Self-Assembly of Amphiphilic Conjugated Diblock Copolymers into

Article pubs.acs.org/Macromolecules

Self-Assembly of Amphiphilic Conjugated Diblock Copolymers into One-Dimensional Nanoribbons Amanda C. Kamps,† Ma. Helen M. Cativo,† Michael Fryd,† and So-Jung Park*,†,‡ †

Department of Chemistry, University of Pennsylvania, 231 South 34th Street, Philadelphia, Pennsylvania 19104, United States Department of Chemistry and Nano Science, Global Top 5 Program, Ewha Womans University, 52 Ewhayeodae-gil, Seodaemun-gu, Seoul 120-750, Korea

‡

S Supporting Information *

ABSTRACT: The colloidal self-assembly of a new conjugated diblock copolymer of a polythiophene derivative, poly[3-(2,5,8,11tetraoxatridecanyl)thiophene]-block-poly(ethylene glycol) (PTOTTb-PEG), led to various well-defined assembly structures such as vesicles, sheets, and nanoribbons. A unique and technologically relevant nanoribbon structure with a dimension reaching tens of micrometers was formed in water when polar protic solvents were used as initial cosolvents. Self-assembly of PTOTT-b-PEG in various solvent compositions and polymer concentrations indicated that the hydrogen bonding between the diblock copolymer and the self-assembly medium plays an important role in determining the self-assembly structure and that the final assembly structure should be the result of a delicate interplay between hydrogen bonding and π−π interactions. This study demonstrates that the addition of hydrogen bonding capability and amphiphilicity in the self-assembly of conjugated polymers can lead to many interesting well-defined assembly structures that are not typically found in conjugated polymers.

S

fundamental interests. In fact, examples of well-defined assemblies of conjugated amphiphilic polymers are mostly limited to the wire-like assemblies of PAT amphiphilic polymers described above.5−8 Herein, we present a novel elongated one-dimensional ribbon-type assembly structure of polythiophene formed from a new amphiphilic conjugated block copolymer, poly[3(2,5,8,11-tetraoxatridecanyl)thiophene]-block-poly(ethylene glycol) (PTOTT-b-PEG) (Scheme 1). One-dimensional assembly structures are particularly interesting for transport properties, alignment, and network formation.18 In this study,

emiconducting conjugated polymers have been widely studied for a number of applications ranging from flexible optoelectronic devices to biological sensing owing to their unique optical and transport properties and solution processability.1,2 On the contrary to conventional inorganic semiconductors, conjugated polymers show properties that are highly dependent on various extrinsic factors including the molecular packing and nanometer scale morphology.3 Therefore, the ability to control the assembly structure of conjugated polymers is important in many of their applications. Solution phase colloidal self-assembly of amphiphilic conjugated polymers offers a way to fabricate nanoscale building blocks of conjugated polymers with well-ordered packing structures.4 For example, amphiphilic polymers containing poly(alkylthiophenes) (PAT) were shown to selfassemble into wire-like structures due to the efficient packing of alkyl side chains as well as π−π stacking interactions.5 Manners and co-workers have shown that elongated nanowires of PAT can be prepared by the crystallization-driven self-assembly of PAT block copolymers.6 We have previously reported that semiconducting nanowires with controllable lengths can be formed from PAT amphiphilic polymers by adjusting the relative block length of conjugated amphiphilic polymers.7 Emrick and co-workers recently reported unique helical assemblies of polythiophene nanowires by the complexation of polymer side chains with added ions.8 However, compared to the vast amount of work done on rod−coil type small molecules9−15 and coil−coil type polymers,16,17 colloidal selfassembly of rod−coil type amphiphilic polymers remains much less understood despite the technological relevance and © 2013 American Chemical Society

Scheme 1. Schematic Description of the Self-Assembly of PTOTT-b-PEG into Nanoribbons

Received: October 18, 2013 Revised: December 17, 2013 Published: December 27, 2013 161

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indicating that polymers are somewhat aggregated in methanol (Figure 2A).20 This is consistent with the lower photo-

tetraethylene glycol side chains on polythiophene and the careful control of self-assembly media resulted in diverse morphologies including vesicles, sheets, and stiff nanoribbons through delicate interplay between hydrogen bonding and π−π interactions.

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RESULTS AND DISCUSSION Synthesis and Self-Assembly of PTOTT-b-PEG into Nanoribbons. PTOTT40-b-PEG108 diblock copolymers were synthesized via the triazole cycloaddition click coupling reaction7,19 between ethynyl-PTOTT and azide-PEG and were characterized by 1H NMR spectroscopy and gel permeation chromatography (Supporting Information). PTOTT-b-PEG is composed of a polythiophene backbone with hydrophilic oligomeric ethylene glycol side chains and a PEG chain covalently attached to the polythiophene derivative. Despite the hydrophilic nature of the PEG block and the tetraethylene glycol side chains, a quick addition of PTOTT-bPEG in water resulted in the formation of large macroscopic precipitates of kinetic aggregate structures. In order to induce self-assembly of PTOTT-b-PEG, the polymer was first dissolved in a common solvent, methanol, followed by a slow addition of water and subsequent dialysis into water. Transmission electron microscope (TEM) images (Figure 1A,B) revealed that PTOTT-b-PEG self-assembles into a

Figure 2. (A) Absorbance and (B) PL spectra of PTOTT40-b-PEG108 in methanol (black) and PTOTT40-b-PEG108 nanoribbons in water (blue). UV−vis and PL spectra of PTOTT40-b-PEG108 dissolved in a good solvent (chloroform) were presented for comparison (red). The polymer concentration in chloroform and methanol solutions was 2 μM. Pictures of solutions under ambient light (top left) and under UV light (top right) are given above the spectra.

luminescence (PL) intensity in methanol than in chloroform (Figure 2B). The UV−vis absorption spectra of nanoribbons showed a blue-shift from those taken in chloroform and methanol (Figure 2A). This blue-shift, which is likely to be caused by a twisted conformation of PTOTT, was found to be an indication of nanoribbon formation (Supporting Information). The PL of PTOTT-b-PEG was completely quenched in the final solvent of water, indicating that PTOTT chains form a closely packed structure in nanoribbons (Figure 2B).21 Effect of Initial Cosolvent on the Assembly Structure. The type of initial common solvent was important in determining the final assembly structure of PTOTT-b-PEG in water. Due to the tetraethylene glycol side chain on the thiophene, PTOTT-b-PEG can be dispersed in a number of polar organic solvents. A range of different solvents were used as initial common solvents for the self-assembly of PTOTT-bPEG, and the resulting morphologies are summarized in Table 1 along with the solubility parameters and hydrogen-bonding Table 1. Characteristics of Common Solvents Used for SelfAssembly and Resulting Morphologies

Figure 1. (A, B) TEM images of PTOTT40-b-PEG108 nanoribbons in water self-assembled from 2 μM PTOTT40-b-PEG108 in methanol. (C) An AFM image of PTOTT40-b-PEG108 nanoribbons placed on a silicon wafer. (D) A height profile of a PTOTT 40 -b-PEG 108 nanoribbon.

solubility parametera (MPa1/2)

hydrogen bondingb

THF

18.6

no

2-propanol acetonitrile

23.5 24.3

yes no

DMF

24.8

no

ethanol methanol

26.0 29.7

yes yes

common solvent

unique nanoribbon structure in the final aqueous dispersion (Scheme 1). Dimensional analysis of TEM images yielded a width distribution of 250 ± 139 nm (Supporting Information) and a length distribution of 18.3 ± 5.8 μm. The height of the nanoribbon was determined to be 38 ± 5 nm by tapping mode atomic force microscopy (TM-AFM) (Figure 1C,D), which closely matches with the height (40.6 nm) of the PTOTT-bPEG bilayer model shown in Scheme 1 (Supporting Information). CryoTEM images confirmed that the structure in the normal TEM and AFM images (Figure 1A−D) represents the solution phase assembly structure (Supporting Information). Without the PEG chain, PTOTT homopolymers aggregated into quasi-spherical micelle-like structures by the same procedure (Supporting Information), demonstrating that the amphiphilic architecture is necessary for the formation of well-defined elongated nanoribbons. The UV−vis spectra of PTOTT-b-PEG in methanol showed a red-shift from that of PTOTT-b-PEG dissolved in a good solvent such as chloroform,

morphology in water micelles,c vesiclesd nanoribbons micelles,c vesiclesd micelles,c vesiclesd nanoribbons nanoribbons

a From ref 22. bThis column indicates the hydrogen-bonding capability of each solvent with PEG. cMicelles denote quasi-spherical micelle-like aggregates. dMicelles are dominant and vesicles are the minor species.

capability of each solvent. TEM images and UV−vis spectra of the assemblies are presented in Figure 3 and in the Supporting Information. Interestingly, the nanoribbon structure was observed only when polar protic solvents such as methanol, isopropanol, and ethanol were used as an initial solvent. When polar aprotic solvents such as THF, DMF, and acetonitrile are used as common solvents, PTOTT-b-PEG assembled into irregular quasi-spherical micelle-like aggregates (Supporting 162

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Figure 3. TEM images of 2 μM PTOTT40-b-PEG108 assemblies in water that were self-assembled from different common solvents: (A) methanol, (B) 2-propanol, (C) ethanol, (D) THF, (E) DMF, and (F) acetonitrile. Figure 5. TEM images of PTOTT40-b-PEG108 assemblies in water formed at different concentrations of PTOTT40-b-PEG108 in methanol: (A) 0.5 μM, (B) 2 μM (typical condition for nanoribbon formation), (C) 5.4 μM, and (D) 10 μM.

Information). Note that despite the similar solubility parameters of acetonitrile (24.3 MPa1/2) and isopropanol (23.5 MPa1/2),22 the self-assembly structure formed using the two solvents are completely different. These results indicate that the hydrogen-bonding capability of the initial solvent with PTOTT-b-PEG is important in controlling the final selfassembly structure in water. Effect of Solvent Composition and Polymer Concentration. The self-assembly process in a methanol/water mixture was monitored at different water addition stages (Figure 4). At 9% water content, PTOTT-b-PEG associates

(Figure 4B, 23% water). The thickness of the sheets was measured to be 7.5 ± 0.8 nm by AFM, which is substantially thinner than that of nanoribbons (38 ± 5 nm). This result suggests that the sheets are likely to adopt the side-by-side parallel packing of PTOTT, which has been observed in other brush copolymers.23−26 With increasing concentration from 0.5 to 2 μM, the morphology changed from sheets to nanoribbons (Figure 5A,B), similarly to the morphology change that occurred with increasing water content in water/methanol mixtures (Figure 4B,C). As the water content or polymer concentration increases, nanoribbons with face-to-face PTOTT packing emerge to minimize the interaction between polythiophene and water. With a further concentration increase beyond 2 μM, the assembly structure evolved from smooth nanoribbons to nanoribbons decorated with budding quasispherical assemblies (Figure 5C) and finally to a mixture of quasi-spherical assemblies and broken nanoribbons (Figure 5D). This micelle-like assembly structure resembles the structure formed when aprotic solvents were used as initial solvents (Figure 3D−F). This result indicates that the ribbonto-micelle transition is likely to occur because other interactions such as π−π interactions become more dominant than hydrogen bonding at high concentrations.

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CONCLUSIONS The solution phase self-assembly of a new π-conjugated amphiphilic polymer, PTOTT-b-PEG, generated various types of well-defined assembly structures from vesicles and nanosheets to nanoribbons with a dimension reaching tens of micrometers. This work demonstrated that the addition of hydrogen-bonding capability to π-conjugated rod−coil polymers can lead to diverse assembly structures of conjugated polymers with well-ordered packing structures and desired nanometer scale morphologies. The self-assembly of PTOTTb-PEG does not follow the typical concentration dependence observed for coil−coil block copolymers where increased polymer concentration leads to morphology transitions from high curvature assemblies such as simple micelles to low curvature assemblies such as bilayers.27,28 Rather, the solvent-

Figure 4. TEM images of PTOTT40-b-PEG108 assemblies formed at a series of different water/methanol (v/v) contents: (A) 9% water, (B) 23% water after overnight incubation, (C) 56% water, and (D) 100% water after dialysis. The assemblies were prepared from 2 μM PTOTT-b-PEG in methanol by the slow water addition method.

into vesicles (Figure 4A). As the water content increases, the morphology evolves from vesicles to sheet-like structures (Figure 4B) to finally nanoribbons (Figure 4C,D). Concentration dependence experiments showed a similar trend (Figure 5). At a low concentration (0.5 μM), the final assembly structure of PTOTT-b-PEG in water adopts the thin sheet-like structure (Figure 5A) observed in a water/methanol mixture 163

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National Science Foundation IGERT DGE02-21664. Use of University of Pennsylvania Nano/Bio Interface Center instrumentation is acknowledged.

and concentration-dependence studies indicated that hydrogenbonding and π−π interactions are important for the selfassembly of PTOTT-b-PEG, and the unique nanoribbon structure is formed due to a delicate interplay between the two interactions. We envision that the solution phase selfassembly of conjugated amphiphilic polymers can be used as a promising alternative to conventional thin film techniques for building active layers of organic devices with desired morphologies.

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EXPERIMENTAL SECTION

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ASSOCIATED CONTENT

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Instrumentation. Transmission electron microscopy (TEM) images were obtained on a JEOL 1400 electron microscope operating at 120 kV accelerating voltage. Tapping-mode atomic force microscopy (TM-AFM) images were collected on a Molecular Imaging PicoPlus system. Electronic absorption spectra were acquired on an Agilent 8453 spectrophotometer. Photoluminescence spectra were acquired on a Spex Fluorolog 3 utilizing a R928 PMT detector. Synthesis of PTOTT40-b-PEG108. PTOTT40-b-PEG108 diblock copolymers were synthesized via the triazole cycloaddition click coupling reaction between ethynyl-PTOTT and azide-PEG. The dibrominated TOTT monomer was synthesized using modified literature procedures29 and was polymerized into ethynyl-PTOTT using the end-functionalized Grignard metathesis polymerization method.30,31 Azide-PEG was synthesized by the mesylation of the hydroxyl terminus of commercial methoxy PEG followed by sodium azide substitution.32,33 The click coupling product was purified by precipitation into diethyl ether and aqueous washing to remove unreacted ethynyl-PTOTT and azide-PEG, respectively. The synthesized block copolymer was characterized by 1H NMR spectroscopy (Figure S5) and gel permeation chromatography (Figure S6). More detailed procedure and characterization data are presented in the Supporting Information. Preparation of PTOTT-b-PEG Assemblies. In a typical experiment, 50 μL of a PTOTT40-b-PEG108 solution (6.3 × 10−5 M) in chloroform was dried down under nitrogen and then redissolved in 1 mL of a common solvent (methanol, ethanol, isopropanol, DMF, THF, or acetonitrile). 300 μL of water (18 MΩ·cm) was slowly added to the block copolymer solution at a rate of 10 μL per 30 s while stirring. The mixture was kept under stirring for 12 h before adding an additional 1 mL of water at a rate of 50 μL per 30 s. Then, the samples were dialyzed against water for 24 h to remove organic solvent. The residual amount of organic solvent was further removed by a series of centrifugation and redispersion in water. S Supporting Information *

Detailed synthesis and characterization of PTOTT-b-PEG, estimation of nanoribbon thickness, additional TEM, cryoTEM, and SEM images of PTOTT-b-PEG nanoribbons, selfassembly and optical properties of PTOTT-b-PEG in selective solvents and at different concentrations, self-assembly and optical properties of PTOTT homopolymer. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (S.-J.P.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by ARO young investigator award (W911NF-09-1-0146), the Camille Dreyfus teacher scholar award, and the Nano/Bio Interface Center through the 164

dx.doi.org/10.1021/ma4021483 | Macromolecules 2014, 47, 161−164