Research on inorganic nanoparticles (NPs) has recently increased rapidly and various applications of inorganic NPs have attracted significant interest, as they exhibit excellent properties such as electronic, optoelectronic, and catalytic properties. For example, NPs have attracted great attention in therapeutic and diagnostic applications due to their unique physical properties coupled with their size-dependent electrical and optical properties.1, 2 However, the interactions between NPs and biomacromolecules should be controlled in order to use NPs in biomedical applications, and thus the surfaces of NPs have been functionalized with various ligands such as small molecules, polymers, and biomolecules, or conjugated with biomolecules for specific recognition or biocompatibility.3
Furthermore, several research groups have studied nanocomposite materials composed of nanofillers in polymer matrix components to broaden the practical use of inorganic NPs in a variety of applications such as photonic band gap materials, electrochemistry, sensors, and memory devices.4–11 For example, Russell and coworkers prepared nanocomposites consisting of a diblock copolymer and either cadmium selenide- or ferritin-based NPs.12 They controlled the orientation of polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) block copolymer thin films by adding tri-n-octylphosphine oxide (TOPO) coated CdSe NPs. The orientation was parallel to the substrate in the pure diblock copolymer thin film. On the other hand, the orientation was perpendicular to the substrates in the diblock copolymer thin film containing the TOPO CdSe NPs. The change in the orientation of the thin film was due to the segregation of the NPs at the air–film interface, which mediates interfacial interactions, even though one of the blocks strongly interacted with the substrate.
Kramer and coworkers also fabricated nanocomposites consisting of lamellar-forming PS-b-P2VP diblock copolymer and surface-tuned gold (Au) NPs with thiol-terminated polymeric ligands; PS-SH, P2VP-SH, and PS-r-P2VP-SH.13 By tailoring the polymeric ligands to tune the surface properties of the Au NPs, they showed a sharp transition in the location of the NPs from the PS domain to the PS/P2VP interface and then to the P2VP domain when the ratio of the PS-SH/P2VP-SH on the surface of the Au NPs was adjusted. Furthermore, by comparing the adsorption energy of amphiphilic and PS-r-P2VP coated Au NPs, they demonstrated the possibility of rearranging thiol-terminated polymeric ligands on the surface of Au NPs.
Among the various NPs used to prepare nanocomposites, Au NPs have attracted significant research interest because their surface is tunable through grafting thiol-terminated polymeric ligands and their size-related optical properties.14–17 However, it is well known that the bond between the Au surface and the thiol group is unstable above 60 °C.18 Thiol-terminated polymeric ligands detach from the surface of the Au NPs at high temperature, and thus the destabilized Au NPs aggregate into larger particles. This serious limitation of Au NPs grafted with thiol-terminated ligands prevents their practical use in a diverse range of applications. In order to solve this issue, several approaches, such as an introduction of more than two thiol-anchoring groups at the polymer chain ends or the shell crosslinking of polymer micelles surrounding the Au NPs, have been proposed previously.19–22 Nonetheless, these methods were limited by complicated fabrication steps or insufficient thermal stability of resulting NPs.
Herein, we discuss the recent advances on the design of thermally stable Au NPs and nanocomposites consisting of a polymer matrix and thermally stable Au NPs. We first present the synthesis of thermally stable Au NPs via shell crosslinking with various functional polymeric ligands. In the second part, we introduce the use of thermally stable NPs as compatibilizers in polymer blends. Then, we review the block copolymer nanocomposites containing the thermally stable NPs, with an emphasis on tuning the surface properties of the Au NPs to control the location of the Au NPs within the block copolymer matrix, and to control the orientation or morphology of the block copolymer films. Lastly, we will show applications of thermally stable NPs in conducting polymer and free-standing multilayer films.
DESIGN AND FABRICATION OF THERMALLY STABLE NANOPARTICLES
“Grafting-from” methods are used to fabricate polymer-tethered NPs by growing polymers directly from active sites on the NP surface. Living radical polymerization, living cationic polymerization, or ring-opening polymerization have been used to prepare the polymeric ligands to decorate the NPs. Among these methods, living radical polymerizations such as atom transfer radical polymerization (ATRP) and reversible addition-fragmentation chain transfer (RAFT) are widely used, because they can be applied to various monomers and are tolerant under various conditions while the ring-opening and ionic polymerizations are very sensitive to moisture, air, and impurities. Unlike the “grafting-to” methods, the grafting-from methods are not limited by the steric hindrance, because the polymer brushes can be grown from the initiators on the NPs surface. Therefore, it is possible to obtain a highly grafted polymer brushes on the NPs surface.
Kotal et al. used a grafting-from method to synthesize poly(methyl methacrylate) (PMMA)-grafted Au NPs with crosslinked thin silica layer around the Au core.23 The Au NPs were first prepared using one-phase method in the presence of 11-mercapto-1-undecanol (MUD), and the MUD on the Au surface was exchanged with 3-mercaptopropyltrimethoxysilane (MPS). Hydrochloric acid was then used to hydrolyze the MPS on the Au NPs and condense the trimethoxysilane groups in the MPS. The hydrolysis resulted in the formation of very thin, dense, crosslinked silica-coated Au NPs with hydroxyl groups on the surface. The hydroxyl groups on the Au surface were reacted with a initiator, [(chloromethyl)phenylethyl] trimethoxysilane (CTMS), to produce initiator-modified Au NPs. The polymerization of PMMA was performed via ATRP from the initiator on the Au surface with the CuCl/2,2′-bipyridyl catalyst system. The ultraviolet–visible (UV–vis) spectra showed no shift in the surface plasmon band after the NPs were heated at 100 °C. This result indicates that the heat treatment did not bring the detachment of the polymer from the NPs and thus the Au NPs were thermally stable.
Moreover, Matyjaszewski and coworkers recently reported the one-pot synthesis of thermally stable Au NPs consisting of a crosslinked shell and linear tethered polymer brushes via grafting-from approach.22 They synthesized the crosslinked polymeric shell by adding n-butyl acrylate (BA) monomer and ethylene glycol dimethacrylate (EGDMA) as a crosslinker via ATRP with initiator-modified Au NPs. The reactivity of BA monomer is much lower than that of the EGDMA crosslinker; therefore, the crosslinker was consumed faster than the monomer and was preferentially incorporated into the crosslinked shell. Subsequent polymerization of the remaining monomer formed the linear brush layer. The crosslinked polymer shell was composed of 13.1 mol % EGDMA and 86.9 mol % BA, and the linear brush consisted of pure BA units. When the resulting core-shell Au NPs solution was heated up to 110 °C for 24 h, the UV–vis spectra exhibited almost the same plasmon band for the as-prepared and heated NPs, indicating that heating did not significantly change the structure of the NPs and thus confirming the thermal stability of NPs.
In “Grafting-to” method, preformed polymers with thiol group at the end of polymer chains are directly attached to the Au surface during the synthesis of Au NPs via two-phase method or one-phase method. Using grafting-to methods, one can precisely control the chain length, architecture, composition, and functionality of polymers to prepare the various NPs with well-defined surface property, although the high grafting density may be limited by the steric hindrance as mentioned earlier.
Yoo et al. designed thermally stable core-shell Au NPs by introducing photo-crosslinkable azido groups (N3) into polymeric ligands. They used the diblock copolymer as ligands to improve the thermal stability and compatibility of Au NPs.24 A thiol-terminated block copolymer consisting of polystyrene (Mn ≈ 2 kg/mol) and azido-polystyrene (Mn ≈ 1 kg/mol), P(S-b-S-N3)-SH, was synthesized via RAFT polymerization. They circumspectly designed the ligands where the short P(SN3) block is placed adjacent to the thiol group. Therefore, the Au cores were protected by crosslinked P(SN3) shells after in situ photo-crosslinking to produce well-defined core-shell nanostructures that consist of linear polystyrene chains surrounding a crosslinked P(SN3) shell, as shown in Figure 1(a). The P(S-b-S-N3)-SH coated Au NPs were prepared by two-phase method, and the core size of the resulting Au NPs was about 3 nm as measured from transmission electron microscopy (TEM). The crosslinking reaction and subsequent thermal stability test of the P(S-b-S-N3)-SH-tethered Au NPs were performed in both solutions and thin films. For the thermal stability test in the solution, the NPs were first dispersed in either dioxane or chloroform, which are transparent to UV absorption and are good solvents for PS. The solution was exposed to UV light (λ = 254 nm) for 1 h under ambient conditions for crosslinking reaction, and then the resulting crosslinked Au NPs were separated by centrifugation. The Au NPs were then redispersed in dibutyl phthalate (DBP), which is a nonvolatile PS-selective solvent. Solutions containing the crosslinked Au NPs in DBP were heated up to 150 °C for several hours. For the control experiment, a DBP solution containing uncrosslinked PS-Au NPs was prepared using the same method and was subjected to the same conditions as the crosslinked Au NPs solutions. In Figure 1(b), the DBP solution of PS-Au NPs became transparent after heating, because the Au-thiol bonds had dissociated causing all the NPs to aggregate and precipitate out of the solution. In contrast, the DBP solution of crosslinked P(S-b-S-N3)-Au NPs showed excellent thermal stability because the solution did not change. As further evidence of the thermal stability of the P(S-b-S-N3)-Au NPs, TEM analysis of the NPs revealed that the size of the Au NP cores did not change as shown in Figure 1(c). For the thermal stability test performed in the thin films, a PS homopolymer containing 10 wt % Au NPs was first spin-casted onto the silicon substrates. The thin films containing the P(S-b-S-N3)-Au NPs were then exposed to UV light in vacuo for 15 min in order to crosslink the shells. After the nanocomposite thin films were annealed in vacuo at 180 °C for 2 days, they were examined by TEM. As a result, it was observed that the core size of the crosslinked P(S-b-S-N3)-Au NPs had not changed during annealing, and the NPs were well-dispersed throughout the PS matrix. The PS-Au NPs, on the other hand, exhibited a significant degree of aggregation after annealing.
Later, Lim et al. fabricated the thermally stable Au NPs with highly grafted polymer chains, by combining RAFT polymerization and the click chemistry of copper-catalyzed azide-alkyne cycloaddition (CuAAC).25 Bromine-functionalized Au NPs (Au-Br) were first synthesized using 11-bromo-1-undecanethiol via the Brust method. The Au-Br NPs were then mixed with NaN3 to substitute the bromine end groups on the Au surface to the azido (N3) group in order to produce Au-N3 NPs. The azido groups on the surface of the Au NPs were used for the CuAAC reaction with alkyne derivatives. A properly designed diblock copolymer consisting of PS and poly(4-benzylchloride) (PSCl) with an alkyne end group, on the other hand, was synthesized via RAFT polymerization using alkyne-terminated RAFT chain transfer agent. The molecular weights of PSCl and PS were determined as 3.3 and 5.5 kg/mol, respectively. The resulting PS-b-PSCl-alkyne was coupled to the Au-N3 NPs through the CuAAC reaction to obtain Au NPs that exhibited a high grafting density of polymer chains. Then, azido groups were substituted on the Au-PSCl-b-PS NPs to produce the Au-PSN3-b-PS NPs. On the basis of the core size of the Au NPs in TEM images and the results of the thermogravimetric analysis (TGA) of Au-PSN3-b-PS NPs, the chain areal density (Σ) of the NPs was 1.22 chains/nm2, which is higher than that of the Au-PS-PSN3-SH NPs (Σ = 0.9 chains/nm2) that were prepared without CuAAC reaction. To increase the thermal stability, azido groups in the Au-PSN3-b-PS NPs were crosslinked by thermal annealing and/or UV irradiation. The thermal stability of the crosslinked Au-PSN3-b-PS NPs prepared by click reaction was subsequently determined by introducing them into a PS matrix (Mn = 56.5 kg/mol) and then annealing at 200 °C for 24 h. Consequently, it was found that the core size of the annealed Au-PSN3-b-PS NPs did not change. In contrast, the PS-Au NPs without crosslinkable groups became aggregated under the same annealing condition, showing a significant increase in particle size from 2.5 to 8.5 nm.
Jang et al. reported a strategy for preparing thermally stable Au-core/Pt-shell NPs containing crosslinked polymeric shells.26 A thiol-terminated poly(styrene-b-1,2&3,4 isoprene) diblock copolymer ligands (PS-b-PI-SH) were synthesized using sequential anionic polymerization with various lengths of polyisoprene blocks. The Au NPs were synthesized using a two-phase toluene/water method in the presence of the thiol-terminated PS-b-PI diblock copolymer with various chain areal densities. Consequently, Au NPs tethered by diblock copolymer ligands that had a vinyl-rich PI shell near the surface of the Au and a PS block outer brush were fabricated, as shown in Figure 2. The shells were then crosslinked through the hydrosilylation reaction between the pendant vinyl groups on the polyisoprene block and the hydrosilane functional groups of 1,1,3,3-tetramethyldisiloxane (TMDS) in the presence of a platinum catalyst. The Pt catalyst used during the hydrosilylation reaction was reduced on the surface of the Au NPs, resulting in the formation of a thin Pt shell instead of a Pt NP. The formation of a Pt shell was characterized using X-ray diffraction, high-resolution transmission electron microscopy (HR-TEM), UV–vis spectroscopy, and X-ray photoelectron spectroscopy (XPS). The thermal stability of the shell-crosslinked Au-Pt NPs was examined by varying the length of the crosslinkable block (PI) in the ligand and the number of ligands per particle. Au NPs tethered with three different ligands, in which PS block lengths were fixed at ∼3 kg/mol but PI block lengths were varied as 0.8, 1.4, or 3.2 kg/mol, were used for thermal test to determine the effect of the PI block length on the thermal stability of the Au NPs. To test the thermal stability, crosslinked Au nanoparticle solutions in toluene were heated up to 130 °C for 1 day. After thermal annealing, the mean diameter of the Au NPs prepared with the shortest PI block increased from 2.5 ± 0.4 to 3.6 ± 1.2 nm, whereas the size of the Au NPs having longer PI blocks did not change significantly, indicating the excellent thermal stability of these Au NPs. They also investigated the thermal stability of NPs as a function of the mean number of chains (f). Three different Au NPs having f of 19, 22, and 39 were synthesized by varying the mole feed ratio of thiol-terminated ligands to the Au precursor. After thermal stability test, the differences in the mean core diameter (ΔDcore) were estimated as 0.7, 0.4, and 0 nm for the Au NPs having f of 19, 22, and 39, respectively. These results emphasize that certain lengths of crosslinkable blocks and mean numbers of chains on one NP were required in order to obtain stable crosslinked shells at elevated temperatures.
POLYMER BLENDS WITH THERMALLY STABLE NANOPARTICLES
Thermally Stable Nanoparticles as Compatibilizers
Most commercial plastic products are produced by blending of various polymers. Blending polymers is a simple way to obtain the advantages of each polymer. However, macrophase separation could cause several problems such as degradation of the mechanical properties or coalescence of the blended polymer during melt processing.27 Various compatibilization strategies have been widely used to stabilize polymer blends in order to prevent the problems associated with thermodynamically unstable polymer blends. Adding a diblock copolymer composed of miscible homopolymer blocks to polymer blends is frequently proposed as a simple method of compatibilization.28–34 Block copolymers can increase the dispersity of minor domains throughout polymer blends, improve the adhesion between the phases, and prevent the coalescence. Macosko et al. used PS-b-PMMA block copolymer as a compatibilizer in a PS/PMMA homopolymer blend system.28 Various ternary blends were fabricated by varying the molecular weight of the homopolymer, and the molecular weight and fraction of the block copolymer. They demonstrated that ∼1 vol % block copolymer can effectively decrease the size of the PMMA minor phase. They also demonstrated that when the PS/PMMA interface was filled with 15–20% block copolymer, the minor PMMA phase in the polymer blend was stable and spherical. Furthermore, it was shown that low-molecular-weight block copolymers are unable to provide static stability because they are not effectively entangled with homopolymers. On the other hand, high-molecular-weight block copolymers were not effective because they form micelles due to the low critical micelle concentration.
As an alternative to block copolymers, NPs can be good candidates for compatibilizers for following reasons. First, surface NPs can be readily tuned with various ligands and they won't form the micelles as block copolymers. Also, the most inorganic NPs have strong contrast with polymers and thus they can be easily traced by electron microscopy. However, NPs have not been actively used as compatibilizers in polymer blends because the most NPs are thermally unstable as mentioned earlier. Yoo et al. recently designed thermally stable Au NPs with a photo-crosslinkable polymeric ligand, PS-b-PS-N3-SH, as already discussed in the previous section. The polymeric ligands anchored on the surface of Au NPs consist of two parts; a polar PS-N3 shell (due to nitrogen atoms) and a nonpolar PS brush. In this case, the difference in polarity between the inner PS-N3 shell and the outer PS brush can allow the balanced enthalpic interactions with polar and nonpolar polymers, respectively. Accordingly, polymer blends composed of nonpolar PS and polar PMMA homopolymers, and the Au NPs were fabricated to examine the compatibilizing effect of the Au NPs on the polymer blends. A blend of PS (56,500 g/mol)/PMMA (57,000 g/mol) (50:50 by volume) was mixed with thermally stable Au NPs (either 5 or 10 wt %). The solutions were drop-cast onto NaCl substrates and were then thermally annealed at 180 °C for 2 days. The thermally annealed samples were then transferred to epoxy supports and were microtomed for cross-sectional TEM. Figure 3(a,b) shows the cross-sectional TEM images of the PS/PMMA blends containing either 5 or 10 wt % of thermally stable Au NPs. In this case, the thermally stable Au NPs were nonselective in both polymer domains and they are dramatically localized at the PS/PMMA interface to decrease the interfacial tension between the PS and PMMA phases. The change of droplet size was then measured to examine the compatibilizing effect of thermally stable Au NPs on PS/PMMA blends. As shown in Figure 3, the PS/PMMA blend prepared without the thermally stable Au NPs has a relatively large droplet size, 0.92 ± 0.33 μm on average. However, when 5 and 10 wt % thermally stable Au NPs were added to the PS/PMMA blends, the average droplet sizes decreased to 0.46 ± 0.14 and 0.32 ± 0.09 μm, respectively. Furthermore, the Au NPs did not agglomerate into larger particles in the polymer blends during thermal annealing. On the basis of these results, it was concluded that the synthesized thermally stable Au NPs can be used as an effective compatibilizer in a PS/PMMA blend systems that require the thermal processing.
Size Effect of Thermally Stable Au NPs on Compatibilization
The size of Au NPs can also affect the compatibilization in immiscible polymer blends.35 Two different Au NPs, small Au NP-1 (core diameter = 2.7 nm) and large Au NP-2 (core diameter = 12.3 nm), were synthesized using photo-crosslinkable PS-b-PSN3-SH polymer ligands to investigate the size effect of Au NPs on the compatibilizing process in immiscible polymer blends. Both Au NPs exhibited thermal stability through the photo-crosslinking of the PS-N3 block. Nonpolar PS and conducting polytriphenylamine (PTPA), which contains a polar nitrogen moiety, were selected as the polymer blend system. In addition, PS-b-PTPA was synthesized using RAFT polymerization to compare the compatibilizing effect of the Au NPs with that of the block copolymer.35 Both the small and large Au NPs had nonpolar PS brushes and polar PS-N3 shells, so that the NPs could effectively compatibilize between the nonpolar PS and the polar PTPA matrix. The nanocomposite samples were prepared by mixing PS/PTPA blends (volume ratio of 8/2) with various contents (vol %) of Au NP-1, Au NP-2, or PS-b-PTPA. Then, the resulting samples were thermally annealed at 200 °C for 2 days. As shown in Figure 4(a), the average size of the PTPA droplets in the PS/PTPA blend prepared without any compatibilizer was 1.4 μm. When either the large or small Au NPs were mixed with the PS/PTPA polymer blends, the Au NPs were mainly located at the polymer/polymer interface, and the NPs did not aggregate. However, it was found that the size of the Au NPs affected their ability to function as compatibilizers in the polymer blend. Figure 4 shows the effect of the Au NP-1 content (φp) on the droplet size in the polymer blend. The PTPA droplet size progressively decreased from 1400 (φp = 0) to 860 (φp = 0.1) to 520 nm (φp = 1 vol %). Au NP-2, which was 3.5 times larger than Au NP-1, was added to the PS/PTPA polymer blend in contents (φp) ranging from 0.1 to 5 vol %. The TEM images show that as φp increases, the amount of Au NP-2 at the interface increases, decreasing the size of the PTPA droplets. However, the compatibilizing effect of Au NP-2 was much smaller than that of Au NP-1 at the same φp. For example, the size of the PTPA droplets in the polymer blend containing Au NP-2 at φp = 1.0 vol % was 700 nm, while the size of the PTPA droplets with smaller Au NP-1 at the same φp was 520 nm. As a result, it was concluded that the smaller Au NPs are more effective compatibilizers, although both Au NPs can function as compatibilizers.
BLOCK COPOLYMER WITH THERMALLY STABLE NANOPARTICLES
Controlling the Location of Nanoparticles in Block Copolymer Matrices
The block copolymers are one of the most fascinating materials used in the field of nanotechnology, as they can microphase separate to from the nanoscopic periodic structures.36–38 The block copolymer has been widely studied and has attracted great attention for use in various applications including microelectronic devices, photonic band-gap devices, membranes, and biosensors.39–41 By incorporating inorganic NPs into the block copolymers, it has been demonstrated that the resulting block copolymer nanocomposites have the enhanced chemical and mechanical properties such as its electrical and thermal conductivity, catalytic activity, and mechanical stiffness.42–47
In this regards, precise control of the location of NPs into either A or B domains or at the A/B interface within block copolymer matrices is critical to fabricate novel functional hybrid materials. Numerous studies on controlling the location of thermally unstable NPs in block copolymer matrices have been well-documented. The location of NPs in diblock copolymers can be affected by sizes of NP cores, the types and compositions of ligands, ligand length, and the grafting density of the ligands on the NP surface.48–50 For example, larger NPs prefer to stay at the polymer chain ends in the center of domains rather than smaller NPs to reduce theconformational entropic penalty. The enthalpy from the interaction between NPs and A/B domains can also affect the location of NPs within the block copolymer matrix. A-type polymer-coated NPs are usually segregated into the A domains and B-type polymer-coated NPs are usually segregated into the B domains in AB diblock copolymers. If the NPs have favorable interactions with the A domain, the location of the NPs can be controlled by varying the grafting density of the B polymer on the surface of the NPs.
However, nanocomposites consisting of block copolymers with thermally stable NPs have not been extensively investigated yet, and there are only a few studies with thermally stable NPs. Yoo et al. developed thermally stable Au NPs with tuned surface property so that they can control their locations within a symmetric PS-b-PMMA diblock copolymer.51 The surface properties of Au NPs were tuned by varying the compositions of methyl methacrylate (MMA) and styrene in the brush part of the polymeric ligands. In consequence, ∼2000 g/mol molecular weight of P(MMA-r-S) containing 80 mol % MMA, 50 mol % MMA, 20 mol % MMA, and 0 mol % MMA were synthesized as outer brush part. A short block of azido styrene (PS-N3, Mn = 700 g/mol) was simultaneously synthesized as a second block for a crosslinked inner shell to impart the thermal stability of Au NPs. The resulting Au NPs were added to the lamellar forming PS-b-PMMA (Mn = 130 kg/mol) in bulk and the samples were annealed in vacuo at 200 °C for 2 days. From the TEM images in Figure 5, it was clearly seen that the Au NPs which have either 80 or 50 mol% MMA in brush part are segregated at the center of the PMMA domain owing to the preferential interaction of the NPs with PMMA, while the Au NPs that only contain 20 mol % MMA in brush block are located at the interface of the PS and PMMA domains. For Au NPs having pure PS brushes, they were located at the PS domain. Consequently, Yoo et al. demonstrated that the location of the Au NPs within the symmetric diblock copolymer could be controlled by varying the composition of brush part in the ligands.
Jang et al. investigated the segregation behavior of unstable Au NPs and shell-crosslinked Au-Pt NPs in lamellar forming PS-b-P2VP (Mn = 114 kg/mol).26 In case of Au NPs without crosslinked shells, it was observed that the Au NPs were located at the center of the PS domains in the composite samples that were solvent annealed using dichloromethane vapor. However, after thermal annealing of these samples, the Au NPs aggregated into larger particles at the center of the PS domains due to the thermal instability. Although the shell-crosslinked Au-Pt NPs (Au-Pt-S-PI-b-PS) with low grafting density did not significantly aggregate into larger particles after thermal annealing, a significant number of NPs was segregated at the grain boundaries, and some NPs were weakly aggregated in the P2VP domains due to the loss of ligands from the NPs surface, resulting in the binding of P2VP to Au-Pt NPs. The shell-crosslinked Au-Pt NPs with higher grafting density were otherwise stable after thermal annealing, and all NPs were localized at the PS/P2VP interface of PS-b-P2VP. It was suggested that the localization of these NPs at the PS/P2VP interface may be due to the favorable interaction between the P2VP block and the residual Pt catalyst in the crosslinked shell of the PI block.
Controlling Orientation of Block Copolymer Film with Nanoparticles
There have been a few previous studies on the effects of NPs on the orientation of block copolymer microdomains. As a pioneering work, Russell and coworkers demonstrated that NPs can direct the orientation of block copolymer microdomains.12 They introduced the TOPO-coated CdSe NPs to a cylinder forming PS-b-P2VP block copolymer. After the nanocomposite films were thermally annealed, the microdomains of the PS-b-P2VP with TOPO-CdSe NPs were oriented normal to the substrate whereas those of the neat PS-b-P2VP film were oriented parallel to the substrate due to the preferential interaction of P2VP with the substrate and the lower surface energy of the PS. They demonstrated that the CdSe NPs were segregated to the surface of the P2VP cylinders in the films. From this result, they suggested that the TOPO-coated NPs having lower surface energy covered the P2VP domains that had higher surface energy so that the balanced surface interaction induced the perpendicular orientation of the microdomains. In the similar manner, Park et al. controlled the orientation of PS-b-PMMA block copolymer thin films by adding hydrophilic NPs after the thin films were annealed under the solvent vapor.52 PEO-grafted Au NPs were incorporated into cylindrical PS-b-PMMA block copolymers and were cast in thin films onto silicon wafers without a neutralization layer. After solvent annealing with rapid solvent evaporation under high humidity conditions (∼90%), it was observed that the microdomains of PS-b-PMMA have perpendicular orientation to the substrate while in case of under low humidity (∼50%) or slow solvent evaporation, the thin films with Au NPs were oriented parallel to the substrate. In this case, it was suggested that the significant amount of Au NPs within the cylindrical PMMA domain interacts with water vapor at the film surface during solvent annealing process, leading to the perpendicular orientation of PMMA cylinders.
Recently, Yoo et al. developed a novel strategy to control the orientation of block copolymer thin films using thermally stable Au NPs with tuned surface chemistry.51 As described in previous section, the surface property of the Au NPs was tuned by adjusting the composition of MMA/styrene in the polymeric ligands. From the results on the location of NPs in PS-b-PMMA block copolymers, the P(MMA-r-S) brush composed of 80 mol % MMA and 20 mol % styrene was chosen to prepare PMMA-selective Au NPs and the P(MMA-r-S) brush composed of 20 mol % MMA and 80 mol % styrene for neutral Au NPs. The resulting PMMA-selective and neutral Au NPs were added to lamellar forming PS-b-PMMA block copolymers (Mn = 94 and 130 kg/mol). After the thin films were thermally annealed, the microdomains of PS-b-PMMA thin films with PMMA selective Au NPs were oriented parallel to the substrate for all concentrations of Au NPs. In contrast, in case of the PS-b-PMMA thin films containing more than 5 wt % neutral Au NPs were oriented perpendicular to the substrate, as shown in Figure 6. From the vertical location of Au NPs, it can be seen that the neutral Au NPs were mainly located near the bottom surface and some Au NPs were located at the top surface, while the PMMA selective Au NPs were segregated within the PMMA domains (Fig. 6). From the self-consistent mean field theory simulation, it was concluded that the neutral Au NPs located at the PS/PMMA interface can move to the substrate to minimize the entropic penalty of the block copolymer chains and neutralize the substrate, resulting in perpendicularly oriented PS-b-PMMA lamellae.
Morphological Change of Block Copolymer Film Induced by Nanoparticles
Kim et al. reported that NPs can also induce the lyotropic order–order transition in a block copolymer films.53 In this work, they introduced the PS-coated Au NPs coated to a symmetric PS-b-P2VP diblock copolymer. The hybrid samples of Au NP/PS-b-P2VP were annealed under solvent vapor. When the volume fraction of Au NPs was below some critical value, the lamellae lay parallel to the film surface, and the Au NPs were located at the center of PS domains, as expected. However, when the volume fraction of Au NPs was above the critical value, the local concentration of Au NPs varied, and different coexisting morphologies were observed according to the film depth instead of a single homogeneous lamellar phase. Near the air–polymer interface, lamellar morphology was observed with low concentrations of Au NPs in the PS domains. However, near the substrate, layers of hexagonal morphology with high concentration of Au NPs in PS domains were observed, showing the layered order–order transition of block copolymer films.
Bicontinuous structures have attracted significant interest because of their potential applications such as transporting ionic species in fuel cells and batteries, separating charge carriers in photovoltaic films, and so on. Some efforts have been devoted to using NPs in order to fabricate block copolymers that exhibit bicontinuous morphology. First, Kim et al. demonstrated the fabrication of bicontinuous morphology of PS-b-P2VP block copolymer using Au NPs which can be located at the interface of domains as surfactants.54 They synthesized the PS-coated Au NPs with chain areal density of 0.92 chains/nm2. When the resulting Au NPs were added to the symmetric PS-b-P2VP block copolymer, they were localized at the PS-b-P2VP interface after samples were solvent annealed. As the volume fraction of Au NPs was increased, the lamellar period decreased first, and then bicontinuous block copolymer microstructures with dimensions <100 nm were observed above a critical volume fraction of Au NPs. To show the versatility of this strategy for the fabrication of bicontinuous morphology, Jang et al. used thermally stable Au-Pt NPs, as mentioned in the previous section, to prepare the interpenetrating bicontinuous structures PS-b-P2VP block copolymers.55 When Au-Pt NPs were added into the lamellar forming PS-b-P2VP block copolymer, they were strongly segregated at the PS/P2VP interface after thermal annealing at 190 °C for 3 days. As with the case of the solvent annealing process, it was observed that the interfacial activity of these NPs also led to decrease in the lamellar period of PS-b-P2VP block copolymers. As the volume fraction of NPs within the PS-b-P2VP composites was increased, the lamellar morphology was distorted and the bicontinuous morphology was developed. Finally, when the volume fraction of Au-Pt NPs was over 0.28, the lamellar phase disappeared, and the bicontinuous morphology was fully developed (Fig. 7)
APPLICATIONS OF THERMALLY STABLE NANOPARTICLES
Conducting Polymer Films Produced Using Thermally Stable NPs
Nanocomposite materials have attracted a great deal of attention for various applications such as display panels, memory devices, and electronic devices.4–7 For example, continuous conducting polymer films can be used for polymer solar cells and light-emitting diodes (LEDs).56–60 However, due to the low processability, low mechanical strength, and high cost of conducting polymers, it is necessary to blend them with a low-cost polymer matrix in order to overcome such limitations. In this case, it is required to produce a continuous conducting polymer phase in polymer blends by adjusting the concentration of conducting minor domain.61 To reduce the percolation threshold of one component, many research groups have used surfactants such as block copolymers and NPs.61–66 Kang et al. employed thermally stable Au NPs as surfactants to prepare continuous polymer blends composed of conducting polymers, PTPA, and insulating template polymers as PS colloids.67 In this case, thermally stable Au NPs were located at the interface of PS-colloid/PTPA domains and thus they were used as compatibilizers. A low volume fraction of Au NPs (φp = 0.35 vol %) was added to PS-colloid/PTPA blends (95:5 vol/vol) in order to determine the role of thermally stable Au NPs as a compatibilizer in the PS-colloid/PTPA blends. Without Au NPs compatibilizers, the PTPA phase partially wetted the free surface of the PS colloids, leaving the void at the PS-colloid/PTPA interface. However, an incorporation of the thermally stable NPs led to the complete infiltration of PTPA phase at the PS-colloid/PTPA interface, demonstrating that the thermally stable Au NPs act as effective compatibilizers in this blend system. Furthermore, the volume fraction of Au NPs (φp) was varied to optimize the compatibilizing effect of the Au NPs on the PS-colloid/PTPA blend and to optimize electrical properties of the PS-colloid/PTPA blend. Figure 8 shows the morphology of PS-colloid/PTPA blend (95:5 vol/vol) for various φp from 0.35 to 1.5%. The TEM images reveal that the Au NPs are located at the interface in the PS-colloid/PTPA blend and did not agglomerate regardless of φp. The conductivity of the PS-colloid/PTPA blend was also changed by varying the volume fraction of the Au NPs and the PTPA in the blend. Adding a small amount of Au NPs (0.35 vol %) to the 95/5 and 90/10 (vol/vol) PS-colloid/PTPA blends dramatically improved the conductivity of the blends. The enhanced conductivity is due to the compatibilizing effect of the Au NPs, which produces a continuous PTPA phase for electrical transport. Increasing the volume fraction of PTPA from 5 to 10 vol % in the blends enhanced the conductivity by two orders of magnitude.
Free-Standing Multilayer Films
By fabricating the nanocomposites into multilayer films, it has been demonstrated that they can be used as various devices such as nonvolatile memory devices, LEDs, electrochemical sensors, and antireflective films.68–71 In this case, it is important that the thickness and functionality of organic/or inorganic individual layer are well controlled to exhibit the desired property. As a representative example, Lee et al. introduced a novel strategy to prepare nanocomposite multilayer films consisting of polymer-coated quantum dots (QDs) and a polymer matrix.72 Two kinds of photo-crosslinkable polymers, PS-r-PSN3 (28 kg/mol) and PS-r-PSN3-SH (6.5 kg/mol), were synthesized via RAFT polymerization to fabricate the inorganic–organic multilayer films with layer-by-layer (LBL) assembly. Green-, red-, and blue-emitting oleic-acid-stabilized QDs were synthesized, and their surface properties were adjusted with PS-r-PSN3-SH photo-crosslinkable ligands. The free-standing nanocomposite multilayer films were prepared by spin-casting a solution of PS-r-PSN3 and PS-r-PSN3-QD blends onto silicon, quartz, or NaCl substrates. The resulting films were photo-crosslinked under UV irradiation (λ = 254 nm), and the same method was then used to fabricate the next layer on top of the previous layer. The entire process was repeated until the desired multilayer films were produced. Figure 9 shows photographs and photoluminescence (PL) intensities of the multilayer (PS-r-PSN3-QDs:PS-r-PSN3)3 nanocomposite films. The fluorescence from the three different kinds of QDs multilayers were adjusted to blue, green, and red according to the size control of QDs. Furthermore, by adjusting the order and the thickness of each color-emitting layer, white-emitting nanocomposite films were fabricated and they could be directly observed by naked eye. They also fabricated free-standing multilayer films by depositing the nanocomposite multilayers onto sacrificial NaCl substrates. As shown in Figure 10, the red-, green-, and blue-emitting nanocomposite multilayers were easily obtained by dissolving the NaCl substrate in water. This simple strategy provided the basis for producing free-standing, transparent, flexible multilayer films whose thickness, optical properties, and light emission were controllable. In addition, these films were very stable under ambient conditions for longer than a few months and showed highly luminescent properties, due to the protection of the crosslinked shell on the QDs against oxidative damage or moisture-induced PL quenching.
In this article, we reviewed recent developments in the design and fabrication of thermally stable inorganic NPs. Several research groups have applied various strategies to develop thermally stable NPs. The basic idea was the design of crosslinked shells around inorganic NP cores to prevent the dissociation of polymeric ligands from the NPs and well-defined linear brushes tethered from the crosslinked shells to control the surface property of NPs. Required properties of NPs could be accomplished by adjusting the type of monomers, the length, composition, and grafting density of the polymeric ligands. The thermally stable NPs were incorporated into polymer blends as well as block copolymer matrices to successfully control the morphological behaviors of these nanocomposites. We also discussed a few examples of applications such as conducting polymer films and free-standing multilayer films. The results indicate the great possibility of thermally stable NPs for practical applications that require heat treatment process. Furthermore, it can be expected that this approach to design the thermally stable NPs can be extended to other inorganic NPs such as Pt, Ag, CdSe, and magnetic NPs, and various applications of these NPs will be further exploited.
This work was supported by National Research Foundation of Korea Grant funded by the Korean Government (MOEHRD) (2011-0027518), and also by the Human Resources Development Program of KETEP grant (No. 20114010203050) funded by the Korea government Ministry of Knowledge Economy.
Joona Bang received his B.S. degree in Chemical Engineering from Seoul National Univ. in 1999. He received his Ph.D. degree from Univ. of Minnesota in 2004 on block copolymer physics. Then, he worked as a postdoctoral fellow at the UCSB. Since 2006, he has been at Korea University as an associate professor. His research interests include the self-assembly of block copolymer thin films, the living free radical polymerization, and the organic/inorganic nanocomposites and their applications.
Misang Yoo is currently working on her Ph.D in Chemical and Biological Engineering under Prof. Joona Bang at Korea University. She received her B.S degree and M.S in chemical and biological engineering from Korea University in 2008 and 2010 respectively. Her research focuses on the fabrication of thermally stable core-shell gold nanoparticles using crosslinkable block copolymer and control the orientation of block copolymer thin films using the surface chemistry tuned gold nanoparticles.
Seyong Kim is currently pursuing his Ph.D. in Chemical and Biological Engineering under the guidance of Dr. Joona Bang at Korea University. He received his B.S. in Chemical and Biological Engineering from Korea University. His research focuses on the controlling the location and dispersity of gold nanoparticles in block copolymer or homopolymer matrix by tuning the surface property of gold nanoparticles with various photo-crosslinkable polymeric ligands.