Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
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Diffusion-Dependent Nanoparticle Assembly in Thin Films of Supramolecular Nanocomposites: Effects of Particle Size and Supramolecular Morphology Jingyu Huang,†,∥ Yiwen Qian,†,∥ Katherine Evans,‡,∥ and Ting Xu*,†,‡,§,∥ Department of Materials Science & Engineering, ‡Department of Chemistry, and §Tsinghua−Berkeley Shenzhen Institute, University of California, Berkeley, California 94720, United States ∥ Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States Downloaded via UNIV OF SOUTHERN INDIANA on July 24, 2019 at 08:32:28 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
†
S Supporting Information *
ABSTRACT: Nonequilibrium structures in nanocomposites provide possibilities to modulate the organization of nanofillers beyond the phase diagram and to fabricate functional materials with targeted properties. However, multicomponent systems, such as nanocomposites, have complex phase diagrams and kinetic pathways. Here, effects of two critical parameters, nanoparticle (NP) size and supramolecular morphology, were systematically evaluated in NP/supramolecule blends. NPs in the size range of 5−25 nm were assembled in cylindrical or lamellar supramolecular nanocomposite thin films with periodicities of 20−30 nm under solvent vapor annealing and rapid solvent removal conditions. The ratio of particle size to supramolecular periodicity was tuned between 0.17 and 1.25. The results showed that the vertical diffusion of NPs toward the film surface depends on the matrix morphology. NP surface migration is more prominent in cylindrical thin films than in lamellar thin films. This is mainly attributed to the higher energetic barriers for interdomain diffusion in lamellar morphology. Supramolecular nanocomposite thin films with ordered structures can be obtained by balancing the size-dependent NP diffusivity and energetic factors affecting NP diffusion during the annealing process for a range of NP sizes. Present studies give insight into how to manipulate the assembly kinetics to access targeted morphologies in nanocomposite thin films.
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gies in thin films,10−14 especially in films containing nanofillers with feature sizes comparable to those of the polymer chains.15 In a multicomponent system, various factors need to be considered during the assembly under SVA, such as the solvent fraction ( fs), the particle size (dNP), and the matrix morphology. Increasing fs in a swollen film reduces the glass transition temperature and the viscosity of the matrix and lowers the effective Flory−Huggins interaction parameters (χeff) among the components. Thus, it enhances the system mobility and reduces the thermodynamic driving force at the same time.13 The ratio of NP size (dNP) to polymer size determines the entropic contributions from NP incorporation. It affects the distribution of NPs in the matrix16−18 and the NP assembly pathways in thin films.13,15 As the ratio of NP size (dNP) to the matrix periodicity (L) exceeds 0.3, the formation of NP assemblies competes with NP segregation at interfaces or in defects due to the entropic penalties associated with polymer chain deformation after NP incorporation.17,19−21 NP’s diffusivity in a polymer matrix increases rapidly when the
INTRODUCTION Hierarchically structured nanocomposite thin films are desirable for the fabrication of functional materials.1,2 Numerous approaches have been explored to control the spatial organization of nanofillers with high precision, including in situ nanoparticle (NP) growth,3 DNA-guided assembly,4 controlled evaporation,5 external field-guided assembly,6 and block copolymer (BCP)-guided assembly.7 Thin films of NP/ polymer blends are particularly attractive due to their scalability, processability, and compatibility with existing device fabrication processes. A range of polymer-guided NP assemblies, such as stacks of two-dimensional NP sheets, NP chains, and three-dimensional NP networks, have been successfully obtained in nanocomposite thin films with NP size smaller than 10 nm.8 The current challenge is to narrow the gap between what can be fabricated and the required spatial arrangement of each building block to access targeted properties. The morphology of a nanocomposite is governed not only by the phase diagram and the energetic driving forces but also by the kinetic pathways during processing. Kinetic control via solvent vapor annealing (SVA)9 has been demonstrated to be an effective approach to accessing nonequilibrium morpholo© XXXX American Chemical Society
Received: February 19, 2019 Revised: July 9, 2019
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DOI: 10.1021/acs.macromol.9b00362 Macromolecules XXXX, XXX, XXX−XXX
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were purchased from Polymer Source, Inc. 3-n-Pentadecylphenol (90−95%) was purchased from Acros Organics. Chloroform (amylene as a preservative) was purchased from Fisher Scientific. Solutions of 5 and 10 nm maghemite (Fe2O3) NPs and 15, 20, and 25 nm magnetite (Fe3O4) NPs were purchased from Ocean Nanotech. The iron oxide NPs are coated by oleic acids and dispersed in chloroform. All of the chemicals and materials were used as received with no further purification. Sample Preparation. The actual NP core sizes in different batches vary and are analyzed in Figure S1. The ligand layer on the NPs was estimated to be ∼2 nm in thickness. Supramolecular solution was prepared by mixing PS-b-P4VP with an appropriate amount of PDP small molecules in chloroform and stirring overnight. NPs were mixed with supramolecules in solution by vigorous shaking. The solution mixture was spin-cast onto a Si wafer to make a film with thickness of ∼100 nm. The NP volume fraction (vol %) in the nanocomposite thin films is ∼9 vol %. The as-cast thin film was placed in a 125 mL top-capped glass vial, and 300 μL of chloroform was injected into the vial for SVA at 22.5 °C. The film thickness was monitored in situ with a white-light interferometer (Filmetrics F20). The solvent volume fraction (fs) in the swollen film was calculated based on the thickness of the film before and during SVA. Once fs reached a targeted value, the vial was uncapped to let the chloroform evaporate freely. The film was removed from the vial quickly, and the swollen film was dried rapidly to its original thickness in < 5 s. Typical solvent swelling profiles in cylindrical and lamellar nanocomposite thin films are shown in Figure S2. The nanocomposite morphology and number of NPs on the film surface depend on fs rather than on annealing time in the time scale of this study, which has also been demonstrated before.15 Atomic Force Microscopy (AFM). AFM imaging was performed on a Bruker Dimension Edge AFM using Si RTESPA-150 probes in tapping mode. The spring constant of the cantilever is 6 N/m with a resonant frequency of ∼150 kHz. The lateral periodicity of a thin film was extracted from the fast Fourier transform of the AFM phase images. The number of NPs on the film surface was counted based on the AFM phase images. All of the results were averaged over six AFM images taken from different spots of the film. Transmission Electron Microscope (TEM). TEM imaging was performed on an FEI Tecnai 12 electron microscope with an accelerating voltage of 120 kV. Samples for cross-sectional TEM studies were prepared in a similar way as mentioned above, except that the Si wafer was replaced by a polystyrene (PS)-coated NaCl disk. (R)-Hydroxyl ω-benzocyclobutene (3 wt %) monomers were incorporated in PS (MW = 30 kg/mol, PDI = 1.21) as the crosslinking agent. PS was dissolved in toluene and spin-cast on the NaCl disk to get a 20−30 nm layer. The disk was then heated at 245 °C for 20 min under nitrogen flow for cross-linking. The substrate was washed with toluene and chloroform in sequence to remove un-crosslinked polymers. Post SVA treatment, the nanocomposite film was floated off the substrate and caught on a cured epoxy block by dipping the salt disk in deionized water. The film was then sectioned using an RMC MT-X ultramicrotome (Boeckeler Instruments). The thin sections, ∼60 nm in thickness, were caught on a 200-mesh carbon film-coated copper grid for imaging under TEM after staining in I2 vapor for 2 h. Grazing-Incidence Small-Angle X-ray Scattering (GISAXS). GISAXS measurements of the thin films were performed at the synchrotron beamline Advanced Photon Source 8-ID-E in Argonne National Lab with a 1.687 Å (7.35 keV) X-ray source. The scattered X-ray intensity distribution was detected using a high-speed version of the Pilatus 1M detector. Images were plotted as intensities (I) versus q, where q = (4π/λ) sin(θ), where λ is the wavelength of the incident X-ray beam and θ is the scattering angle. Line-cut profiles of GISAXS patterns were extracted using Igor Pro with the Nika package.
NP size gets smaller than that of a polymer. Small NPs only experience the microscopic viscosity from their local polymer environment, which is much smaller than the experimentally measured macroscopic viscosity of the polymer matrix.22−26 Meanwhile, the dynamics of polymer diffusion in a nanocomposite is affected by the NPs’ size,27 loading,27,28 polydispersity,27 anisotropy,29,30 and surface modifications.31,32 Within a nanostructured matrix, the diffusion of a component can be deconvoluted into two directions, i.e., parallel (Dpara) and perpendicular (Dperp) to the interfaces of microdomains.33,34 Anisotropic polymer diffusion dynamics in BCPs35−40 have been extensively investigated in disordered41 and ordered states,34 in lamellar,34−37,39 cylindrical,38,40 and spherical42 morphologies, and in entangled36 and unentangled39 BCP matrices. The diffusion across the interfaces of the two blocks (Dperp) is usually much slower than that within one microdomain (Dpara), since the diffusion across interfaces needs to overcome a higher energetic barrier that is proportional to χN,36,43 where N is the BCP degree of polymerization. However, there is a lack of systematic studies on NP diffusion in microphase-separated systems. In BCP-based supramolecular systems, the small molecules, often hydrogen-bonded to one of the BCP blocks, selectively compatibilize NPs within one type of a microdomain.44 The PS-b-P4VP(PDP) supramolecule, composed of polystyreneblock-poly(4-vinylpiridine) (PS-b-P4VP) and the hydrogenbonded small molecule, 3-pentadecylphenol (PDP),45,46 has been successfully applied to assemble a series of alkylpassivated NPs such as Au, CdSe, CdS, PbS, CuS, and Fe3O4 for a range of NP sizes, forming well-ordered assemblies independent of the particle core chemistry.13,17,47−49 However, in thin films with dNP/L > 0.3, NPs were expelled toward the film surface, provided they have enough mobility.15,17 This expulsion is due to the low surface energy of the NPs’ alkyl ligands19,20 and can also relieve entropic penalties associated with deforming polymer chains.15,17 The formation of the supramolecule relies on the noncovalent attachment of small molecules, which also changes the chain architecture from coil−coil to coil−comb. All of these factors significantly increase the system complexity. However, the ability to achieve highly ordered NP assemblies and the existing knowledge of their phase behavior make the supramolecular system an ideal model for performing systematic studies on the diffusiongoverned assembly process. Here, we investigate the effects of particle size and supramolecular morphology on the diffusion-dependent NP assembly in nanocomposite thin films via SVA with rapid solvent removal. The thin films contain 5−25 nm NPs blended with cylindrical or lamellar supramolecules with 20−30 nm periodicities, resulting in dNP/L ratios ranging from 0.17 to 1.25. This study mainly focuses on the size ratios > 0.5, which have not been systematically studied before. In films containing 15−25 nm NPs, the fast diffusion of NPs toward the film surface is observed in cylindrical films at fs > 0.5−0.6. However, NP surface migration is not as prominent in lamellar films under similar annealing conditions. This result is mainly attributed to the larger total energetic barriers for NP interdomain diffusion (Dperp) in the lamellar films than those for NP intradomain diffusion (Dpara) in the cylindrical films.
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RESULTS AND DISCUSSION In the PS-b-P4VP(PDP)r supramolecule, r represents the molar ratio of PDP/4VP monomer.45,46 Thin films of PS(19
MATERIALS AND METHODS
Materials. BCPs of PS(19 kDa)-b-P4VP(5.2 kDa) [polydispersity index (PDI) = 1.10] and PS(40 kDa)-b-P4VP(5.6 kDa) (PDI = 1.10) B
DOI: 10.1021/acs.macromol.9b00362 Macromolecules XXXX, XXX, XXX−XXX
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shown that in cylindrical films, 5.5 nm Au NPs51 preferentially fill the top layers of the film at low particle loadings, whereas 9.3 nm PbS NPs17 and 20 nm Fe3O4 NPs15 migrate toward the film surface after SVA. Previous studies have probed the kinetic pathways of NP assembly in thin films.13−15 The terminal solvent fraction ( fs) before rapid solvent removal was determined to be the key parameter for achieving a high degree of order.13,15 In cylindrical films, the best order was reached at fs = 0.31 for 6 vol % of 5.5 nm Au NPs13 and at fs = 0.62 for 9 vol % of 20 nm Fe3O4 NPs.15 In lamellar films, the best order was reached at fs = 0.43 for 9 vol % of 3 and 12 nm Au NPs.14 NP Assembly in Cylindrical Nanocomposite Thin Films. Figure 1 shows AFM phase images of ∼100 nm films containing 9 vol % of 5−25 nm NPs and the cylindrical supramolecule PS(19 kDa)-b-P4VP(5.2 kDa)(PDP)1.7. The films were annealed under chloroform vapor to various fs followed by rapid solvent removal. The as-cast (fs = 0) films are disordered with randomly distributed NPs. The patterns of the bright cylindrical PS domains are disrupted by dark NP clusters in films with 5.6 nm NPs at fs = 0.35−0.55 (Figure 1a) and in films with 10.7 nm NPs at fs = 0.45−0.55 (Figure 1b), suggesting NP segregation on the film surface under these SVA conditions. Ordered morphologies without an obvious NP surface segregation were obtained at fs = 0.3 in films with 5.6 nm NPs, similar to previously published results using 6 vol % of 5.5 nm Au NPs13 and at fs = 0.35 in films with 10.7 nm NPs. In films with 15.2 nm NPs (Figure 1c), the supramolecule forms parallel cylindrical patterns at fs ≤ 0.4, and gradually transforms into micelle-like structures at fs ∼ 0.4−0.6, where
kDa)-b-P4VP(5.2 kDa)(PDP)1.7 and PS(40 kDa)-b-P4VP(5.6 kDa)(PDP)1 supramolecules form parallel cylindrical and parallel lamellar morphologies after SVA, and have periodicities of 30 and 20 nm on Si wafers, respectively.47 Iron oxide NPs (5−25 nm) passivated with oleic acid ligands were used as model nanofillers. The alkyl ligands and PDP small molecules have favorable van der Waals interactions, and NPs can be selectively included in the P4VP(PDP) domain.44 There is a batch-to-batch NP size variation as shown in Figure S1. The actual sizes of NPs used in this study are listed in Table 1. The Table 1. Actual Sizes of 5−25 nm Iron Oxide NPs Used in the Current Study NP batches (nm) 5 10 15 20 25
(Fe2O3) (Fe2O3) (Fe3O4) (Fe3O4) (Fe3O4)
batch 1, dNP (nm) batch 2, dNP (nm) batch 3, dNP (nm) 5.6 10.7 14.6 18.8 24.5
11.3 15.2 20.4
16.4 22.5
parameter dNP/L governs the entropic contributions after NP incorporation and influences the NP distribution16,17 and assembly pathways13,15 in nanocomposites. The dNP/L ratio ranges from 0.17 to 1.25 in this study. Thermodynamically, the incorporation of a small quantity of NPs stabilizes the defects or grain boundaries in BCP/NP blends.16,50 However, increasing the volume fraction or size of NPs leads to the segregation of NPs on the film surface.17,51 For the supramolecular nanocomposites studied here, it was
Figure 1. AFM phase images of ∼100 nm PS(19 kDa)-b-P4VP(5.2 kDa)(PDP)1.7 supramolecular thin films containing 9 vol % of (a) 5.6 nm and (b) 10.7 nm Fe2O3 NPs and 9 vol % of (c) 15.2 nm, (d) 20.4 nm, and (e) 24.7 nm Fe3O4 NPs as a function of solvent volume fraction (fs) before solvent removal (scale bar: 100 nm). PS is shown as the brighter domain in the AFM phase images. NPs (15.2, 20.4, and 24.7 nm) appear as bright dots in films. Highlighted images show the NP assemblies with the best order in the top layer for each particle size. C
DOI: 10.1021/acs.macromol.9b00362 Macromolecules XXXX, XXX, XXX−XXX
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Figure 2. Top row: AFM phase images of ∼100 nm PS(19 kDa)-b-P4VP(5.2 kDa)(PDP)1.7 supramolecular thin films containing 9 vol % of 5−10 nm Fe2O3 NPs and 15−25 nm Fe3O4 NPs (scale bar: 100 nm). Bottom row: the corresponding top-view schemes showing the component organizations and lateral periodicities on the film surface (yellow: NP, blue: P4VP(PDP), purple: PS, black: arrangement of the coil−comb supramolecular chains and distribution of PDP small molecules).
the NP resides in the P4VP(PDP) block and the PS block forms the outer shell, as schematically shown in Figure 2d,e. At fs = 0.4, NPs reside in the distorted cylindrical microdomains or get wrapped in micelle-like structures. The numbers of NPs and micelle-like structures in the top layer increase with fs. At fs = 0.6, a mixed supramolecular morphology containing a majority of micelle-like structures and a small fraction of short cylindrical structures forms. The number of NPs in each micelle-like structure varies from one to three. In films with 20.4 (Figure 1d) and 24.7 (Figure 1e) nm NPs, NP density in the top layer also increases with fs. A similar morphological transition appears around fs ∼ 0.5−0.6, and most micelle-like structures contain only one particle. Long-range order of hexagonally packed NP arrays is observed at fs = 0.62 in films with 20.4 nm NPs. Higher fs is required for larger NPs to form fully packed particle structures on the film surface, and this may be due to size-dependent particle diffusion. The surface morphologies of nanocomposite films containing 5−25 nm NPs are summarized in Figure 2. Films with 5.6 and 10.7 nm NPs show supramolecule-guided NP assemblies forming parallel cylindrical morphologies (Figure 2a,b) with lateral periodicities of 30 and 35 nm, respectively. The film with 15.2 nm NPs starts to show NP/supramolecule coassembly. NPs (15.2 nm) are incorporated either in the distorted cylindrical domains with an expanded periodicity of 41 nm or in the micelle-like structures with an interparticle distance (LIP) of 41 nm (Figure 2c). The micelle-like structures seem to be a direct transformation from the distorted cylindrical structures, where the PS cylinders bend and connect in the center part of two adjacent NPs, as schematically illustrated in Figure 2c. As the particle size further increases to 20.4 or 24.7 nm, NPs are mostly incorporated in the micelle-like structures and spread across the entire surface layer with LIP of 46 and 51 nm, respectively (Figure 2d,e). GISAXS was performed to investigate the ordering of 15−25 nm NPs in the thin films. Figure 3a shows the GISAXS horizontal line-cut profiles of films containing 20.4 nm NPs annealed to fs = 0.3−0.62. Structure factors (Figure 3b) of NP packing were extracted by dividing the overall GISAXS intensity in Figure 3a by the fitted form factors of the NPs. As fs increases above 0.6, the structure factors of hexagonally
Figure 3. Horizontal GISAXS line-cut profiles of PS(19 kDa)-bP4VP(5.2 kDa)(PDP)1.7 supramolecular thin films containing (a) 9 vol % of 20.4 nm NPs as a function of fs and (b) the extracted structure factors of NP packing in (a); (c) 9 vol % of 15.2, 20.4, and 24.7 nm NPs (the same films as shown in Figure 2c−e) and (d) the extracted structure factors of NP packing in (c). For all of the line-cut profiles, qz = 0.035 Å−1 and the incident angle is 0.15°.
packed NP arrays start to appear, consistent with the AFM images in Figure 1d. Figure 3c shows the GISAXS horizontal line-cut profiles of films containing 15−25 nm NPs, the same films as those shown in Figure 2c−e. Based on the structure factors in Figure 3d, 15.2 and 24.7 nm NPs form arrays with poor order as evidenced by the broad first- and second-order peaks, while 20.4 nm NPs form hexagonally packed arrays. The poor order of 24.7 nm NPs could be due to the larger polydispersity of the particle size, as shown in Figure S1. The interparticle distances extracted from the GISAXS profiles are consistent with the values obtained from the AFM images in Figure 2c−e. Based on the above observations, the formation of ordered morphologies depends on the NP size. The incorporation of 15−25 nm NPs deforms the supramolecular microdomains (Figure 2c−e). Thus, ordered cylindrical structures similar to the supramolecule-guided assembly of 5 and 10 nm NPs (Figure 2a,b) cannot form. Instead, the formation of micelleD
DOI: 10.1021/acs.macromol.9b00362 Macromolecules XXXX, XXX, XXX−XXX
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Figure 4. (a−f) Top row: AFM phase images of ∼100 nm PS(19 kDa)-b-P4VP(5.2 kDa)(PDP)1.7 supramolecular thin films containing 9 vol % of NPs in the size range of 14.6−22.5 nm (scale bar: 100 nm). Middle row: Delaunay triangulation of NPs in the top layer of the films. The mass centers of NPs are marked as colored dots, and the neighboring particles are connected with blue lines. Red, blue, green, or black particles have 6, 5, 7, or other numbers of neighbors, respectively. Bottom row: the angular distributions of the blue-lined triangles and the curves in Gaussian fitting. (g) Mean and FWHM of the Gaussian-fitted angular distribution as a function of NP size. (h) Interparticle distance (LIP) and the ratio of dNP to LIP as a function of NP size.
like structures was observed in these films as a result of bending PS cylinders around the NPs. Figure 4a−f shows the AFM phase images of ∼100 nm films containing 9 vol % of 14.6−22.5 nm NPs forming micelle-like structures. NPs in the AFM images are processed with Delaunay triangulation [details in Supporting Information (SI)]. Each NP is connected with its neighboring particles using blue lines. NPs with six neighbors are marked as red dots. Hexagonally packed NPs with better order have a smaller full width at halfmaximum (FWHM) of angular distribution and a mean angle value closer to 60° in the fitted Gaussian curves. Based on Figure 4g, 14.6 and 15.2 nm NPs show poor hexagonal packing, 16.4 nm NPs show partial hexagonal packing, and 18.8−22.5 nm NPs show good hexagonal packing. As dNP increases from 14.6 to 22.5 nm, LIP increases from 41 to 49 nm, and the ratio of dNP to LIP increases from 0.37 to 0.46 (Figure 4h). Based on Figure 4, the formation of hexagonally packed micelle-like structures wrapped around single NPs is less favored for 14.6 and 15.2 nm NPs than for NPs larger than 16.4 nm. NP Assembly in Lamellar Nanocomposite Thin Films. Figure 5 shows AFM phase images of ∼100 nm films containing 9 vol % of 5−25 nm NPs and the lamellar
supramolecule PS(40 kDa)-b-P4VP(5.6 kDa)(PDP)1. The films were annealed to various fs resembling the experimental conditions for the cylindrical films in Figure 1. Due to a batchto-batch NP size variation, some of the NPs shown in Figure 5 have different sizes from the NPs in Figure 1. The supramolecule forms a parallel lamellar morphology on the Si substrate with the P4VP(PDP) domain at the film/air interface due to the low surface tension of the P4VP(PDP) block.47 In films with 5.6 nm NPs (Figure 5a), particles are present on the film surface. In some regions, NPs pack hexagonally as seen in the zoomed-in image in Figure S3, similar to the assembly of 6 vol % of 6.1 nm Au NPs in the lamellar films.47 In films with 11.3 (Figure 5b) and 16.4 (Figure 5c) nm NPs, particles form chains or networks on the film surface. In films with 22.5 (Figure 5d) and 24.7 (Figure 5e) nm NPs, NP densities in the top layer are close to each other but largely reduced compared to those of 16.4 nm NPs. NPs either disperse or form chain structures on the film surface. Since the iron oxide NPs may have magnetic attraction that leads to NP chain formation, the assembly of 10 nm Au NPs in the lamellar thin films was performed as the control study (Figure S4). Au NP chains were also observed at the particle loading of 6 vol %, indicating that the NP chain E
DOI: 10.1021/acs.macromol.9b00362 Macromolecules XXXX, XXX, XXX−XXX
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Figure 5. AFM phase images of ∼100 nm PS(40 kDa)-b-P4VP(5.6 kDa)(PDP)1 supramolecular thin films containing 9 vol % of (a) 5.6 nm and (b) 11.3 nm Fe2O3 NPs and 9 vol % of (c) 16.4 nm, (d) 22.5 nm, and (e) 24.7 nm Fe3O4 NPs as a function of solvent volume fraction (fs) before solvent removal (scale bar: 100 nm).
Figure 6. Cross-sectional TEM images and the corresponding profile analysis of 100−120 nm PS(40 kDa)-b-P4VP(5.6 kDa)(PDP)1 supramolecular thin films containing 9 vol % of (a) 5.6 nm and (b) 11.3 nm Fe2O3 NPs after SVA to fs = 0.3, 0.45, or 0.6, and (c) 9 vol % of 16.4 nm and (d) 18 vol % of 22.5 nm NPs after SVA to fs = 0.45 (scale bar: 30 nm). The section thicknesses are ∼60 nm in (a) and (b) and ∼120 nm in (c) and (d). The TEM profile analysis is averaged through the boxed region in (a) and (b) and the entire image in (c) and (d).
and the bottom of the films is slightly rough due to the NaCl disk substrate. The TEM profile was analyzed by averaging the boxed region in each image, where the film/substrate interface is flat. For 5.6 and 11.3 nm NPs, particles are well confined within the P4VP(PDP) domain, and the best order of parallel lamellar structures was observed at fs = 0.45 with a periodicity of 21−22 nm (Figure 6a,b). This is consistent with our previous observation of 9 vol % of 3 and 12 nm Au NPs.14 At fs = 0.3 and 0.6, the parallel lamellae are not well-ordered due to the synergetic effects of the solvent field and the thermodynamic driving force during rapid solvent removal.14 The assembly behavior of 10 nm Fe3O4 NPs (Figure S5) is similar to the 12 nm Au NPs14 and 11.3 nm Fe2O3 NPs in the lamellar films. This further confirms that the assembly of supra-
formation in Figure 5 is not mainly due to the magnetic attraction forces between the iron oxide NPs. This is reasonable, since the critical size for the transition from superparamagnetic to ferrimagnetic of Fe3O4 NPs is ∼30 nm.52 In fact, anisotropic NP chain or patch formation has been reported in colloidal spherical NPs before.53,54 To probe the lamellar ordering, cross-sectional TEM images were collected in films containing 5−20 nm NPs after SVA to fs = 0.3, 0.45, or 0.6 and are shown in Figure 6. To prepare films for cross-sectional TEM, the films were fabricated on NaCl disks coated by cross-linked PS. No differences in nanocomposite behavior were observed between the films fabricated on Si wafers and NaCl disks based on AFM results and previous studies.47,51 The film surface is facing upward, F
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macroscopic viscosity of the polymer matrix.22 We first estimated the diffusivities of 5−25 nm NPs in homopolymer matrices as a function of fs. All of the NPs can easily diffuse 100 nm in less than 10 ms at fs > 0.3, as shown in Figure S6 (details are given in SI). Based on Figure 7, NP diffusivity in the supramolecular matrices is much slower than that in homopolymer matrices. Such hindered NP diffusion rates can be attributed to the energy barriers from the microphaseseparated nanostructures. In microphase-separated BCPs, the diffusion of polymer chains can be slowed by the existence of interfaces and Dperp decays exponentially with increasing χN.40 Large diffusion anisotropy (Dpara/Dperp), as high as 40−80, has been observed in unentangled lamellar39 and cylindrical38 BCPs. NP diffusion toward the film surface can be achieved within a continuous matrix of P4VP(PDP) in the cylindrical films (Dpara) but requires the penetration of the PS microdomains in the lamellar films (Dperp). The activation energy for NP diffusion (ΔG) in the nanocomposites consists of the changes in interfacial energy (ΔHPS/P4VP(PDP), ΔHPS/NP) and supramolecular chain conformational entropy (ΔSSM). ΔH is proportional to χ (1 − fs ) ΔA , 13,55 where ΔA is the interfacial area change and increases with NP size. Both ΔHPS/P4VP(PDP) and ΔHPS/NP decrease with increasing fs. ΔSSM depends on the entropic penalties associated with deforming P4VP(PDP) and PS microdomains by the NP. As fs increases above 0.2−0.3, the rigid P4VP(PDP) comb block softens13 and ΔSSM contributes less to ΔG. In the cylindrical films, 5−10 nm NPs are not expected to deform the PS domains (Figure 2a,b), but movements of 15−25 nm NPs should deform PS cylinders (Figure 2c−e). Thus, the diffusion of 15−25 nm NPs in the P4VP(PDP) matrix is expected to have higher energy barriers than 5−10 nm NPs. Compared to that in the cylindrical films, the diffusion of 5−25 nm NPs toward the film surface in the lamellar films has to overcome extra energy barriers from ΔHPS/NP during interdomain diffusion. This results in the much slower surface migration rate of NPs in the lamellar films (Dperp) than in the cylindrical films (Dpara), as shown in Figure 7. Solvent fraction fs is a critical parameter for NP diffusivity in the films. At low fs < 0.3, both χeff and the matrix viscosity are large enough to prohibit the NP’s mobility.13 At moderate fs ∼ 0.3−0.45, 5−10 nm NPs gain enough mobility to assemble.13,14 At high fs > 0.5, χeff and the matrix viscosity are further reduced. In the cylindrical films, the system is in a miscible state as free PDP becomes compatible with the PS block at fs > 0.5,15 which explains the large enhancement of Dpara for 15−25 nm NPs, as shown in Figure 7a. In the lamellar films, the diffusion of 10−15 nm NPs toward the film surface is not apparent, whereas there is a slight enhancement of Dperp for 20−25 nm NPs at fs > 0.5, as shown in Figure 7b. This could be explained by the lamellar fluctuation after the incorporation of 20−25 nm NPs. Based on our previous in situ GISAXS,14 the lamellar periodicity in the swollen film is ∼34 nm at fs ∼ 0.5. The sizes of 20−25 nm NPs are larger than the thickness of the P4VP(PDP) domain. Thus, the inclusion of 20−25 nm NPs requires bending the lamellar microdomains. Such fluctuations are similar to those observed in BCPs under electric field56,57 or during order-toorder transition58 and have intrinsic periodicities close to the lamellar periodicities.56 The energy barriers for the diffusion of 20−25 nm NPs across the fluctuated microdomains might be smaller than the barrier for the diffusion of 10−15 nm NPs
molecular nanocomposites is independent of the particle core chemistry within the scope of our studies. Parallel lamellar structures of NP arrays are also seen in cross sections of films containing 9 vol % of 16.4 nm NPs and 18 vol % of 22.5 nm NPs after SVA to fs = 0.45 (Figure 6c,d). The section thicknesses are ∼60 nm in Figure 6a,b and ∼120 nm in Figure 6c,d. The section thickness and/or particle loading were increased to increase the total number of NPs in the thin films. With a small quantity of NPs, it is hard to confirm if 16.4 and 22.5 nm NPs can still form ordered structures. According to the TEM profile analysis, NP surface enrichment as a function of fs is not obvious in films with either 5.6 or 11.3 nm NPs (Figure 6a,b). Unlike the fast NP surface migration at fs > 0.5− 0.6 in cylindrical films as shown in Figure 1c−e, a 15−25 nm NP diffusion toward the film surface is not as obvious in the lamellar films based on the AFM images in Figure 5c−e. Comparison of NP Assembly in Cylindrical and Lamellar Nanocomposite Thin Films. NP assembly in ∼100 nm nanocomposite thin films was studied by varying the particle size, supramolecular morphology, and annealing condition. Migration of NPs toward the film surface is more prominent in the cylindrical films than in the lamellar films, as summarized in Figure 7. In the cylindrical films, NP surface
Figure 7. NP densities (number of particles in a unit area) in the top layer of (a) cylindrical (PS(19 kDa)-b-P4VP(5.2 kDa)(PDP)1.7) and (b) lamellar (PS(40 kDa)-b-P4VP(5.6 kDa)(PDP)1) supramolecular thin films containing 9 vol % of 10−25 nm NPs, as summarized in Figures 1 and 5. The actual sizes of NPs used in the experiments are listed in the parentheses of the legends.
segregation was observed at fs = 0.35 for 5.6 nm NPs and at fs = 0.45 for 10.7 nm NPs (Figure 1a,b). The densities of 15−25 nm NPs in the top layer of the cylindrical films increase abruptly when fs > 0.5−0.6 (Figure 7a). In the lamellar films, the densities of 5.6 and 11.3 nm NPs in the top layer do not change much with increasing fs (Figure 6a,b). The densities of 11.3 and 16.4 nm NPs remain fairly constant with increasing fs (Figure 7b). For 22.5 and 24.7 nm NPs, there appears to be a slight increase of NP diffusion rate at fs > 0.48 (Figure 7b). In summary, NP diffusion toward the film surface depends on dNP, fs, and supramolecular morphology. The vertical diffusion of 5-25 nm NPs is less obvious in films with a parallel lamellar morphology than in films with a parallel cylindrical morphology, where the matrix periodicity is ∼20−30 nm. In a homopolymer matrix, NP diffusion depends on the ratio of particle size (dNP) to the characteristic lengths of a polymer such as the radius of gyration (Rg), correlation length (ξ) in unentangled polymers, and entanglement mesh size or tube diameter (aT) in entangled polymers.25 The diffusivity of NPs with a size smaller than the characteristic polymer length deviates from the Stokes−Einstein relation and experiences a much smaller viscosity than the experimentally measured G
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Macromolecules across the microdomains with sharper interfaces. Free PDP is another critical parameter for NP diffusivity. When the PDP/ 4VP molar ratio in the cylindrical films is reduced to 1, the supramolecule PS(19 kDa)-b-P4VP(5.2 kDa)(PDP)1 shows a perforated morphology on the film surface (Figure S7a). Such perforated morphologies are usually observed in thin films at a P4VP(PDP) volume fraction between the lamellar and PS cylindrical regimes after SVA. At fs > 0.5, the diffusivity of 20 nm NPs toward the film surface in the perforated films is larger than that in the lamellar films but slower than that in the cylindrical films (Figure S7e). In another cylindrical supramolecule PS(23 kDa)-b-P4VP(16.5 kDa)(PDP)1, the diffusivity of 20 nm NPs is similar to that in the perforated films at fs > 0.5 (Figure S7e). (More details are given in the Supporting Information.) These observations further qualitatively confirm that the energy barriers for NP interdomain diffusion (Dperp) in the lamellar morphology are larger than those for NP intradomain diffusion (Dpara) in the cylindrical morphology when there is no extra free PDP. Moreover, the presence and redistribution of free PDP in the cylindrical films could possibly further reduce the energy barriers for NP diffusion at high fs. Overall, the surface migration of 5−25 nm NPs in the lamellar films (Figures 5 and 6) is still largely prohibited compared to that in the cylindrical films (Figure 1), mainly due to larger total energy barriers in the lamellar films.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Ting Xu: 0000-0002-2831-2095 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division under Contract No. DEAC02-05-CH11231 (Organic−Inorganic Nanocomposites KC3104). GISAXS was done at the Advanced Photon Source, and the 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. Scattering study at the Advanced Light Source is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract DE-AC02-05-CH11231.
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CONCLUSIONS In summary, the assembly of 9 vol % of 5−25 nm NPs in cylindrical and lamellar supramolecular nanocomposite thin films with periodicities of 20−30 nm was systematically studied under SVA and rapid solvent removal. The ratio of particle size to supramolecular periodicity was varied from 0.17 to 1.25. The effects of particle size and supramolecular morphology on the diffusion-dependent NP assembly were investigated. Ordered NP assemblies for a range of NP sizes can be obtained in thin films by balancing the energetic driving force and kinetic pathways of the NP diffusion toward the film surface during SVA. In the cylindrical films, 5−10 nm small NPs show supramolecule-guided assembly at moderate fs ∼ 0.3−0.35 and 15−25 nm large NPs show hexagonally packed micelle-like morphologies through supramolecule/NP coassembly at high fs ∼ 0.6−0.65. In the lamellar films, a solvent fraction of ∼0.45 results in parallel lamellar NP structures with the best order. The diffusion of 5−25 nm NPs toward the film surface is much less prominent in films with a parallel lamellar morphology than in films with a parallel cylindrical morphology. This is mainly attributed to the higher energetic barriers for interdomain diffusion in the lamellar films. The current study provides insights into how the relative NP size versus polymer size and the polymeric matrix morphology can affect the diffusion-dependent NP assembly in nanocomposite thin films. These results also demonstrate the feasibility of fabricating thin films of functional materials with targeted morphologies through the control of kinetic pathways.
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additional cross-sectional TEM images and AFM images of the nanocomposite thin films, and size-dependent NP diffusivity in polymer matrices (PDF)
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b00362. NP size analysis, swelling profiles of thin films during SVA, Delaunay triangulation of the AFM images, H
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