microscopic images taken at intervals of 5 min, indicating the “growth” of the hybrid gel due to self-healing nature.

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Figure 8. Rheological analysis of the tuning properties of self-healing in QPD-MMT-PAD gel with various concentrations of QPD in MMT. The G′ value in the time sweep plot reaches the initial G′ value in the strain % sweep plot, demonstrating the “tunability” of the self-healing.

the fibers are stable in the presence of clay sheets. TEM images of nanocomposites and nanocomposite gel are given in Figure S4. It is reasonable to assume that the stability of the composites is relatively high compared to that of fibrous gel. In order to test this, we have carried out rheological studies of the samples. Figures 6a and 6b show the characterization of PAD and QPDMMT-PAD by rheological studies. Figure 6a shows the strain sweep curve (i.e., G′ or G″ versus strain at constant frequency) with G′ value greater than G″ in both the cases. The G′ value of QPD-MMT-PAD is higher than G′ of PAD, which means that QPD-MMT induces high strength and stiffness to the system. When the strain is increased, G′ value falls below G″, indicating the breakdown of the gel. Figure 6b shows the frequency sweep curve of PAD and QPD-MMT-PAD at constant strain. The G′ value of QPD-MMT-PAD is higher than the G′ value of PAD and little change for the G′ value of QPD-MMT-PAD has been observed upon increasing the frequency. All these observations suggest that the hybrid gel is reasonably strong and stiff. The time sweep curve of QPD-MMT-PAD is shown in Figure S5, where the G′ value is not showing significant changes during a period of time. The hardness of the gel is due to the hydrogen bonding interaction of poly(aryl ether) with the clay surface and QPD and an extra hydrophobic interaction between the PAMAM periphery and PAD. Scheme 2 shows the photographs taken during hybrid gel formation, along with a cartoon representation of the selfassembly and gel formation. Tuning of Self-Healing. While PAD form robust gel systems, they possess poor self-healing ability, even in the presence of MMT. Figure S6 shows photographs of PAD based gels in the presence and absence of MMT, and the images clearly convey that there is no self-healing property in the systems. As mentioned before, a combination of QPD and PAD also fails to form robust gel systems. However, the gels formed upon addition of QPD-MMT to PAD show excellent selfhealing property. Figure 7 a−c shows the photographic evidence of mechanical strength as well as self-healing ability of QPD-MMT-PAD. The hybrid gel was cut into three pieces, and one of the pieces was colored using rhodamine B. The three pieces of gel were mechanically placed together and kept for 30 min. After 30 min, it was found that the three pieces were joined and their whole weight could be held by keeping the gel above two supports, where the supports were separated by 3 cm distance. QPD-MMT-PAD maintained a definite shape, along with the excellent self-healing property. In order to

get an insight into the self-healing behavior of the system, the QPD-MMT-PAD mixture was examined using optical microscopy. The gel was made into a thin film, and a crack was made in the film by using a spatula. The crack was monitored at definite intervals of time by using the optical microscope, and it was observed that the crack was healed by 35 min due to the self-healing nature of the film. Figures 7d−7j show the optical microscopic images of QPD-MMT-PAD as a function of time. Since QPD can play a key role in regulating the noncovalent interactions in the hybrid gel, the self-healing nature of the hybrid gel has been carried out at different concentrations of QPD. Three different concentrations (3 × 10−5 mol, 4.5 × 10−5 mol, and 6 × 10−5 mol) of QPD have been used. Figure 8 shows plots of G′ vs strain % and G′ vs time for the hybrid gel at three different concentrations of QPD. The strain was increased until the hybrid gel lost its gelation property (until the gel broke), and healing time of 400 s was allowed. After 400 s, the G′ value was measured as a function of time. Upon increasing the concentration of QPD, the G′ value in the time sweep curve gets closer to the initial G′ value of the strain sweep curve, which is attributed to the enhancement of selfhealing nature of the hybrid gel, as a function of the concentration of QPD. The difference in the modulus values (ΔG) was 8659 and 10760 Pa for QPD concentrations 4.5 × 10−5 mol and 6 × 10−5 mol, respectively. This suggests that the hybrid gel was gaining remarkable self-healing capacity as QPD concentration was increased. The tunability of the self-healing property is due to the extra hydrogen bonding between glucose units in PAD and the PAMAM, along with the strong hydrophobic interaction between the PAMAM periphery and poly(aryl ether) part of the gel. Adsorption Studies of Polycyclic Aromatic Hydrocarbons. We found out that these nanocomposites and hybrid gels were an excellent adsorbent for selective polycyclic aromatic hydrocarbons from water. Adsorption of polycyclic aromatic hydrocarbons (PAH) from water was carried out using QPD-MMT-PAD, QPD-MMT(G4), and QPD-MMT(G1). Pyrene, phenanthrene, and β-naphthol were chosen as adsorbates, and the initial concentration of the pollutants (pyrene, phenanthrene, and β-naphthol) was kept at 1 ppm. The QPD-MMT nanocomposites (10 mg) and dried QPDMMT-PAD gel (10 mg) were introduced into water containing 1 ppm pollutant and stirred for 1 h. The samples (3 mL) were withdrawn at definite time intervals, and quantitative measurement was done using fluorescence spectroscopy. The adsorption yield was determined using eq 1, F

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Figure 9. Variation of adsorption yield for QPD-MMT-PAD, QPD-MMT(G1), and QPD-MMT(G4) with time where the pollutants are (a) βnaphthol, (b) pyrene, and (c) phenanthrene.

Figure 10. Langmuir adsorption isotherm plots containing (a) q versus concentration and (b) c/q versus concentration.

⎛ C − Ce ⎞ V q=⎜ o ⎟ × 100 ⎝ Co ⎠ m

QPD-MMT-PAD. These three adsorbents could remove 80− 98% PAH from water within 1 h. High adsorption capacity has been shown in the case of QPD-MMT-PAD, where more hydrophobic moieties from the poly(aryl ether) part are present. Thus, it can be concluded that the adsorption is mainly due to hydrophobic interaction between PAH and the dendritic periphery. A detailed study of adsorption was carried out using an adsorption isotherm as shown in Figure 10a,b. Figure 10a shows the quantity of PAH adsorbed (q) versus concentration of PAH remaining in the solution (c). It was determined that the q value increased with increasing c value and it reached a constant value. Figure 10b represents the c/q versus c plot using eq 2,

(1)

where q is the adsorption amount at equilibrium, C e corresponds to equilibrium concentration, and Co is the initial concentration to polycyclic aromatic hydrocarbons in water; V and m are the volume of the aqueous solution containing polycyclic aromatic hydrocarbons and mass of the hybrid gel, respectively. The concentration terms were replaced by corresponding fluorescence intensities. The fluorescence spectra from the samples at regular intervals are shown in Figure S7. Figures 9a−9c show adsorption yield versus time plots of different PAH using QPD-MMT(G1), QPD-MMT(G4), and G

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c 1 c = + q NKb N

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(2)

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CONCLUSION The experimental results taken together suggest that induction and tunability of self-healing property are feasible in LMWGs by judicious choice of additives. We have synthesized a novel hybrid gel using poly(aryl ether) dendron derivative and quaternized PAMAM intercalated MMT. While the poly(aryl ether) derivative based gel did not exhibit any self-healing property, the hybrid gel exhibits self-healing properties at room temperature. We have also shown that the self-healing property of the hybrid gel can be improved by 2 orders of magnitude. The significant improvement in the self-healing ability of the gel was demonstrated by rheological experiments where the distorted hybrid gel regained the initial modulus value before distortion, when the concentration of quaternized PAMAM dendrimer was doubled. The hybrid gel has been used for adsorbing PAH pollutants from water with average efficiency of 95%. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.6b00935.

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where N and Kb are the total number of adsorption site per unit weight of adsorbent and binding constant, respectively. From the linear plot, the values of N and Kb are obtained as 0.0997 and 5.172 cm3/mg, respectively. These values are comparable to values obtained from other systems known for removing organic pollutants from water.61 The correlation coefficient value is greater than 0.987, which indicates a best fit for the Langmuir adsorption model. The adsorption is concentration dependent and increases with initial concentration of pollutants. Control experiments were carried out where adsorption of pyrene, β-naphthol, and phenanthrene on nanoclay (without intercalated clay) has been carried out. The adsorption capacity was negligibly small compared to adsorption capacity of the hybrid gel, indicating that the nanoclay alone has little adsorption capacity. The results are given in Figure S8.

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*E-mail: [email protected]. Phone: 044-2257-4232. Fax: 0442257-4202. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS 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 and DST Unit of Nanoscience IIT Madras for TEM facility. H

DOI: 10.1021/acs.jpcb.6b00935 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcb.6b00935 J. Phys. Chem. B XXXX, XXX, XXX−XXX