Spherical Polymer Brushes in Solvents of Variable Quality: An

Letter pubs.acs.org/Langmuir

Spherical Polymer Brushes in Solvents of Variable Quality: An Experimental Insight by TEM Imaging Qian Zhang, Yin Liao, Lipeng He, 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 City, Gansu Province, China S Supporting Information *

ABSTRACT: The spherical micelle and vesicle composed of [PW12O40]3− and poly(styrene-b-4-vinylpyridinium methyl iodide) are regarded as a model system to study spherical polymer brushes (SPBs) in solvents of various quality. The pure repulsions occur for the brush chains in the chloroform solution and chloroform/ methanol mixture with a methanol volume ratio of 9.1%, where the grafted polystyrene chains have a relatively extended conformation. Further increase in the methanol concentrations leads to the presence of the intra/inter-brush van der Waals attractions. Transmission electron microscopy studies show that there is a coexistence of isolated and oligomeric SPBs and multi-SPB aggregates (MSAs) with the methanol content from 17% to 23%. Only MSAs are detected with the increasing methanol content. Both the corona and core shrink significantly in the isolated and oligomeric SPBs and MSAs. The full interpenetration of the grafted chains is observed between the cores in the oligomeric SPBs and MSAs.

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INTRODUCTION

the solution-based applications mentioned above, have not been concerned. Polyoxometalates (POMs) are anionic metal-oxide clusters and have remarkable structural and electronic versatility.15,16 One of the topics in this field is the fabrication of POM-based polymeric nanocomposites by the electrostatic self-assembly with positive polymers to explore the functional performance in catalysis and materials science. For example, the electrostatic bonding of POMs to quaternary ammonium sites of the alkylated polyethyleneimine allows the preparation of recyclable catalysts with hydrophobic regions and lipophiloselectivity.17,18 Multiple molybdenum cluster rings are threaded by a conjugated polymer bearing pendant ammonium ion groups to form an inorganic/organic polypseudorotaxane.19 The POM-based supramolecular star polymers formed by the electrostatic combination of POMs and cationic-terminated polymers can self-assemble into vesicular aggregates in chloroform/methanol mixtures.20 The incorporation of POMs into cationic block copolymers leads to the formation of nanocomposites with micellar and vesicular morphologies21−24 in selective solvents and highly ordered hexagonal films.23,24 We previously fabricated spherical micelles and vesicles in toluene by the electrostatic combination of Keggin-type polyoxometalate (POM), [PW12O40]3−, and poly(styrene-b-4vinylpyridinium methyl iodide) (Sn-b-Vm).23 The presence of POMs allows us to directly image the micellelike nanostructures by transmission electron microscopy (TEM) without any

Spherical polymer brushes (SPBs) are composed of a core of a nanoparticle and a shell of polymer chains, in which the polymer chains are grafted to the surface of the nanoparticle by one end via a covalent bond with tunable grafting densities.1−6 The desired properties can be achieved by an elegant combination of both nanoparticle and polymer chains, which has resulted in potential applications in the fields of catalysis1,2 and nanocomposite materials.3−6 For these solution-processable applications, the intra/inter-brush interactions in solvents of various quality should be fully understood in the ongoing efforts to control, predict, and thus design the structures and properties of SPBs. The theoretical studies have predicted that the interaction between the segments of grafted chains in a single isolated SPB is purely repulsive in good solvents due to entropic effects, and then the polymer chains have an extended conformation there.7,8 When the solvent quality is worsened, a van der Waals attraction occurs together with the reducing repulsion, which leads to significant shrinkage of the grafted chains.9−11 Similarly, the interaction between SPBs is purely repulsive in good solvents, while an attraction force appears with the repulsion being reduced for worsening solvent quality.11 However, only two SPBs are involved, which is really different from the actual solution. Several experimental studies reveal that the grafted chains are remarkably stretched under good solvent conditions, which is qualitatively consistent with the above-mentioned theoretical predictions.12−14 However, the experimental study for SPBs under variable solvent conditions, which is of fundamental and practical interest in the context of © 2013 American Chemical Society

Received: January 4, 2013 Revised: March 5, 2013 Published: March 21, 2013 4181

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Figure 1. Schematic drawings of the (a) micelle and (d) vesicle, (b) isolated and oligomeric micelles and (e) vesicles, and (c and f) multi-SPB aggregates in the chloroform/methanol mixtures with an increase in the methanol volume ratio.

the methanol content from 42% to 67% yielded an average Dh at 200 ± 30 nm. Further increase in the methanol content resulted in the presence of the precipitate. Similarly, Dh’s of SVP-3 at 108 nm did not show a significant change when the methanol content increased from 0% to 23% in the chloroform/methanol mixtures (Figure S1 of the Supporting Information). SVP-3 precipitated with the additional increase in the methanol content. However, such size evolution was incompatible with the theoretical picture that a single SPB should show a reduced size due to significant shrinkage of the graft chains with worsening solvent quality, as noted in the Introduction. To get the fundamental insight on the intra/inter-SPB interactions in solvents of variable quality, the chloroform/ methanol mixture solutions of SVP-3 and SVP-6 were cast onto carbon-coated copper grids for TEM observations. The TEM image of SVP-6 obtained from the chloroform/methanol mixture containing 9.1% methanol showed micellar aggregates in a quasi-hexagonal array (Figure 3a). The average core and overall micelle diameters and corona thickness were determined to be 32 ± 6, 55 ± 7, and 13 nm, respectively, consistent with the TEM results obtained from the toluene and chloroform solutions.23 The core radius (16 nm) was in good agreement with the length of the fully stretched V57 block (14.25 nm). Therefore, the micelle reached superstrong segregation (SSS) regime, where the interface between the core and the corona was completely occupied by S−V junction points.25−27 The core sizes obtained from the previous smallangle X-ray scattering measurements were consistent with the TEM results, and thus the micellar cores are stable upon the solvent evaporation.23 Therefore, the grafting density of S480 (σ0) was estimated to be 0.28 chains/nm2, according to the interfacial area per chain of the micelle (3.5 nm2).23 In combination of these results with the above-mentioned DLS data, the corona thickness of SVP-6 in the solutions could be calculated to be 39 nm by the subtraction of the diameter of the dark core (ca. 32 nm) from Dh of the micelle (ca. 110 nm). This hydrodynamic thickness was much longer than the unperturbed end-to-end distance of the S480 chain (R0 = 0.456n0.595 = 18 nm)26 and smaller than the fully stretched S480

other staining. These spherical nanoobjects have an S corona and a hybrid core filled with POMs, iodides, and nitrates binding to V blocks via Coulomb force, which could be conceptually described as a model system for spherical polymer brushes. Here, we report the first experimental insight on the intra/inter-SPB interactions in solvents of variable quality by TEM imaging the above POM-based model system (Figure 1).

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RESULTS AND DISCUSSION SVP-3 and SVP-6 were prepared as described previously23 and composed of S480-b-V57 with 18 and 8 [PW12O40]3− anions, respectively. They, respectively, formed spherical vesicles and micelles in organic solvents of very low polarity, such as toluene, chloroform, and dichloromethane (Figure 1). Both SVP-3 and SVP-6 were dispersed first in chloroform, and then methanol was added with volume ratios from 0% to 67%, where chloroform and methanol are the good and poor solvents for the brushlike models, respectively. Their final concentrations in the solvent mixtures were controlled at 0.33 mg/mL. These solutions were first subjected to dynamic light scattering (DLS) measurements. The hydrodynamic diameters (Dh’s) of SVP-6 kept almost constant at 110 nm with the increasing chloroform/methanol volume ratio from 0 to 23% (Figure 2). When the amount of methanol was 33% in the mixture solvent, Dh increased sharply to 280 ± 100 nm. The increase in

Figure 2. The hydrodynamic diameters of SVP-6 were measured from DLS in the chloroform/methanol mixtures, where the methanol volume ratio increased from 0% to 67%. 4182

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Figure 3. TEM images of SVP-6 obtained from the chloroform/methanol mixtures containing (a) 9.1%, (b−k) 17%, (l) 43%, and (m) 50% methanol. (n) SEM and (o) AFM images obtained from the chloroform/methanol mixtures containing 50% methanol.

length (contour length, 0.25 × n = 120 nm). Therefore, the corona chains were relatively stretched in the chloroform solution and chloroform/methanol mixture containing 9.1% methanol. This observation was consistent with previous theoretical and experimental studies on SPBs under good solvent conditions,7,8,12−14 where the pure repulsions were present between the segments of grafted chains in SPBs due to the entropic effects. When the amount of methanol was 17% in the mixture solvent, three forms of SVP-6 occurred in the TEM images. (1) A typical single isolated micelle was observed in Figure 3b. The average core diameter and corona thickness were 22 ± 5 and 8 ± 2 nm, respectively, which showed considerable shrinkage in comparison with the data listed above. (2) The oligomeric micelles formed from 2 to 9 micelles were also observed in the same copper grids (Figure 3, panels c−j). The resulting core diameter and outer corona thickness were totally consistent with the values obtained in the isolated micelle. In the dimer and trimer, the distance between the micellar cores was estimated to 8 ± 2 nm, which was consistent with the outer corona thickness. The close contact between the micellar cores occurred in the tetramer and pentamer and their overlapping started from the hexamer. (3) In addition, the multimicelle aggregates (MMAs) were clearly observed with diameters ranging from 120 to 200 nm, in which the micelles packed closely (Figure 3k). The outer corona thickness, micellar core diameter, and distance between the micellar cores were consistent with the values in the isolated micelles and oligomeric aggregates. The occurrence probabilites for the single isolated and oligomeric micelles and MMAs were counted to be 10%, 30%, and 60%, respectively. A similar situation was also observed in the chloroform/methanol mixtures containing 20% and 23% methanol. Further increase in the methanol ratio from 33% to 67% resulted in the

complete presence of MMAs with similar sizes. The typical TEM images were obtained at the methanol ratio of 43% (Figure 3l) and 50% (Figure 3m). The spherical morphology was further revealed by both the scanning electron microscope (Figure 3n) and the atomic force microscope (Figure 3o) images at the methanol ratio of 50%. The MMAs obtained at the chloroform/methanol (v/v = 1) mixture could be stable for ca. 2 months without significant size changes. The parameters such as the diameter of the core, corona thickness, and the distance between the micellar cores were summarized in Table 1. Table 1. Packing Parameters of the Model SPBs in Solvents of Various Quality sample SVP-6 SVP-3

volume ratio of methanol

DCD (nm)a

DCT (nm)b

σ (chains/ nm2)c

DDM (nm)d

0−9.1% 17−67% 0−9.1%

32 (m) 22 (m) 38 (vd) 8 (vt) 31 (vd) 6 (vt)

39 8 35

0.28 0.59 0.16

8

8

0.24

8

17−23% a

Core diameter of micelles (m), outer core diameter (vd), and core thickness of vesicles (vt). bCorona thickness. cGrafting density of S480.23 dDistance between the micellar cores in oligomeric species and MSAs.

The corona thickness (8 nm) in the isolated and oligomeric SPBs and MMAs was even smaller than the R0 value of the S480 chain (18 nm). Such significant shrinkage was assigned to the weakening solvent quality with the methanol volume content from 17% to 67% and thus the presence of van der Waals attractive interactions between the segments of the S480 chains. 4183

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Figure 4. TEM images of SVP-3 obtained from the chloroform/methanol mixtures containing (a) 9.1%, (b−n) 17%, and (o) 43%.

MMAs was a delicate balance between van der Waals attractive forces and repulsive entropic effects between the brushes in the chloroform/methanol mixtures. To generalize the experimental insight of SPBs in solvents of variable quality, the vesicle formed by SVP-3 (Figure 1d) was further imaged with the increase in methanol volume ratio from 0% to 23%. Similar morphological evolution was also observed in SVP-3. The vesicles formed in both the chloroform solution and the chloroform/methanol mixture containing 9.1% methanol (Figure 4a). Their outer core diameter and core thickness were estimated to be 38 ± 5 and 8 ± 2 nm (Table 1), respectively, consistent with the reported values from the toluene solution.23 Taking the DLS (Figure S1 of the Supporting Information) and TEM results into account, the grafting density and hydrodynamic thickness of the S480 chains in the model SPB were determined to be 0.16 chains/nm2 and 35 nm, respectively. Further increase in the methanol ratio from 17% to 23% in the chloroform/methanol mixture led to the formation of the isolated (Figure 4b) and oligomeric vesicles (Figure 4, panels c−l) and multivesicle aggregates (MVAs, Figure 4, panels m and n) with occurrence probabilities of 20%, 48%, and 32%, respectively. The additional increase in methanol content resulted in the appearance of precipitate within 10 min. The sample was cast onto a carbon-coated copper grid before the precipitate occurred. The corresponding MVAs were then imaged (Figure 4o). The outer corona thickness, outer core diameter, and core thickness were 8 ± 3, 31 ± 5, and 6 ± 1 nm (Table 1), respectively, which again showed significant shrinkage when the methanol content was larger than 17%. The grafting density was correspondingly magnified from 0.16 to 0.24 chains/nm2. The distance between the outer cores in both the oligomeric vesicles and MVAs was 8 ± 3 nm, consistent with the full interpenetration of the S480 chains. These evolution trends in SVP-3 were consistent with the weakening solvent quality with the methanol content ≥17%

Unexpectedly, the distances between the cores were almost identical to the outer corona thickness in both the oligomers and aggregates. This conflicting observation was reasonably explained by assuming the full interdigitation of the S480 chains, as demonstrated in Figure 1 (panels b and c). Similar interdigitation of alkyl chains has also been observed in bilayer assemblies, where the van der Waals attractions are usually believed to be maximized.28−30 In our case, the full interdigitation of the S480 chains was attributed to the van der Waals attractive interactions between the model brushes with the weakening solvent quality. The present observations for the grafting chains were qualitatively consistent with the previous theoretical studies of SPBs in the solvents of variable quality.9−11 Of note was that the sizes of the POM-based cores were reduced from a diameter of 32 to 22 nm, where the cores were obtained by the electrostatic self-assembly of [PW12O40]3− with V57 blocks. Therefore, the corresponding grafting density of S480 (σ) increased from 0.28 to 0.59 chains/nm2 (Table 1) on the basis of the following equation: σ = σ0(r0/r)2, where r0 and r are the core radii of the micelles obtained from the chloroform/ methanol mixture with methanol contents at 0−9.1% and 17− 67%, respectively. It should be emphasized that the micellar core produced from the chloroform/methanol mixture with methanol contents at 0−9.1% was in the SSS regime due to the rather strong interfacial tension between the ionic species and solvent/S blocks.23,25−27 However, the hybrid cores were completely incompatible with both chloroform and methanol. Such significant compactness should again be assigned to the rather strong van der Waals attractions between the segments of the model SPBs in the chloroform/methanol mixture containing 17−67% methanol. The coexistence of the isolated and oligomeric SPBs and MMAs was obtained from the chloroform/methanol mixture containing 17−23% methanol. We therefore inferred that the driving force for the formation of 4184

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in University (Grant NCET-10-0462), the Specialized Research Fund for the Doctoral Program of Higher Education (Grant 20100211110023), the Fundamental Research Funds for the Central Universities (Grants lzujbky-2012-k14, lzujbky-201129, and lzujbky-2010-162), the Gansu Natural Scientific Foundation (Grant 1107RJZA214), and the Open Project of State Key Laboratory of Supramolecular Structure and Materials of Jilin University (Grant sklssm201214).

and thus the presence of van der Waals attractive interactions between the segments of the S480 chains. Recently, we have fabricated POM-based supramolecular star polymers (PSPs) by an electrostatic combination of a Keplerate cluster of [Mo132O372(CH3COO)30(H2O)72]42− (d = 3 nm) and cationic-terminated polystyrenes.20 The resulting PSPs can be regarded as a model SPB, in which the core diameter is much smaller than the R0 values. PSPs are highly soluble in good solvents of chloroform, toluene, or tetrahydrofuran. When the solvent quality is worsened by increasing volume ratios of methanol as a poor solvent in chloroform/methanol mixtures, the isolated PSPs can self-assemble into vesicular aggregates. In the isolated and oligomeric micelles and MMAs of SVP-6, the core diameter was determined to be 22 nm by TEM imaging, which was rather comparable to the R0 of S480 (18 nm). SVP-6 formed the relatively stable MMAs in the chloroform/methanol mixture containing methanol volume ratios from 33% to 67%. However, the MVAs of SVP-3 precipitated rapidly in the chloroform/methanol mixture with the methanol content ≥28%, where the outer core diameter (31 nm) much larger than the R0 of S480. On the basis of these results, we therefore inferred that the aggregate structures of SPBs in solvents of variable quality depended on the core size. This conjecture will be further confirmed by other SPB systems with variable core diameters. Such a work is now in progress.

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CONCLUSION In conclusion, we have demonstrated a first experimental insight into SPBs in solvents of variable quality by TEM imaging, where the spherical micelle (SVP-6) and vesicle (SVP3) fabricated by the electrostatic self-assembly of [PW12O40]3− with S480-b-V57 were used as SPB models.23 The brush chains have a relatively extended conformation in the chloroform solution and chloroform mixture containing 9.1% methanol. When the methanol content increases from 17% to 23%, the isolated and oligomeric SPBs and MSAs are discovered all together by TEM imaging. Further increase in the methanol content leads to the complete formation of MSAs. Both the outer coronas and cores show a remarkable shrinkage. The polymer chains between the cores are fully interpenetrated in the oligomers and MSAs. These features are assigned to the intra/inter-SPB van der Waals attractions with the weakening solvent quality. Such implementation would benefit the development and understanding of programmable and hierarchical self-assembly of brushlike macromolecules into complex materials.

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ASSOCIATED CONTENT

S Supporting Information *

Instrumental part and additional DLS data. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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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 National Natural Scientific Foundation of China (Grants 20901034, 51173073, and 20931003), the Program for New Century Excellent Talents 4185

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