Article pubs.acs.org/Langmuir

Interfacial and Phase Transfer Behaviors of Polymer Brush Grafted Amphiphilic Nanoparticles: A Computer Simulation Study Jiaqi Dong,† Jiaying Li,‡ and Jian Zhou†,* †

School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, P.R. China School of Science, South China University of Technology, Guangzhou 510640, P.R. China

‡

ABSTRACT: Nanoparticles’ phase transfer behaviors at the oil− water interface have many respects in common with lipid bilayer crossing behavior and the Pickering emulsion formation. Hence, the interfacial behavior and phase transfer behavior are intuitive indicators for the application potential of nanoparticle materials, e.g., on the emulsion formation and biomedical applications. Polymer brush modification enables nanoparticles to behave differently in hydrophilic solvent, hydrophobic solvent, and their interface region. In the present work, phase transfer behaviors of triblock polymer brush modified gold nanoparticles are explored by using coarse-grained simulations. The nanoparticles grafted with hydrophobic/weak hydrophilic/hydrophobic triblock brushes are found to have the best phase transfer performance, and the enhanced flexibility and mobility of head blocks are found to be the most vital factors. The inherent mechanism of interfacial behavior and phase transfer process are investigated and explained as perturbation effect and traction effect. According to our results, middle blocks dominate the brush morphology and decide whether NPs can be transferred into another phase. However, the inner blocks show higher dominance for the phase transfer behavior of nanoparticles restricted in the interface region, while the outer ones shows higher dominance for the nanoparticles departing from the interface region. Otherwise, interesting flatJanus morphologies are found. Special applications in two-phase interface including emulsion stabilization could be expected. This work could provide some guidance for the molecular design and applications of polymer−nanoparticle composite materials.

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INTRODUCTION Polymer brush grafted spherical inorganic nanoparticles (NPs)1−4 are frontier research fields in macromolecule science and nanomaterial technology. Combining the stability from inorganic NPs themselves with the versatility of polymer brushes, unique morphology forms and special features are added to this kind of composite. Mixed or blocked modification of different polymer brushes with diverse characteristics on the surface of NPs provide the possibility of integrating a variety of intelligent responsiveness on NP. The application of this series of composite materials have received keen anticipation in areas such as detection, imaging, sensing, drug controlled release, microfluidics, biocompatibility intelligent medical materials, smart materials and other fields.5−8 Today, more and more attentions have been paid to human health, thus the research of biomedical applications9,10 is of particular importance. By grafting polymer brushes with different chemical properties, grafting density, and chain length, we can achieve different functional modification for the polymer brush-inorganic NPs composite system. On the one hand, for the biomedical applications of polymer brush modified NPs, it is vital to enhance the biocompatibility of the composite materials in the biological environment. Under above consideration, current modifications are mostly based on the hydrophilic polymer such as poly(ethylene glycol) (PEG), a classic hydrophilic polymer. In addition, the © 2014 American Chemical Society

hydrophilic polymer modified composite materials also have an advantage on the dispersion of NPs.11 In this field, PEG has also been fully demonstrated for its good biological compatibility.9,12 However, if we use such polymer brush− NPs composite materials for disease diagnosis and treatment, biocompatibility in vivo is only one of our concerns. Another critical problem is that how NPs can successfully cross the cell membrane. On the other hand, some NPs are concentrated in the interface area. Although they are unable to cross the phase interface, their stability in the interface area could be expected to have other interesting potential applications, e.g., the formation of Pickering emulsions.13 For cell membrane passage, the inner side of lipid bilayer is hydrophobic, while the solvent of bioenvironment is mainly hydrophilic. Also, for nuclear passage, the hydrophobic proteins are the key component of nuclear pores.14 Obviously, reasonable modification of hydrophobic NPs’ can effectively improve the efficiency of membrane crossing and nuclear pores crossing of NPs.15 The phase transfer behaviors at the oil− water interface have many respects in common with the lipid bilayer membrane crossing behavior of NPs. Several previous studies16−18 were focused on this phenomenon. It has been Received: February 13, 2014 Revised: April 19, 2014 Published: April 23, 2014 5599

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isopropylacrylamide) (PNIPAm). When the NPs are heated to above 35 °C, the NPs originally in the aqueous phase can pass through the interface region into the oil phase (chloroform), and when they are cooled to below 35 °C, they can return to the water phase. No matter what kind of polymer brush grafted NPs, the controllability of interfacial behavior and transmembrane behavior is needed. Besides, factors like blocks with different polarity and block length are very important for the design of NPs for specific application. Therefore, microscopic understanding of the phase transfer mechanism and the structure− property relationship of polymer brush modified NPs is particularly important. With the progress of computer technology, the applications of computer simulations are increasingly widespread in the fields of chemistry, material science, biology, and engineering. In the aspects of cost and time, as well as the investigation of microscopic mechanism, computer simulation has incomparable advantages over traditional experimental characterization. For the microscopic study of interactions between inorganic NPs and polymers, the traditional experimental operation is complicated and costly. Computer simulation will provide researchers microscopic explanation for the macroscopic properties of materials. So computer simulation studies are currently playing more and more important roles. Hall and co-workers contributed a series of valuable works on the computer simulation of polymer30 and polymer/nanoparticle composites.31 Considering that the most common inorganic NP systems have particle sizes between 2 and 50 nm, efforts have been made to study such systems with molecular dynamics (MD) in all-atom level efficiently with limited computing resource.32 However, the polymer modified NP systems could be even larger and more complex, all-atom molecular dynamics (AAMD) simulations can be very costly for such a big system. Therefore, though with higher accuracy, AAMD simulations are only performed fairly small systems.33 Hence, coarse-grained (CG) simulations are more suitable for complex systems. Coarse-grained molecular dynamics (CGMD) was applied to study the polyelectrolyte polymer brush aggregation behavior by Sandberg et al.34 Among various CG approaches, the most important two methods are dissipative particle dynamics (DPD) and MARTINI force field based CG method. DPD is a mesoscopic simulation method proposed and developed by Hoogerbrugge et al.,35 based on MD and Lattice Gas Automata theory development, which has been widely adopted on the investigation of polymer materials, even smart responsive polymers.36 Utilizing this method, Juan37 et al. investigated agglomeration behavior of thiol-terminated oligohydroquinonyl ether (TTOE) wrapped in AuNPs in solution. Chen et al.38 used DPD method to simulate the double-shell morphology formation of AuNPs caused by the self-assembly and stabilizing effect of poly(ethylene oxide)-poly(phenylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) block copolymers. In their results,38 hydrophobic phenylene oxide (PO) fragments were adsorbed on the surface of AuNPs while the hydrophilic ethylene oxide (EO) segments were exposed to water environment, which successfully reproduced the experimental aggregation morphology and the size of AuNPs. Tsao and coworkers39,40 studied various large scale nanoparticle−polymer systems by DPD simulations. They investigated polystyrene chains loaded silica NPs with unique shapes to explore the effect of NP shape parameters and graft polymers’ various properties on the behaviors of the composite system.44

confirmed that NPs at the oil−water interface tend to accumulate in the interface region rather than completely accomplish the movement across the interface, due to the energy differences between NPs and phase interface. Wang et al.18 studied the poly(methyl methacrylate)-b-poly(glycidyl methacrylate)-b-poly(tert-butyl methacrylate) (PMMA-bPGMA-b-PtBMA) modified silica NPs and found that NPs would form the Janus hydrophobic/hydrophilic morphology at the oil−water interface. The amphiphilicity could be adjusted through changing the relative length of polymer blocks. However, the NPs always gathered at the interface region, and they did not cross through the interface and transfer to another phase. Requirements of biocompatibility (mainly related to hydrophilicity) and membrane crossing capability (mainly related to hydrophobicity) are both essential to the biomedical NPs. In this sense, modification of amphiphilic block polymer brush is a reasonable solution. Several articles and reviews19−21 demonstrated the possibilities of achieving phase transfer movement by regulating the solvent composition. However, whether adding external trigger substances or changing environmental conditions, exterior changes must be taken to achieve phase transfer behavior, which could be more or less difficult in the organism environment. Thus, how to completely achieve the phase transfer behavior through the conformation change of polymer brushes themselves is an urgent issue.22 Niikura23 and co-workers found that, by preparing alkyl (mercaptan) chains which capped PEG on both ends (i.e., an alkyl chain-b-PEG-balkyl chain) of triblock polymer brush, small and flexible alkyl and PEG ligands can provide an amphiphilic feature to the surface of NPs, thereby allowing NPs to cross from the water phase to an organic phase.23 The NP can be automatically dragged from the aqueous phase into the oil phase. Amphiphilic NPs concentrated in the interface region could have different type of brush conformation, including the Janustype (which have distinctly different regions on two hemispheres) and irregular-type. Amphiphilic NPs with Janus-type conformation have interfacial behaviors in common with the originally Janus-modified NPs. They could adapt well to water phase with their hydrophilic hemisphere, and adapt well to oil phase with their hydrophobic hemisphere. It has been confirmed that Janus NPs have the desorption energy about three times higher than corresponding homogeneous nanoparticle.24 Such property could be expected not only in the stabilization of emulsions by Janus NPs,25 but also in the inverse production of Janus NPs by Pickering emulsions.26 The structure-properties relationship between Janus NPs and Pickering emulsions provide attractive prospect in various application field, e.g., intelligent surface and phase-selective catalysis. With surface modification on inorganic NPs by polymer brushes with the environment responsive characteristics, the switchable surface chemistry and phase transfer can be achieved by adjusting the solution pH,27 salt concentration,24,25 or temperature.28 Li et al.29 studied and confirmed that PNIPAm (poly(N-isopropylacrylamide)) grafted AuNPs can achieve good dispersion in water, tetrahydrofuran, alcohol, and chloroform, which offer them potentials as biomedical nanocarrier materials. In the water−toluene two-phase system, the NP composites tend to gather in the interface area as many other researches proved. However, in the water-chloroform system, the composite material will form some conspicuous features due to the unique thermoresponsiveness of poly(N5600

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MARTINI force field was developed for CG studies of biomolecules by Marrink and co-workers.41 Its initial intention was to describe the movement and behavior of biofilm systems, and then more and more attentions were paid by other researchers. The applications of this force field soon expanded to the research field of polymers. Lee et al.42 used the MARTINI force field to establish the CG PEO and PEG chain models and studied the hydrodynamics and conformation changes after being grafted on the surface. Rossi et al.43 used two kinds of mapping methods to describe PS chain and studied its properties in solvents. Recently, they44 also used the similar method to establish the CG model of polyelectrolyte brushes; they successfully reproduced the experimental glass transition temperature, which truly revealed the effect of crosslink by curing agent for the glass transition temperature and the temperature responsiveness of the plastic properties. These studies well proved that MARTINI force field can be well applied to polymers. For polymer brush-AuNP systems, the researches using MARTINI force field are focusing on alkyl thiol-modified AuNP systems. Chan et al.45 simulated the membrane and capsule formation of alkyl sulfhydryl modified AuNPs. Lin et al.46 worked on the interaction between membrane and short alkyl sulfhydryl modified AuNP and its membrane penetration. In addition to these two major CG approaches, Brownian Dynamics (BD) and Monte Carlo (MC) are also performed. Jayaraman and co-workers47−49 accomplished a series of studies on polymer grafted NPs, discussing the effect of blockiness, chain length, chemical and physical heterogeneity using MC method. Hooper et al.50 used BD to simulate the supramolecular self-assembly of PEO modified C60 in aqueous solutions. Almusallam and Sholl51 adopted BD to investigate the copolymer (random/block) protected metal NPs’ behaviors at the oil−water interface and the effects of block sequence and interaction strength. However, to the best of our knowledge, for polymer brush grated NP systems, rare research has been focused on their microscopic relationship between phase transfer behavior and structure by molecular modeling. A series of systematic research based on proper CG models are needed to set up. In this work, we will construct the CG model for amphiphilic triblock polymer grafted AuNPs as a typical system and systematically study the phase transfer behavior at the water−oil interface of AuNPs modified with amphiphilic brushes composed of different block components and block sequences. The microscopic mechanism of phase transfer behavior and structure− property relationships will be discussed from the molecular level to make effective guidance for the design and development of these modified NPs with broad application prospects.

Figure 1. Coarse-graining scheme for AuNPs.

field type for Au beads (AU) was SC4 in MARTINI V2.0 force field.41 The bond lengths between neighboring AU beads were set to 0.408 nm, the same as lattice constant of gold atom lattice, in order to keep the particle internally rigid.45 Grafted polymer brushes were linked together with AU beads on the surface of NP, and brushes are distributed evenly on the surface. It should be mentioned that the shape (e.g., sphere, pyramid, rice, cone, rod, or cube) and surface charge condition also play an important role under external interactions. Sureshkumar and co-workers53 found that such factors have vital influences in the nanoparticle−lipid layer interactions due to different electrostatically repelling effect and contact area. However, for fairly small gold nanoparticles’ behavior in free solvents, especially when the nanoparticles are covered with a dense layer of ligands45 or even longer polymer brushes, the shape effect and charge condition of the gold core could be nonsignificant factors for the behavior of the whole nanoparticle−polymer brush composite material. For hydrophobic and hydrophilic polymer brush blocks, the CG scheme was based on the train-like chain single-bead model, which indicating that each monomer was described by only one CG bead. The hydrophobic chains were all made up of C1 beads (indicating the basic force field type in standard MARTINI v2.0 force field,41 similarly hereafter), a kind of beads with strong hydrophobicity similar to alkyl chains. For the hydrophilic chains, two kinds of beads were discussed, P2 beads with quite strong hydrophilicity (similar to PVA) and N0 beads with weaker hydrophilicity (similar to PEO). Water and oil solvents with different polarities were represented by beads with different force field types: water by polar P4 beads, while oil by apolar C3 beads as explained in MARTINI force field.41 Equilibrium bond length and force constant were adopted from the standard MARTINI V2.0 force field.41 AuNP Systems. Many factors can cause differences in physical or chemical properties of brushes, such as grafting density, composition, chain-length, asymmetry and solvent quality.54,55 The most straightforward influence factors on the properties of brush modified AuNP are obviously the monomer sequence and the ratio of blocks, which are also main factors considered in this work. We will discuss four different triblock sequences made up by hydrophilic polymer N0 or P2 beads and hydrophobic polymer C1 beads: C1−N0−C1 (from inner to outer, similarly hereafter), C1−P2−C1, N0−C1−N0, and P2−C1−P2. For each sequence, five different compositions are discussed: 6:18:6, 6:6:18, 10:10:10, 12:6:12, and 18:6:6. The triblock polymer brush modified AuNP systems studied in this work are listed in Table 1. Experimentally, the diameters of the gold core were mostly between 1.5−6 nm5; while the grafting densities of PEO polymer brushes and alkanethiols5 were around 0.3−4.8 chains·nm−2, in which higher grafting densities

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METHODS Coarse-Grained Model. The CG MARTINI force field developed by Marrink and co-workers41 was used in this work. For the construction of AuNP model, the cuboctahedral structure was adopted as the same as used in our previous work.52 It had been proved that such a cuboctahedral structure can successfully reflect the AuNP behaviors,45 especially when there exists a relatively thick and dense polymer-brush modification layer on the surface of the NPs. Therefore, adopting this cuboctahedral structure is feasible in this work. A cuboctahedral AuNP model consists of 147 Au beads with a diameter of about 2.5 nm was established (Figure 1). The Au beads were arranged into the face centered cubic (fcc) lattice and coarse-grained with 4:1 as Juan et al.37 described. The force 5601

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the van der Waals interaction is 1.2 nm. After equilibration in water, we change the solvent from pure water to water−oil twophase mixed solvent and use the same parameters with a time step of 20 fs to perform sampling stage for 400 ns. In this stage, the central bead of AuNP is restrained in the center of the XOY plane, while along the Z axis, the particle can move freely.

Table 1. Twenty Systems Studied in This Work head/tail blocks

C1

C1

N0

P2

middle blocks

N0

P2

C1

C1

block compositions

C16-N06C118 C16-N018C16 C110-N010C110 C112-N06C112 C118-N06C16

C16-P26C118 C16-P218C16 C110-P210C110 C112-P26C112 C118-P26C16

N06-C16N018 N06-C118N06 N010-C110N010 N012-C16N012 N018-C16N06

P26-C16P218 P26-C118P26 P210-C110P210 P212-C16P212 P218-C16P26

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RESULTS AND DISCUSSION Analyses including the phase transfer behavior, position of NP center, morphology, and the distribution of brush blocks are carried out. Phase transfer behavior is compared with the experimental results and described by the position of NP center. Also, describing the morphology and microstructure via the block distribution is an important indicator for discussing the mechanism of phase transfer and the different structure properties of studied systems. In this section, first, we directly describe the phase transfer behavior and analyze the center position of different particles in water−oil two-phase system. Second, the description of the morphology is introduced and the brush distributions along Z axis are investigated by density profiles. It should be mentioned that though the size of nanoparticles studied in the present work is fairly small and the size of experimental studied gold nanoparticles23 is about 20 nm in diameter, phase transfer phenomenon in the experimental study of alkyl chain capped brushes coated AuNPs is successfully reproduced in the present study with a CG model. In other words, the size effect of nanoparticle does not show significant influence on the structure and phenomenon in present diameter range according to our results, which had been also proved by another simulation research.45 Phase Transfer Behavior and Gyration Radius Analysis. In the work of Niikura and co-workers,23 triblock polymer brushes with the composition of alkyl:PEG:alkyl 10:13:8 were modified on AuNPs and such hairy NPs successfully break the interfacial energy barrier and transferred into the oil (chloroform). Bent-straight conformational change form of modified ligands are supported by proton nuclear magnetic resonance analysis. In terms of phenomenon, the similar transfer process is well reproduced in the present work. The system C110-N010C110 brush grafted AuNP in P4/C3 two-phase environment has a fairly similar block composition (around 1:1:1), block

require specific preparation method. With comprehensively considering experimental data and simulation efficiency, the diameter of AuNP in our work was set as 2.5 nm and the grafting density was about 1.63 chains·nm−2. Both the grafting density and diameter of Au core are reasonable according to the experimental range. In other words, on each AuNP, 32 brushes were modified. For each brush, the number of monomers was uniformly set as 30. Simulation Details. In present work, we investigated the interfacial behavior of polymer brush modified AuNPs in water−oil two-phase system. All CGMD simulations were performed by using the GROMACS 4.5.3 package.56 It is worth noting that because of smoothed energy barrier in MARTINI force field, the effective time that each system goes through is four times longer than the simulation sampling time.57 Hereafter in this paper, the effective time will be adopted. First, energy minimization using steepest descent method was performed for 1250 steps on the NP model to get a preliminary optimization. Optimized NPs were put in the middle of box (18 × 18 × 36 nm3) with ∼95 000 solvent beads surrounding them. We optimized the system with 2500 steps of steepest descent energy minimization again. Then, 40 ns isobaric−isothermal ensemble (NPT)-MD equilibrium was performed with a time step of 20 fs to make sure that the NP was in equilibrium with water environment. During this stage, the central bead of AuNP was always restrained in the center of the simulation box. Temperature is regulated at 300 K and pressure is controlled to be 1.0 bar by the Berendsen method.58 The cut-off distance of

Figure 2. Phase transfer process of system C110-N010-C110 (1. Equilibrium state in water: brushes bent; 2. Contact with interface: brushes start to stretch; 3. Dragged into oil phase: more brushes stretched; 4. Phase-transfer finished: brushes completely stretched. In stage 1 and 4, the gyration radius Rg of each block is marked in the graph, which will be further discussed in Figure 3. Yellow: hydrophobic C1 beads; blue: weak hydrophilic N0 beads; gold: Au beads.) 5602

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polarity, and solvent quality with the experimental system mentioned above. However, microscopic transfer process and inherent microstructure-phenomenon relationship still need to be investigated by computer simulations. As shown in Figure 2, the similar result as experimental study23 can prove the validity of our models and methods. First, the hairy NPs achieve equilibrium in pure water (P4) environment and all the brushes are bent on the surface (Figure 2-1). A “buckle” conformation is formed due to the exposure of middle N0 blocks and the collapse of head and tail C1 blocks, which is similar to the “PEG exposed form” in Niikura and co-workers’ study.23 Second, when some of the C1 head contact with oil phase, they soon freely extend out and become straight (Figure 2-2). The enhanced flexibility of the brushes offers the NP more mobility and further make more brushes contacted with the oil phase, as the analysis of gyration radius shown in Figure 3. Almost all of the brushes in each system have an increment on gyration radius after contacting the oil phase, even though the oil phase is a poor solvent for hydrophilic blocks (N0 and P2). It can be interpreted as two main reasons according to the observation on the equilibrium conformations and simulation trajectory: (1) perturbation effect: it indicates that the active extension and moving of any brushes can fluctuate and activate other brushes around it; (2) traction effect: dragging relatively good extended polymer blocks could enhance the mobility of linked blocks. From Figure 3, we can also find that the blocks farther away from the particle center have more significant enhancement on brush flexibility and mobility, which indicates the mobility of outer blocks have significant contribution on the phase transfer process. In this way, the NP is gradually dragged into the oil phase with more brushes stretched out (Figure 2-3). Lastly, the phase transfer finishes when all brushes completely stretch and the whole particle accomplishes its moving into the oil phase (Figure 2-4). With this comparison, the validity of our simulation study can be proved. Equilibrium Positions of Modified NPs. Distribution of the center of AuNPs along the Z axis is calculated by density profiles. The water−oil interface is set as the zero position in the Z direction. The positions where the most AuNPs center concentrated are assumed as the equilibrium positions (see Table 2) in the following discussion. Negative values represent the water phase while the positive ones are for the oil phase. To show results as clearly as possible, we defined the range of Z coordinate from −2 to 2 nm as the interface region. Considering about the size of NPs themselves, the equilibrium position of NPs that completely breaks away from interface region should be higher than 6 nm. In this way, only systems with brushes made up by C1 and N0 blocks could break through the interface region and be transferred into the oil phase. For systems containing P2 blocks, none can accomplish the phase transfer. Even if the inner block content of the C1− P2−C1 brush is over 60%, the C1−P2−C1 brush modified AuNP can only accomplish the phase transfer partially, which is still trapped in the interface region. Comparing two groups of systems made up of hydrophobic C1 and hydrophilic N0 chains with each other, we find that when C1 chains act as the middle block, the NP can hardly across the interface completely, while if the content of C1 chains is over 60%, the phase transfer could be expected. However, when C1 chains act as the head or tail blocks of the NP, they can always break the interfacial energy barrier easily and be completely drawn into the oil phase, except for the

Figure 3. Gyration radius of blocks in different systems.

Table 2. Equilibrium Positions of AuNP Center in 20 Systems block sequences

C1−N0− C1

block compositions 6:6:18 12:6:12 10:10:10 6:18:6 18:6:6

N0−C1− N0

C1−P2− C1

P2−C1− P2

equilibrium position (Z coordinate) 10.70 8.90 6.92 2.97 9.26

4.04 3.86 4.76 5.31 2.07

1.53 2.07 1.89 1.17 2.97

0.27 −0.63 −0.27 0.45 −0.45

situation that the content of middle N0 blocks in the polymer brush reaches 60% (system C16-N018-C16) since the strong hydrophilicity of middle blocks make the NP unable to break the interfacial energy barriers to cross the interface. 5603

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Morphologies and Microstructures. 1. C1−N0−C1 Systems. These model systems correspond to the alkyl-PEGalkyl brush modified AuNPs in the experimental study. Typical core−shell NPs with hydrophobic shells on oil−water interface are found in all five systems. As the ratio of blocks changes, both morphologies of NPs and equilibrium positions in twophase systems are quite different. Comparing the system C16-N018-C16 (Figure 4-2) with system C110-N010-C110 (Figure 4-3) and system C112-N06-C112 (Figure 4-4), we can see that when the content of middle blocks (N0) reach 60%, middle N0 block densely distributed at the oil−water interface. The affinity between N0 blocks and

All in all, NPs with phase transfer ability need over 40% C1 block content in the brush and the C1 block acts as the head and tail block rather than the middle one. The higher the content of C1 is, the better the phase transfer can be achieved. In other words, with more C1 blocks in polymer brushes, the NP can stay farther from the interface and is more stabilized in the oil phase. For systems with C1 blocks as middle block and N0 blocks as head/tail block, phase transfer can be expected only when the content of C1 blocks is very high. Our conclusion is well consistent with Niikura and co-workers’ experiments.23 Such phenomenon could be explained as the activity of the high mobility and flexibility of alkyl chain which contributes a lot in breaking the interfacial energy barrier. As mentioned above, only the systems with C1 blocks as head/tail block have potential phase transfer ability. However, for such systems, different roles of three blocks still need to be discussed. The C16-N018-C16 system cannot accomplish perfect phase transfer as other C1−N0−C1 systems do (these perfectly transferred nanoparticles are named “transferred NPs” in the following discussion, while the others are named “restricted NPs”). From this phenomenon, we can conclude that the middle blocks take a more dominant role in deciding whether the particles can accomplish the phase transfer. More specifically, the inner and outer C1 blocks also play different roles in the phase transfer and interfacial behaviors. In Table 2, we can see that for the “restricted” C1−P2−C1 NPs, when the composition of inner and outer C1 blocks are enhanced together from 10:10:10 to 12:6:12, the equilibrium position is only slightly shifted toward the oil phase (from 1.89 to 2.07 nm); when only the inner block’s composition is increased and the outer block’s is decreased (from 10:10:10 to 18:6:6), the equilibrium position is significantly shifted (from 1.89 to 2.97 nm); when the composition is changed to 6:6:18, the equilibrium position is even more restricted to the interface (from 1.89 to 1.53 nm). In this way, it is clear that though the outer blocks are more important for the nanoparticles’ mobility, for the “restricted” NPs, the inner blocks play a much more dominant role in its equilibrium position and interfacial behaviors. However, for the C1−N0−C1 systems that mostly remain stable in oil phase perfectly, when the composition of inner and outer C1 blocks are changed from 10:10:10 to 12:6:12 or 18:6:6, the equilibrium position increment is quite close (increased from 6.92 to 8.90 and 9.26 nm). When the composition of outer block is increased to 6:6:18, the equilibrium position becomes extremely high (10.70 nm). In this way, we can find that for the “transferred” nanoparticles, the length of outer blocks provide greater influence on the phase transfer behavior and its stability. That is to say, the enhanced mobility of hydrophobic blocks is the most important underlying driving force on the phase transfer behavior, especially the enhanced mobility of outer blocks contribute a lot for the successful departure from the interface region. However, as the linker on the particle surface, the relatively better extended inner blocks also have important effects on this process, especially before the nanoparticle completely leave the interface region. We can conclude from above findings that the length of outer blocks are more important for the “transferred” NPs when they are departing from the interface; however, the inner blocks are more important for the “restricted” NPs when they are being stabilized in the interface region.

Figure 4. Brush density profiles and equilibrium conformations of C1−N0−C1 systems with different ratios. [(a) density profile; (b) equilibrium conformation in water system; (c) equilibrium conformation in water−oil two phase system. The number “1” to “5” in front of letters represents different ratio of block length: 6:6:18, 6:18:6, 10:10:10, 12:6:12, and 18:6:6, respectively.] 5604

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water phase would trap the NP near the interface region, though the entire NP is in the oil phase. Comparing system C118-N06-C16 (Figure 4-5) and system C16-N06-C118 (Figure 4-1), the different sequences of three same blocks bring some small but important differences on the particle position and conformation. With the longer C1 blocks in the inner side, these blocks show significant dominance on the particle position due to the traction effect. In Figure 4-5a, the distribution of the central AU bead is much more concentrated than that in Figure 4-1a. This could be accounted for the weaker mobility-improving capacity of inner blocks comparing with outer blocks. As the results of five systems show, when the content of middle blocks (N0) was less than 1/3, the mobility and traction effect of hydrophobic tail is strong enough so that the NP could always successfully cross the interface and become stabilized in the oil phase regardless of the block sequence. In drug delivery systems or other advanced nanomaterials, this phase transfer behavior has a number of potential applications. 2. N0−C1−N0 Systems. This group of model system corresponds to the PEG:alkyl:PEG brush modified AuNP system. We can see that whatever the composition of three blocks is, the NPs can always stand on the oil-side because of the strong hydrophobicity of C1 blocks. However, they cannot leave the interface region due to the hydrophilic tail being trapped by water phase. As shown in Figure 5-2 and -3, when the content of middle block reaches 33% and even 60%, the NP tends to be separated from the interface region except some of the outer C1 blocks. The positions of the middle blocks are also at the equilibrium around 5 nm. It could be expected that when the content of hydrophobic blocks become even higher, the NP can completely cross the interface and stably stay in oil phase. Comparing the brush distribution and particle center position of system N018-C16-N06 (Figure 5-5) with those of system N06-C16-N018 (Figure 5-1), the same conclusion as we discussed above is also found. With the same brush composition, the inner blocks show much higher dominance on the interfacial behavior than the outer blocks do. It should be mentioned that such NPs with hydrophilic tails may have better biocompatibility. From this perspective, these series of systems could also be expected to have good performance in biomaterials and cellular drug delivery systems with proper regulation on the composition of the middle block. 3). C1−P2−C1 and P2−C1−P2 Systems. As the hydrophilicity of middle block is increased to P2 (e.g., the poly(vinyl alcohol)), the morphology of brushes in the interface region can be quite different. The most common phenomenon is that due to the strong traction effect by C1 blocks, P2 blocks can hardly extend to water phase freely. Instead, they have to keep themselves in polar solvent phase as much as possible along the interface and radially spread around from the center axis of NPs (Figures 6 and 7). In this way, AuNPs tend to stay inside the interface region instead of transferring into any phase. However, in P2−C1−P2 systems (Figure 7), the C1 blocks’ distribution on Z axis is also mainly restricted, for example within about 3 nm for P210-C110-P210 and P218-C16-P26 systems, both of which have quite short outer blocks. Although these groups of systems do not show their ability on phase transfer, their flat and discal morphology with restricted vertical dimensions on the oil−water interface are special and worthy of attention. Such “flat Janus” NPs could be expected for

Figure 5. Brush density profile Equilibrium conformation of N0−C1− N0 systems. [(a) density profile; (b) equilibrium conformation in water system; (c) equilibrium conformation in water−oil two phase system. The number “1” to “5” in front of letters represents different ratio of block length: 6:6:18, 6:18:6, 10:10:10, 12:6:12, and 18:6:6, respectively.]

interesting applications including but not limited to smart surface, films and emulsions formation.18,59,60 Conclusions can be drawn that for the “restricted” NPs, the middle block is also of great influence on deciding their brush morphologies. When hydrophobic C1 blocks act as the middle block, strong hydrophilic P2 blocks are closely restricted in the interface region. Indeed, this hauling effect is mutual. The distribution ranges of C1 blocks are also apparently fairly narrow due to the restriction effect of strong hydrophilic blocks, especially when C1 blocks are restricted on both ends in P2− C1−P2 systems. 5605

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Figure 6. Brush density profile equilibrium conformation of C1−P2− C1 systems. [(a) density profile; (b) equilibrium conformation in water system; (c) equilibrium conformation in water−oil two phase system. The number “1” to “5” in front of letters represents different ratio of block length: 6:6:18, 6:18:6, 10:10:10, 12:6:12, and 18:6:6, respectively.]

Figure 7. Brush density profile Equilibrium conformation of P2−C1− P2 systems. [(a) density profile; (b) equilibrium conformation in water system; (c) equilibrium conformation in water−oil two phase system. The number “1” to “5” in front of letters represents different ratio of block length: 6:6:18, 6:18:6, 10:10:10, 12:6:12, and 18:6:6, respectively.]

CONCLUSIONS Effects of surface chemistry, particle size, brush distribution on the interfacial behaviors, and phase transfer of triblock polymer brush modified amphiphilic AuNPs in oil−water two-phase systems were investigated by CGMD simulations. Block polymer grafted AuNP systems mostly form typical core− shell structure, while special block sequence and different polymer hydrophilicity will offer us fairly abundant morphologies and phase behaviors. Furthermore, the inherent structure−property relationship is discussed from the microscopic view. Most of AuNP modified by brushes made up with hydrophobic (C1) and weak hydrophilic (N0) blocks can break the interface region and draw the particle into the oil phase. However, with hydrophobic blocks as head and tail ends, in

most cases, the particle can wholly move into the oil phase (except that the content of middle block reaches 60%), while with hydrophobic blocks as middle block, the particle would be trapped by the vicinity of interface region and form a partial phase transfer (if the content of block is increased to be more than 60%, the situation might be changed). For NP systems containing the fairly strong hydrophilic blocks (P2), both of the hydrophobic and strong hydrophilic blocks can hardly extend to their good solvents freely in the interface region due to the mutual hauling effect, which is particularly evident for strong hydrophilic blocks. They have to keep themselves as much as possible in good solvents and mostly centrally distributed in the good solvent side at the interface region and radially spread around from the center axis

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of NPs. Hence, interesting “flat Janus” morphologies are formed and might have extensive application prospects on surface regulating and emulsions stabilization, though they do not show any phase transfer behavior. However, some parts of brushes that are distant from interface region, could not interact with similar blocks in interface region, so they have to behave quite independently. The different roles of three blocks are also discussed and concluded as the following rules. Block length and sequence have significant effects on the interfacial behaviors. The content of middle blocks is dominant on deciding whether a nanoparticle can accomplish perfect phase transfer, in other words, deciding a particle to be a “restricted” one or a “transferred” one. For all systems, especially the “restricted” and even “flat-Janus” ones, the middle blocks show great dominance on the brushes morphology. However, in the matter of waterto-oil phase transfer behavior of particles with hydrophobic head/tail, the length of inner and outer blocks show higher dominance than the middle blocks with the same content. The inner blocks play a more important role for “restricted” ones while the outer blocks are more important for the “transferred” ones. We can expect that, by regulating and adjusting various factors, rich phase transfer and interfacial behaviors of NP products can be observed more easily. Our simulation study could offer some useful information and guidance for the development and applications of this series of nanomaterials.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by National Key Basic Research Program of China (No. 2013CB733500, National Natural Science Foundation of China (Nos. 21376089, 91334202) and the Fundamental Research Funds for the Central Universities (SCUT-2013ZM0073). The computational resources for this project are provided by SCUTGrid at South China University of Technology.

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