Janus Composite Nanorod from Molecular Bottlebrush Contained

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Janus Composite Nanorod from Molecular Bottlebrush Contained Block Copolymer Fan Jia, Fuxin Liang, and Zhenzhong Yang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b04074 • Publication Date (Web): 22 Dec 2017 Downloaded from http://pubs.acs.org on December 25, 2017

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Janus Composite Nanorod from Molecular Bottlebrush Contained Block Copolymer Fan Jia,†, ‡ Fuxin Liang,† and Zhenzhong Yang*, † †

State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese

Academy of Sciences, Beijing 100190, China ‡

Beijing Research Institute of Chemical Industry, SINOPEC, Beijing 100013, China

*E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT: The asymmetric ABC type Janus polymer composite nanorods are synthesized by in situ preferential growth of functional materials against the molecular bottlebrush contained triblock copolymer of poly(ethylene oxide)-b-poly(2-methacryloyloxyethyl pentynoate-gpoly(acrylic acid))-b-polystyrene. PEO and PS single chains are terminated onto the opposite ends of the composite nanorods. The two polymer chains are responsible for amphiphilic performance, while the composite nanorod for the functionality. The Janus nanorods can stand vertically at an emulsion interface, making the interfaces easily functionalized and manipulated. Protection of the PAA molecular bottlebrush via electrostatic interaction is important to obtain individual nanorod at high solid contents. A huge family of Janus composite nanorods is expected by changing compositions of the two polymer chains and the nanorod. KEYWORDS: Janus, Nanorod, Composite, Molecular bottlebrush, Block copolymer


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1. INTRODUCTION Janus materials with varied constituents thus functions distinctly compartmentalized onto the same object, have gained growing academic and industrial interests.1-5 One-dimensional (1D) nanomaterials such as nanorods (or nanowires) are important owing to their anisotropic shape dependent optical and electronic properties.6-8 It is interesting to render the nanorods with amphiphilic performance thus achieving Janus nanorods, which are important as building blocks, interconnects and functional components for assembling nanodevices. Janus gold nanorods resembling an amphiphilic ABA triblock copolymer, can self-assemble into a variety of superstructures.9,10

The

gold

nanorod

is

covered

with

a

hydrophilic

bilayer

of

cetyltrimethylammonium bromide at the central block. It is symmetric with the two ends terminated with the same polymer. Polymeric Janus nanocylinders have been synthesized by disassembly of the crosslinked microphase-segregated lamellar-cylinder morphology from triple block copolymer.11 The separation plane is parallel to the cylinder axis. The opposite ends contain the same constituent. Strict processing conditions and narrow molecular distribution of the copolymers are prerequisites for the uniform Janus nanocylinders. It remains challenging to synthesize asymmetric (ABC) type Janus nanorod with two single different polymer chains terminated at the opposite ends of the nanorod. A larger family of superstructures will be available from the ABC type Janus nanorods by more flexibly tuning the directional interaction variables. It is noticed that a molecular bottlebrush is cylindrical, which carries a linear or dendritic polymer brush as the side chain.12,13 Functional groups of the polymer side chains can induce a preferential growth of 1D nanocomposites within the molecular bottlebrush. The side chain structure is tunable. When it is block copolymer, the bottlebrush shows a core/sheath cable


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structure. A cylindrical hybrid nanostructure is derived by selective modification of the core for example metallization.14-17 Similarly, a semiconducting (CdS) or superparamagnetic (Fe3O4) nanowire is obtained by a favorable growth in the poly(acrylic acid) (PAA) core. Although many 1D functional nanostructures are derived from molecular bottlebrushes, they are still symmetric with the same constituents at the opposite ends. Herein, we present a facile approach toward asymmetric ABC type polymer composite Janus nanorods with two different single polymer chains at the opposite ends against molecular bottlebrush contained block copolymers (Scheme 1). The molecular bottlebrush is composed of a main backbone of poly(2-methacryloyloxyethyl pentynoate) (PMAPA) and PAA side chains. Single poly(ethylene oxide) (PEO) and polystyrene (PS) chains are terminated at the opposite ends. The PAA side chain as a nanosized reactor, can induce a favorable growth of functional materials. Meanwhile, the molecular bottlebrush is converted into a functional nanorod after the synchronous crosslinking. At the opposite ends of the composite nanorod, single PEO and PS chains are terminated. The asymmetric ABC type Janus composite nanorods are derived. The two polymer chains are responsible for the amphiphilic performance, while the composite nanorod is responsible for the functionality. Protection of the PAA brush with octadecylamine forming electrostatic interaction is significant to achieve individual Janus nanorods at high solid contents. Scheme 1. Illustrative synthesis of the asymmetric ABC type Janus composite nanorod against a molecular bottlebrush contained triple block copolymer.


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2. EXPERIMENTAL SECTION 2.1. Materials. Methoxypoly(ethylene oxide) (PEO-OH) (Mn,NMR = 5.0k, Mw/Mn = 1.03, Aldrich) was dried azeotropically with anhydrous toluene. 2-Bromoisobutyryl bromide (98%, Aldrich) was distilled at room temperature under vacuum. Triethylamine (TEA) was dried over KOH. 2-(Trimethylsilyloxy)ethyl methacrylate (HEMA-TMS) was synthesized.18 CuBr was purified by washing with acetic acid and acetone. Styrene (98%, Beijing Chemical Factory) and tert-butyl acrylate (tBA) (99%, Alfa Aesar) were purified by vacuum distillation prior to use. Potassium fluoride (KF) (99%, Acros) was dried at 200 °C for 12 h prior to use. 4-Pentynoic anhydride was synthesized from 4-pentynoic acid.19 Paraffin wax (Tm = 52-54 °C) was purchased from Nan Yang Wax Fine Chemical Plant. Dichloromethane was dried by purging with nitrogen and passing through alumina column prior to use. Anisole was washed with 5% aqueous NaOH three times and water until the aqueous phase became neutral, dried over anhydrous Na2SO4 and CaH2 overnight and distilled under reduced pressure. Tetrahydrofuran (THF) was distilled over Na prior to use. N,N-Dimethylformamide (DMF) was dried over CaH2 and distilled under reduced pressure prior to use. 4-(Dimethylamino)pyridine (DMAP) (99%, Alfa Aesar), 4,4’-di(5nonyl)-2,2’-bipyridine (dNbpy) (97%, Aldrich), N,N,N’,N”,N”-pentamethyldiethylenetriamine (PMDETA) (98%, Aldrich), tetrabutylammonium fluoride (TBAF) (98%, Acros), ethyl 2bromoisobutyrate (EBiB) (98%, Aldrich), ascorbic acid (99%, Sinopharm Chemical Reagent), CuSO4·5H2O (99%, Sinopharm Chemical Reagent), poly(ethylene glycol) (PEG) (Mn = 2.0k, Mw/Mn = 1.03, Aldrich) and other chemicals were used as received. 2.2. Synthesis of the samples. PEO-Br

macroinitiator.

PEO-Br

macroinitiator

was

synthesized

by

reacting

methoxypoly(ethylene oxide) with 2-bromoisobutyryl bromide in the presence of triethylamine.


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Triethylamine (0.70 g, 7.0 mmol) and DMAP (0.92 g, 7.5 mmol) were added to a solution of PEO-OH (12.5 g, Mn,NMR = 5.0k, 2.5 mmol) in 65.0 mL of anhydrous dichloromethane at room temperature. After 2-bromoisobutyryl bromide (2.88 g, 12.5 mmol) was dissolved in 20.0 mL of anhydrous dichloromethane and added dropwise over 1 h to the PEO-OH solution in an ice bath, the mixture was warmed to room temperature and stirred for 24 h. The solvent was removed by rotary evaporator. The crude product was dissolved in THF, filtered and precipitated in diethyl ether. The PEO-Br macroinitiator was collected by filtration, and dried under vacuum at room temperature. PEO-b-P(HEMA-TMS)-Br diblock copolymer. The macroinitiator PEO-Br (0.13 g, Mn,NMR = 5.1k), dNbpy (40.4 mg, 0.10 mmol), HEMA-TMS (4.00 g, 19.8 mmol) and anisole (1.7 mL) were charged into a 25 mL Schlenk flask equipped with a stir bar under vacuum. After the mixture was degassed by three cycles of freeze-evacuate-thaw, CuBr (7.2 mg, 0.05 mmol) was added. The flask was sealed under vacuum. The polymerization was conducted in an oil bath at 90 °C for 43 h. After the same treatment procedures as the macroinitiator, the PEO-b-P(HEMATMS)-Br diblock copolymer was obtained. PEO-b-P(HEMA-TMS)-b-PS triblock copolymer. The PEO-b-P(HEMA-TMS)-Br diblock copolymer was used as another macroinitiator to synthesize PEO-b-P(HEMA-TMS)-b-PS triblock copolymer. PEO-b-P(HEMA-TMS)-Br (0.40 g, Mn,NMR = 74.1k, Mw/Mn = 1.13), PMDETA (1.9 mg, 0.01 mmol), styrene (0.67 g, 6.4 mmol) and anisole (2.8 mL) were charged into a 25 mL Schlenk flask equipped with a stir bar under vacuum. After the mixture was degassed by three cycles of freeze-evacuate-thaw, CuBr (1.6 mg, 0.01 mmol) was added. The flask was sealed under vacuum. After the polymerization at 90 °C for 44 h, the reaction was terminated by cooling and exposing to air. The crude product was passed through a neutral


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alumina column and purified by precipitating in a large amount of methanol/water mixture (7:3 vol/vol). The composition was determined by 1H NMR spectrum. PEO-b-PMAPA-b-PS triblock copolymer. PEO-b-P(HEMA-TMS)-b-PS (0.50 g, Mn,NMR = 80.4k) was dissolved in 40.0 mL of dry THF under nitrogen. After potassium fluoride (0.37 g, 6.4 mmol) was added, tetrabutylammonium fluoride (42.0 µL, 1 mol/L THF, 0.042 mmol) in THF (5.0 mL) was added dropwise. The mixture was stirred at room temperature for 2 h. After DMAP (0.13 g, 1.1 mmol) was added, 4-pentynoic anhydride (2.24 g, 12.6 mmol) in THF (5.0 mL) was added dropwise. The reaction was conducted at room temperature for 24 h. The crude product was purified by precipitating in a large amount of water and a methanol/water mixture (1:1 vol/vol). The composition was determined by 1H NMR spectrum. Azido-terminated PtBA (PtBA-N3). Bromo- terminated PtBA chain (PtBA-Br) was prepared by ATRP using the catalyst of CuBr/PMDETA and initiator of EBiB. EBiB (195.0 mg, 1.0 mmol), PMDETA (86.5 mg, 0.5 mmol), tBA (6.40g, 50.0 mmol) and acetone (3.5 mL) were added into a 25 mL Schlenk flask equipped with a stir bar under vacuum. After the mixture was degassed by three cycles of freeze-evacuate-thaw, CuBr (72.0 mg, 0.5 mmol) was added. After the flask was sealed under vacuum, the polymerization was conducted at 60 °C for 18 h. The reaction was terminated by cooling and exposing to air. The crude product was passed through a neutral alumina column and purified by precipitating in a large amount of methanol/water mixture (1:1 vol/vol). After PtBA-Br (5.00 g, Mn,NMR = 5.2k) was dissolved in DMF, NaN3 (10 times molar excess to the bromo group) was added. The reaction was conducted under stirring at 40 °C for 48 h. After precipitation in a methanol/water mixture (1:1 vol/vol), the crude polymer was dissolved in dichloromethane and washed with water. After drying over anhydrous


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magnesium sulfate and vacuum evaporation of the solvent, PtBA-N3 was obtained. The composition was determined by 1H NMR spectrum. PEO-b-P(MAPA-g-PtBA)-b-PS by click reaction. PEO-b-PMAPA-b-PS (63.0 mg, Mn,NMR = 83.1k), ascorbic acid (91.6 mg, 0.5 mmol), PtBA-N3 (2.00 g, Mn,NMR = 5.2k) and DMF (12.0 mL) were added into a 25 mL Schlenk flask. After the mixture was degassed by three cycles of freeze-evacuate-thaw, CuSO4·5H2O (13.0 mg, 0.05 mmol) was added. Toluene (0.05 mL) was added as an internal standard to calculate the conversion of PtBA-N3 via GPC measurement. After the flask was sealed under vacuum, the click reaction was carried out at 50 °C for 24 h. The reaction was terminated by cooling and exposing to air. While one portion was diluted by DMF/LiBr for GPC measurement, the other portion was precipitated in a large amount of methanol/water mixture (10:1 vol/vol) three times. The composition was determined by 1H NMR spectrum. PEO-b-P(MAPA-g-PAA)-b-PS by hydrolysis. PEO-b-P(MAPA-g-PtBA)-b-PS (0.41 g, Mn,NMR = 1805.2k) was dissolved in dichloromethane, and 5-fold molar excess of CF3COOH (with respect to the tert-butyoxycarbonyl group) was added. The mixture was stirred at room temperature for 48 h for the hydrolysis. PEO-b-P(MAPA-g-PAA)-b-PS was precipitated in dichloromethane. The crude product was purified by precipitating in a large amount of diethyl ether. Polymer/Au composite Janus nanorod. PEO-b-P(MAPA-g-PAA)-b-PS (0.10 g, Mn,NMR = 1075.8k, 1.24 mmol AA unit) was dissolved in 1.0 mL of DMF. Octadecylamine (50.1 mg, 0.19 mmol) was added under ultrasonication for the protection. An aqueous solution of HAuCl4·4H2O (0.01 mL, 50.0 mg/mL) was added under stirring at room temperature for 24 h for a preferential absorption of HAuCl4 within the PAA brush. After removing residual HAuCl4 by centrifugation,


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the polymer composite was dispersed in DMF (1.0 mL). After NaBH4 aqueous solution (0.01 mL, 5.0 mg/mL) was added under stirring, the reaction was carried out at room temperature for 24 h. Au formed within the PAA brush by in situ reduction. After centrifugation and dialysis against acidic methanol solution and DMF to remove octadecylamine, the polymer/Au composite Janus nanorod was obtained. Polymer/SiO2 composite Janus nanorod. PEO-b-P(MAPA-g-PAA)-b-PS (0.10 g, Mn,NMR = 1075.8k, 1.24 mmol AA unit) was dissolved in 1.0 mL of DMF. Octadecylamine (50.1 mg, 0.19 mmol) was added under ultrasonication for the protection. After 3-(triethoxysilyl)propyl isocyanate (0.05 mL) was added, the mixture was stirred at room temperature for 24 h allowing a preferential absorption of the silica precursor within the PAA brush. After removing residual silane by centrifugation, the composite was dispersed in DMF (1.0 mL). After aqueous HCl (2 mol/L) was added under stirring at room temperature, the reaction was carried out for 24 h. SiO2 formed within the PAA brush by in situ sol-gel process. After centrifugation and dialysis against acidic methanol solution and DMF to remove octadecylamine, the polymer/SiO2 composite Janus nanorod was obtained. Polymer/Ni composite Janus nanorod. PEO-b-P(MAPA-g-PAA)-b-PS (0.10 g, Mn,NMR = 245.9k, 1.10 mmol AA unit) was dissolved in 1.0 mL of DMF. Octadecylamine (47.4 mg, 0.17 mmol) was added under ultrasonication for the protection. After an aqueous solution of Ni(NO3)2·6H2O (0.01 mL, 50.0 mg/mL) was added, the mixture was stirred at room temperature for 24 h allowing a preferential absorption of Ni(NO3)2 within the PAA brush. After removing residual Ni(NO3)2 by centrifugation, the composite was dispersed in DMF (1.0 mL). After NaBH4 solution (0.01 mL, 5.0 mg/mL) was dropped under stirring, the reaction was carried out at room temperature for 24 h. Ni formed within the PAA brush by in situ reduction. After


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centrifugation and dialysis against acidic methanol solution and DMF to remove octadecylamine, the polymer/Ni composite Janus nanorod was obtained. 2.3. Janus performance of the composite nanorod. A given amount of the polymer/Au composite Janus nanorod was added in a mixture of water (2.0 mL) and cyclohexane (0.4 mL). A trace of oil soluble dye dil-C18 was introduced into cyclohexane for easy observation. After the mixture was vigorously shaken, an oil-in-water emulsion formed. In another experiment, 0.4 g of melt paraffin wax (Tm = 52-54 °C) and 2.0 g of water were emulsified at 70 °C in the presence of a given amount of the polymer/Au composite Janus nanorod. The wax phase was solidified after the mixture was cooled down to room temperature. 2.4. Characterization. Gel permeation chromatography (GPC) was performed by a set of four Waters Styragel columns (HT 2, HT 3, HT 4, and HT 5), a Waters 515 isocratic HPLC pump, and a Waters 2414 RI detector using DMF with LiBr (1.0 g/L) as the eluent at a flow rate of 1.0 mL/min. 1H NMR spectra were recorded on a Bruker Advance 400 spectrometer with CDCl3 as solvent at room temperature. FT-IR spectroscopy was recorded by a deuterated triglycine sulfate (DTGS) detector on a Bruker EQUINOX 55 spectrometer with the samples/KBr pressed pellets. Morphology of the samples was characterized using scanning electron microscopy (SEM, Hitachi S-4800 at 15 kV), transmission electron microscopy (TEM, JEOL 1011 at 100 kV) and high-resolution transmission electron microscopy (HRTEM, JEOL 2100F at 200 kV). The samples for SEM observation were ambient dried and vacuum sputtered with Pt. The samples for TEM observation were prepared by spreading very dilute dispersions onto carbon-coated copper grids. Atomic force microscope (AFM) measurements were performed with a Digital Instrument Multimode Nanoscope IIIA at the tapping mode. The sample solutions were spin cast onto a freshly cleaved mica. Size and size distribution of the samples in DMF dispersions were


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measured using Zeta sizer (Nano Series, Malvern Instruments) at 25 °C. Thermogravimetric analysis (TGA) was performed using the PerkinElmer Pyris 1 thermogravimetric analyzer under air at a scanning rate of 10 °C/min. 3. RESULTS AND DISCUSSION Synthesis of Janus composite nanorods from a molecular bottlebrush contained block copolymer is shown as Scheme 1. The molecular bottlebrush of P(MAPA-g-PAA) is terminated with single PEO and PS chains at the opposite ends, which was derived by selective hydrolysis of the grafted PtBA side chain of PEO-b-P(MAPA-g-PtBA)-b-PS. After a preferential growth of inorganic or metallic materials within the PAA brush, the molecular bottlebrush was converted into a functional nanorod. The nanorod is Janus with PEO and PS single chains terminated at the opposite ends. PEO-b-P(MAPA-g-PtBA)-b-PS was synthesized by a sequential ATRP following by a click grafting (Fig. S1). PEO-Br macroinitiator was synthesized by reacting single hydroxyl group capped PEO113 with 2-bromoisobutyryl bromide. Both GPC (curves 1 and 2, Fig. S2) and 1H NMR spectra (runs 1 and 2, Table 1) indicates a slight increase in molecular weight after the substitution. An example diblock copolymer of PEO-b-P(HEMA-TMS)-Br was further synthesized by ATRP. Polymerization degree of P(HEMA-TMS) block is 341 determined by 1H NMR (run 3, Table 1), while molecular weight distribution keeps narrow with a polydispersity index (PI) of 1.13 (curve 3, Fig. S2). The third block of PS was further synthesized by ATRP with a polymerization degree of 60 (run 4, Table 1). GPC measurement reveals a further increase in molecular weight with a narrow distribution with PI of 1.18 (curve 4, Fig. S2). After deprotection of trimethylsilyl, a triblock copolymer of PEO113-b-PMAPA341-b-PS60 was derived by in situ functionalization with 4-pentynoic anhydride (run 5, Table 1). Molecular weight is


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further increased (curve 5, Fig. S2). No trimethylsilyl group is detected by 1H NMR (Fig. S3), indicating that the deprotection is completely achieved. PtBA-N3 was synthesized by the azidation of PtBA-Br with NaN3. A new absorption band at 2111 cm-1 assigned to azido group appears (Fig. S4). A complete azidation is confirmed by 1H NMR (Fig. S5). PEO-b-P(MAPA-gPtBA)-b-PS was synthesized by copper catalyzed azido-alkyne cycloaddition click reaction of PtBA-N3 onto the PMAPA backbone. In order to increase the grafting density, excess PtBA-N3 (molar ratio of 1.5:1) was added. After the click reaction, a new peak corresponding to higher molecular weight coexists with the residual PtBA-N3 (curves 1 and 2, Fig. S6). One GPC peak remains after removal of the residual PtBA-N3 (curve 3, Fig. S6). Molecular weight measurement by 1H NMR (Fig. S7) reveals that the grafting density is 98%. Eventually, an example copolymer PEO113-b-P(MAPA-g-PtBA39)341-b-PS60 was achieved. PAA side chain was derived by selective hydrolysis of PtBA. A broad new peak around 3200 cm-1 assigned to carboxylic acid group appears, while the absorption peak of tert-butyl group at 1370 cm-1 nearly vanishes (Fig. S8). Table 1. Characteristics of some reagents for the synthesis of PEO113-b-P(MAPA-g-PtBA39)341b-PS60. Run

Polymer

Mna (kg/mol)

PIb

1

PEO113-OH

5.0

1.07

2

PEO113-Br

5.1

1.07

3

PEO113-b-P(HEMA-TMS)341-Br

74.1

1.13


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4

PEO113-b-P(HEMA-TMS)341-b-PS60

80.4

1.18

5

PEO113-b-PMAPA341-b-PS60

83.1

1.23

6

PtBA39-N3

5.2

1.11

7

PEO113-b-P(MAPA-g-PtBA39)341-b-PS60

1805.2

1.20

a b

Calculated by 1H NMR spectrum. Determined by GPC with PS standard.

Morphology of PEO113-b-P(MAPA-g-PAA39)341-b-PS60 was observed via AFM after spin coating of the dilute solution in DMF onto a freshly cleaved mica. Worm-like molecular bottlebrush is clearly visualized (Fig. 1a). The bottlebrush is long ~140 nm with a height of ~5 nm and width of ~40 nm. 3D AFM height image reveals that the bottlebrush is rod-like in shape (Fig. 1b). Both height (thus width) and length of the bottlebrush are tunable. When the polymerization degree of PAA side chain is decreased to 25 from 39, the molecular bottlebrush is high ~1.5 nm and wide ~25 nm (Fig. 1c). The bottlebrush becomes slightly short ~110 nm. This is explained by a weakening repulsion between the side chains. When the middle backbone length is decreased to 108 from 341, the bottlebrush becomes much shorter ~45 nm and appears spherical (Fig. 1d). It is high ~3 nm and wide ~30 nm, implying a shrinkage of the short bottlebrush.


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a.

b.

c.

d.

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Figure 1. AFM images of some copolymers: (a, b) PEO113-b-P(MAPA-g-PAA39)341-b-PS60, inset a schematic structure. (c) PEO113-b-P(MAPA-g-PAA25)341-b-PS60. (d) PEO113-b-P(MAPA-gPAA25)108-b-PS60. PEO113-b-P(MAPA-g-PAA39)341-b-PS60 was selected as the model copolymer to derive Janus composite nanorods. The PAA brush can serve as a nanosized reactor for preferential growth of functional materials.14-17 As an example, Au can form within the PAA brush by a preferential absorption of HAuCl4 following by in situ reduction. In the first attempt, a heavy aggregation occurred even at a low solid content of ~1 mg/mL (Fig. S9a). It is caused by intermolecular crosslinking between the bottlebushes. In order to avoid the aggregation, the individual molecular bottlebrush should be isolated by protection with octadecylamine achieving electrostatic repulsion. The strong absorption at 1707 cm-1 is corresponded to the carbonyl


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stretch of carboxylic acid group in the molecular bottlebrush (curve 1, Fig. 2a). After protection with octadecylamine (curve 2, Fig. 2a), a new peak at 1547 cm-1 appears while the peak intensity at 1707 cm-1 becomes weaker (curve 3, Fig. 2a). This indicates the formation of amino/carboxyl complexes. The peak at 1547 cm-1 becomes stronger when octadecylamine content increases from 5 mol% to 15 mol% of AA units, whilst the peak at 1707 cm-1 becomes much weaker (curve 4, Fig. 2a). When protecting with 5 mol% of octadecylamine, all the polymer/Au composite nanorods are individual at the solid of ~1 mg/mL. At a higher solid content for example 100 mg/mL, aggregation appeared (Fig. S9b). After increasing the protection extent with 15 mol% of octadecylamine, all the polymer/Au composite nanorods are individual at such high solid contents. The dispersion looks pink yet transparent, revealing that no aggregation occurs. The composite nanorod is self-stained thus visible under TEM (Fig. 2b). The continuous crystalline lattices clearly demonstrate the presence of Au nanoparticles within the middle block (Fig. S10). In comparison, the molecular bottlebrush contained block copolymer without staining is invisible. The rod shape is verified by SEM image (inset Fig. 2b). The nanorod is ~10 nm in diameter and long ~70 nm. The DLS result shows that the composite nanorod in dispersion (curve 3, Fig. 2c) is slightly smaller than the copolymer (curve 1, Fig. 2c). The peak is single and narrow, implying that the nanorods are individual in the dispersion. Au content in the polymer/Au composite nanorods is measured 0.2 wt.-% (curve 1, Fig. S11). The approach is general. Along the similar approach, the polymer/SiO2 composite nanorod was achieved by absorption of 3-(triethoxysilyl)propyl isocyanate following by a sol-gel process at high solid contents (Fig. 2d). The nanorods are individual in the dispersion (curve 2, Fig. 2c). SiO2 content in the polymer/SiO2 composite nanorods is measured 9.8 wt.-% (curve 2, Fig. S11). Characteristic size of the composite nanorod is tunable by using molecular bottlebrushes with


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varied length. As an example, a shorter polymer/Au composite nanorod was derived from PEO113-b-P(MAPA-g-PAA25)108-b-PS60 (Fig. S12). Similarly, a polymer/Ni composite nanorod was synthesized by using Ni(NO3)2 (Fig. 2e). The nanorods are individual in the dispersion (curve 4, Fig. 2c). Ni content in the polymer/Ni composite nanorods is measured 0.1 wt.-% (curve 3, Fig. S11). It is noted that PS and PEO single chains are terminated at the opposite ends of the nanorods. The Janus nanorods are ABC type asymmetric. They are amphiphilic and well dispersible both in water and oil. In the aqueous dispersion of the polymer/Ni composite Janus nanorod, the nanorod can be completely collected with a magnet (Fig. 2f). Upon removal of the magnet, the Janus nanorod becomes re-dispersible.

a.

b.

c.

d.


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e.

f.

Figure 2. (a) FT-IR spectra of PEO113-b-P(MAPA-g-PAA39)341-b-PS60 (curve 1); octadecylamine (curve 2); the amino/carboxyl complex at varied fraction of octadecylamine of AA units: (curve 3) 5 mol%, (curve 4) 15 mol%. (b) TEM and inset SEM images of the polymer/Au composite Janus nanorod, inset a schematic Janus nanorod. (c) DLS results of PEO113-b-P(MAPA-gPAA39)341-b-PS60 (curve 1); the long polymer/SiO2 composite Janus nanorod (curve 2); the long polymer/Au composite Janus nanorod (curve 3); the short polymer/Ni composite Janus nanorod (curve 4). (d) TEM image of the long polymer/SiO2 composite Janus nanorod. (e) TEM image of the short polymer/Ni composite Janus nanorod. (f) The polymer/Ni composite Janus nanorod dispersion in water (left) and collection with a magnet (right). The polymer/Au composite Janus nanorod (as shown in Fig. 2b) was selected as a solid surfactant to stabilize emulsions. Cyclohexane and water are typically immiscible (left, Fig. 3a). A trace of oil soluble dye dil-C18 is added into cyclohexane for easy observation. In the presence of the Janus nanorod, a cyclohexane-in-water (1:5 vol/vol) emulsion formed (right, Fig. 3a). The emulsion keeps stable over months. In order to observe orientation of the Janus nanorods at the emulsion interface, a paraffin wax (Tm = 52-54 °C) was used to form a melt wax-in-water emulsion at 70 °C. Orientation of the Janus nanorods was frozen upon cooling to room temperature. The emulsion droplets are 3-5 µm in diameter (Fig. 3b). No Janus nanorods are


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found either in water or wax phase. All the nanorods are located at the interface, and their ends are protruded from the wax sphere surface. Similarly, a solid wax-in-PEG blend formed. After dissolution of PEG with water, wax spheres were obtained (Fig. 3c). All the nanorods penetrate across the interface. After wax was dissolved with n-hexane from the fracture surface, many voids were left (Fig. 3d). All the nanorods penetrate across the interface, whose two ends are visualized from both sides of the interface. All the Janus nanorods adopt a vertical orientation at the interface rather than random distribution. In the presence of the polymer/Ni composite Janus nanorod, a cyclohexane-in-water emulsion formed (left, Fig. 3e). All the dispersed oil droplets can be driven to move toward a magent (~0.5 T). Eventually, they are completely collected, leaving colorless transparent water phase (middle, Fig. 3e). Upon removal of the magnet, the emulsion is recovered. When using a stronger magnet (~0.7 T), the emulsion is de-stabilized and separated into three phases: oil, water and the Janus composite nanorod (right, Fig. 3e). The Janus composite nanorods are easily recycled.

a.

b.


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c.

d.

e. Figure 3. (a) A cyclohexane/water mixture (left) and a cyclohexane-in-water emulsion stabilized with the polymer/Au composite Janus nanorod (right), oil soluble dye dil-C18 added into cyclohexane for easy observation. (b) SEM image of the wax-in-water emulsion droplet after frozen, inset a magnified SEM image. (c) SEM image of the wax sphere from the wax/PEG blend after selective washing PEG with water, inset a magnified SEM image. (d) SEM image of the fractured PEG after selective dissolution of wax with n-hexane. (e) A cyclohexane-in-water emulsion stabilized with the polymer/Ni composite Janus nanorod (left), collection of the emulsion droplets with a magnet (middle), de-emulsification with a stronger magnet while the Janus composite nanorod was recycled (right). 4. CONCLUSIONS


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A facile approach is proposed toward the asymmetric ABC type composite Janus nanorods with two single different polymer chains terminated at the opposite ends. The nanorods are derived by a preferential growth of functional materials within the PAA bottlebrush of an example triple block copolymer PEO-b-P(MAPA-g-PAA)-b-PS. Protection of the PAA brush via electrostatic interaction is significant to obtain individual Janus nanorods at high solid contents. The Janus composite nanorods integrate functionality from the composite and amphiphilic performance from the two polymer chains. The nanorods are Janus and adopt vertical orientation at an emulsion interface. Performance of the Janus composite nanorods can be broadly extended by changing compositions of the functional materials and the polymer chains. Manipulation of emulsions becomes easier by using functional Janus nanorods for example magnetic responsive ones. The asymmetic ABC type Janus nanorods can provide more variables to tune directional interactions, and a larger family of superstructures are expected. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.XXXXXXX. Schematic synthesis of PEO-b-P(MAPA-g-PAA)-b-PS; GPC traces, 1H NMR spectra, and FT-IR spectra of some reagents for the synthesis of PEO113-b-P(MAPA-g-PAA39)341-b-PS60; TEM images of the polymer/Au composites and the short polymer/Au composite Janus nanorod; HRTEM image of the polymer/Au composite Janus nanorod; TGA curves of the Janus composite nanorods (PDF) AUTHOR INFORMATION Corresponding Author


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*E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by NSF of China (51233007, 51173191).


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For Table of Contents Use Only:

Title: Janus Composite Nanorod from Molecular Bottlebrush Contained Block Copolymer Authors: Fan Jia, Fuxin Liang, and Zhenzhong Yang*


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Janus Composite Nanorod from Molecular Bottlebrush Contained

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