Letter Cite This: ACS Macro Lett. 2019, 8, 363−367
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Capillary Force Driving Directional 1D Assembly of Patchy Colloidal Discs Shuping Zhao,†,‡ Yuanyuan Wu,† Wensheng Lu,§ and Bing Liu*,†,‡ †
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Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, China ‡ University of Chinese Academy of Sciences, Beijing, 100149, China § Key Laboratory of Colloid, Interface and Chemical Thermodynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, China S Supporting Information *
ABSTRACT: Self-assembly from individual colloidal building units to complex superstructures provides a simple yet effective way for the fabrication of functional materials. A rational design of the unit interactions is essential for it to proceed in a desired manner. Here we show that nondirectional capillary force can be used for directional onedimensional (1D) assembly of colloidal discs having a designed patch distribution, and colloidal discs with two liquid patches can assemble into long colloidal chains where the stacked colloidal discs have a well-matched configuration with parallel orientation. The length of the chains can be controlled by controlling the experimental parameters. We also found when liquid patches gradually turn into sticky patches, hydrophobic attraction comes into play and becomes dominant, which can also result in chains with continuously increasing length. This method opens an effective avenue for obtaining colloidal chains (or fibers), which can be adapted for the fabrication of other superstructures.
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attraction between patches, but only a few systems show that the chains formed have both long chain length and welldefined structures.25−27 Solid patches are usually used, and liquid patches have been obtained only in a few cases.28−30 Self-assembly of colloids with liquid patches has rarely been reported. When colloidal particles have a layer of liquid on their surfaces, they can interact by forming capillary bridges among the particles. However, this normally results in irregular aggregates. To turn the aggregates into 1D chain-like structure, an external magnetic field is required.31 When patchy colloids are used, capillary bridging drives the formation of colloidal clusters and other complex assemblies.32,33 Despite these advances, up to date, due to its nondirectional nature, capillary force driving directional 1D assembly of colloidal particles remains a challenge. Here, we show that, with a designed liquid patch distribution, capillary bridging can be used for directional 1D assembly and can drive colloidal particles to form 1D colloidal chains. As a proof of concept, patchy colloidal discs were used. As shown in Figure 1a, patchy PS discs we employed have two types of surfaces, one is the top and bottom surfaces, which are covered by a thin layer of liquid decane, and stabilized by polyvinylpyrrolidone (PVP), and another is the side surface, which is not wetted by decane.34 The decane patches contain the dissolved poly(2-ethylhexyl methacrylate) (PEHMA).
elf-assembly of colloidal building units provides a powerful bottom-up approach for the fabrication of mesoscale materials with hierarchical structures from one dimension (1D) to three dimensions (3D).1−4 In particular, 1D chain structures offer great opportunities for many applications, including a model system for visualized polymer chain physics studies, flexible electronics for wearable devices, chemical sensors, responsive photonic materials, and directed transport of electrons or heat.5−11 Generally, a self-assembled structure is governed by an interplay between colloidal interactions and dynamic self-assembly processes. Unlike 2D or 3D assemblies, which usually can be realized from isotropic colloidal interactions, 1D assembly of colloids usually requires directional interactions and thus is more challenging. A frequently used way for achieving 1D colloidal chains is using an external electric or magnetic field.12−18 These fields can induce a dipole moment inside each particle, thus, leading to an attraction between particles along the field direction. Unfortunately, the formed structures in the fields are not exclusively 1D chains. 2D sheets or 3D crystals are inevitably encountered when the concentration of colloidal particles is high enough.19 In addition, these chains disassociate to individual particles when the fields are turned off. To permanently link the connected particles, extra experimental steps have to be used.6,18 Template-assisted assemblies are also used for 1D chains, and their major limitation is low efficiency.20−24 Patchy colloids can provide directional interactions and, thus, possibly self-assemble into 1D chains by utilizing the © XXXX American Chemical Society
Received: December 20, 2018 Accepted: February 20, 2019
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DOI: 10.1021/acsmacrolett.8b00985 ACS Macro Lett. 2019, 8, 363−367
Letter
ACS Macro Letters
adding of ethanol can remove decane but not PEHMA, therefore, PEHMA retains on the surfaces of PS discs. Meanwhile, after decane is mostly removed, PEHMA decane patches on PS discs should become highly viscous and thus not flowing anymore, so that no halos can be observed at high E/ M/W ratios. That means, only PEHMA cannot make halos, and the halos originate from the liquidity of PEHMA decane patches. Similar halos were observed for the dried θ-shaped PS disc droplets (Figure S4), for which the liquidity of patches were confirmed by optical microscopy (Figure S1a). The nanoscale rough structure on dried PS disc surfaces is from the dried PEHMA (Figure 1c, right). Additionally, we separated the patchy PS discs from the continuous phase by centrifugation and then removed the supernatant as much as we can, and then coated them with a thin layer of SiO2. If decane was on PS disc surfaces, SiO2 should be coated on the surface of decane according to our previous study.30 The following removal of decane will result in the occurrence of hollow chambers. The SiO2-coated PS discs are shown in Figure S5. Two symmetric hollow chambers on the flat surfaces were observed inside each particle, also implying that decane was indeed on PS disc surfaces. The assembly of patchy discs was performed via a regular centrifugation as PS discs are dynamically stable and they do not spontaneously self-assemble (Figure S6). Besides centrifugation can provide an effective compression force to liquid-patch PS discs, the reason we chose centrifugation is that it is a very simple way, and the experimental procedures can be performed by using a regular setup and be done for only a few minutes. It must be mentioned that other means should also work for this assembly as long as it can provide a driving force for the formation of capillary bridging for the approaching PS discs. For example, when sedimentation was used, we found similar assembly behavior (Figures S7−S9). A typical image of the assembled colloidal chains from PS discs was shown in Figure 1d. From the inset, well-matched configuration with parallel orientation can be clearly observed. To search for an optimum centrifugal force (Fc), we investigated the assembly behavior of patchy discs by using different centrifugal speeds. For each Fc, the centrifugal time was determined by the time that all discs traveled to the bottom of the centrifuge tube. By comparing the length of the formed chains and the conversion, we found the optimum centrifugal force is 606g (g, gravitational acceleration), which corresponds a centrifugal speed of 3000 rpm for our setup. In this case, the formed chains have an average length of 23.6 ± 9.1 discs and the conversion is 96.5%. It should be noted that for all centrifugal experiments only well-defined chains formed, and no clusters and jagged chains were observed. Interestingly, we found that the chain length continuously increased by repeated centrifugation for all the centrifugal forces we investigated from 17−6740g (Figure S10). It implied that the formed chains have living ends and can further form longer chains by an interchain reaction. Especially for the case with 606g, the longest colloidal chain we observed consisted of more than 200 discs (Figure S10d). The chain growth and the increase of conversion can be clearly observed from SEM images with an increase in the number of centrifugation (Figure 2a−d). No individual discs can be observed anymore after five repeated centrifugations. We unexpectedly found that the increase of Lchain stopped after 10 repeated centrifugations (Figure 2e), and the reason is possibly due to the unoptimized experimental parameters, such as the initial disc concentration,
Figure 1. (a) Schematic assembly mechanism of PS discs with two liquid patches. (b) Optical microscopy images of typical patchy PS discs (E/M/W = 6.0/1.0/0.2). (c) SEM images of a pure PS disc (left), a PS disc that was shown in (b) (middle), and a PS disc that was obtained at E/M/W = 18.0/1.0/0.2 (right). Half of the halo in the middle image was marked as yellow. (d) SEM image of the representative colloidal chains assembled and the inset is shown in high magnification. Scale bar in the inset of (d) is 2.0 μm.
These PS discs are dynamically stable due to the adsorbed PVP and thus they do not spontaneously self-assemble into chains or other complex structures during stirring. Centrifugation was employed as an external driving force to compress the discs and promote the formation of capillary bridging. Our results show that by this way long colloidal chains can be prepared and PS discs in these long chains have a well-matched configuration with parallel orientation. Moreover, the formed chains can be permanently linked by PEHMA after the liquid decane was removed. To prepare liquid-patch PS discs, θ-shaped PS disc droplets were first prepared by a phase separation process taking place inside decane-swollen PS/PEHMA microspheres (Figure S1a).30,35 Each droplet consists of a PS disc sandwiched by two attached decane droplets. Due to the phase separation, the side surfaces of PS discs are not covered by PEHMA so that they are not wetted by decane.34 The droplets are stabilized by PVP. The dissolved PEHMA is 28.6 wt % by an NMR analysis (Figure S2). The content of decane in the patches was tuned by adding ethanol to the dispersions (Figure S1b,c). In this case, the continuous phase is a mixture of ethanol, methanol, and water. Because ethanol can increase the solubility of decane in the continuous phase, the content of decane on PS discs gradually decreases when more and more ethanol is added. Unfortunately, the decane on PS discs cannot be directly observed from optical microscopy images (Figure 1b). To verify the existence of decane, the PS disc dispersions shown in Figure 1b were directly dropped onto a silicon wafer for SEM observation. PS discs that were extensively washed to remove PEHMA and decane are called “pure PS disc” and shown in Figure 1c, left. In comparison to pure PS discs, as expected, there appears a halo of rougher structure, half of which is marked as yellow, surrounding the PS disc (Figure 1c, middle). This phenomenon can be clearly observed for the ratios of ethanol/methanol/water (E/M/W) less than and equal to 6.0/1/0.2 (Figure S3). When more ethanol was added, for example, for the E/M/W ratio is 18.0/1/0.2, no halos can be observed anymore (Figure 1c, right). Because the 364
DOI: 10.1021/acsmacrolett.8b00985 ACS Macro Lett. 2019, 8, 363−367
Letter
ACS Macro Letters
Figure 2. (a−d) SEM images of the formed chains with the increasing number of centrifugations: (a) 1, (b) 3, (c) 5, and (d) 10. (e) The chain length Lchain as a function of the number of centrifugations and the inset shows the results from three parallel measurements. (f) Long colloidal chains formed by rotational mixing of the short chains shown in (d) for 3 days.
the type of centrifuge tubes and the direction of centrifugal force. This can certainly be improved by building an optimum setup. Alternatively, the formed chains can self-assemble into longer chains by utilizing hydrophobic attraction partially replacing capillary attraction. The hydrophobic attraction was realized by gradually adding more fresh ethanol during the repeated centrifugation so that decane can be gradually removed from the liquid patches and, thus, liquid patches turned into highly viscous sticky patches. When the hydrophobic attraction comes into play, the formed short chains via capillary force can further form longer chains driven by hydrophobic attraction and this process of chain growth can spontaneously perform without the need of centrifugation. By this way, long colloidal fibers with an average length of 251 ± 94 discs was achieved (Figures 2f and S11). The 1D assembly process of patchy PS discs can be understood by capillary attraction. Because the patches are liquid PEHMA decane solution, they form a capillary bridging when two PS discs approach with the parallel orientations. To verify if the assembly is governed by capillary attraction and exclude van der Waals attraction, we did a model calculation by comparing the two forces as a function of the separation between two PS discs (please see Supporting Information for the details of calculations and Figures S12 and S13). It was found that the van der Waals force was dominant over capillary force only when the separation was smaller than 15 nm even with the lowest interfacial tension (0.001 mN/m) we used (Figure 3a). This separation is obviously less than the thickness of the liquid patches. The interfacial tension in experiments was estimated to be the order of several tens μN/ m by following a reported method.36 Higher interfacial tension further shifts the crossover to smaller separation. This indicates the assembly is indeed governed by capillary force. One of the merits through capillary force driving assembly is that the discs can almost perfectly match in the formed chains, which is due to the equal wetting ability of liquid patches on the top- and bottom- surfaces of PS discs and the deformable ability of liquid patches (Figure 3b). When decane is removed from the liquid patches, capillary attraction will be gradually replaced by hydrophobic interaction. As the same with capillary attraction, hydrophobic attraction can be used to further increase the length of short chains, as the case shown in
Figure 3. (a) Comparison of capillary and van der Waals attraction between two parallel discs. (b) Schematic showing capillary force can lead to two connected discs with well-matched configuration. (c, d) Effect of hydrophobic force on the morphology of the chains, E/M/W = 6.0/1.0/0.2 (c1, d1), 30.0/1.0/0.2 (c2, d2), and 120.0/1.0/0.2 (c3, d3).
Figure 2f. However, it is important to control the amount of decane in the patches, because too little liquid in the patches increases the probability of a mismatch between two short chains. For example, in Figures 3c,d and S14, we compared the formed chains and found that the chains became more jagged when more ethanol was added into the system, and thus, the system is governed by hydrophobic interaction. 1D assembly of patchy colloids is regarded as a colloidal analog of step-growth polymerization of polymers because they have similar reaction dynamics.7,37 Two-patch colloids are bifunctional and can polymerize into linear chains, and multipatch colloids, being similar to multifunctional molecular cross-linker, have a potential to cross-link linear chains to form more complex colloidal networks. However, the synthesis of multipatch colloids is never simple. Instead, we found that patchy PS discs, only with larger diameters, can serve as crosslinker for the forming of branch-like colloidal chains and colloidal networks. As an example, we mixed about 7.5% of large discs (2.56 ± 0.09 μm or 4.28 ± 0.10 μm) into 95% of small discs (1.90 ± 0.04 μm) and performed 1D assembly along a similar procedure. Both branch-like colloidal chains and colloidal networks were observed (Figure 4). The functionality of the colloidal cross-linkers relies on the ratio of the diameters. When the ratio is 1.35, the effective functionality is 2−4, and when it is 2.25, the effective functionality is 2−6. In conclusion, we have developed a simple yet effective method for the preparation of colloidal chains by employing PS discs with two liquid patches and demonstrated that an attractive capillary force can drive colloids for directional 1D assembly. In particular, the use of liquid patches ensures that the stacked colloidal discs have a well-matched configuration with parallel orientation. When liquid patches turn into sticky 365
DOI: 10.1021/acsmacrolett.8b00985 ACS Macro Lett. 2019, 8, 363−367
Letter
ACS Macro Letters
NSFC (21774136), the Hundred Talents Program of ICCAS, and the 1000 Young Talents program of China.
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Figure 4. SEM images of branch-like colloidal chains and colloidal networks. The ratio of the diameters is 1.35 (a−d) and 2.25 (e−i). The red lines represent larger discs and the yellow dash lines represent the chains formed by smaller discs, and the numbers in white are the effective functionality. Scale bars are 2 μm in all images.
patches, hydrophobic attraction comes into play and becomes dominant, which results in chains of jagged morphology. This study enables the possibility of preparing polymer fibers from colloidal self-assembly route without external fields and points to the direction of engineering new distribution of liquid patches for colloids to fabricate more complex colloidal superstructures.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.8b00985. Materials, synthetic and assembly methods, characterization, theoretical calculations, and Figures S1−S14 (PDF).
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REFERENCES
(1) Whitesides, G. M.; Grzybowski, B. Self-assembly at all scales. Science 2002, 295, 2418−2421. (2) Glotzer, S. C.; Solomon, M. J. Anisotropy of building blocks and their assembly into complex structures. Nat. Mater. 2007, 6, 557−562. (3) Cademartiri, L.; Bishop, K. J. Programmable self-assembly. Nat. Mater. 2015, 14, 2−9. (4) Kruglova, O.; Demeyer, P.-J.; Zhong, K.; Zhou, Y.; Clays, K. Wonders of colloidal assembly. Soft Matter 2013, 9, 9072−9087. (5) Kuei, S.; Garza, B.; Biswal, S. L. From strings to coils: Rotational dynamics of DNA-linked colloidal chains. Phys. Rev. Fluids 2017, 2, 104102. (6) Vutukuri, H. R.; Demirors, A. F.; Peng, B.; van Oostrum, P. D.; Imhof, A.; van Blaaderen, A. Colloidal analogues of charged and uncharged polymer chains with tunable stiffness. Angew. Chem., Int. Ed. 2012, 51, 11249−11253. (7) Liu, K.; Nie, Z.; Zhao, N.; Li, W.; Rubinstein, M.; Kumacheva, E. Step-growth polymerization of inorganic nanoparticles. Science 2010, 329, 197−200. (8) Luo, W.; Cui, Q.; Fang, K.; Chen, K.; Ma, H.; Guan, J. Responsive hydrogel-based photonic nanochains for microenvironment sensing and imaging in real time and high resolution. Nano Lett. 2018, DOI: 10.1021/acs.nanolett.7b04218. (9) Su, M.; Li, F.; Chen, S.; Huang, Z.; Qin, M.; Li, W.; Zhang, X.; Song, Y. Nanoparticle based curve arrays for multirecognition flexible electronics. Adv. Mater. 2016, 28, 1369−1374. (10) Solis, D., Jr.; Willingham, B.; Nauert, S. L.; Slaughter, L. S.; Olson, L.; Swanglap, P.; Paul, A.; Chang, W. S.; Link, S. Electromagnetic energy transport in nanoparticle chains via dark plasmon modes. Nano Lett. 2012, 12, 1349−1353. (11) Hu, Y.; He, L.; Yin, Y. Magnetically responsive photonic nanochains. Angew. Chem., Int. Ed. 2011, 50, 3747−3750. (12) Townsend, J.; Burtovyy, R.; Galabura, Y.; Luzinov, I. Flexible chains of ferromagnetic nanoparticles. ACS Nano 2014, 8, 6970− 6978. (13) Byrom, J.; Han, P.; Savory, M.; Biswal, S. L. Directing assembly of DNA-coated colloids with magnetic fields to generate rigid, semiflexible, and flexible chains. Langmuir 2014, 30, 9045−9052. (14) Bharti, B.; Findenegg, G. H.; Velev, O. D. Co-assembly of oppositely charged particles into linear clusters and chains of controllable length. Sci. Rep. 2012, 2, 1004. (15) Amali, A. J.; Saravanan, P.; Rana, R. K. Tailored anisotropic magnetic chain structures hierarchically assembled from magnetoresponsive and fluorescent components. Angew. Chem., Int. Ed. 2011, 50, 1318−1321. (16) Zhou, J.; Meng, L.; Feng, X.; Zhang, X.; Lu, Q. One-pot synthesis of highly magnetically sensitive nanochains coated with a highly cross-linked and biocompatible polymer. Angew. Chem., Int. Ed. 2010, 49, 8476−8479. (17) Benkoski, J. J.; Bowles, S. E.; Korth, B. D.; Jones, R. L.; Douglas, J. F.; Karim, A.; Pyun, J. Field induced formation of mesoscopic polymer chains from functional ferromagnetic colloids. J. Am. Chem. Soc. 2007, 129, 6291−6297. (18) Bannwarth, M. B.; Kazer, S. W.; Ulrich, S.; Glasser, G.; Crespy, D.; Landfester, K. Well-defined nanofibers with tunable morphology from spherical colloidal building blocks. Angew. Chem., Int. Ed. 2013, 52, 10107−10111. (19) Dassanayake, U.; Fraden, S.; van Blaaderen, A. Structure of electrorheological fluids. J. Chem. Phys. 2000, 112, 3851−3858. (20) Schürings, M.-P.; Nevskyi, O.; Eliasch, K.; Michel, A.-K.; Liu, B.; Pich, A.; Böker, A.; von Plessen, G.; Wöll, D. Diffusive motion of linear microgel assemblies in solution. Polymers 2016, 8, 413. (21) Zhang, X.; Lv, L.; Ji, L.; Guo, G.; Liu, L.; Han, D.; Wang, B.; Tu, Y.; Hu, J.; Yang, D.; Dong, A. Self-assembly of one-dimensional nanocrystal superlattice chains mediated by molecular clusters. J. Am. Chem. Soc. 2016, 138, 3290−3293.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Wensheng Lu: 0000-0002-5822-0909 Bing Liu: 0000-0003-4297-157X Author Contributions
All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge Prof. Zhenzhong Yang in Tsinghua Univ., Prof. Hongxia Guo in ICCAS, and Prof. Xinhua Zhang in BJTU for helpful discussions. The research was supported by 366
DOI: 10.1021/acsmacrolett.8b00985 ACS Macro Lett. 2019, 8, 363−367
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ACS Macro Letters (22) Xia, Y.; Yin, Y.; Lu, Y.; McLellan, J. Template-assisted selfassembly of spherical colloids into complex and controllable structures. Adv. Funct. Mater. 2003, 13, 907−918. (23) Yin, Y.; Xia, Y. Self-assembly of spherical colloids into helical chains with well-controlled handedness. J. Am. Chem. Soc. 2003, 125, 2048−2049. (24) Jiang, L.; de Folter, J. W.; Huang, J.; Philipse, A. P.; Kegel, W. K.; Petukhov, A. V. Helical colloidal sphere structures through thermo-reversible co-assembly with molecular microtubes. Angew. Chem., Int. Ed. 2013, 52, 3364−3368. (25) Groschel, A. H.; Walther, A.; Lobling, T. I.; Schacher, F. H.; Schmalz, H.; Muller, A. H. Guided hierarchical co-assembly of soft patchy nanoparticles. Nature 2013, 503, 247−251. (26) Nie, Z.; Fava, D.; Kumacheva, E.; Zou, S.; Walker, G. C.; Rubinstein, M. Self-assembly of metal-polymer analogues of amphiphilic triblock copolymers. Nat. Mater. 2007, 6, 609−614. (27) Tigges, T.; Heuser, T.; Tiwari, R.; Walther, A. 3D DNA origami cuboids as monodisperse patchy nanoparticles for switchable hierarchical self-assembly. Nano Lett. 2016, 16, 7870−7874. (28) Kraft, D. J.; Vlug, W. S.; van Kats, C. M.; van Blaaderen, A.; Imhof, A.; Kegel, W. K. Self-assembly of colloids with liquid protrusions. J. Am. Chem. Soc. 2009, 131, 1182−1186. (29) Sacanna, S.; Korpics, M.; Rodriguez, K.; Colon-Melendez, L.; Kim, S. H.; Pine, D. J.; Yi, G. R. Shaping colloids for self-assembly. Nat. Commun. 2013, 4, 1688. (30) Luo, Z.; Liu, B. Shape-tunable colloids from structured liquid droplet templates. Angew. Chem., Int. Ed. 2018, 57, 4940−4945. (31) Bharti, B.; Fameau, A. L.; Rubinstein, M.; Velev, O. D. Nanocapillarity-mediated magnetic assembly of nanoparticles into ultraflexible filaments and reconfigurable networks. Nat. Mater. 2015, 14, 1104−1109. (32) Bharti, B.; Rutkowski, D.; Han, K.; Kumar, A. U.; Hall, C. K.; Velev, O. D. Capillary bridging as a tool for assembling discrete clusters of patchy particles. J. Am. Chem. Soc. 2016, 138, 14948− 14953. (33) Zheng, X.; Liu, M.; He, M.; Pine, D. J.; Weck, M. Shape-shifting patchy particles. Angew. Chem., Int. Ed. 2017, 56, 5507−5511. (34) Wu, Y.; Luo, Z.; Liu, B.; Yang, Z. Colloidal rings by siteselective growth on patchy colloidal disc templates. Angew. Chem., Int. Ed. 2017, 56, 9807−9811. (35) Fujibayashi, T.; Okubo, M. Preparation and thermodynamic stability of micron-sized, monodisperse composite polymer particles of disc-like shapes by seeded dispersion polymerization. Langmuir 2007, 23, 7958−7962. (36) Vis, M.; Blokhuis, E. M.; Erne, B. H.; Tromp, R. H.; Lekkerkerker, H. N. W. Interfacial tension of phase-separated polydisperse mixed polymer solutions. J. Phys. Chem. B 2018, 122, 3354−3362. (37) Pavlopoulos, N. G.; Dubose, J. T.; Hartnett, E. D.; Char, K.; Pyun, J. Colloidal random terpolymers: controlling reactivity ratios of colloidal comonomers via metal tipping. ACS Macro Lett. 2016, 5, 950−954.
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DOI: 10.1021/acsmacrolett.8b00985 ACS Macro Lett. 2019, 8, 363−367
