Letter pubs.acs.org/Langmuir
Tunable Interactions of Polyoxometalate-Based Brushlike Hybrids in Solvents of Variable Quality: From Self-Recognition to Supramolecular Recognition Qian Zhang, Yin Liao, and Weifeng Bu* Key Laboratory of Nonferrous Metals Chemistry and Resources Utilization of Gansu Province, State Key Laboratory of Applied Organic Chemistry, and College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, Gansu, 730000 PR China S Supporting Information *
ABSTRACT: The controllable interactions of a spherical polymer brush modeled by a poly(styrene-b-4-vinylpyridinium methyl iodide)-polyoxometalate composite micelle, SVP-6, with a polyoxometalate-based supramolecular star polymer, PSP-4, in solvents of variable quality allow us to tune their selfassembly behaviors from self-recognition to supramolecular recognition. In the former case, isolated, contractive spheres together with a few vesicles formed by PSP-4 coexist with multimicelle aggregates formed by SVP-6, whereas SVP-6 is hosted inside the vesicle of PSP-4 in the latter case. This work represents an important step toward the development and understanding of programmable self-assembly of brushlike polymers into complex materials.
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INTRODUCTION Spherical polymer brushes (SPBs) have a spherical particle core and a polymer shell in which the polymer chains are grafted to the surface of the particle by one end via a covalent bond.1−7 The flexible tunability in both the spherical particles and grafted chains allows for the preparation of a wide range of organizational SPBs from starlike polymers to polymer brushes on nearly flat substrates. These nanostructures together with their functionalities have resulted in potential applications in the fields of catalysis1,2 and nanocomposite materials.3−7 Furthermore, the intrabrush and interbrush interactions in solvents of various quality have been predicted by theoretical studies.8−12 The interactions between the grafted chains are purely repulsive in good solvents because of an entropic effect, and the resulting grafted chains have a relatively extended conformation. However, when the solvent quality is worsened, van der Waals attractions in terms of an enthalpic contribution appear together with the reducing repulsion, leading to the significant shrinkage of the grafted chains in an isolated SPB through intrabrush attractions and the effective interpenetration of the grafted chains between two SPBs through interbrush attractions. Polyoxometalates (POMs) are anionic nanosized metal oxide clusters with tremendous versatility in their structures and electronic properties.13,14 The electrostatic combination of these anionic metal oxide clusters with cationic polymeric species leads to the formation of POM-based functional hybrids with hierarchical nanostructures, such as nanowires,15,16 micelles,17−20 and vesicles,17,18,20,21 as well as highly ordered hexagons17,18,20 and porous nanocomposites.22,23 These nanostructures are proposed to be directly relevant to the functional performance in catalysis and materials science. Among these © 2013 American Chemical Society
different structures, the spherical POM-based polymeric hybrids have been regarded as a good model system for studying the intrabrush and interbrush interactions of SPBs in solvents of various quality because they can be directly imaged by transmission electron microscopy (TEM) without any other staining.17,18,21 For example, POM-based supramolecular star polymers (PSPs) formed by the electrostatic combination of a Keplerate cluster of [Mo132O372(CH3COO)30(H2O)72]42− (Mo132, d = 3 nm) with quaternary-ammonium-terminated polystyrenes (Sn+) are highly soluble in good solvents chloroform, toluene, and tetrahydrofuran as a result of the purely repulsive intra-PSP and inter-PSP interactions (Figure 1a).21 In this case, the core diameter is much smaller than the unperturbed end-to-end distances of the Sn+ chains (R0 = 0.456n0.595).24 When the solvent quality is weakened by increasing the volume ratio of methanol as a poor solvent in the chloroform/methanol mixtures, the van der Waals attractions appear together with the reducing repulsions, and the discrete PSPs can self-assemble into vesicular aggregates (Figure 1c). POM-based nanocomposites formed by poly(styrene-b-4-vinylpyridinium methyl iodide) (Sn-b-Vm) and POMs ([PW12O40]3− anions) (SVPs) can form spherical micelles and vesicles with a polystyrene corona and a POMbased ionic hybrid core in solvents with very low polarity, such as toluene, chloroform, and dichloromethane (Figure 1d).17,18 Unlike PSPs, the core diameters of SVPs are comparable to or larger than the R0 values of the Sn chains. The polystyrene chains show a relatively stretched conformation in the Received: July 1, 2013 Revised: August 7, 2013 Published: August 8, 2013 10630
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fabricated from an S480-b-V57 molecule with eight [PW12O40]3− anions can form spherical micelles with an ionic core diameter of 32 nm and a hydrodynamic corona thickness of 39 nm in chloroform.17,18 PSP-4 and SVP-6 were first mixed in chloroform, and then methanol was directly added in volume ratios of 33, 43, and 50%, where chloroform and methanol are good and poor solvents for polystyrene, respectively. The concentrations of both PSP-4 and SVP-6 were controlled at 0.33 mg/mL with a weight ratio of 1. Accordingly, the number ratio of the starlike polymer of PSP-4 against the SPB of SVP-6 can be determined to be 95:1 (Supporting Information). The resulting dispersions were allowed to age for 2 weeks and then were subjected to DLS and TEM measurements.
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RESULTS AND DISCUSSION The DLS pattern of the dispersion of PSP-4 and SVP-6 in the chloroform/methanol mixture with a methanol volume ratio of 43% provided a hydrodynamic diameter (Dh) of 116 nm (Figure S1a), which was smaller than those of both PSP-4 (150 nm, Figure S1b) and SVP-6 (230 nm)18 in the same solvent mixtures with the same concentrations. The mixture dispersion was cast onto a carbon-coated copper grid for both BF-TEM (Figure 2a,b) and HAADF-STEM (Figure 2c) observations. In the latter image, the components with heavy elements are bright against the dark carbon matrix background. The BFTEM image revealed spherical nanoparticles with a host−guest form of the SVP-6@PSP-4 vesicle with an occurrence probability of 78%, where a dark core was encapsulated by a dark ring together with two alternating gray polystyrene domains. In these host−guest nanostructures, both tungsten and molybdenum were detected by an energy-dispersive X-ray spectroscopy analysis (Figure S2). The core diameter and ring thickness were determined to be 23 ± 5 and 3 nm, respectively. The thicknesses of the polystyrene corona and stripe between the ring and core were estimated to be 8 ± 4 (Figure 2d) and 11 ± 4 nm (Figure 2e), respectively. This was in sharp contrast to the vesicles (Figure 2f) and multimicelle aggregates (MMAs)18 individually formed by PSP-4 and SVP-6 in the same solvent mixtures with the same concentrations. The average vesicle diameter, corona, and core thicknesses were determined to be 90 ± 30, 8 ± 4, and 3 nm, respectively (Figure 2f). As described previously,18 the total diameter of the MMAs ranged from 120 to 200 nm, and the core diameter, outer corona thickness, and distance between the micellar cores were 22 ± 5, 8, and 8 nm, respectively. With these results in mind, the dark core in the center was therefore assigned to the ionic domain of [PW12O40]3− anions binding to V57 blocks via electrostatic interaction in SVP-6, whereas the dark ring with an average thickness of 3 nm corresponded to the Mo132 cluster in PSP-4. The outer corona thickness (8 nm) was much smaller than the R0 of the S290+ chain (13 nm),24 which was due to the worsening solvent quality of the chloroform/methanol solvent with a methanol volume content of 43% resulting in the presence of strong intra- and inter-PSP-4 van der Waals attractions. The gray stripe between the ring and core was assigned to the polystyrene mixture from both S290+ and S480 chains. Unexpectedly, the average thickness of this stripe (11 nm) was smaller than the sum of the corona thicknesses of S290+ (8 nm) and S480 chains (8 nm) obtained from the vesicle of PSP-4 and the MMA of SVP-6 in the chloroform/methanol solvent containing 43% methanol. Such a phenomenon was reasonably explained by assuming significant interdigitation of the S290+ chains in PSP-4 with the S480 chains in SVP-6,18,25−27 which stemmed from the rather strong van der Waals attractions between the internal brush of the vesicles of PSP-
Figure 1. Schematic drawings of (a−c) PSP-4 and (d−f) SVP-6 and (g−i) a mixture of PSP-4 and SVP-6 with a weight ratio of 1:1 in the chloroform/methanol mixtures with a stepwise increase in the methanol volume ratios.
chloroform/methanol mixtures with low methanol volume ratios as a result of the repulsive interactions.17,18 The increase in the volume ratio of methanol leads to the decrease in solvent quality and thus the direct observation of isolated and oligomeric SPBs and multi-SPB aggregates by TEM imaging, where both the corona and core shrink significantly (Figure 1e,f). The compact corona is assigned to the presence of strong intrabrush van der Waals attractions. In the latter two assembles, the grafted chains fully interdigitate between the POM-based ionic cores, which is a typical feature of interbrush van der Waals attractions. With these theoretical and experimental studies in mind, we ask what happens when mixing SPBs with substantially different core sizes mentioned above in solvents of variable quality. This will provide deeper insight into the intrabrush and interbrush interactions of the repulsions due to an entropic effect and van der Waals attractions due to an enthalpic contribution and thus is very important for exploring SPB-based advanced functional materials. Herein, we report the tunable interactions of an SPB with a PSP and thus their controllable self-assembly behaviors from self-recognition to supramolecular recognition in the chloroform/methanol mixtures with a stepwise increase in the methanol volume ratios (Figure 1g−i).
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EXPERIMENTAL SECTION
Both bright-field TEM (BF-TEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging was performed with an FEI Tecnai F30 operating at 300 kV. Dynamic light scattering (DLS) measurements were carried out on a Brookhaven BI-200SM spectrometer. PSP-4 that has a Mo132 core and 25 S290+ chains can self-assemble into vesicular aggregates in chloroform/methanol mixtures as described previouly.21 SVP-6 10631
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Table 1. Packing Parameters of the Mixture of SVP-6 and PSP-4 in Solvents of Variable Quality volume ratio of methanol (%) 33
43 50
aggregate form SVP-6 MMA PSP-4 sphere PSP-4 vesicle SVP-6@PSP-4 vesicle SVP-6@PSP-4 vesicle PSP-4 sphere PSP-4 vesicle
DCD (nm)a 22 (m) 3 (s) 3 (rt) 22 (m) 3 (rt) 22 (m) 3 (rt) 3 (s) 3 (rt)
DCT (nm)b
DDM (nm)c
8 8 6 8
8 (m)
11 (st)
8
11 (st)
8 6
a
Core diameter of the multimicelle aggregates (MMAs) of SVP-6 (m), isolated sphere of PSP-4 (s), and ring thickness of vesicles of PSP-4 (rt). bCorona thickness. cDistance between the micellar cores in MMAs (m) and the stripe thickness between the ring and core in the host−guest form of SVP-6@PSP-4 vesicle (st).
(78%, Figure S4). In some cases, the average distance between the dark cores was estimated to be 9 ± 4 nm (Figure S3c,f,g−l), which was consistent with the distance between the micellar cores (8 nm) and smaller than the 2-fold outer corona thickness of the MMAs of SVP-6 (8 × 2 = 16 nm) under the same solvent condition. As addressed above and previously,18,25−27 this situation was attributed to the almost full interdigitation of the S480 chains as a result of the strong interbrush van der Waals attractions. However, such interaction showed an occurrence probability of only 10% in the present case, which should be mostly suppressed by the van der Waals attractions of the S480 brush of SVP-6 with the internal S290+ chains in the vesicle of PSP-4. These host−guest nanostructures could be further applied in the nanoreactors for nanoparticle synthesis within a confined space28 and in the preparation of Mo/WOxCy nanocomposite catalysts.29 When the methanol volume content was increased to 50% in the chloroform/methanol solvent, the DLS plot offered two modes with Dh values of 19 and 133 nm after aging for 2 weeks (Figure 3a). This picture was very different from those of both PSP-4 and SVP-6 in the same solvents and with the same concentrations as reported previously, where they showed Dh values of only 12021 and 170 nm,18 respectively. The first Dh at 19 nm was similar to an intermediate state (29 nm)21 in the previous DLS observation of PSP-4 in the chloroform/ methanol solvent containing 33% methanol, which should correspond to isolated spheres of PSP-4. Additional BF-TEM and HAADF-STEM images revealed three aggregate forms (Figure 3b,c and Table 1). (1) Isolated nanoparticles with a diameter of 19 ± 3 nm were observed. This was consistent with the first DLS signal but much smaller than that of dried spheres (33 nm) cast from the chloroform and toluene solutions of PSP-4.21 Considering the diameter of the Mo132 core (3 nm), we estimated the corona thickness to be 8 nm, which was much smaller than the R0 of the S290+ chain (13 nm). Such significant shrinkage was attributed to the worsening solvent quality of the chloroform/methanol solvent with a methanol volume content of 50% resulting in strong intra-PSP van der Waals attractions between the segments of the S290+ chains. This was consistent with previous theoretical studies on starlike polymers in solvents of variable quality.8−12 However, for the first time, we detected isolated, contractive starlike polymers in a solvent of worsening quality by TEM imaging. (2) Vesicles were clearly evidenced by the higher
Figure 2. (a, b) BF-TEM and (c) HAADF-STEM images of the mixture of PSP-4 and SVP-6 (weight ratio 1:1) obtained from the chloroform/methanol mixture containing 43% methanol. The thickness distributions of the (d) polystyrene corona and (e) stripe between the ring and core were counted with 500 encapsulating nanostructures, where the solid lines represented the Gaussian fits (R2 = 0.99) and the average thicknesses were determined to be 8 ± 4 and 11 ± 4 nm, respectively. (f) BF-TEM image of PSP-4 obtained from the chloroform/methanol mixture containing 43% methanol.
4 and the brush of SVP-6. The diameter of the dark core (23 nm) was smaller than those of the micelles of SVP-6 (32 nm) obtained from the chloroform dispersion and chloroform/ methanol mixture with a methanol volume ratio of 9.1%, but completely consistent with those of the isolated and oligomeric micelles and MMAs (22 nm) obtained from the chloroform/ methanol mixtures with methanol volume ratios of 17−67%. In the present case, the core shrinkage should be similarly attributed to the presence of strong intra-SVP-6 van der Waals attractions. The encapsulating nanostructure was further confirmed by the HAADF-STEM imaging (Figure 2c). Parameters such as the diameter of the core, corona thickness, and distance between the ring and core are summarized in Table 1. In addition, one vesicle also hosted 2−5 SPBs of SVP-6 (Figure S3), where the core diameter, corona thickness, and distance between the ring and core were consistent with those listed above. Their occurrence probabilites were counted to be 14, 3.8, 2.5, and 1.0%, respectively, which were much smaller than that of the one-to-one host−guest form mentioned above 10632
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nanostructures were consistent with those obtained from the chloroform/methanol mixtures with a methanol volume ratio of 50%. In the latter nanostructures, the outer corona thickness, micellar core diameter, and distance between the micellar cores were determined to be 8, 22, and 8 nm, respectively, consistent with the values of the MMAs of SVP-6 obtained from the chloroform/methanol mixture with methanol volume ratios of 17−67%.18 However, we did not detect the DLS signal for the isolated, contractive spheres as shown in the TEM image (Figure 3d). In the DLS measurements, a much stronger signal intensity can be obtained from large particles than from small particles.30 That is to say, a small number of large particles can even scatter incident light more strongly than many more small particles. Therefore, the signal for the isolated, contractive spheres of PSP-4 (ca. 20 nm) should be suppressed by the much stronger DLS signals for the vesicle of PSP-4 (77 nm) and the MMA of SVP-6 (290 nm). The low occurrence probability of the vesicular aggregates revealed that the interactions between the starlike polymers are relatively weak, consistent with the situation of PSP-4 obtained from the chloroform/methanol mixture with a methanol volume ratio of 33%.21 However, we did not observe any host−guest nanostructures as addressed above. Such a self-recognition phenomenon should be attributed to the much stronger van der Waals interactions between the brushes of SVP-6 than between the S290+ chains of PSP-4 and between the internal brush of the vesicle of PSP-4 and the brush of SVP-6 under this solvent condition.
Figure 3. (a) DLS plot and (b) BF-TEM and (c) HAADF-STEM images of the mixture of PSP-4 and SVP-6 (weight ratio 1:1) obtained from the chloroform/methanol mixture containing 50% methanol. (d) BF-TEM image of the mixture of PSP-4 and SVP-6 (weight ratio 1:1) obtained from the chloroform/methanol mixture containing 33% methanol.
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CONCLUSIONS We have demonstrated solvent-tunable self-assembly behaviors of PSP-4 with SVP-6 by controlling their intrabrush and interbrush interactions in chloroform/methanol mixtures with variable methanol volume ratios. When the methanol volume ratio is 33%, supramolecular self-recognition occurs in the mixed dilute dispersion of PSP-4 and SVP-6. The former forms isolated, contractive spheres together with a few vesicular aggregates, and the latter self-assembles into MMAs. Further increasing the methanol contents to 43 and 50% leads to supramolecular recognition, where SVP-6 is hosted inside the vesicles of PSP-4 and no MMA is observed. This work represents the first step toward the development and understanding of the hierarchical self-assembly of brushlike macromolecules into complex materials.
transmission in the center than around the peripheral ring. The average vesicle diameter, corona, and core thicknesses were determined to be 60 ± 15, 6 ± 2, and 3 nm, respectively, consistent with the previous TEM result obtained from the chloroform/methanol solvent containing 50% methanol.21 The considerable contraction of the S290+ corona (6 nm) was again attributed to the rather strong van der Waals attractions between polystyrene segments within a PSP-4 molecule and between PSP-4 molecules. (3) The vesicles were filled with 1− 5 SPBs of SVP-6, and their occurrence probabilities were comparable to those obtained in the chloroform/methanol solvent with a methanol volume content of 43% (Figures 3b,c, S5, and S6). The core diameter, stripe, corona, and ring thicknesses were 22 ± 5, 11 ± 5, 9 ± 4, and 3 nm, respectively, consistent with those described in the chloroform/methanol solvent with a methanol volume content of 43%. Such coexistence of the contractive spheres, vesicles, and host− guest nanostructures was in sharp contrast to the picture in the chloroform/methanol mixture with a methanol volume content of 43%. This should be due to the kinetic control of the strong van der Waals interactions within an isolated PSP-4, between PSP-4 molecules, and between PSP-4 and SVP-6 during the preparation process of the chloroform/methanol mixture dispersion with a higher methanol content of 50%. Therefore, the second signal (133 nm) in the DLS plot was due to the presence of the vesicles and host−guest nanostructures. The solvent quality was improved by reducing the methanol volume ratio to 33% in the chloroform/methanol mixture. In its DLS plot, two modes were observed at Dh values of 77 and 290 nm (Figure S7), which were assigned to the vesicles of PSP-4 and the MMAs of SVP-6, respectively. The TEM image revealed the presence of the contractive spheres, vesicles, and MMAs with occurrence probabilities of 85, 5, and 10%, respectively (Figure 3d and Table 1). The former two
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ASSOCIATED CONTENT
S Supporting Information *
Additional DLS and TEM data. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by the NSFC (51173073 and 20931003), the Program for New Century Excellent Talents in University (NCET-10-0462), the Specialized Research Fund for the Doctoral Program of Higher Education (20100211110023), the Fundamental Research Funds for the 10633
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(21) Zhang, Q.; He, L.; Wang, H.; Zhang, C.; Liu, W.; Bu, W. Starlike Supramolecular Polymers Fabricated by a Keplerate Cluster with Cationic Terminated Polymers and their Self-Assembly into Vesicles. Chem. Commun. 2012, 48, 7067−7069. (22) Haimov, A.; Neumann, R. An Example of Lipophiloselectivity: The Preferred Oxidation, in Water, of Hydrophobic 2-Alkanols Catalyzed by a Cross-Linked Polyethyleneimine−Polyoxometalate Catalyst Assembly. J. Am. Chem. Soc. 2006, 128, 15697−15700. (23) Haimov, A.; Cohen, H.; Neumann, R. Alkylated Polyethyleneimine/Polyoxometalate Synzymes as Catalysts for the Oxidation of Hydrophobic Substrates in Water with Hydrogen Peroxide. J. Am. Chem. Soc. 2004, 126, 11762−11763. (24) Förster, S.; Zisenis, M.; Wenz, E.; Antonietti, M. Micellization of Strongly Segregated Block Copolymers. J. Chem. Phys. 1996, 104, 9956−9970. (25) Israelachili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic Press: New York, 1991. (26) Kimizuka, N.; Kawasaki, T.; Hirata, K.; Kunitake, T. Supramolecular Membranes. Spontaneous Assembly of Aqueous Bilayer Membrane via Formation of Hydrogen Bonded Pairs of Melamine and Cyanuric Acid Derivatives. J. Am. Chem. Soc. 1998, 120, 4094−4104. (27) Bu, W.; Li, H.; Li, W.; Wu, L.; Zhai, C.; Wu, Y. SurfactantEncapsulated Europium-Substituted Heteropolyoxotungstates: Structural Characterizations and Photophysical Properties. J. Phys. Chem. B 2004, 108, 12776−12782. (28) Li, H.; Yang, Y.; Wang, Y.; Li, W.; Bi, L.; Wu, L. In Situ Fabrication of Flower-like Gold Nanoparticles in Surfactant-Polyoxometalate-Hybrid Spherical Assemblies. Chem. Commun. 2010, 46, 3750−3752. (29) Lunkenbein, T.; Rosenthal, D.; Otremba, T.; Girgsdies, F.; Li, Z.; Sai, H.; Bojer, C.; Auffermann, G.; Wiesner, U.; Breu, J. Access to Ordered Porous Molybdenum Oxycarbide/Carbon Nanocomposites. Angew. Chem., Int. Ed. 2012, 51, 12892−12896. (30) Liu, G.; Liu, T.; Mal, S. S.; Kortz, U. Wheel-Shaped Polyoxotungstate [Cu20Cl(OH)24(H2O)12(P8W48O184)]25‑ Macroanions Form Supramolecular “Blackberry” Structure in Aqueous Solution. J. Am. Chem. Soc. 2006, 128, 10103−10110.
Central Universities (lzujbky-2012-k14), and the Open Project of the State Key Laboratory of Supramolecular Structure and Materials of Jilin University (sklssm201314).
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