Tandem Self-Assembly of Block Copolymer: From Vesicles to Stacked

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Tandem Self-Assembly of Block Copolymer: From Vesicles to Stacked Bowls Xiaoqing Wang,†,‡,∥ Songlin Liu,§,∥ Shida Cao,† Fei Han,§ Hong Wang,*,† and Hongyu Chen*,† †

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Institute of Advanced Synthesis and School of Chemistry and Molecular Engineering, Jiangsu National Synergetic Innovation Center for Advanced Materials, Nanjing Tech University, Nanjing 211816, P. R. China ‡ College of Science, Nanjing Forestry University, Nanjing 210037, P. R. China § Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore 637371, Singapore S Supporting Information *

ABSTRACT: We report a three-step tandem polymer selfassembly, from PS-b-P4VP macromolecules to vesicles, from vesicles to chains, and from chains to stacked bowls. Once aggregated, the soft vesicles and the hydrogen-bonded interface favor the gradual increase of the contact area, thus promoting concerted optimization toward stacked bowls. The continual structural modification opens a new route for exploring sophisticated synthetic control and complex shapes.

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INTRODUCTION Self-assembly of simple components into an ordered architecture is a common phenomenon in nature, for example, the assembly of cell membranes, the construction of protein superstructures, etc. In nanoscience, self-assembly is a critical capability of joining together basic building blocks for their concerted properties. In recent years, spontaneous organization of nanoparticles into various superstructures has receive increasing amount of interest due partially to the fundamental importance of assembly skills,1−6 and partially to the potential applications in sensing, catalysis, drug delivery, energy conversion, and storage.7−16 The resulting superstructures include threedimensional (3D) superlattices,17 films of two-dimensional (2D) arrays,18 and one-dimensional (1D) chains.4 It is important to note that most of such assemblies in the literature involve hard spheres that are incapable of further shape transformation. As such, it is difficult to achieve tandem assembly and multistep structural modifications, which are critical capabilities for advanced nanosynthesis and also the core concepts of total synthesis19 and tool fabrication. Polymer micelles are an interesting class of nanoparticles in that they are soft and capable of shape transformation. They are typically formed by amphiphilic block copolymers (BCP) giving various shapes such as spheres, ellipsoids, cylinders, lamellae, vesicles, and disks.20−28 In addition, complex morphologies and their control and transformation of BCP have been extensively employed. Examples range from polymersome stomatocytes, striped ellipsoid, convex lens, onionlike particles, to 3D confined assembly.29−34 While it is common to view these structures as basic building blocks in nanoscience, there has been few studies on assembling them © XXXX American Chemical Society

into regular superstructures, and particularly no report on the self-assembly of vesicles exists. In this work, we present an interesting case of assembly of polystyrene-block-poly(4-vinylpyridine) (PS 222 -b-P4VP 43 ) vesicle to give stacked bowls. In the process, there are three levels of tandem self-assembly: (1) the self-assembly of polymer molecules into vesicles; (2) the linear aggregation of vesicles into chains; and (3) the transformation of vesicle chains into stacked bowls. The soft vesicles and their mutually attractive interaction are of critical importance in this new assembly pathway. The guest−host interaction among the neighboring bowls limits the structural freedom to give 1D chains of stacked bowls.

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RESULTS AND DISCUSSION First, PS222-b-P4VP43 was dispersed in a solvent mixture (tetrahydrofuran (THF)/H2O = 4:1, v/v) and then incubated at 80 °C to allow self-assembly and structural evolution. After 4 h, the solution was cooled to room temperature, and then an excess amount of water was added to quench the reaction and remove THF from the resulting polymer aggregates. After deswelling, the PS domains of these aggregates became glassy,35,36 allowing direct characterization by transmission and scanning electron microscopy (TEM and SEM). As shown in Figures 1 and S1, stacked bowls several micrometers in length were obtained, together with the remaining free vesicles. Defined as the number of aggregated vesicles/bowls over all vesicles/bowls, the yield is estimated to be 45% (the detailed calculation is shown in the Supporting Received: June 27, 2019 Revised: August 9, 2019

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DOI: 10.1021/acs.macromol.9b01341 Macromolecules XXXX, XXX, XXX−XXX

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aggregation of indented vesicles or random indentation in vesicle chains is expected to cause the random orientation of the bowl openings, disagreeing with our observation. Hence, the consistent orientation of the stacked bowls indicates a concerted evolution. To investigate the formation process, the intermediates were isolated at different times. At 15 min, the products were mainly vesicles (Figure 2a), which have been well-studied in the literature.37−40 Polymer vesicles are typically hollow spheres made of a bilayer membrane with hydrophobic cores and hydrophilic coronas at both internal and external surfaces.41 On this basis, our PS222-b-P4VP43 vesicles should have a similar structure as that of a hydrophobic PS wall and hydrophilic P4VP coronas. PS-b-P4VP vesicles have been prepared by a variety of methods,42−45 but there is no report on chain aggregation or shape transformation. After 30 min, the vesicles aggregated into chains. As shown in Figure 2b, the vesicles still maintain a spherical shape, with curved joints and minimal contact area with their neighbors. When the incubation time was extended to 1 h, the vesicles became squashed and the contact area increased (Figure 2c). The middle partitioning walls appeared partially overlapped and merged as their combined thickness decreased to half to reach roughly the same thickness as that of the outer walls. After 2 h, the squashed vesicles broke up, with their rims opened outward from the middle partitioning walls and the opening oriented in the same direction, giving partially embedded bowls (Figure 2d). At 4 h, deeply stacked bowls were obtained, with overlapped rims and a single bottom layer separating the neighboring bowls (Figure 2e). With further incubation, the bowls became thinner and larger in diameter, with more extensive stacking (Figures 2f and S2). It should be noted that there are multiple morphologies in a sample (Figure S3). Figure 2g shows the percentage analysis (Z-axis) of various intermediates (Y-axis) with different incubation times (X-axis). The intermediates are arranged according to the order of evolution as shown in Figure 2a−e. Although the morphologies are inhomogeneous, all species show a clear trend of evolution, with the dominant species changing from free vesicles to chains, squashed vesicle chains, partially embedded bowls, and finally to the deeply stacked bowls.

Figure 1. Morphologies of the stacked bowls. (a) TEM and (b) SEM images of PS222-b-P4VP43 stacked bowls. The encircled middle section shows a scalelike appearance with overlapped rims (inset of a), and the tail section contains vesicles that retain the spherical shape (c). (e) A less embedded stack of bowls with clearer boundaries. It should be more appropriately assigned as stacked bowls (d), rather than a chain of indented vesicles (f). Scale bar: 250 nm.

Information). The openings of the bowls are oriented in the same direction so that the bowls stack closely together in series. Most of the bowls are 200−400 nm in diameter, but large bowls (2−3 μm) can also be found (Figure 1b). Because the polymers swell differently in solvents, even structures with similar assemblies show different sizes. The stacking of the rims gives the overall structure a scalelike appearance with a zigzag outline, but it is hard to distinguish the internal structures. As shown in Figure 1c, the vesicles at the narrower end are not fully evolved to bowls and retained near-spherical shapes. Figure 1e gives an example of the wider end, where the bowls are embedded to a less degree, so that the boundary of each bowl can be clearly assigned. It is important to note the single layer at the bottom of the bowls. Should the bowls be indented vesicles, there should be two layers between the overlapping bowls: one layer being the bottom of one vesicle and the other being the folded rim of another vesicle (Figure 1f). A schematic diagram is shown in Figure 1d for a clearer assignment of this structure. Judging from the starting materials and the remaining vesicles at the narrower end, it appears that the stacked bowls should evolve from aggregated vesicles. However, random

Figure 2. Formation process of the stacked bowls. TEM images of the dominant intermediates at different times of reaction: (a) 15 min, (b) 30 min, (c) 1 h, (d) 2 h, (e) 4 h, and (f) 8 h. (g) Temporal evolution (X-axis) of these samples, showing the percentage (Z-axis) of the various intermediates (Y-axis) represented in (a)−(f). Scale bar: 200 nm. B

DOI: 10.1021/acs.macromol.9b01341 Macromolecules XXXX, XXX, XXX−XXX

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Figure 3. Separation of the stacked bowls upon acidification. TEM and SEM images showing the products of 4 h after quenching by HCl solution: (a, b) the separated bowls with a clean cleavage; (c) the opening of the separated bowl stack with a clear view of the single-layer rim; (d, e) the interlocked chains of double-layer vesicles; and (f) the severed chain remaining in the formation. (h) Molecular diagrams showing the attractive hydrogen bonding between partially protonated P4VP chains, which would be turned into repulsion upon heavy protonation by acid treatment (i) and deprotonation by base treatment (g). Scale bar: 500 nm.

unfavorable stacking (Figure 2e). It is possible that the bowl may “feel” the orientation of its neighbor through strain, for example, the released strain as a vesicle breaks up. Study of such a mechanical force at a nanoscale would be extremely difficult, but the concerted action along the vesicle chain is highly interesting and deserves further study. Given the unchanged solvent environment, we expect the P4VP chains of Figure 2a−f to have a similar degree of protonation. We speculate that during early stages of polymer self-assembly, the vesicles seek individual optimization; once aggregated, the overall architecture with a hydrogen-bonded interface favors concerted optimization. In other words, the vesicle is a thermodynamically stable state by itself,41,47 but the favorable hydrogen bonding among them means that the aggregated vesicles would further evolve to lower-energy states (stacked bowls, etc.). To test this hypothesis, we added acid to the mixture to quench the products for 4 h to cause severe protonation. The resulting pH = 3.0 is significantly lower than the pKa of pyridine (5.25). As shown in Figure 3a−e, the stacked structures showed a thinning of the walls and clear signs of separation. The spacing between the bowls widened so that the bowls appeared to be dislodged from their original pockets, probably due to increased charge repulsion. The clean cleavage of the bowls indicates that the fusion never really occurs extensively between them (Figure 3a,b). Some of the large openings can be visualized by SEM (Figures 3c and S4), where the rim has a single layer rather than folded 2 layers. These observations provide a strong support for the assignment of bowls rather than indented vesicles.

In our experiments, all samples underwent the same centrifugation and drying processes. Given the clear trends in the temporal evolution of the samples, these processes are not expected to be the key factors. In our previous studies of polystyrene-block-poly(acrylic acid) (PS-b-PAA) structures, aggregation and merging are common phenomena,4,46 but the process essentially involves the fusion of smaller micellar domains into large ones. In contrast, our PS-b-P4VP vesicles never fuse into a single domain; their overlapped walls are well-preserved during the aggregation and transformation. Indeed, such a characteristic is unprecedented in all known systems of polymer structures. We note that it is easy to develop strong hydrogen bonds among the P4VP chains. As illustrated in Figure 3g−i, partially protonated P4VP subunits could form extensive hydrogen bonds, and repulsion by positive charges would only occur when they are heavily protonated. Such attractive interaction is likely responsible for the aggregation and transformation, where the adhesion of vesicle walls, the split of vesicles into bowls, and the stacking of bowls are all means to increase the contact area between the polymer domains. The strongly bonded interface not only provides a driving force for the tandem self-assembly but also helps in preserving the interface from mixing up during transformation. The consistent orientation of the stacked bowls is intriguing. Going from Figure 2c,d, obviously a bowl “knows” the orientation of its neighbor. Considering the small degree of overlap between the bowl walls at this stage, the wrong orientation would not cause much difference in the contact area. It is unlikely that the bowls would be able to “predicate” that, hours later, the random orientation would lead to C

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bowls in the first sample and scattered vesicles in the second (Figure 4), highlighting the importance of the aggregation step. Combined with the knowledge of previous studies in the literature, the overall mechanism of the tandem self-assembly can be presented as follows (Figure 5). Initially, the polymer

Among the structures newly formed with the addition of acid, Figure 3d,e shows the chains of double-layer vesicles interlocked by guest−host interactions. It appears that the separated bowls now seek lower energy states by returning to vesicle-like shapes. As the bowl opening tightens, the neighboring bowl is squeezed to form a neck. Thus, each bowl gives two connected spherical compartments: one as the host and the other as the guest remaining inside the neighboring host. Eventually, the connected compartments are severed as the squeezing continues, giving double-layer vesicles that remain in the chain formation (Figure 3f). Hence, once separated, the bowls seek individual optimization again, as they can no longer feel the hydrogen bonding from their neighbors. With the vesicle being in a thermodynamically stable state, the bowl opening gradually tightens and the overall shape turns spherical.46 As predicted from the chemical property of P4VP, extensive deprotonation would also cause repulsion among the polymer chains, albeit with a weaker intensity (Figure 3g). NaOH was added to the solution (pH = 6.7 or 12.8) to neutralize the protonated P4VP. As a result, the stacked bowls also started to separate (Figure S5), though not as extensively as those in the acidic conditions. The concerted optimization of vesicles is a new concept. With basic logic, the distinction is whether a vesicle can feel the presence of its neighbors. We carried out a control experiment, as shown in Figure 4, where the solution of mainly

Figure 5. Schematics illustrating the three levels of tandem selfassembly. (a, b) Assembly of PS-b-P4VP molecules into vesicles. (c, d) Aggregation of vesicles into chains. (e−g) Transformation of the vesicle chains into stacked bowls.

molecules randomly aggregate into clusters, which undergo simultaneous phase separation (Figure 5a,b).41 The low polarity of the solvent (THF/H2O = 4:1) causes significant swelling of the PS domain,35,36 thus promoting the mobility of the embedded macromolecules and facilitating phase separation.46,48 The high temperature (80 °C) is an additional promoter.49 The vesicle concentration quickly increases and the rate of vesicle aggregation would increase even more abruptly (Figure 5c). Considering the small contact area between spherical vesicles in the initial aggregates (Figure 5d), the specific chain aggregation mode can be better attributed to long-range charge repulsion,47 rather than the tendency to form polymer cylinders.4 The partially protonated P4VP renders the vesicle surface with positive charges. Once in contact, the short-range hydrogen bonding dominates47 and, thus, the vesicles deviate from spherical shape to increase the contact area between them (Figure 5e). We believe that the hydrogen-bonded interface would be as strong as the interactions within the PS domain, if not more. Thus, the interface is well-preserved in the shape transformation to deeply stacked bowls. Upon addition of acid, the high degree of protonation brings about a significant increase of charge density, which, in turn, favors thinner walls47,50 and causes the stacked bowls to separate. Guest−host interactions are a hot topic in molecular chemistry,51 and the docking interactions between proteins can be viewed as guest−host interactions at the nanoscale. However, so far, there are only few examples of guest−host interactions between artificial nanostructures. In this work, there are several kinds of guest−host structures, including the closely stacked bowls in Figure 2d,e, the loosely packed bowls with large separation in Figure 3a,b, and the interlocked double-layer vesicle chains in Figure 3d,e.

Figure 4. Dependence of concerted optimization on vesicle concentration. The vesicles had a chance to meet each other before dilution turned into stacked bowls (a), whereas those that did not have the chance remained scattered (b).

vesicles was divided into two portions. One portion was directly incubated for 2 h. Then, the solution was diluted 3fold with the same solvent and incubated for another 2 h. At the same time, the other portion was immediately diluted 3fold and then incubated for 4 h. Thus, the two samples underwent almost identical reaction conditions, except that the vesicles in the first portion had the chance to “meet” each other by aggregation, whereas those in the second portion did not. The probability of a collision should be of a second order with respect to the concentration of the vesicles, and multiple collision events are required to achieve long chains. Thus, a 3fold dilution has a significant impact on such cascade events. But once concerted optimization starts, the subsequent dilution would have no effect. The products show stacked D

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Author Contributions

CONCLUSIONS In summary, we demonstrated an interesting example of a three-step tandem self-assembly, where block copolymer nanostructures can undergo continual structural modifications. The idea of concerted optimization would allow the intrinsic properties of polymer structures to cooperate and compete in a complex platform, opening doors to new morphologies and synthetic controls.

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (21703103 and 21673117), recruitment Program of Global Experts, Jiangsu Science and Technology Plan (BK20170966), Jiangsu Provincial Foundation for Specially-Appointed Professors, the Start-up Fund from Nanjing Tech University (39837115 and 39837102), and SICAM Fellowship from Jiangsu National Synergetic.

METHODS

Chemicals and Materials. All chemical reagents were purchased from commercial suppliers and used without further purification. Amphiphilic block copolymer poly(styrene)-block-poly(4-vinylpyridine) (PS222-b-P4VP43, Mn = 23 000 for the PS block and Mn = 4500 for the P4VP block, Mw/Mn = 1.10) was purchased from Polymer Source, Inc. Deionized water (resistance > 18.2 MΩ cm−1) was used in all experiments. Copper specimen grids (200 meshes) with a carbon support film (referred to as TEM grids in the text) were purchased from Beijing ZJKY Technology co., ltd. Characterization and Sample Preparation. TEM images were collected from a HITACHI HT-7700, Talos L120C TWIN, and JEOL JEM-1400 transmission electron microscope operated at 100 and 120 kV, respectively. TEM grids were treated with oxygen plasma in a Harrick plasma cleaner for 1 min to increase surface hydrophilicity. SEM images were collected from a Quanta 250 FEG Scanning Electron Microscope operated at 10 kV. One or two drops of the solution were added to the hydrophilic surface of the TEM grid, and excess solution was wicked off with a filter paper. After drying in atmosphere for 30 min, the grid was underwent TEM and SEM characterizations. All pH measurements were determined using a Sartorius PB-10 pH meter. Synthesis of BCP Vesicles and Stacked Structures. In a typical experiment, 10 mg of PS222-b-P4VP43 powder was first dissolved in 1 mL of THF in a 5 mL glass vial and 250 μL of water was directly dropped into the mixture, followed by vortex to give thorough mixing of the ingredients. Then, the vial was capped and sealed to avoid solvent loss, and the mixture was placed in a silicon oil bath and incubated under 80 °C for a corresponding period of time. After heating, the vial was taken out of the oil bath and slowly cooled down in the air to room temperature. After cooling, 100 μL of the product solution was extracted and mixed with 1 mL of water or a solution of certain acidity to quench the polymer aggregates. Then, the mixture was centrifuged at 13 000 rpm for 7 min to condense the solution, and these concentrated aggregates were further used for TEM and SEM characterizations.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b01341. Explanation of the acidity of deionized water; the calculation process of the yield of the aggregates; TEM images of the stacked bowls; SEM images of the multilayered structures; TEM images of the multiple morphologies in a sample; SEM images showing the openings of the bowls, TEM images of the structures upon basification (PDF)

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X.W. and S.L. contributed equally to this work.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.W.). *E-mail: [email protected] (H.C.). ORCID

Hongyu Chen: 0000-0002-5325-9249 E

DOI: 10.1021/acs.macromol.9b01341 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.9b01341 Macromolecules XXXX, XXX, XXX−XXX