Preparation of Ordered Monolayers of Polymer Grafted Nanoparticles

Article pubs.acs.org/Macromolecules

Preparation of Ordered Monolayers of Polymer Grafted Nanoparticles: Impact of Architecture, Concentration, and Substrate Surface Energy Justin Che,†,‡ Kyoungweon Park,†,§ Christopher A. Grabowski,†,§ Ali Jawaid,†,§ John Kelley,†,§ Hilmar Koerner,† and Richard A. Vaia*,† †

Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright Patterson Air Force Base, Ohio 45433, United States ‡ National Research Council, Washington, D.C. 20001, United States § UES, Inc., Dayton, Ohio 45432, United States S Supporting Information *

ABSTRACT: Rapid fabrication of large area, ordered assemblies of polymer grafted (hairy) nanoparticles (PGNs) will enable additive manufacturing of novel membrane, electronic, and photonic elements. Herein, we discuss the relationship between select processing conditions, substrate surface energy, and canopy architecture on the hierarchical structure of sub- to monolayer PGN assemblies. Varying concentrations (10, 20, and 70 nM) of polystyrene (PS) grafted (σ ∼ 1 chain/nm2) gold nanoparticles (AuNP, r0 = 9 nm) were flowcoated onto surface-modified silicon wafers (γs ∼ 20 mN/m, hydrophobic to 80 mN/m, hydrophilic). The profile of an isolated gold−polystyrene (PS) PGN depends on substrate−canopy interface energy. At low substrate−PS interface energy (20 mN/m), the PS canopy spreads to maximize contact with the surface, whereas at high substrate−PS interface energy (80 mN/m), the chains minimize contact area resulting in a more compact, thicker PGN corona. This behavior is translated up to monolayer assemblies, where rougher, less-ordered assemblies with smaller AuNP−surface separation form on substrates with low interface energy. These films are also thinner with greater Au volume fraction, indicating that the segment density within the PS canopy depends on substrate surface energy. The impact of these processing parameters on PGN film formation parallels classic colloidal deposition even though the PS concentration is within the Landau−Levich regime for film formation from linear chains. The factors influencing local morphology, however, resemble those that affect polymer thin films. Using this understanding, we demonstrate fabrication within seconds of large area monolayer films with close-packed order.

■

INTRODUCTION Printed and hybrid electronics are emerging technologies to fabricate compliant flexible monolithic packages with integrated sensing, data processing, storage, communication, and power components for the internet-of-things.1 Nanoinks are a crucial enabling material technology necessary for numerous components, including interconnects, resistors, capacitors, and optical elements. Continual improvements of processing and formulation methods are required to enhance film quality, transforming random nanoparticle dispersions to ordered morphologies. Issues, such as aggregation, incomplete solidification, local disorder, residual additives, nonuniform interfaces, surface roughness, and porosity, all diminish film properties.2−4 Recently, polymer grafted nanoparticles (PGNs), also known as hairy nanoparticles (HNP), have been developed5−8 as potential candidates for nanoinks. The inherent single-component nature of PGNs9 eliminates issues related to miscibility and aggregation of constituents. Furthermore, PGNs differ from classic ligand functionalized © XXXX American Chemical Society

(stabilized) nanoparticles. Rather than relatively short molecules (1−3 nm) at high packing density (1−4 chains/nm2), surface functionalization of the PGN consists of long chain, high molecular weight polymers at lower surface densities. This affords entanglement and interpenetration between PGNs that lead to unique properties and morphologies.10−19 How the architecture of the PGN canopy (i.e., core radius (R), graft density (σ), and degree of polymerization (N)) interacts with processing parameters, such as solvent, substrate, and temperature, to influence the formation, structure, and stability of PGN films, however, is not well understood. Solution-based assembly of ligand stabilized nanoparticles has been extensively studied.20−26 Hierarchical structures with ordering across many length scales are fabricated via one of two general approaches:27,28 (1) liquid interface-mediated assembly Received: December 16, 2015 Revised: February 5, 2016

A

DOI: 10.1021/acs.macromol.5b02722 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

assembly process is driven by the evaporative regime.51,52 However, Stafford et al. reported that thicker, multilayer films of linear soft polymers can be obtained using faster deposition velocities and the assembly process occurs in the Landau− Levich regime.41 The difference in the film formation between colloidal nanoparticles and linear polymers is attributed to the disparities in solution properties and the corresponding evaporative and viscous forces as described by eq 1. PGNs are an example of soft colloids, whose characteristics and properties can be tuned between the two extremes of a hard sphere and soft linear polymer based on canopy architecture.53 This implies that the processing behavior of PGNs is anticipated to be an intermediate between, or with a combination of, factors from hard-sphere colloids and linear flexible polymers. Herein, we discuss the impact of PGN concentration, graft chain molecular weight, deposition velocity, and substrate surface energy on the morphology and degree of order of PGN monolayers formed via flow-coating. These factors impact assembly’s structure and quality across a range of length scaleslocal particle−particle spacing to near-neighbor order to uniformity across mm2in a nontrivial manner. This work is structured into three sections: (1) the influence of PGN− substrate interactions on individual PGN morphology; (2) the influence of PGN−substrate and PGN−PGN interactions on monolayer film morphology with respect to concentration and substrate surface energy; and (3) optimization of processing conditions to obtain a uniform surface coverage of PGN over a large surface area. Using a combination of grazing-incidence small-angle X-ray scattering (GISAXS), atomic force microscopy (AFM), and X-ray reflectivity (XRR), the local and global structure depends on substrate surface energy in a nonintuitive manner. When the interface energy between substrate and canopy is large and thus favoring poor wettability, the polymer canopy collapses around the nanoparticle core, particle− particle distance is minimized, and the assembly is locally ordered into a hexagonally close-packed (HCP) arrangement. When the difference in interface energy is small, the polymer canopy extends to interact with the substrate, resulting in isolated clusters of PGN strings and poorer local ordering. This understanding of the correlation between these processing conditions (i.e., deposition velocity and concentration) and the structural morphology is a critical step to understand additional processing parameters (i.e., temperature, pressure, solvent) and to optimize fabrication to maximize ordering and minimize defect density in thin films of PGNs.

or (2) direct assembly on solid substrates. Langmuir−Blodgett (LB) and Langmuir−Schaefer (LS) techniques at air−water interfaces29−34 are the most common liquid interface methods. Relative surface energies drive short-ligand grafted nanoparticles to the dynamic interface, where evaporation and capillarity force close-packed structures. Mechanical transfer to a solid support yields thin films suitable for fundamental studies. Printing and additive manufacturing, however, require direct assembly on solid surfaces via fewer, simpler steps. Dropcasting, spin-coating, and spray-coating35−40 are common options; however, obtaining highly ordered, uniform assemblies across large surface areas is challenging. Alternatively, horizontal and vertical deposition of colloids via convective assembly yields large scale, close-packed structures, but the process is extremely slow and requires long time scales (several hours to days).28 Horizontal deposition via flow-coating,41−47 meanwhile, enables faster film formation (several minutes) by balancing the capillary forces holding the solution between the blade and substrate with the frictional drag exerted on the solution as the relative position of the blade and substrate is translated at a fixed velocity (v). Depending on the relative viscous and evaporative forces, two regimes can be observed the Landau−Levich and evaporative47−49which are defined by the crossover velocity (v*) ⎛ γ ⎞2/5 ν* = k ⎜ ⎟ Q evap3/5 ⎝ μ⎠

(1)

where k is a constant related to the geometry, γ is the solution surface tension, μ is the solution viscosity, and Qevap is related to the evaporative flux of the solvent. Film formation in the Landau−Levich regime (v > v*) occurs when solvent evaporation is slower than the coating process (i.e., low-volatile solvents).49,50 Viscous forces pull liquid from the meniscus at the blade, resulting in the formation of thicker films at faster velocities. The deposited film is wet and continues to evolve locally as the remaining solvent evaporates. Conversely, the evaporative region occurs when evaporation is faster than the coating process (v < v*). The film thickness decreases with increasing deposition velocity, and assembly is driven by convective flow of particles and capillary forces. Ordering requires low particle−surface interactions so that the particles can readily diffuse across the substrate. Classic convective assembly of colloids use extremely slow v (typically μm/s) and low volatile solvents (e.g., H2O), whereas flow-coating techniques employ faster v (mm/s) and more volatile solvents (organics). Thus, understanding the impact of PGN architecture, solvent, and substrate on film formation is crucial to determine the maximum deposition velocity that yields largearea, uniform highly ordered PGN assemblies within the processing constraints of an application.28 Thin film formation of close-packed colloidal nanoparticles via flow-coating has been recently explored; however, its translation to PGNs is not straightforward due to the unique hard-core/soft-shell architecture of a PGN. In general, the surface morphology (i.e., discontinuous islands, bicontinuous network, continuous film) and thickness (submonolayer to multilayers) of the thin films formed via flow-coating requires precise control of processing conditions and solution properties, including deposition velocity, concentration, solvent, surface tension, and viscosity. For example, Prevo et al. reported that thicker, multilayer films of colloidal nanoparticles can be obtained using slower deposition velocities and that the

■

EXPERIMENTAL SECTION

Synthesis of Polystyrene Grafted Gold Nanoparticles (AuNP-PS). Au NP Synthesis. Gold nanoparticle (AuNP) synthesis followed the standard citrate reducing methodologies developed by Turkevich and Frens.54,55 350 mL of deionized water was boiled in a round-bottom flask while stirring. The temperature of the flask was maintained at 100 °C. 87.5 mg of hydrogen tetrachloroaurate trihydrate (HAuCl4·3H2O) (Sigma-Aldrich) was added to a glass vial and dissolved with 5 mL of H2O. The resulting solution was added to the boiling water. 150 mg of tribasic sodium citrate (Na3C6H5O7· 2H2O) (Sigma-Aldrich) was added to a glass vial and dissolved with 10 mL of H2O. The resulting solution was added to the reaction and allowed to stir with heat for 15 min. The color of the solution changed from pale yellow to clear (after 1 s), to black (after 10 s), to purple (after 20 s), to ruby red (after 2 min), which is a direct indication of the growth of the AuNPs. After 15 min, the solution was removed from heat and allowed to cool in a cold water bath. Once the solution B


Article

Macromolecules was cooled, the aggregates were filtered using a 0.45 μm polypropylene (PP) syringe filter. The solution was then purified and concentrated to 10 mL via diafiltration using an Omega 70k membrane (Pall Corporation). Polystyrene Grafting to AuNPs. In order to prepare gold nanoparticles capped with polystyrene, 5 mL of 10 nM AuNP solution was added to 5 mL of 25 mg/mL of thiol-terminated polystyrene with Mw of 12 000 and 53 000 g/mol (Polymer Source, Inc.) in tetrahydrofuran (THF) (Sigma-Aldrich) into a 20 mL glass vial. The resulting solution was vigorously mixed for 1 min. The reaction leads to instantaneous aggregation, and the aggregates stick to the side wall of the glass vial, leaving behind a mixture of colorless water and THF mixture. The water and THF mixture was decanted and the remaining moisture in the glass vial was removed by drying with a heat gun at 100 °C for 3 min. The resulting material consists of AuNPs grafted with thiol-terminated polystyrene (AuNP-PS), which were redispersed in 1 mL of THF and sonicated for 15 min to obtain a perfect dispersion of particles. The resulting solution was centrifuged three times at 12 000 rpm to remove free polystyrene. AuNP-PS Characterization. Table 1 summarizes the characterization of the AuNP-PS with 20 nm core diameters grafted with

reduced the free polymer content to 8%. It is to noted that the increase in number of centrifugations leads to an instability of the polymer chains that are grafted to the core. The additional centrifugation step that reduces free polymer content by 2% was not significant as compared to the increase in polydispersity of the grafting density. Thus, we will focus only on the samples that have been centrifuged three times. It is also interesting to note that the SEC results for free polymer content of thiol-terminated PS contain a bimodal distribution. This is associated with the tendency of two thiol groups to couple with each other to form a disulfide bond. Thermal gravimetric analysis (TGA) (Q500, TA Instruments) was used to determine the graft density (σ) or the number of chains grafted to the surface of each NP. The weight loss obtained from TGA was accounted for the free polymer content, and the graft density of Au20-PS12k and Au20-PS53k was determined to be 1.1 ± 0.10 and 0.6 ± 0.05 chains/nm2, respectively. Note that if the free polymer was not considered, the graft density would increase by 0.3 chains/nm2. Fabrication of Si Substrates with Controlled Surface Energies. Silicon wafers with controlled surface energies were prepared following established silane treatment procedures by Pasternack and Kannan et al.57,58 Silicon (100) wafers (Ted Pella, Inc.) were cut into 2.5 × 1.0 cm2 pieces and UVO treated for 20 min to remove organic contaminants and activate surface silanol groups. The wafers were then sequentially rinsed and sonicated with ethanol, 50% v/v ethanol:toluene, and toluene for 2 min to ensure the removal of contaminants and excess water at the surface. The rinsed wafers were immediately submersed into either a 1 wt % 3-aminopropyltriethoxysilane (APTES) (Sigma-Aldrich) or n-octyltrichlorosilane (OTS) (Gelest, Inc.) for 1 h. After silanization, the wafers were rinsed and sonicated sequentially using the same solvents in the reverse order for 2 min to remove any unreacted silanes. The substrates were subsequently rinsed with deionized water and blown dry with nitrogen. Finally, the wafers were then dried in an oven at 120 °C for 1 h under vacuum to promote complete condensation of the silane monolayers. Contact angles on the treated substrates were measured using sessile drop mode on an Attension optical tensiometer (Biolin Scientific). Substrate surface energies (SE) were calculated using Owens−Wendt geometric mean approach59 using water and methylene iodide. The thickness and surface roughness of the silanized layers were measured using X-ray reflectivity (XRR) (SmartLab, Rigaku) as shown in Figure S3. The XRR curves were fitted using MotoFit60 macro from Igor Pro (WaveMetrics). Table 2 summarizes the water contact angle, surface energies, silanized layer thickness, and surface roughness of various treated substrates with UVO, APTES, and OTS.

Table 1. Characteristics of AuNP-PS Samples core radius (nm) Mn (g/mol) σ (chains/nm2) core volume (%)

Au20-PS12k

Au20-PS53k

9.1 ± 1.3 12000 1.1 ± 0.1 12.7

9.1 ± 1.3 53000 0.6 ± 0.05 6.2

molecular weights (Mn) 12 000 and 53 000 g/mol thiol-terminated polystyrene (denoted as Au20-PS12k and Au20-PS53k, respectively). The full analytical characterization of AuNP-PS PGN is shown in the Supporting Information Figures S1 and S2. Small-angle X-ray scattering (SAXS) (S-MAX3000, Rigaku) and transmission electron microscopy (TEM) (CM200, Philips) were used to determine the core size of the gold nanoparticles. From SAXS, the intensity versus the scattering vector, q, was fitted with the form factor of a sphere, F(q), with the addition of a Schultz distribution to account for the polydispersity of the system:

∫0

|⟨F(q)⟩|2 = |

∞

F(q , R ) = 4πR3

F(q , R )f (R ) dR |2

(2)

sin(qR ) − qR cos(qR ) (qR )3

⎛ z + 1 ⎞ z + 1 Rz exp[− (z + 1)R /R m] f (R ) = ⎜ ⎟ Γ(Z + 1) ⎝ Rm ⎠

(3)

Table 2. Characteristics of Surface Treated Silicon Substrates (4) substrate

where R is the radius of the sphere, Rm is the mean radius, Γ is the Gamma function, and z is the width of the size distribution. The full analytical solution of eq 2 can be found elsewhere.56 Dilute concentrations in THF (1 mg/mL) were used to determine the core size radius for both Au20-PS12k and Au20-PS53k to be 9.1 ± 1.3 nm, which are consistent with the core diameters obtained from TEM. In addition, there was no upturn at low scattering vectors, which confirms the dispersion of the PGNs in solution without aggregation. Since these PGNs are grafted with PS chains, the polymer canopy acts as a shield that can sufficiently remove the van der Waals (vdW) attraction between the NP cores and prevent aggregation. Size exclusion chromatography (SEC) was used to determine the free polymer content in each sample prior to casting films using an injection system (1260 Infinity, Agilent Technologies) coupled to a differential refractive index detector (Optilab T-rEX, Wyatt Technologies) eluting with THF at a flow rate of 1.0 mL/min. For the AuNP-PS PGNs used in this work, after three times centrifugation, the free polymer content was estimated from SEC to be approximately 10% as shown in Figure S2. An additional fourth centrifugation only

UVO cleaned Si APTES-Si OTS-Si

water contact angle (deg)

surface energy (mN/m)

6.3 ± 0.9

80.8 ± 1.5

65.1 ± 3.3 95.0 ± 5.4

44.8 ± 1.8 23.3 ± 1.2

thickness (nm)

roughness (nm) 0.4 ± 0.1

2.0 ± 0.3 1.2 ± 0.2

0.4 ± 0.1 0.5 ± 0.1

Formation and Characterization of PGN Films. Flow-Coating. In this work, a custom-built flow-coater (similar to the design by Stafford et al.41) with a motorized translational stage was used to prepare thin films of polymer grafted nanoparticles on substrates. The angle and gap height between the blade and substrate were 5° and 200 μm, respectively. 10 μL of PGN dispersion in THF was deposited between blade and surface. The substrate was moved via a translation stage at constant speeds (0.1−10 mm/s) and air-dried. Concentration of the AuNP-PS deposition solution was determined with UV−vis spectroscopy (Cary 300 Bio, Varian), where the extinction coefficient at the surface plasmon peak of 530 nm is 7.1 × 108 M−1 cm−1 (based on the size of the AuNP61). C


Article

Macromolecules

Figure 1. Structural morphology of an isolated PGN on different substrate surface energies. (a) AFM images of an individual PGN of Au20-PS12k and Au20-PS53k on various substrate surface energies. The scale bar for all the images represents 25 nm. (b) Radially averaged line scans of each AFM image of Au20-PS12k (left) and Au20-PS53k (right): 20 mN/m (green), 40 mN/m (blue), and 80 mN/m (red). Note that the height and lateral dimensions are on different scales, and the profiles should be flatter than it appears. (c) Schematic model representing the changes in PGN structure with different substrate SE εP−S corresponds to the interfacial interaction between the polymer canopy and substrate. It is interesting to note that the flow-coated thin films ( q1, as well as the breadth of S(q2), both increases due to the appearance of the (11) reflection (q2/q1 = 31/2). The best-fit model is based on the ability to capture these features and to obtain a statistically better standardized residual and R2.

Figure 2. Volume of PS canopy for isolated Au20-PS12k (squares) and Au20-PS53k (circles) on different substrate surface energies. The dashed lines correspond to an independent determination of canopy volume assuming bulk PS density (1.05 g/cm3) and the measured volume fraction of PS in PGNs.

■

difference between the total volume of a PGN (VPGN) determined from AFM (see Experimental Section) and the volume of the NP core (VNP). The dashed lines correspond to an independent determination of canopy volume assuming bulk PS density (1.05 g/cm3) and the measured volume fraction of PS in PGNs (Table 1). Overall, the canopy volume is consistent with composition, irrespective of shape. Also within experimental uncertainty, VCanopy decreases slightly with increasing substrate SE, which would imply that the substrate impacts the extent of polymer densification. It is interesting to note that the isolated PGNs on lower substrate SE (20 mN/m) show a degree of distortion from spherical to ellipsoidal shape (20%) that was found to be independent of the flow-coating direction employed during sample preparation. In addition, this distortion was constant irrespective of the angle between the sample and scan direction, indicating that it is not an AFM artifact but arises from the interaction of the PGN with the substrate. Similar distortion was previously observed in polystyrene-b-poly(1,1′,2,2′tetrahydroperfluorooctyl methacrylate) (PS-b-PFOMA) micelles on silicon nitride due to a strong preferential affinity between PFOMA block and the polar substrate.72 Further investigation is necessary to elucidate the relative role of favorable enthalpic interactions and conformational constraints imposed by tethering on the symmetry breaking of the PS canopy on the low SE substrates. Structural Morphology of PGN Films: Surface Energy. The structure and assembly of a PGN monolayer will be determined by the attractive and repulsive forces between the PGN−substrate (WPGN−Subs) and PGN−PGN (WPGN−PGN). The total interaction energy (W) is then the sum of these contributions as

RESULTS AND DISCUSSION Morphology of Isolated PGNs: Surface Energy. Figure 1 summarizes the structure of an isolated Au20-PS12k and Au20-PS53k on silicon substrates with different surface energies (SE) deposited via flow-coating a dilute THF solution (1 nM). The shape and dimensions of the PGNs were obtained by taking radially averaged profiles through each individual PGN as shown in Figure 1a,b. In general, the shape of the PGN depends on the interaction energy between the polymer canopy and the substrate. As the interaction energy decreases, the width of the PGN increases and its height decreases. This trend was observed for PGNs with grafted chains of different molecular weights. A schematic model of the changes in PGN structure with substrate SE is shown in Figure 1c. Lelievre et al. showed that a grafted PS shell of 4.4 nm on a AuNP (σ ∼ 1.0 chains/nm2, Mn PS = 6500 g/mol) was sufficient to completely overcome the vdW attraction between core and substrate.24 Since the AuNP-PS PGNs discussed here have thicker canopies comprising higher molecular weight chains, core−substrate interactions should be minor relative to the interactions between the polymer canopy and surface. The total energy is therefore dominated by the interaction between the grafted PS chains and the terminal functional groups on the surface of the substrate (εP−S). The interfacial interaction between nonpolar PS and native oxide layer of silicon yield a large positive Hamaker constant.68−71 These nonfavorable interactions (80 mN/m, large εP−S) result in a collapsed polymer canopy, where the grafted PS chains contract around the Au core to minimize the contact area with the polar silanol groups on the surface. In contrast, the nonpolar OTS surface E


Article

Macromolecules

Figure 3. AFM images (low, medium, and high magnification) of Au20-PS12k (left) and Au20-PS53k (right) films formed by flow coating THF solutions at various concentrations and substrate surface energies. The scale bars for each row are shown in the first column. Features from the AFM images include PGNs, free polymer that resides in the samples (dashed white circles at 10, 20, and 70 nM), and defects (dotted black circles at 70 nM). Enlarged AFM images are shown in Figures S4−S9. W = WPGN − Subst + WPGN − PGN ∼ εC − S + εP − S + εC − C + εP − P (8)

interactions between two gold cores can be neglected when the interparticle distance is greater than 10 nm.24,73 Assuming core−core and core−substrate attractions can be negligible for the PGNs examined here since the interparticle distances are greater than 10 nm (see X-ray scattering results below), the total interaction energy can be simplified to

where εC−S, εP−S, εC−C, and εP−P are the relative interaction energies between the nanoparticle core−substrate, the polymer canopy−substrate, the nanoparticle core−core, and polymer− polymer, respectively. Figure S15 shows the calculated interparticle potentials for Au20-PS12k and Au20-PS53k based on Hamaker’s derivation for the vdW attraction between two spheres and the de Gennes’ approximation of steric repulsion between spheres with concentrated polymer brushes. Because of the graft density and molecular weight of the PS chains, the vdW attraction between the gold cores is screened by the canopy, even in the assembly. This is consistent with Yockell−Lelievre findings that also demonstrated that the vdW

W ∼ εP − S + εP − P

(9)

This contrasts alkane-capped metallic nanoparticles, where the strong vdW attractions between the metallic cores and substrate play a significant role. Since to an initial approximation εP−P is the same for all the polystyrene functionalized nanoparticles, the impact of substrate surface energy on film structure and F


Article

Macromolecules

been attributed to depletion attraction arising from a reduced conformational freedom of tethered chains along the axis of closest approach or due to corona architectures with shortrange hard and long-range soft-repulsive potentials that lead to an intermediate plateau in the particle−particle potential.75,76 Experimentally though, the relationship between string density and order on SE indicates that the substrate is playing the crucial, but yet to be elucidated, role in string formation. Figure 5 summarizes the surface profiles of the monolayer assemblies at various SE surfaces. The thickness of the film

assembly in a given solvent should follow similar trends to those observed for isolated PGNs. Figure 3 summarizes the morphology of sub- to monolayer films of Au20-PS12k and Au20-PS53k flow-coated (10 mm/s) from THF solutions (10, 20, and 70 nM) on different SE substrates (γs = 20, 40, and 80 mN/m). The general trends and features are similar for both PS grafts of different molecular weight as anticipated from the above energetic arguments. At 80 mN/m, PS’s unfavorable interfacial interaction with the native oxide of silicon leads to PGN aggregation, and εP−S > εP−P. Figure 4 shows the corresponding AFM line scans at 80

Figure 5. Corresponding AFM line scans from Figure 3 of Au20PS12k (left) and Au20-PS53k (right) monolayer films formed by flow coating 70 nM on various substrate SE.

Figure 4. Corresponding AFM line scans from Figure 3 of Au20PS12k (left) and Au20-PS53k (right) films formed by flow coating THF solutions at various concentrations on substrate SE = 80 mN/m.

slightly decreases as substrate SE decreases, consistent with the results for an isolated PGN. The amplitude of the profile is bimodal, with fluctuations on the order of the diameters of the PGN (∼20 nm) along with deviations of 1−3 nm. The former reflects the quantized nature of the thin film, which consists of n-layers of PGNs. The latter corresponds to the local features of the PGN, packing, and the grafted polymer. This fine scale surface roughness is significantly greater for Au20-PS12k than Au20-PS53k for all surfaces, potentially reflecting the greater ability of higher molecular weight chains to fill space. Lastly, it is interesting to note the presence of holes the size of a few particles in the monolayers (Figure 3, dotted black circles). Similar holes were reported by Muralidharan et al. for liquid− air interface assembly of AuNP-PS monolayer formation and transfer to silicon.73 For Au20-PS12k and Au20-PS53k, the number of particles per defect was uniform on all substrate SE, 1.8 ± 0.9 and 6.0 ± 1.4 particles, respectively (Figure S10). This implies that the cluster of missing particles is a stable intrinsic defect of the PGN assembly. The size and energy of this defect depend on canopy architecture and are independent of substrate−surface interactions. However, the exact origin, free energy, and dynamics of these defects still remain to be determined. Finally, a small fraction of free polymer resulting from the grafting-to process used to prepare the PGNs can be seen in the AFM. In this case, the free thiol-terminated PS and disulfidelinked PS (Figure S2) leads to a ratio of grafted chain (P) to matrix (N) molecular weight of P/N ≥ 1 with 1/Rcore ∼ 0.1. Recent studies of the miscibility of free polymer with the PGN78−81 indicate that for these parameters the linear polymer will be immiscible with chain grafts. This immiscibility is clearly

mN/m. At low concentrations (10 and 20 nM), submonolayer films are composed of discrete clusters of highly ordered, hexagonally close-packed PGNs. The size and number of these clusters increase with increasing concentration. At 70 nM, a continuous monolayer was obtained. AFM height profiles reflect the progress of clusters to a monolayer with missing PGNs (defects). Overall, the thicknesses of the clusters and uniform film are comparable and consistent with the dimensions of a single PGN. For lower substrate SE (20 and 40 mN/m), a more favorable interaction between the polymer canopy and substrate results in a larger fraction of isolated particles in submonolayer films (e.g., 10 nM), and thus εP−S < εP−P. The number of small agglomerates and the length of strings of interconnected particles increase with concentration, indicating that attractive polymer−polymer interactions also play a crucial role. If particle−substrate interactions were strongly attractive and dominate, the morphology would reflect a random sequential adsorption process.74 Additionally, the number and length of strings are greater for the higher molecular weight grafts, which is consistent with the ability of PS to make entanglements between particles (Me(PS) = 16 600 g/mol).77 At 70 nM, continuous monolayers are also obtained. Note that if these chains and clusters were preformed in solution, the dependence of morphology on SE would not been seen. Prior reports have implied that chain formation is a result of a particle potential with short-range attractive and long-range repulsive components.13 As noted above, however, due to the amount of PS in the canopy, the particles cannot approach sufficiently close for attractive core−core interactions. String formation has also G


Article

Macromolecules

Figure 6. GISAXS images of Au20-PS12k (left) and Au20-PS53k (right) at various concentrations and substrate surface energies. A scale bar of the scattering vector for all the GISAXS images is shown in the bottom left. The incidence angle (α) is 0.10°.

Figure 7. In-plane structure factor (S(qy)) profiles (obtained at constant qz = 0.03 A−1) of Au20-PS12k and Au20-PS53k at various concentrations and substrate surface energies [20 mN/m (green), 40 mN/m (blue), and 80 mN/m (red)]. The model fitting for each curve using paracrystalline (when S(q1) > 2.85) or Percus−Yevick (when S(q1) < 2.85) is shown in black. The paracrystalline model was applied only to Au20-PS12k (80 mN/ m at 10 and 20 nM) and Au20-PS53k (80 mN/m at 10 nM). All other curves were fitted using Percus−Yevick. The profiles are offset vertically for clarity. Short horizontal gray lines right-hand side of each curve denote S(q) = 1 values.

seen in the AFM of submonolayer films from low concentration THF solutions (shown as dashed white circles at 10 and 20 nM). For example, at high substrate SE, the free polymer is found at the periphery of the PGN clusters due to the relatively unfavorable interactions with the substrate. On the other hand, PS has more favorable interactions with lower SE substrates that lead to isolated clusters of free polymer dispersed across the surface. At concentrations sufficient to form a monolayer (70 nM), the free polymers at all SE surfaces are situated in

isolated clusters at the air interface (shown as dashed white circles for the low-magnification AFM). Quantitative information about the degree of local ordering within the PGN assemblies can be obtained from GISAXS. Figure 6 summarizes GISAXS patterns from the various PGN films. Figure S11 summarizes horizontal line cuts of intensity, I(qy), versus the in-plane scattering vector, qy. Strong GISAXS reflections are observed from both Au20-PS12k and Au20PS53k on 80 mN/m surface, consistent with the AFM H


Article

Macromolecules

Figure 8. In-plane structure of PGN assemblies on different substrate surface energies. (a) Intensity of the first structure factor peak, (S(q1)), and (b) in-plane interparticle distances with respect to substrate surface energies for Au20-PS12k (left) and Au20-PS53k (right) at various concentrations [10 (filled circles), 20 (filled triangles), and 70 nM (filled squares)]. The dotted line in (a) represents the order−disorder transition at S(q1) = 2.85.

high SE substrate decreases as a monolayer is approached (circle to triangle to square), indicating that even though the surface contains more particles, the fraction that is highly correlated is lower. In other words, the PGNs in the isolated clusters of the submonolayer (circles) are locally more ordered than they are in the complete monolayer (squares). The SE transition between these trends has a dependence on canopy molecular weight, as seen from the data for Au20-PS12k and Au20-PS53k at 40 mN/m. Also on the high SE substrate, the degree of order within the submonolayers surpasses the criteria for a crystalline lattice as reflected by the Hansen−Verlet criterion.82 The Hansen−Verlet rule of freezing defines order when the intensity of the first structure factor peak (SHV(q1)) exceeds a critical value of 2.85 and has been previously applied to colloidal systems to distinguish between densely disordered and ordered assemblies.83−85 The long-range lattice correlations are also manifested in the position of the higher order reflections, which exhibit a qn/q1 ratio of 1:31/2:2:71/2 (n = 1− 4). This represents a hexagonally close-packed (HCP) structure. A paracrystalline model provided the statistically best fit (based on the standardized residual and R2) to these experimental curves (Figure 7, black lines). The model describes ordered, densely correlated systems, but with a progressive loss of long-range ordering due to finite size effects and lattice defects (details shown in Figure S12). Conversely, the positions of higher order reflections of films below the criterion value (i.e., low SE substrates) had a qn/q1 of 1:2:3.

observations. The intensity of these reflections decreases as a submonolayer approaches a monolayer, indicating that packing constraints decrease local PGN order. As the substrate SE decreases (20 and 40 mN/m), the number and intensity of reflections decrease, also consistent with the small islands, strings, and less-defined features seen in the AFM. Figures 7 and 8 summarize the in-plane structure factor, S(qy), and associated analysis from the various Au20-PS12k and Au20-PS53k films. Recall that the X-ray scattered intensity is dominated by the AuNP core due to its substantially greater contrast in electron density with respect to the PS canopy (form factor is determined only by AuNP). However, the polymer chains indirectly impact the scattering pattern; that is, they modulate the AuNP core correlations (structure factor is AuNP and PS). S(qy) reflects the probability of finding partiles that have correlations at interparticle distance of 2π/qy. The intensity of the first peak (S(q1)) is proportional to the number density (i.e., fraction) of particles that have correlations at distance 2π/q1. Thus, larger values of S(q1) correspond to higher local order and longer range correlations, whereas smaller values reflect more local disorder. When S(q1) is 1, the particles are dilute or have no interparticle correlations in a monolayer. On low SE substrates, the intensity S(q1) is similar as submonolayers converge to a monolayer assembly (Figure 8a), indicating local PGN correlations are weak irrespective of the particle surface density. In contrast, the intensity S(q1) from the I


Article

Macromolecules Here, the experimental S(q) was described by a Percus−Yevick model of a disordered system with weak correlation and shortrange order. Overall, this trend implies an order−disorder transition from crystalline to liquid-like (i.e., absence of longrange translational symmetry) occurs as particle surface density increases on higher SE surfaces. Srivastava et al. reported analogous behavior for solution assembly (Langmuir techniques) of gold nanoparticles grafted with DNA chains.86 By increasing the ionic strength, the interparticle interactions were switched from electrostatic repulsion to site-specific attraction between DNA canopies. In concert, the local morphology transitioned from an ordered HCP assembly to isolated stringlike clusters of particles, respectively. Finally, the distance between Au cores within the PGN films depends on substrate SE, surface particle density, and molecular weight of the PS graft (Figure 8b). Recall that the shape of the PS canopy of isolated PGNs also depended on SE (Figure 1). A schematic model of the changes in morphology of the PGN assemblies with respect to SE is shown in Figure S13. For monolayers of Au20-PS53k (high molecular weight graft), the interparticle spacing is approximately 47 nm, irrespective of substrate SE. This is consistent with simple volume filling arguments (45 nm). However, how the interparticle distance within the submonolayer features evolves with surface particle density is drastically different for low (20 mN/m) and high SE (80 mN/m) surfaces. For example, at SE of 80 mN/m, the particle distance expands slightly from 43 to 47 nm as clusters of Au20-PS53k merge into a monolayer. The core−core distances within the small agglomerates and chains on the low and intermediate SE substrates, however, are much shorter (∼30 and 40 nm on 20 and 40 mN/m, respectively). Similar general behavior is seen for PGNs with a lower molecular weight graft (Au20-PS12k). Here, the interparticle spacing within the monolayer on intermediate and high SEs is approximately 31 nm, also consistent with volume filling arguments (34 nm). However, the interparticle spacing is substantially shorter on low SE (26 nm). These results are consistent with the structural morphology of an isolated PGN. Although the width of an isolated PGN is broader on low SE (Figure 1), the surface roughness of the film increases as submonolayer converges to monolayer assembly (Figures 4 and 5). At low SE, the canopy preferentially interacts with the substrate and thus geometrically enables neighboring particles to approach closer than the spherical closest approach. Additionally, as particle surface density increases, more canopy interpenetration increases resulting in more canopy−canopy interpenetration due to the finite attractive εP−P. To obtain a 3D description of the monolayer assemblies, XRR was used to obtain quantitative information on layered structures perpendicular to the substrate (i.e., along the vertical direction of the PGN assemblies). The extent of interaction between the polymer canopy and substrate will dictate how far the AuNP cores are situated from the substrate surface. Figure 9 shows the experimental XRR results for monolayer assemblies of Au20-PS53k on various SE surfaces. Assuming the AuNP cores are uniformly situated within the monolayer, the measured XRR profiles were fit to a model of the PGN assembly consisting of three layers (1: PS; 2: AuPS; 3: PS) on a surface modified silicon substrate. Details of the model (Figure S14) and fitting procedure are discussed in the Supporting Information. Table 3 summarizes the model fit parameters to the scattering length density (SLD), thickness, and roughness of each of the layers on various SE surfaces. Note that the layer

Figure 9. XRR of out-of-plane structure of PGN assemblies. (a) Fivelayer model to represent PGN assemblies on surface modified silicon substrate. (b) Experimental XRR profiles of Au20-PS53k monolayer assemblies at 70 nM on various substrate surface energies [20 mN/m (green), 40 mN/m (blue), and 80 mN/m (red)]. The inset represents the enlarged reflectivity profile at low qz.

containing the AuNPs (layer 2) is not 100% AuNP but contains both AuNP and PS. Therefore, the SLD of this layer is much lower (SLD ∼ 16 × 10−6 A−2) than that of pure Au (SLD = 123 × 10−6 A−2). The distance between the AuNP and substrate can be observed in the thickness of layer 3. With decreasing substrate SE, the AuNPs are situated closer to the substrate, which is consistent with the AFM results observed for isolated PGNs and monolayer assemblies. Conservation of volume will then imply either an increase in the Au−Au spacing within layer 2 (and commensurate reduction in SLD) or an increase in the thickness of the PS surface layer 1. Unfortunately, the roughness associated with layer 1 is too great to draw a conclusion about its change in thickness. As for the SLD of AuPS layer 2, it increases slightly with decreasing substrate SE, rather than decreasing. This is clear from the shift in the critical angle of the assembly to higher qz (shown in the inset in Figure 9). The critical angle of the AuPS layer [θc = (2·SLD)1/2] was calculated to be qz = 0.0464, 0.0467, and 0.0471 A−1 for 80, 40, and 20 mN/m, respectively. The resulting mass density (ρ) within the AuPS layer 2 was calculated to be 2.20, 2.23, and 2.27 g/cm3 for 80, 40, and 20 mN/m, respectively. The increased density of layer 2 corresponds to a decrease in the lateral interparticle distance and is consistent with the slight variation in particle−particle distance observed in GISAXS (Figure 8b). Overall, this implies that as the particles are coming into closer contact with the substrate at low SE surfaces, the AuNP cores are also packing closer together. Interestingly, the overall thickness of the film (obtained from the total sum of the thickness of each layer) is also smallest for lower SE surfaces albeit the absolute certainty of this conclusion is challenged by the aforementioned trend in surface roughness. However, this trend is also consistent with the aforementioned AFM. The volume fraction of AuNP to PS in the direction perpendicular J


Article

Macromolecules Table 3. XRR Analysis of Out-of-Plane PGN Assemblies 80 mN/m material

X-ray SLD (10−6 A−2)

0 1 2

air PS Au PS

3 4 5

PS SiO2/APTES/OTCS Si

1.00 9.61 16.2 (80 mN/m) 16.4 (40 mN/m) 16.7 (20 mN/m) 9.61 18.9/8.90/10.3 20.1

layer

40 mN/m

20 mN/m

thickness (nm)

roughness (nm)

thickness (nm)

roughness (nm)

thickness (nm)

roughness (nm)

14.0 ± 2.1 18.0 ± 1.8

4.0 ± 0.6 0.5 ± 0.1

14.0 ± 2.1 18.0 ± 1.8

5.0 ± 0.8 0.5 ± 0.1

14.0 ± 2.1 18.0 ± 1.8

6.0 ± 0.9 1.0 ± 0.2

10.8 ± 1.6 0.5 ± 0.1

3.0 ± 0.5 0.1 ± 0.0 0.1 ± 0.0

9.0 ± 1.4 2.0 ± 0.3

3.8 ± 0.6 0.4 ± 0.1 0.5 ± 0.1

8.5 ± 1.3 1.2 ± 0.2

3.5 ± 0.5 0.5 ± 0.1 0.3 ± 0.1

increased or concentration decreased, the quantity of PGN deposited decreased. For example, multilayer films were observed at low deposition velocities, whereas submonolayers were observed at higher deposition velocities. This implies that due to the volatile solvent (THF) and low PGN concentrations ([nM]), film formation occurs within the evaporative regime. This behavior is more akin to colloidal deposition as opposed to polymer coating, where linear polymer flow-coating is conducted in the Landau−Levich regime.47,48 For example, in the latter, Stafford et al. reported that the thickness of PS thin films (using [μM] in toluene) was linearly dependent on solution concentration and deposition velocity.41 It is interesting to note that the [nM] of PGN in solution corresponds to [μM] of grafted chains in solution, which is comparable to those used by Stafford et al. The unique architecture of a PGN leads to processing conditions that reflect colloidal behavior, but the local structure within PGN assemblies is determined by polystyrene. In summary, the regime of optimal deposition conditions to form monolayer films are denoted as the blue line in Figure 10. An example of the preparation of large-scale, highly ordered monolayers of PGN is shown in the high-resolution SEM and optical images in Figure 11. Here, Au20-PS53k (70 nM) were flow-coated at 5 mm/s on 80 mN/m substrate up to ∼4 × 2 cm2 in dimension. Large scale, uniform ordered assemblies of PGNS can be obtained up to macro dimensions using precise processing conditions.

to the surface was calculated to be 6.0, 6.9, and 7.1% for 80, 40, and 20 mN/m, respectively. Since the volume of the AuNP core will not change under these conditions, this could imply that conservation of the PS canopy volume does not occur across different substrate SE and that the relative energy of canopy−substrate and canopy−canopy interactions impacts local segment density. This is analogous to prior conclusions drawn from Tg investigations of ultrathin film polymer studies, where substrate-dependent shifts in Tg and associated cooperative segment relaxation have been related to the nearsurface segment density profile.87−91 Structural Morphology of PGN Films: Deposition Velocity. The deposition speed relative to the solvent evaporation rate is a crucial parameter to determine the mechanism dominating film formation (Landau−Levich or evaporative). Thicker films can be obtained using a faster deposition velocity when the coating process is more rapid than solvent evaporation (Landau−Levich), whereas slower velocity produces thicker films when solvent evaporation is faster than coating process (evaporative regime). Figure 10 summarizes AFM images that map the correlation between deposition velocity (0.1−10 mm/s) and concentration (10−100 nM) on the formation of Au20-PS53k films on high SE substrate (80 mN/m). Recall that systems on 80 mN/m provided monolayers with the highest order. As deposition velocity

■

CONCLUSION Fabrication of large area, ordered assemblies of PGNs will enable additive manufacturing of novel membrane, electronic, and photonic elements. Because of the single component nature of these hybrids, aggregation and phase separation common in blended polymer nanocomposites are not an issue. In summary, the formation of a monolayer on a substrate is dependent on PGN architecture, PGN concentration, and substrate surface energy. The structure of an isolated PGN with densely grafted chains as well as its corresponding monolayer depends on the interaction energy between the canopy and substrate. When the interfacial energy between substrate and canopy is large, the polymer canopy collapses around the NP core, and the core is situated further away from substrate surface. The repulsive interactions between the PGN and substrate result in the attraction of adjacent polymer canopies (εP−S > εP−P) and the formation of ordered HCP clusters. When the difference in interface energy is small, the polymer canopy extends to interact with the substrate and the core is situated closer to substrate surface. Also, these films are thinner with greater Au volume fraction, indicating that the segment density within the PS canopy also depends on substrate surface

Figure 10. AFM images and correlation between deposition velocity and concentration on the film formation of Au20-PS53k on 80 mN/m substrate. The scale bar (shown at the bottom right) for all the images represents 2 μm. K


Article

Macromolecules

Figure 11. Large-scale, highly ordered assemblies of PGN. High-resolution SEM image of Au20-PS53k (70 nM) flow-coated at 5 mm/s on 80 mN/ m substrate. Low-magnification optical (bottom left) and high-magnification SEM (top right) images are shown in the insets.

assembly not only determines properties from refractive index and permeability to gas transport and mechanical robustness but also utilization of PGN assemblies as etch masks, active layers in devices, and metamaterials.

energy. The PGN−substrate interactions are stronger than the interparticle interactions (εP−S < εP−P); hence, the particles tend to adhere to the substrate and result in disordered, isolated small islands and strings of interconnected particles. This behavior is translated up to monolayer assemblies, where rougher, less-ordered assemblies form on substrates with low interface energy (20 mN/m). Overall, the process of PGN film formation parallels classic colloidal deposition, even though the concentration of polymer in solution reflects linear polymer flow coating in the Landau−Levich regime. Thus, large-area (cm2), highly ordered monolayer PGN assemblies can be fabricated on substrates with high interface energy (80 mN/m) within seconds using flow-coating and a volatile solvent (THF). Overall, flow-coating is a simple, cost-effective, and controllable technique that can be extended to other PGNs with thick canopies, where the assembly is determined by only the chain composition and irrespective of the core. While the work presented here focuses on PGN with high graft density and chain configurations in the concentrated polymer brush (CPB) regime, it will be of particular interest to expand to other canopy architectures (i.e., mushroom or semidilute polymer brush) to refine the understanding of the relationship between PGN structure, processing order, and subsequent properties. Other factors such as the core, polymer, and substrate interactions and their influence on final film morphology need to be examined. Equally important is to comprehend the role of substrate−canopy interactions and the tethered architecture on chain packaging and local segment density. All these insights are critical to optimize postprocessing conditions to improve the structural ordering and minimize defect concentration. Other processing parameters (i.e., temperature, pressure, solvent, etc.) must also be examined to gain a complete understanding of the processing−structure relationship of these PGNs. In conclusion, processing-based control of the internal structure and overall order of the

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b02722. Figures S1−S15 (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (R.A.V.). Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors thank the Air Force Office of Scientific Research (AFOSR), Air Force Research Laboratory Materials & Manufacturing Directorate (AFRL/RX), and National Research Council (NRC) for their financial support. We also thank Dr. Alexander Hexemer, Mr. Eric Schaible, and Dr. Chenhui Zhu for guidance, setup, and data collection at beamline 7.3.3 at Advanced Light Source/Lawrence Berkeley National Laboratory. The Advanced Light Source is supported by the Director of the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract DE-AC02-05CH11231.

■

REFERENCES

(1) Kawahara, Y.; Hodges, S.; Cook, B. S.; Zhang, C.; Abowd, G. D. In Instant inkjet circuits: lab-based inkjet printing to support rapid

L


Article

Macromolecules prototyping of UbiComp devices, Proceedings of the 2013 ACM international joint conference on Pervasive and ubiquitous computing, ACM, Zurich, Switzerland, 2013; pp 363−372. (2) Ahn, B. Y.; Lewis, J. A. Amphiphilic Silver Particles for Conductive Inks with Controlled Wetting Behavior. Mater. Chem. Phys. 2014, 148, 686−691. (3) Compton, B. G.; Lewis, J. A. 3D Printing of Lightweight Cellular Composites. Adv. Mater. 2014, 26, 5930−5935. (4) Muth, J. T.; Vogt, D. M.; Truby, R. L.; Menguc, Y.; Kolesky, D. B.; Wood, R. J.; Lewis, J. A. Embedded 3D Printing of Strain Sensors within Highly Stretchable Elastomers. Adv. Mater. 2014, 26, 6307− 6312. (5) Matyjaszewski, K.; Xia, J. Atom Transfer Radical Polymerization. Chem. Rev. 2001, 101, 2921−2990. (6) Pyun, J.; Matyjaszewski, K. Synthesis of Nanocomposite Organic/ Inorganic Hybrid Materials Using Controlled/ “Living” Radical Polymerization. Chem. Mater. 2001, 13, 3436−3448. (7) Voudouris, P.; Choi, J.; Gomopoulos, N.; Sainidou, R.; Dong, H.; Matyjaszewski, K.; Bockstaller, M. R.; Fytas, G. Anisotropic Elasticity of Quasi-One Component Polymer Nanocomposites. ACS Nano 2011, 5, 5746−5754. (8) Fernandes, N. J.; Wallin, T. J.; Vaia, R. A.; Koerner, H.; Giannelis, E. P. Nanoscale Ionic Materials. Chem. Mater. 2014, 26, 84−96. (9) Fernandes, N. J.; Koerner, H.; Giannelis, E. P.; Vaia, R. A. Hairy Nanoparticle Assemblies as One-Component Functional Polymer Nanocomposites: Opportunities and Challenges. MRS Commun. 2013, 3, 13−29. (10) Akcora, P.; Liu, H.; Kumar, S. K.; Moll, J.; Li, Y.; Benicewicz, B. C.; Schadler, L. S.; Acehan, D.; Panagiotopoulos, A. Z.; Pryamitsyn, V.; Ganesan, V.; Llavsky, J.; Thiyagarajan, P.; Colby, R. H.; Douglas, J. F. Anisotropic Self-Assembly of Spherical Polymer-Grafted Nanoparticles. Nat. Mater. 2009, 8, 354−359. (11) Pryamtisyn, V.; Ganesan, V.; Panagiotopoulos, A. Z.; Liu, H.; Kumar, S. K. Modeling the Anisotropic Self-Assembly of Spherical Polymer-Grafted Nanoparticles. J. Chem. Phys. 2009, 131, 221102. (12) Dukes, D.; Li, Y.; Lewis, S.; Benicewicz, B.; Schadler, L.; Kumar, S. K. Conformational Transitions of Spherical Polymer Brushes: Synthesis, Characterization and Theory. Macromolecules 2010, 43, 1564−1570. (13) Lafitte, T.; Kumar, S. K.; Panagiotopoulos, A. Z. Self-Assembly of Polymer-Grafted Nanoparticles in Thin Films. Soft Matter 2014, 10, 786−794. (14) Goel, V.; Pietrasik, J.; Dong, H.; Sharma, J.; Matyjaszewski, K.; Krishnamoorti, R. Structure of Polymer Tethered Highly Grafted Nanoparticles. Macromolecules 2011, 44, 8129−8135. (15) Chevigny, C.; Dalmas, F.; Di Cola, E.; Gigmes, D.; Bertin, D.; Boue, F.; Jestin, J. Polymer-Grafted-Nanoparticles Nanocomposites: Dispersion, Grafted Chain Conformation, and Rheological Behavior. Macromolecules 2011, 44, 122−133. (16) Srivastava, S.; Agarwal, P.; Archer, L. A. Tethered NanoparticlePolymer Composites: Phase Stability and Curvature. Langmuir 2012, 28, 6276−6281. (17) Maillard, D.; Kumar, S. K.; Fragneaud, B.; Kysar, J. W.; Rungta, A.; Benicewicz, B. C.; Deng, H.; Brinson, L. C.; Douglas, J. F. Mechanical Properties of Thin Glassy Polymer Films Filled with Spherical Polymer-Grafted Nanoparticles. Nano Lett. 2012, 12, 3909− 3914. (18) Kumar, S. K.; Jouault, N.; Benicewicz, B.; Neely, T. Nanocomposites with Grafted Nanoparticles. Macromolecules 2013, 46, 3199−3214. (19) Ganesan, V.; Jayaraman, A. Theory and Simulation Studies of Effective Interactions, Phase Behavior and Morphology in Polymer Nanocomposites. Soft Matter 2014, 10, 13−38. (20) Grabar, K. C.; Freeman, R. G.; Hommer, M. B.; Natan, M. J. Preparation and Chracterization of Au Colloid Monolayers. Anal. Chem. 1995, 67, 735−743. (21) Korgel, B. A.; Fullam, S.; Connolly, S.; Fitzmaurice, D. Assembly and Self-Organization of Silver Nanocrystal Superlattices: Ordered “Soft-Spheres. J. Phys. Chem. B 1998, 102, 8379−8388.

(22) Motte, L.; Lacaze, E.; Maillard, M.; Pileni, M. P. Self-Assemblies of Silver Sulfide Nanocrystals on Various Substrates. Langmuir 2000, 16, 3803−3812. (23) Santhanam, V.; Liu, J.; Agarwal, R.; Andres, R. P. Self-Assembly of Uniform Monolayer Arays of Nanoparticles. Langmuir 2003, 19, 7881−7887. (24) Lelievre, H. Y.; Desbiens, J.; Ritcey, A. M. Two-Dimensional Self-Organization of Polystyrene-Capped Gold Nanoparticles. Langmuir 2007, 23, 2843−2850. (25) Colson, P.; Henrist, C.; Cloots, R. Nanosphere Lithiography: A Powerful Method for the Controlled Manufacturing of Nanomaterials. J. Nanomater. 2013, 948510, 1. (26) von Freymann, G.; Kitaev, V.; Lotsch, B. V.; Ozin, G. A. Bottom-up Assembly of Photonic Crystals. Chem. Soc. Rev. 2013, 42, 2528−2554. (27) Vogel, N.; Weiss, C. K.; Landfester, K. From Soft to Hard: The Generation of Functional and Complex Colloidal Monolayers for Nanolithography. Soft Matter 2012, 8, 4044−4061. (28) Vogel, N.; Retsch, M.; Fustin, C. A.; del Campo, A.; Jonas, U. Advances in Colloidal Assembly: The Design of Structures and Hierarchy in Two and Three Dimensions. Chem. Rev. 2015, 115, 6265−6311. (29) Heath, J. R.; Knobler, C. M.; Leff, D. V. Pressure/Temperature Phase Diagrams and Superlattices of Organically Functionalized Metal Nanocrystal Monolayers: The Influence of Particle Size, Size Distribution, and Surface Passivant. J. Phys. Chem. B 1997, 101, 189−197. (30) Bourgoin, J.-P.; Kergueris, C.; Lefèvre, E.; Palacin, S. LangmuirBlodgett Films of Thiol-Capped Gold Nanoclusters: Fabrication and Electric Properties. Thin Solid Films 1998, 327−329, 515−519. (31) Lenzmann, F.; Li, K.; Kitai, A. H.; Stover, H. D. H. Thin-Film Micropatterning Using Polymer Microspheres. Chem. Mater. 1994, 6, 156−159. (32) van Duffel, B.; Ras, R. H. A.; De Schryver, F. C.; Schoonheydt, R. A. Langmuir-Blodgett Deposition and Optical Diffraction of Two Dimensional Opal. J. Mater. Chem. 2001, 11, 3333−3336. (33) Reculusa, S.; Ravaine, S. Synthesis of Colloidal Crystals of Controllable Thickness through Langmuir-Blodgett Technique. Chem. Mater. 2003, 15, 598−605. (34) Sirotkin, E.; Apweiler, J. D.; Ogrin, F. Y. Macroscopic Ordering of Polystyrene Carboxylate-Modified Nanospheres Self-Assembled at the Air-Water Interface. Langmuir 2010, 26, 10677−10683. (35) Hall, D. B.; Underhill, P.; Torkelson, J. M. Spin-Coating of Thin and Ultrathin Polymer Films. Polym. Eng. Sci. 1998, 38, 2039−2045. (36) Yang, C. C.; Josefowicz, J. Y.; Alexandru, L. Deposition of Ultrathin Films by a Withdrawal Method. Thin Solid Films 1980, 74, 117−127. (37) Hammond, P. T. Form and Function in Multilayer Assembly: New Applications at the Nanoscale. Adv. Mater. 2004, 16, 1271−1293. (38) Ozaydin-Ince, G.; Coclite, A. M.; Gleason, K. K. CVD of Polymeric Thin Films: Applications in Sensors, Biotechnology, Microelectronics/Organic Electronics, Microfluidics, MEMS, Composites and Membranes. Rep. Prog. Phys. 2012, 75, 1. (39) Shepard, K. B.; Priestley, R. D. MAPLE Deposition of Macromolecules. Macromol. Chem. Phys. 2013, 214, 862−872. (40) Mihi, A.; Ocana, M.; Míguez, H. Oriented Colloiadl-Crystal Thin Films by Spin-Coating Micospheres Dispersed in Volatile Media. Adv. Mater. 2006, 18, 2244−2249. (41) Stafford, C. M.; Roskov, K. E.; Epps, T. H., III; Fasolka, M. J Generating Thickness Gradients of Thin Polymer Films via Flow Coating. Rev. Sci. Instrum. 2006, 77, 023908. (42) Meredith, J. C.; Karim, A.; Amis, E. J. Combinatorial Materials Science for Polymer Thin-Film Dewetting. Macromolecules 2000, 33, 9747−9756. (43) Smith, A. P.; Sehgal, A.; Douglas, J. F.; Karim, A.; Amis, E. J. Combinatorial Mapping of Surface Energy Effects on Diblock Copolymer Thin Film Ordering. Macromol. Rapid Commun. 2003, 24, 131−135. M


Article

Macromolecules

(68) Reiter, G. Dewetting of Thin Polymer Films. Phys. Rev. Lett. 1992, 68, 75−78. (69) Xie, R.; Karim, A.; Douglas, J. F.; Han, C. C.; Weiss, R. A. Spinodal Dewetting of Thin Polymer Films. Phys. Rev. Lett. 1998, 81, 1251−1254. (70) Morariu, M. D.; Schaffer, E.; Steiner, U. Capillary Instabilities by Fluctuation Induced Forces. Eur. Phys. J. E: Soft Matter Biol. Phys. 2003, 12, 375−381. (71) Hong, S. W.; Xia, J.; Lin, Z. Spontaneous Formation of Mesoscale Polymer Patterns in an Evaporating Bound Solution. Adv. Mater. 2007, 19, 1413−1417. (72) Meli, L.; Li, Y.; Lim, K. T.; Johnston, K. P.; Green, P. F. Templating of Gold Nanocrystals in Micellar Cores of Block Copolymer Films. Macromolecules 2007, 40, 6713−6720. (73) Muralidharan, G.; Sivaraman, S. K.; Santhanam, V. Effect of Substrate on Particle Arrangement in Arrays Formed by Self-Assembly of Polymer Grafted Nanoparticles. Nanoscale 2011, 3, 2138−2141. (74) Feder, J. Random Sequential Adsorption. J. Theor. Biol. 1980, 87, 237−254. (75) Malescio, G.; Pellicane, G. Stripe Phases from Isotropic Repulsive Interactions. Nat. Mater. 2003, 2, 97−100. (76) Malescio, G.; Pellicane, G. Stripe Patterns in Two-Dimensional Systems with Core-Corona Molecular Architecture. Phys. Rev. E 2004, 70, 021202. (77) Fetters, L. J.; Lohse, D. J.; Richter, D.; Witten, T. A.; Zirkel, A. Connection Between Polymer Molecular Weight, Density, Chain Dimensions, and Melt Viscoelastic Properties. Macromolecules 1994, 27, 4639−4647. (78) Wang, X.; Foltz, V. J.; Rackaitis, M.; Bohm, G. G. A. Dispersing Hairy Nanoparticles in Polymer Melts. Polymer 2008, 49, 5683−5691. (79) Meli, L.; Arceo, A.; Green, P. F. Control of Morphology and Its Effects on the Optical Properties of Polymer Nanocomposites. Soft Matter 2009, 5, 533−537. (80) Chen, X. C.; Green, P. F. Structure of Thin Film Polymer/ Nanoparticle Systems: Polystyrene coated-Au Nanoparticle/Tetramethyl Bisphenol-A Polycarbonate Mixtures (TMPC). Soft Matter 2011, 7, 1192−1198. (81) Green, P. F. The Structure of Chain End-Grafted Nanoparticle/ Homopolymer Nanocomposites. Soft Matter 2011, 7, 7914−7926. (82) Hansen, J. P.; Verlet, L. Phase Transitions of the Lennard-Jones System. Phys. Rev. 1969, 184, 151. (83) Wang, Z.; Alsayed, A. M.; Yodh, A. G.; Han, Y. TwoDimensional Freezing Criteria for Crystallizing Colloidal Monolayers. J. Chem. Phys. 2010, 132, 154501. (84) Dillmann, P.; Maret, G.; Keim, P. Comparison of 2D Melting Criteria in a Colloidal System. J. Phys.: Condens. Matter 2012, 24, 464118. (85) Lowen, H.; Palberg, T.; Simon, R. Dynamical Criterion for Freezing of Colloidal Liquids. Phys. Rev. Lett. 1993, 70, 1557−1560. (86) Srivastava, S.; Nykypanchuk, D.; Fukuto, M.; Halverson, J. D.; Tkachenko, A. V.; Yager, K. G.; Gang, O. Two-Dimensional DNAProgrammable Assembly of Nanoparticles at Liquid Interfaces. J. Am. Chem. Soc. 2014, 136, 8323−8332. (87) Keddie, J. L.; Jones, R. A. L.; Cory, R. A. Size-Dependent Depression of the Glass Transition Temperature in Polymer Films. Europhys. Lett. 1994, 27, 59−64. (88) Kawana, S.; Jones, R. A. L. Effect of Physical Ageing in Thin Glassy Polymer Films. Eur. Phys. J. E: Soft Matter Biol. Phys. 2003, 10, 223−230. (89) Priestley, R. D. Physical Aging of Confined Glasses. Soft Matter 2009, 5, 919−926. (90) Pye, J.; Rohald, K. A.; Baker, E. A.; Roth, C. B. Physical Aging in Ultrathin Polystyrene Films: Evidence of a Gradient in Dynamics at the Free Surface and Its Connection to the Glass Transition Temperature Reductions. Macromolecules 2010, 43, 8296−8303. (91) Gray, L. A. G.; Roth, C. B. Stability of Polymer Glasses Vitrified Under Stress. Soft Matter 2014, 10, 1572−1578.

(44) Crosby, A. J.; Fasolka, M. J.; Beers, K. L. High-Throughput Craze Studies in Gradient Thin Films Using Ductile Copper Grids. Macromolecules 2004, 37, 9968−9974. (45) Lee, J. Y.; Crosby, A. J. Crazing in Glassy Block Copolymer Thin Films. Macromolecules 2005, 38, 9711−9717. (46) Mittal, M.; Niles, R. K.; Furst, E. M. Flow-Directed Assembly of Nanostructured Thin Films from Suspensions of Anisotropic Titania Particles. Nanoscale 2010, 2, 2237−2243. (47) Davis, R. L.; Jayaraman, S.; Chaikin, P. M.; Register, R. A. Creating Controlled Thickness Gradients in Polymer Thin Films via Flow-coating. Langmuir 2014, 30, 5637−5644. (48) Le Berre, M.; Chen, Y.; Baigl, D. From Convective Assembly to Landau-Levich Deposition of Multilayered Phospholipid Films of Controlled Thickness. Langmuir 2009, 25, 2554−2557. (49) Landau, L.; Levich, B. Dragging of a Liquid by a Moving Plate. Dynamics of Curved Fronts 1988, 141. (50) Mayer, H. C.; Krechetnikov, R. Landau-Levich Flow Visualization: Revealing the Flow Topology Responsible for the Film Thickening Phenomena. Phys. Fluids 2012, 24, 052103. (51) Prevo, B. G.; Velev, O. D. Controlled, Rapid Deposition of Structured Coatings from Micro- and Nanoparticle Suspensions. Langmuir 2004, 20, 2099−2107. (52) Prevo, B. G.; Fuller, J. C., III; Velev, O. D. Rapid Deposition of Gold Nanoparticle Films with Controlled Thickness and Structure by Convective Assembly. Chem. Mater. 2005, 17, 28−35. (53) Vlassopoulos, D.; Fytas, G. From Polymers to Colloids: Engineering the Dynamic Properties of Hairy Particles. High Solid Dispersions: Advances in Polymer Science; Springer: Berlin, 2009; Vol. 236, pp 1−54. (54) Turkevich, J.; Stevenson, P. C.; Hillier, J. The Formation of Colloidal Gold. J. Phys. Chem. 1953, 57, 670−673. (55) Frens, G. Controlled Nucleation for the Regulation of the Particle Size in Monodisperse Gold Suspensions. Nature, Phys. Sci. 1973, 241, 20−22. (56) Kotlarchyk, M.; Chen, S. Analysis of Small-Angle Neutron Scattering Spectra from Polydisperse Interacting Colloids. J. Chem. Phys. 1983, 79, 2461−2469. (57) Pasternack, R. M.; Amy, S. R.; Chabal, Y. J. Attachment of 3(Aminopropoyl)triethoxysilane on Silicon Oxide Surfaces: Dependence of Solution Temperature. Langmuir 2008, 24, 12963−12971. (58) Kannan, B.; Higgins, D. A.; Collinson, M. M. Aminoalkoxysilane Reactivity in Surface Amine Gradients Prepared by Controlled-Rate Infusion. Langmuir 2012, 28, 16091−16098. (59) Owens, D. K.; Wendt, R. C. Estimation of the Surface Free Energy of Polymers. J. Appl. Polym. Sci. 1969, 13, 1741−1747. (60) Nelson, A. Co-refinement of Multiple-Contrast Neutron/X-ray Reflectivity Data Using MOTOFIT. J. Appl. Crystallogr. 2006, 39, 273−276. (61) Liu, X.; Atwater, M.; Wang, J.; Huo, Q. Extinctional Coefficient of Gold Nanoparticles with Different Sizes and Different Capping Ligansd. Colloids Surf., B 2007, 58, 3−7. (62) Xu, S.; Arnsdorf, M. F. Calibration of the Scanning (Atomic) Force Microscopy with Gold Particles. J. Microsc. 1994, 173, 199−210. (63) Li, Y.; Lindsay, S. M. Polystyrene Latex Spheres as a Size Calibration for the Atomic Force Microscopy. Rev. Sci. Instrum. 1991, 62, 2630−2633. (64) Westra, K. L.; Mitchell, A. W.; Thomson, D. J. Tip Artifacts in Atomic-Force Microscope Imaginign of Thin-Film Surfaces. J. Appl. Phys. 1993, 74, 3608−3610. (65) Villarrubia, J. S. Algorithms for Scanned Probe Microscope Image Simulation, Surface Reconstruction, and Tip Estimation. J. Res. Natl. Inst. Stand. Technol. 1997, 102, 425−454. (66) Hexemer, A.; Bras, W.; Glossinger, J.; Schaible, E.; Gann, E.; Kirian, R.; MacDowell, A.; Church, M.; Rude, B.; Padmore, H. A SAXS/WAXS/GISAXS Beamline with Multilayer Monochromator. J. Phys: Conf. Ser. 2010, 247 (012007), 1−11. (67) Llavsky, J. Nika: Software for Two-Dimensional Data Reduction. J. Appl. Crystallogr. 2012, 45, 324−328. N


Preparation of Ordered Monolayers of Polymer Grafted Nanoparticles

Recommend Documents