Self-Assembly of Polyoxometalate-Based Starlike Polymers in

Article pubs.acs.org/Macromolecules

Self-Assembly of Polyoxometalate-Based Starlike Polymers in Solvents of Variable Quality: From Free-Standing Sheet to Vesicle Yin Liao, Nijuan Liu, Qian Zhang, 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, China S Supporting Information *

ABSTRACT: Polyoxometalate-based supramolecular star polymers, where an anionic polyoxometalate is closely encapsulated by a hydrophobic shell of cation terminated polystyrenes, are conceptually regarded as a model of star polymers to study their intra- and interstar interactions and self-assembled behaviors in solvents of variable quality. These model stars can self-assemble to form unilamellar freestanding sheets in the toluene/methanol mixture solvent with a methanol volume ratio of 50%. An increase in the methanol content to 75% or 80% results in a coexistence of free-standing sheets with vesicles, where several intermediates are captured for the morphological evolution from sheet to vesicle. With further increasing methanol content to 90%, pure vesicle phases form with unilamellar or oligolamellar features. All these aggregates are packed by the totally hydrophobic starlike polymers, where the core−shell structures are retained. This is distinctly different from the bilayered nanostructures formed by amphiphiles such as surfactants and block copolymers in selective solvents.

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INTRODUCTION

in the context of the solution-based applications mentioned above. On the other hand, polyoxometalates (POMs) are anionic metal oxide clusters with nanoscaled sizes ranging from 0.5 to 6 nm, and their structural and electronic versatilities have been well documented.7−9 The negative charges of POMs allow an electrostatic self-assembly with polymeric cations to fabricate POM-based polymeric functional composites with desired hierarchical nanostructures.10−19 The introduction of POMs allows these nanostructures to be directly imaged by transmission electron microscopy (TEM) without any other staining. Therefore, the core−shell structure, where the POM anion core is closely covered by a shell of cation terminated polymers, can be regarded as an easily obtained model of star polymers to study their intra- and interstar interactions and self-assembled behaviors in the real solutions with worsening quality. In a recent preliminary report,16 we described the preparation of POM-based supramolecular star polymers (PSPs) by electrostatic combination of the nanosized POMs with quaternary ammonium terminated polystyrenes (Sn+) and their self-assembly into unilamellar vesicles in the chloroform/ methanol mixture solvents. In this case, chloroform and methanol are good and poor solvents of polystyrene, respectively. These vesicular nanostructures are completely different from those theoretical predictions,1,5,6 in which only single- and twostar systems are considered, as noted above. Therefore, the

Star polymers consist of more than three linear arms covalently connected onto a central core by one end.1−4 They have topologically branched nanostructures and thus lower viscosities in solution and melt states in comparison to their linear analogues with the same molecular weights. Potential applications have been proposed for star polymers in materials science and catalysis.1−4 These solution-processable applications require a full understanding of interactions between the segments of grafted chains within a single star polymer, between two stars, and among more stars in solvents of variable quality. As already addressed by theoretical studies,1,5,6 in good solvents, purely repulsive interactions occur between the segments of grafted chains within a single isolated star polymer due to entropic effects. This leads to a swollen-coil conformation of the grafted chains there. When the solvent quality is worsened, van der Waals attractions increase along with the reducing repulsions. The resulting polymer chains shrink considerably. Star polymers collapse finally into a compact globule in poor solvents. Similarly, the corresponding interactions between two stars are purely repulsive in good solvents, while the decrease in solvent quality leads to the presence of van der Waals attractions with the repulsions being reduced. In addition to the comparable chain shrinkage to that in a single and isolated star, the grafted chains interpenetrate effectively between two stars as a result of interstar van der Waals attractions.6 To the best of our knowledge, the experimental study of star polymers in the real solutions with tunable solvent qualities has not been addressed. However, this is of fundamental and practical interest © XXXX American Chemical Society

Received: June 30, 2014 Revised: September 21, 2014

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dx.doi.org/10.1021/ma501343s | Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. Schematic drawings of self-assembly of PSP-1 (a−d) and PSP-2 (e−h) from free-standing sheet to vesicle in the toluene/methanol mixtures with the stepwise increase in the methanol volume ratios.

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RESULTS AND DISCUSSION Self-Assembly of PSP-1 into Free-Standing Sheet. In PSP-1 (Figure 1a),16 a Keplerate cluster of [Mo132O372(CH3COO)30(H2O)72]42− (Mo132, d = 2.8 nm)31 was closely tethered by a hydrophobic shell of 25 S290+ chains.32 PSP-1 was first dissolved in toluene, and then methanol was directly added with a volume ratio of 50%, where its final concentration was controlled at 0.33 mg/mL. The resulting solution was found to be opaque, which was then cast onto a copper grid coated with a perforated polymer membrane for TEM observation. The holes in the polymer membrane have diameters ranging from 1 to 5 μm. A typical bright-field TEM (BF-TEM) image of PSP-1 revealed a free-standing sheet with a planar size of a few micrometers in a pore (Figure 2a). A magnified TEM image indicated that the sheet was extremely smooth and uniform without any defects and pinholes observed (Figure 2b). However, the Mo132 cluster was not seen in the BF-TEM images. The energydispersive X-ray (EDX) spectrum (Figure 2c) indicated that molybdenum did appear in the sheet. The invisible Mo132 cluster in the BF-TEM images should be due to the really comparable electron density of the contractive polystyrene shell with Mo132 and/or a low mass concentration of Mo132 in PSP-1 (2.87 wt %).16,17 To confirm the presence of the Mo132 cluster, the free-standing sheet was further examined by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), in which the components with heavy elements appear bright against the dark carbon matrix background. In the corresponding HAADF-STEM image (Figure 2d), isolated white spots were clearly observed with a diameter of ca. 3 nm, consistent with the cluster size of Mo132.31 The inbetween areas were accordingly assigned to the contractive S290+ chains. By measuring more than 350 spacing intervals between the Mo132 clusters in the HAADF-STEM images, the average length of the S290+ chains was determined to be 3 ± 1 nm (Figure 2d,e and Table 1). The free-standing sheets were really flexible and readily curved. The resultant curling up allowed us to directly observe the cross-sectional area and thus

vesicle formation has been tentatively attributed to a delicate balance between van der Waals attractive forces and repulsive entropic effects within a single PSP and among PSPs in the solvent mixtures with worsening quality.16 That is to say, manystar interactions actually occur in the real solutions with variable solvent quality even under dilute conditions. Spherical polymer brushes (SPBs) are analogues of star polymers but with a much larger core size.20,21 The electrostatic self-assembly of Na3[α-PW12O40] with poly(styrene-b-4-vinylpyridinium methyl iodide) (Sn-b-Vm) leads to the formation of spherical micelles and vesicles in toluene.13 These spherical nanoobjects have an Sn corona and a hybrid ionic core filled with [α-PW12O40]3−, iodides, and nitrates binding to Vm blocks via Coulomb force, which can be conceptually described as a model system for SPBs.14,17 Of difference is that these model SPBs self-assemble to form multimicelle aggregates (MMAs), micelle-like nanostructures in the chloroform/methanol mixture solvents. Therefore, the formation of vesicles and not just micelle-like nanostructures for PSPs implies that the many-star interactions are anisotropic to a certain degree in the chloroform/ methanol mixture solvents. These experimental results together with the previous theoretical works1,5,6 further prompt us to explore hierarchical self-assembly of PSPs induced by the anisotropic many-star interactions in solvents of varying quality. Herein, we report controllable self-assembly of completely hydrophobic PSPs from unilamellar free-standing sheet to uni- or oligolamellar vesicle in the toluene/methanol mixtures with the stepwise increase in the methanol volume ratios (Figure 1). Furthermore, several intermediates between the sheet and vesicle are carefully captured by TEM that provides important mechanistic insights for this morphological transition. In these assemblies, PSPs maintain their core−shell structures. Such a picture is distinctly different from the bilayered nanostructures formed by amphiphilic molecules such as surfactants and block copolymers due to a hydrophilic−hydrophobic character, where the hydrophilic heads interact with the water and the hydrophobic chains contact each other.22−30 B


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Figure 2. BF-TEM images (a, b) revealed that PSP-1 formed free-standing sheets in the toluene/methanol mixture with a methanol volume ratio of 50%. The EDX spectrum (c) of the sheet confirmed the presence of molybdenum. The Mo132 clusters were clearly observed in the HAADF-STEM image (d). The length of the S290+ chains between the Mo132 clusters was estimated to be 3 ± 1 nm by counting more than 350 spacing intervals (e). The flexibility and curling up of the sheets were imaged by both BF-TEM (f, g) and SEM (h, i). The cross-sectional areas as shown by the arrows suggested that the sheet occupied a thickness of 10 nm.

the arrows in Figure 2i, the thickness of the free-standing film was determined to be 17 ± 2 nm. Considering the total thickness of gold layers deposited on both surfaces (8 nm), the free-standing sheet occupied a thickness of about 9 nm. Within the limits of experimental error, this value was consistent with the TEMimaging thickness listed above. By subtracting the diameter of Mo132 (2.8 nm), the corona thickness of the S290+ chains was estimated to be approximately 3.6 nm, which was much smaller than the unperturbed end-toend distance of the S290+ chain (R0 = 0.456n0.595 = 13 nm).33 Such remarkable shrinkage was similar to the situation of the unilamellar vesicle of PSP-1 obtained from the chloroform/ methanol mixture containing 50% methanol, where the corona thickness was 6 nm.16,17 Such significant shrinkage was assigned to the presence of intra-PSP-1 van der Waals attractive interactions between the segments of the S290+ chains that originated from the weakening solvent quality with a methanol volume ratio of 50% in chloroform or toluene. Unexpectedly, the horizontal length of the S290+ chains (3 nm) between the Mo132 clusters was almost consistent with their vertical corona thickness (3.6 nm). This unusual picture could be reasonably documented as a result of the full interdigitation of the S290+ chains (Figure 1b), which was a typical feature of inter-PSP-1 van der Waals attractive interactions in solvents of weakening solvent quality. Similar interpenetration between the grafted chains of SPBs and between the internal chains of the vesicle of PSP-1 and the grafted chains of SPBs has been well addressed

Table 1. Packing Parameters (in nm) of Free-Standing Sheets and Vesicles Obtained from the Toluene/Methanol Mixtures with Methanol Volume Ratios of 50% and 90% vol ratio of methanol

50%

90%

free-standing sheet

vesicle

sample

Dca

Tsb

Tcsc

Tid

Dve

Trf

Tcvg

Nah

PSP-1 PSP-2

2.8 1.04

10 8

3.6 3.5

3 3.5

260 51

3 1

45 6.4

59

a

Diameter of the clusters. bThickness of sheets. cCalculated corona thickness of sheets. dSpacing intervals between the clusters. eDiameter of vesicles. fRing thickness of vesicles. gCorona thickness of vesicles. h Aggregation number.

determine the sheet thickness (Figure 2f,g). The observed thickness was 10 ± 2 nm. The surface morphology of the free-standing sheet was further examined with scanning electron microscopy (SEM). To avoid electric charging and obtain high quality SEM images, a 4 nm thick gold layer was deposited onto the free-standing sheet by using a mild ion-sputter. The resulting images showed that the free-standing sheet was uniform and flexible (Figure 2h,i), which agreed well with the above-mentioned TEM observations. The latter feature also allowed us to capture the cross-sectional area of the free-standing sheet during the SEM imaging process and thus to observe the sheet thickness directly. As indicated by C


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Figure 3. TEM images of PSP-1 obtained from the toluene/methanol mixture solvent with a methanol volume of 25% revealed an occasional sheet (a) together with a large number of single PSP-1 (b, c).

by TEM imaging.14,17 Such interdigitated chains originated from interbrush van der Waals attractions in the solvents of weakening quality, which were in principle consistent with our present case. Therefore, PSP-1 self-assembled to form unilamellar free-standing sheet, where the Keplerate cluster was closely encapsulated with a shell of the cationic terminated polystyrenes. The contractive and interpenetrated S290+ chains could be conceptually described as the intra- and interstar van der Waals attractions, respectively, in the solvents of worsening quality. Such observations were essentially consistent with the previously theoretical insights of star polymers in solvents of variable quality as briefly noted in the Introduction.1,5,6 The unilamellar free-standing sheets with almost the same thicknesses were also observed when the volume ratios of methanol decreased and increased respectively to 43% and 67% in the toluene/methanol mixture solvents (Figures S1 and S2). Upon decreasing the methanol volume content to 25%, the BFTEM images of PSP-1 showed that the free-standing sheet was rarely observed (Figure 3a) and coexisted with a large number of dark spots with a diameter of ca. 3 nm (Figure 3b,c). The dark spots were then assigned to the Keplerate cluster as in PSP-1. When the toluene solution of PSP-1 was cast onto a carboncoated copper grid, no free-standing sheet was observed, and only Mo132 clusters were clearly imaged.16 In the toluene/methanol mixture with a methanol volume ratio of 25%, where the solvent quality was slightly weakened, PSP-1 was in the act of selfassembly to form free-standing sheets; the pure sheetlike assemblies were not obtained yet. Further weakening the solvent quality of these solvent mixtures with methanol volume ratios of 43%, 50%, and 67% resulted in the formation of total freestanding sheets (Figure 2, Figures S1 and S2). Combining these collective features with the previously theoretical1,5,6 and experimental16 studies, the formation of free-standing sheets was tentatively attributed to the increase in the intra- and interstar van der Waals attractions together with the reducing repulsions in the toluene/methanol mixture solvents with the stepwise increase in the methanol volume ratios. It should be emphasized that the intra- and interstar van der Waals attractions should be cooperatively anisotropic in a many-star way in the present case. Generalization of This Strategy for Fabricating POMBased Sheets. PSP-2 has a Keggin anion core of [α-SiW12O40]4− (d = 1.04 nm)34 and four S290+ chains (Figure 1e). Both the core size and arm number in PSP-2 are much smaller than in PSP-1. Therefore, PSP-2 was chosen as a building block to generalize the strategy for exploring POM-based sheets. To this end, the solution of PSP-2 in the toluene/methanol mixture solvent containing a methanol volume ratio of 50% (0.33 mg/mL) was cast onto a perforated polymer membrane coated copper grid for TEM observations. The BF-TEM image revealed that PSP-2

packed into the free-standing sheets and the sheet layers were clearly captured on the perforated polymer membrane (Figure 4a). In the sheet, tungsten was detected by the EDX measurement (Figure 4b). Figures 4c and 4d show typical high-resolution BF-TEM and HAADF-STEM images of the sheet, respectively. Both the dark and white spots were clearly recognized with a diameter of 1 nm, consistent with the Keggin anion size. This clear observation should be due to a larger electron density of [SiW12O40]4− than those of both the contractive polystyrene and Mo132 cluster in the free-standing sheet formed by PSP-1. Noticeably, these Keggin-type clusters were isolated. Therefore, the S290+ chains occupied an average length of 3.5 ± 1.8 nm in the in-between areas of the Keggin anions (Figure 4e and Table 1). The sheet thickness was determined to be 8 ± 2 nm from the folded free-standing sheet (Figure 4f), which was further supported by the SEM imaging (Figure 4h,i). The corona thickness of the S290+ chains was therefore calculated to be 3.5 nm by the subtraction of the diameter of the Keggin-type anion (1.04 nm) from the observed thickness of the sheet (8 nm). This value was much smaller than the R0 value of the S290+ chain (13 nm)33 and in complete agreement with that in the free-standing sheet of PSP-1 obtained from the same solvent condition. Therefore, the strong intra-PSP-2 van der Waals attractions under this solvent condition were responsible to the significant shrinkage of the S290+ chains in the free-standing sheet. Again, the horizontal length of the S290+ chains (3.5 nm) between the Keggin anions was consistent with their vertical corona thickness (3.5 nm), which was accordingly attributed to the full interdigitation of the S290+ chains (Figure 1f). This was a typical feature of interstar van der Waals attractions in solvents of weakening solvent quality. Such contractive and interpenetrated S290+ chains were totally consistent with those in the free-standing sheet of PSP-1 obtained from the same solvent condition. Further decrease and increase in the methanol contents to 43%, 67%, and 75% still yielded free-standing sheets with an average thickness of 8 ± 2 nm (Figures S3 and S4), which was consistent with that of PSP-2 obtained from the toluene/ methanol mixture solvent with a methanol volume ratio of 50%. When the solvent quality was improved by the decrease in the methanol volume ratio to 25%, the coexistence of single PSP-2 and sheetlike assemblies was imaged by BF-TEM (Figure S5). Accordingly, the formation of the free-standing sheets was due to the anisotropic intra- and inter-PSP-2 van der Waals attractions in the toluene/methanol mixture solvent with worsening solvent quality as similarly addressed in the case of PSP-1 (vide supra). To be briefly summarized, the free-standing sheets with highly uniform and smooth features could be generally fabricated by self-assembling PSPs in the toluene/methanol mixture solvents with weakening quality. It should be highlighted that D


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Figure 4. Several layers of free-standing sheet formed by PSP-2 were clearly recognized in the BF-TEM image (a) obtained from the toluene/ methanol mixture with a methanol volume ratio of 50%. The EDX signals of tungsten (b) were clearly detected in the sheet. Both BF-TEM (c) and HAADF-STEM images (d) revealed the presence of the clusters of [α-SiW12O40]4−. The S290+ chains between the Keggin anions occupied a length of 3.5 ± 1.8 nm by counting more than 500 spacing intervals (e). The flexibility and curling up of the sheets in both BF-TEM (f, g) and SEM (h, i) yielded the cross-sectional areas with a thickness of 8 nm as shown by the arrows.

quality was further worsened by increasing the methanol volume ratio in the toluene/methanol mixture solvent to increase the van der Waals attractions along with the additionally reducing repulsions. When the methanol volume ratio increased to 75% in the toluene/methanol mixture solvent, the BF-TEM of PSP-1 showed a coexistence of free-standing sheets with vesicles (Figure 5). As shown from the folded part (Figure 5g), the thickness of the sheet was estimated to be 10 ± 2 nm, which agreed well with those values obtained from the toluene/methanol mixture solvents with methanol volume ratios of 43%, 50%, and 67% (vide supra). The vesicular aggregates showed a considerably broad size distribution ranging from 100 to 550 nm. Further scrutiny of these images revealed that four forms of the vesicles occurred. (1) Some of vesicles were actually embedded into the free-standing sheets, where the interface between the vesicles and sheets was rather sharp as shown in the BF-TEM images (Figure 5a−d). (2) Several embedded vesicles were protruding from the free-standing sheets but were not yet detached completely (Figure 5b−d). (3) Complete detachment from the sheets led finally to the formation of isolated vesicles (Figure 5e−g). The vesicular character was clearly evidenced by the higher transmission in the center than around the peripheral ring. The average thickness of the darker rings was estimated to be ca. 3 nm, consistent with the diameter of Mo132. The gray corona domains were accordingly assigned to the S290+ chains with an average thickness of 7 nm. This situation was totally consistent

the sheetlike assemblies appeared as wrinkled platelets and were usually folded into bilayers (Figures 2−4 and Figures S1−S5). These observations demonstrated that the free-standing sheets formed in the toluene/methanol mixture solvents rather than on the substrate during the drying process. Intermediates of Morphological Evolution from FreeStanding Sheet to Vesicle. As described previously,16,17 PSP-1 underwent a self-assembly process to form a pure vesicle phase with unilamellar nature in the chloroform/methanol mixture solvents with methanol volume ratios of ≥43%. Similar vesicular aggregates of PSP-2 were also observed with TEM imaging under these solvent conditions (Figure S6). However, the unilamellar free-standing sheets were formed by both PSP-1 and PSP-2 in the toluene/methanol mixture solvents with methanol volume ratios of 43%, 50%, and 67%. This sharp contrast was unexpected completely. We inferred that the solvent molecules (toluene and chloroform) were presumably involved in the hierarchically self-assembled processes of these starlike polymers in the solvents of worsening quality, leading to the larger anisotropic degree of the van der Waals attractive forces in the toluene/methanol mixture solvents than in the chloroform/ methanol mixture solvents. These unilamellar nanostructures together with the easy wrinkledness of the sheet raised an important question concerning a solvent-tunable morphological transformation from sheet to vesicle. To this aim, the wrap-up of sheets to form vesicles required more energy penalty than the formation of the edges of lamellae.35−37 Therefore, the solvent E


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Figure 5. BF-TEM images of PSP-1 obtained from the toluene/methanol mixture with a methanol volume ratio of 75% revealed several intermediates of morphological evolution from free-standing sheets to vesicles, including embedded vesicles into sheets (a−d), vesicles being protruding from sheets but not yet completely detached (b−d), and isolated vesicles from sheets (e−g) as well as vesicle networks with small-size sheets embedded there (d, h, and i).

vesicles formed by PSP-1 than formed by PSP-2 under the corresponding solvent conditions. This matched well with the formation of vesicle networks in the former case. As a brief summary, all these TEM images showed that the free-standing sheets of PSP-1 and PSP-2 were captured in the act of wrap-up to form vesicles; they had not yet resulted in a pure vesicle phase under the present solvent conditions (Figures 1c,g and 5 and Figure S7). Such intermediate states were in sharp contrast to the pure sheet phase in those better solvents. Under these worse solvent conditions, the freestanding sheets had a strong tendency to reduce their own surface energies and energy costs and thus minimize the contacting areas of the stars with the solvent mixtures. Consequently, the morphological evolution occurred from sheet to vesicle. We therefore inferred that the anisotropic degree of the intra- and interstar van der Waals attractions decreased with the increasing methanol contents in the toluene/methanol mixture solvents. The additional increase in the methanol volume ratio of the toluene/methanol mixtures would further reduce the surface energies and energy costs, leading to the final formation of the pure vesicle phase, where the anisotropic degree of the van der Waals attractions was further reduced. Self-Assembly of PSP-1 and PSP-2 To Form Pure Vesicle Phases. When the methanol volume ratio was further increased to 90%, PSP-2 formed a pure vesicle phase. The vesicular feature was clearly evidenced by the higher transmission in the center than around the peripheral ring (Figure 6a,b). The vesicle occupied an average diameter of 51 ± 10 nm, and this value was further corroborated by SEM imaging (Figure S8a). The average thickness of the darker rings was estimated to

with the unilamellar vesicles formed by packing a single layer of PSP-1 in the chloroform/methanol mixture solvents.16,17 It should be highlighted again that in these vesicles and sheets the starlike supramolecular polymers still held a core−shell structures, in which the Keplerate cluster was closely encapsulated with a hydrophobic shell of quaternary ammonium terminated polystyrenes. (4) The vesicles were docked together or interdigitated significantly, leading to the formation of vesicle networks together with several small-size sheets embedded there (Figure 5d,h,i). This should be attributed to substantial contacting or interdigitating of the S290+ chains as a result of the strong van der Waals attractions occurring between the vesicular coronas in the toluene/methanol mixture solvent with a methanol volume ratio of 75%. Such vesicle networks were morphologically similar to docking or multicompartment vesicles occurring in the process of vesicle fusion formed by amphiphiles and others.38−42 In the case of PSP-2, the coexistence of free-standing sheets with vesicles was also captured by TEM imaging but with a methanol volume ratio up to 80% in the toluene/methanol mixture solvent. Both embedded vesicles into the sheet and isolated vesicles from the sheet were clearly recognized in the TEM images (Figure S7). In addition to this observation, several vesicles were breaking away from that sheet, but not yet completely (Figure S7a,b). However, the vesicle networks formed by PSP-1 were not detected in the present case. It should be stressed again that PSP-1 has 25 S290+ chains attached onto a Keperalate cluster and PSP-2 has 4 S290+ chains attached onto a Keggin cluster. Therefore, it was reasonably believed that much larger van der Waals attractions occurred among the F


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unilamellar vesicle, we calculated the diameter of this hollow sphere to be 37 nm, yielding a surface area of 4,301 nm2. The aggregation number was consequently calculated to be 59 (Table 1). Under this solvent condition, PSP-1 also formed vesicular aggregates but together with a coexistence of nanospheres observed in the BF-TEM and SEM images (Figure 7a,b, and

Figure 6. Both BF-TEM (a, b) and HAADF-STEM images (d) showed that PSP-2 formed unilamellar vesicles in the toluene/ methanol mixture with a methanol volume ratio of 90%. (c) The unilamellar vesicle had a corona thickness of 6.4 ± 2.6 nm.

be ca. 1 nm, consistent with the diameter of the [SiW12O40]4− anion (1.04 nm). The gray corona domains were assigned to the S290+ chains with a thickness 6.4 ± 2.6 nm (Figure 6c and Table 1). These assignments were further substantiated by HAADF-STEM imaging (Figure 6d). The single Keggin-type clusters were clearly recognized in both the ring and center of the vesicles. The corona thickness was determined to be 7 ± 2 nm, which agreed well with the value obtained from the BF-TEM images. This corona thickness was much smaller than the R0 value of the S290+ chain (13 nm). The contractive S290+ corona was consistent with the strong intra-PSP-2 van der Waals attractions induced by worsening solvent quality of the toluene/ methanol mixture solvent with a methanol volume content of 90%. We therefore estimated the diameter of an isolated and contractive PSP-2 to be 14 nm (6.4 × 2 + 1.04 nm). The inbetween microdomains of the Keggin anions were attributed to the contractive S290+ chains and occupied an average length of 4.3 ± 1.2 nm by carefully analyzing the HAADF-STEM images. This value was slightly smaller than the corona thickness of the unilamellar vesicle. Such a phenomenon was again attributed to the full interdigitation of the contractive S290+ chains originating from the inter-PSP-2 van der Waals attractions at this solvent condition. These data revealed that PSP-2 self-assembled to form the unilamellar vesicles, where the core−shell structure of PSP-2 was retained and the S290+ chains between the Keggin anions were fully interpenetrated (Figure 1h). Such vesicular nature was consistent with that of the unilamellar vesicles of both PSP-1 and PSP-2 achieved in the chloroform/methanol mixture solvents (Figure S6)16,17 or toluene/methanol mixture solvents with methanol volume ratios of 75 or 80% as mentioned above. As already documented above and previously,16,17 the vesicle formation was similarly due to the anisotropic interstar van der Waals attractions but with a less degree relative to those for the sheet fabrication. The stars of PSP-2 were assumed to be uniformly packed with an almost hexagonal closest form on a spherical surface.43 Therefore, the star occupied an average area of 73 nm2 there. By subtracting the diameter (14 nm) of an isolated and contractive star with the total diameter (51 nm) of the

Figure 7. PSP-1 self-assembled to form oligolamellar vesicles in the toluene/methanol mixture with a methanol volume ratio of 90% as verified by both BF-TEM (a, b) and HAADF-STEM images (d). The statistical corona thickness was 45 ± 17 nm (c).

Figure S8b). Their diameters were estimated to be 260 ± 50 and 19 ± 3 nm, respectively. Considering the diameter of the Mo132 core (3 nm), the corona thickness of the nanospheres was calculated to be 8 nm. This value was rather comparable to the corona thicknesses of vesicles obtained from the chloroform/methanol16,17 or toluene/methanol mixture solvents as already addressed above. Therefore, these nanospheres were assigned to the isolated, contractive star-like polymers of PSP-1, which were consequently due to the strong intrastar van der Waals attractions. The isolated and contractive nanospheres of PSP-1 were also observed by BF-TEM imaging from the chloroform/methanol mixture solvent with a methanol volume ratio of 50%.17 These isolated and contractive nanospheres were in principle consistent with the compact globules of star polymers in poor solvents as predicted by the theoretical work.5,6 The occurrence probabilities of the vesicles and nanospheres were calculated to be 90% and 10%, respectively. Considering the much larger diameter of the vesicles than that of the nanospheres, we therefore estimated that more than 99% of PSP-1 self-assembled into the vesicular aggregates. In addition, the corona and ring thicknesses of the vesicles were 45 ± 17 nm (Figure 7c) and 3 nm, respectively. The latter value was again consistent with the cluster size of Mo132. In contrast, this corona thickness was much larger than those of the vesicles formed by PSP-2 under the same solvent condition (6.4 nm), by PSP-1 (Figure S9)16,17 and PSP-2 (Figure S6) in the chloroform/methanol mixture solvents (6−8 nm), the R0 of the S290+ chain (13 nm), and the total diameter of the isolated and contractive nanospheres of PSP-1 (19 nm, vide supra).17 The HAADF-STEM image revealed the presence of white spots with a diameter of ca. 3 nm in both the corona and center G


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Figure 8. BF-TEM images of the mixture of PSP-2 (0.33 mg/mL) and SVP-6 (0.33 mg/mL) were obtained from the toluene/methanol mixture solvent with a methanol volume ratio of 90%, in which the micelles of SVP-6 were wrapped with the vesicle interiors of PSP-2.

PSP-1 or PSP-2 will lead to the formation of inclusion nanostructures, which should be logically imaged by TEM. PSP-1 or PSP-2 was first mixed with SVP-6 in toluene, and then methanol was directly added with volume ratios 75% or 80% and 90%. Their concentrations were controlled at 0.33 mg/mL. The resulting solutions were drop-cast onto a carbon-coated copper grid for TEM observations. When the methanol volume was 90%, the TEM images of the mixture of PSP-2 and SVP-6 revealed the presence of spherical nanostructures with three host−guest forms as follows. (1) A single dark core was hosted into a dark ring together with two alternating gray polystyrene domains (Figure 8a). (2) Dark cores ranging from 2 to 7 were clearly observed in the dark rings (Figure 8b−g). (3) In addition, polymeric dark cores were also packed into the dark rings (Figure 8h,i). The core diameter and ring thickness were determined to be 23 ± 5 and 1 nm, respectively. The former value was completely consistent with those of the ionic cores occurring in the isolated and oligomeric micelles and MMAs of SVP-6 (22 nm) obtained from the chloroform/methanol mixtures with methanol volume ratios of 17%−67%14,17 and in the MMAs obtained from the toluene/methanol mixture with a methanol volume ratio of 90% (Figure S10). And the latter value was in good agreement with the diameter of the Keggin cluster (1 nm). These results showed that the internal lumens of the vesicles included the micelles formed by SVP-6. In the host−guest nanostructures, the S290+ corona occupied a thickness of 6.3 ± 1.2 nm, while the length of the microdomain between the ring and core was 10 ± 2.6 nm. The former value was consistent with the corona thickness of the vesicles formed by PSP-2 in toluene/methanol mixture solvent with a methanol volume ratio of 90%. The area between the ring and core was the mixture of the S290+ and S480 chains originated from the

domains of the vesicles (Figure 7d). The white spots were accordingly assigned to the Keplerate cluster as in PSP-1. These results suggested that the vesicles of PSP-1 were not unilamellar, but oligolamellar in nature. This was in sharp contrast to the unilamellar vesicles formed by PSP-2 under the same solvent condition and by PSP-116,17 and PSP-2 in the chloroform/ methanol mixture solvents (Figures S6 and S9). The former difference was probably due to much more S290+ chains in PSP1 than in PSP-2 and thus much stronger interstar van der Waals attractions for the formation of the oligolamellar vesicles in PSP-1. The latter difference should be attributed to the different solvent molecules (toluene versus chloroform) and thus much larger interstar van der Waals attractions in a toluene/ methanol mixture solvent with a methanol volume ratio of 90%. This vesicle formation mechanism was further verified with a coexistence of the rarely observed nanospheres, isolated and contractive stars of PSP-1 together with a large number of vesicles. When considering the significant interpenetration of the S290+ chains in these vesicles as a result of strong interstar van der Waals attractive forces,16,17 the mean packing layer number per oligolamellar vesicle was approximately four (Figure 1d). Encapsulation of Guest Nanoobjects in the Vesicle Interiors. To further substantiate the formation of vesicles, a nanoobject with heavier species should be incorporated into the inside lumens. SVP-6 fabricated by poly(styrene-b-4-vinylpyridinium methyl iodide) (S480-b-V57) and eight [PW12O40]3− anions can self-assemble to form spherical micelles with an S480 corona and a [PW12O40]3−/V57 hybrid core.13,14,17 Such an ionic core has a much larger electron density than the Keplerate and Keggin clusters as in PSP-1 and PSP-2, respectively. The encapsulation of the micelle of SVP-6 by the vesicle lumens of H


Macromolecules

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These results provided a deeper insight into the intra- and interstar interactions of star polymers in solvents of variable quality. The solvent-tunable interactions and thus controllable self-assembly behaviors in solvents of variable quality offer a new avenue for exploring hierarchical nanostructures with desired functions on the basis of various types of organic/inorganic polymeric composites. This has recently been targeted for selfassembling both metallosupramolecular brushes41 and planar− coil metallosupramolecular block copolymers42 into fusion vesicles in the chloroform/methanol mixture solvents with a methanol volume ratio of 50%.

vesicle interior of PSP-2 and the micellar corona of SVP-6, respectively. Unexpectedly, its length (10 nm) was smaller than the sum of the corona thicknesses of S290+ (6 nm) and S480 chains (8 nm, Figure S10) respectively obtained from the vesicle of PSP-2 and the MMAs of SVP-6 in the toluene/ methanol mixture solvent with a methanol volume ratio of 90%. This situation was attributed to the rather strong van der Waals attractions between the internal brush of the vesicle of PSP-2 and the brush of the micelle of SVP-6, leading to their significant interpenetration in these host−guest nanostructures. Similarly, the micelles of SVP-6 were also encapsulated by the internal lumens of the vesicles formed by PSP-2 in a toluene/ methanol mixture solvent with a methanol volume ratio of 80% (Figure S11) and by PSP-1 in the toluene/methanol mixture solvents with methanol volume ratios of 75% and 90% (Figure S12). These host−guest forms were consistent with our previously reported supramolecular recognition that SVP-6 was encapsulated into the inside lumens of vesicles formed by PSP-117 and by metallosupramolecular block copolymers42 in the chloroform/methanol mixtures. Therefore, these results confirmed that PSPs self-assembled to form vesicles under the present solvent conditions. A New Self-Assembled System in Contrast to Amphiphilic Molecules. Small molecule surfactants and their macromolecular analogues, block copolymers, can undergo a spontaneous self-assembly to generate spherical micelles, wormlike micelles, vesicles, and sheetlike aggregates.22−30 The driving forces behind these self-assembled nanostructures are essentially originated from a delicate competition of several interactions, including the hydrophobic attraction at the hydrocarbon−water interface, the hydrophilic, ionic, or steric repulsion of the headgroups, and entropic penalty for hydrophobic chain deformation. In the present case, however, the sheetlike and vesicular aggregates were formed by the totally hydrophobic PSPs in organic solvents with stepwise weakening quality. Combining previously theoretical studies1,5,6 with our experimental insights on star polymers,16,17 we inferred that the major forces for the formation of sheets and vesicles were assigned to a delicate balance of two opposing forces between the segments of the grafted chains: van der Waals attraction due to enthalpic contribution and repulsive interaction due to entropic effect within a single PSP and among PSPs in the solvent mixtures with worsening quality. The finally balanced forces were cooperatively anisotropic. The present pictures were completely different from bilayered nanostructures occurring in selective solvents of conventional surfactants and block copolymers. Usually, small molecule surfactants form oligo- or multilamellar vesicles in solutions.22−24,44−47 However, these ordered nanostructures are rarely observed in the case of block polymers, the macromolecular analogues of the surfactants. Only very recently, similar vesicles with an oligolamellar feature were captured during a polymerization-induced self-assembly of block copolymers.37,48 In the present case, PSP-1 selfassembled into vesicles with an oligolameller feature as a result of strong intra- and interstar van der Waals attractions (vide supra). The morphological transitions from lamellae to vesicles were only observed from intermediates between wormlike micelles and vesicles formed by block copolymers.35−37 However, as far as we are aware, the present work is the first example of controllable morphology evolution of starlike polymers from lamellae to vesicles occurring in organic mixture solvents with stepwise weakening quality (Figures 1 and 5 and Figure S7).

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CONCLUSIONS In summary, we have demonstrated an experimental insight of star polymers in solvents of variable quality by using POMbased polymeric composites as a star model, where POM anion cores are tightly wrapped by a hydrophobic shell of cationterminated polystyrenes. The tunable interactions between the stars in the toluene/methanol mixture solvents with worsening quality by increasing methanol contents induce a controllable self-assembly and thus predictably hierarchical nanostructures from free-standing sheets to vesicles. Moreover, close inspection of TEM imaging reveals several intermediate nanostructures, which provide important mechanistic insights for this morphological evolution. All these aggregates are packed by the completely hydrophobic starlike polymers, where the core− shell structures are retained. This picture is in sharp contrast to the conventional bilayered nanostructures formed by amphiphiles such as surfactants and block copolymers in selective solvents. The specific mechanism of the aggregate formation concerns a delicate competition of tunable intra- and interstar steric repulsions and van der Waals attractions in the toluene/ methanol mixture solvents with worsening quality. This, together with highly available core−shell structures containing an inorganic core and polymeric shell,49 will enable a systematical exploration of advanced functional materials with controllable and complex nanostructures.

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EXPERIMENTAL SECTION

Materials and Instruments. PSP-1,16 S290+Br (Mn = 30 683 g/ mol, PDI = 1.16),16,32 and K4[α-SiW12O40]·20H2O50 were synthesized according to the procedures described in the literature. Infrared spectra (IR, KBr) were performed with a Nicolet NEXUS 670 spectrometer. Elemental analyses were measured with an Elementar VarioELcube (C, H, and N) and an IRIS Advantage ICP-EMIS (Mo and W). The bright-field TEM and HAADF-STEM images, and EDX measurements were carried out with an FEI Tecnai F30 operating at 300 kV or JEM-2100 operating at 200 kV. The SEM measurements were performed on a field emission Hitachi S-4800, before which we deposited a 4 nm thick gold layer on the specimen by using Hitachi E1045 ion sputter. This coating can prevent efficiently electric charging generated during the process of SEM measurements, leading to highquality SEM images. To obtain the weight-average molar masses (Mw) of both PSP-1 and PSP-2, their dilute toluene solutions were subjected to static light scattering (SLS) measurements on a multiangle Brookhaven BI-200SM spectrometer with a He−Ne laser (532 nm). Considering the low weight fractions and small sizes of the POM anions (2.23%, 1.04 nm for PSP-1 and 2.87%, 2.8 nm for PSP-2), the chemical heterogeneity was negligible in the star polymers. Therefore, the specific refractive index increment for both PSP-1 and PSP-2 was consistent with that for polystyrene in toluene, dn/dc = 0.11. The collected data were further analyzed by Zimm plots (Figures S13 and S14). PSP-2 Was Prepared as Follows. K4[α-SiW12O40]·20H2O (5.6 mg, 1.63 × 10−6 mol) was dissolved in 20 mL of water. The resulting solution was treated with solid powders of S290+Br− (200 mg, 6.52 × 10−6 mol) I


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with vigorous stirring for 3 days. Such a long reaction allowed a full ionic exchange of K4[SiW12O40]·20H2O with S290+Br−. The resulting suspension was filtered, and the white powder was collected. The solid product was further washed with copious water and dried in vacuo. IR (KBr): 3081, 3059, 3025, 2920, 2848, 1942, 1871, 1801, 1742, 1601, 1492, 1451, 1542, 1367, 1311, 1154, 1068, 1027, 966, 923, and 803 cm−1. These IR absorption bands were assigned in detail as shown in Table S1. Elemental analysis calcd for PSP-2, [Br(CH3)2COO(CH2)11N(CH3)3(C8H8)290]4[SiW12O40] (Mn = 1.25 × 105 g/mol, Mw = 1.45 × 105 g/mol): C 89.71, H 7.59, N 0.04, W 1.76. Found: C 89.68, H 7.27, N 0, W 1.06. Elemental analysis calcd for PSP-1,16 [Br(CH3)2COO(CH2)11N(CH3)3(C8H8)290]25(NH4)17[(H2O)50⊂Mo132O372(CH3COO)30(H2O)72] (Mn = 7.87 × 105 g/mol; Mw = 9.11 × 105 g/mol): C 89.24, H 7.60, N 0.07, Mo 1.61. Found: C 89.69, H 7.10, N 0, Mo 1.60. The Mn and Mw values in the parentheses were respectively calculated from the elemental analyses of PSP-1 and PSP-2. The SLS data, analyzed through the Zimm plot (Figures S13 and S14), revealed Mw values of (8.3 ± 1.5) × 105 and (3.12 ± 0.11) × 105 g/mol for PSP-1 and PSP-2 in toluene, respectively. The former value was consistent with the calculated Mw of 9.11 × 105 g/mol from the elemental analysis. After subtracting the molecular weight of Mo132, the number of the S290+ arms was determined to be 23 ± 4 in PSP-2, which agreed well with the value obtained from the elemental analysis (25 ± 2). However, the experimental Mw of PSP-2 ((3.12 ± 0.11) × 105 g/mol) was 2 times larger than the calculated value from the elemental analysis (1.45 × 105 g/mol). It was therefore inferred that PSP-2 should form dimers in toluene. On the other hand, DODAencapsulated Keggin clusters could be conceptually regarded as smallmolecule analogues of PSP-2 (DODA: dimethyldioctadecylammonium). They self-assembled to form multilamellar vesicles in low-polarity solvents as a result of complete phase separation between the DODA chains and Keggin anions.44,51 However, due to the polymer nature of PSP-2, the S290+ chains were almost impossible to be completely reorganized on the surface of [α-SiW12O40]4−. We therefore inferred that the dimer formation of PSP-2 could be assigned to partial phase separation between S290+ and [α-SiW12O40]4− in toluene. Alternatively, the total molecular weight of PSP-2 was much smaller than that of PSP-1, and the element of W was much heavier than that of Mo. The latter feature was confirmed by the invisible Mo132 (Figure 2b) and visible [α-SiW12O40]4− (Figure 4c) in the respective BF-TEM images of the free-standing sheets formed by PSP-1 and PSP-2. In this sense, the chemical heterogeneity could not be negligible in PSP-2, leading to the overestimated Mw.

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ASSOCIATED CONTENT

S Supporting Information *

Additional TEM and SEM images. 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] (W.B.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the NSFC (51173073), the Program for New Century Excellent Talents in University (NCET-100462), the Fundamental Research Funds for the Central Universities (lzujbky-2013-238 and lzujbky-2014-74), and the Open Project of State Key Laboratory of Supramolecular Structure and Materials of Jilin University (sklssm201405).

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REFERENCES

(1) Grest, G. S.; Fetters, L. J.; Huang, J. S.; Richter, D. Adv. Chem. Phys. 1996, 94, 67−163. J


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