Graft Architectured Rod–Coil Copolymers Based on Alternating

Article pubs.acs.org/Macromolecules

Graft Architectured Rod−Coil Copolymers Based on Alternating Conjugated Backbone: Morphological and Optical Properties Wonho Lee,† Jin-Seong Kim,† Hyeong Jun Kim,† Jae Man Shin,† Kang Hee Ku,† Hyunseung Yang,† Junhyuk Lee,† Jung Gun Bae,‡ Won Bo Lee,*,§ and Bumjoon J. Kim*,† †

Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Republic of Korea ‡ Department of Chemical and Biomolecular Engineering, Sogang University, Seoul 121-742, Republic of Korea § School of Chemical and Biological Engineering, Institute of Chemical Processes, Seoul National University, Seoul 151-742, Republic of Korea S Supporting Information *

ABSTRACT: Controlling the self-assembly of conjugated copolymers is of great importance in tuning their physical and optoelectronic properties, offering potential pathways to greatly enhance the performance of organic electronics. Here, we report the synthesis of rod−coil graft copolymers containing an electroactive conjugated rod-like backbone and polymer coils as grafts and demonstrate the control of their ordered nanostructures. As a model system, we synthesized light-emitting poly(fluorene-altphenylene) (PFP) alternating copolymers and then grafted poly(2-vinylpyridine) (P2VP) chains with different lengths via a “click” reaction to produce a series of PFP-g-P2VP graft copolymers with various P2VP volume fractions ( f P2VP). Interestingly, PFP-g-P2VP rod−coil copolymers assembled into well-ordered cylinders and lamellae depending on f P2VP values that resembled those of the coil−coil type block copolymers, but with very different f P2VP values for the morphological transitions (i.e., cylinders to lamellae). The morphological behavior of these graft copolymers was investigated using self-consistent-field theory simulations. Furthermore, by fully exploiting the controlled nanostructures of PFP-g-P2VP and the strong emitting properties of the PFP backbone, we developed multicolor colloidal particles that emit a broad range color spectrum from blue, white, and orange light. Our synthetic approach paves a new method for modulating the self-assembled nanostructures of rod−coil copolymers and their optoelectronic properties.

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INTRODUCTION Conjugated polymers have attracted significant attention for their applications in light-emitting diodes, polymer solar cells (PSCs), transistors, and sensors.1−10 Physical, optical, and electrical properties of the conjugated polymers should be considered and controlled suitable for specific purposes of the applications.1,11−13 In particular, alternating conjugated copolymers, composed of two different monomers, have emerged as a promising approach because their optical and electrical properties can be easily manipulated and tailored by changing the combination of two different monomers that have different electron affinities.1,13−17 In particular, the alternating push−pull type copolymers consisting of electron-rich (D) and electrondeficient (A) units (D−A alternating copolymers) have shown superior optoelectrical properties relative to homopolymers (i.e., poly(3-hexylthiophene) (P3HT)) in organic optoelectronic devices.13,14,18−21 For example, D−A alternating © XXXX American Chemical Society

copolymers for applications in PSCs have been intensively developed to produce tunable bandgaps and optimize light harvesting, resulting in the state-of-the-art PSCs.13,22−28 Ordered nanostructures of self-assembled block copolymers have been considered as ideal active-layer candidates for highperformance organic electronics that require continuous charge transport pathways with desirable nanoscale domain sizes.29−31 To date, various rod−coil block copolymers, containing poly(thiophene), 3 2 −3 6 poly(fluorene), 3 7 −3 9 or poly(phenylenevinylene)40−42 as the rod block, have been reported to produce self-assembled nanostructures. However, the choice of the rod block of the rod−coil copolymers has been limited to the conjugated homopolymers. This is mainly due to the Received: May 19, 2015 Revised: July 31, 2015

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DOI: 10.1021/acs.macromol.5b01068 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Synthetic Routes for PFP-g-P2VP

tuning their self-assembly structures and optoelectronic properties.52 Herein, we describe the synthesis of a novel series of rod− coil graft copolymers based on rod backbones of alternating conjugated copolymers and investigate their self-assembled structures. As a model structure, we selected alternating conjugated copolymer, poly(fluorene-alt-phenylene) (PFP), as a rod backbone due to their simple structure and excellent optoelectronic properties. Then, alkyne-terminated poly(2vinylpyridine) polymers (P2VP-alkyne) with different number-average molecular weights (Mn) were incorporated via the azide−alkyne “click” reaction to produce a series of poly(fluorene-alt-phenylene)-graf t-poly(2-vinylpyridine) (PFP-gP2VP) with various P2VP volume fractions (f P2VP). Their self-assembly behaviors were examined using transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS). Remarkably, well-ordered cylinders and lamellae nanostructures that resembled those of the coil−coil type copolymers were successfully produced, and their morphological behaviors were also compared with self-consistent-field theory (SCFT) simulations. To demonstrate the potential use of PFP-g-P2VP polymers with ordered nanostructures, we presented multicolor colloidal inks, which was achieved by selective incorporation of different amounts of rhodamine 6G dye within the P2VP domains of the well-ordered nanostructures.

difficulty in synthesizing the rod−coil block copolymers based on alternating conjugated copolymers as the rod block. While the synthesis of alternating conjugated copolymers typically requires step-growth polycondensation catalyzed by Pd (i.e., Pd-catalyzed Suzuki−Miyaura or Stille polycondensations), controlling the functionality of their end-group has been a great challenge, which is critical step before sequential growth of second block to complete the synthesis of the block copolymers.43,44 Thus, the resulting polymers often have a mixture of functional groups on each termini, which could lead to significant amount of triblock copolymer or homopolymer impurities. Therefore, new synthetic approaches are needed for producing well-defined alternating conjugated polymer-based rod−coil copolymers.44−46 Graft copolymers belong to an interesting class of copolymers with a variety of well-ordered nanomorphologies.47−50 Recently, we demonstrated the tunable selfassembled structures from P3HT homopolymer-based graft copolymers as a model system. In this approach, the rod−rod interactions between P3HT could be systematically modulated, resulting in well-ordered, nonfibril nanostructures, including lamellae and cylindrical structures, which have not been reported previously in P3HT-based rod−coil conventional block copolymers.51 In particular, the graft approach can produce rod−coil copolymers without involving any ill-defined end-groups on the rod backbone. Therefore, introducing the graft architecture can be a powerful solution for developing rod−coil copolymers based on conjugated copolymer rods and B

DOI: 10.1021/acs.macromol.5b01068 Macromolecules XXXX, XXX, XXX−XXX

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RESULTS AND DISCUSSION Scheme 1 describes the synthetic routes for the PFP-g-P2VP copolymers. A series of PFP-g-P2VP copolymers were prepared via a “click” reaction between azido-group-incorporated poly(fluorene-alt-phenylene) alternating conjugated copolymer (PFP-azide) prepared by Pd-catalyzed polycondensation and P2VP-alkyne polymers synthesized by reversible addition− fragmentation chain transfer polymerization (RAFT) (Scheme 1). In order to synthesize PFP-azide, 2,7-dibromofluorene monomer with a bromo group at the end of alkyl chains (2) was prepared according to the modified literature procedure.53,54 The random copolymerization was performed by Suzuki−Miyaura polycondensation between boronic esters monomer (1) and brominated monomers (2 (10 mol %) and 3 (90 mol %)). After the polymerization, the bromo group at the end of alkyl chain was converted into an azido group (N3) with sodium azide (NaN3) to produce PFP-azide polymers. Therefore, the composition of N3 grafting points, randomly placed along the PFP backbones, was controlled to be 10 mol %. The substitution of the bromo groups by azido groups was confirmed by 1H NMR and FT-IR spectroscopy. The 1H NMR spectra showed the proton peak next to the azido group at 3.14 ppm as well as the disappearance of the proton peak next to bromo group at 3.30 ppm (Figure S1). FT-IR spectra also indicated the presence of N3 groups from the peak at 2100 cm−1 (Figure 1a). Meanwhile, a series of P2VP-alkynes as graft

toward higher molecular weight and a monomodal distribution, indicating the successful formation of graft copolymers (Figure 1b and Figure S2). Consequently, architecturally well-defined PFP-g-P2VP copolymers were prepared with an average number of grafted P2VP chains per copolymer of 3.7, and the Mn of P2VP varied from 5.5 to 18.4 kg/mol to systematically tune the f P2VP values. For convenience, we have denoted PFP-g-P2VP copolymers with different f P2VP values as P1 ( f P2VP = 0.51), P2 ( f P2VP = 0.61), and P3 ( f P2VP = 0.77) (Figure S3 and Table S1). Table 1 summarizes the characteristics of the synthesized PFP-g-P2VP copolymers. We investigated the self-assembly behaviors of PFP-g-P2VP by employing TEM and SAXS. The samples for morphological study were prepared by a microwave-assisted solvent annealing process that was reported to greatly reduce annealing time and provide low defect densities.56,57 First, thick films were produced by drop-casting a solution of PFP-g-P2VP in chloroform onto silicon substrates. Then, the samples were annealed at 130 °C for 10 min in a microwave reactor. The detailed procedure of microwave-assisted annealing is described in the literature.57 To observe the cross-sectional morphology, samples were microtomed to produce 50 nm thick films and then treated with iodine vapor to selectively stain the P2VP domains of the polymers. Further details of the sample preparation are provided in the Supporting Information. The TEM image in Figure 2a shows that the film prepared from P1 has P2VP cylinders embedded in a PFP matrix, but their structure was not highly ordered. The domain spacing (d) was determined as 16.1 nm from the scattering peak (q1 = 0.39 nm−1) of the SAXS data (Figure S4). As the f P2VP was increased from 0.51 to 0.61, a morphological transition from cylinder to lamellar phases was observed (Figure 2b). The domain spacing of the lamellar phase in P2 film was found to be d = 16.1 nm (q1 = 0.39 nm−1) by SAXS measurements (Figure S4). The lamellar structures in the P2 film were highly ordered, containing long continuous stacks oriented parallel to the substrate through the entire bulk film (Figure S5). As the f P2VP was increased further to 0.77, the P3 graft copolymer selfassembles into PFP hexagonal phase within P2VP matrix as shown in Figure 2c. SAXS measurements confirmed that P3 film had hexagonal ordering scattering peaks at q1 = 0.35 nm−1 and q2 = 0.61 nm−1 (q1:q2 = 1:√3, d = 17.9 nm) (Figure S4). Thus, PFP-g-P2VP copolymers exhibit self-assembly behavior similar to conventional coil−coil copolymers, i.e., P2VP cylinders, lamellae, and PFP cylinders. We note that this is the first report on the self-assembled nanostructures based on rod−coil copolymers containing a rod-like alternating conjugated polymer backbone. Notably, a shift in the phase boundaries of PFP-g-P2VP melts with respect to conventional coil−coil diblock copolymers was observed. This difference in phase behavior is attributed mainly to molecular asymmetry.47,58 As a result, higher f P2VP value was required to form the lamellar phase compared to that for linear diblock copolymers. To gain a deeper understanding of phase behavior of PFP-gP2VP, self-consistent-field theory (SCFT) simulations were performed.49,59−61 We assumed that the number of constituting graft chains per rod-like backbone was four by considering the number of grafted P2VP chains per PFP backbone to be on average 3.7. In addition, the distances between two grafting points on a given polymer backbone were assumed to be equivalent to simplify the calculation. For comparison with the experimental results of PFP-g-P2VP copolymers showing cylindrical and lamellar nanostructures, we calculated the

Figure 1. (a) FT-IR spectra of PFP-azide and PFP-g-P2VP copolymers. (b) SEC trace of P1 after purification. The graft copolymers were obtained by repeated centrifugations using methanol as a selective solvent for ungrafted P2VP.

chains were synthesized with different number-average molecular weights (Mn) values by RAFT polymerization according to the literature.55 Then, a series of PFP-g-P2VP copolymers having different f P2VP values were synthesized by a “click” reaction between azido groups of PFP and alkyne groups of P2VP. Completion of the reaction was confirmed by the disappearance of the azide peak (2100 cm−1) in the FT-IR spectra (Figure 1a). In addition, size exclusion chromatography (SEC) of the PFP-g-P2VP copolymers showed a peak shift C

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Macromolecules Table 1. Characteristics of PFP-g-P2VP Copolymers graft copolymer

grafting densitya (mol %)

MnPFPb (kg/mol)

PFP Đb

MnP2VPb (kg/mol)

P2VP Đb

f P2VPc

structured

de (nm)

P1 P2 P3

9.8

15.0

1.68

5.5 8.4 18.4

1.20 1.20 1.14

0.51 0.61 0.77

P2VP cylinder lamellar PFP cylinder

16.1 16.1 17.9

a Determined from 1H NMR spectra of PFP-azide. bDetermined from SEC of PFP-Br calibrated with PS standards. cCalculated based on the integration of 1H NMR with the density of 0.95 g cm−3 for PFP and of 1.14 g cm−3 for P2VP. dDetermined from TEM and SAXS. eDetermined from SAXS.

stable HEX phase was predicted for graft copolymers having f P2VP values as high as 0.447, for which LAM morphology is expected in the case of linear diblock copolymers. Additionally, the boundary of the LAM region was expanded to f P2VP value of 0.769. The experimental results for P2 are in agreement with the mean-field phase diagram, both of which show lamellar nanostructures at f P2VP = 0.61. In the case of P1 (P2VP cylinders) and P3 (PFP cylinders), there is a little discrepancy between experimental and SCFT results. SCFT results predict that f P2VP value for P1 is located at the phase boundary between GYR and LAM, and the f P2VP value for P3 is located in the region of GYRII. The discrepancy may come from the use of simplified (Gaussian) chain model and assumptions neglecting variations in grafting points and thermal fluctuation effects. In addition, the rigid rod backbone may suppress the stability of the bicontinuous double gyroid phase, which may expand the stable HEXII phase into the GYRII region. Interestingly, the increased domain spacing of PFP cylinders (d = 17.9 nm) in P3 film compared to that (d = 16.1 nm) of P2VP cylinders in P1 film also can be explained well by the SCFT simulations. For example, the comparison of the simulated images of HEX and HEXII in Figure 3 revealed the increased lattice spacing of the HEXII phase relative to that of HEX. Table S2 provides quantitative comparison of the lattice parameters of the HEX and HEXII phases normalized by the lamellar lattice spacing dL. This result is reasonable because Ngraft of HEXII is larger than that of HEX while Nbackbone value is fixed. To exploit the strong emitting behavior of PFP and the controlled nanostructures from PFP-g-P2VP, we demonstrated multicolor colloidal inks that emit a wide range of colors. Colored light-emitting colloidal particles are useful in various applications such as white lighting, inks for displays, high contrast bioimaging, and fluorescent sensors.62−67 PFP-g-P2VP copolymers are particularly promising for templating small molecules and inorganic nanoparticles (i.e., quantum dots) and for controlling their 3D spatial arrangements through hydrogen bonding or electrostatic interactions with P2VP domains.68−71 To examine this possibility, we demonstrated the fabrication of color-tunable polymeric particles by incorporating rhodamine 6G into the P2VP chain of P2 particles through hydrogen bonding. We chose rhodamine 6G because it exhibits strong light-emitting behavior and an emission wavelength (λ) of 560 nm, complementary to that of PFP so that a broad color range can be achieved simply by tuning the concentration of rhodamine 6G in the PFP-g-P2VP colloids. PFP-g-P2VP colloidal particles were prepared by the slow release of chloroform at 40 °C from PFP-g-P2VP/chloroform droplets stabilized by 1 wt % cetyltrimethylammonium bromide (CTAB) in an aqueous solution (see Supporting Information for details). As shown in the SEM images of Figure S6, P2 and P3 colloidal particles were successfully prepared with the average diameters of 235 ± 75 and 265 ± 69 nm, respectively. Figures 4a and 4b show TEM images of P2 and P3 colloidal

Figure 2. Cross-sectional TEM images of (a) P1, (b) P2, and (c) P3 films.

mean-field phase diagram by tuning the degree of polymerization of the grafted chain from Ngraft = 25 to Ngraft = 220, which provided graft copolymers with a range of f P2VP values from 0.33 to 0.82 (see Supporting Information for additional details). Figure 3 shows the mean-field phase diagram where hexagonal cylinder (HEX), lamellar (LAM), and bicontinuous double gyroid (GYR) phases are considered for determining phase boundaries. Consistent with experimental results, all phase boundaries are shifted to higher f P2VP values in the phase diagram compared to linear diblock copolymers. For example, a D

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Figure 3. Mean-field phase diagram for a graft copolymer at χ = 0.2. Nbackbone = 200 and Ngraft = 25−220. Labeled phases are HEX (hexagonal cylinders), GYR (bicontinuous double gyroid), LAM (lamellae), GYRII (inverted bicontinuous double gyroid), and HEXII (inverted hexagonal cylinders).

successful example of the synergistic effect of supramolecular assembly of organic dyes in highly emissive rod−coil copolymers with well-ordered nanostructures.

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CONCLUSIONS We described the synthesis of a series of the graft copolymers based on a PFP alternating conjugated copolymer and P2VP grafts using well-controlled polymerizations and “click” chemistry. The resulting PFP-g-P2VP graft copolymers selfassembled into well-ordered P2VP cylinders, lamellae, and PFP cylinders depending on f P2VP values in both bulk films and colloidal particles. Interestingly, a shift in the phase behavior of PFP-g-P2VP compared to conventional coil−coil diblock copolymers was observed in experiments and confirmed by SCFT simulations. With the ordered nanostructures of PFP-gP2VP, multicolor emitting colloidal particles were developed by incorporating rhodamine 6G into the P2VP grafts that emitted a broad range of colors, including blue, orange, and white emissions. The graft architecture can be potentially applicable to any electroactive conjugated copolymers that are currently used for solution-processed organic electronics, allowing advanced applications with tunable photophysical and morphological properties.

Figure 4. TEM images of (a) P2 and (b) P3 colloidal particles. (c) Photographs and PL spectra of P2 particles with different concentrations of rhodamine 6G under 365 nm irradiation: (1) 0, (2) 0.2, (3) 0.4, (4) 0.8, and (5) 1.6 wt %.

particles. The ordered onion-like multilayered and cylindrical structures were successfully formed in P2 and P3 colloids, respectively, which is consistent with the morphologies observed in other lamellar and cylindrical forming BCP systems confined in spherical particles.72,73 The initial amount of rhodamine 6G in the P2VP domains was controlled quantitatively to generate various light emissions combined with the strong blue emission of the PFP block. As shown in Figure 4c, P2 particles without dye molecules exhibited bright blue emissions with a peak maximum of 420 nm (1). Then, the intensity ratios of the emission of PFP chains (I420) to the emission of rhodamine 6G (I560) were tuned simply by controlling the amount of rhodamine 6G. As a result, various colors of light emission were observed, including light blue (2), white (3), light orange (4), and orange (5), as the ratio of I420 to I560 increased. TEM images of P2 particles with rhodamine 6G indicated that the onion-like morphology of the P2 colloids was well preserved upon the addition of rhodamine 6G (Figure S7). To further confirm the selective inclusion of rhodamine 6G in P2VP chains through hydrogen bonding, we also prepared colloidal particles using PFP-Br homopolymers with 1.6 wt % rhodamine 6G (same condition with (5) in Figure 4c) as a control sample. As shown in Figure S8, the emission of rhodamine 6G (I560) in the PFP-Br colloids is almost negligible compared to that of P2 colloids. These features provided a

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b01068. Detailed experimental procedures and characterization data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (B.J.K.). *E-mail: [email protected] (W.B.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Global Frontier R&D Program on Center for Multiscale Energy System (2012M3A6A7055540) and by the National Research E

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Foundation Grant (2012M1A2A2671746, 2015R1A2A2A01007379), funded by the Korean Government. This research was also supported by the Research Projects of the KAIST-KUSTAR and the CRH (Climate Change Research Hub) of KAIST. Experiments at PLS were supported by MEST and POSTECH.

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DOI: 10.1021/acs.macromol.5b01068 Macromolecules XXXX, XXX, XXX−XXX