Improved Mechanical Stability of Acetoxypropyl Cellulose upon

Nov 22, 2013 - and Somobrata Acharya*. ,†. †. Center for Advanced ... An enhancement in stability of the blend is observed upon PbS nanowire incor...
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Improved Mechanical Stability of Acetoxypropyl Cellulose upon Blending with Ultranarrow PbS Nanowires in Langmuir Monolayer Matrix Subrata Maji,† Sudarshan Kundu,‡ L. F. V. Pinto,‡ M. H. Godinho,*,‡ Ali Hossain Khan,† and Somobrata Acharya*,† †

Center for Advanced Materials, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India CENIMAT/I3N, Departamento de Ciência dos Materiais, Faculdade de Ciências e Tecnologia, FCT, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal

‡

S Supporting Information *

ABSTRACT: Cellulose and cellulose derivatives have long been used as membrane fabrication. Langmuir monolayer behavior, which naturally mimics membranes, of acetoxypropyl cellulose (APC) and lead sulfide (PbS) nanowire mixtures at different volume ratios is reported. Surface pressure (π)−area (A) isotherms of APC and PbS nanowires mixtures at different volume ratios show a gradual decrease in the monolayer area with increasing volume fraction of PbS nanowires. Change of surface potential with monolayer area at different volume ratios also reveals a gradual increase in the surface potential indicating incorporation of PbS nanowires within APC matrix. The compressibility and elastic constants measurements reveal an enhancement of the elasticity upon incorporation of PbS nanowires up to certain volume fractions. An enhancement in stability of the blend is observed upon PbS nanowire incorporation to the APC matrix. Rheological measurements also support the robustness of the mixture of APC and PbS nanowires in 3D bulk phase. Such robust ultrathin films of cellulose based-nanowire blend obtained by means of the Langmuir technique may lead to novel routes for designing cellulosic-based thin films and membranes.

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INTRODUCTION Cellulose is the major constituent of plant cells and the most abundant polymer in nature. Cellulose is made up of β-Dglucopyranose monomers consisting of one primary and two secondary hydroxyl groups which can undergo esterification and etherification.1 Introduction of substituents could make cellulose water-soluble owing to reconstruction of crystalline regions, which arise from inter- and intramolecular hydrogen bonding among hydroxyl groups. Hydroxypropylcellulose (HPC), a derivative of cellulose, can generate aqueous liquid crystalline solutions, and some HPC esters are found to form thermotropic and lyotropic phases.2−4 Isotropic solutions of cellulose and cellulose derivatives can be successfully electrospun into fibers of submicrometer diameters using the action of electrostatic forces.5−8 On the other hand, thin polymer films made out of the cellulose derivatives have enormous possible applications in areas such as electronics, dielectric coatings, lubricants, adhesives, paints, biological membranes, etc.9−15 Dewetting of polymer films is an interesting phenomenon considering its vast application in polymer coating on electronic devices and paints. Barnes et al.16 showed the control over dewetting of ultrathin polystyrene and polybutadiene thin films by addition of low amount of fullerene nanoparticles, which acts as nanofillers and stabilize the films against dewetting. Luo et al.17 showed that the dewetting of polymer thin films on © 2013 American Chemical Society

substrates can be retarded or totally prevented by addition of nanosized filler. One-dimensional (1D) nanostructures, such as nanotubes and nanowires, have drawn enormous attention because of their unique structures, size and shape dependent physical properties, and novel possibility for a variety of applications.18−23 Lead sulfide (PbS) quantum particles are a class of unique materials that are extremely important for both fundamental scientific studies and technological applications. The large exciton Bohr radius (20 nm) owing to the nearly equal contribution from electron and holes allows an enhanced level of quantum confinement compared to II−IV or III−V type nanomaterials.24 From a technological perspective, PbS is an extremely promising material for a large number of applications in the mid- and near-infrared emission detection range,25 for biological applications,26 and for optoelectronic devices.27,28 There has been great interest in designing PbS nanocrystals of various sizes and shapes.29−32 When mixed with cellulose, PbS nanowires may interact with cellulose as nanofillers, enhancing the stability of the resultant mixture (blend). Cellulose has long been used in membranes for controlling water flow.11 Received: April 17, 2013 Revised: November 13, 2013 Published: November 22, 2013 15231

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Scheme 1. Idealized Representation of an Anhydroglucose Unit of Acetoxypropyl Cellulose (APC) with Mole Equivalent (ME) and Mole Substitution (MS) Prepared from Hydroxypropyl Cellulose (HPC) with Degree of Substitution (DS) and Mole of Substitution (MS)

without further purification. All the solvents used in this work were purged with dry nitrogen for few minutes before starting a reaction. Synthesis of Acetoxypropyl Cellulose. Acetoxypropyl cellulose (APC) was synthesized from hydroxypropyl cellulose (HPC) of molecular weight of ∼100 000. 50 g of HPC was added to 150 mL of glacial acetic anhydride in a three-neck reactor. 13.5 mL of acetic acid was finally added to the resulting mixture with continuous stirring. The reaction appears very viscous at the initial stage. During 8 days, the solution was heated at 60 °C for 4 h every day. After 8 days, the reaction was stopped with the addition of water. The product immediately precipitates, forming a whitish viscous solid. The crude product was washed several times with pure water until neutral pH is achieved. The product was dried several days under vacuum at 60 °C; the final yield was around 75%. Characterization of as-synthesized APC was done by 1H NMR (Figure S1) and FTIR (Figure S2). The number of acetyl groups per residue was evaluated by 1H NMR and was around 2. The acetylation reaction is shown in Scheme 1. PbS Nanowire Synthesis. First, lead hexadecylxanthate was prepared from potassium hexadecylxanthate (prepared using hexadecanol, carbon disulfide, and potassium hydroxide) and lead nitrate as demonstrated in the literature.37,38 Hexadecanol (0.04 mol, Aldrich, 95%) and KOH (0.04 mol, AnalaR, 85%) are heated to 150 °C in a round-bottom flask. The suspension was cooled to ice temperature where 0.05 mol of CS2 (Aldrich, 99.9%) was added. The thick suspension was washed using petroleum ether to obtain purified hexadecylxanthate. Resultant hexadecylxanthate was reacted with lead nitrate (0.0014 mol) in water to obtain Pb hexadecylxanthate. For the synthesis of PbS nanowires, lead hexadecylxanthate was added (0.065 g) in one shot to 1.6 mL of TOA at 65 °C with continuous stirring under nitrogen.37,38 A grayish-milky color appeared after 5 min, and then the temperature was increased to 80 °C. Annealing was carried for 35 min at 80 °C. Finally, the temperature was reduced to 40 °C, and the wires were collected by washing two times with methanol (centrifuge at 3000 rpm for 3 min) and with a mixture of dichloromethane and methanol (5:40 by volume) to remove excess TOA. Langmuir Trough Experiments. A Langmuir trough from Nima Technologies was used for all experiments at the air−water interface. APC was dissolved in chloroform at a concentration of 0.5 mg/mL. Surfactant coated PbS nanowires was dispersed in chloroform at a concentration of 1 mg/mL. PbS nanowire solution was then added to APC at five different ratios of 5:1, 5:2, 5:3, 5:4, and 5:5 by volume. For isotherm measurements, 70 μL of APC was spread at the air−water interface, and the barrier was compressed after allowing 15 min for evaporation of solvent. In case of pure PbS nanowires, 700 μL of solution was spread, and the barrier was compressed after allowing 60 min for evaporation of solvent. For mixed monolayers, 150 μL solution was spread at the air−water interface for each experiment. We have measured the surface pressure versus area curves for pure PbS only,

Preparation of high-flow semipermeable membranes using cellulose for ion separation from saline solutions is one of the important applications of cellulose.14 Hence, improving the stability of cellulose membrane with nanomaterial additives is of fundamental importance in membrane technology for water purification. The Langmuir monolayer, which naturally mimics membranes at the air−water interface,33 is an advantageous technique for sophisticated processability of 2D membranes since the packing, orientation, and long-range ordering of the constituent monolayer can be precisely tailored with perfect control and reliable thickness.34−36 Hence, studying the integrated behavior of blends consisting of nanomaterials and cellulose at air water interface might promote membrane fabrication mechanics with enhanced stability and better efficiency. In this work, we report on the study of Langmuir monolayers of acetoxypropylcellulose (APC) blended with PbS nanowires at the air−water interface. Pure APC shows rigid Langmuir film with no hysteresis between the compression and expansion cycles, however, with less stability. A finite hysteresis has been observed upon mixing PbS nanowires in APC matrix, which indicates the organization of nanowires with in the APC matrix. Surface potential measurement confirms the alignment of dipoles in near parallel orientation of the PbS nanowires within the APC matrix. Elasticity and compressibility studies using thermodynamic formulation reveal improved robustness of the monolayer upon incorporation of PbS nanowires within the APC matrix. The stability curves confirm that incorporation of PbS nanowires in the APC matrix enhances the stability of the mixed monolayer. The enhanced stability of the blends is further corroborated from the measurements of steady shear viscosity by means of rheological experiments which reveal an enhanced shear thinning behavior at higher shear rate for the blends of APC with PbS nanowires in comparison to pure APC. Such ultrathin films of nanowire blended cellulose obtained by means of the Langmuir−Blodgett (LB) technique may offer potential applications in designing membranes required for water purification technology.

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EXPERIMENTAL SECTION

Chemicals. Hydroxypropyl cellulose (HPC), glacial acetic anhydride, acetic acid, hexadecanol, carbon disulfide, lead nitrate, and trioctylamine (TOA) were purchased from Aldrich and used 15232

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Figure 1. (a) TEM image of PbS nanowires revealing ∼2 nm diameter and 100−150 nm in length. (b) HRTEM images of PbS nanowires showing well-resolved (200) and (220) lattice planes with interplanar distances of 0.29 and 0.21 nm. (c) SAED pattern from the nanowires shows the rocksalt cubic structure with 200 and 220 diffraction rings, in line with the interplanar distances of 0.2969 and 2.099 nm of PbS bulk rock-salt structure (JCPDS 05-0592).

Figure 2. (a) Surface pressure−area isotherm of pure PbS nanowires. A large hysteresis between the compression (solid line with up-arrow) and expansion (dotted line with down-arrow) cycle is observed. The bottom axis presents the surface area of the Langmuir trough since the actual concentration of the nanowires is complicated to quantify due to the presence of the ligand molecules. (b) Surface pressure−area isotherm pure APC and five different mixtures of PbS and APC in different volume ratios taking APC as a standard. The compression and expansion cycles are shown by solid line with up-arrow and dotted line with down-arrow, respectively. The hysteresis between the compression and expansion cycles of pure APC and all the mixtures of APC and PbS nanowires are also shown. Pure APC does not show hysteresis. A gradual increase of the hysteresis is observed upon incorporation of PbS nanowires, indicating a strong interaction of PbS nanowires and APC molecules. The inset shows the color code of pure APC and as well as different mixtures of PbS in APC matrix. All the isotherms are taken at a temperature of 22 °C. since the precise molecular weight of the PbS nanowires including the TOA coating is unknown. We have used a molecular weight of 700 g/ mol, which was tentatively derived using thermal gravimetric analysis (TGA) and modeling.35 However, this molecular weight is tentative, and hence a representation of the area per molecule for the PbS nanowire will contain error; rather, a representation of molecular area (in terms of trough area) is realistic. For measuring the surface pressure versus area per molecules of the mixtures of APC and PbS nanowires, we have used the molecular weight of APC as standard in all carves, while the contribution of the PbS nanowires comes from the volume fraction of APC and PbS nanowires in the mixtures. We have used the following equation for calculating overall concentration for the APC and PbS mixtures: c=

x1v1 + x 2v2 v1 + v2

mixture of APC to PbS) was transferred onto lacey carbon-coated copper grid by using the Langmuir−Schaefer method. Rheological Experiments. Shear viscosities of pure APC and different mixtures of APC and PbS nanowires at different volume ratios were measured by a stress controlled rheometer (Anton-Paar MCR 302) with cone−plate geometry (cone diameter 50 mm and cone angle 1°) equipped with a Peltier base temperature control. A sample cover was used with the instrument to minimize the solvent evaporation during the experiment. Viscosity measurements of all the samples were conducted at a shear rate of 0.01−2 s−1. All the measurements were done at constant temperature of 20 °C.

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RESULTS AND DISCUSSION Figure 1a shows the transmission electron microscopy (TEM) images of TOA-coated nanowires of ∼2 nm in diameter and 100−150 nm in length. The high-resolution TEM (HRTEM) images of individual nanowires clearly reveal a diameter of ∼2 nm (Figure 1b) with well-resolved lattice planes corresponding to interplanar distances of 0.29 ± 0.02 and 0.21 ± 0.02 nm consistent with the (200) and (220) d-spacings of the PbS bulk rock-salt structure. The HRTEM images of the nanowires also indicate an orientation in which the ⟨110⟩ crystallographic axis is parallel to the long axis of the nanowires. The selected area electron diffraction (SAED) patterns of the PbS nanowires (Figure 1c) also shows the rock-salt cubic structure with predominant 200 and 220 diffraction rings, corresponding to

(1)

where c is the overall concentration (net concentration) of the mixture solution, x1 and x2 are the concentrations of APC and PbS nanowires in chloroform, and v1 and v2 are the volumes of APC and PbS nanowires taken to prepare the mixture. Here the concentrations of APC and PbS were calculated using the measured weight (mg) to the volume of the solvent (mL). Surface potentials at the air−water interface were measured using Trek Electrometer 320. The same volume of pure and mixed materials was spread for surface potentials measurements at the air−water interface. Transmission electron microscopy (TEM) was carried out using a JEOL JEM 2010. For TEM measurements, the Langmuir film of mixed monolayer (5:5 15233

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Figure 3. (a) Surface potential isotherms of pure PbS nanowires (black square), pure APC (red full circle), and five different mixtures of PbS nanowires to APC at different volume ratios: 5:1 (blue hollow circle), 5:2 (cyan triangle), 5:3 (magenta half circle), 5:4 (dark yellow hexagon), and 5:5 (purple star). (b) TEM image of PbS nanowires in the APC matrix for 5:5 mixture lifted using the Langmuir−Schaefer method. The size and shape of the PbS nanowires are preserved upon incorporation in the APC matrix. The monolayer film is lifted onto a TEM grid at surface pressure of 22 mN/m at 22 °C.

alkyl chains (the capping agent) surrounding the PbS nanowires and APC molecules. The PbS nanowires are synthesized using trioctylamine (TOA) as a capping agent. The alkylamines are Lewis bases; the long alkyl chain lengths provide an increased electron density on the nitrogen site, thus inducing metal coordination. Note the reduced size of the PbS nanowires imply an increased surface to volume ratio which increases the number surface atoms. Since the surface atoms consist of both lead and sulfur atoms, there are a large number of lead atoms on the surface of PbS nanowires for coordination with the ligand TOA. The presence of large number of metal atoms on the surface in PbS nanowires will allow better passivation with the ligands. It is expected that due to its greater nucleophilicity, TOA should bind to the lead ions of the inorganic core of the PbS nanowires, while the alkyl chains are exposed outward making the PbS nanowire surfaces hydrophobic. The hydrophobic nature of the PbS nanowires renders them surface active. The incorporation of PbS nanowires within APC monolayer might have increased the resultant area of the mixed monolayer. On the other hand, an unchanged area of the APC monolayer is expected for squeezing out of the PbS nanowires from the APC monolayer surface. The decrease in the area of the pure APC upon mixing the PbS nanowires thus indicates strong interaction of PbS nanowires with the APC molecules. The hydrophobic moieties of the APC molecules interact with the hydrophobic PbS nanowires creating strong hydrophobic−hydrophobic interactions, which results in squeezing of the APC monolayer at the air−water interface. Upon increasing the volume ratio of the PbS nanowires, the hydrophobic interaction increases to a greater extent, resulting in the further decrease in the mixed monolayer area. Importantly, the collapse pressure increases upon increasing the ratio of the PbS nanowires supporting increased hydrophobicity of the mixed monolayer. Hysteresis between the compression and expansion isotherms is also induced upon PbS nanowires incorporation. The hysteresis increases with incorporation of PbS nanowires in a smaller volume, which becomes stable upon increasing the ratio of PbS nanowires, indicating that no permanent structural change in the monolayer is occurring upon incorporation of the PbS nanowires in a larger volume. Surface potential measurement of the monolayer is a useful method for the study of reorganization processes of molecular dipoles within Langmuir monolayers.45,46 Figure 3a shows the

the interplanar distances of 0.2969 and 0.21 nm of PbS bulk rock-salt structure (JCPDS file #05-0592). Since APC is insoluble in water, it forms a monolayer at the air−water interface at room temperature. Figure 2b shows the compression−expansion pressure−area isotherm curve for pure APC. The isotherm at room temperature shows a low-pressure, 2D liquid-expanded phase followed by a liquid condensed region, which collapses slightly above 30 mN/m. The limiting APC molecular area obtained by extrapolation of the steep portion of the isotherms to zero surface pressure is 94.4 nm2. The Langmuir film of the APC is rigid as reflected from the lack of hysteresis between the compression and expansion isotherm cycles. Within the experimental uncertainty of our results, the molecular areas and surface pressures at which the liquidexpanded to liquid-condensed39 takes place are independent of temperature. The APC isotherms were independent of the time delay (up to 4 h) between spreading of the material at the air− water interface and recording of the isotherms, indicating sufficiently rapid equilibration. The alternative possibility, of a relaxation slower than the time scale of the experiment, is ruled out on the basis of the reversibility of the isotherms below the collapse pressure. The surfactant coating around PbS nanowires makes them excellent candidates for LB assembly.40,41 At zero or low surface pressure, the nanowires are arranged randomly with a relatively large separation between them.40 The nanowires come in closer proximity owing to the surface pressure and the surface pressure starts rising sharply in the isotherm curve. Importantly, a large hysteresis between the compression and expansion isotherm cycles is observed for PbS nanowires (Figure 2a), indicating a structural change in the monolayer.40,41 In fact, the advantage of the LB technique for inducing nanowire alignment on a large scale into 2D assemblies in a single step has now been well established.40,42−44 The surface pressure area isotherms of mixtures of APC and PbS nanowires at different volume ratios show a gradual decrease in the monolayer area along with increasing surface pressure upon increasing volume fraction of PbS nanowires (Figure 2b). The isotherms are also independent of the time delay (up to 4 h) between spreading of the film and recording of the isotherms, indicating robust stability of the monolayer. An increase in the collapse pressure with decreasing monolayer area while increasing the volume ratio of PbS nanowires can be understood in terms of interaction between the hydrophobic 15234

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Figure 4. (a) Elastic constants of pure APC (red bar) and different ratios of APC to PbS nanowires: 5:1 (blue bar), 5:2 (cyan bar), 5:3 (magenta bar), 5:4 (deep yellow bar), and 5:5 (purple bar). (b) Compressibility−surface pressure curve for pure APC, pure PbS, and five different mixtures of APC and PbS nanowires. The inset shows the color code of pure APC and PbS monolayers as well as different mixtures of PbS nanowires in APC matrix. The color codes are maintained the same as in (a).

the elastic constants of pure APC, PbS nanowires, and different ratios of PbS nanowires to APC. The monolayers of pure APC and pure PbS nanowires show the lowest elastic character, while the same for the mixed monolayers at different ratios of PbS nanowires reveal higher elasticity in comparison to the pure components. Interestingly, the mixed monolayer with lowest PbS concentration shows the highest elasticity and the highest stability (discussed later in the stability section). Compressibility study of Langmuir monolayer is another useful tool to characterize the inherent transitions that occur during compression.52−55 The compressibility coefficient (β) is calculated using the following standard thermodynamic (eq 3) relation in two dimensions:

dependence between the effective molecular dipole moment and the area of the pure and mixed monolayer as a function of the PbS nanowires concentration in the subphase. The effective molecular dipole moment is defined as the product of the surface potential (ΔV) and the average area per molecule (A).47 Using this simplified form, the influence of the orientational change of mixed monolayer between the electrodes of the Kelvin probe is estimated. At the beginning of compression, large mean area is characterized by a small value of the effective molecular dipole moment. A rapid compression-induced increase of the effective molecular dipole moment is observed at higher surface pressure (Figure 3a). The further behavior of the surface potential curves strongly depends on the PbS nanowires concentration within the mixed monolayer. The surface potential of pure APC is different from the mixed monolayer at higher surface pressure, while the nature of PbS nanowires is completely different from pure APC. However, in the case of the mixtures of APC and PbS nanowires, incorporation of PbS nanowires, even in small amounts, greatly influences the value of surface potential of the mixed monolayer. The maximum value of the dipolar moment is expected to correspond to a nearly parallel arrangement of molecules. Obviously, the change in orientation of the APC molecules is different from PbS nanowires. When PbS nanowires are incorporated within the monolayer, a gradual increase in the surface potential is observed. The PbS nanowire is supposed to have an inherent dipole moment even in the cubic crystallographic phase.41,48−50 During the monolayer compression the PbS nanowires within the APC molecules turn up in near parallel configuration, thereby increasing the value of the dipole moment. Indeed, such a near parallel orientation of the PbS nanowires within the APC matrix is observed, when the mixed monolayer is transferred onto a TEM grid by using Langmuir−Schaefer (LS) deposition technique (Figure 3b). The behavior of Langmuir monolayer is greatly important for understanding the structure and stability of molecularly thin films. By analyzing the π−A isotherms, one would be able to understand the state of monolayer, for example, molecular arrangement, phase structure, inherent phase transitions, and elasticity, etc. The elasticity of a monolayer can be defined as E = −AΔσ /ΔA = −A dπ /dA

β = −(1/A)(dA /dπ )T

(3)

The inherent phase transition is reflected as a peak in the β−π curve, and the maximum compressibility (βmax) of the monolayer represents the lowest stability of a monolayer. The asymmetry of the peaks in the β−π curve indicates that phase transition may involve several steps.52 Figure 4b shows the β−π curve for the pure APC, pure PbS, and mixed monolayers at different PbS nanowires to APC volume ratios. It can be observed that the compressibility of pure APC is quite higher than the other mixtures or the PbS nanowires monolayer. The compressibility of pure APC show sharp peaks at lower, intermediate, and high surface pressures, indicating several reorganization processes occur during compression which might lead to the instability of the monolayer. However, the lack of hysteresis in the isotherm cycles of APC (Figure 2b) suggests that such reorganization processes occur faster than the time scale of the measurements. In comparison, pure PbS nanowires show a prominent phase transition at around 25 mN/m, indicating the possibility of irreversible phase transition at higher surface pressure. Such an observation is in line with the fact that a monolayer of PbS nanowires remain in random orientations at lower surface pressure which undergoes ordering with a preferred direction of the long axes parallel to the barrier. At higher surface pressure, PbS nanowires may undergo permanent structural changes, as reported earlier.40,41 Notably, all the mixtures of PbS nanowires and APC show a much lesser compressibility with less fluctuation compared to the pure components. The mixtures show broader peaks at their phase transitions points owing to the reorganization process, indicating that the compressibility of mixtures are more rigid in comparison to pure components.

(2)

where π = σ0 − σ is the difference in surface tension in the absence and presence of monolayer constituents, π is the surface pressure, and A is the mean molecular area. The static elasticity can be directly calculated from the slope of the π−A isotherm according to the eq 2.51 Figure 4a shows 15235

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Figure 5. Stability curves of pure APC, pure PbS nanowires, and different ratios of APC to PbS nanowires at different surface pressures: (a) at 15 mN/m; (b) at 25 mN/m at temperature 22 °C. Pure APC (red curve), pure PbS nanowires (black curve), and different blends with 5:1 (blue curve), 5:2 (cyan curve), 5:3 (magenta curve), 5:4 (deep yellow curve), and 5:5 (purple curve) are shown. The color codes of the respective curves are the same for all figures.

Our observation is further supported from the measurement of the stability of monolayer by holding at constant surface pressure and simultaneously observing the area vs time decay carves. Figure 5 shows the normalized surface area (At/A0) versus time stability curves of pure APC, PbS nanowires, and for five different mixtures of APC and PbS nanowires at constant surface pressures of 15 and 25 mN/m, respectively. Here, A0 is the area of the monolayer at the beginning of the decay measurements at time t = 0, and At is the area of the monolayer at time t. Pure APC shows a faster decay corresponding to faster decrease in the monolayer area with time at all the constant surface pressures. The addition of PbS nanowires to APC increases the stability of the mixed monolayer as evidenced from a slower decrease of the surface area with time for the mixed monolayers in comparison to pure APC. We rule out the decrease in the monolayer area with time owing to the dissolution of APC or PbS nanowires in water because of their hydrophobic nature. Interestingly, when the monolayer is kept at lower surface pressure of 15 mN/m (Figure 5a), fluctuations in the monolayer area with time are observed upon incorporation of the smaller volume of PbS nanowires, which might point out the stabilization of PbS nanowires in APC matrix due to a minimum-energy configuration while the process of hydrophobic interactions. When the monolayer is kept at higher surface pressure of 25 mN/m (Figure 5b), much smaller fluctuations are observed owing to the tighter packing density of the monolayer constituents at higher surface pressure. Upon increasing the volume ratio of PbS nanowires, the area versus time curve becomes less stable compared to the low volume fraction incorporation of PbS nanowires in APC, which indicates that there is a critical concentration of PbS nanowires incorporation within the monolayer of APC matrix, which is an APC:PbS ≈ 5:1 mixture. In order to confirm that incorporation of PbS nanowires within APC matrix indeed increases the stability of the resultant blends, we have corroborated the results obtained from 2D surface behavior with the 3D bulk phase. Different blends in chloroform are prepared retaining the same volume ratios as used for surface behavior for viscosity measurements in 3D. Figure 6 shows the plot of steady shear viscosity against shear rate for pure APC and different blends with PbS nanowires at different volume ratios. At the lower shear rate values, all the samples follow Newtonian behavior followed by shear thinning behavior at higher share rate values.56 All the samples were found to exhibit the phenomena of shear thinning; however, the overall shear rate values are low. Analysis of Newtonian

Figure 6. Double-logarithmic plot of viscosity vs shear rate of APC (black curve) and different ratios of PbS nanowires to APC: 5:1 (red), 5:2 (blue), and 5:3 (magenta) at constant temperature 20 °C.

region reveals that the zero-shear viscosity of the blends are 364 Pa·s for the APC:PbS ≈ 5:1 mixture, 190 Pa·s for the APC:PbS ≈ 5:2 mixture, and 474 Pa·s for the APC:PbS ≈ 5:3 mixture, indicating that upon incorporation of PbS nanowires in APC matrix the blends become more viscous than pure APC with zero shear viscosity, 11.5 Pa·s. This observation is in line with the increase in the elastic character observed for the blends (Figure 4a). The other interesting observation is the increase of shear thinning behavior of the blends in comparison to pure APC (Figure 6). This indicates that pure APC is deformable or breakable at the lower shear rate compared to the blends with PbS nanowires, suggesting that the blends require more shear stress to deform.57 This observation suggests a stronger interaction of APC with PbS nanowires exist revealing the role of strong hydrophobic−hydrophobic interaction. A strong interaction between the hydrophobic acetyl groups of APC and the alky chains of TOA coating ligand of PbS nanowires is the origin of such interaction. Such an increase in the stability is also observed at the air−water interface (Figure 6), suggesting that blends are more stable than the pure APC. We have compared the UV−vis absorption spectra of APC, PbS solution, and the mixed films at APC:PbS ∼ 5:1 with different layer number prepared by the LS deposition technique (Figure 7a). Pure APC shows a prominent peak at 280 nm due to the n−Π* transition of CO bond,58 while the PbS nanowires shows humps at 320 and 420 nm. The LS films prepared from the monolayer of APC:PbS ∼ 5:1 show feature less spectra with a week hump at 350 nm, which is largely redshifted compared to the APC solution spectra. A comparison of this peak position with pure PbS nanowires reveals that the peak at 350 nm is a contribution from PbS nanowires within the blend. Interestingly, the n−Π* transition of pure APC is 15236

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Figure 7. (a) UV−vis absorption spectra of pure PbS nanowires (black circle), pure APC solution (red triangle), and the mixed LS films of APC:PbS = 5:1 with different layer number: 1 layer (blue curve), 3 layer (cyan curve), 5 layer (magenta curve), 7 layer (dark yellow curve), and 10 layer (purple curve) on the hydrophilic quartz substrates. Inset figure shows the absorbance maxima at 350 nm as a function of the number of layers, which indicates that regular and uniform deposition was carried out by the LS deposition technique. (b) FT-IR spectra of pure PbS nanowires (black curve), pure APC (red curve), and five different mixtures of PbS nanowires to APC at different volume ratios: 5:1 (blue curve), 5:2 (cyan curve), 5:3 (magenta curve), 5:4 (dark yellow), and 5:5 (purple).

Figure 8. Typical structure of a cellulose molecule.

absent in the mixed LS films, which is presumably due to the strong van der Waals interaction of the alkyl group of the PbS nanowires with the carbonyl portion of the acetyl group present in the APC molecule. Such interaction leads to the nonbonding electrons on oxygen atom getting restricted in the n-orbital, which does not contribute to the absorption process. The LS deposition process results in a uniform transfer of the monolayer with a transfer ratio ∼1 (inset of Figure 7a). Here Figure 7b shows the Fourier transform infrared spectroscopy (FTIR) spectra of pure PbS, pure APC, and LS films of five different mixtures of PbS to APC. PbS nanowires shows peaks in the region 3000−2850 cm−1 corresponding to C−H stretching frequency of alkylamine. The peak at 1230 cm−1 corresponds to the C−N stretching frequency of alkylamine. FTIR spectra of pure APC show CO stretching frequency at 1740 cm−1 and peaks in the region of 1240−1050 cm−1 corresponding to the C−O stretching frequency.59 The LS films of PbS nanowires and APC mixtures show peaks corresponding to C−O stretching at 1110 cm−1. Interestingly,the C−O peak appears stronger in intensity compared to pure APC for all the mixtures of PbS to APC. This implies a stronger interaction of the van der Waals force between the electron donating alkyl chain of PbS nanowires to the acetyl part of APC. Pure APC shows a weak peak at 610 cm−1. The peak becomes sharp and intense for different mixtures, which originates due to COCO deformation of the acetyl group, indicating strong alkyl−acetyl interaction within the mixtures. The Langmuir films of mixed monolayers show mainly the reorganization process of PbS nanowires within APC matrix owing to hydrophobic interaction. APC is a derivative of cellulose which possesses at least two distinct intramolecular hydrogen bonds between OH-3 and O-5′ and between OH-6 and OH-2′ positions (Figure 8).60 When hydroxyl groups in positions 2 and 6 are substituted by hydroxypropyl groups, the compound becomes HPC. In such a case, the intramolecular

hydrogen bonds become weak and the hydrophobicity of the macromolecule decreases, which makes HPC water-soluble. Further esterification of HPC in the presence of acetic acid substitutes hydroxyl groups of HPC by an acetyl group to yield APC. Because of the substitution of hydroxyl groups of HPC in positions 2 and 6, APC is hydrophobic in nature. Noticeably, the stiffness and stability of the mixed monolayer increase when TOA-coated PbS nanowires are mixed with APC to make a blend. In the monolayer, van der Waals interaction between acetyl group of APC and alkyl groups of TOA becomes dominant, enhancing the stiffness and stability. Indeed, the UV−vis and FTIR spectroscopy support the interaction between the acetyl group of APC and alkyl groups of TOA. It is well-known that cracks and holes appear in polymer thin films owing to thermal fluctuations,60 which could be responsible for the lower stability of the cellulose membranes. When PbS nanowires are incorporated to APC monolayer, it seems nanowires could work as nanofillers welding cracks and holes and thus imparting enhanced stability to the mixed monolayers. However, such a process continues up to a certain critical concentration of the nanowires corresponding to APC:PbS ≈ 5:1 mixture. Upon increasing percentage of PbS nanowires in the mixed monolayer, the acetyl−alkyl interaction decreases while the alkyl−alkyl interaction dominants, showing a trend toward pure PbS nanowires.

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CONCLUSIONS The mixed monolayer behavior of APC/PbS nanowire blend by incorporating nanowires in cellulose polymer monolayers at the air−water interface is reported here. A decrease in the monolayer area of pure APC upon mixing PbS nanowires revealed strong interaction of PbS nanowires with the APC. The reorganization of APC and PbS nanowires mixtures has been observed upon monolayer compression. Surface potential measurements confirm the alignment of dipoles in near parallel 15237

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orientation of the PbS nanowires within the APC matrix. Elasticity and compressibility studies using thermodynamic relations reveal improved robustness of the monolayer upon incorporation of PbS nanowires within the APC matrix. The area versus time curves reveals an enhanced stability upon incorporation of PbS nanowires up to a certain volume ratio. The rheological measurements also support the robustness of the mixture of APC and PbS nanowires in 3D bulk phase. van der Waals interaction between acetyl group of APC and alkyl groups of TOA becomes dominant, leading to the strong hydrophobic interaction to improve the stiffness and stability of the blends. Enhancement of stability of the cellulose monolayer blended with inorganic nanowires may be useful in designing and fabrication of robust polymer thin films or membranes useful for water purification technology, dielectric coatings, and biomedical membranes.

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ASSOCIATED CONTENT

S Supporting Information *

Figures S1 and S2. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (M.H.G.). *E-mail [email protected] (S.A.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank DST, India, and FCT, Portugal, for financial support under the Indo-Portugal bilateral proposal. S.K. thanks grant BPD/34096/2006, FCT Portugal, for financial support. S. Maji and A. H. Khan gratefully acknowledge CSIR, India, fellowship.

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