Spiral and Mesoporous Block Polymer Nanofibers Generated in

Dec 29, 2014 - Cylindrical Micelles of PS-b-P2VP (with Different. Compositions) polymer sample unconfined micelles confined micelles. PS27.7k-b-P2VP4...
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Spiral and Mesoporous Block Polymer Nanofibers Generated in Confined Nanochannels Peilong Hou, Hailong Fan, and Zhaoxia Jin* Department of Chemistry, Renmin University of China, Beijing, 100872, P. R. China S Supporting Information *

ABSTRACT: Spiral-like or various porous polymer nanofibers have great applications in biosensor, bioengineering, and template-fabrication of functional inorganic materials. However, the fabrication of polymer nanostructures with controllable porous or spiral morphology in one process is a big challenge. Here we first demonstrated a general and easy method to generate spiral or porous block copolymer (BCP) nanofibers by using geometric confinement of nanochannels to disturb the self-assembly of BCP while nonsolvent is induced into BCP solution. Continuous spiral polymer nanofibers and polymer nanofibers with hierarchical porous nanostructures can be easily generated within channels of anodic aluminum oxide (AAO) membranes by tuning the composition and concentration of BCP. This study first reports the influence of cylinder confinement to the arrangement of BCP micelles. These spiral and porous BCP nanostructures are not only good templates to generate functional inorganic nanostructures, but also promising candidates to create biosensors or to load catalyst because their enlarged surface area enables high guest concentrations.

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INTRODUCTION Spirals and helical morphologies, like the shells of snails, righthanded helical DNA double-strands, are often observed in nature in organisms. These fascinating helical nanostructures are usually associated with biomolecules, but absent in synthetic materials. The fabrication of ceramics arrays (ZnO, SiO2, TiO2) with biomimic nanostructures attracts much attention of researchers.1,2 Several strategies for fabricating materials with helical nanostructures have been developed.3 The reported methods include: (1) by using chiral surfactants, chiral reaction field4,5 or bioscaffold6 as templates, (2) via morphological transformation accompanied by a reduction in surface free energy,7 (3) selective electroless metallization on a phospholipid microtubule template,8 (4) combining sol−gel reaction with supramolecular self-assembly of sugar-based amphiphile9 or organogelators,10 and so on. Recent studies observed that electromagnetic fields with strong optical chirality can be generated in the near field of chiral plasmonic nanostructures.11−14 Asymmetric gold nanoshells on ZnO achiral nanopillars behave like chiral materials.15 Twisted or helical polymer nanostructures are good templates for generating inorganic helical nanomaterials. For creating twisted or helical polymer nanofibers, building a homochiral blocks in polymer is a general route to create homochiral helical nanofibers.16 In particular, chiral block copolymer (BCP) comprised of chiral entities can form helical nanostructures based on selfassembly.17,18 Except above-mentioned strategies with the addition of chiral molecules or using chiral scaffolds, physical factor also helps the formation of twisted nanostructures. Zhao et al. observed that © XXXX American Chemical Society

space confinement is the most important factor for the formation of carbon nanotube array double helices.19 Chen et al. further reported a general methodology of using emulsions as space confinement for coiling various nanofilaments, such as carbon nanotubes, Pd nanowires and MnO2 nanowires.20 Polymeric colloids are also observed to deform polymertethered gold nanowires which are confined inside.21 In contrast, spatial confinement also significantly influences the morphology and even properties of polymers themselves.22−26 Confinement not only induces the miscibility of polymer blends,27 but also disturbs the self-assembly and crystallization of BCP.28−36 Russell’s group has reported that confinement (cylindrical pores in alumina membranes with diameters of 33− 45 nm) will induce the transformation from cylinders to helices of phase separation morphology of polystyrene-b-polybutadiene (whose equilibrium period L0 ∼ 29.1 nm).37 Lodge et al. studied the hierarchical nanostructures of BCP confined within bicontinuous microemulsion-derived nanoporous polyethylene.38 Confined assembly of silica−surfactant composite mesostructures have also been reported by Wu et al.2 Commensurability is the structural-directing factor in these studies.33,37−39 However, although the confinement to BCP is a hot topic in recent studies, there is no attempt to investigate if the confinement is also a critical factor in the self-assembly of BCP solutions. Moreover, in above-mentioned studies of confined BCP, the interaction between BCP and boundary Received: September 18, 2014 Revised: November 29, 2014

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of poly(styrene)-b-poly(4-vinylpyridine) in a mixed solvent of THF and DMF.51−54 The type of BCP micelles generated in THF/DMF and the nonsolvent-induced phase separation is believed a critical factor to determine the pore structure.51,52,54 Recently, Oss-Ronen et al. reported their study of PS-b-P4VP micellization in solutions and dried films by using cryogenic high-resolution scanning electron microscopy, cryogenic transmission electron microscopy and small-angle neutron scattering.50 They observed that micelles are formed in BCP solution and P4VP is as micelle core. As water is introduced in BCP solution, at equilibrium micelles structures have PS core and P4VP corona. In our study, the process of micelles formation is confined in cylindrical channels of AAO membranes. The slower speed of solvent exchange in confined space will advance the formation of micelle structures at equilibrium. Scheme 1

walls is occasionally dominant in determining the transformation of phase-separation morphologies.29,40,41 However, for polymer solutions, solvent property, i.e., its affinity to boundary walls, will break and even diminish the interaction between polymer and boundary walls, resulting in an isolated polymer system in a confined space.42 It gives us an opportunity to investigate how confinement disturbs the selfassembly of BCP in solution without the interference of interaction between boundary wall and BCP. To answer this question, we studied BCP solution system confined in cylindrical channels of AAO membranes. Nonsolvent was induced into the system to lead micelles solidification in cylindrical channels. Spiral-like, double helical, and hierarchical mesoporous nanofibers are obtained by tuning the composition and concentration of BCP solutions. This study first presents how confinement impacts on BCP micelles arrangement. It is also an easy strategy to generate spiral-like nanofibers and hierarchical mesoporous nanotubes in one-pot which have great applications in many fields further.

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Scheme 1. Schematic Illustration Showing Experimental Process for Preparing Spiral-Like or Mesoporous PS-b-P2VP Nanostructures

EXPERIMENTAL SECTION

Polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) samples with different molecular weights were obtained from Polymer Source Inc. (Canada) and used as received. Analytical tetrahydrofuran (THF), dimethylformamide (DMF) and sodium hydroxide were purchased from Sinopharm Chemical Reagent Beijing Co., Ltd., and used as received. Deionized water (Millipore Q > 18 MΩ cm) was used for all aqueous solutions. Anodic aluminum oxide (AAO) membrane (Anodic 13, 0.2 μm) was the product of Whatman Ltd. The membranes were thoroughly rinsed by organic solvents and deionized water and then annealed at 150 °C for 1 h in vacuum before use. Table S1 (Supporting Information) presents all detailed information on these BCP samples we used in our studies. PS-b-P2VP was dissolved in mixed solvent (WTHF:WDMF = 3:7) in different concentrations (5, 10, 20, 30, and 40 wt %). Then these BCP solutions were imbibed into AAO membrane and the AAO membrane filled with BCP solutions was immediately immersed into large amount of water at room temperature and soaked for 12 h. Finally, AAO membrane with frozen BCP micelles inside was dried and AAO membrane was dissolved in 3 mol/L NaOH solution to release BCP micelles nanostructures. PS32.5kb-P2VP7.8k and PS47k-b-P2VP24k (20 wt %) dissolved in THF/DMF mixed solvents with different weight ratios, 1:0, 7:3, 5:5, 3:7, and 0:1, were also used to generate BCP nanostructures in AAO channels. Obtained BCP nanostructures were collected and washed by fresh water several times and then characterized by using scanning electron microscope (SEM) and transmission electron microscope (TEM). In some cases, SEM characterizations were directed conducted by using BCP nanostructures@AAO membrane. Scanning electron microscope (SEM, JEOL 7401) characterization was performed at an accelerating voltage of 5 kV. The samples were coated with a thin layer of gold before SEM characterization. Transmission electron microscope (Hitachi TEM, H-7650B) was operated at an accelerating voltage of 100 kV. A droplet of PS-b-P2VP nanofibers suspension in water was placed onto copper grids for TEM analysis. To increase the contrast, the sample was stained by I2 vapor at 60 °C for 30 min before observation.

illustrates the detailed experimental process. Solid micelles were exposed and collected after dissolving AAO membrane in NaOH solution. For a comparison, all BCP solutions were also coated on silicon wafer and then dipped into water bath to induce phase separation. Obtained unconfined micelle nanostructures were presented in Figure S1. Figure 1 shows SEM images of PS-b-P2VP spiral-like nanofibers obtained via confined self-assembly. Morphological characterization before dissolving AAO membrane clearly presents spiral-like nanofibers stacked inside cylindrical channels (Figure 1b). Obviously, confinement in cylindrical channels induces the cylindrical micelles arranged in a closely packed way (Figure 1c). From the image, which shows the connection part of BCP nanostructures, from outside of AAO membranes to AAO channels (Figure 2), we can find that BCP nanofibers which are cylindrical micelles in BCP solutions are randomly dispersed outside AAO channels, however, when they contact AAO channels, nanofibers start to twist and arrange like spirals to accommodate the narrow space, resulting in arrays of spiral-like BCP nanofibers. Micelles fibers with a helical arrangement in channels have advantages in stacking density compared with those parallel to channels. These spiral-like nanofibers are not unique for PS23.6k-b-P2VP10.4k. We have tested six PS-b-P2VP samples with different molecular weights and volume fractions of two blocks, most of them will generate spiral nanofibers in a range of BCP concentrations (Figure S2). Maritan et al. have investigated the optimal shape of closely packed compact

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RESULTS AND DISCUSSION Self-assembly of BCP in solutions not only is a research topic of fundamental interests but also is related to practical applications, such as the fabrication of nanoporous membranes.43−45 Membranes generated by using BCP self-assembly have a high density of uniform pores.46,47 Nonsolvent-induced phase separation, named as SNIPS, is a simple way to generate hierarchically porous films.48−50 Peinemann et al. first demonstrated the process combining SNIPS and self-assembly B

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Figure 1. SEM images of PS23.6k-b-P2VP10.4k spiral like nanofibers arrays generated in AAO membranes. The concentration of PS-bP2VP is 30 wt %. Mixed solvent contains THF and DMF in 3:7 weight ratios. (a) Arrays of spiral-like BCP nanofibers, (b) spiral-like nanofibers inside AAO channels, and (c) magnified image of closepacked spiral-like BCP nanofibers.

Figure 3. Images of double helices of PS148.5k-b-P2VP19k nanofibers generated in AAO membrane. The concentration of BCP is 10 wt %. (a) SEM image of double helical BCP nanofibers inside channels, (b) TEM image of double helical BCP nanofibers, and (c) SEM image of double helical BCP nanofibers arrays.

BCP cylindrical micelles. We compared diameter values of different BCP nanofibers and presented them in Table 1. On Table 1. Diameter Values (nm) of Unconfined and Confined Cylindrical Micelles of PS-b-P2VP (with Different Compositions) polymer sample PS27.7k-b-P2VP4.3k PS23.6k-b-P2VP10.4k PS32.5k-b-P2VP7.8k PS47k-b-P2VP24k PS125k-b-P2VP58.5k PS148.5k-b-P2VP19k

unconfined micelles 25.1 23.2 29.6 36.5 59.0 68.9

± ± ± ± ± ±

1.7 2.0 2.4 2.4 4.6 3.9

confined micelles 24.9 22.9 31.7 37.1 58.5 69.7

± ± ± ± ± ±

3.0 2.2 3.2 3.0 3.2 3.4

a

All values were measured based on TEM images of unconfined and confined micelles.

Figure 2. SEM image of the connection part of PS32.5K-b-P2VP7.8K cylindrical micelles from outside to AAO membrane. The concentration of BCP is 20 wt %. Randomly dispersed cylindrical micelle fibers are twisted inside AAO channels. The stacking density of helical arrangement is higher than that in parallel way.

the basis of these values, we observed that the size of cylindrical micelles of PS148.5K-b-P2VP19K is over twice that of micelles composed of PS23.6K-b-P2VP10.4K. To these larger micelles, the narrow geometric confinement forces them to twist closely, forming double helical nanofibers. Morphological fluctuations and size distributions of BCP nanostructures may be due to the influence of transient morphologies and solvent properties. On the other hand, the concentration shows significant influence to the morphology of BCP confined in AAO channels. In the lower concentration, generally from 5−10 wt % for PS-b-P2VP (except PS148.5K-b-P2VP19K), the frozen nanostructures are uncontinuous, isolated spherical micelles or mixture of spherical and cylindrical micelles (Figure 4a and 4b). When the concentration of BCP solutions is up to 20 wt %, continuous nanostructures are generated (Figure 4c). The further increase of BCP concentration will lead to morphological transformation, from spiral (Figure 5a,d) to mesoporous (Figure 5b,e), and finally to continuous multilayer nanofibers (Figure 5c,f). Chen et al. have reported the effect of nonsolvent on the formation of polymer nanospheres and nanorods in the AAO channels.56 With the increasing of polymer concentration,

strings, which is commonly encountered in biology, chemistry, and physics.55 They found that for bulk-like solutions which are not influenced by boundary effects, helices are optimal arrange, that is also the way of natural proteins to choose. We believe that the optimal packing is also the underlying mechanism of helical arrangement of BCP cylindrical micelles in AAO channels. In particular, we noticed special double-helices nanostructures were generated from PS148.5K-b-P2VP19K. Figure 3 shows clear double-helical BCP nanofibers obtained in AAO channels by using PS148.5K-b-P2VP19K. Two nanofibers are closely twisted together to form double helices to fit them into the narrow space. This sample has larger molecular weight compared with other BCP samples, so we wonder whether the large molecular weight may induce the size difference of BCP micelles, leading to the change of obtained nanostructures. Because these nanofibers are frozen from original BCP cylindrical micelles, the diameter value of BCP nanofibers will correspond to the size of C

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Figure 4. SEM images of frozen micelle nanostructures inside AAO channels of PS32.5K-b-P2VP7.8K at various concentrations. (a) 5 wt %, (b) 10 wt %, and (c) 20 wt %.

Figure 5. Morphological characterizations of PS32.5K-b-P2VP7.8K nanostructures generated at different BCP concentrations. (a, d) 20 wt %, (b, e) 30 wt %, and (c, f) 40 wt %.

40 wt % BCP solution showed multilayered feature. They somehow resemble with multilamellar micelles. Similarly, PS148.5K-b-P2VP19K with a smaller volume fraction of P2VP also shows that continuous nanostructure generated in AAO channels is appearing in advance (Figure 2). Other BCPs, such as PS125 K-b-P2VP58.5K, although it also has large molecular weight, the happening of continuous nanostructure is still at 20 wt % (Figure S3, PS125 K-b-P2VP58.5K spherical micelles inside AAO channels at 10 wt %). In Table 2, we summarized typical morphological features with various concentrations of all BCP samples we obtained (Figure S4 provides more detailed information on these different morphologies). We noticed that spiral-like BCP nanofibers are formed in BCP samples with moderate P2VP volume fractions (∼20% ∼ 33.3%). In addition, Stegelmeier et al. recently reported their systematic investigation of the structure formation pathways and transient morphologies in the formation of mesoporous PS-b-P2VP membranes.58 They provided a phase diagram of PS-b-P2VP/THF/DMF ternary system. With varying of initial solvent composition, membranes’ morphology changes significantly. We also compared the influence of solvent composition to BCP nanostructures generated in AAO channels. Figure S5 shows PS32.5K-b-P2VP7.8K nanostructures generated in different initial solvents, in which weight ratio of THF/DMF is changed from 1:0, 7:3, 5:5, and 3:7 to 0:1. Spiral-

the obtained PMMA nanostructure shows increasing aspect ratio, from nanospheres (5 wt %), to long nanorods (32 wt %).56 Dorin et al. have investigated the relationship between porous structures of asymmetric BCP membrane and the concentration. They revealed a concentration-dependent onset of ordered structure formation.57 In our experiments, spherical and cylindrical micelles clearly are observed in 5 and 20 wt % for PS32.5K-b-P2VP7.8K, multilayer BCP nanofibers are at 40 wt %, the structural feature for 30% BCP samples contains both characteristics of cylindrical micelles and multilayer, presenting hierarchical mesoporous nanofibers. Moreover, the volume fraction of two blocks in PS-b-P2VP shows some influence to obtained nanostructures. When we compared PS27.7K-b-P2VP4.3K which has smaller fraction of P2VP with other BCPs (Figure 6), we noticed that continuous interconnected strands are entangled together without identical spiral-like morphology as other BCP samples have showed (Figure 6a). Moreover, this connected structure is generated at a lower concentration (10 wt %), at which other BCP samples form only isolated spherical or cylindrical micelles, without continuous nanostructures (Figure 4). When the BCP concentration reaches 20 wt %, in which other BCPs form continuous spiral nanofibers, PS27.7K-b-P2VP4.3K changes to mesoporous nanofibers, and the multilayered characteristic shows obvious in 30 wt % (Figure 6c). These nanofibers from D

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Figure 6. Morphological characterizations of PS27.7K-b-P2VP4.3K nanostructures generated at various BCP concentrations: (a−c) 10 wt %, (d−f) 20 wt %, (g−i) 30 wt %, and (j−l) 40 wt %. All SEM images in left column show these obtained nanostructures confined in AAO channels; those in middle column present their release nanostructures; TEM images in right column show their inner structural features.

Table 2. List of Different BCP Nanostructures Generated in AAO Channels from PS-b-P2VP with Various Concentrations and Compositions structure for polymer concn (wt %) PS27.7k-b-P2VP4.3k PS23.6k-b-P2VP10.4k PS32.5k-b-P2VP7.8k PS47k-b-P2VP24k PS125k-b-P2VP58.5k PS148.5k-b-P2VP19k

10

20

30

40

mesoporous spherical spherical spherical spherical double-helices

mesoporous spiral-like spiral-like spiral-like mesoporous multilayer

mesoporous spiral-like mesoporous mesoporous

multilayer multilayer

tendency with varying solvent compositions, showing that it may be common for cylinder-forming BCPs. In particular, it is worth noting that these one-dimensional BCP nanostructures generated by our confined-SNIPS share some structural features with mesoporous nanostructures generated from selective-swelling59,60 or selective-etching61,62

like nanofibers can be obtained in weight ratios of THF/DMF at 5:5 and 3:7, which correspond to strongly segregated regions in PS-b-P2VP/THF/DMF phase diagram, disordered cylinder (DC) and ordered cylinder (OC) regions. Morphological variation of PS47K-b-P2VP24K nanostructures also shares similar E

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of BCP nanofibers or nanotubes. These mesoporous BCP nanostructures have been used as templates to fabricate various inorganic mesoporous materials, showing promising prospects in applications. For example, mesoporous carbon micro- and nanowires are fabricated by carbonization of the one-dimensional nanostructure created by using triblock copolymer Pluronic F127, phloroglucinol, formaldehyde, and traces of HCl in ethanol/water.63 One-dimensional semiconductor nanostructures with helical, stacked-doughnut-like, interconnected tubular morphologies are generated through atomiclayer deposition based on mesoporous 1D BCP nanostructures.64,65 Compared with selective-swelling or selectiveetching, our strategy to generate helical and mesoporous onedimensional BCP nanostructures has great advantages in its simple process and large quantities. It will further advance the development of using twisted one-dimensional BCP nanostructure as templates in generation of inorganic functional materials.

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CONCLUSIONS Herein, we reported a general methodology to prepare springlike, interconnected-strands like, double-helical and hierarchical mesoporous BCP nanostructures via a simple process, which combined the self-assembly of BCP in solution with nonsolvent-induced phase separation confined in AAO channels. We demonstrated that when this process is conducted in cylindrical nanochannels of AAO membrane, geometric confinement disturbs the arrangement of BCP cylindrical micelles, resulting in unprecedented transformation of BCP nanostructures: spring-like, double-helical, interconnected strands or mesoporous BCP nanofibers are generated inside channels. Multilayered BCP nanofibers will be formed at a higher BCP concentration. Different nanostructures can be produced in one process by simply tuning the BCP composition or concentration. A further study of utilizing these one-dimensional BCP nanostructures is ongoing. This unique and simple method generates hierarchical one-dimensional BCP nanostructures, which are useful as templates to generate inorganic functional materials with chiral or mesoporous structural features.

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ASSOCIATED CONTENT

S Supporting Information *

Table with detailed information on BCP samples, SEM images, and generated BCP nanostructures. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the National Natural Science Foundation of China (Grants 21374132, 51173201) for financial support.

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REFERENCES

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DOI: 10.1021/ma501933s Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/ma501933s Macromolecules XXXX, XXX, XXX−XXX