Fabrication of Amphiphilic Nanoparticles via Mixed Homopolymer

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

pubs.acs.org/Macromolecules

Fabrication of Amphiphilic Nanoparticles via Mixed Homopolymer Brushes and NMR Characterization of Surface Phase Separation Brenda Guzman-Juarez, Ahmed Abdelaal, Kuenhee Kim, Violeta Toader, and Linda Reven* Quebec Center for Advanced Materials (QCAM), Department of Chemistry, McGill University, 801 Sherbrooke St. W., Montreal, QC H3A 0B8, Canada

Macromolecules Downloaded from pubs.acs.org by UNIV OF GOTHENBURG on 12/01/18. For personal use only.

S Supporting Information *

ABSTRACT: “Patchy particles”, where the surface is anisotropically patterned through variation in the surface composition, can assemble into different colloidal crystal structures as well as act as interface stabilizers, heterogeneous reaction catalysts, and targeted drug delivery agents. Patchy nanoparticles (NPs) can be formed by adsorbing two chemically different polymer chains that will spontaneously phase separate. Although there is growing interest in polymer-based patchy nanoparticles, the majority of the studies have been theoretical rather than experimental due to difficulties in preparing significant quantities of nanoparticles with controlled polymer ratios. Likewise, characterization of the phase separation on the nanoparticle surface is challenging. Here we simultaneously overcome the synthesis and characterization hurdles by developing a facile, versatile protocol to produce sufficient quantities of patchy NPs for quantitative solid-state NMR measurements of the patch fractions, degree of phase separation, and morphology. Monodisperse 3.5 nm ZrO2 nanocrystals with polystyrene (PS) and poly(ethylene oxide) (PEO) ligands, covering the entire possible composition range, were reproducibly prepared through a simple exchange process. This approach has the advantage of well-defined polymer molecular weights and NP sizes, allowing experimental validation of theoretical predictions for nanophase separation in NPs with mixed homopolymer brushes. Upon exposure to a nonselective solvent, the nanoparticles assemble into different morphologies, namely micelles and vesicles, as a function of the PEO:PS ratios. termed “binary hairy nanoparticles”. For sufficiently long chain lengths relative to the particle diameter, the chains can freely rearrange to phase separate, forming patches of different polymers on the particle surface. The number and distribution of these patches will depend on a number of variables, including the polymer molecular weight (chain length or coil size), the attachment density, the surface curvature (i.e., particle diameter), and exposure to good versus selective solvents.13 Although there is growing interest in polymer-based patchy nanoparticles, the majority of the studies have been theoretical rather than experimental.14 Theoretical studies indicate that phase-separated mixed homopolymer brushes can be used to program the spatial arrangement of nanoparticles into different crystalline superstructures if the size ratio between the nanoparticle core and polymer coil is controlled.15 As a result of the influence of particle environment on the responses of the constituent surface patches, patchy particles have the potential to be used in a broad range of applications including interface stabilization, biosensing, and targeted drug delivery.16 The bias for theoretical over experimental research into binary hairy nanoparticle assemblies is due to the difficulty in

1. INTRODUCTION In nature, surface morphologies normally present anisotropic arrangements that provide localized functionalities and confer the ability to selectively respond to different environmental stimuli. In an effort to add this type of functionality to artificial colloidal systems, “patchy particles”, which bear surface anisotropy through variation in surface composition, are a topic of current experimental1,2 and theoretical research.3 Patchy particles have been defined as patterned particles with at least one well-defined patch through which the particles can experience anisotropic and directional interactions with other particles or surfaces.3 The directional interactions allow the assembly of patchy particles into novel superstructures.4 The general approaches to producing patchy particles include use of microfluidic devices (combined with lithographic patterning), Pickering emulsions, electrohydrodynamic jetting, techniques exploiting phase separation in confined volumes, and emulsion polymerization.1 There are numerous efforts to control the patch number per particle, the patch size, and even the positions of the patches. When the particle size is reduced to the nanometer size and decorated with mixed ligand shells, patchy particles can form spontaneously through surface phase separation.5−12 Patchy nanoparticles (NPs) can also be formed by adsorbing two chemically different polymer chains that will spontaneously phase separate when mixed.13 These particles are also © XXXX American Chemical Society

Received: September 10, 2018 Revised: November 13, 2018

A

DOI: 10.1021/acs.macromol.8b01959 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

2. EXPERIMENTAL SECTION

preparing significant quantities of nanoparticles with mixed polymer ligands as well as controlling the ratios of the different polymer ligands. Many of the successful systems reported to date required a considerable synthetic investment to make triblock copolymers for attachment to nanoparticles,17−19 combining “grafting to” and “grafting from” approaches,20,21 or the in situ synthesis of inorganic nanoparticles within block copolymer micelles.22,23 Custom synthesized block copolymer ligands offer precise control over the molecular weight and polymer ratios but are prohibitively expensive to produce in large quantities. The “grafting to” method onto premade NPs, that is, the direct attachment of terminally functionalized polymers, has been extensively used to make binary hairy NPs. Binary hairy NPs are an alternative that has been barely explored for making patchy NPs but have been developed as stimuli-responsive NPs for a variety of applications.24,25 Theoretical predictions indicate that a wide variety of patchy nanoparticles that can spontaneously be formed by mixed polymer ligand shells. This provides a strong motivation to develop simpler synthetic protocols, accompanied by appropriate characterization tools. Janus NPs, where the two hemispheres have different chemical properties, have been shown to form spontaneously through the adsorption of mixtures of hydrophilic and hydrophobic thiolated polymer ligands on larger gold NPs (diameter >10 nm).26−28 Smaller binary hairy gold NPs produced by direct synthesis using a mixture of thiolated polymers were also found to form Janus NPs.29,30 However, these preparations often yield a wide NP size distribution, a common disadvantage as compared to using premade NPs. Characterization of nanophase separation on the smaller patchy NPs remains a major challenge. Various imaging methods have been applied to the larger NPs in combination with staining27,31,32 or treatment to grow silica shells.26 Scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS) were combined for direct imaging of the different polymer domains.27 1H NOESY NMR is routinely applied with the absence of cross-peaks taken as evidence for phase separation. However, this technique does not give information regarding the dimensions or morphology of the different domains.22,26 Small-angle neutron scattering (SANS) has been applied to detect phase separation in small binary hairy NPs but requires the use of deuterated polymers.29 This work presents several significant advances in the production of polymer-based patchy NPs. First of all, we apply a simple and reproducible ligand exchange approach to functionalize monodisperse 3.5 nm ZrO2 NPs with different ratios of polystyrene (PS) and poly(ethylene oxide) (PEO) ligands. The monodisperse ZrO2 nanocrystals can be made in gram quantities using a simple solvothermal reaction, yielding sufficient quantities for characterization and testing different assembly conditions. Second, solid-state (SS) NMR methods used to characterize spatial heterogeneity and phase separation in bulk polymers represent a powerful addition to the nanomaterials characterization toolbox, providing noninvasive, quantitative measurements of the composition and phase separation within the mixed polymer brush corona. Finally, we show that the NPs display amphiphilic properties and can be assembled into different morphologies through variation of the ligand shell composition and exposure to nonselective solvents.

2.1. Materials. The zirconium(IV) isopropoxide−isopropanol complex (99.9%) and anhydrous benzyl alcohol (≥99%) were commercially obtained from Aldrich and used as received without further purification. Monodisperse ZrO2 nanoparticles were synthesized using a modified nonaqueous solvothermal procedure in the presence of benzyl alcohol.33 Zirconium(IV) isopropoxide−isopropanol (1.99 g) was weighed into a glass vial located in a Teflon reaction cup, and 30 mL of anhydrous benzyl alcohol (≥99%) was added under an inert atmosphere. The Teflon cup was inserted into a steel reactor and heated at 230 °C for 96 h. The reactor was allowed to cool for 1 day. The ZrO2 NPs were washed with anhydrous tetrahydrofuran (THF) under an inert atmosphere and retrieved by centrifugation. The syntheses of phosphonic acid-terminated poly(ethylene oxide) (PEO115-PO3H2) and carboxylic acid-terminated polystyrene (PS48COOH) are described in the Supporting Information. For the functionalization of ZrO2 NPs with PS48-COOH, the required amount of ligand was calculated based on the ligand footprint and the NP surface. PS48-COOH (MW = 5000 g/mol) was dissolved in THF (2 wt %) and added dropwise to the suspension of ZrO2 in THF under an inert atmosphere. The mixture was left stirring for 3 days at room temperature to ensure the NP surface saturation with the ligand. The resulting mixture was centrifuged to remove aggregates, and the NPs dispersed in the supernatant were purified using quantitative precipitation with methanol, followed by redispersion in THF. To remove remaining free ligands, the NPs were dialyzed against DMF/water for 3 days using a Spectra/Por 3 standard RC dialysis tube. The bath was replaced every 24 h. The purified NPs contained in the dialysis tube were retrieved and allowed to dry, yielding a white powder. For the preparation of mixed polymer ligand stabilized ZrO2-NPs via ligand exchange, the amount of PEO115-PO3H2 (MW = 5000 g/ mol) was calculated based on the grafting density of PS-functionalized ZrO2 calculated from TGA results (Supporting Information S3). The PS-ZrO2 NPs were dispersed in THF, and a solution of PEO−PO3H2 in THF/methanol (5 wt %) was added dropwise. The mixture was left under stirring for 3 days at room temperature. NPs with mixed ligand shells were purified via dialysis against a mixture of DMF/water for 3 days. The bath was replaced every 24 h. The functionalized NPs were allowed to dry under atmospheric pressure at room temperature, yielding a white powder. After the exchange reaction, 31P NMR was used to detect any free PEO-PO3H2. Only a broad 31P signal for surface bound PEO-PO3H2 was observed. The ligand ratios of PEO/ PS were determined by solid-state 1H magic angle spinning (MAS) NMR using a spinning rate of 25 kHz at 60 °C to ensure that any crystallized PEO was melted. For the self-assembly of PEO/PS-ZrO2, the NPs were dispersed in THF (7.25 mL, 2 wt %), a cosolvent for both ligands. Water (22 mL) was then added dropwise to get a final THF:H2O ratio of 1:3. The solution was left under stirring at room temperature and atmospheric pressure until complete evaporation of THF was achieved. The final assemblies of NPs were determined using transmission electron microscopy (TEM). 2.2. Methods. 1H solid-state NMR experiments were conducted on a Bruker Avance 600WB using a 2.5 mm probe and a spinning rate of 25 kHz. The 1H π/2 pulse length was 2.5 μs, and the 1H longitudinal relaxation times (T1) were measured using the saturation recovery method at 60 °C. The same 1H T1 value was observed for both components (PEO and PS) in all compositions, ranging from 0.5 to 0.8 s. A recycle delay of 5 s was enough to avoid T1 saturation effects. 2D 1H double-quantum (DQ) MAS NMR spectra of PEO/ PS-ZrO2 were acquired using the back-to-back (BABA) sequence; two rotor periods were used at a spinning rate of 25 kHz.34 Nuclear Overhauser effect spectroscopy (NOESY) experiments were performed on a Bruker 500 MHz AV500. The mixing times for each spectrum ranged from 25 ms to 1.1 s with 16 scans for each spectrum. B


Article

Macromolecules Table 1. Ligand Shell Composition, Grafting Densities, and 1H MAS NMR Spectra

a

f PS = PS fraction. bPolymer ratios expected after the ligand exchange process. cExperimental polymer ratios calculated by integrating signals using H SS-NMR. The peaks at ∼2 ppm correspond to the nonaromatic (methine and methylene) protons of polystyrene and increase in intensity with increasing f PS. 1

1

H spin diffusion NMR measurements were performed on a Bruker Avance 600WB using a 4 mm probe, operating at a 1H frequency of 600.13 MHz under static conditions and MAS frequencies of 2, 4, 6, 8, 10, 12, and 14 kHz. The 90° pulse length was 3.5 μs. The dipolar filter sequence35 was used to select the mobile signal of PEO. The pulse train is formed by 12 cycles with an interpulse delay time τ of 3 μs. A six-cycle experiment was needed to suppress the rigid signal of PS. The mixing times used go from 0.5 ms to 1 s in 16 steps. The relaxation delay was 3 s, and the number of scans was 128 for each mixing time. The final spin diffusion data were corrected for the T1 relaxation decay.

Thermogravimetric analysis (TGA) was performed on a TA Instruments Q500. Samples were heated from 25 to 700 °C at a heating rate of 20 °C/min, under nitrogen flow, switching gas to synthetic air at 600 °C. Dynamic light scattering (DLS) measurements were performed on a Malvern Zetasizer Nano ZS instrument, equipped with a He−Ne 633 nm laser. The polymer functionalized ZrO2 NPs were dispersed in THF. Differential scanning calorimetry measurements (DSC) were performed on a TA Instruments Q2000; about 5 mg of each sample was introduced and sealed in preweighed aluminum pans and then C


Article

Macromolecules scanned from −10 to 150 °C at a rate of 10°/min using a six-cycle experiment. Transmission electron microscopy (TEM) observations were performed on a Philips CM200 microscope with an acceleration voltage of 200 kV with an AMT XR40B CCD camera and EDAX Genesis analysis system. Solution-cast samples were prepared by depositing a drop of 1 mg/mL PS/PEO-ZrO2 solution in THF on a copper grid (300 mesh), coated with an amorphous carbon film. The grids were dried at room temperature overnight before imaging. Scanning electron microscopy (SEM) observations were conducted on a FEI Quanta 450 environmental scanning electron microscope (FE-ESEM) with EDAX Octane Super 60 mm2 SDD and TEAM EDS analysis system. Samples were prepared in the same way as described for TEM.

with a melting temperature of ∼59 °C for the highest f PEO which decreased to 53 °C for the lowest PEO fraction where the melting endotherm was still detected, f PEO = 0.18 (Supporting Information S5 and S6). The melting endotherms also broaden considerably with decreasing PEO content, reflecting the smaller and broader range of crystallite sizes. The melting temperatures are similar to those observed for 10 nm diameter SiO2 NPs with PEO brushes of the same molecular weight (5000 g/mol) and grafting density (1.5 chains/nm2).39 This study showed that at high grafting densities the PEO ligands tend to crystallize as planar zigzag unit cells confined to the polymer corona. At low grafting densities, 3D helix units dominate and crystals contain chains from neighboring NPs so that spherulite structures, like free PEO chains, are observed. The PEO crystalline/amorphous distribution in mixed brush NPs is a structural variable that should be addressed in future studies. The ratios of the two polymers in the ligand shells were calculated from the integrated intensities of the 1H NMR spectra at 60 °C to avoid having to fit an extra component for the crystallized PEO chains. The theoretical PEO:PS ratios assume that all the added PEO-PO3H2 displaces the surface bound PS-COOH. As seen from the experimental data in Table 1, this exchange method allows us to vary over the entire composition range with the fraction of PS ranging from f PS = 0.08 to 0.93. Most of the ratios were repeated to demonstrate the high reproducibility of this protocol. 3.2. 1H Spin Diffusion NMR Measurements: Surface Phase Separation. As expected, the 1H NOESY NMR experiments of the NPs dispersed in chloroform with long mix times of 700 ms did not show cross-peaks which can be taken as evidence of phase separation and the formation of a Janus nanoparticle (Supporting Information S7). Likewise, 2D double quantum (DQ) 1H NMR, which can detect proton proximities less than ∼0.35 nm,34 did not show cross-peaks (data not shown). More information can be extracted from 1H spin diffusion NMR measurements, widely used to detect and characterize spatial heterogeneity in polymeric materials.35 Spin diffusion experiments consist of suppressing (magnetically randomizing) one component, known as “the sink”, followed by a mix time during which the magnetization from the remaining component, “the source”, is transferred to the sink regions via proton dipole coupled driven spin diffusion.35 In the spin diffusion experiment, the nonequilibrium spin polarization created in one region of a sample diffuses to the other regions and equilibrates via “flip-flop” (zero quantum) transitions that are an exchange of spin states but not energy. The recovery of the magnetization due to spin diffusion is measured as a function of the mix time, τmix. The shape of the recovery curve can be analyzed to extract the domain sizes and, to a lesser extent, the dimensionalities and morphologies. The selection of one phase over another is usually based on differences in the chemical shifts or dipolar couplings, as reflected by the line widths and relaxation parameters. Dipolar filters, based on different T2 relaxation times, are commonly used to select the mobile component (PEO) whereas double quantum filters (DQF) can be used to select the more rigid component (PS) based on differences between the dipolar coupling strengths. A dipolar filter was used to suppress the signal of the aromatic component (the sink), leaving magnetization remaining in the more mobile PEO component (the source). Then, during the mixing time, the magnetization from the source is allowed to transfer to the sink. The spin

3. RESULTS AND DISCUSSION 3.1. Synthesis of ZrO2 NPs with PEO/PS Mixed Brushes. A ligand exchange method developed by the group of Förster was first used to produce a dense corona of polystyrene chains on monodisperse ZrO2 NPs. In these reactions, the small molecules typically used for the nanoparticle synthesis (oleic acid, phosphines, and phosphonates) are exchanged with an excess of polymer ligands with stronger binding terminal groups (carboxylic acids, phosphonic acids, and multidentate amines).36 In our case, the mixed ligand shells are produced by first functionalizing ZrO2 NPs, weakly stabilized with benzyl alcohol, with PS48-COOH, which has a moderate binding strength to zirconia. Quantitative precipitation, an essential step, consisted of adding a solvent that is a good solvent (methanol) for the small molecule ligand (benzyl alcohol) but a common nonsolvent for the NPs and polymer ligands (PS48-COOH). This effectively increases the polymer ligand concentration and simultaneously depletes the small ligand to promote the exchange, yielding NPs with a dense corona of PS chains.36 Phosphonic acid-functionalized poly(ethylene oxide), PEO115-PO3H2, is then added which replaces some of the PS48-COOH. The displacement of adsorbed carboxylic acids by phosphonic acids on inorganic surfaces has been studied previously for low molecular weight ligands.37 Schadler and co-workers likewise found that low molecular weight poly(dimethylsilane) (PDMS) with COOH terminal groups adsorbed on ZrO2 NPs could be displaced by a higher molecular weight PDMS-PO3H2 ligands to create “bimodal” polymer brushes.38 In their case, the chain density (0.2 chain/ nm2) and molecular weight (1000 mol/g) were much lower than in our samples (∼1.5 chains/nm2 and 5000 g/mol). However, we found that the displacement of the PS48-COOH ligands was quite efficient despite the possibility of steric hindrance for the incoming PEO115-PO3H2 ligands. In fact, adding an excess of PEO115-PO3H2 to the ZrO2 NPs resulted in the complete displacement of PS48-COOH by PEO115PO3H2. The properties of a series of mixed PEO/PS brush NPs, prepared by this exchange procedure, are listed in Table 1 along with the 1H magic angle spinning (MAS) NMR spectra. The fast MAS (νr ∼ 25 kHz) allows for the separation of the aliphatic protons of PEO from the aromatic protons of PS. At the higher PEO concentrations, there is the possibility for crystallization so 1H NMR spectra were also run at 60 °C, above the PEO melting temperature. For PEO fractions higher than ∼0.4−0.5, the proton line width for PEO is strongly reduced upon heating at 60 °C as compared to room temperature, indicating chain crystallization. DSC scans indicated that the PEO domains were partially crystallized D


Article

Macromolecules diffusion recovery curve for the sample 3a with f PS = 0.37 under a MAS rate of 14 kHz is shown in Figure 1. The

simply estimated from the proton line widths (eqs 3 and 4). Such experiments require a large difference in the line widths of the two components for accurate deconvolution to extract the integrated intensities (Supporting Information S8). Because the relatively small quantities of polymer in our samples as compared to a bulk polymer sample made deconvolution of the small broad signals more difficult, the spin diffusion experiments were run under MAS rather than static conditions. Under MAS, the chemical shift difference between the aliphatic and aromatic protons allows one to distinguish between the two components. However, MAS reduces the 1H−1H dipolar couplings, and corrections for the effect on the spin diffusivity must be carried out. Fast MAS also increases the temperature which will also reduce the spin diffusivity rate, so care must be taken to maintain a constant temperature. We followed the work of Jia et al.42 to correct for the reduction of the spin diffusion coefficients by MAS. Sorte and co-workers recently validated this approach for faster MAS rates to extract domain sizes from 1H DQF spin diffusion experiments,43 using the following equation to correct the spin diffusion coefficients: ÄÅ ÉÑ ÅÅÅ π D2(νR ) ÑÑÑ D(νR ) ÑÑ = tanÅÅÅ ÅÅÅ 2 D0 2 ÑÑÑÑ D(0) Ç Ö ÅÄÅ ÑÉÑ ÅÅÅ ÑÑ ÅÅ 2αD0νR 2 ln 2 ÑÑÑÑ 1 Å × ÅÅ ÄÅ 2 ÉÑ + ÑÑÑ 0 ÅÅ ÅÅ π D (0) ÑÑ D(0)Δν1/2 ÑÑ ÅÅ tanÅÅ Ñ ÑÑ 2 Ñ ÅÅÇ ÅÅÇ 2 D0 ÑÑÖ ÑÖ (2)

Figure 1. Recovery of the signal intensity of the PS signal as a function of the square root of the mix time. The red dashed curve is a fit to a Boltzman function, I(tmix ) = a − 2a/1 + exp (( tmix − t0 )/Δ), with a = 100.65, t0 = 0.75, and Δ = 3.2, for extracting the initial rate approximation curve.42 The horizontal line represents the limiting signal intensity at long mix times.

magnetization buildup at short mix times is highly linear, indicating a sharp interface, as expected for two immiscible polymers, while the plateau indicates equilibration of the magnetization and reflects sample uniformity. The widely used, initial rate approximation (IRA) method was applied to estimate the domain sizes. In the IRA analysis, the initial buildup of magnetization is assumed to be linear with tmix . The intersection of this line with that of the

Here D(νR) and D(0) are the diffusion coefficients under MAS frequency, νR, and static conditions, respectively, D0 is the rigid lattice limit of the spin diffusion coefficient, Δν01/2 is the proton rigid lattice line width, and α is a cutoff in the Lorentzian fit of the mobile component.42 D(0) is calculated from the following equations for Gaussian (rigid component) and Lorentzian (mobile component):44,45

* , is used equilibrium magnetization at long mix times, tmix with the following equation to calculate the domain size, d:40

ij ρH ϕPS + ρH ϕPEO yzij 4εϕ yz dis PEO zzjj d = jjjj PS zzj 1/2 zzz j z ϕ ϕ { PS PEO k {k π ij yz (DPSDPEO)1/2 j zz * zz τ × jjjj 1/2 1/2 j ρH DPS + ρH DPEO zz mix (1) PS PEO k { The volume fraction of the dispersed (minor) phase ϕdis is calculated from the PEO/PS ratios, ε is the dimensionality, defined as the number of orthogonal directions in 3D coordinates relevant for the spin diffusion process (1 = lamellar, 2 = cylindrical, and 3 = spherical morphologies), and the proton densities are ρHPS = 0.081 g cm−3 and ρHPEO = 0.103 g cm−3.41 In the case of static NMR experiments, the spin diffusion coefficients of PS and PEO, DPS and DPEO, can be

DPS =

1 12

DPEO =

π ⟨r 2⟩Δν1/2 2 ln(2)

(3)

1 2 ⟨r ⟩ αΔ ν1/2 6

(4)

Δν1/2 is the full width at half-intensity obtained from the experimental static spectra (PEO: Δν1/2 = 3.833 kHz; PS: Δν1/2 = 28.33 kHz), α is a cutoff in the Lorentzian fit of the mobile component,42 and r2 is the mean-square distance between the nearest proton spins. For PEO r2 = 5.032 Å2, and for PS r2 = 5.83 Å2.45 The effect of MAS on the spin diffusion coefficients is shown in Figure S9. The spin diffusion parameters for sample 3a, which had ligand shell composition of f PS = 0.37 and f PEO = 0.63, are listed in Table 2. This sample was chosen since it is near the middle of the composition range with sufficient 1H NMR signal intensities for both polymers.

Table 2. Spin Diffusion Parameters and Domain Size spin diffusion coefficients (nm2/ms) D(0)

sample 3a f PS

f PEO

√(τmix)

D(14 kHz)

PEO

PS

PEO

PS

0.016

0.207

0.011

0.134

1/2

(ms ) 0.37

0.63

7.15

domain sizes lamellar (nm)ε = 1

patches (nm)ε = 2

3D (nm)ε = 3

2.3

4.6

6.8

E

ϕPEO

ϕPS

0.63

0.37


Article

Macromolecules

Figure 2. (a) Schematic showing the relevant dimensions of phase separated ligand shells for f PS = 0.37 and f PEO = 0.63 and shell thickness of ∼4 nm. (b) Schematic illustrating the dimensionality parameter, ε. Arrows indicate the orthogonal directions of spin diffusivity for 1D, 2D, and 3D geometries of the dispersed phase.

Table 3. Polymer Parameters

PEO PS

Mn (g/mol)

N

Kuhn monomer length, ba (nm)

contour length, L (nm)

radius of gyration, Rg (nm)

end-to-end distance, R ∼ b√N (nm)

grafting density,b σ (chain nm−2)

5000 5000

115 48

1.1 1.8

40 12.5

2.7 1.9

11.7 12.4

∼1.8 ∼ 4.8

a b ≡ R2/Rmax, the ratio of the mean-square end-to-end distance and the maximum chain length. bGrafting density is scaled by the Kuhn segment length, b, expressed in terms of chains/b2.

polymer brush regime (SDPB), the ligand shell architecture consists of a densely packed inner shell with looser polymer conformations in the outer shell. A cutoff distance, rc, marks the transition between the condensed polymer brush (CPB) and SDPB regimes as theoretically estimated by extending the Daoud−Cotton model for star polymers to polymer-grafted nanospheres.47 For the NPs with f PS = 0.37 (sample 3a), rc = rnp σ * /ν* ∼ 14 or 22 nm for PEO versus PS, respec-

The domain sizes are that of the polystyrenethe dispersed phase in the case of sample 3a with f PS = 0.37. A geometry (lamellar, patches, or 3D structure) must be assumed to interpret the proton spin diffusion data. In the context of proton spin diffusion, the dimensionality, ε, is the number of orthogonal directions that the magnetization can diffuse (Figure 2b).We carefully considered the dimensionality given that this new application of proton spin diffusion is neither a bulk material nor a flat polymer thin film but the spherical shell of a NP with a small radius. We assume that the dispersed domain is a section of the highly curved shell, either a stripe or a short cylinder (more accurately, a truncated spherical cone), but the shape is likely to be irregular rather than circular as shown in Figure 2. Because no magnetization diffuses to/from the inorganic core and interparticle spin diffusion contribution is assumed to be negligible, as discussed below, then dimensionalities of ε = 1 for lamellar (stripes) or ε = 2 for cylindrical (patches) are possible. From the interparticle distances as measured from the TEM images, the polymer shell thickness is ∼4 nm, similar to that found for NP superlattices formed by 3.8 nm gold NPs with dense polystyrene brushes of the same molecular weight.46 If we then consider a core−shell structure with core and outer shell radii of Rnp ∼ 1.75 nm and Rshell ∼ 5.75 nm, along with the relative fractions of PS and PEO, then a single PS patch would occupy core and shell arc lengths of 3.7 and 13 nm, respectively, whereas the lengths for two symmetrical PS patches are 1.9 and 7.5 nm as shown in Figure 2a. Given the assumed geometry of Figure 2a, where the polymer domain is a truncated spherical cone which has a dimensionality of ε = 2, the polystyrene domain size could conceivably correspond to 1 or 2 patches. This picture assumes minimal interpenetration between the polymer coronas of neighboring nanoparticles. The extent of entanglement between the polymer chains of neighboring nanoparticles depends on the surface curvature, grafting density, and chain length.47 In the case of a very low grafting density, the tethered chains take on a mushroom conformation, and the ligand shell is easily distorted. At higher grafting densities, the semidilute

tively. Here rnp is the radius of the nanoparticle, σ* is the reduced grafting density given by σ* = σb2 where b2 (nm2) is the cross-sectional area per monomer unit in terms of the Kuhn monomer length, and ν* = ν / 4π = 0.17 is a rescaled excluded volume parameter (Flory exponent), assuming ν = 0.6. Given that the hydrodynamic brush thickness is ∼8 nm as estimated from the DLS data (Supporting Information S4), our polymer ligand shell fits into the condensed polymer brush (CPB) regime. Here the nanoparticles, surrounded by a shell of dense, highly stretched polymer chains, are predicted to interact more as hard spheres with relatively noninterpenetrating polymer coronas. However, a contribution from spin diffusion between polymer coronas cannot be ruled out and merits future investigation by repeating the same experiments on samples with lower grafting densities and/or higher polymer molecular weights that are in the dilute or semidilute regimes where extensive interpenetration of neighboring polymer coronas is expected. Now we compare our system to theoretical studies of phase separation in mixed homopolymer brushes grafted to nanoparticles. The relevant parameters, listed in Table 3, are the NP radius, grafting density (chains per Kuhn length squared), and polymer sizes. Wang and co-workers used self-consistent field theory (SCFT) to simulate the phase separation of two polymers A and B grafted to NPs as a function of the ratio of grafting densities of two mixed homopolymers (σA/σB), relative chain lengths (NA and NB) and core size (Rnp).48 If we compare our system to the cases examined in this study, the two patch (dumbbell) structure (Figure 7a of ref 48) matches most closely. This figure corresponds to a NP radius, Rnp = 0.5, a total grafting density, σ = 1, monomer numbers, NA = NB =

F


Article

Macromolecules

Figure 3. TEM images of PEO/PS-ZrO2 NPs deposited from (a) THF/H2O before complete evaporation of THF and (b) a chloroform/H2O Pickering emulsion. The PS fractions in the ligand shells were 0.23 and 0.40, respectively.

Table 4. Morphologies of NP Assemblies

G


Article

Macromolecules 20, and a Kuhn length, b = 1, so the ratio of the polymer to NP size, b N /Rnp = 1 20 /0.5 ∼ 9, is close to the ratios for our system R(PS)/RNP ∼ 10 and R(PEO)/RNP ∼ 9. Our grafting densities are higher as σ was defined in this study in terms of b, the Kuhn monomer length. A more recent theoretical study, combining SCFT with a fluctuating dynamic mean-field theory (DMFT), examined the effects of a nonuniform grafting density and fluctuations for mixed brush grafted NPs in a nonselective solvent.49 They also varied the grafting densities and defined the NP to polymer size as Rnp/Rg, using the radius of gyration rather than the mean end-to-end distance of the polymer chain. Our mixed polymer brush NP with Rnp/Rg ∼ 0.46 or 0.65 and σ ∼ 1.8 or 4.8 for PEO or PS, respectively, corresponds most closely to the morphologies predicted for Rnp/Rg = 2/3 in their phase diagram (Figure 2 of ref 49). Considering also the polymer−solvent and polymer−polymer interaction parameters, these simulations would qualitatively predict ABA (two patch) or AB (Janus) structures. The ABA structure is predicted for a good solvent, χs ∼ 0.5, and upon drying corresponds to χs ∼ 1.0, transforming into a Janus structure in the phase diagram. This study found that the Janus phase is always preserved with the introduction of defects (nonuniform grafting) and fluctuations. They concluded that the formation of a Janus phase is experimentally more realistic rather than the defect-free multivalent structures predicted by other studies.15 3.3. Self-Assembly of Mixed Brush NPs. As seen in Figure 3a, the ZrO2 NP cores are quite uniform with an average size of 3.5 ± 0.5 nm (Supporting Information S2). When dispersed as a monolayer from THF, the particles form a 2D hexagonal lattice with an average interparticle distance of 8.5 ± 1 nm. In a good solvent, the extended brush length for the PEO is estimated to be ∼17 nm and the collapsed dry brush as measured by TEM is ∼4 nm. These numbers are in good agreement with the 2D superlattices formed by 3.8 nm inorganic NPs functionalized with PS ligands of the same molecular weight.46 The ZrO2 NPs with mixed polymer brushes were first tested to see whether they behave as amphiphiles to stabilize a water/chloroform Pickering emulsion. A stable emulsion was observed, and the TEM image of the emulsion shows large spherical aggregates, held together by the PS glassy state. (Figure 3b) Next assembly was induced by adding a selective solvent. The nanoparticles were first dispersed in THF, a common solvent, and then a selective solvent for PEO, water, was added for a THF:H2O ratio of 1:3. The THF was evaporated overnight, and TEM images were taken after drop casting from the aqueous dispersion onto a grid and letting it dry overnight. Table 4 summarizes the observed structures as a function of the polymer ligand shell composition. The control sample, NPs with only PS chains, assembled into large dense spheres very similar to those reported by Sánches-Iglesias et al.50 The NPs with the lowest PS content, f PS = 0.08, did not form compact shapes in water using this protocol since the structures rely on the PS glassy state to hold the structures together. When the PS fraction was increased to f PS ∼ 0.2, dense spherical micelles with a narrow size range around ∼30 nm form which increase to an average size of ∼50 nm for f PS = 0.4. At the higher PS contents, f PS > 0.6, vesicles appear. SEM images were also taken to confirm the formation of hollow vesicles (Supporting Information S10). Most diblock copolymer assemblies formed via this type of selective solvent approach are made using copolymers with

long hydrophobic (typically PS) blocks and short hydrophilic blocks (typically PEO or PAA). The parameter space that determines the resulting morphologies of these assemblies is quite large and includes the molecular weights of the two blocks, polymer concentration, amounts and rates of water addition, added salt, and so forth. Moffitt and co-workers prepared mixed diblock copolymer micelles for the in situ synthesis of CdSe quantum dots to produce PS and PMAA binary hairy nanoparticles. They used selective solvents to assemble these NPs in a manner analogous to amphiphilic block copolymers and showed that increasing the fraction of PS was equivalent to increasing the PS block length.22 Our system follows the same trend as the amphiphilic NPs formed from mixed block copolymer micelles. Increasing the PS fraction led to an increase in the average size of the spherical supermicelles that transform into large vesicles at the higher PS fractions. In comparison to previously reported mixed polymer brush NPs, our synthetic protocol is simple and more versatile in terms of the types of polymers and NPs, with well-defined NP sizes and polymer molecular weights. These advantages will allow us to access a larger variety of assembly morphologies.

4. CONCLUSIONS A simple, highly reproducible protocol to graft two immiscible polymers to metal oxide nanoparticles by polymer−polymer exchange over the entire possible composition range was developed. Because phosphonic and carboxylic acids adsorb with similar relative bonding strengths to a wide range of technologically important metal oxides (TiO2, Al2O3, ITO, etc.) and semiconductor surfaces like CdSe, this approach can be applied to other types of premade NPs, providing the advantage of precise control over the NP to polymer size ratios. The polymer phase separation on the NP, characterized by proton spin diffusion NMR experiments, supports recent theoretical simulations predicting a two patch (dumbbell) or Janus structure for the NP to polymer size ratio studied here. The amphiphilic properties, analogous to block copolymers, were demonstrated by the self-assembly of the NPs as a function of the ligand shell composition upon the addition of a selective solvent. Future work will vary the polymer molecular weights and thus the NP/polymer size ratios to study the effect on the surface nanoscale phase separation. Whereas phase separation of mixed polymer brushes on flat surfaces is well understood both experimentally and theoretically, there has been little experimental verification of the large number of theoretical and simulation studies predicting the formation of “multivalent” particles from mixed polymer brush NPs. The ability to precisely control the composition and polymer/NP size ratios, along with quantitative SSNMR characterization of the nanophase separation, provides an excellent opportunity to validate and further advance theoretical models. The interplay of nanophase separation with confinement effects on the polymer mobility and crystallization, already studied by NMR for single component polymer brush NPs,39,51 is another topic of interest. The facile synthesis of these mixed polymer brush NPs will allow exploration of the large parameter space for amphiphilic self-assembly where different solvents, salts, and other parameters can be tested. The solid-state assembly of the patchy nanoparticles into hierarchical superlattices through thermal annealing is another topic that can now be more easily explored. In particular, theoretical studies predict the H


Article

Macromolecules

alkanethiolates on the surface of a gold core. ACS Nano 2016, 10, 9316−9325. (8) Meena, S. K.; Goldmann, C.; Nassoko, D.; Seydou, M.; Marchandier, T.; Moldovan, S.; Ersen, O.; Ribot, F.; Chanéac, C.; Sanchez, C.; Portehault, D.; Tielens, F.; Sulpizi, M. Nanophase segregation of self-assembled monolayers on gold nanoparticles. ACS Nano 2017, 11, 7371−7381. (9) Posocco, P.; Gentilini, C.; Bidoggia, S.; Pace, A.; Franchi, P.; Lucarini, M.; Fermeglia, M.; Pricl, S.; Pasquato, L. Self-organization of mixtures of fluorocarbon and hydrocarbon amphiphilic thiolates on the surface of gold nanoparticles. ACS Nano 2012, 6, 7243−7253. (10) Jackson, A. M.; Myerson, J. W.; Stellacci, F. Spontaneous Assembly of Subnanometre-Ordered Domains in the Ligand Shell of Monolayer-Protected Nanoparticles. Nat. Mater. 2004, 3, 330−336. (11) Singh, C.; Ghorai, P. K.; Horsch, M. A.; Jackson, A. M.; Larson, R. G.; Stellacci, F.; Glotzer, S. C. Entropy-Mediated Patterning of Surfactant-Coated Nanoparticles and Surfaces. Phys. Rev. Lett. 2007, 99, 226106. (12) Ong, Q.; Luo, Z.; Stellacci, F. Characterization of ligand shell for mixed-ligand coated gold nanoparticles. Acc. Chem. Res. 2017, 50, 1911−1919. (13) Chen, L.; Klok, H. A. Multifaceted” polymer coated, gold nanoparticles. Soft Matter 2013, 9, 10678−10688. (14) Chen, C.; Tang, P.; Qiu, F. Binary hairy nanoparticles: Recent progress in theory and simulations. J. Polym. Sci., Part B: Polym. Phys. 2014, 52, 1583−1599. (15) Chen, C.; Zhang, T.; Zhu, L.; Zhao, B.; Tang, P.; Qiu, F. Hierarchical Superstructures Assembled by Binary Hairy Nanoparticles. ACS Macro Lett. 2016, 5, 718−723. (16) Yi, C.; Zhang, S.; Webb, K. T.; Nie, Z. Anisotropic selfassembly of hairy inorganic nanoparticles. Acc. Chem. Res. 2017, 50, 12−21. (17) Zubarev, E.; Xu, J.; Sayyad, A.; Gibson, J. AmphiphilicityDriven Organization of Nanoparticles into Discrete Assemblies. J. Am. Chem. Soc. 2006, 128, 15098−15099. (18) Hu, J.; Wu, T.; Zhang, G.; Liu, S. Efficient Synthesis of Single Gold Nanoparticle Hybrid Amphiphilic Triblock Copolymers and Their Controlled Self-Assembly. J. Am. Chem. Soc. 2012, 134, 7624− 7627. (19) Jiang, X.; Zhong, G.; Horton, J. M.; Jin, N.; Zhu, L.; Zhao, B. Evolution of phase morphology of mixed poly (tert-butyl acrylate)/ polystyrene brushes grafted on silica particles with the change of chain length disparity. Macromolecules 2010, 43, 5387−5395. (20) Song, J.; Huang, P.; Chen, X. Preparation of plasmonic vesicles from amphiphilic gold nanocrystals grafted with polymer brushes. Nat. Protoc. 2016, 11, 2287−2299. (21) Song, J.; Cheng, L.; Liu, A.; Yin, J.; Kuang, M.; Duan, H. Plasmonic vesicles of amphiphilic gold nanocrystals: self-assembly and external-stimuli-triggered destruction. J. Am. Chem. Soc. 2011, 133, 10760−10763. (22) Moffitt, M. G. Self-assembly of polymer brush-functionalized inorganic nanoparticles: from hairy balls to smart molecular mimics. J. Phys. Chem. Lett. 2013, 4, 3654−3666. (23) Coleman, B. R.; Moffitt, M. G. Amphiphilic Inorganic Nanoparticles with Mixed Polymer Brush Layers of Variable Composition: Bridging the Paradigms of Block Copolymer and Nanoparticle Self-Assembly. Chem. Mater. 2018, 30, 2474−2482. (24) Zhao, B.; Zhu, L. Mixed polymer brush-grafted particles: a new class of environmentally responsive nanostructured materials. Macromolecules 2009, 42, 9369−9383. (25) Motornov, M.; Roiter, Y.; Tokarev, I.; Minko, S. Stimuliresponsive nanoparticles, nanogels and capsules for integrated multifunctional intelligent systems. Prog. Polym. Sci. 2010, 35, 174− 211. (26) Percebom, A. M.; Giner-Casares, J. J.; Claes, N.; Bals, S.; Loh, W.; Liz-Marzán, L. M. Janus gold nanoparticles obtained via spontaneous binary polymer shell segregation. Chem. Commun. 2016, 52, 4278−4281.

formation of unusual non-close-packed simple cubic crystal lattices15 for the NPs studied here. Programmable lattices of semiconductors and ferroelectric or magnetic NPs, assembled from patchy NPs, would represent a new class of polymer nanocomposites for sensor, energy storage, and actuator applications.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b01959. Synthesis of PS-COOH and PEO-PO3H2 ligands, histograms of the ZrO2 NP sizes and interparticle distances, grafting densities from TGA, hydrodynamic diameters from DLS, 1H MAS NMR of ZrO2 NPs at 60 °C, DSC endotherms, 2D 1H NOESY NMR spectrum, line shape deconvolution of 1H spectrum, spin diffusion coefficients versus MAS rate, and the SEM and TEM images of vesicles formed from NPs with f PEO = 0.07 (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (L.R.). ORCID

Linda Reven: 0000-0002-6643-6371 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This research was supported by a Discovery Grant from the National Science and Engineering Research Council (NSERC). B. Guzman Juarez acknowledges financial support from CONACyT (Mexico) and FRQNT, Bourse d’excellence pour étudiants étrangers (Quebec), for Ph.D. scholarship grants. We thank Professor Baohui Li of Nankai University for useful discussions regarding the interpretation of theoretical simulations.

■

REFERENCES

(1) Du, J.; O’Reilly, R. K. Anisotropic particles with patchy, multicompartment and Janus architectures: preparation and application. Chem. Soc. Rev. 2011, 40, 2402−2016. (2) Lunn, D. J.; Finnegan, J. R.; Manners, I. Self-assembly of “patchy” nanoparticles: a versatile approach to functional hierarchical materials. Chem. Sci. 2015, 6, 3663−3673. (3) Zhang, Z.; Glotzer, S. C. Self-assembly of patchy particles. Nano Lett. 2004, 4, 1407−1413. (4) Pawar, A. B.; Kretzschmar, I. Fabrication, assembly, and application of patchy particles. Macromol. Rapid Commun. 2010, 31, 150−168. (5) Carney, R. P.; DeVries, G. A.; Dubois, C.; Kim, H.; Kim, J. Y.; Singh, C.; Ghorai, P. K.; Tracy, J. B.; Stiles, R. L.; Murray, R. W.; Glotzer, S. C.; Stellacci, F. Size Limitations for the Formation of Ordered Striped Nanoparticles. J. Am. Chem. Soc. 2008, 130, 798− 799. (6) Harkness, K. M.; Balinski, A.; McLean, J. A.; Cliffel, D. E. Nanoscale phase segregation of mixed thiolates on gold nanoparticles. Angew. Chem., Int. Ed. 2011, 50, 10554−10559. (7) Ş ologan, M.; Marson, D.; Polizzi, S.; Pengo, P.; Boccardo, S.; Pricl, S.; Posocco, P.; Pasquato, L. Patchy and Janus nanoparticles by self-organization of mixtures of fluorinated and hydrogenated I


Article

Macromolecules (27) Rossner, C.; Tang, Q.; Müller, M.; Kothleitner, G. Phase separation in mixed polymer brushes on nanoparticle surfaces enables the generation of anisotropic nanoarchitectures. Soft Matter 2018, 14, 4551−4557. (28) Deng, H.; Zhong, Y.; Du, M.; Liu, Q.; Fan, Z.; Dai, F.; Zhang, X. Theranostic self-assembly structure of gold nanoparticles for NIR photothermal therapy and X-Ray computed tomography imaging. Theranostics 2014, 4, 904−918. (29) Kim, S.; Kim, T. H.; Huh, J.; Bang, J.; Choi, S. H. Nanoscale phase behavior of mixed polymer ligands on a gold nanoparticle surface. ACS Macro Lett. 2015, 4, 417−421. (30) Liu, L.; Zhang, J.; Wu, C.; Zhao, H. Surface-Active Gold Nanoparticles with Mixed Polymer Brushes as Surfactants in the Preparation of Polystyrene Colloid Particles. Macromol. Rapid Commun. 2008, 29, 45−51. (31) Tang, S.; Lo, T. Y.; Horton, J. M.; Bao, C.; Tang, P.; Qiu, F.; Ho, R. M.; Zhao, B.; Zhu, L. Direct visualization of three-dimensional morphology in hierarchically self-assembled mixed poly (tert-butyl acrylate)/polystyrene brush-grafted silica nanoparticles. Macromolecules 2013, 46, 6575−6584. (32) Fox, T. L.; Tang, S.; Horton, J. M.; Holdaway, H. A.; Zhao, B.; Zhu, L.; Stewart, P. L. In situ characterization of binary mixed polymer brush-grafted silica nanoparticles in aqueous and organic solvents by cryo-electron tomography. Langmuir 2015, 31, 8680− 8688. (33) Garnweitner, G.; Goldenberg, L. M.; Sakhno, O. V.; Antonietti, M.; Niederberger, M.; Stumpe, J. Large-Scale Synthesis of Organophilic Zirconia Nanoparticles and their Application in Organic− Inorganic Nanocomposites for Efficient Volume Holography. Small 2007, 3, 1626−1632. (34) Feike, M.; Demco, D. E.; Graf, R.; Gottwald, J.; Hafner, S.; Spiess, H. W. Broadband multiple-quantum NMR spectroscopy. J. Magn. Reson., Ser. A 1996, 122, 214−221. (35) Clauss, J.; Schmidt-Rohr, K.; Spiess, H. W. Determination of domain sizes in heterogeneous polymers by solid-state NMR. Acta Polym. 1993, 44, 1−17. (36) Ehlert, S.; Taheri, S. M.; Pirner, D.; Drechsler, M.; Schmidt, H. W.; Förster, S. Polymer ligand exchange to control stabilization and compatibilization of nanocrystals. ACS Nano 2014, 8, 6114−6122. (37) Bauer, T.; Schmaltz, T.; Lenz, T.; Halik, M.; Meyer, B.; Clark, T. Phosphonate-and carboxylate-based self-assembled monolayers for organic devices: a theoretical study of surface binding on aluminum oxide with experimental support. ACS Appl. Mater. Interfaces 2013, 5, 6073−6080. (38) Li, Y.; Wang, L.; Natarajan, B.; Tao, P.; Benicewicz, B. C.; Ullal, C.; Schadler, L. S. Bimodal “matrix-free” polymer nanocomposites. RSC Adv. 2015, 5, 14788−14795. (39) Kim, S. A.; Archer, L. A. Hierarchical structure in semicrystalline polymers tethered to nanospheres. Macromolecules 2014, 47, 687−694. (40) Clauss, J.; Schmidt-Rohr, K.; Spiess, H. W. Determination of domain sizes in heterogeneous polymers by solid-state NMR. Acta Polym. 1993, 44, 1−7. (41) Buda, A.; Demco, D. E.; Bertmer, M.; Blümich, B.; Reining, B.; Keul, H.; Höcker, H. Domain sizes in heterogeneous polymers by spin diffusion using single-quantum and double-quantum dipolar filters. Solid State Nucl. Magn. Reson. 2003, 24, 39−67. (42) Jia, Z.; Zhang, L.; Chen, Q.; Hansen, E. W. Proton spin diffusion in polyethylene as a function of magic-angle spinning rate. A phenomenological approach. J. Phys. Chem. A 2008, 112, 1228−1233. (43) Sorte, E. G.; Abbott, L. J.; Frischknecht, A. L.; Wilson, M. A.; Alam, T. M. Hydrophilic domain structure in polymer exchange membranes: Simulations of NMR spin diffusion experiments to address ability for model discrimination. J. Polym. Sci., Part B: Polym. Phys. 2018, 56, 62−78. (44) Demco, D. E.; Johansson, A.; Tegenfeldt, J. Proton spin diffusion for spatial heterogeneity and morphology investigations of polymers. Solid State Nucl. Magn. Reson. 1995, 4, 13−38.

(45) Bonk, F. A.; Caldarelli, S.; Phan, T.; Bertin, D.; Deazevedo, E. R.; Mantovani, G. L.; Bonagamba, T. J.; Plivelic, T. S.; Torriani, I. L. Investigation by combined solid-state NMR and SAXS methods of the morphology and domain size in polystyrene-b-polyethylene oxide-bpolystyrene triblock copolymers. J. Polym. Sci., Part B: Polym. Phys. 2010, 48, 55−64. (46) Ye, X.; Zhu, C.; Ercius, P.; Raja, S. N.; He, B.; Jones, M. R.; Hauwiller, M. R.; Liu, Y.; Xu, T.; Alivisatos, A. P. Structural diversity in binary superlattices self-assembled from polymer-grafted nanocrystals. Nat. Commun. 2015, 6, 10052. (47) 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. (48) Wang, Y.; Yang, G.; Tang, P.; Qiu, F.; Yang, Y.; Zhu, L. Mixed homopolymer brushes grafted onto a nanosphere. J. Chem. Phys. 2011, 134, 134903. (49) Koski, J. P.; Frischknecht, A. L. Fluctuation Effects on the Brush Structure of Mixed Brush Nanoparticles in Solution. ACS Nano 2018, 12, 1664−1672. (50) Sánchez-Iglesias, A.; Grzelczak, M.; Altantzis, T.; Goris, B.; Perez-Juste, J.; Bals, S.; Van Tendeloo, G.; Donaldson, S. H., Jr.; Chmelka, B. F.; Israelachvili, J. N.; Liz-Marzán, L. M. Hydrophobic interactions modulate self-assembly of nanoparticles. ACS Nano 2012, 6, 11059−11065. (51) Zhu, L.; Wang, X.; Gu, Q.; Chen, W.; Sun, P.; Xue, G. Confinement-induced deviation of chain mobility and glass transition temperature for polystyrene/Au nanoparticles. Macromolecules 2013, 46, 2292−2297.

J


Fabrication of Amphiphilic Nanoparticles via Mixed Homopolymer

Recommend Documents