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Dec 29, 2015 - In this work, we report the preparation of complex nano-objects by means of a stepwise self-assembly of two polymer-polyoxometalate hyb...
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Tube-graf t-Sheet Nano-Objects Created by A Stepwise SelfAssembly of Polymer-Polyoxometalate Hybrids Jing Tang,† Xue-Ying Li,† Han Wu,† Li-Jun Ren,† Yu-Qi Zhang,† Hai-Xia Yao,‡ Min-Biao Hu,† and Wei Wang*,† †

Center for Synthetic Soft Materials, Key Laboratory of Functional Polymer Materials of Ministry of Education and Institute of Polymer Chemistry, Nankai University, and Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300071, China ‡ State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China S Supporting Information *

ABSTRACT: In this work, we report the preparation of complex nano-objects by means of a stepwise self-assembly of two polymer-polyoxometalate hybrids (PPHs) in solution. The PPHs are designed and synthesized by tethering two linear poly(εcaprolactone)s (PCL) of different molecular weights (MW) on a complex of a Wells-Dawson-type polyoxometalate (POM) cluster and its countraions. The higher MW PCL−POM self-assembled into nanosheets, while the lower MW PCL−POM assembled into nanotubes just by altering the ratio of water in the DMF−water mixed solvent system. The two nano-objects have a similar membrane structure in which a PCL layer is sandwiched by the two POM-based complex layers. The PCL layer in the nanosheets is semicrystalline, while the PCL layer in the nanotubes is amorphous. We further exploited this MW-dependence to self-assemble the nanotubes on the nanosheet edges to create complex tube-graf t-sheet nano-objects. We found that the nanotubes nucleate on the four {110} faces of the PCL crystal and then further grow along the crystallographic b-axis of the PCL crystal. Our findings offer hope for the further development of nano-objects with increasing complexity.

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INTRODUCTION In chemistry, organic polymers and inorganic clusters are two types of chemical compounds with totally different structures and properties. Normally, organic polymers, which are synthesized by polymerization of monomers, have a linear structure with chain flexibility and a high molecular weight (MW). Most of their most properties exhibit a strong MWdependence.1 Polymers have good processability and mechanical properties thus are the most widely used materials. Inorganic clusters are atomically precise ensembles of bound atoms or molecules with nanoscale sizes and unique shapes.2 Many elements are involved in the constitution of clusters. Thus, they exhibit an amazing diversity in structure, chemical composition, shape, and functionality. Covalently linking organic polymers and inorganic clusters is an efficient strategy for the development of novel polymer−cluster hybrid materials with a desire that they can combine both properties of polymers and functions of clusters.3−9 The simplest hybrids are © 2015 American Chemical Society

composed of a linear polymer and an approximately spherical cluster.10−47 The experimental10−44 and simulated45−47 studies have demonstrated that different structure and properties between organic polymers and inorganic clusters allow them to self-assemble into nanoaggregates in solution and nanostructures in bulk. These studies also indicate that the differences in their geometrical shapes and interactions play a vital role in formation of diverse self-assembled nano-objects or nanostructures. Polyoxometalates (POMs) are early transition metal−oxygen anion clusters and have a wide range of attractive functions.48 Aiming at improving and creating more functionalities based on these intriguing clusters, they have been covalently linked with polymers, such as polystyrene (PS), poly(ε-caprolactone) Received: December 10, 2015 Revised: December 26, 2015 Published: December 29, 2015 460

DOI: 10.1021/acs.langmuir.5b04504 Langmuir 2016, 32, 460−467

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−CH2CH2CH2O−) (Supporting Information Figure S1). MS (MALDI-TOF): calcd monoisotopic mass for 19mer [M19· Na]+: 2247.1 Da (Supporting Information Figure S2); found: m/z 2247.3. GPC: M̅ w = 2.3 kg/mol, M̅ w/M̅ wn = 1.15 (Supporting Information Figure S6). Thus, the product was denoted as PCL19−OH. PCL19−COOH. PCL19−OH (1.2 g, 0.5 mmol), succinic anhydride (75 mg, 0.75 mmol), and triethylamine (Et3N) (61 mg, 0.6 mmol) were dissolved in 50 mL of fresh distilled dichloromethane (CH2Cl2), The reaction mixture was stirred at 40 °C under argon for 24 h and the CH2Cl2 solution was washed with hydrochloric acid (HCl) solution (pH = 2) (50 mL × 3), distilled water (50 mL × 3). Then the organic portion was collected and dried over anhydrous sodium sulfate overnight. The polymer was precipitated in cold methanol. The purified product was dried under vacuum until a constant weight was observed. The product was denoted as PCL19− COOH. Yield: 92%. 1H NMR (400 MHz, CDCl3, δ): 4.68 (d, 2H, −OCH2CCH), 4.06 (t, −CH2OCO), 2.65 (m, 4H, −CH2CH2COOH), 2.48 (t, 1H, −CH2CCH), 2.31 (t, −CH2CO), 1.69−1.60 (m, −CH2CH2CO, −CH2CH2O), 1.42−1.33 (m, −CH2CH2CH2O−) (Supporting Information Figure S3). PCL19−Tris. To a 100 mL round-bottomed flask equipped with a condenser and a magnetic stirrer was added PCL19− COOH (0.9 g, 0.35 mmol), EEDQ (112 mg, 0.46 mmol), and Tris (42 mg, 0.42 mmol), followed by addition of fresh distilled 20 mL of acetonitrile (CH3CN) and 20 mL of CH2Cl2 to fully dissolved all the samples. The solution was stirred and refluxed at 85 °C under argon for 24 h. The reaction was stopped by cooling to room temperature. The solution was concentrated to the minimum. The concentrated liquid was then added dropwise to cold methanol to collect a white solid. The purified product was dried under vacuum until a constant weight was observed. The product was denoted as PCL19-Tris. Yield: 83%. 1H NMR (400 MHz, CDCl3, δ): 6.74 (s, 1H, −(CH2)3CNHCO−), 4.68 (d, 2H, −OCH2CCH), 4.06 (t, −CH2OCO), 3.69 (s, 6H, −(CH2)3CNHCO−), 2.71 (t, 2H, −NHCOCH2−), 2.51 (t, 2H, −NHCOCH2CH2−), 2.48 (t, 1H, −CH2CCH), 2.31 (t, −CH2CO), 1.69−1.60 (m, −CH2CH2CO, −CH2CH2O), 1.42−1.33 (m, −CH2CH2CH2O−) (Supporting Information Figure S4). PCL19−POM Hybrid. The (Bu4N+)6H3(P2W15V3O62)9− (0.6 g, 0.11 mmol) and PCL19-Tris (0.29 g, 0.11 mmol) were dissolved in DMF (40 mL). The reaction was kept at 80 °C for 7 days under argon and was then cooled and concentrated to a minimum. The solution was then precipitated in diethyl ether. The yellow solid was redissolved in the minimum volume of CH3CN and then reprecipitated in diethyl ether. The purified product was dried under vacuum until constant weight was reached. The product was denoted as S-POM−PCL19. Yield: 88%. 1H NMR (400 MHz, d6-DMSO): 7.60 (s, 1H, −(CH2)3CNHCO−), 5.52 (s, 6H, −(CH2)3CNHCO−), 4.67 (d, 2H, −OCH2CCH), 3.98 (t, −CH2OCO), 3.17 (t, 48H, NCH 2 CH 2 −), 2.27 (t, −CH 2 CO), 1.49−1.62 (m, −CH2CH2CO, −CH2CH2O, NCH2CH2−), 1.26−1.37 (m, −CH 2 CH 2 CH 2 O−, NCH 2 CH 2 CH 2 −), 0.94 (t, 72H, NCH2CH2CH2CH3) (Supporting Information Figure S5). GPC: M̅ w = 8.6 kg/mol, M̅ w/M̅ wn = 1.14 (Supporting Information Figure S6). The same method was carried out using to prepare POM− PCL43. MS (MALDI-TOF) of PCL43−OH: calcd monoisotopic mass for 43mer [M19·Na]+: 4985.4 Da (Supporting Information

(PCL), poly(ethylene glycol) (PEG), poly(N,N-diethylacrylamide) (PDEAAm), to create novel polymer−POM hybrid materials.9,25,27,31,33,34,38,41−43 In their solutions, these polymer−POM hybrids can self-assemble into hybrid aggregates with different supramolecular structures, such as hybrid vesicles,25,33 POM nanotubes,27 nanosheets,41 hybrid latexes42,43 and so on. In bulk, a hybrid of poly(ethylene glycol) (PEG) and POM cluster forms a hybrid lamellar structure composed of alternatively arranged PEG and POM layers.38 Importantly, counterion-mediated electrostatic interaction between the POM clusters results in a stable POM layer in the temperature above the melting points of the PEG block thus mechanical properties (such as shear storage modulus) of the hybrids are improved. Meanwhile, nanocomposites of the pure PEG and the self-assembled aggregates of PEG−POM hybrid have been prepared and their shear storage moduli can be tailored over many orders of magnitude at temperatures above the PEG melting point.41 Thus, the nanosized aggregates act as a nanoenhancer to efficiently improve the performance of the nanocomposites. Here we report a new strategy for the creation of complex nano-objects from simple aggregates through a stepwise selfassembly process of polymer−POM hybrids with different molecular weights (MWs) of the polymer blocks. More precisely, a careful selection of MWs of the polymer blocks is a key step to achieving a MW-dependent self-assembly process of the hybrids in their solutions. Because of a change in solubility of the hybrids, both the aggregation condition and the supramolecular structure of self-assembled aggregates are MWdependent. We further utilize this MW dependence to perform a stepwise self-assembly of the hybrids to construct complex nano-objects composed of the two simple aggregates. The detailed self-assembly process and mechanism will be discussed in the text.

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EXPERIMENTAL SECTION MATERIALS

The (Bu4N+)6H3(P2W15V3O62)9− complex was synthesized according to the literature.49 Tin(II) 2-ethylhexanoate (Sn(Oct)2; 96%), ε-caprolactone (ε-CL; 99%), succinic anhydride (99%), tris(hydroxymethy1) aminomethane (Tris; 99.8%), and 2-ethoxy-1-ethoxycarbonyl-1,2-dihydroquinoline (EEDQ; 99%) were purchased from Aldrich. Other reagents were purchased from major chemical supplies and used as received unless otherwise noted. All the solvents were dried and distilled prior to use. ε-CL was purified by drying over CaH2 followed by distillation under reduced pressure before being stored over molecular sieves in a refrigerator. Synthetic Procedures. PCL19−OH. Propargyl alcohol (50 mg, 0.9 mmol), ε-CL (4.0 g, 35 mmol), and 10 mL of anhydrous toluene were added into a flame-dried Schlenk flask. After three freeze−pump−thaw cycles, Sn(Oct)2 (91 mg, 0.23 mmol) was added under argon. The Schlenk flask was then placed in an oil bath of 80 °C. After 16 h, the flask was cooled in an ice water bath and the mixture was precipitated into cold methanol twice. The white solid was collected after filtration and dried at 25 °C under vacuum overnight to give a white solid. Yield: 73%. 1H NMR (400 MHz, CDCl3, δ): 4.68 (d, 2H, −OCH 2 CCH), 4.06 (t, −CH 2 OCO), 3.65 (t, 2H, −CH2OH), 2.48 (t, 1H, − CH2CCH), 2.31 (t, −CH2CO), 1.69−1.60 (m, −CH2CH2CO, −CH2CH2O), 1.42−1.33 (m, 461

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Langmuir Figure S7); found: m/z 4985.7. GPC: M̅ w = 6.1 kg/mol, M̅ w/ M̅ wn = 1.19 (Supporting Information Figure S6). PCL43−POM Hybrid. Yield: 81%. 1H NMR (400 MHz, d6-DMSO) shown in Supporting Information Figure S8. GPC: M̅ w = 12.5 kg/mol, M̅ w/M̅ wn = 1.14 (Supporting Information Figure S6). Preparations. To ensure reproducibility, all experiments were repeated at least three times. PCL19−POM Nanotubes. PCL19−POM nanotubes were prepared by dissolving 3.6 mg PCL19−POM in 1.2 mL DMF and then adding 0.6 mL deionized water. The temperature of the mixture solution was increased to 80 °C and kept for 2 h. After complete dissolution, the solution was allowed to cool to 25 °C for at least 3 days. PCL43−POM Nanosheets. PCL43−POM nanosheets were prepared by dissolving 3 mg PCL43−POM in 1.25 mL dry DMF in a vial and then adding 0.25 mL deionized water. The temperature of the mixture solution was increased to 80 °C and kept for 4 h. It was then quenched to 25 °C for at least 5 days to allow the hybrid to crystallize. Tube-graf t-Sheet Nano-Objects. Nine hundred microliters of the above solution of PCL43−POM lamellar single crystals was sedimentated-filtered to remove excess of PCL43− POM molecule. The PCL43−POM nanosheets were redispersed in a mixture of 540 μL DMF and 108 μL deionized water. To this solution was added 60 μL of DMF containing 1.8 mg of PCL19−POM and then added 192 μL deionized water with 0.16 μL/min at 30 °C. The mixture was aged for 5 days at 25 °C. Characterization. The 1H and 31P nuclear magnetic resonance (NMR) experiments were performed on a Varian Unity Plus-400 NMR spectrometer. Size exclusion chromatography (SEC) was carried out in 0.2% LiBr/N, N-dimethylformamide (DMF) with the flow rate of 0.6 mL/min at 60 °C on a Viscotek 270max system equipped with a VE2001 SECmax isolated pump, an autosampler, a P3000 column, a VE3580 RI detector, a 270 max light scattering detector and a viscometer. Matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectra were recorded on a Bruker Autoflex MALDI TOF-TOF III LRF200 CID. Differential scanning calorimetry (DSC) measurements were carried out on a DSC thermal analysis system (Diamond from PerkinElmer). The samples were heated from room temperature to 80 °C at a heating rate of 5 °C/min under nitrogen atmosphere. X-ray diffractometry (XRD) measurements were performed using a Rigaku D/Max-2500 X-ray diffractometer equipped with a Cu-Ka radiation (λ = 0.154 nm) source operating at 40 kV/100 mA along a range of 0.6° < 2θ < 40°. The samples were prepared by casting solutions on a freshly cleaved glass surface. The mixed solvent was rapidly removed by using a piece of filter paper and then dried under vacuum at 25.0 °C for 24 h. Transmission electron microscopy (TEM) and energydispersive X-ray spectroscopy (EDX) were performed using a field emission transmission electron microscopy FEI Tecnai G2 F20 operating at an acceleration voltage of 200 kV. The selected area electron diffraction (SAED) pattern was obtained using a JEOL JEM-2100 transmission electron microscope operated at 200 kV. The samples were prepared by casting solutions on carbon film-coated copper grids. The mixed solvent was rapidly removed by using a piece of filter paper and then dried under vacuum at 25.0 °C for 24 h.

Atomic force microscopy (AFM) images were recorded using a multimodel atomic force microscope (Digital Instrumental Nanoscope IV) in tapping mode. The samples were prepared by dropping a drop of the solution on a freshly cleaved mica surface. The mixed solvent was rapidly removed by using a piece of filter paper and then dried under vacuum at 25.0 °C for 24 h.

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RESULTS AND DISCUSSION Design and Synthesis of Polymer−Cluster Hybrids. In this work, we will report self-assembly of the two polymer− cluster hybrids in solution into complex nano-objects. Two linear PCL blocks with different MWs and a Wells-Dawsontype POM (P2W15V3O62)9− cluster49 are, respectively, selected as the polymer and cluster building blocks (Figure 1a). This

Figure 1. (a) Structure of the PCL−POM hybrids. (b) Mass spectra showing molecular weights and molecular weight distributions of the PCL blocks.

rigid POM core has a calculated molecular weight of 3969 Da and an ellipsoid with a long axis of 1.2 nm and a short axis of 1.0 nm. In this study, the anionic core forms a complex with six counterions (tetrabutylammoniums, Bu4N+). (For simplification, the POM−ligand complex is denoted as POM cluster hereafter.) The complex has a calculated molecular weight of 5422 Da and was dissolved in dimethylformamide (DMF). The PCL blocks are flexible and nonpolar. The two PCL blocks used have number-average MWs, M̅ n = 2220 Da and M̅ n = 4960 Da determined using mass spectrometry (Figure 1b) and MW distributions, M̅ w/M̅ n = 1.15 and 1.19 determined using size exclusion chromatography (SEC) (Figure S6 and Table S1, Supporting Information). The corresponding degrees of polymerization are NPCL = 19 and 43, so the two PCL blocks are denoted as PCL19 and PCL43. The structure of the PCL− POM hybrids are also shown in Figure 1a. Their synthetic procedure is shown in Scheme S1 in the Supporting Information. The two PCL−POM hybrids are composed of the POM cluster head and the PCL chain tails. They are denoted as PCL19−POM and PCL43−POM, respectively. The absolute weight-average MWs, M̅ w of the two PCL−POM hybrids are M̅ w = 8800 Da and M̅ w = 12 500 Da, respectively, with a narrow MW distribution M̅ w/M̅ n < 1.2 determined using SEC (Figure S6 and Table S1, Supporting Information). The degrees of crystallinity, f PCL, were f PCL = 0.69 for the two pure PCL blocks but f PCL = 0 for PCL19−POM and f PCL = 0.42 for 462

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Figure 2. Self-assembled hybrid nanosheets of the PCL43−POM. (a,b) TEM images showing hexagonal single- and multilayer nanosheets. (c) The ED pattern of a single-layer nanosheet. (d) The 3D AFM height image of multilayer nanosheets. (e) XRD pattern. (f) Proposed models of the hybrid sheet (f-1) and the arrangement of the POM clusters and PCL chains (f-2). The {110} and {100} faces and crystallographic a- and b-axes of the PCL crystal are highlighted.

PCL43−POM (Figure S9 and Table S2, Supporting Information). The melting and crystallization temperatures and the degree of crystallinity of the two PLC blocks are MWdependent and lower than those of the corresponding pure PCLs. MW-Dependent Aggregation in Solution. The PCL− POM hybrids dissolve well in DMF to form transparent solutions because DMF is a good solvent of the two building blocks. Drop-wise addition of deionized water results in the transparent solutions becoming turbid. Figure S10a,b (Supporting Information) show Tyndall scattering at DMF/water volume ratios r = 5:1 v/v for the PCL43−POM and at r = 2:1 v/v for the PCL19−POM, respectively. The final hybrid concentration in the DMF/water solvent mixture is ca. 2 mg/ mL. Obviously, the addition of water causes the hybrids to aggregate because the PCL blocks are not water-soluble. The MW-dependent minimal water content is reasonable: A longer PCL block corresponds to a lower minimal water content. The aggregates were carefully characterized using transmission electron microscopy (TEM), atomic force microscopy (AFM), and X-ray diffractometry (XRD). The samples were prepared by dropping a water dispersion of the aggregates onto the carbon coated grids for TEM and a freshly peeled mica surface for AFM. The dried samples were used for XRD. Hexagonal Nanosheets of PCL43−POM. The PCL43− POM self-assembles into hexagonal nanosheets (Figure 2), that resemble lamellar single crystals of PCL homopolymers, at r = 5:1 v/v. Two TEM images (Figure 2a,b) show single- (a) and multilayer (b) nanosheets. The energy-dispersive X-ray (EDX) analysis of the nanosheets shows signals of phosphorus, tungsten, and vanadium elements to confirm the existence of POM clusters (Figure S11, Supporting Information). A

selected-area electron diffraction (SAED) pattern was performed from a selected single-layer nanosheet (Figure 2c). The (110) and (200) diffraction spots are completely identical to those obtained in single crystals of the PCL homopolymers.50,51 Evidently, the PCL43 block in these PCL43−POM nanosheets crystallizes into lamellar single crystals. A three-dimensional (3D) AFM image of hexagonal multilayer aggregates is displayed in Figure 2d. The determined single-layer thickness is 12.3 ± 0.2 nm (Figure S12, Supporting Information). This indicates that the multilayer nanosheets are composed of a stack of the single-layer nanosheets. The XRD pattern of the dried sample (Figure 2e) displays two narrow diffraction peaks at 2θ = 21.2° and 23.5° relating to the PCL crystal50,51 and a broad diffraction peak at 2θ = 6.42° coming from the POM cluster.38,41 This suggests that the POM cluster and PCL block form the independent structures within the nanosheets. The d-spacing between the POM clusters is d = 1.38 nm. The peak broadening in comparison with that of the pure POM (Figure S13, Supporting Information) suggests that the POM arrangement in the nanosheets is less ordered than that in the crystalline sample of the pure POM. We suggest a model to describe the hybrid structure of the hexagonal PCL43−POM nanosheets (Figure 2f). Scheme f-1 depicts a nanosheet suggesting {110} and {100} faces on the crystallographic a- and b-axes of the PCL crystal. First, the nanosheets have a hybrid structure in which a crystalline PCL layer is sandwiched by two single POM layers (Scheme f-2 in Figure 2f).51 Clearly, there are the two PCL chains existing inbetween the two face-to-face POM clusters. Second, the formation of the flat nanosheets requires an equal crosssectional area (CSA) of the POM complexes and the PCL crystal. We estimated the CSA of individual POM complexes to 463

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Figure 3. Self-assembled hybrid nanotubes of the hybrid PCL19−POM. (a) TEM image showing a large amount of nanotubes. (b) TEM image showing four isolated nanotubes. The scheme depicts a tube and its corresponding TEM image when observed along the direction indicated as a red arrow. (c) Zoomed-in TEM image showing a fine structure of the tube wall. The yellow lines highlight the outline of a membrane. (d) The 3D AFM height image of a nanotube. (e) XRD pattern. (f) Suggested models for the tubes (f-1) and hybrid membrane (f-2).

A zoomed-in TEM image shows a fine structure of the dark tube wall with a 6.4 ± 0.7 nm wall thickness (Figure 3c and Figure S15b, Supporting Information). Here, the two yellow lines highlight the thin bright zone between the two dark tube tapes. A 3D AFM image shows a ca. 3 μm long nanotube (Figure 3d). From the height profile, we obtained a 12.7 ± 0.6 nm height (Figure S16, Supporting Information). This height is smaller than the diameter (21.7 nm) measured from the TEM image. This means that the soft nanotubes are flattened during the sample preparation. The XRD pattern of the dried sample in Figure 3e displays a broad diffraction peak at 2θ = 6.22° that corresponds to a 1.42 nm d-spacing. Its peak width and dspacing, which are wider and larger than those found in the PCL43−POM nanosheets, suggests that the arrangement of the POM clusters within the wall membrane is worse than that in the nanosheets. Disappearance of the diffraction peaks at 2θ = 21.2° and 23.5° related to the PCL crystal suggests the PCL block being amorphous. We also propose a model for the nanotube (f-1) as well as a membrane of the PCL chains and POM clusters (f-2) in Figure 3f. This membrane also has a hybrid structure in which an amorphous PCL layer is sandwiched by two POM layers. The formation of the hybrid membrane suggests that the CSA of the POM complexes is approximately equal to the CSA of the PCL. Because the wall thickness is 6.4 nm and the single POM layer thickness is 1.2 nm, the amorphous PCL layer sandwiched in between the two POM layers is only 4.0 nm thick. Discussion on MW-Dependence of Formation of Nanosheets and Nanotubes. Here, we discuss on the formation of the nanosheets and nanotubes of the two hybrids. In fact, both have the same hybrid membrane structure, that is, a PCL layer sandwiched by the two POM layers because such a

be 1.48 nm2, from the XRD peak at 2θ = 6.42° (Figure 2e). This value is equal to the 1.49 nm2 CSA of the eight PCL cells because the CSA of a single unit cell is 0.186 nm2. Third, to meet this balance each PCL chain must be folded three times to form a thrice-folded chain crystal. Nanotubes of PCL19−POM. The results, obtained in the PCL19−POM samples at r = 2:1 v/v, are summarized in Figure 3. Clearly, the PCL19−POM self-assembles into long nanotubes. When the nonstaining samples are observed directly under bright-field conditions we clearly see a large amount of long nanotubes gathering into bundles in which they arrange in a parallel way (TEM image in Figure 3a). The EDX analysis also confirms the existence of tungsten and vanadium in the POM cluster (Figure S14, Supporting Information). Because metals scatter more electrons than nonmetals the dark areas in the TEM micrographs correspond to the POM clusters. This enables us to directly visualize the structural characteristics in the nonstaining specimens under bright-field conditions. The long nanotubes have a wormlike configuration and a length of more than a few micrometers. A suspension of the nanotubes were further diluted, and the bundles were disbanded with a mild stirring. The TEM image in Figure 3b evidently displays four isolated nanotubes from 100 to 300 nm, which is shorter than those in Figure 3a. This means that the long nanotubes could be broken during diluting and stirring. The four nanotubes consist of the two parallel dark lines and the bright central regions. Noticeably, the bright central regions are still darker than the blank. This is a typical feature of nanotubes when viewed under bright-field TEM (shown schematically in the inset).52,53 The measured diameter is 21.7 ± 1.5 nm (Figure S15a, Supporting Information). 464

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Figure 4. Complex nano-objects. (a) TEM image showing tube-graf t-sheet objects composed of the tubes of PCL19−POM growing from the sheet edges of PCL43−POM. (b) High-resolution TEM image showing detailed structures. The blue line highlights the {110} faces and red arrow shows the crystallographic b-axis of the PCL crystal. (c) The 3D AFM height image of tube-graft-sheet objects showing a height difference between the darker zone and the nanosheet. (d) Height profile, obtained from an AFM image, showing a height change from the darker zone (regime I) to the nanosheet (regime II). (e) Proposed two-step self-assembly pathway from simple objects to complex tube-graft-sheet objects.

face of the PCL single crystals. We can see a darker zone in the fronts of the two {110} faces. This darker zone corresponds to a higher electron density. In other words, it should contain the more POM clusters. An SAED pattern obtained in the darker zone (Figure S17b, Supporting Information) is completely identical to the pattern in Figure 2c. The complex nano-objects were also characterized by AFM (see Figure 4c). Clearly, there is a thickness difference between the darker zone and the nanosheet. We further determined a height profile to reflect a height change from the darker zone to the nanosheet (Figure S18, Supporting Information). Regimes I and II in Figure 4d correspond to the darker zone and the nanosheet with heights (or thicknesses)of h ≈ 14.5 nm and h ≈ 12.5 nm, respectively. The cartoons in Figure 4e schematically depict the stepwise self-assembly for generation of this tube-graf t-sheet nano-object in the DMF/water solvent mixtures with the different DMF/ water ratios. The first step is the crystallization of the PCL43 block inducing aggregation of the PCL43−POM into the nanosheets at r = 5:1 v/v. The second step is the self-assembly of the PCL19−POM into the nanotubes at r = 2:1 v/v selectively on the {110} faces of the POM−PCL43 semicrystalline nanosheets to form the tube-graf t-sheet nano-objects. This means that at r = 2:1 v/v, the PCL43 semicrystalline edges of the PCL43−POM nanosheets are nuclei that initiate the PCL19−POM self-assembly into the nanotubes. The reason that the face-selective nucleation of the PCL19−POM and the growth of the self-assembled nanotubes along definite directions is because the {110} faces are the fast-growth direction of the PCL crystal. In this way, the PCL19−POM nanotubes are eventually selectively grafted on the four {110} faces of the PCL43−POM nanosheets to generate the tubegraf t-sheet nano-objects.

structure can decrease the contact of the polar DMF/water mixture with the nonpolar PCL layer. Once formed, nanosheets or nanotubes will grow laterally or axially for reducing the free energy caused by sheet edges or tube ends. Normally, a flexible membrane prefers to form a tube to further reduce its free energy. However, the rigidity of the semicrystalline PCL layer prevents them being curved into tubes. Here, we demonstrate that the geometric shape of the self-assembled nano-objects can be tuned by changing the MW of the PCL blocks. Tube-graf t-Sheet Nano-Objects and Stepwise SelfAssembly. In this study, we further performed a stepwise selfassembly process of the two hybrids by controlling the DMF/ water volume ratios to access the complex nano-objects by combining the two simple nano-objects together. We first prepared a suspension of the PCL43−POM nanosheets at r = 5:1 v/v, and then dropped the DMF solution of PCL19−POM into the suspension. Finally, we immediately added water into the suspension to change the ratio to r = 2:1 v/v. The suspension also shows Tyndall scattering (Figure S10c, Supporting Information). The self-assembled nano-objects were also characterized using TEM and AFM. The TEM image in Figure 4a clearly shows complex tubegraf t-sheet nano-objects, that is, the PCL19−POM tubes are grafted onto the edges of the PCL43−POM sheets. A highresolution TEM image shows more detailed structural features (Figure 4b). The measured diameter is 20.4 ± 2.1 nm (Figure S17a, Supporting Information). Interestingly, the tubes are grafted only on the four {110} faces. Their growth direction is parallel to the direction of the crystallographic b-axis of the PCL crystals. We can see the nanotubes growing from each crystal vertex between the two {110} faces as well as many nearby vertexes. There are no tubes on the {100} faces. It is worthy to mention that the {110} faces are the crystal growth 465

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CONCLUSIONS In summary, we designed and synthesized two hybrids, PCL43− POM and PCL19−POM. In the DMF/water solvent mixture, the PCL43−POM self-assembles into nanosheets at r = 5:1 v/v and the PCL19−POM self-assembles into the nanotubes at r = 2:1 v/v. This MW-dependent self-assembly behaviors are due to the different solubility of the PCL43 and PCL19 blocks at different DMF/water ratios. The two nano-objects have the same membrane structure: a PCL layer is sandwiched by the two POM layers. The rigidity of the semicrystalline PCL43 layer results in the flat nanosheets, while the flexibility of the amorphous PCL19 layer leads to the membranes that can further curve into nanotubes. By further exploiting the MW-dependence, we performed a two-step self-assembly to create complex tube-graf t-sheet nanoobjects by grafting the PCL19−POM nanotubes on the edges of the flat PCL43−POM nanosheet. This process is divided into two steps: (1) The PCL43 crystallization induces the aggregation of the PCL43−POM to form the nanosheets. (2) The face-selective nucleation of the PCL19−POM on the four {110} faces of the PCL crystal and further growth along the crystallographic b-axis of the PCL crystal consequently create the complex tube-graf t-sheet nano-objects. This method should be extended to combine other simple nano-objects with various geometries and properties together to create more complex nano-objects.

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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.langmuir.5b04504. Experimental methods and additional figures. (PDF)

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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 We appreciate the financial support of the National Natural Science Foundation of China for Grants 21274069 and 21334003, PCSIRT (IRT1257), and Open Research Fund of State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry. We also thank Professor. Shou-Ke Yan (State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology) for assistance in obtaining the selected area electron diffraction pattern.

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DOI: 10.1021/acs.langmuir.5b04504 Langmuir 2016, 32, 460−467