Achieving 3-D Nanoparticle Assembly in Nanocomposite Thin Films

Mar 3, 2017 - Nanocomposite thin films containing well-ordered nanoparticle (NP) assemblies are ideal candidates for the fabrication of metamaterials...
0 downloads 0 Views 5MB Size
Download PDF
Article pubs.acs.org/Macromolecules

Achieving 3‑D Nanoparticle Assembly in Nanocomposite Thin Films via Kinetic Control Jingyu Huang,† Yihan Xiao,† and Ting Xu*,†,‡,§ †

Department of Materials Science & Engineering and ‡Department of Chemistry, University of California, Berkeley, Berkeley, California 94720, United States § Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States S Supporting Information *

ABSTRACT: Nanocomposite thin films containing well-ordered nanoparticle (NP) assemblies are ideal candidates for the fabrication of metamaterials. Achieving 3-D assembly of NPs in nanocomposite thin films is thermodynamically challenging as the particle size gets similar to that of a single polymer chain. The entropic penalties of polymeric matrix upon NP incorporation leads to NP aggregation on the film surface or within the defects in the film. Controlling the kinetic pathways of assembly process provides an alternative path forward by arresting the system in nonequilibrium states. Here, we report the thin film 3-D hierarchical assembly of 20 nm NPs in supramolecules with a 30 nm periodicity. By mediating the NP diffusion kinetics in the supramolecular matrix, surface aggregation of NPs was suppressed and NPs coassemble with supramolecules to form new 3-D morphologies in thin films. The present studies opened a viable route to achieve designer functional composite thin films via kinetic control.

■

INTRODUCTION

optical, plasmonic, dielectric, and mechanical properties of the nanocomposites. Different approaches have been explored to overcome the NP surface aggregation. The phase behavior of nanocomposite thin films is thermodynamically governed by the polymer−NP ligand interaction5,19−22 and the energy penalties associated with polymer chain deformation upon NP incorporation.12,13,23−26 Strong intermolecular interactions, such as hydrogen bonding, have been engineered to strengthen NP− polymer matrix interaction.21,22 In NP/BCP blends, the BCP periodicity has been increased substantially13,27 to ensure that the ratio between the NP diameter (d) and the BCP periodicity (L), d/L, is less than 0.312,18 to minimize the entropic effects. However, there has been limited exploration to control the NP assembly kinetics to access nonequilibrium states. We recently studied the ordering process in thin films of supramolecular nanocomposites via solvent vapor annealing (SVA).10 The presence of solvent affects the glass transition temperature (Tg), the viscosity (η) of the polymers, and the effective interaction parameter (χeff) among various components.10,28 The solvent volume fraction ( fs) in a swollen thin film determines the diffusion coefficient of NPs, the intermolecular interactions among NPs, polymers and small molecules, and the kinetic pathways of NP assembly.10 SVA opens a new route to

Thin films of nanocomposites containing hierarchically structured 3-D assembly of nanoparticles (NPs) are highly desirable to fabricate metamaterial via a bottom-up approach for applications in sensors, barriers, optoelectronics, and storage devices.1,2 Multilayers of NPs have been fabricated via layer-bylayer deposition, where NPs get expelled to the surface of polymer film in each cycle.3 Block copolymers (BCPs) have been used to direct NP assemblies both in bulk and in thin films by optimizing NP−polymer interactions and the ratio between NP size (d) and BCP periodicity (L).4,5 BCP-based supramolecules6−8 are simple, yet versatile systems to coassemble with a wide range of NPs since the small molecule mediates the NP/polymer interactions and modulates the supramolecule conformation and assembly kinetics.9−11 However, for all of these cases, there is a restriction on the NP size to achieve 3-D NP assembly in thin films. There is a critical particle size above that the NPs aggregate on the film surface and/or in defects.12,13 In blends of NPs and homopolymers or copolymers, NPs, as small as 5 nm in size, can be expelled to the film surface or interface to minimize entropic penalties.14−17 In supramolecular nanocomposite thin films with a lateral periodicity of 30 nm, NPs, 7 nm in size, aggregate on the film surface.18 In some cases, surface aggregation of NP is desirable for film postmodification, stabilizing defects, and to fabricate layered structure. However, it represents a significant hurdle to obtain 3-D NP assembly in a thin film to optimize the © XXXX American Chemical Society

Received: January 10, 2017 Revised: February 28, 2017

A

DOI: 10.1021/acs.macromol.7b00063 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

grid. To perform cross-sectional TEM, the floated film was picked up by an epoxy block (Araldite 502, Electron Microscopy Sciences) and left at room temperature overnight to ensure the adhesion of the thin film to the epoxy. The sample was then microtomed using an RMC MT-X ultramicrotome (Boeckler Instruments) with the section thickness of 60 nm. The thin sections were then caught on TEM grid for imaging. Grazing-Incidence Small-Angle X-ray Scattering (GISAXS) and X-ray Reflectivity. GISAXS samples were prepared on Si wafers with the dimension of 1 × 2 cm2 coated with an ∼25 nm cross-linked PS brush layer. GISAXS experiments were performed at beamline 8ID-E at Advanced Photon Source in Argonne National Laboratory. The X-ray wavelength is 1.687 Å, and the scattered X-ray intensity distribution was detected using a high-speed version of the Pilatus 1M detector. Images were plotted as intensities (I) vs q, where q = (4π/λ) sin(θ), λ is the wavelength of the incident X-ray beam, and 2θ is the scattering angle. Line-cut profiles of GISAXS patterns were extracted using Igor Pro with the Nika package. Form factor was fitted using Igor Pro with the NCNR & SANS package provided by NIST. For in situ GISAXS, samples were annealed under chloroform vapor in a chamber designed for in situ measurements. The film thickness was monitored in situ with a Filmetrics F20 interferometer, and the exposure time was chosen to be 5 s for each measurement.

kinetically control NP assembly pathway in the film interior before the surface aggregation of NPs occurs. Here, we show experimentally that 3-D hierarchically structured NP assembly containing 20 nm NPs (d/L = 0.67) can be readily fabricated in thin films by controlling the assembly kinetics via SVA. Fundamentally, present studies show a new regime of mesoscale self-assembly, where the resultant 3-D thin film NP assemblies are attributed to balancing various energetic driving forces and modulating diffusion kinetics of NPs.

■

EXPERIMENTAL SECTION

Materials. PS(19 kDa)-b-P4VP(5.2 kDa) (PDI = 1.10) was purchased from Polymer Source, Inc. 3-n-Pentadecylphenol (90%− 95%) was purchased from ACROS Organics. Chloroform (Amylene as Preservative) was purchased from Fisher Scientific. Iron oxide nanoparticles with the core size of 20 nm were purchased from Ocean Nanotech. All chemicals and materials were used as received with no further purification. Sample Preparation. The magnetite (Fe3O4) NPs used in the experiments have a core size ∼20 nm. The oleic acid ligand layer on the NPs was estimated to add an additional ∼2 nm thickness on the iron oxide cores. Nanoparticles were dispersed in chloroform at the concentration of 25 mg/mL. Supramolecule solution was prepared by simply mixing 20 mg/mL PS-b-P4VP with an appropriate amount of small molecules PDP in chloroform. Nanoparticle suspension was mixed with supramolecule solution by vigorous shaking. The ratio of the NP solution and supramolecular solution was carefully controlled to get the desired NP volume percentage. Si wafers, 1 cm × 2 cm in size, were used as the substrates. Nanocomposite thin films were prepared by spin-coating the solution mixture onto the Si wafers at the spinning speed ranging from 1000 to 3000 rpm for 10 s. The spinning speeds were carefully chosen to get the desired film thicknesses, which were then measured with the Filmetrics F20 interferometer. The asprepared thin film sample was placed in a 125 mL top-capped glass vial for solvent vapor annealing at 22.5 °C by injecting 300 μL of chloroform into the vial. Film swelling rate was also changed by adding 100, 400, or 600 μL of chloroform for annealing. Films with 20 nm NPs were swollen in chloroform vapor to reach 260%−280% of the original film thickness. Once solvent vapor annealing was completed, the vial was uncapped to let the chloroform evaporate freely. The sample was removed from the vial quickly, and the swollen film was dried rapidly to its original thickness in less than 5 s. Slow solvent evaporation was achieved by slightly unfasten the cap of the vial. Film thickness was monitored with the Filmetrics F20 during solvent evaporation in order to control the drying speed. Atomic Force Microscopy (AFM). AFM imaging was performed on a Bruker Dimension Edge AFM under the tapping mode. The model of the probes used in measurements is RTESPA-150. The spring constant of the cantilever is 6 N/m with a resonant frequency ∼150 kHz. Substrate Modification for Transmission Electron Microscopy (TEM) Studies. Sodium chloride (NaCl) disk was coated with an ∼25 nm layer of cross-linked PS brush and used as the substrate for TEM studies. R-hydroxyl ω-benzocyclobutene (BCB) incorporated polystyrene (8 wt % BCB, MW = 90 800 g/mol, PDI = 1.41) was dissolved in toluene to prepare 6 mg/mL PS brush solution. The Si wafer was spin-coated with the PS brush solution at 3000 rpm and then heated at 240 °C for 20 min in a vacuum oven for the crosslinking of BCB. After thermal heating, Si wafer was washed with toluene and chloroform in sequence to get rid of the un-cross-linked PS. Finally, a Filmetrics F20 interferometer was used to measure the thickness of the PS brush layer on the Si wafer. Transmission Electron Microscopy (TEM). TEM imaging was performed on a FEI Tecnai 12 TEM at an accelerating voltage of 120 kV. Thin film samples for TEM studies were prepared in the similar way as mentioned above, except that Si wafer was replaced by PS brush-coated NaCl disk. Thin films were floated off the substrates by dipping the salt disks in deionized water. To perform in-plane TEM, the floated film was caught on a 200-mesh carbon film-coated copper

■

RESULTS AND DISCUSSION The supramolecule, PS(19K)-b-P4VP(5.2K)(PDP)1.7, is based on polystyrene-block-poly(4-vinylpyridine) and hydrogenbonded 3-pentadecylphenol (PDP) with the molar ratio of PDP to 4VP monomer of 1.7.6−8 The PS(19k)-b-P4VP(5.2k) (PDP)1.7 forms hexagonally packed PS cylinders parallel to substrate with a lateral periodicity (L) of 30 nm post solvent annealing under chloroform vapor.29 NPs, 3−5 nm in size (d/L < 0.3), are incorporated into the P4VP(PDP) microdomain and form 3-D hexagon grids or hexagonally packing 1-D NP chains.10,18,30 However, NPs larger than 7 nm are expelled to the film surface.18 Monodispersed magnetite (Fe3O4) NPs coated by oleic acid with the particle core diameter (d) of 20 nm (Figure S1) were used here, and the d/L is 0.67. Similar to previous studies of supramolecular nanocomposites,9,18 no morphological difference was noticed when using NPs of same size but with different chemical composition in the core. We first determined NP diffusion in nanocomposite thin films as a function of solvent volume fraction at the end of SVA. Figure 1 shows the solvent annealing profiles (Figure 1a), NP density on the film surface (Figure 1b), and the surface morphologies (Figure 1c−e) of 120 nm thin films containing 9 vol % 20 nm Fe3O4 NPs. Same as reported previously,10 we observed that the solvent swelling rate does not affect the morphology on the film surface (Figure 1a). It is the solvent volume fraction (fs) at the end of solvent annealing that determines the final morphology.10 During the solvent annealing process, NPs gradually migrated to the top layer of the film with increasing fs (Figure 1b). At fs = 0.32 (Figure 1a, region I), there was little morphological change on the film surface even after the solvent annealing was extended to 20 min (Figure 1c). A significant increase of the number of NPs on the film surface happened at fs ∼ 0.56 (intersection of the dashed line in Figure 1b). At fs = 0.62 (Figure 1a, region II), the number of NPs on the film surface increased rapidly and the film surface showed hexagonally packed arrays of 20 nm NPs (Figure 1d). As fs was further increased, the system finally macrophase separated at fs = 0.8 and NPs segregated on the film surface (Figure 1e). It is worthwhile to notice that previous in situ GISAXS studies showed that the supramolecular nanocomposite thin films with 5 nm NPs order at fs = 0.31 B

DOI: 10.1021/acs.macromol.7b00063 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

The 3-D structure of hexagonally packed NP assembly obtained at fs = 0.62 was further characterized using AFM and TEM. The fast Fourier transform (FFT) (Figure 2a, inset) of the AFM image in Figure 2a shows hexagonally packed 2-D NP arrays on the surface layer with the interparticle distance a ∼ 44 nm, consistent with the zoomed-in AFM image in Figure 2b. Analysis of the AFM height profile indicated that NPs are embedded in the thin film rather than sitting on the film surface. To probe the internal structure, series of in-plane TEM images at different tilt angles were taken, confirming the formation of multilayered NPs inside of the film (Figure 2c−f). As the TEM tilt angle changed from 0° to 45°, the orientation of the multilayered NP arrays shifted around 120° as shown by the dotted white lines in Figure 2c,f. A 3-D model of NP distribution in the thin film was reconstructed with AutoCAD and was in good agreement with the in-plane TEM images (Figure 2c−f insets). Based on the model, the unit cell of NP arrays is proposed and shown schematically in Figure 2g. Red, green, and blue balls represent NPs located at the top, middle, and bottom layer in the unit cell, respectively. The model shows that NPs in each layer are hexagonally packed, and the second layer of NPs has a translational offset of a/2 relative to the first and third layers along the x-axis (Figure S3). The model can also be successfully applied to explain the orientation change of NP arrays with TEM tilt angle at different imaging spots and in different samples (Figure S4). Cross-sectional TEM image of the thin film corresponding to the y−z plane view is shown in Figure 2h, where NPs packed in multilayers in the vertical direction with an interlayer distance around a/2. Grazing-incidence small-angle X-ray scattering (GISAX) and X-ray reflectivity were performed to further investigate the structure of NP assemblies over macroscopic areas in thin films. As shown in Figure 2i, the GISAXS patterns are primarily dominated by the form factor of NPs due to the high electron density contrast between NP and supramolecule and the

Figure 1. The 120 nm nanocomposite thin films containing 9 vol % of 20 nm Fe3O4 NPs and PS(19K)-b-P4VP(5.2K)(PDP)1.7: (a) the profiles of solvent volume fraction (fs) vs annealing time at different film swelling rates controlled by the amount of chloroform added for annealing; (b) the density of NPs on the surface layer vs fs analyzed based on AFM phase images; AFM phase images of films annealed to (c) fs = 0.32, (d) fs = 0.62, and (e) fs = 0.8. NPs are sparsely dispersed at grain boundaries or in defects in (c), hexagonally packed in micellelike structures in (d), and aggregated on the film surface in (e). The density of NPs on film surface increases with fs. (PS domains show a brighter color than the P4VP(PDP) domain in the AFM phase images; NPs are shown as the bright dots.)

and begin to disorder at fs = 0.5.10 Thus, the significant increase in the NP diffusion rate can be attributed to the reduction in the viscosity of supramolecular matrix and the elimination of interface between the two microdomains at fs > 0.6.

Figure 2. The 120 nm nanocomposite thin films containing 9 vol % of 20 nm Fe3O4 NPs and PS(19K)-b-P4VP(5.2K)(PDP)1.7: (a) AFM phase image; (inset: FFT image); (b) zoomed-in AFM phase image; (c−f) 0°, 15°, 30°, and 45°-tilted TEM images and the corresponding calculated 3-D NP organizations based on the model (scale bar = 100 nm); (g) simulated 3-D model of the unit cell of NP arrangements in the thin film (dotted white lines in parts c and d indicate the orientation change of the linear NP arrays with TEM tilt angle); (h) cross-sectional TEM image corresponding to the view of the y−z plane in part g; (i) GISAXS patterns as a function of beam incident angle; (j) GISAXS line-cut profiles at qz = 0.035 Å−1; (k) extracted structure factor of the NP assembly; (l) X-ray reflectivity profile of a 300 nm thin film. Inset: in-plane TEM image of multilayer hexagonally packed NPs in the 300 nm thin film. C

DOI: 10.1021/acs.macromol.7b00063 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 3. In-plane TEM images of (a) 45, (b) 60, (c) 130, and (d) 250 nm nanocomposite thin films containing 9 vol % of Fe3O4 NPs and PS(19K)b-P4VP(5.2K)(PDP)1.7 annealed to fs = 0.62. (e) X-ray reflectivity profile of the 60 nm film and the fitting. (f) Scattering length density profile of the 60 nm film (yellow: NP; purple: supramolecule/polymer; gray: Si wafer).

films were solvent annealed to fs ∼ 0.62 (Figure 3a−d). On the film surface, hexagonally packed NP assemblies similar to that in Figure 1d were observed for all films. NPs preferentially filled the top layers of the film, which agrees with the observation in the previously reported films with 5 nm NPs.30 In the 45 nm film, the amount of NPs was just enough to fill the surface layer, and one layer of hexagonally packed NPs can be clearly seen in the in-plane view of the TEM image in Figure 3a. In the 60 nm film, the amount of NPs was not enough to occupy the second layer. NPs formed hexagonally packed arrays on the top layer but dispersed randomly in-plane in the second layer (Figure 3b). A fitting of the X-ray reflectivity profile of the 60 nm film (Figure 3e) verified the existence of two layers of NPs in the film. The out-of-plane order of the first layer is good with a roughness of ∼6 nm, while the second layer has relatively poor out-of-plane order (Figure 3f). In the 130 and 250 nm films, TEM images (Figure 3c,d) display multilayer NP arrays similar to that in Figure 2c. Similar results to Figure 3a−d can be obtained in nanocomposite thin films with the same film thickness of 120 nm but varied NP concentrations of 3, 4.5, 9, and 18 vol %, respectively. Thus, the formation of 3-D NP assembly is a surface-directed process30 controlled by the kinetics of NP diffusion in the presence of solvent in the thin films. The number of NP layers can be controlled by the total number of NPs in the film. Successfully utilizing kinetic control to access nonequilibrium states requires one to achieve a good balance among various processes, such as NP diffusion, small molecule distribution and diffusion, polymer mobility, and the order−disorder transition of supramolecular system. To achieve 3-D NP assembly in a thin film, the system kinetics needs to be manipulated such that there is enough mobility of 20 nm NPs in the matrix, but the NP surface aggregation can be suppressed. The observed results in nanocomposite thin films containing 20 nm NPs differ from those based on 3−5 nm NPs studied previously10,18,29,30 in several aspects. Thermodynamically, 3−5 nm NPs (d/L ∼ 0.1− 0.17) can act as fillers to stabilize the supramolecular interstitial sites, while conformational entropic penalties to deform the polymer become the major driving force to expel NP > 5 nm to the film surface.18 Kinetically, the diffusivity of NPs smaller than polymer size deviates from the Stokes−Einstein relation, and NPs experience a much smaller viscosity than the experimentally measured macroscopic viscosity of the polymer matrix.32,33 Although fs ∼ 0.3 is the optimal condition for the supramolecular-directed assembly of 3−5 nm NPs in thin films,10 the diffusion coefficient (D) of 20 nm NPs, especially at the direction normal to the supramolecular interface Dperp, is very low at fs ∼ 0.3 (Figure 1b) due to the large particle size

monodispersity of NPs. Horizontal line-cut profiles at qz = 0.035 Å−1 (Figure 2j) were used to analyze the in-plane structure of NP assemblies. Form factor of the NPs was fitted at the qy range of 0.004−0.14 Å−1 (red dashed curve in Figure 2j) to avoid scattering from the lamellae of the comb block at 0.16 Å−1.31 The structure factor (Figure 2k), the scattering from the order of NP packing, was obtained from dividing the overall scattering intensity by the NP form factor. The interparticle distance was calculated to be ∼44 nm, and the structure factor appeared to be characteristic of the hexagonal packing, which agrees well with the AFM and TEM results in Figure 2a−f. The strong scattering from the NP form factor made it difficult to decipher the vertical packing of the NPs. To get the vertical interlayer distance of the NP assemblies, X-ray reflectivity profile of a 300 nm thin films is shown Figure 2l. The film thickness of 300 nm was selected instead of 120 nm simply because the 300 nm film contains more layers of NPs and shows clearer Bragg peaks. Based on the Bragg reflections originated from multilayer ordered NP assemblies, the interlayer distance was estimated to be ∼22 nm from the Δqz value of the adjacent Bragg peaks, which generally agrees with the cross-sectional TEM (Figure 2h) of the 120 nm film. Inplane TEM image of the 300 nm film (Figure 2f inset) shows multilayered NPs similar to that in Figure 2c. Together, all these results confirmed 3-D assemblies of NPs in the nanocomposite thin films as opposed to surface aggregation of NPs. Thus, the results validated the hypothesis of kinetic control and confirmed the experimental feasibility of arresting the assembly process of supramolecular nanocomposite thin films as the NP size approaches to that of supramolecular matrix. It significantly opened the window of NP size that can be incorporated, decoupled the NP size from that of the soft matrix, and expanded the types of NPs that can be used to fabricate functional nanocomposites. We previously showed that small NP (5 nm) assembly in supramolecular nanocomposite thin films is a surface-directed process.30 NPs assemble in the top layer first, then the second, and so on. The number of layer of NP assembly is governed by the total number of NPs in the film, which can be tailored by varying the NP concentration and the film thickness. We hypothesize that this mechanism should also apply for large NPs (20 nm) as long as we can balance the kinetics of NP surface aggregation and local organization. This may further expand control over the NP distribution along the film depth in addition to the inplane ordering of NPs in a film. Nanocomposite thin films with 9 vol % of NPs were prepared with film thickness ranging from 45 to 250 nm. All the D

DOI: 10.1021/acs.macromol.7b00063 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 4. (a) In-situ GISAXS line-cut profiles at qz = 0.035 Å−1 of 120 nm nanocomposite thin films containing 9 vol % of 20 nm Fe3O4 NPs and PS(19K)-b-P4VP(5.2K)(PDP)1.7 at the incident angle of 0.22°. (b−d) Schematic drawing of in-plane arrangements of NPs and supramolecules during and post SVA. (e) Zoomed-in view of arrangements of different components post rapid solvent removal, where some free PDP remains in the PS microdomain (yellow: NP; blue: P4VP(PDP) domain; purple: PS-rich domain; orange head and black tail: PDP).

and χeff. As a result, the nanocomposite thin films containing 20 nm NPs (d/L ∼ 0.67) shows very little morphological change post SVA at fs ∼ 0.3 (Figure 1c). When fs is higher than ∼0.5, previous in situ GISAXS studies confirmed that the supramolecular matrix is disordered.10 The PDP molecule also becomes soluble in the PS microdomain at fs = 0.6. Since the stoichiometry ratio between PDP:4VP is higher than 1, there is a good fraction of unbound PDP in the PS domain acting as plasticizer.10,34 Together, the low viscosity, η, and reduction in χeff of the supramolecular matrix enable the diffusion of NPs toward the film surface. However, there should be preferential interaction between the P4VP(PDP) block and the alkyl passivated NPs. In-situ GISAXS studies were also carried out to validate these hypotheses. The GISAXS qz line-cut profiles of the 120 nm nanocomposite thin films with 9 vol % of 20 nm NPs are shown in Figure 4a. The NP and supramolecular arrangements during the process are schematically displayed in Figure 4b,c. At fs ∼ 0.3, the structure factor of NP packing showed very limited change compared to as-cast film. This is consistent with the AFM results in Figure 1. At fs ∼ 0.6 the structure factor associated with NP packing disappeared, and the system was in a disordered state. During the solvent removal, the supramolecular nanocomposite started to order, leading to the structure factor of hexagonally packed NPs similar to Figure 2j. However, the ordering process is mainly governed by the short-range diffusion due to the reduced fs and is affected by the presence of NPs. The preferential interaction between the P4VP(PDP) and alkyl ligands leads to the formation of micelle-like structure as schematically shown in Figure 4e. The rapid solvent removal also discourages the free PDP from returning back to the P4VP(PDP) microdomain. The free PDP redistribution in the PS microdomain changes the volume ratio between the two blocks and possibly stabilizes the formation of micelle-like structures around NPs as seen in the AFM images in Figures 1d and 2b. The obtained micellelike structures appear to be metastable states kinetically frozen through fast solvent evaporation postannealing. This speculation was confirmed by varying the solvent removal rate. An extension of the solvent removal rate from 5 s to 30 min results in a disordered state as shown in Figure S5. During the slow solvent evaporation, PDP in the PS domain has enough time to move back to the P4VP domain and destroy those preformed micelle-like structures in the system.

0.67. A 3-D assembly of 20 nm NPs blended with supramolecules with periodicity of 30 nm was achieved by modulating the assembly kinetics via solvent vapor annealing. Furthermore, the NP assembly along the film depth can also be controlled by varying the total amount of NPs in the thin films. The results validated the feasibility and opened a new pathway to construct hierarchically structured 3-D nanocomposites with NPs tens of nanometers in size or d/L > 0.3 for a range of applications such as plasmonic, electronic devices, energy, and data storage via kinetic control.

■

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b00063. Detailed analysis of NP diffusion under SVA and 3-D packing of NPs in thin films (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Ph 510-642-1632; Fax 510-6435792 (T.X.). ORCID

Ting Xu: 0000-0002-2831-2095 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors thank Dr. Joseph Strzalka and Dr. Zhang Jiang at beamline 8-ID-E in Advanced Photon Source and Dr. Eric Schaible and Dr. Chenhui Zhu at beamline 7.3.3 in Advanced Light Source for their inputs and assistance on the GISAXS studies. This work was supported by the Department of Energy, Office of Basic Energy Science, under Contract DE-AC0205CH11231 through the “Organic−inorganic Nanocomposites” program at Lawrence Berkeley National Laboratory. Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract DE-AC02-06CH1135.

■

■

REFERENCES

(1) Hoheisel, T. N.; Hur, K.; Wiesner, U. B. Block CopolymerNanoparticle Hybrid Self-Assembly. Prog. Polym. Sci. 2015, 40, 3−32. (2) Sarkar, B.; Alexandridis, P. Block Copolymer−nanoparticle Composites: Structure, Functional Properties, and Processing. Prog. Polym. Sci. 2015, 40, 33−62.

CONCLUSION In summary, the thin film coassembly of NP/supramolecule blends under solvent vapor annealing was investigated where the supramolecule and NP have similar feature sizes at d/L = E

DOI: 10.1021/acs.macromol.7b00063 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules (3) Krishnan, R. S.; Mackay, M. E.; Duxbury, P. M.; Pastor, A.; Hawker, C. J.; Van Horn, B.; Asokan, S.; Wong, M. S. Self-Assembled Multilayers of Nanocomponents. Nano Lett. 2007, 7, 484−489. (4) Bockstaller, M. R.; Mickiewicz, R. A.; Thomas, E. L. Block Copolymer Nanocomposites: Perspectives for Tailored Functional Materials. Adv. Mater. 2005, 17, 1331−1349. (5) Chiu, J. J.; Kim, B. J.; Kramer, E. J.; Pine, D. J. Control of Nanoparticle Location in Block Copolymers. J. Am. Chem. Soc. 2005, 127, 5036−5037. (6) Ruokolainen, J.; Saariaho, M.; Ikkala, O.; ten Brinke, G.; Thomas, E. L.; Torkkeli, M.; Serimaa, R. Supramolecular Routes to Hierarchical Structures: Comb-Coil Diblock Copolymers Organized with Two Length Scales. Macromolecules 1999, 32, 1152−1158. (7) Ruokolainen, J.; ten Brinke, G.; Ikkala, O. Supramolecular Polymeric Materials with Hierarchical Structure-Within-Structure Morphologies. Adv. Mater. 1999, 11, 777−780. (8) Ikkala, O.; ten Brinke, G. Hierarchical Self-Assembly in Polymeric Complexes: Towards Functional Materials. Chem. Commun. 2004, 60, 2131−2137. (9) Zhao, Y.; Thorkelsson, K.; Mastroianni, A. J.; Schilling, T.; Luther, J. M.; Rancatore, B. J.; Matsunaga, K.; Jinnai, H.; Wu, Y.; Poulsen, D.; et al. Small-Molecule-Directed Nanoparticle Assembly towards Stimuli-Responsive Nanocomposites. Nat. Mater. 2009, 8, 979−985. (10) Kao, J.; Thorkelsson, K.; Bai, P.; Zhang, Z.; Sun, C.; Xu, T. Rapid Fabrication of Hierarchically Structured Supramolecular Nanocomposite Thin Films in One Minute. Nat. Commun. 2014, 5, 4053. (11) Kao, J.; Thorkelsson, K.; Bai, P.; Rancatore, B. J.; Xu, T. Toward Functional Nanocomposites: Taking the Best of Nanoparticles, Polymers, and Small Molecules. Chem. Soc. Rev. 2013, 42, 2654−2678. (12) Thompson, R. B.; Ginzburg, V. V.; Matsen, M. W.; Balazs, A. C. Predicting the Mesophases of Copolymer-Nanoparticle Composites. Science 2001, 292, 2469−2472. (13) Bockstaller, M. R.; Lapetnikov, Y.; Margel, S.; Thomas, E. L. Size-Selective Organization of Enthalpic Compatibilized Nanocrystals in Ternary Block Copolymer/Particle Mixtures. J. Am. Chem. Soc. 2003, 125, 5276−5277. (14) Gupta, S.; Zhang, Q.; Emrick, T.; Balazs, A. C.; Russell, T. P. Entropy-Driven Segregation of Nanoparticles to Cracks in Multilayered Composite Polymer Structures. Nat. Mater. 2006, 5, 229−233. (15) Meli, L.; Arceo, A.; Green, P. F. Control of the Entropic Interactions and Phase Behavior of Athermal Nanoparticle/homopolymer Thin Film Mixtures. Soft Matter 2009, 5, 533−537. (16) Kim, J.; Green, P. F. Phase Behavior of Thin Film Brush-Coated Nanoparticles/Homopolymer Mixtures. Macromolecules 2010, 43, 1524−1529. (17) Kim, J.; Green, P. F. Time Evolution of the Topography of Structured Hybrid Polymer/Nanoparticle Systems. Macromolecules 2012, 45, 3496−3502. (18) Kao, J.; Bai, P.; Lucas, J. M.; Alivisatos, A. P.; Xu, T. SizeDependent Assemblies of Nanoparticle Mixtures in Thin Films. J. Am. Chem. Soc. 2013, 135, 1680−1683. (19) Fogg, D. E.; Radzilowski, L. H.; Blanski, R.; Schrock, R. R.; Thomas, E. L. Fabrication of Quantum Dot/Polymer Composites: Phosphine-Functionalized Block Copolymers as Passivating Hosts for Cadmium Selenide Nanoclusters. Macromolecules 1997, 30, 417−426. (20) Xu, C.; Ohno, K.; Ladmiral, V.; Composto, R. J. Dispersion of Polymer-Grafted Magnetic Nanoparticles in Homopolymers and Block Copolymers. Polymer 2008, 49, 3568−3577. (21) Jang, S. G.; Khan, A.; Hawker, C. J.; Kramer, E. J. Morphology Evolution of PS-B-P2VP Diblock Copolymers via Supramolecular Assembly of Hydroxylated Gold Nanoparticles. Macromolecules 2012, 45, 1553−1561. (22) Jang, S. G.; Kramer, E. J.; Hawker, C. J. Controlled Supramolecular Assembly of Micelle-Like Gold Nanoparticles in PSB-P2VP Diblock Copolymers via Hydrogen Bonding. J. Am. Chem. Soc. 2011, 133, 16986−16996.

(23) Lee, J.-Y.; Thompson, R. B.; Jasnow, D.; Balazs, A. C. Entropically Driven Formation of Hierarchically Ordered Nanocomposites. Phys. Rev. Lett. 2002, 89, 155503. (24) Akcora, P.; Liu, H.; Kumar, S. K.; Moll, J.; Li, Y.; Benicewicz, B. C.; Schadler, L. S.; Acehan, D.; Panagiotopoulos, A. Z.; Pryamitsyn, V.; et al. Anisotropic Self-Assembly of Spherical Polymer-Grafted Nanoparticles. Nat. Mater. 2009, 8, 354−359. (25) Chiu, J. J.; Kim, B. J.; Yi, G.-R.; Bang, J.; Kramer, E. J.; Pine, D. J. Distribution of Nanoparticles in Lamellar Domains of Block Copolymers. Macromolecules 2007, 40, 3361−3365. (26) Spontak, R. J.; Shankar, R.; Bowman, M. K.; Krishnan, A. S.; Hamersky, M. W.; Samseth, J.; Bockstaller, M. R.; Rasmussen, K. Ø. Selectivity- and Size-Induced Segregation of Molecular and Nanoscale Species in Microphase-Ordered Triblock Copolymers. Nano Lett. 2006, 6, 2115−2120. (27) Song, D.-P.; Lin, Y.; Gai, Y.; Colella, N. S.; Li, C.; Liu, X.-H.; Gido, S.; Watkins, J. J. Controlled Supramolecular Self-Assembly of Large Nanoparticles in Amphiphilic Brush Block Copolymers. J. Am. Chem. Soc. 2015, 137, 3771−3774. (28) Gu, X.; Gunkel, I.; Hexemer, A.; Gu, W.; Russell, T. P. An In Situ Grazing Incidence X-Ray Scattering Study of Block Copolymer Thin Films During Solvent Vapor Annealing. Adv. Mater. 2014, 26, 273−281. (29) Kao, J.; Bai, P.; Chuang, V. P.; Jiang, Z.; Ercius, P.; Xu, T. Nanoparticle Assemblies in Thin Films of Supramolecular Nanocomposites. Nano Lett. 2012, 12, 2610−2618. (30) Kao, J.; Xu, T. Nanoparticle Assemblies in Supramolecular Nanocomposite Thin Films: Concentration Dependence. J. Am. Chem. Soc. 2015, 137, 6356−6365. (31) Kao, J.; Tingsanchali, J.; Xu, T. Effects of Interfacial Interactions and Film Thickness on Nonequilibrium Hierarchical Assemblies of Block Copolymer-Based Supramolecules in Thin Films. Macromolecules 2011, 44, 4392−4400. (32) Tuteja, A.; Mackay, M. E.; Narayanan, S.; Asokan, S.; Wong, M. S. Breakdown of the Continuum Stokes−Einstein Relation for Nanoparticle Diffusion. Nano Lett. 2007, 7, 1276−1281. (33) Liu, J.; Cao, D.; Zhang, L. Molecular Dynamics Study on Nanoparticle Diffusion in Polymer Melts: A Test of the Stokes− Einstein Law. J. Phys. Chem. C 2008, 112, 6653−6661. (34) van Zoelen, W.; Polushkin, E.; ten Brinke, G. Hierarchical Terrace Formation in PS-B-P4VP(PDP) Supramolecular Thin Films. Macromolecules 2008, 41, 8807−8814.

F

DOI: 10.1021/acs.macromol.7b00063 Macromolecules XXXX, XXX, XXX−XXX