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Controlling the Radial Position of Nanoparticles in Amphiphilic Block-Copolymer Assemblies Brenda L. Sanchez-Gaytan,† Shan Li,† Amanda C. Kamps,† Robert J. Hickey,† Nigel Clarke,‡ Mike Fryd,† Bradford B. Wayland,†,§ and So-Jung Park*,† † ‡
Department of Chemistry, University of Pennsylvania, 231 South 34th Street, Philadelphia, Pennsylvania 19104, United States Department of Physics and Astronomy, The University of Sheffield, Sheffield S3 7RH, United Kingdom
bS Supporting Information ABSTRACT: Unique radial arrays of quantum dots can be formed by the cooperative self-assembly of quantum dots and amphiphilic block-copolymers. Here, we report the effect of nanoparticle volume fractions as well as the length and concentration of polymers on the self-assembly structure. It was found that the size of the assemblies and the radial position of nanoparticles can be effectively controlled by changing the volume fraction of nanoparticles. Contrary to the typical trend observed for homogeneous incorporation of nanoparticles, increases in nanoparticle volume fractions resulted in decreases in the size and size distribution of the assemblies over a wide nanoparticle volume fraction range. The strong segregation theory calculations indicate that the structural change is due to a balance between the chain stretching energy and the interfacial energy between the two blocks. In addition, the coassemblies became larger with increasing nanoparticle sizes at maximum nanoparticle volume fractions. However, the polymer length did not significantly affect the structural parameters at maximum nanoparticle volume fractions. These findings indicate that the design rules established for the self-assembly of amphiphilic block-copolymers do not directly apply to the coassembly structure with nanoparticles and that the nanoparticles play an active role in the self-assembly.
’ INTRODUCTION Hybrid materials of polymers and inorganic nanoparticles have received significant attention in the past decade as a promising way of creating new materials with enhanced mechanical, conductive, and electronic properties.14 The net properties of polymer/ nanoparticle composite materials are determined not only by the intrinsic properties of each component but also by the spatial distribution of nanoparticles in polymer matrixes.5,6 While various nanoparticles and polymers can be readily synthesized by wellestablished synthetic methods, the control of nanoparticle dispersion in polymer matrices remains a difficult and important issue in this field. Addressing the critical problem, there has been a growing interest in the formation of ordered arrays of nanoparticles in polymer thin films through the cooperative self-assembly of nanoparticles and block-copolymers (BCPs). In this approach, the locations of nanoparticles are conveniently controlled by segregating nanoparticles into a favorably interacting polymer domain or at the interface between two polymers.710 It has been shown that the size and volume fraction of nanoparticles as well as the chemical nature of nanoparticle surface and polymers affect the preferential position of nanoparticles,1113 demonstrating that these parameters can be adjusted to obtain nanoparticle/polymer composite materials with desired structures. In the solution phase, amphiphilic BCPs self-assemble into various assembly structures such as spherical and rod-like micelles, r 2011 American Chemical Society
vesicles, and lamellae.14 Researchers have exploited the self-assembly capability of BCPs to encapsulate nanoparticles in such polymer assemblies for various applications such as biological imaging, medicine, and nanofabrication.1519 For example, Taton and coworkers have shown that a range of different types of hydrophobic nanostructures can be solubilized in water by encapsulating them in prototypical amphiphilic block-copolymer assemblies of polystyrene and poly(acrylic acid) (PAA-b-PS).20 However, currently not much is known about the effect of nanoparticles on the self-assembly behavior of amphiphilic block-copolymers in the solution phase. Indeed, in most solution-phase studies, nanoparticles were considered to be simple solutes, and they were incorporated in polymer matrix without a particular order.19,21,22 Recently, we have reported a unique radial assembly structure of quantum dots (QDs) and PAA-b-PS.23,24 In the study, QDs spontaneously accumulate at a spherical interface inside BCP micelles, forming a cavity-like structure as described in Scheme 1. This work demonstrated that it is possible to form ordered arrays of nanoparticles inside colloidal BCP assemblies by manipulating the interactions between nanoparticles and BCPs.23,24 The radial segregation of nanoparticles in PAA-b-PS assemblies was shown to be a general behavior for Received: September 14, 2010 Revised: February 3, 2011 Published: April 06, 2011 7836
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as-synthesized alkyl-terminated nanoparticles such as trioctylphosphine oxide (TOPO)-stabilized CdSe nanoparticles and oleic acidstabilized iron oxide nanoparticles.24,25 Here, we show that the radial position of nanoparticles as well as the size of the assemblies can be systematically controlled by tuning the volume fraction of nanoparticles (ΦQD). The strong segregation theory26 calculations indicate that the structural change is due to a balance between the stretching energy of chains and the interfacial energy between the two blocks. We also show that the nanoparticle volume fraction and the nanoparticle size are more effective in controlling the coassembly structure than the polymer length, a parameter typically used to control BCP morphologies. These factors should be taken into consideration when fabricating hybrid materials of nanoparticles and BCPs to obtain predesigned structure and properties.
’ EXPERIMENTAL SECTION Synthesis of Block-Copolymers. The synthesis of poly(tbutyl acrylate)x-block-poly(styrene)y (PtBAx-b-PSy) was performed using sequential reversible additionfragmentation chain transfer (RAFT) method.27 A representative protocol to make PtBA38-b-PS154 is as follows. To a 100 mL bulb with a vacuum adaptor was added a 50 mL acetone solution of (4-cyano-4-[(dodecylsulfanylthiocarbonyl)sulfanyl]pentanoic acid) (807.3 mg, 2 mmol), 4,40 -azobis(4-cyanovaleric acid) (112.1 mg, 0.4 mmol), and freshly distilled tert-butyl acrylate (11.5 mL, 79 mmol). This mixture was degassed by three freezepumpthaw cycles and heated at 80 °C overnight. Next, the reaction solution was concentrated by rotary evaporation and precipitated with methanol. The light yellow precipitate was filtered, washed twice with 2 mL of methanol, and dried under vacuum to a constant weight. To one twentieth of this product was added a 1.5 mL acetone solution of 4,40 -azobis(4-cyanovaleric acid) (5.6 mg, 02 mmol) and styrene (6.9 mL, 60 mmol). The resulting mixture was degassed by three freezepumpthaw cycles and heated at 80 °C for 3 h. The reaction solution was then concentrated by rotary evaporation and slowly added into 20 mL of methanol. The resulting off-yellow precipitate was washed by methanol and dried under vacuum. Poly(acrylic acid)x-block-poly(styrene)y (PAAx-b-PSy) was prepared by hydrolyzing PtBAx-b-PSy following a literature method.28 Typically, 0.2 mL of concentrated aqueous solution of hydrochloric acid (NHCl = 12.1) was added to a 1.67 mM solution of PtBAx-b-PSy in dioxane (3 mL). The solution was heated to reflux for 2.5 h and concentrated by rotary evaporation. The oily residue was slowly added to 10 mL of methanol, which resulted in off-yellow precipitates. The precipitates were then filtered, washed twice with 2 mL of methanol, and dried under vacuum to a constant weight.
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Synthesis of Quantum Dots. ZnS-coated CdSe QDs were synthesized by a modified literature procedure.29 To prepare CdSe nanoparticles, 0.50 g of TOPO, 0.50 g of hexadecylamine (HDA), 0.12 g of TDPA, and 12.8 mg of CdO were loaded in a three-neck flask. After the mixture was purged with nitrogen, the temperature was raised to 290 °C. When the solution became clear, a SeTOP solution (1 mL, 1.0 M) was quickly injected into the reaction pot. Subsequently, the temperature was set to 250 °C, and nanoparticles were grown to a desired size at the temperature. The synthesized CdSe nanoparticles were isolated and coated with ZnS. Briefly, approximately 8 108 mol of CdSe nanoparticles (2.8 nm in diameter) was precipitated with methanol and redispersed in chloroform. Next, the nanoparticle solution was added to the mixture of 1.8 g of TOPO, 1.5 g of HDA, and 1.5 g of TOP. Chloroform was then evaporated from the mixture by heating at 50 °C. To coat CdSe nanoparticles with ZnS, 4.0 mL of a TOP solution of Zn(Et)2 (26 μL) and (TMS)2S (52 μL) was added to the reaction pot in a dropwise fashion over about 10 min at 160 °C. Subsequently, the CdSe/ZnS nanoparticles were annealed at 100 °C for ∼1 h. Finally, the synthesized QDs were purified by a series of precipitations with methanol and acetone. Purified QDs were dispersed in chloroform. The diameter of coated QDs was determined to be 4.6 ( 0.38 nm by transmission electron microscopy (TEM). To examine the effect of nanoparticle sizes on the self-assembly structure, additional batches of nanoparticles were synthesized by adjusting the reaction time. The diameters of the nanoparticles were determined to be 4.7 ( 0.4, 5.8 ( 0.4, and 8.3 ( 0.7 nm by TEM. Unless otherwise noted, 4.6 nm particles were used throughout the study. Self-Assembly of QDs and PAA-b-PS. The assemblies of QDs and PAA-b-PS were prepared as described previously.23 In a typical experiment, 500 μL of a PAAx-b-PSy solution (∼1.5 μM) in dimethylformamide (DMF) was mixed with 25 μL of QD solution (3.2 μM) in chloroform. For experiments with different volume fractions of nanoparticles, the initial polymer concentration was changed from 3 to 30 μM, or the nanoparticle concentration was changed from 0.31 to 14 μM, while keeping all of the other conditions the same. Under stirring, additional DMF (1 mL) was added to the solution, and then 300 μL of water (18 MΩ) was slowly added to the mixture at a rate of 10 μL per 30 s to induce the assembly of BCP and QDs. The solution was stirred for 15 h and then dialyzed against water for 24 h. Finally, the assemblies were purified by a series of centrifugation. Instrumentation. The molecular weight of PtBA-b-PS was estimated with a gel permeation chromatography (GPC) system from Shimadzu equipped with Polymer Laboratories columns (guard; 106, 104, and 5 102 Å), a UV detector (SPD-10AV) at 600 nm, and a refractive index detector (RID-10A) calibrated against linear polystyrene standards in THF. Proton NMR spectra were obtained on a Bruker-DMX300 interfaced to an Aspect 3000 computer at ambient temperature. IR spectra were obtained on a Perkin-Elmer system 2000 FTIR spectrometer. UVvis spectra were measured with a Hewlett-Packard 8452A diode array spectrometer. TEM was performed on a JEOL TEM2010F operating at 200 kV accelerating voltage.
’ RESULTS AND DISCUSSION The Effect of Nanoparticle Volume Fractions. PAA38-bPS247 was self-assembled with TOPO-stabilized QDs by the procedure depicted in Scheme 1.23 Briefly, QDs and PAA38-b-PS247 7837
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Figure 1. (AC) TEM images of QD/BCP assemblies formed at ΦQD of 0.012 (A), 0.047 (B), and 0.103 (C). For this set of experiments, ΦQD was adjusted by changing the polymer concentration while keeping the nanoparticle concentration constant. The faint lines indicated by the gray arrow are artifacts commonly seen in polymer TEM samples. (D) A plot of overall radius (b), shell thicknesses (O), and core radius (9) of QD/BCP assemblies versus ΦQD, showing that all three parameters increase with increasing BCP concentration (i.e, decreasing ΦQD). The shell thicknesses of asymmetric assemblies are the average values of the smallest and the largest shell thicknesses. The lines were added to guide the eyes.
Figure 2. (AC) TEM images of QD/BCP assemblies formed at ΦQD of 0.01 (A), 0.013 (B), 0.047 (C), and 0.09 (D). For this set of experiments, ΦQD was adjusted by changing the nanoparticle concentration while keeping the BCP concentration constant (0.15 mg/mL). (E) A plot of overall radius (b), shell thicknesses (O), and core radius (9) of QD/BCP assemblies versus ΦQD. The lines were added to guide the eyes. (E) Relative position of nanoparticles inside the assembly at different ΦQD. The parameters rC and rA are the radius of the core and the assembly, respectively.
were mixed in a cosolvent, and the self-assembly was induced by slowly adding water to the mixture. The resulting coassemblies were transferred into water by dialysis and centrifugation. This assembly process led to a three-layered structure composed of a BCP core, a BCP shell, and QDs arranged at a spherical PS/PS interface between the polymer core and the shell (Scheme 1, Figure 1). As previously reported,23 the polymer core is comprised of one or more reverse micelles of BCP, and the polymer shell is composed of a BCP monolayer. The PAA at the exterior solubilizes the assemblies in water. Except for the incorporated QDs, the assembly structure resembles large compound micelles, reported by Eisenberg and co-workers.14,30 As we have previously shown,24 the nanoparticle volume fraction (ΦQD, defined as the volume of QDs over the combined volume of BCP and QDs) should be carefully controlled to form well-defined coassemblies. When ΦQD was larger than a threshold value (ΦQD-max), BCPs and QDs were macroscopically phase-segregated and precipitated out of solution upon water
addition.24 To examine the effect of the nanoparticle volume fraction on the coassembly structure, QDs and PAA38-b-PS247 were self-assembled at a series of different ΦQD. The diameter of ZnS-coated CdSe nanoparticles used for the self-assembly was determined to be 4.6 ( 0.38 nm by TEM. The TOPO layer adds approximately 1.4 nm to the diameter,31 bringing the size of the entire nanoparticle to 6.0 nm. Throughout the study, the ΦQD was estimated using nanoparticle diameters that include the TOPO layer. For one set of experiments presented in Figure 1, the ΦQD was controlled by varying the BCP concentration while keeping the QD concentration constant (5.3 108 M). The size and the size distribution of the assemblies were determined by TEM and plotted as a function of nanoparticle volume fraction in Figure 1. Both the radius and the shell thickness measured by TEM exclude the outmost thin PAA layer because the PAA layer is not visible by TEM. Typically, the most uniform assemblies with the narrowest size distributions were formed at ΦQD-max (Figure 1D). The standard deviations of the assembly size and the shell thickness at ΦQD-max were measured to be 8% and 6%, respectively. At ΦQD smaller than the ΦQD-max, BCPs and QDs self-assemble into the typical radial assembly structure with structural parameters varying with ΦQD. As shown in Figure 1, the size of the assemblies as well as the shell thickness progressively increased with increasing polymer concentration. It is wellknown that the aggregation number (Nagg) of polymer assemblies increases with the BCP concentration,32 which can cause the observed change in the assembly size and the shell thickness shown in Figure 1. When the polymer concentration becomes too high, assemblies with asymmetric shells and off-centered QD cavities start to appear (Figure 1A). This behavior has been attributed to the inclusion of polymer aggregates (i.e., reverse micelles) in the outer polymer layer.24 As Nagg increases further, the polymer stretching energy becomes too high. This can cause the inclusion of polymer aggregates in the shell, which leads to the asymmetric shell structure with off-centered QD cavities presented in Figure 1A. Similar concentration-dependent morphological changes are common in the self-assembly of BCP.33 In support of this hypothesis, the shell thickness of the asymmetric assemblies was frequently larger than the length of fully stretched PS (∼62 nm), and multiple QD cavities were also often observed in single assemblies.24 To determine whether the structural change shown in Figure 1 is due solely to BCP concentration, another set of experiments was carried out by controlling ΦQD through the nanoparticle concentration rather than the BCP concentration. In this set of 7838
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Figure 3. (A) Schematic illustration of the wedge used to approximate a segment of the spherical coreshell structure with a nanoparticle layer at the interface. (B) Calculated values of the assembly radius excluding the outmost PAA layer (solid line, rA), PS shell thickness (dotted lines, rA rC), and core radius (dashed line, rC) for κ = 104, dNP = 6 nm, NPS = 247, and NPAA = 38. (C) Corresponding values of the relative position of nanoparticles inside the QD/BCP assemblies.
experiments, the polymer concentration was kept constant, and the assemblies were formed at a series of different nanoparticle concentrations. The results presented in Figure 2 reveal similar ΦQD dependence when the nanoparticle volume fraction is sufficiently high (ΦQD > 0.013). At very low nanoparticle volume fraction range (ΦQD < 0.013), the size of the assemblies grows with ΦQD. However, it quickly reaches a maximum, and the assembly size decreased with a further increase of the nanoparticle concentration. Again, the most uniform coassemblies with the smallest diameter and polydispersity were formed at the maximum nanoparticle volume fraction (Figure 2D). This result indicates that the nanoparticle volume fraction is an important factor in determining the structural parameters of coassemblies of QDs and BCPs. For homogeneous incorporation, the size and size distribution of block-copolymer micelles become larger with the addition of solutes because of the volume taken up by the solutes. This behavior has been previously demonstrated with homopolymers34 and nanoparticles.35 On the contrary, in the current study described here, adding more nanoparticles reduces the size and size distribution of BCP assemblies for a wide range of nanoparticle volume fractions. Note that the local density of nanoparticles does not significantly change with the nanoparticle volume fraction. It is the radial location of nanoparticles or the shell thickness that changes with the nanoparticle volume fraction. Dynamic light scattering (DLS) measurements also showed the progressive decrease in size with ΦQD, confirming that the size data collected from TEM measurements reflect the trend in solution-phase morphologies (Figure S1). In control experiments where TOPO was added to the system at several different concentrations, there was no clear trend in the size of micelles, excluding the possibility that the residual amount of TOPO causes the observed morphology changes. In addition, Figure 2E shows how the relative position of QDs (rc/r) changes with
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ΦQD, demonstrating that the assemblies with various combinations of micelle size and relative nanoparticle position can be prepared by adjusting the ΦQD. The Strong Segregation Theory Calculations. To understand the origin of the nanoparticle-induced structural change described above, we applied the strong segregation theory26 to this system and determined the dimensions of equilibrium structures. In the theory, it is assumed that the only contributions to the free energy arise from the chain stretching and the interfacial energy, and there is no mixing between the A and the B blocks. As shown in Figure 3A, the core is assumed to be a single spherical reverse micelle, with the PAA on the inside of the sphere and PS on the outside. We also describe the nanoparticles as occupying a layer of a uniform thickness that corresponds to the diameter of the nanoparticles, dNP; thus, we neglect the possibility of partial packing of the layer and the impact that the curvature of nanoparticles might have on the chain configurations at the copolymer/nanoparticle interface. Despite these simplifications, this model qualitatively reproduces the observed structural change with nanoparticle volume fractions. The key parameter in the theory is the dimensionless quantity: k¼
9π2 b3 ðlA lB Þ1=2 8Ω2 γ
ð1Þ
which determines the relative contribution of the stretching and interfacial energies to the free energy per chain within a core shell structure. In eq 1, lA and lB are monomer segment lengths, Ω is the volume of a single chain, γ is the PAA/PS interfacial tension, and b is a reference length scale, which we take to be 1 nm. Further details of the theory have been described previously.24 As shown in Figure 3, the predicted equilibrium structures with the minimum total free energy show a trend similar to that of the experimental data (Figure 2); the initially increasing assembly dimension shows a maximum at the nanoparticle volume fraction of 0.05 and then decreases with the nanoparticle volume fraction. The qualitative agreement between the theory and the observations indicates that chain stretching and interfacial energies are the main contributions to the structural changes, even for these complex assemblies studied here. For a fixed polymer concentration, a higher number of nanoparticles can be accommodated by increasing either the core radius (rc) or the number of coassemblies. In the limit that rc . dNP, the number of coassemblies is inversely proportional to the volume of each coassembly. Within a given assembly, the key factor controlling the dimensions of the structure is the balance between the chain stretching within the core and within the shell. At volume fractions less than 0.05, as ΦQD increases, the nanoparticle layer moves further from the center of the structure to accommodate more nanoparticles. The increased core dimension means increased stretching of core polymers. At the same time, the increased core reduces the curvature and the thickness of the shell, which relieves its chain stretching in the shell (Figure 3). However, the changes in the shell thickness are not as big as the changes in core radius, and thus the overall coassembly radius increases with the nanoparticle volume fraction. In the experimental results shown in Figure 2, the core dimension increases rapidly in the regime when ΦQD < 0.012, showing the same trend predicted from the calculated structures. The shell dimension, however, also slightly increases with the nanoparticle volume fraction. This small discrepancy between 7839
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Figure 4. (AC) TEM images of QD/BCP assemblies prepared with three different length polymers, PAA38-b-PS108 (A), PAA38-b-PS154 (B), and PAA38-b-PS247 (C), at the same ΦQD (0.047). (D) Plot of shell thicknesses (O) and radius (b) of QD/BCP assemblies versus NPS. The lines were added to guide the eyes. (E) Dependence of ΦQD-max on the length of PS (NPS). (F) Plot of structural parameters of QD/BCP assemblies formed with four different length BCPs (PAA38-b-PS108, PAA38-b-PS154, PAA38-b-PS189, and PAA38-b-PS247) at their ΦQD-max, showing that the assembly structures at ΦQD-max do not change significantly with polymer lengths. (G) Plot of shell thicknesses (O) and radius of QD/BCP assemblies formed with different sized nanoparticles at their ΦQD-max.
the calculated dimensions and the experimentally measured values arises from the simplified assumptions made in the calculation that the shell is composed of a monolayer of blockcopolymers. For the asymmetric structures made at small nanoparticle volume fractions, the shell structure should be more complex than the simple monolayer. Nonetheless, the shell thickness change is small in the regime, and the general trend matches well with the calculated data. Above a nanoparticle volume fraction of 0.05 (Figure 3), we find that the penalty from chain stretching within the core prevents further increase in the core radius, and the addition of nanoparticles must instead be accommodated by an increase in the number of coassemblies. This increase, along with the roughly constant value for the core radius, means that the total number of polymers that are within cores also increases. Hence, the total number of chains available for the shells must decrease with increasing nanoparticle volume fraction. Thus, in this regime, the shell thickness decreases, not due to relieving chain stretching, but due to the conservation of the total amount of polymer. The maximum in the coassembly radius thus arises from the initial increase in stretching of the core and the subsequent reduction in the number of chains in the shell. Once a maximum in core stretching has been reached, the addition of further nanoparticle is accommodated by an increase in the number of coassemblies. The Effect of Polymer Lengths. To evaluate the effect of relative BCP lengths, which is another key factor that determines the BCP assembly structure, QD/BCP assemblies were prepared using a set of different length polymers. As presented in Figure 4, QDs were self-assembled with three different length BCPs (i.e., PAA38-b-PS108, PAA38-b-PS154, and PAA38-b-PS247) at the same ΦQD (0.047). The TEM analysis of the samples revealed that the size of the assemblies and the shell thickness gradually increase with the length of the PS block as expected (Figure 4D). This general behavior has been shown for PAA-b-PS assemblies without nanoparticles36 and also for polymers adsorbed on surfaces.20,37 BCP lengths also affect the ΦQD-max values. Longer
polymers can incorporate a larger amount of nanoparticles and have larger ΦQD-max values (Figure 4E). This behavior has an interesting consequence for the assembly structure formed at ΦQD-max for different length polymers. Figure 4F presents the structural parameters of the assemblies formed at ΦQD-max for four different length polymers (PAA38-b-PS108, PAA38-b-PS154, PAA38-b-PS189, and PAA38-b-PS247). Because increasing PS lengths increase ΦQD-max (Figure 4E) and increasing ΦQD decreases the size of the assemblies (Figure 2), the assemblies formed at ΦQD-max possess similar structural parameters (i.e., assembly size and radial position of nanoparticles) despite different polymer lengths (Figure 4F). As mentioned above, at ΦQD-max, QD/BCP assemblies have the most well-defined structure with the smallest assembly size and shell thickness, the narrowest polydispersity, and the highest QD loading. The data presented in Figure 4F show that the assembly structure with the maximum nanoparticle content does not change with the polymer length. The Effect of Nanoparticle Sizes. To further examine the structure-directing role of nanoparticles, several different sized QDs were self-assembled with PAA38-b-PS154 at ΦQD-max. As previously reported, ΦQD-max increases with the size of nanoparticles.24 The radii of QDs used in this study were 4.7 ( 0.4, 5.8 ( 0.4, and 8.3 ( 0.7 nm, and their ΦQD-max values were determined to be 0.014, 0.047, and 0.11 for 4.7, 5.8, and 8.3 nm particles, respectively. The structural parameters of the assemblies formed at their correspondent ΦQD-max are shown in Figure 4F. It is apparent from the data that the core radius increases with the radius of nanoparticles while the shell thickness remains constant. We attribute this behavior to the stretching of shell polymers. Incorporated nanoparticles create valleys that needed to be filled by polymers, which was ignored in the model shown in Figure 3. Larger nanoparticles create deeper valleys in the shell and induce higher polymer stretching. For a given nanoparticle size, the extra volume in the shell relative to the total shell volume becomes smaller with increasing core size. Thus, the assemblies with bigger nanoparticles adopt a larger 7840
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The Journal of Physical Chemistry C core to reduce the extra polymer stretching and keep the minimum shell thickness. In BCP assemblies without nanoparticles, the relative block length of polymers is a key factor that dictates the assembly structure.38 The results presented in Figure 4 show that in coassemblies of nanoparticles and BCPs, other factors related to nanoparticles can have a greater impact on the assembly structure than does the polymer length.
’ CONCLUSIONS We examined how the length and concentration of blockcopolymers (BCPs) and the volume fraction of nanoparticles (ΦQD) affect the structural parameters of the radial assembly of nanoparticles and amphiphilic block-copolymers. The volume fraction of nanoparticles was found to be the prevailing factor that controls the self-assembly structure. Most uniform assemblies were formed at the maximum nanoparticle volume fraction (ΦQD-max). Below ΦQD-max, radial assemblies of nanoparticles and BCPs were formed with structural parameters (i.e., size and size distribution of the assemblies and the radial position of nanoparticles) that were highly dependent on ΦQD. At very low ΦQD, the size of the assemblies increases with ΦQD because of the increase in the radial position of nanoparticles. When ΦQD became sufficiently high, the increases in ΦQD resulted in decreases in the size and size distribution of the coassemblies over a wide range of ΦQD. This behavior is opposite of what is expected when nanoparticles act as simple solutes. The equilibrium structures calculated by the strong segregation theory qualitatively matched with the experimental data, indicating that the ΦQD-dependent structural change is due to the chain stretching energy and the interfacial energy between the two blocks. Last, the polymer length did not significantly affect the structural parameters of the coassemblies at ΦQD-max. In contrast, the core radius and the overall size of the coassemblies significantly increased with nanoparticle sizes. These results indicate that the self-assembly principles established for blockcopolymers do not directly apply to the cooperative self-assembly of nanoparticles and block-copolymers and that nanoparticles have a significant impact on the self-assembly structure. ’ ASSOCIATED CONTENT
bS
Supporting Information. DLS analysis of coassembly structures. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Present Addresses §
Department of Chemistry, Temple University, Philadelphia, Pennsylvania 19122, United States.
’ ACKNOWLEDGMENT S.-J.P. is thankful for financial support from a MRSEC seed award (DMR 05-20020), NSF career award, and ARO young investigator award. B.B.W. acknowledges the support from NSFCHE: 0809395. N.C. thanks the EPSRC for an Overseas Travel Grant (EP/E050794/1).
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