Template-Free Bottom-Up Method for Fabricating Diblock Copolymer

Apr 24, 2016 - Patchy particles are one of most important building blocks for hierarchical structures because of the discrete patches on their surface...
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Template-Free Bottom-Up Method for Fabricating Diblock Copolymer Patchy Particles Xianggui Ye,† Zhan-Wei Li,*,‡ Zhao-Yan Sun,‡ and Bamin Khomami*,† †

Materials Research and Innovation Laboratory (MRAIL), Sustainable Energy Education and Research Center (SEERC), Department of Chemical and Biomolecular Engineering, The University of Tennessee, Knoxville, Tennessee 37996, United States ‡ State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China ABSTRACT: Patchy particles are one of most important building blocks for hierarchical structures because of the discrete patches on their surface. We have demonstrated a convenient, simple, and scalable bottom-up method for fabricating diblock copolymer patchy particles through both experiments and dissipative particle dynamics (DPD) simulations. The experimental method simply involves reducing the solvent quality of the diblock copolymer solution by the slow addition of a nonsolvent. Specifically, the fabrication of diblock copolymer patchy particles begins with a crew-cut soft-core micelle, where the micelle core is significantly swelled by the solvent. With water addition at an extremely slow rate, the crew-cut soft-core micelles first form a larger crew-cut micelle. With further water addition, the corona-forming blocks of the crew-cut micelles begin to aggregate and eventually form well-defined patches. Both experiments and DPD simulations indicate that the number of patches has a very strong dependence on the diblock copolymer compositionthe particle has more patches on the surface with a lower volume fraction of patch-forming blocks. Furthermore, particles with more patches have a greater ability to assemble, and particles with fewer patches have a greater ability to merge once assembled. KEYWORDS: patchy particle, self-assembly of block copolymer, bottom-up method, dissipative particle dynamics simulation, PS-b-P4VP, nonsolvent

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mixed homopolymer brushes or amphiphilic block copolymers grafted onto particles are also able to create particles with a patchy or patterned surface.12−15 Further, an asymmetrical alternating AB block copolymer in a poor solvent for both blocks has been theoretically known to collapse into a patchy nanoparticle.16,17 Finally, confining diblock copolymers in spherical nanopores has been demonstrated to allow fabrication of patchy particle-like structures.18−22 A common approach to produce desired aggregate structures from copolymers is to first dissolve the copolymer in a common solvent that dissolves all the blocks, then gradually add a selective precipitant of one of the blocks, and finally remove the common solvent.23−30 We recently demonstrated that polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) with a wide range of molecular weights and P4VP composition in dimethylformamide (DMF) forms soft-core spherical micelles,

atchy particles possess discrete surface patches and often have an isotropic polymer core. The specific interactions between patches can provide a driving force for assembly of a targeted structure. Thus, the anisotropic nature of the patchy particle surface affords a certain degree of control over the assembly process. Therefore, patchy particles are one of most important building blocks for hierarchical structure, with potential applications in fabricating photonic crystals, targeted drug delivery, sensors, and electronics.1−9 Many experimental methods have been successfully developed to fabricate patchy particles,4,9 including templating, colloidal assembly, particle lithography, glancing-angle deposition, nanoparticle lithography, and capillary fluid flow. It is also possible to fabricate patchy particles by taking advantage of phase separation of chemically different polymers. Specifically, two different diblock copolymers (AB and BC) with a common hydrophobic block (B) but sufficiently dissimilar hydrophilic blocks (A and C)3 or amphiphilic ABC linear terpolymers in a nonsolvent for the B block5,7,10,11 have been predicted or demonstrated to obtain various micellar structures with the features of patchy nanoparticles. Similarly, © XXXX American Chemical Society

Received: January 29, 2016 Accepted: April 24, 2016

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DOI: 10.1021/acsnano.6b00742 ACS Nano XXXX, XXX, XXX−XXX

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ACS Nano where the micelle core is a PS block and the micelle corona is a P4VP block.31 Specifically, when the PS block is the majority component, the soft-core micelles in solution are crew-cut. In this context, with a crew-cut soft-core micelle in DMF, it is possible to fabricate patchy particles by adding a liquid that is a nonsolvent for the PS-forming core and a slightly inferior solvent for the corona-forming P4VP block. Water is a nonsolvent for PS and has reduced solubility for P4VP in comparison that with DMF.26,32 Therefore, the addition of water can result in micelle morphology changes or aggregation.

RESULTS AND DISCUSSION External agitation enhances merging of the micelles and chain exchange between micelles.33,34 To avoid any external agitation in our experiments, we added water by exposing the solution to moist air. Transmission electron microscopy (TEM) imaging in regular mode and scanning mode (STEM) was performed to characterize the copolymer micelle morphologies. Figure 1

Figure 2. Micelle morphologies obtained by stepwise decrease of the solvent quality for a corona-forming block from DPD simulation.

1.0 × 106 time steps were performed using a time step of 0.04. For each system, five independent simulations with different initial configurations were performed to ensure the accuracy of our results. The solvent used was a nonsolvent for block A and a selective solvent for block B. As expected, when the interaction between the solvent and corona-forming block is αBS = 25.0, that is, a good solvent for block B, the crew-cut-like micelle was easily observed. When the solvent quality was slightly reduced by increasing the value of αBS to 30.0, the corona-forming blocks began to aggregate. When the solvent quality was reduced even further by increasing the value of αBS to 35.0, well-ordered patches formed on the surface of the spherical particle. Therefore, the simulation provides further evidence that it is definitely feasible to fabricate patchy particles by reducing solvent quality, starting with a crew-cut soft-core micelle. When the P4VP block composition of PS-b-P4VP is increased, patches formed by P4VP tend to merge, and ultimately, the core−shell structure is attained. Therefore, the number of patches should decrease with the increase of patchforming block composition. DPD simulations provide a similar prediction, as shown in Figure 3. When the corona-forming

Figure 1. STEM HAADF images of PS-b-P4VP in DMF after exposure to air for 36 h; the bright areas are iodine-stained P4VP block-forming domains. Left: PS(252 K)-b-P4VP(43 K). Right: PS(110 K)-b-P4VP(107 K).

shows the STEM high-angle annular dark-field (HAADF) image of particle morphologies after exposure to moist air for 36 h for PS(252 K)-b-P4VP(43 K) (left image) and PS(110 K)b-P4VP(107 K) (right image). For both systems, uniform spherical particles with patchy or bumpy surfaces were observed. The patches or bumps on the surface are brighter in the HAADF image, which clearly indicates that patches are formed by P4VP blocks because P4VP blocks have been selectively stained by iodine vapor. Specifically, for PS(252 K)b-P4VP(43 K), about nine patches can be observed by visual examination of TEM images, and for PS(110 K)-b-P4VP(107 K), only one or two patches can be observed. Obviously, with an even higher P4VP volume fraction of PS-b-P4VP diblock copolymer, the resulting particles will be of core−shell-type instead of patchy particles. To further test whether the above concept (beginning with crew-cut soft-core micelle and then reducing the solvent quality) is feasible to fabricate patchy particles, we performed dissipative particle dynamics (DPD)35 simulation of A42B7 asymmetrical diblock copolymers in solvent S by stepwise reduction of the solvent quality, as shown in Figure 2. The repulsive interaction parameters are as follows: αAA = αBB = αSS = 25.0, αAS = αAB = 50.0, and αBS = 25.0−35.0. The simulation box size is 40 × 40 × 40; number density is 3.0, and polymer concentration is about 5% (200 polymer chains in the simulation box). All of the simulations were performed with the aid of the GALAMOST software package.36 For each αBS,

Figure 3. Patch number as a function of corona-forming block composition of a diblock copolymer.

block composition increases from 14 to 29%, the number of patches decreases from 6 to 4. When the corona-forming block composition is further increased to 43%, only two patches are observed. These DPD results are consistent with the experimental results for both PS(252 K)-b-P4VP(43 K) and PS(110 K)-b-P4VP(107 K) systems. The patch formation reduces the unfavorable contact between block PS and P4VP and also reduces the contact between P4VP and solvent while simultaneously reducing the entropy of the patch-forming block. In other words, patch forming on the micelle surface serves as a confinement for the patch-forming block. With a longer patch-forming block, the entropy penalty imposed by B

DOI: 10.1021/acsnano.6b00742 ACS Nano XXXX, XXX, XXX−XXX

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ACS Nano confining P4VP blocks within the patch will eventually surpass the energy gain by reducing the unfavorable contact between PS and P4VP; thus, patches will finally merge to form the shell of a core−shell particle. The favorable interactions between patches can act as a driving force for patchy particle assembly; therefore, particles with different numbers of patches have a different ability to assemble and merge. Specifically, with more patches in the PS(252 K)-b-P4VP(43 K) system, a larger variation of branched assemblies of the patchy particle are observed (Figures 1 and 4). This is attributed to the fact that a greater

Figure 5. Micelle size for PS(252 K)-b-P4VP(43 K) and PS(110 K)b-P4VP(107 K) as a function of exposure time: (a) Z average from dynamic light scattering (the error bar is smaller than the symbol); (b) micelle size measured from TEM by analyzing a few hundred micelles.

dynamic radius for PS(110 K)-b-P4VP(107 K) is attributed to the significantly longer P4VP blockthe micelles with a longer P4VP block keep the DMF in the micelle core for a longer time period, and the longer P4VP block damps the Brownian motion of the micelle further. The individual micelle sizes for both systems cease to increase significantly earlier than the hydrodynamic radius. Specifically, the individual micelle size from TEM stops increasing at about 36 h for PS(252 K)-b-P4VP(43 K) and about 72 h for PS(110 K)-b-P4VP(107 K). Further, the individual micelle size of PS(252 K)-b-P4VP(43 K) ceases to increase earlier than that of PS(110 K)-b-P4VP(107 K), which is due to the significantly longer P4VP block for PS(110 K)-bP4VP(107 K). As expected, this longer P4VP block for PS(110 K)-b-P4VP(107 K) keeps the DMF in the PS-forming micelle core longer and thus facilitates merging of the micelle, which is clearly supported by the discrete particle size distribution in the inset of Figure 4 for PS(110 K)-b-P4VP(107 K). Therefore, within approximately 36 h for PS(252 K)-b-P4VP(43 K) and about 72 h for PS(110 K)-b-P4VP(107 K), the hydrodynamic radius increase is due to the merging of the micelles; after the aforementioned times, the hydrodynamic radius increase is due to self-assembly of patchy particles. Based on the above discussions, a mechanism for patchy particle formation is proposed as follows. With the addition of water, the soft-core crew-cut micelle is driven to form a larger micelle, which is supported by both DLS and TEM results (Figure 5). With the further addition of water, the DMF content in the PS-forming core decreases significantly. The reduction of DMF in the PS-forming core results in (1) increasing the unfavorable contact between PS and P4VP, namely, increasing the interfacial energy,39 and (2) preventing further merging of the micelles into a larger micelle. To reduce the unfavorable contact between PS and P4VP and thereby decrease the interfacial energy, the continuous corona breaks into distinct areas that form patches resulting in patchy particles.

Figure 4. Bright-field TEM images for PS(252 K)-b-P4VP(43 K) (left image) and PS(110 K)-b-P4VP(107 K) (right image) in DMF after exposure to air for 36 h. The size distributions in the inset are from measuring the individual micelles in the TEM images.

number of patchy particles can aggregate from different directions. For PS(110 K)-b-P4VP(107 K), short chain-like aggregations (mainly three micelles in a chain) are formed. When the solvent quality is decreased by adding water slowly, thermodynamic forces drive the patchy particles to aggregate to reduce the surface energy of the entire system. Kinetically, the particle with more patches will have a higher chance of forming energetically favorable aggregation because it is the patch− patch interaction that drives particle aggregation.37 However, once a large aggregation forms, the patches become an energy barrier for further merging because of the unfavorable interaction between the patch-forming block and the coreforming block. The above assertion is clearly supported by more uniform particle size distribution for the PS(252 K)-bP4VP(43 K) system and a handful of particles with a diameter at 70−80 nm and about 100 nm for the PS(110 K)-bP4VP(107 K) system (inset, Figure 4). With water addition, the content of DMF in the PS homopolymer phase significantly decreases, as shown in Figure 8 of ref 26. To have a full picture of how the crew-cut soft-core micelles respond to the water addition, we measured both the effective hydrodynamic radius of the micelle by dynamic light scattering (DLS) and the dry micelle size by TEM as a function of exposure time (see Figure 5). Obviously, the measured hydrodynamic radius from DLS for both PS(252 K)-bP4VP(43 K) and PS(110 K)-b-P4VP(107 K) increases until it reaches a plateau after being exposed to moist air for about 300 h. Note that the plateau is reached when water addition by exposure to moist air ceases because the water in the liquid and air gradually reach an equilibrium.38 Note also that the DLS measured size for PS(110 K)-b-P4VP(107 K) is larger than that for PS(252 K)-b-P4VP(43 K). However, the TEM measured size for PS(110 K)-b-P4VP(107 K) is smaller than that for PS(252 K)-b-P4VP(43 K). The significantly larger hydro-

CONCLUSIONS In summary, we have demonstrated a convenient, simple, and scalable bottom-up approach to fabricate patchy particles using a generic diblock copolymer by slowly decreasing the solvent quality. Based on our experiments and DPD simulations, a mechanism for formation of patchy particles is proposed. Specifically, with the addition of water, the soft-core crew-cut micelles first merge to form large spherical micelles. With further water addition, solvents leave the micelle core, and the C

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Z.Y.S. also thank the National Basic Research Program of China (973 Program, 2012CB821500) and the National Science Foundation of China (21474110, 21474111, 21222407) for financial support.

micelle becomes a hard-core micelle, which increases the unfavorable contact between PS and P4VP, and finally results in formation of patchy particles. Furthermore, both experiments and simulation indicate that the number of patches has a very strong dependence on the diblock copolymer composition: there are more patches on the particle surface with a lower volume fraction of patch-forming blocks. In addition, our experiments show that particles with more patches possess a greater ability to merge and particles with fewer patches have a greater tendency to merge once assembled. Gradually adding a selective precipitant of one of the blocks into the copolymer in a solvent that dissolves all the blocks is a widely used method for obtaining aggregate structures. Therefore, the patchy particle formation of block copolymers in solution with a slow reduction of solvent quality should improve our understanding of the hierarchical morphology formation and finally facilitate the discovery of appropriate pathways for obtaining desired structures.

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EXPERIMENTAL SECTION Materials and Sample Preparation. PS-b-P4VP block copolymers were purchased from Polymer Source in Canada. Specifically, we used the following copolymers: PS(252 K)-b-P4VP(43 K) (PI = 1.09) and PS(110 K)-b-P4VP(107 K) (PI = 1.15). PI is the polydispersity index, which is defined as the ratio of the weight-average molecular weight (Mw) to the number-average molecular weight (Mn). The numbers in parentheses following PS and P4VP represent the Mn of each block, for example, 252 K for PS(252 K)-b-P4VP(43 K) means that the Mn of PS is 252 000. Dimethylformamide was supplied by Fisher Scientific and used as received. PS-b-P4VP copolymers were dissolved in DMF to obtain 0.1 wt % solutions. After this solution was kept in a sealed vial overnight, water was very slowly added by exposing the solution to moist air. After 36 h of exposure, the total amount of water in solution was about 5.0 wt %. Transmission Electron Microscopy. A drop (about 5 μL) of 0.1 wt % PS-b-P4VP in DMF solution was placed onto thin carbon-coated 400-mesh copper grids that had been glow-discharged for 20 s to render them hydrophilic. After 1 min, the excess sample solution was blotted away with filter paper. Next, the grid was dried at room temperature under vacuum for 12 h and stained by exposure to iodine vapor for 3 h, which selectively stains the P4VP blocks. Stained samples were examined with a Zeiss Libra 200 MC TEM microscope operating at an acceleration voltage of 200 kV. Images were recorded with a Gatan UltraScan 1000XP. The HAADF image was also taken based on the STEM with an accelerating voltage of 200 kV. Dynamic Light Scattering. DLS measurements were taken with a Malvern Zetasizer Nano Series (Nano-ZS, model number zen3500) at 20 °C. Solutions of 0.1 wt % PS-b-P4VP in DMF as a function of time of exposure to moist air were investigated. All sizes obtained from DLS data were collected using 173° backward scattering and averaged over at least five experimental runs, each of which was averaged over 12−13 time correlations fitted by the in-built software of the Zetasizer.

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors acknowledge the National Science Foundation (EPS-1004083) as well as the Sustainable Energy Education and Research Center (SEERC) at The University of TennesseeKnoxville for supporting this work. Z.W.L. and D

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DOI: 10.1021/acsnano.6b00742 ACS Nano XXXX, XXX, XXX−XXX