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Nanostructured organic–inorganic hybrid materials constitute one of the most promising and rapidly emerging research areas in materials chemistry, as shown by examples such as optical materials,1, 2 electromagnetic devices,3 metal catalysts,4 and novel structural features.5 In particular, block copolymer/nanoparticle (NP) hybrids offer diverse scientific challenges because of their ability to microphase-separate into a wide range of ordered nanostructures; the block copolymer can assemble to form alternating layers, complex cubic structures, cylinders packed in hexagonal lattices, and spheres in body-centered lattices, and NPs can then be sequestered into one domain to form ordered hybrids.6–8 Because of their synergistic interaction of individual constituents, block copolymer/NP hybrids can produce a wealth of novel structural features,9–11 and the rational design of hybrid morphology can be based on the current understanding of the phase behavior of block copolymers and copolymer–homopolymer mixtures.12–17
When block copolymer/NP hybrids are prepared by the blending of presynthesized NPs with a block copolymer,18, 19 not by in situ reduction of metal ions within one of the block domains,20–25 the surface of NPs can be decorated with surfactant molecules with a chemical structure similar to that of a selected block, and those NPs will diffuse into the compatible block domain. In this case, the addition of NPs can alter the characteristic dimension or sometimes even the morphology of the ordered block copolymer structure26, 27 because the miscibility of NPs with a polymer matrix (or with one of the blocks) and the resulting NP stability are crucial factors. As a result, a desirable morphology can be obtained by the control of the copolymer composition, the volume fraction of NPs, and so forth.5, 18, 28
In particular, in the case of hybrids involving ferromagnetic NPs such as iron29 and cobalt,30 the strong magnetic interactions among NPs can cause clustering or extensive aggregation31, 32 because block copolymers are often weakly associated with NPs. Consequently, for the microphase-separated block copolymers, the addition of strongly interacting magnetic NPs can change the characteristic dimensions or even the morphology of the ordered copolymer structures.33 In other words, a novel change in the ordered morphology can be realized by the control of the competition between NP–NP interactions and NP–block interactions. These fascinating features of interacting magnetic NPs have recently prompted much fundamental and applied research.
Recently, we investigated the effect of a casting solvent on the phase behavior of a poly(styrene-block-isoprene) (PS-b-PI) diblock copolymer mixed with magnetic maghemite (γ-Fe2O3) NPs.34 The competition of the interactions between NPs and the ordering of a block copolymer against the interactions of NP themselves leads to different hybrid morphologies. In other words, for PS-b-PI/γ-Fe2O3 NP mixtures cast with a neutral solvent for both polystyrene (PS) and polyisoprene (PI) blocks, latticelike aggregates of γ-Fe2O3 NPs are formed by the dominant magnetic dipole interactions, whereas the NPs are selectively incorporated into the PI domains when PI-selective hexane is used.
In this study, we re-examine the phase behavior of PS-b-PI/γ-Fe2O3 NP mixtures cast with a neutral solvent by varying the size and concentration of the magnetic NPs. The phase behavior of block copolymer/silver (Ag) NP mixtures with weak particle–particle interactions is also investigated as a reference. PS-b-PI can serve as a model system to investigate the subtle changes in the morphology upon the addition of NPs because PS-b-PI has a narrow interface between the two blocks. Moreover, we can refer to existing theoretical models of NP/block copolymer mixtures35, 36 on the basis of the strong segregation limit approximation. A hexagonal cylinder (hex) forming a PS-b-PI diblock copolymer was used as a structure-directing matrix. The structural information was obtained by the combination of small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM).
Monodisperse γ-Fe2O3 NPs (7 or 14 nm in diameter) and monodisperse Ag NPs (6 nm in diameter), with both surfaces modified with oleic acids, were synthesized by the reaction of iron pentacarbonyl, silver trifluoroacetate, and oleic acid.37 A PS-b-PI diblock copolymer with a PS molecular weight of 39,200 g/mol and a PI molecular weight of 12,100 g/mol was synthesized by standard anionic polymerization,38 and the prepared sample was designated SI(39–12); it had a polydispersity index of 1.02, as verified by gel permeation chromatography. Mixtures of NPs and SI(39–12) (0/100, 1/99, 3/97, 5/95, 7/93, 10/90, and 15/85 w/w) were prepared via solution casting with toluene as a casting solvent. All the samples were annealed in a vacuum oven for 7 days at 120 °C, which was well above the glass-transition temperatures of both PS and PI.
TEM was performed on a JEOL JEM-3000F microscope. The specimens were stained by exposure to osmium tetroxide (OsO4) vapor for 20 min. For TEM measurements, electron-transparent films with nominal thicknesses of 50–80 nm were cryomicrotomed at −90 °C with a diamond knife on a Leica Ultracut UCT and transferred to Cu grids.
Synchrotron SAXS measurements were carried out at the 4C1 SAXS beam line at the Pohang Light Source (λ = 1.54 Å and Δλ/λ = 5 × 10−4) consisting of Si (111) double-crystal monochromators, ion chambers, and a two-dimensional position-sensitive detector with 2048 pixels × 2048 pixels. The typical beam size was smaller than 1 × 1 mm2, and the sample-to-detector distance was 2.16 m. The sample temperature was controlled within ±0.2 °C with a thermostated brass block, and at least 5 min of equilibration time was allowed at a given temperature before measurement. The resulting two-dimensional scattering images were averaged azimuthally to obtain traces of the intensity versus scattering wave vector q [q = 4πsin(θ/2)/λ, where θ is the scattering angle].
RESULTS AND DISCUSSION
To characterize the structural changes in the block copolymer associated with different NP concentrations, TEM was first performed. The stained PI phase appears dark, whereas PS is bright in the TEM micrographs. The micrographs reveal the following novel features. We can observe the clear morphological transition of hex (without NP) → undulated hex (with 5 wt % 7-nm γ-Fe2O3 NPs) → cubic spheres (with 10 wt % 7-nm γ-Fe2O3 NPs). In our case, 1 wt % of 7-nm γ-Fe2O3 NPs is equivalent to 0.4 vol % of γ-Fe2O3 NPs on the basis of the densities of γ-Fe2O3 NPs (2.583 g/cm3), PS (1.05 g/cm3), and PI (1.015 g/cm3). With an increase in the NP concentration, the magnetic NPs clearly form strings, and this behavior confirms the previous study,39 in which magnetic interactions between NPs favor clustering, particularly in the form of strings. For an NP concentration of 5 wt %, as shown in Figure 1(a), extensively aggregated strings can be observed, and TEM confirms the fluctuation or undulation of the ordered block copolymer perturbed by the existence of stringlike γ-Fe2O3 NP aggregates. For an NP concentration of 10 wt %, the magnetic NPs aggregate into self-assembled lattices, and the block copolymer morphology shows body-centered-cubic (bcc) spheres, as shown in Figure 1(b).
To better understand the morphological changes with the variation of the NP concentration, synchrotron SAXS measurements were undertaken for 7-nm γ-Fe2O3 NP/SI(39–12) mixtures, as shown in Figure 2. For neat SI(39–12), the q/q* ratios of 1:√3:√7:√9 (the lower trace in Fig. 2), where q* = 2π/d100 and d100 is the domain spacing between hex microdomains, as indicated by inverted, open triangles, represent the hex morphology of the block copolymer. The √4q* reflection is relatively weak because of the coincident extinction with the cylinder form factor. The scattered intensity of the first peak gradually decreases, and high-order peaks are much smeared with an increase in the NP concentration up to 5 wt %; this is in good agreement with the TEM result in Figure 1(a) showing undulated cylinders. However, the main peak intensity is again increased above the NP concentration of 5 wt %, and a well-developed bcc phase is finally obtained for 15 wt % NP, as indicated by inverted, filled triangles with q/q* ratios of 1:√2:√3:√4:√6:√8:√9 (the upper trace in Fig. 2) where q* = 2π/d110, where d110 is the domain spacing between bcc microdomains. At the same time, the scattering intensity around q = 0.73 nm−1, originating from the NP correlation, significantly increases, signaling dominant NP aggregation. The unique morphological transformation is again confirmed with TEM in Figure 2. Without an NP loading, the TEM image clearly shows the hex morphology viewed in the  direction. In contrast, with the 15 wt % NP loading, the block copolymer morphology is completely transformed into the bcc structure. Three zone axes have been obtained on the basis of the feature of each TEM image, and they are all in excellent agreement with the TEM image simulation using the CaRIne Crystallography packages.40 It should be emphasized at this point that no order–order transition has been observed for neat SI(39–12) up to 200 °C. In addition, SI(39–12) is in the vicinity of the bcc and hex phase in the block copolymer phase diagram, and this makes the morphological transition between hex and bcc rather easy if triggered by external means. In this study, the external mean should be coexisting magnetic NPs or, more specifically, extensively aggregated, latticelike NPs.
During the hex-to-bcc transition induced by coexisting magnetic NPs, we observed an interesting change in the q* value as a function of the NP concentration. Figure 3 shows that the q* value decreases abruptly for the 5 wt % NP concentration, which is the same concentration at which undulated cylinders are observed. After the formation of cubic spheres with a 10 wt % NP concentration, no considerable change in the q* value was observed. d100 is 296 Å without γ-Fe2O3 NPs, whereas d110 is 330 Å with 15 wt % γ-Fe2O3 NPs. As a result, the difference in the domain spacing between bcc and hex is about 11.4%, which is in good agreement with the conservation of volume, which requires the diameter of hex cylinders to be smaller than that of spherical microdomains.
The hex → bcc transformation of block copolymers is a well-known mechanism, and the existence of a metastable or even long-lived undulated hex phase during the transition has been reported.41 However, the phenomena observed in this study are quite intriguing because morphological transitions have typically been predicted when small NPs, which are much smaller than the domain size of a block copolymer, are selectively dispersed into one of the blocks.42 In this respect, one might expect the origin of the hex-to-bcc transition to be the preferential distribution of NPs into the PS domains. However, the interfacial tensions, which are related to the surface tensions (γPS = 40.7 dyn/cm, γPI = 30.5 dyn/cm, and γoctadecene = 28.1 dyn/cm at 20 °C) and the polarity of the two contiguous phases,43 are calculated as γPS/NP = 7.38 dyn/cm and γPI/NP = 0.33 dyn/cm on the basis of the harmonic mean equation.44 This implies that the NP surfaces prefer the PI block over the PS block.
It is also worthwhile to observe that a majority of γ-Fe2O3 NPs are macroscopically segregated from the block copolymer domains, and as a result, the hex cylinder → cubic sphere transition is not due to the selective dispersion of γ-Fe2O3 NPs into one of the blocks. Figure 4 clearly confirms that the γ-Fe2O3 NP aggregates are macroscopically segregated from the block copolymer domains at a 10 wt % NP loading. Compared with the bright-field TEM image [Fig. 4(a)], a dark-field TEM image [Fig. 4(b)] using the (311) reflections of γ-Fe2O3 NPs serves as definite evidence for the segregation of NPs from block copolymer domains. To the best of our knowledge, this morphological transition of a block copolymer triggered by coexisting latticelike NP aggregates has never been reported before.
Because the new bcc structure of the block copolymer emerges through the undulated hex triggered by coexisting NPs, we can now address the following important questions: What is the role of coexisting magnetic NPs in causing such a peculiar morphological transition? Is it truly general behavior in the sense that the hex phase follows the same phase sequence (i.e., hex → bcc) upon mixing with other types of NPs? A particular emphasis is placed on the case without significant particle–particle interactions such as metal NPs. From a theoretical point of view,35, 36 both dynamic mean-field density functional theory and Monte Carlo simulations, when the particle–particle interaction is not taken into account, predict that as the NP volume fraction is increased, a two-phase coexistence, macroscopically phase-separated NP aggregates and block copolymers, emerges when the size of the NPs becomes comparable to the radius of gyration of a minority block. The formation of such phases is due to the interplay between the particle–particle excluded-volume interaction, the preferential interaction between the particles and one of the blocks, and the enthalpic and stretching interactions within the bock copolymer. However, the morphological transition between ordered structures of a block copolymer perturbed by coexisting NPs has not been reported, and a disordered morphology is instead expected when the NP concentration is increased.
To investigate the effect of particle–particle interactions on the block copolymer morphology, we introduced Ag NPs 6 nm in diameter into SI(39–12). Synchrotron SAXS profiles for Ag NP/SI(39–12) mixtures with increasing NP concentration are shown in Figure 5. For 1 and 3 wt % NP concentrations, inverted, filled triangles represent the hex phase with high-order peaks of 1:√3. With an increase in the NP concentration, however, the hex morphology gradually loses its long-range order, as confirmed by the abrupt decrease in the scattered intensity of the first peak and the disappearance of high-order peaks; this is also consistent with previous theories35, 36 but is in striking contrast to the case of γ-Fe2O3 NPs. The scattering peaks at q = 0.87 nm−1 corresponding to the NP correlation indicate that NP aggregation is also feasible with metal NPs, as already predicted by theory.35, 36 However, in contrast to γ-Fe2O3 NPs of a similar size, the scattering intensity of the NP correlation is not much dependent on the NP concentration, and this implies the random aggregation of Ag NPs; this is also confirmed by the TEM image of Ag NP/SI(39–12) (10/90 w/w), as shown in the inset of Figure 5. Interestingly, the TEM image shows a gray band of PI blocks 12 nm thick preferentially wetting the aggregated Ag NPs. This distinct wetting of the block copolymer plays an important role in the disordering of the block copolymer morphology as a function of the NP concentration.
We also examined the interplay between the block copolymer and magnetic NPs of different sizes. When larger magnetic NPs (i.e., 14-nm γ-Fe2O3 NPs instead of 7-nm NPs) are employed in the mixture, the same hex → bcc transformation can be observed even with a lower NP concentration of 3 wt %, presumably because of the enhanced magnetic interactions. Figure 6(a) shows the cross-sectional TEM images of 14-nm γ-Fe2O3 NP/SI(39–12) (3/97 w/w) stained with OsO4 in different regions. The block copolymer morphology is again in good agreement with the bcc structure. Extensive hexagonal packing of self-assembled 14-nm γ-Fe2O3 NPs, instead of stringlike aggregates whose size measures from a few hundred nanometers to micrometers, has also been confirmed with TEM as well as SAXS.
The effect of the polydispersity of the size of NPs on the morphological transition is also examined. We deliberately synthesized polydisperse γ-Fe2O3 NPs with an average size of 7 nm, whereas the NP size varies from 4 to 10 nm because of the introduction of humidity during the synthesis. To our surprise, the initial hex morphology remains with an increase in the NP concentration, and the NP aggregates cannot form regular, latticelike aggregates up to a 20 wt % NP loading. Figure 6(b) shows a representative example with a 10 wt % NP loading. In this case, with an increase in the NP concentration, the number of NP aggregates 50–100 nm in size increases instead of the increased size of NP aggregates for the case with monodisperse NPs. This implies that the hex-to-bcc transition, shown with monodisperse NPs, does not occur with polydisperse NPs, and this could also serve as indirect evidence for the role of NP lattices in the morphological transformation.
Finally, we infer that the unique morphological behavior observed in this study is due to the specific particle–particle interaction leading to latticelike aggregates. For magnetic NPs, the effect of the magnetic dipole interaction is huge, and it can significantly expand the range of interparticle interactions. Thus, we expect strong magnetostatic interactions between magnetic dipole moments as NP strings are formed,45 and the magnetic attraction is estimated to be most dominant for 14-nm γ-Fe2O3 NPs coated with oleic acids.46 In this study, different interparticle interactions are shown by different aggregation behaviors when different types of NPs are mixed with a given block copolymer. As shown in Figure 7, with a fixed NP loading of 5 wt %, the high-order peaks of 1:2:3 for 7-nm γ-Fe2O3 NP and 1:√3:√4: √7:√9:√11:√12… for 14-nm γ-Fe2O3 NP represent the formation of strings and hexagonal lattices, respectively, whereas a single peak for Ag NP aggregates indicates randomly aggregated NPs. When latticelike γ-Fe2O3 NP aggregates are formed, the PI block chains preferentially diffuse into the interstitial space of NP lattices without perturbing the NP lattices, and at the same time, the NP surfaces are wetted by PI chains. We believe that this could induce packing frustration near the NP aggregates, and the ordering wave emanating from the regular, undulated surfaces of latticelike γ-Fe2O3 NP aggregates can cause interfacial fluctuations between PS and PI block domains. Eventually, at a higher NP loading, the ordering wave triggers the ultimate transformation from hexagonal, cylindrical domains into bcc spheres.
The phase behavior of a PS-b-PI diblock copolymer, that is, cylinder-forming SI(39–12), doped with metal (Ag) and magnetic (γ-Fe2O3) NPs, has been investigated by the combination of SAXS and TEM. When the NP size is larger than the radius of gyration of a minor block, the domains of NP aggregates are observed for both magnetic and metal NPs as the NP concentration is increased. However, the block copolymer morphology significantly depends on the strength and range of particle–particle interactions. Ag NP aggregates induce a disordered micellar morphology of the block copolymer because of the preferential wetting of PI domains on Ag NP aggregates and the significant number of block copolymer chains trapped within the aggregate. In contrast, when magnetic NPs are mixed with PS-b-PI, segregated domains of self-assembled magnetic NPs are formed with an increase in the NP concentration, and regular, undulated surfaces of latticelike γ-Fe2O3 aggregates covered with low-surface-energy surfactants strongly perturb the ordering of the block copolymer. In the case of 7-nm γ-Fe2O3 NPs, when the NP concentration is increased above 5 wt %, fluctuations or undulations of cylindrical PI domains start to occur and eventually lead to the bcc ordered structure for higher NP concentrations. The interplay between magnetic NPs and the block copolymer has also been tested with magnetic NPs of different sizes. Notably, when the particle–particle interactions are enhanced by the size of γ-Fe2O3 NPs being increased from 7 to 14 nm, these peculiar morphological transitions are observed even with lower NP concentrations. This implies that the block copolymer microstructure, whether it is in hex or bcc ordered domains, can be tuned by the simple variation of the coexisting magnetic NP concentration without the alteration of the block copolymer composition. The fact that the morphology of a block copolymer can be altered by the existence of latticelike, magnetic NP aggregates, as clearly demonstrated in this study, can add more flexibility in controlling nanostructures, opening up potential for future applications.
This work was supported by the NANO Systems Institute of the National Core Research Center of the Korea Science and Engineering Foundation and by the Brain Korea 21 Program, which is endorsed by the Ministry of Education of Korea. The authors are very grateful for the use of the Pohang Light Source, which is supported by the Ministry of Science and Technology of Korea.