Cylinder Nanoparticles

Article pubs.acs.org/Macromolecules

Multigeometry Nanoparticles: Hybrid Vesicle/Cylinder Nanoparticles Constructed with Block Copolymer Solution Assembly and Kinetic Control Yingchao Chen,† Ke Zhang,‡,∥ Xiaojun Wang,§ Fuwu Zhang,‡ Jiahua Zhu,† Jimmy W. Mays,§ Karen L. Wooley,‡ and Darrin J. Pochan*,† †

Department of Material Science and Engineering, University of Delaware, Newark, Delaware 19716, United States Department of Chemistry, Chemical Engineering, and Material Science & Engineering, Texas A&M University, College Station, Texas 77842, United States § Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, United States ∥ Department of Chemistry and Chemical Biology, Northeastern University, Boston, Massachusetts 02115, United States ‡

S Supporting Information *

ABSTRACT: We report an experimental study of the assembly of various multicompartment and multigeometry nanoparticles constructed from mixtures of two diblock copolymers. By simply blending the amphiphilic block copolymers poly(acrylic acid)block-polyisoprene (PAA-b-PI) and poly(acrylic acid)-block-polystyrene (PAA-b-PS), we constructed complex nanostructures including multigeometry vesicle−cylinder particles, multicompartment vesicles, or multigeometry cylinder−disk nanoparticles via kinetic control of solution assembly. In our assembly strategy, the PAA common domain in both block copolymers was first complexed with organic diamine molecules in organic solution to form a particle core. Therefore, the two different hydrophobic blocks from the two different polymers were trapped into the same particle. After addition of water, the particles form the structure having hydrophobic cores and PAA-amine hydrophilic shells; the two hydrophobic domains locally nanophase-separated in the core to form the multicompartment and multigeometry nanoparticles. Importantly, detailed control of the solvent mixing rates helps dictate the final hybrid nanostructures formed at higher water content. TEM and cryoTEM with selective staining were performed to characterize the hybrid nanoparticles. use of the multiple internal/external compartments.8 For example, MCN and MGN are being studied as multifunctional therapeutic delivery vehicles for incompatible drug payloads,9 substrates for inorganic materials, and possible optical and electronic nanomaterial.10 Hybrid, extraordinary geometries can be created in one nano-object by different assembly methods.11 For instance, the Lodge and Hillmyer groups have pioneered work on multicompartment nanostructure assembly,12 built up from star-like triblock copolymers having two immiscible hydrophobic domains. Yan’s group has studied multicompartment micelles assembled from hyperbranched star-like polymer with polycationic and fluoropolymer segments.13 More recently, the Winnik and Manners groups have built noncentrosymmetric cylindrical nanoparticles by multistep solution assembly of crystalline-coil block copolymer and selective

1. INTRODUCTION Amphiphilic block copolymers, consisting of two or more polymeric blocks that are covalently linked and chemically different, provide excellent molecular tools for solution assembly. Because of the association of insoluble blocks and expansion of soluble blocks in selective solvent, amphiphilic block copolymers will assemble into well-defined nanoparticles. A wide variety of nanostructures has been achieved with block copolymer assembly including spheres, cylinders, vesicles,1 toroids,2 and helices,3 successfully built by solution assembly of linear diblocks,1 linear triblocks,4 and star-like triblock copolymers.5 Some recent work reported on altering traditional nanostructures such as vesicles by changing the block copolymer compositions.6 Recently, increasing interest in nanotechnology has focused research on creating more complex nanoobjects with well-defined and novel geometries. Multicompartment and multigeometry nanoparticle (MCN and MGN) assemblies are of broad interest.7 Extensive research has been devoted to MCN because various applications could make © XXXX American Chemical Society

Received: April 10, 2015 Revised: August 3, 2015

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DOI: 10.1021/acs.macromol.5b00752 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

Figure 1. Schematic of MCN/MGNs via binary blends of PAA100-b-PI150 and PAA99-b-PS169 diblock copolymers in different water addition rates, in THF/H2O mixtures (v/v = 1:4). Pure vesicles and pure cylinders are assembled from separate assembly of PAA-b-PI and PAA-b-PS, respectively. When the two different block copolymers are coassembled, separate population of cylinders and vesicles are formed in extremely slow water addtion; vesicle−cylinder connected particles are assembled with a faster rate; disk−cylinders and hybrid vesicles are formed in fast and extremely fast water addition conditions. The blue color represents PAA block complexed with organic diamine; the red represents PI block and the green represents PS block.

micelle corona cross-linking.14 The Müller group has used the hierarchical coassembly of triblock terpolymers to produce patchy nanoparticles to further build multicompartment nanostructures with predictable and tunable nanosize periodicities.15 All of these examples produced extraordinary MCN and MGNs through designed solution assembly. The ability of amphiphilic block copolymers to associate into complex morphologies depends not only on the molecules used but also on the intermolecular assembly strategy. Frequently used methodologies to create novel nanosized morphologies include the design and synthesis of different block copolymers, changing the solution conditions (pH,16 temperature,17 ionic strength18) and variation of solution assembly kinetic pathways.19 In an effort to minimize the need to develop new polymer synthetic routes,8b management of assembly strategies offers an alternative for novel nanostructure formation. Protocols involving solvent processing,15,20 flash nanoprecipitation,21 and polymerization of crystalline-coil micelles14 are good examples of new processes in engineering nanoscale materials. However, there is clear opportunity for new methods of forming MCN and MGNs via simple assembly strategies.

Compared to small molecule amphiphiles, a signature character of large molecular weight amphiphilic block copolymers is their slow-to-nonexistent chain exchange rate with solvent. The slow solution exchange kinetics, which slows down or eliminates global system equilibration,22 depends on the respective block chemistry and length as well as solvent composition. Consequently, block copolymers are readily kinetically trapped into desired nanoparticles with solution processing. Our assembly pathway allows us not only to take advantage of the kinetic trapping of block copolymers but also to purposefully trap multiple types of block copolymers into the same nanoparticle. By mixing two distinct block copolymers that separately can form a different geometry of nanoparticles, one can produce mixed nanoparticles with hybrid shapes. Sphere−cylinder, sphere−sphere, disk−cylinder, and patchy disks have been assembled via our method of block copolymer mixing and kinetic control.23 To create the multigeometry/multicompartment nanoparticles, we first dissolve two amphiphilic block copolymers, both containing PAA but each having a unique hydrophobic block, in pure THF to produce a homogeneous solution. One B


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Macromolecules Table 1. Summary of PAA100-b-PI150/PAA99-b-PS169 Blending Results in Different Conditions PAA100-b-PI150:PAA99-b-PS169molar ratio H2O to THF volume ratio

9:1 4:1

1:1 4:1

1:9 4:1

extremely slow (0.8 mL/h)a middle rate (5 mL/h) fast rate water (30 mL/h) extremely fast (≫100 mL/h)

vesicle + cylinder vesicle−cylinder hybrid vesicle hybrid vesicle

vesicle + cylinder vesicle−cylinder disk−cylinder disk−cylinder

vesicle + cylinder vesicle−cylinder disk−cylinder disk−cylinder

a

Water addition rates were listed in a way of volume of water added per hour to 1 mL of initial THF solution. Disk−cylinder represents the diskshaped exotic nanoparticles having the immiscible domains of PI disk piece surrounded by PS cylinder rim. Some disk−cylinder MCNs are with cylinder tails as in Figure 5C.

block molecular weight, solvent composition, and acid−base complexation in the particle shells, are able to further evolve internally and between particles to reach a lower free energy state. Significantly, while discussed in the context of the model amphiphilic block copolymers herein, the solution assembly strategy is applicable to a wider variety of block copolymers and block copolymer mixtures for future nanomaterials construction.

of the polymers contains polystyrene (PS) hydrophobic block, and the other contains a polybutadiene (PB) or polyisoprene (PI) block. Following polymer dissolution, organic diamine molecules are added to complex with the PAA domains. Inverse micelle-like particles are formed with PAA complexed with diamine in the core and mixed hydrophobic blocks residing on the outside. Finally, water is added to trigger hydrophobic collapse and hydrophilic solubilization/expansion. At higher water contents, various shapes of nanoparticles with mixed block copolymers can be formed. Importantly here, during the solvent processing, the unlike hydrophobic blocks from the two mixed block copolymers are locally trapped together due to the PAA complexation with diamine. During the water addition process, nanophase separation between the distinct hydrophobic domains can occur since the two hydrophobic blocks are kinetically trapped together locally within nanoparticles. Another parameter that can be manipulated during solution assembly is the water addition rate. Different solvent mixing rates can result in different final particle geometries23a in block copolymer blends as well as in single diblock/triblock copolymer assembly.24 In an effort to gain control of the assembly of nanoobjects with new shapes, it is important to discuss the influence of solvent mixing rate on the assembly results. Herein, we used four different water addition rates at three blending ratios of two different block copolymers in order to construct new MCN and MGNs. Two different block copolymers that form vesicles and cylinders, respectively, were selected as the building blocks for coassembly. Examples of the particles observed in this work are hybrid vesicle−cylinders, hybrid vesicles, and cylinder−disk nanoparticles as shown in Figure 1. Importantly, strong correlations between water addition rate and assembled nanostructures were observed. Generally, faster water addition rates provide for hybrid nanoparticles with different block copolymers trapped within the same particle while slow solvent mixing rates produce separate populations of nanoparticles with separate, constituent block copolymers. At intermediate solvent mixing rates, there are subtle effects on the morphologies of formed, hybrid nanoparticles in which different block copolymers are trapped together. The particles observed inform construction strategies of other hybrid nanostructures with other block copolymer blend systems. Therefore, the use of solvent mixing rate is discussed as an additional and necessary tool to partially control the morphology of mixed block copolymer hybrid nanoparticles. Since all of the nanoparticles produced with kinetic control are in metastable, kinetically trapped states, the evolution of nanoparticles over time is also discussed. Interestingly, unanticipated morphologies can develop over long periods of time (e.g., several months) of aging during which block copolymers, still trapped locally within the particles due to

2. EXPERIMENTAL SECTION 2.1. Materials. The unsaturated diblock copolymer poly(tert-butyl acrylate)-block-polyisoprene was synthesized by anionic polymerization employing high vacuum techniques and sequential polymerization of isoprene and tert-butyl acrylate according to a developed protocol.25 The subsequent dialysis process to obtain poly(acrylic acid)-block-polyisoprene (PAA100-b-PI150: Mw: 27 370 Da; Mw/Mn = 1.19) was performed according to a method described elsewhere.25 The diblock copolymer poly(tert-butyl acrylate)-block-polystyrene (PtBA-PS) was synthesized by successively incorporating tert-butyl acrylate (tBA), and styrene (S) through an atom transfer radical polymerization (ATRP) procedure.26 The tert-butyl esters of PtBA-PS were selectively cleaved via reaction with trifluoroacetic acid (TFA) in dichloromethane, producing the amphiphilic block copolymer poly(acrylic acid)-block-polystyrene (PAA99-b-PS169: Mw: 26 927 Da; Mw/ Mn = 1.05). The organic diamine, 2,2′-ethylenedioxybis(ethylamine) (EDDA, 99%), was purchased from Sigma-Aldrich and was used as obtained. 2.2. Self-Assembly Procedure and Sample Characterization. The polymer solutions were first prepared by dissolving PAA100-b-PI150 and PAA99-b-PS169 separately into neat tetrahydrofuran (THF) to reach a concentration of 2 mg/mL. The solution was stirred for 24 h for complete dissolution. The blending solution was made by mixing the two presolutions at different molar ratios of the two diblocks (PAA100-b-PI150: PAA99-b-PS169 = 9:1, 1:1, 1:9) in THF. Organic diamine (1 vol % concentration in THF) was then added to reach a polymer concentration of 0.1 wt % and an amine:acid ratio of 0.5:1. EDDA was chosen as the diamine molecule due to its demonstrated ability to provide control of the final nanoparticle morphology and allow the local, intraparticle phase separation of unlike hydrophobic blocks from different block copolymers trapped in the same nanoparticle.4 The solution was stirred for 24 h for sufficient mixing and complexation between amine and acid groups. Solutions of individual block copolymers for assembly were prepared the same way as the blend samples with EDDA added to either PAA99-PS169 or PAA100-PI150 solution. Next, water was added to 1 mL of a desired block copolymer THF solution at different addition rates as listed in Table 1 to reach 80% water concentration. Final polymer assemblies was thus obtained in 5 mL total mixed solvent (THF/H2O = 1/4 v/v) with a final polymer concentration at 0.018 wt %. Different water addition rates were achieved through use of a syringe pump (KD Scientific Syringe Pump, KDS 100). The assembly solutions were constantly stirred with a magnetic stir bar during water addition, with the same stirring rates in all the experiments. The assembly solutions were stored in glass vials sealed with PTFE covers and Teflon tape to limit solvent evaporation to undetectable levels. During aging, the solutions were not stirred further. The parameters of titration rates, C


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Macromolecules blending ratios, and corresponding morphologies are listed in Table 1. Sample microscopy images were obtained after solutions were aged for 1 day after water addition. The aging effects shown in Figure 6 were performed after 1, 2, or 3 months of aging. 2.3. Transmission Electron Microscopy and Selective Staining. TEM observations of dried samples were performed on a FEI Tecnai G2-12 microscope operated at 120 kV, equipped with a Gatan CCD camera. Grids were pretreated with oxygen plasma to increase the surface hydrophilicity. TEM samples were prepared by applying a drop of nanoparticle suspension (about 3 μL) directly onto a carbon-coated copper TEM grid, and the drop was allowed to evaporate under ambient conditions. After the grids were completely dried, they were stained by exposure to 1 mL of osmium tetroxide aqueous solution (purchased from Electron Microscopy Sciences, 4 wt % OsO4) in a closed glass vessel. Different time periods (6−24 h) were used for the staining of different samples. 2.4. Cryogenic Transmission Electron Microscopy (CryoTEM) and Selective Staining. CryoTEM observations were performed on a FEI Tecnai G2-12 microscope, operated at 120 kV and equipped with a Gatan CCD camera. Lacey carbon grids were pretreated using oxygen plasma to increase the surface hydrophilicity right before sample preparation. Approximately 0.6 mL of nanoparticle suspension was held in a centrifuge tube (2 mL total volume). The tube was then put into a staining vessel having 0.5 mL of 4 wt % OsO4 solution in the bottom. Staining occurred for 10−20 min of staining vapor exposed to the nanoparticle solution in the closed 100 mL glass vessel. It was observed that the clear solutions turned to be slightly yellow after staining. CryoTEM sample grids were prepared directly after staining using a Vitrobot vitrification system. A droplet of ∼1.2 μL of stained nanoparticle suspension was pipetted onto a lacey carbon grid. Multiple blotting times were used (usually 2 times) with each blot lasting 2 s in order to obtain suitable liquid film thickness for imaging. The sample was allowed to relax for ∼3 s postblotting and then quickly plunged into a liquid ethane reservoir cooled by liquid nitrogen. The vitrified sample was transferred into a Gatan 626 cryoholder and cryo-transfer stage cooled by liquid nitrogen. During the observation of the vitrified samples, the cryo-holder temperature was maintained around −176 °C to limit sublimation of vitreous water. The images were recorded digitally with a Gatan CCD camera. Longer exposure time (3−5 s) at a lower dose was used to acquire data and to limit beam damage. For each assembly solution, 10−15 cryoTEM/TEM images were taken from different areas, with more than 100 nanoparticles analyzed, in order to assess the distribution in structures observed and to discuss representative nanoparticle behavior in the results. Specifically in both extremely fast water addition (≫100 mL/h) and fast addition conditions (30 mL/h), the diameters of resultant hybrid vesicles and disk−cylinder particles formed were measured using ImageJ and summarized for comparison (Figure S7).

nanoparticles, the immiscible nature of PI and PS affords local nanophase separation and separated PI and PS compartment formation. Scheme 1. Chemical Structures of Diblock Copolymers and Diamine Molecule

When assembled individually, PAA100-b-PI150 and PAA99-bPS169 form vesicles and cylinders, respectively, at 80% water condition (Figure 2 and Figure S1). The bilayer wall thickness of the vesicles is approximately 35 nm (34.9 ± 1.1 nm), and the diameter of the cylinders is approximately 40 nm. The measurement of the wall thickness was achieved by using ImageJ software. A histogram of the wall thickness measurements across many different particles is in the Supporting Information (Figure S8). The uncertainty in vesicle wall thickness can be attributed to the dispersity in molecular weight and block composition of the constituent PAA−PI block copolymer. The dimensional similarity of the different molecules and assemblies is important to form hybrid nanoparticles. Interestingly, after blending the two diblock copolymers in pure THF at 1:1 molar ratio, then adding EDDA for an amine to acid ratio of 0.5:1, and finally adding water at a same rate of 5 mL/h, vesicle−cylinder connected nanoparticles were observed (Figure 3B and Figure S3). Osmium tetroxide was then used to distinguish the PI and PS phases. 3.2. Effect of Water Addition Rates on the Assembled Nanostructures at Different Blending Ratio. Previous block copolymer blending results indicate that with proper kinetic control unique nanoparticle geometries can be confined into one nanoparticle such as hybrid sphere−sphere, sphere− cylinder, disk−cylinder, and disk−sphere particles.23 However, the study of the effects of kinetic solvent mixing rates has not been presented. The solvent mixing rate plays a crucial role in the design and formation of metastable nanoparticles with new hybrid shapes. Four different water addition rates were used to explore the impacts and possible nanostructure construction (0.8, 5, 30, and ≫100 mL/h). Also, three blending ratios of the two block copolymers were used to observe the influence of block copolymer blend composition on final nanostructure formation. With the extremely slow addition rate (0.8 mL/h), cryoTEM images show that separate populations of PAA100-b-PI150 vesicles and PAA99-PS169 cylinders are formed (Figure 3A and Figure S2A). Clearly, each diblock copolymer was able to organize into separate particles with preferred curvature during slow water titration as opposed to the formation of multigeometry and multicompartment nanoparticles. However, when the water addition rate was faster (5 mL/h), connected vesicle−cylinder MCN/MGNs were produced (Figure 3B). The PAA100-b-PI150 and PAA99-b-PS169 block copolymers expressed their own preferred curvatures locally while connected together and shared the same PAA/amine complexed shell. All vesicles in these hybrid nanoparticles exhibit the same wall thickness (∼35 nm) as in the pure PAA100-b-PI150 vesicle assembly. Similarly, the cylinder former

3. RESULTS AND DISCUSSION 3.1. Block Copolymers for Binary Blending and Solution Assembly. PAA-b-PI (A-B) and PAA-b-PS (A-C) diblocks were chosen as the two block copolymers to study A− B/A−C binary blending via desired solution assembly pathway. PAA100-b-PI150 was selected due to its demonstrated ability to form vesicles at water-rich condition in the presence of EDDA diamine (H2O = 80%, amine:acid = 0.5:1). Also, PAA100-b-PI150 provides unsaturated double bonds for increasing mass− thickness contrast via OsO4 selective staining. PAA99-b-PS169 was selected due to its ability to form cylinders at the same solution condition in which the PAA100-b-PI150 forms vesicles. The identical hydrophilic PAA domains shared by the two block copolymers are complexed with diamine in neat THF solution which acts to confine the PAA-b-PI and PAA-b-PS into the same aggregate with a PAA-diamine core and mixed PI and PS in the shell. After addition of water and consequent confinement of the PI and PS blocks into the core of formed D


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Figure 2. CryoTEM images of nanoparticles assembled from PAA100-b-PI150 and PAA99-b-PS169 diblock copolymers in mixed solvent with THF:water volume ratio v/v = 1:4 and an amine to acid ratio = 0.5:1.0 due to added EDDA. (A) CryoTEM images of vesicles assembled by PAA100b-PI150 with vesicle wall thickness equal to approximately 35 nm, formed with a water addition rate of 5 mL/h. (B) CryoTEM images of cylinders assembled by PAA99-b-PS169 with the diameter = 40 nm, with water addition rate at 5 mL/h.

illustrate the general utility of the kinetic trapping strategy with multiple block copolymers. In the extremely fast water addition, water is added by using a pipet to immediately change the solvent condition (≫100 mL/h). Similar to the fast addition rate (30 mL/h), disk− cylinder shaped MCN/MGNs are formed with flat polyisoprene-core domains bordered by half polystyrene-core cylindrical rims. The disk−cylinder particles display a more homogeneous size as compared to the fast addition rate (30 mL/h) with a rare, discrete cylinder also observed (Figure 3D and Figure S2D). The comparison of the overall sizes of the disk−cylinder MCNs at the two different rates was performed by measuring the diameters of numbers of nanoparticles in different imaging areas (100−150 nanoparticles in 5−10 different images). Slight differences of the diameters are observed as shown in the Supporting Information (Figure S7). By changing the blending ratio to one dominated by the vesicle former (PAA100-b-PI150:PAA99-b-PS169 = 9:1), unblended or blended MCN/MGNs are formed depending on the different water addition rates. At the extremely slow water addition rate (0.8 mL/h), again, separate populations of vesicles and cylinders are formed (Figure 4A). Vesicle−cylinder connected nanoparticles with more vesicles linked by fewer cylinders are observed in the middle water addition rate (5 mL/ h) (Figure 4B). However, when the fast rate (30 mL/h) is used, the cylindrical morphology is no longer obtained. Instead, novel kinetically trapped multicompartment vesicles are formed. Because of the dominant molar fraction of PAA100-b-PI150 being a vesicle former, the overall particle is shown as a vesicular shape with the PAA99-b-PS169 (despite being a cylinder former) constrained inside the same bilayer wall. The partial dark and partial light domains shown in Figure 4C are attributed to the stained PI and unstained PS blocks, respectively. When the extremely fast water addition is used (≫100 mL/h), hybrid vesicles (Figure 4D and Figure S4D,E) with a quite uniform diameter were observed. To compare the difference, diameters of the hybrid vesicles formed with the two

PAA99-b-PS169 exhibits the same diameter (∼40 nm) as in the separate PAA99-b-PS169 assembly. Without OsO4 staining, it is observed that the vesicular head and cylindrical tail in the blended nanoparticle have the same contrast (Figure S3E). After staining, the vesicle domain is darker than the cylinder domains due to the staining of the isoprene block. Additional cryoTEM images from different imaging areas are provided in Supporting Information to indicate the formation of vesicle− cylinder connected nanostructure (Figure S3A−D). In the blended assemblies, diamine molecules play a significant role in helping to trap the different hydrophobic blocks inside the PAA/amine condensed shell. Without the diamine molecules, the diblock copolymer blends are assembled into ill-defined aggregates at different water addition rates (Figure S6). The construction of nanoparticles with desired formation of different compartments in the core domain relies on the amine/acid electrostatic complexation in the shared PAA/ amine shell. All the well-controlled blended/unblended nanoparticles shown here need the functions of diamine molecules. When an even faster rate of water addition was used (30 mL/ h), mixed disk−cylinder nanostructures were formed in the blended system, rather than the discrete vesicles and cylindrical domains shown with slower rates. In each disk−cylinder hybrid nanoparticle, the center disk piece (80−150 nm diameter) is dark due to the staining of the polyisoprene block and is surrounded by a light border with the thickness equal to half of the PAA99-b-PS169 cylinder diameter (∼20 nm) (Figure 3C). The PAA/amine condensed shell also stains slightly and is shown as the darker, thin circle edge surrounding the whole disk−cylinder hybrid nanoparticles. A few detached cylinders are observed in the images (Figure 3C and Figure S2C). The detailed interpretation of TEM images of disk−cylinder nanostructures as well as the comparison with vesicle structures is presented in Supporting Information (Figures S9−S11). These disk−cylinder nanoparticles have been reported in previous work with different diblocks.23b The new disk− cylinder particle results using two different block copolymers E


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Figure 3. CryoTEM and TEM images of hybrid nanoparticles assembled from blends of PAA100-b-PI150 and PAA99-b-PS169 diblock copolymers (molar ratio = 1:1) with EDDA at amine to acid ratio = 0.5:1, in a mixed solvent of THF: water at a volume ratio = 1:4 (1 mL THF initial, 5 mL final mixed solution) with different water addition rates. (A) CryoTEM of separated vesicles and cylinders at a water addition rate: 0.8 mL/h H2O. (B) CryoTEM of vesicle−cylinder connected nanoparticles assembled at a water titration rate: 5 mL/h. (C) TEM of disk−cylinder hybrid nanoparticles assembled at a water addition rate: 30 mL/h H2O. (D) TEM of disk−cylinder hybrid nanoparticles assembled at the fasted water addition rate by pipet addition (≫100 mL/h). All solutions were imaged 1 day after the assembly process and vapor stained by OsO4 for 10−20 min.

observed at the same water addition rates but at different block copolymer blending ratio is obtained. Separate populations of vesicles and cylinders (now with fewer vesicles and more cylinders) are formed with the extremely slow water addition rate (0.8 mL/h, Figure 5A). Connected vesicle−cylinder MCN/MGNs, with more cylinders around each vesicle, are observed in the middle water addition rate (5 mL/h, Figure 5B). For the fast water addition rate (30 mL/h, Figure 5C), a small population of disk−cylinder nanoparticles was observed along with many separate cylinders as well as cylinders with disk end-caps. When water is added in an ultrafast way (≫100 mL/h), small disk−cylinder shaped nanoparticles are formed with rare short cylinders as indicated by the arrows (Figure 5D). The inner disk pieces are much smaller (5−50 nm) than observed in the disks (10−80 nm) assembled with the equivalent blending ratio of block copolymers in Figure 3D.

different rates are measured (Figure S7). Smaller and more monodisperse hybrid vesicles are formed with the faster water addition rate. It is understood that as water is added in an extremely fast way, the water and THF mixed in an extremely short time. Smaller seeds for the nucleation and self-assembly formed during the solvent mixing, and smaller nanoparticles are then expected. In order to form the hybrid vesicles, mixed hydrophobic chains from the two block copolymers must locally separate and migrate into different domains in the same vesicle wall. Since the dominant block copolymer component is a vesicle former, the overall morphology of the mixed nanoparticle remains a vesicle, as opposed to the stabilization of disk particles observed in the 1:1 blending ratio at faster water addition rates in Figure 3. At blending molar ratio of 1:9 of PAA100-b-PI150 to PAA99-bPS169, a trend of nanostructure formation similar to what was F


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Figure 4. CryoTEM images of hybrid nanoparticles assembled from blends of PAA100-b-PI150 and PAA99-b-PS169 diblock copolymers (molar ratio = 9:1) with EDDA added to form an amine to acid ratio = 0.5:1. The final solvent mixture is THF:water volume ratio = 1:4. Each image resulted from a different water addition rate. (A) Separated vesicles and cylinders at a water addition rate of 0.8 mL/h. (B) Vesicle−cylinder connected hybrid nanoparticles, assembled at 5 mL/h. (C) Hybrid vesicles with partial dark wall domain (PI) and light wall domain (PS) assembled at 30 mL/h added H2O. (D) Hybrid vesicles assembled at an extremely fast rate; H2O was added into THF by using pipet addition. All solutions were investigated 1 day after the assembly process and vapor stained by OsO4 for 10−20 min.

behavior in the hybrid particles formed, showing the importance of the differences in local nanophase separation on the final particle structure. Since the hybrid particle formation is the result of a kinetic process, the structure is only metastable and may change over time. The structure evolution of nanoparticles is an important topic in macromolecule self-assembly due to the formation of unusual nanostructures resulting from diverse evolution mechanisms. Structure evolutions could be caused by a change in chemical structure, addition of external stimuli to the assembly solution, and interparticle or intraparticle chain exchange in selective solvent.27 For example, the Hillmyer group has reported the incorporation of pH-sensitive degradable blocks in star-like triblock copolymer and the subsequent pH-tuning block degradation and micelle evolution.28 Another good example is the micelle-to-micelle

Additional representative images are provided in the Supporting Information (Figure S5). 3.3. Structure Evolution of Multicompartment Nanoparticles. It is clear when comparing nanoparticles formed by different water addition rates that there are gross differences. For example, macrophase separation clearly is possible between unlike block copolymers during the extremely slow water addition despite the fact that the unlike block copolymers were initially complexed together in pure THF solution by the presence of the organic amines. This macrophase separation consistently produced separate nanoparticles of exclusively vesicle or cylinder geometry with no hybrid structures observed. However, at faster water addition rates, the unlike block copolymers are forced to reside in the same nanoparticle and produce particles with hybrid geometry. Even within the faster water addition rates, there is a rich variety of assembly G


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Figure 5. CryoTEM and TEM images of hybrid nanoparticles assembled from blends of PAA100-b-PI150 and PAA99-b-PS169 diblock copolymers (molar ratio = 1:9) with EDDA at an amine to acid ratio = 0.5:1, THF:water volume ratio = 1:4 and different water addition rate. (A) Separated vesicles and cylinders at a water addition rate of 0.8 mL/h H2O with cryoTEM. (B) Vesicle−cylinder connected nanoparticles assembled at 5 mL/h water titration rate with cryoTEM. (C) TEM of cylinder−disk hybrid nanoparticles assembled at 30 mL/h H2O. (D) TEM of disk−cylinder hybrid nanoparticles assembled at an extremely fast water addition. Arrows point to the short cylinders in disk−cylinder structure. All solutions were observed 1 day after assembly process and vapor stained by OsO4 for 10−20 min.

morphology evolution reported by the Lodge group in which immiscible PEE (polyethylethylene) and PMCL (polymethylcaprolactone) hydrophobic blocks are covalently linked in a triblock copolymer, and intramicelle structure evolution happened in selective solvents via dynamic slow chain exchange.29 In the current assembly system, multicompartment nanoparticle evolution occurred in the hybrid vesicle systems, as observed in Figures 6C and 6D. During the ultrafast water addition process, two immiscible hydrophobic blocks were confined into the vesicle geometry. The trapped PI and PS blocks experience chain reorganization in mixed solvent. It was clearly shown in Figure 4 that the mixed vesicles contain the segregated phases of the PI and PS blocks in the vesicle walls. After aging, further transformations occurred, with hybrid vesicles (Figure 6A) seemingly joining together to form larger,

compound vesicles with a preference of the PI membrane portions to lie on the interior of the hybrid particles (Figure 6C,D). The wheel-like structures stay stable after three months of aging (Figure 6D). The overall diameter of the nanoparticles changes from around 100 nm (Figure 6.1) to 140 nm (Figure 6.4). The wheel-like structures seem like aggregates of several small vesicles from previous time stage. The majority phase is still PI, and the minority phase PS may joint together to stabilize the junction. Since all of the nanoparticles are kinetically trapped, fluid assemblies, transitions with aging are always possible. The transition of the hybrid vesicles is simply shown for illustrative, descriptive purposes at this point. The transition only occurs with apparent interparticle interaction over time since the PI-rich membrane portions seem to remain intact as well as the PS-rich domains. If one wishes to guarantee the persistence of a particular hybrid particle size/shape after H


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Figure 6. Aging results of multicompartment hybrid vesicles. (A) CryoTEM image of hybrid vesicles constructed from blends of PAA100-bPI150:PAA99-b-PS169 = 9:1 in an extremely fast manner (≫100 mL/h) after sample aging for 1 day. (B) Same sample after aged for one month. (C) Sample aged for two months. (D) Sample aged for three months. Insets 1−4 are selected snapshots from images A, B, C, and D to show the signature micelle evolution status in different aging points: 1, hybrid vesicles; 2, asymmetric hybrid vesicles with extended PS wall and PI phase; 3, asymmetric wheel-like hybrid vesicle; 4, larger magnification of asymmetric wheel-like hybrid vesicle from aggregates of hybrid vesicles. Inserted scale bar: 100 nm.

formation, then covalent trapping of the particle is suggested30 in order to prevent aging and further structure evolution. Further evolution of the hybrid vesicles may occur over time as the PS-containing block copolymer seeks a higher interfacial curvature and cylindrical geometry, although we have not observed this transition over several months of aging.

4. CONCLUSION Formation of various multicompartment and multigeometry nanoparticles is discussed here made through solution assembly of binary blends of PAA-b-PI and PAA-b-PS. Different controlling parameters were utilized: the solvent addition I


Macromolecules

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rate, the block copolymer blending ratio, and aging effects. Separate vesicles and cylinders, novel connected vesicle− cylinder multigeometry nanoparticles, multicompartment disk− cylinders, and multicompartment hybrid vesicles are formed at specific solution conditions. Specifically, the new structures reported here shed light on multifunctional nanomedicine vehicle or nanoreactor construction with advantages from both vesicular and cylindrical shapes.31 Overall rate−morphology correlations showed that smaller, blended nano-objects are formed at faster water addition rates. The overall size and size dispersity of the hybrid nanoparticles become smaller at faster water addition rate along with the smaller size of local immiscible hydrophobic domains. The evolution of hybrid vesicles was presented here to fully understand the advantages of kinetic control methods. Nanoparticles with untraditional shapes were evaluated over long time aging. The observed gross differences in assembly behaviors showed the importance of kinetic control. By taking advantage of fast solution assembly methods, other hybrid multicompartment nanoparticles are anticipated; hybrid cylinders, hybrid disks, and hybrid lamellar via other material blends. Moreover, systems that incorporate more building units such as degradable blocks and functional groups for postassembly modification are promising and are under investigation, targeting for well-defined asymmetric and anisotropic multifunctional nanomaterials.

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REFERENCES

(1) Zhang, L.; Eisenberg, A. Multiple Morphologies of “Crew-Cut” Aggregates of Polystyrene-b-poly(acrylic-acid) Block Copolymers. Science 1995, 268, 1728−1731. (2) Pochan, D. J.; Chen, Z.; Cui, H.; Hales, K.; Qi, K.; Wooley, K. L. Toroidal Triblock Copolymer Assemblies. Science 2004, 306, 94−97. (3) Zhong, S.; Cui, H.; Chen, Z.; Wooley, K. L.; Pochan, D. J. Helix self-assembly through the coiling of cylindrical micelles. Soft Matter 2008, 4, 90−93. (4) (a) Cui, H.; Chen, Z.; Wooley, K. L.; Pochan, D. J. Controlling Micellar Structure of Amphiphilic Charged Triblock Copolymers in Dilute Solution via Coassembly with Organic Counterions of Different Spacer Lengths. Macromolecules 2006, 39, 6599−6607. (b) Li, Z.; Chen, Z.; Cui, H.; Hales, K.; Qi, K.; Wooley, K. L.; Pochan, D. J. Disk Morphology and Disk-to-Cylinder Tunability of Poly(Acrylic Acid)-bPoly(Methyl Acrylate)-b-Polystyrene Triblock Copolymer SolutionState Assemblies. Langmuir 2005, 21, 7533−7539. (c) Li, Z.; Chen, Z.; Cui, H.; Hales, K.; Wooley, K. L.; Pochan, D. J. Controlled Stacking of Charged Block Copolymer Micelles. Langmuir 2007, 23, 4689−4694. (5) Jain, S.; Bates, F. S. On the Origins of Morphological Complexity in Block Copolymer Surfactants. Science 2003, 300, 460−464. (6) Yoshida, E. Morphology control of giant vesicles by composition of mixed amphiphilic random block copolymers of poly(methacrylic acid)-block-poly(methyl methacrylate-random-methacrylic acid). Colloid Polym. Sci. 2015, 293, 249−256. (7) Moughton, A. O.; Hillmyer, M. A.; Lodge, T. P. Multicompartment Block Polymer Micelles. Macromolecules 2012, 45, 2−19. (8) (a) Zhu, J.; Zhang, S.; Zhang, F.; Wooley, K. L.; Pochan, D. J. Hierarchical Assembly of Complex Block Copolymer Nanoparticles into Multicompartment Superstructures through Tunable Interparticle Associations. Adv. Funct. Mater. 2013, 23, 1767−1773. (b) Gröschel, A. H.; Schacher, F. H.; Schmalz, H.; Borisov, O. V.; Zhulina, E. B.; Walther, A.; Müller, A. H. E. Precise hierarchical self-assembly of multicompartment micelles. Nat. Commun. 2012, 3, 710. (c) Christian, D. A.; Tian, A.; Ellenbroek, W. G.; Levental, I.; Rajagopal, K.; Janmey, P. A.; Liu, A. J.; Baumgart, T.; Discher, a. D. E. Spotted vesicles, striped micelles and Janus assemblies induced by ligand binding. Nat. Mater. 2009, 8, 843−849. (9) (a) Holowka, E. P.; Sun, V. Z.; Kamei, D. T.; Deming, T. J. Polyarginine segments in block copolypeptides drive both vesicular assembly and intracellular delivery. Nat. Mater. 2007, 6, 52−57. (b) Geng, Y.; Dalhaimer, P.; Cai, S.; Tsai, R.; Tewari, M.; Minko, T.; Discher, D. E. Shape effects of filaments versus spherical particles in flow and drug delivery. Nat. Nanotechnol. 2007, 2, 249−255. (10) Du, J.; O’Reilly, R. K. Anisotropic particles with patchy, multicompartment and Janus architectures: preparation and application. Chem. Soc. Rev. 2011, 40, 2402−2416. (11) Li, X.; Liu, G.; Han, D. Wrapping amino-bearing block copolymer cylinders around carboxyl-bearing nanofibers: a case of hierarchical assembly. Soft Matter 2011, 7, 8216−8223. (12) (a) Li, Z.; Hillmyer, M. A.; Lodge, T. P. Control of Structure in Multicompartment Micelles by Blending. Macromolecules 2006, 39, 765−771. (b) Saito, N.; Liu, C.; Lodge, T. P.; Hillmyer, M. A. Multicompartment Micelles from Polyester-Containing ABC Miktoarm. Macromolecules 2008, 41, 8815−8822. (13) Mao, J.; Ni, P.; Mai, Y.; Yan, D. Multicompartment Micelles from Hyperbranched Star-Block Copolymers Containing Polycations and Fluoropolymer Segment. Langmuir 2007, 23, 5127−5134. (14) Rupar, P. A.; Chabanne, L.; Winnik, M. A.; Manners, I. NonCentrosymmetric Cylindrical Micelles by Unidirectional Growth. Science 2012, 337, 559−562. (15) Groschel, A. H.; Walther, A.; Lobling, T. I.; Schacher, F. H.; Schmalz, H.; Müller, A. H. Guided hierarchical co-assembly of soft patchy nanoparticles. Nature 2013, 503, 247−251. (16) Liu, C.; Hillmyer, M. A.; Lodge, T. P. Multicompartment Micelles from pH-Responsive Miktoarm Star Block Terpolymers. Langmuir 2009, 25, 13718−13725. (17) Jiang, X.; Zhao, B. Tuning Micellization and Dissociation Transitions of Thermo- and pH-Sensitive Poly(ethylene oxide)-b-

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b00752. Figures S1−S11 (PDF)

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (D.J.P.). Author Contributions

Y.C. and D.J.P. conceived the experiments and cowrote the paper. Y.C. designed and carried out the experiments and collected and analyzed the data. K.Z., X.Z., F.Z., J.W.M., and K.L.W. provided the block copolymer materials. All authors discussed the results and commented on the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the financial support from the National Science Funding DMR-0906815 (D.J.P. and K.L.W.) and DMR-1309724 (D.J.P. and K.L.W.). We are also thankful for the cooperative agreement 70NANB12H239 from NIST, U.S. Department of Commerce, and the UD-NIST Center for Neutron Science. The statements, findings, conclusions and recommendations are those of the authors and do not necessarily reflect the view of U.S Department of Commerce and NIST. We are thankful to Dr. Chaoying Ni and Frank Kriss for the help of TEM and cryoTEM facilities in Advanced Microscopy and Microanalysis lab at University of Delaware and the NMR facilities in Texas A&M University and University of Tennessee. We are thankful for the help of Dr. Hao-Jan Sun from University of Pennsylvania for 3D cartoon modeling. J


Article

Macromolecules poly(methoxydi(ethylene glycol) methacrylate-co-methacrylic acid) in Aqueous Solution by Combining Temperature and pH Triggers. Macromolecules 2008, 41, 9366−9375. (18) Jansson, J.; Schillen, K.; Olofsson, G.; Cardoso da Silva, R.; Loh, W. The Interaction between PEO-PPO-PEO Triblock Copolymers and Ionic Surfactants in Aqueous Solution Studied Using Light Scattering and Calorimetry. J. Phys. Chem. B 2004, 108, 82−92. (19) (a) Hayward, R. C.; Pochan, D. J. Tailored Assemblies of Block Copolymers in Solution: It Is All about the Process. Macromolecules 2010, 43, 3577−3584. (b) Cui, H.; Chen, Z.; Zhong, S.; Wooley, K. L.; Pochan, D. J. Block Copolymer Assembly via Kinetic Control. Science 2007, 317, 647−650. (20) (a) Meeuwissen, S. A.; Kim, K. T.; Chen, Y.; Pochan, D. J.; van Hest, J. C. M. Controlled Shape Transformation of Polymersome Stomatocytes. Angew. Chem., Int. Ed. 2011, 50, 7070−7073. (b) Kim, K. T.; Zhu, J.; Meeuwissen, S. A.; Cornelissen, J. J. L. M.; Pochan, D. J.; Nolte, R. J. M.; van Hest, J. C. M. Polymersome Stomatocytes: Controlled Shape Transformation in Polymer Vesicles. J. Am. Chem. Soc. 2010, 132, 12522−12524. (21) Zhang, C.; Pansare, V. J.; Prud’homme, R. K.; Priestley, R. D. Flash nanoprecipitation of polystyrene nanoparticles. Soft Matter 2012, 8, 86−93. (22) (a) Choi, S.-H.; Lodge, T. P.; Bates, F. S. Mechanism of Molecular Exchange in Diblock Copolymer Micelles: Hypersensitivity to Core Chain Length. Phys. Rev. Lett. 2010, 104, 1−4. (b) Choi, S.-H.; Bates, F. S.; Lodge, T. P. Molecular Exchange in Ordered Diblock Copolymer Micelles. Macromolecules 2011, 44, 3594−3604. (c) Lu, J.; Choi, S.; Bates, F. S.; Lodge, T. P. Molecular Exchange in Diblock Copolymer Micelles: Bimodal Distribution in Core-Block Molecular Weights. ACS Macro Lett. 2012, 1, 982−985. (d) Lu, J.; Bates, F. S.; Lodge, T. P. Chain Exchange in Binary Copolymer Micelles at Equilibrium: Confirmation of the Independent Chain Hypothesis. ACS Macro Lett. 2013, 2, 451−455. (23) (a) Pochan, D. J.; Zhu, J.; Zhang, K.; Wooley, K. L.; Miesch, C.; Emrick, T. Multicompartment and multigeometry nanoparticle assembly. Soft Matter 2011, 7, 2500−2506. (b) Zhu, J.; Zhang, S.; Zhang, K.; Wang, X.; Mays, J. W.; Wooley, K. L.; Pochan, D. J. Diskcylinder and disk-sphere nanoparticles via a block copolymer blend solution construction. Nat. Commun. 2013, 4, 1−7. (24) Cui, H.; Chen, Z.; Wooley, K. L.; Pochan, D. J. Origins of toroidal micelle formation through charged triblock copolymer. Soft Matter 2009, 5, 1269−1278. (25) Wang, X.; Chen, J.; Hong, K.; Mays, J. W. Well-Defined Polyisoprene-b-Poly(acrylic acid)/Polystyrene-b-Polyisoprene-b-Poly(acrylic acid) Block Copolymers: Synthesis and Their Self-Assembled Hierarchical Structures in Aqueous Media. ACS Macro Lett. 2012, 1, 743−747. (26) Ma, Q.; Wooley, K. L. The Preparation of t-Butyl Acrylate, Methyl Acrylate, and Styrene Block Copolymers by Atom Transfer Radical Polymerization: Precursors to Amphiphilic and Hydrophilic Block Copolymers and Conversion to Complex Nanostructured Materials. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 4805−4820. (27) Denkova, A. G.; Mendes, E.; Coppens, M. O. Non-equilibrium dynamics of block copolymer micelles in solution: recent insights and open questions. Soft Matter 2010, 6, 2351−2357. (28) Saito, N.; Liu, C.; Lodge, T. P.; Hillmyer, M. A. Multicompartment Micelle Morphology Evolution in Degradable Miktoarm Star Terpolymers. ACS Nano 2010, 4, 1907−1912. (29) Liu, C.; Hillmyer, M. A.; Lodge, T. P. Evolution of Multicompartment Micelles to Mixed Corona Micelles Using Solvent Mixtures. Langmuir 2008, 24, 12001−12009. (30) (a) O’Reilly, R. K.; Hawker, C. J.; Wooley, K. L. Cross-linked block copolymer micelles: functional nanostructures of great potential and versatility. Chem. Soc. Rev. 2006, 35, 1068−83. (b) Zhang, F.; Elsabahy, M.; Zhang, S.; Lin, L. Y.; Zou, J.; Wooley, K. L. Shell crosslinked knedel-like nanoparticles for delivery of cisplatin: effects of crosslinking. Nanoscale 2013, 5, 3220−3225. (31) (a) Duncan, R.; Gaspar, R. Nanomedicine(s) under the microscope. Mol. Pharmaceutics 2011, 8, 2101−2141. (b) Vriezema,

D. M.; Comellas Aragonès, M.; Elemans, J. A. A. W.; Cornelissen, J. J. L. M.; Rowan, A. E.; Nolte, R. J. M. Self-Assembled Nanoreactors. Chem. Rev. 2005, 105, 1445−1489.

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