Structure–Photophysical Property Relationship of Conjugated Rod

Note pubs.acs.org/Macromolecules

Structure−Photophysical Property Relationship of Conjugated Rod− Coil Block Copolymers in Solutions Jui-Hsiang Hung Institute of Polymer Science and Engineering, National Taiwan University, Taipei, Taiwan 106, R.O.C

Yung-Lung Lin and Yu-Jane Sheng* Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan 106, R.O.C

Heng-Kwong Tsao* Department of Chemical and Materials Engineering, Department of Physics, National Central University, Jhongli, Taiwan 320, R.O.C S Supporting Information *

T

According to the extensive studies on dye molecules which often assemble into large aggregates in a parallel way, the hypsochromic (blue) shift is attributed to the formation of Haggregates while the bathochromic (red) shift is accounted for by the formation of J-aggregates.12 Dyes are normally very large aromatic molecules consisting of many linked rings. The direction of spectral shifts, red or blue, is largely dictated by the angle of slippage (α) of successive conjugated planes.12 As shown in Figure 1, the slippage angle is defined as the angle

he self-assembly of amphiphilic coil−coil block copolymers in a selective solvent has a wide range of potential applications, including formulations, pharmaceuticals, and separations, due to their unique physical and chemical properties. Lately, much attention has been focused on the aggregation behavior of the rod−coil copolymer, which is further complicated by the inflexible rodlike segments of these molecules. The rigid chain conformations come from πconjugation along the polymer backbone (as in conjugated polymers),1 helical secondary structures (as in biomolecules),2 or mesogenic units (as in liquid crystals).3 Photoresponsive rod−coil block polypeptides which can undergo a reversible aggregation−dissolution process are used as viable model systems for photoinduced drug release applications.2e As for the π-conjugation rod−coil copolymers, due to their distinct photoactive and electroactive properties, they have advanced applications in optoelectronic materials, including lasers,4 photovoltaic cells,5 and polymer light-emitting diodes for fullcolor displays,6 to name a few. It has been shown that the photophysical properties of πconjugated rod−coil copolymers are highly correlated to their molecular structures and microscopic arrangements of rod blocks. The combination of the molecular amphiphilic nature and the preference of orientational ordering of rod blocks drives rod−coil block copolymers to self-assemble into a variety of intriguing morphologies such as structural sphere, inverted cylindrical vesicle, and segmented network.7,8 These supramolecular morphologies often induce a significant variation in the optical or electronic properties of the π-conjugated segments. That is, different self-assembled patterns of conjugated rod blocks affect the electronic structure of the system and lead to different photophysical behaviors examined by UV−vis and photoluminescence spectroscopy. For example, the spectral shift is commonly observed as conjugated rod−coil copolymer systems proceed from the dispersed phase to the aggregated phase in solution or solid state.9−11 Thus, a detailed investigation of the morphological structures and internal rod arrangements is of great importance in understanding the photophysical properties for future polymer technologies and optoelectronic applications. © 2012 American Chemical Society

Figure 1. Schematic representations of the included angle (θ) and slippage angle (α) in rod−coil systems. Note that 0 ≤ θ ≤ 90° and 0 ≤ α ≤ 90°.

between the line-of-centers of a column of molecules and the long axis of any one of the parallel molecules. It is believed that planes with slippage angle larger than a certain value leads to bathochromic shifts, while planes with slippage angle less than that value result in hypsochromic shifts. Note that the larger the slippage angle, the less the slippage extent (planar offset). Although dye molecules tend to be parallel to each other for both types of aggregates, the slippage extent is larger for Jaggregates and smaller for H-aggregates. To demonstrate this principle applicable to rodlike molecules, the largest absorption wavelength (λmax) of the πconjugated molecule F3T8 which contains three fluorene units and eight thiophene units at different molecular arrangements Received: January 6, 2012 Revised: February 5, 2012 Published: February 16, 2012 2166

dx.doi.org/10.1021/ma300039d | Macromolecules 2012, 45, 2166−2170

Macromolecules

Note

is calculated by using the VAMP package13 based on quantum semiempirical calculations. For the dispersed phase, λmax ≈ 308 nm. The variation of λmax with α is illustrated in Figure 2. When

conservative, dissipative, and random forces. The interaction parameters for the conservative forces are listed in Table 1. The Table 1. Interaction Parameters (aij) for the Rod−Coil Block Copolymer Systems coil rod solvent

coil

rod

solvent

25 35 30

25 45

25

polymeric beads are bound together by spring forces to keep the neighboring beads at an equilibrium distance. The rigidity of the rod block is provided by additional spring forces.7 Since conjugated rods have a tendency to orient parallel to one another due to the π−π interaction when situated in close proximity, an orientational potential is further employed, Uo = −k(1 − r/rc) cos2 θ as r ≤ rc and zero otherwise.7 The strengths as well as the causes of the π−π interaction vary strongly. It is generally accepted that the π−π strength ranges between 0 and 50 kJ/mol.7c Therefore, the strength of the orientational force k, which essentially corresponds to the Maier−Saupe parameter, is adjusted between 0 and 10. Note that all the physical variables are scaled by m, rc, and kBT. The volume fraction of rod−coils is kept at 0.05. In most rod−coil block copolymer solutions, the solvent is selective for the coil block, leading to the formation of aggregates with the soluble coils exposed to the solvent. The self-assembly morphology is influenced by the molecular structure, including architecture, rod length (y), coil length (x), and the π−π interaction. First, we consider the archetypical rod−coil systems, diblock copolymers with y:x. Though the aggregate morphology is quite complicated and intriguing, the optoelectronic property is closely related to the inner rod arrangement. Therefore, the internal structure of the rod domain is analyzed via both orientational and positional order. The extent of rod alignment can be reflected via the distribution of the included angle θ (as defined in Figure 1) between two neighboring rods, P(θ), and the local order parameter, S = ⟨(3 cos2 θ − 1)/2⟩. Note that θ = 0 indicates two neighboring rods being perfectly aligned in parallel and the value of S is on the order of 0.3−0.8 for a typical liquid crystal sample. The effects of the rod length and π−π interaction on the alignment of rod blocks are illustrated in Figure 3. Consider P(θ) of rod−coil diblocks with y:x = 6:3. When the orientational force is absent (k = 0), the included angle is uniformly distributed, indicating an isotropic structure in the rod domain. However, once the orientational force is introduced, the rod blocks tend to align with each other and the distribution is highly skewed toward θ = 0. As k is further increased, the width of the included angle distribution gets narrower. In terms of the order parameter, S grows quickly from around zero to greater than 0.8 for 0 ≤ k ≤ 10, as shown in the inset for rod length = 6. In addition to π−π interaction (k), the rod alignment can also be enhanced by increasing the rod length (y).7a As also can be seen in the inset of Figure 3, for k = 0, the rod domain for rod−coils with y = 3 is essentially isotropic and S ≃ 0. However, as the rod length is varied from 3 to 15, the order parameter rises due to entropy effect associated with excluded volume interactions, and the crossover from isotropy to alignment is observed at y ≈ 8. The presence of the orientation force raises the orientational order furthermore and

Figure 2. UV−vis absorption wavelength versus slippage angle of F3T8 using VAMP. The dashed line indicates the absorption wavelength of the dispersed state at 308 nm. The critical slippage angle for the direction of spectral shift is found around 50°.

the slippage angle of the parallel aggregate consisting of three F3T8 molecules is larger than a critical value (say 50°), λmax of such H-aggregates becomes shorter. On the contrary, λmax turns longer for J-aggregate with smaller slippage angle. Evidently, the spectral shift varies with the stacking pattern in the aggregate in terms of the slippage angle, which affects the electronic structure and thereby the photophysical behavior. However, the structural packing and the self-assembly morphology is generally determined by the molecular architecture. Though considerable efforts have been dedicated to the knowledge of the self-assembly behaviors of conjugated rod− coils, there are no manifest and self-contained guidelines to date that can be used to account for the relationships between the fundamental molecular architectures as well as the morphologies and spectral behaviors associated with their self-assembly. To tackle this significant issue, in this Note we explore the self-assembly of conjugated rod−coil copolymers via the dissipative particle dynamics (DPD). Structural effects including molecular architectures, rod/coil block length, and strength of π−π interaction on the orientational order and selfassembled patterns are thoroughly investigated. Our study shows that the gaps between the molecular architectures and the aggregation patterns can be bridged by the classification of rod−coils with various architectures (multiple blocks, star, brush, dendron, etc.) into rod−coil diblocks (RC-type) or coil−rod−coil triblocks (CRC-type) structures. The former tends to have parallel arrangements of rod blocks with smaller slippage extent while the latter displays the stacking structure with larger slippage extent. The DPD method is a coarse-grained particle based, mesoscopic simulation technique.14 DPD beads with the mass m and diameter rc are clusters of several atoms or molecules that have similar properties and they obey Newton’s equation of motion. The interaction between any pair of DPD beads is soft and consists of three intrinsic pairwise-additive forces: 2167

dx.doi.org/10.1021/ma300039d | Macromolecules 2012, 45, 2166−2170

Macromolecules

Note

denoted as smectic CI and that with α̅ < 60° is depicted by smectic CII. The former shows a wide distribution skewed toward α = 90° while the latter displays a peak significantly less than 60°. The distinct difference of the rod arrangement between RC and CRC can be demonstrated by the variation of the mean slippage angle with the rod length at a specified coil length, as shown in the inset of Figure 4. Although α̅ varies with the rod length, it is evident that the mean slippage angle of the RC is always significantly greater than that of the CRC. Their differences can also be manifested by the feature of the distributions of α, as illustrated in Figure 4. There exists a peak at lower values of α for the CRC while the maximum is skewed toward 90° for the RC. These characteristics associated with the slippage angle distribution are closely related to the morphology of the rod domain. The photophysical phase diagram (coil length versus rod length) is shown in Figure 5a for rod−coil diblocks with strong

Figure 3. Distribution of the included angle for rod−coil diblocks (6− 3) with k varying from 0 to 10. In the inset, the orientational order parameter is plotted against the rod length with the coil length fixed at 3.

one has S > 0.3 for short rods (y = 3) when k > 4. Note that in this case the coil length is fixed at 3 and an increase of the coil length hinders the tendency of rod alignment due to the bulky end, but such an effect is relatively weak.7a,b The change of the spectral properties takes place in the rod domain with parallel arrangements and is dependent on the positional order, which can be realized through the slippage angle measurement. Without loss of generality, the arrangement with larger mean slippage angle, α̅ ≥ 67.5°, is defined as smectic A, of which the slippage angle distribution exhibits a peak at α = 90°, as shown in Figure 4. In contrast, that with

Figure 5. Phase diagram of the structure of the rod domain for RC (a) and CRC (b) with k = 10. I, SA, and SC represent the isotropic, smectic A, and smectic C arrangements of the rod domains. The corresponding morphologies are also shown.

enough π−π interaction (k = 10). When the rod length is small enough, i.e. y = 3, the isotropic arrangement in the rod domain is observed regardless of the coil length. Thereby, no spectral shift is expected. As the rod length is long enough, however, the smectic A aggregates are formed and the blue-shift behavior prevails. Note that at a given rod length the increment of the coil length leads to the decline of α, and thus the arrangement of rods tends to shift from smectic A to smectic C type. This consequence reveals that RC molecules tend to form Haggregate with large slippage angles (small slippage extent) and thus exhibit blue-shift spectra. This is consistent with the majority of the experiment findings.10,15,16 Now let us consider coil−rod−coil triblocks (x:y:x). The diagram of the aggregation pattern is illustrated in Figure 5b through coil length against rod length and the π−π interaction

Figure 4. Slippage angle distribution for RC (15−3 and 15−12) and CRC (8−15−8). The inset shows the average slippage angle versus rod length for RC and CRC with varied coil lengths and k = 10.

smaller angle, α̅ < 67.5°, is defined as smectic C, of which the peak occurs at α < 90°. To further demonstrate the degree of the slippage between rods, the aggregate with α̅ = 60°−67.5° is 2168

dx.doi.org/10.1021/ma300039d | Macromolecules 2012, 45, 2166−2170

Macromolecules

Note

k = 10. Similar to RC molecules, the isotropic arrangement is observed for short rod length y = 3, and the rod blocks prefer being parallel to each other at long rod length (y ≥ 6). The smectic CI structure is observed for y = 6 with x ≥ 2, while the smectic A structure is seen for x = 1 with y ≥ 6. Except the aforementioned triblock configurations, the rest of the CRC structure result in the smectic CII aggregates. That is, CRC molecules tend to form J-aggregate with small slippage angles (large slippage extent) and thus exhibit red-shift spectra.10,15,17 Note that the blue-shift behavior corresponding to smectic A can only be observed in a very narrow regime.11 The rod domain of the RC displays a trunklike shape for smectic A phase and a slanted trunk for smectic CI phase, as shown in Figure 5a. On the other hand, the morphology of the rod domain of the CRC is very interesting. The presence of the small slippage angle is accompanied by a leaf-like shape (8:15:8) or a funnel-like structure (2: 15: 2), as illustrated in Figure 5b. It is evident that the morphologies of the rod domain vary with both rod and coil lengths significantly. DPD simulation results indicate that the rod−coil diblocks favor the formation of H-aggregates while the coil−rod−coil triblocks tend to form J-aggregates. What happens for rod− coils with complicated molecular architectures, such as multiblock, star, brush, and dendron? DPD simulations are also performed to explore the influence of molecular architecture, and similar phase diagrams are obtained. Their spectral behavior can be simply classified into two categories: RC type and CRC type. The schematic representations of the general RC-type and CRC-type structures are shown in Figure 6. For CRC-type copolymers, both ends of all rod blocks are

Figures S4−S6 show the typical slippage angle distributions of the CRC-type copolymers: CRCRC, CRC-star, and CRC-graft block copolymers. As one can clearly see, the RC-type copolymers tend to form H-aggregates with large slippage angles and CRC-type copolymers tend to form J-aggregates with small slippage angles. In a single lamellar sheet, densely grafted coils are highly stretched. The preference of smectic A or smectic C phase in the rod domain depends on the competition between the deformation energy of coil stretching and the interfacial energy between rod and coil domains.18 The coil stretching penalty can be lowered by the reduction of the apparent grafting density of coils. For RC-type copolymers, the interfacial energy is dominant and smectic A phase is favored. The apparent grafting density reduction is achieved by the head-to-tail arrangement, as in the puck micelle model, for rod−coil diblocks, multitailed RC, and CRCR or is attained by the common share of coils for RCR, RC-stars, and RC-graft. On the contrary, for CRC-type copolymers, the deformation energy becomes dominant and smectic CII phase prevails. Since their apparent grafting density cannot be reduced by simply changing the rod orientation as in the RC-type, rod blocks of the CRCtype dislocate significantly at the expense of the increment of interfacial area to reduce the crowdedness among coils. In CRC systems the crossover from smectic CI to CII phase occurs as the rod length increases. Different from the driving force for the transition from smectic A to C phase, the crossover between smectic CI and CII phases is due to the effect of π−π interactions between two rods, which grow with the rod length. As a result, longer rods become more parallel-aligned. This result can be demonstrated through the included angle distribution P(θ), as shown in Figure S7. Evidently, longer rods possess higher probability of parallel alignment (θ = 0) and their distributions skew toward θ = 0. For CRC with short rods, the π−π attraction is weak and the rods are less parallel-aligned. This conformation has more free space for the coils and the interfacial energy becomes more important than the coil stretching energy. The competition between the two energies results in small slips between rods. As a result, smectic CI phase is formed. The schematic representation of smectic CI phase is shown in Figure S8. For CRC with long rods, the π−π attraction is strong and most of the rods in close proximity align in parallel formations. Large slips between rods are needed to prevent coils from stretching. Consequently, smectic CII phase takes shape. The schematic representation of smectic CII phase is also shown in Figure S8. Conjugated rod−coil copolymers in a coil-selective solvent self-assemble into aggregates with parallel arrangements of rod blocks. The spectral properties of the aggregative polymers are significantly affected by their supramolecular morphologies, which in turn are greatly influenced by the molecular architectures of rod−coils. In this Note the relationship between spectral properties and molecular architecture is established based on DPD simulations, which incorporates large-scale molecular features while avoiding atomic scale details. It is found that rod−coil copolymers can be qualitatively classified into two types: RC-type and CRC-type. RC-type is generally with large slippage angle (smectic A), and blue-shift behavior prevails. CRC-type is normally with small slippage angle (smectic CII) and thus exhibits red-shift spectra. The structural packing in the rod domain is the consequence of the competition between the deformation energy of coil stretching and the rod/coil interfacial energy. The interfacial energy is

Figure 6. Classification of the spectral behavior of rod−coils with various architectures into the general RC-type and CRC-type structures with their schematic representations.

connected with coil blocks, while for RC-type copolymers, only one end of some rod blocks is attached with coil blocks. This distinct difference plays the major role in determining the feature of the slippage angle distribution. Some simulation results of the slippage angle distributions (Figures S1−S6) are presented as Supporting Information. Figures S1−S3 demonstrate the typical slippage angle distributions of the RC-type copolymers: RCR, RC-star, and RC-graft block copolymers. 2169

dx.doi.org/10.1021/ma300039d | Macromolecules 2012, 45, 2166−2170

Macromolecules

Note

Ha, C. S. Prog. Polym. Sci. 2003, 33, 581. (d) Akcelrud, L. Prog. Polym. Sci. 2003, 28, 875. (7) (a) Chou, S. H.; Tsao, H. K.; Sheng, Y. J. J. Chem. Phys. 2011, 134, 034904. (b) Chou, S. H.; Wu, D. T.; Tsao, H. K.; Sheng, Y. J. Soft Matter 2011, 7, 9119. (c) Steed, J.; Atwood, J. Supramolecular Chemistry; Wiley & Sons: Chichester, 2000. (8) Chen, X. L.; Jenekhe, S. A. Macromolecules 2000, 33, 4610. (9) (a) Jenekhe, S.; Chen, X. Science 1998, 279, 5358. (b) Rubatat, L.; Kong, X.; Jenekhe, S.; Ruokolainen, J.; Hojeij, M.; Mezzenga, R. Macromolecules 2008, 41, 1846. (10) Huo, H.; Li, K.; Wang, Q.; Wu, C. Macromolecules 2007, 40, 6692. (11) Bu, L.; Qu, Y.; Yan, D.; Geng, Y.; Wang, F. Macromolecules 2009, 42, 1580. (12) (a) Harrison, W.; Mateer, D.; Tiddy, G. J. Phys. Chem. 1996, 100, 2310. (b) McRae, E.; Kasha, M. J. Chem. Phys. 1958, 28, 721. (c) Czikkely, V.; Forsterling, H.; Kuhn, H. Chem. Phys. Lett. 1970, 6, 11−14. (d) Norland, K.; Ames, A.; Taylor, T. Phot. Sci. Eng 1970, 14, 295−307. (13) Accelrys Software Inc., Materials Studio Release Notes, Release 4.4, Accelrys Software Inc., San Diego, CA, 2008. (14) (a) Hoogerbrugge, P. J.; Koelman, J. M. V. A. Europhys. Lett. 1992, 19, 155. (b) Español, P.; Warren, P. Europhys. Lett. 1995, 30, 191. (c) Groot, R. D.; Warren, P. B. J. Chem. Phys. 1997, 107, 4423. (15) Lin, C.; Tung, Y.; Ruokolainen, J.; Mezzenga, R.; Chen, W. Macromolecules 2008, 41, 8759−8769. (16) Tung, Y.; Wu, W.; Chen, W. Macromol. Rapid Commun. 2006, 27, 1838−1844. (17) (a) Lee, M.; Kim, J.; Hwang, I.; Kim, Y.; Oh, N.; Zin, W. Adv. Mater. 2001, 13, 1363. (b) Cuendias, A. D.; Ibarboure, E.; Lecommandoux, S.; Cloutet, E.; Cramail, H. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 4602−4616. (c) Cuendias, A.; Hellaye, M.; Lecommandoux, S.; Cloutet, E.; Cramail, H. J. Mater. Chem. 2005, 15, 3264−3267. (18) (a) Halperin, A. Europhys. Lett. 1989, 10, 549−553. (b) Halperin, A. Macromolecules 1990, 23, 2724−2731. (c) Semenov, A. Mol. Cryst. Liq. Cryst. 1991, 209, 191−199. (d) Williams, D.; Fredrickson, G. Macromolecules 1992, 25, 3561−3568. (e) Sary, N.; Mezzenga, R.; Brochon, C.; Hadziioannou, G.; Ruokolainen, J. Macromolecules 2007, 40, 3277.

dominant in smectic A-aggregates while the deformation energy dominates in smectic C-aggregates. Our predictions of rod−coil diblocks exhibiting blue-shift spectra and coil−rod−coil triblocks showing red-shift spectra are consistent with experimental observations.

■

ASSOCIATED CONTENT

S Supporting Information *

Detailed simulation results of the slippage angle distributions of for some RC-type and CRC-type copolymers; typical slippage angle distributions of the RC-type copolymers CRCR, RC-star, and RC-graft (Figures S1−S3); typical slippage angle distributions of the CRC-type copolymers CRCRC, CRCstar, and CRC-graft (Figures S4−S6). Figure S7 shows the included angle distributions for CRC triblocks. Figure S8 are the schematic representations of the smectic C (I and II) phases. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Y.-J.S.); [email protected] (H.-K.T.). Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS This research work is supported by National Science Council of Taiwan. REFERENCES

(1) (a) Yesodha, S. K.; Pillai, C. K. S.; Tsutsumi, N. Prog. Polym. Sci. 2004, 29, 45. (b) Cho, M. J.; Choi, D. H.; Sullivan, P. A.; Akelaitis, A. J. P.; Dalton, L. R. Prog. Polym. Sci. 2008, 33, 1013. (c) Moliton, A. Optoelectronics of Molecules and Polymers; Springer: New York, 2005. (d) McCulloch, I.; Yoon, H. J. Polym. Sci., Part A: Polym. Chem. 1995, 33, 1177. (2) (a) Cornelissen, J.; Fischer, M.; Sommerdijk, N.; Nolte, R. J. M. Science 1998, 280, 1427. (b) Klok, H. A.; Langenwalter, J. F.; Lecommandoux, S. Macromolecules 2000, 33, 7819. (c) Checot, F.; Lecommandoux, S.; Gnanou, Y.; Klok, H. A. Angew. Chem., Int. Ed. 2002, 41, 1339. (d) Rodriguez-Hernandez, J.; Lecommandoux, S. J. Am. Chem. Soc. 2005, 127, 2026. (e) Kotharangannagari, V. K.; Sánchez-Ferrer, A.; Ruokolainen, J.; Mezzenga, R. Macromolecules 2011, 44, 4569. (3) (a) Lee, M.; Cho, B. K.; Ihn, K. J.; Lee, W. K.; Oh, N. K.; Zin, W. C. J. Am. Chem. Soc. 2001, 123, 4647. (b) Lee, M.; Cho, B. K.; Oh, N. K.; Zin, W. C. Macromolecules 2001, 34, 1987. (c) Braun, P. V.; Osenar, P.; Tohver, V.; Kennedy, S. B.; Stupp, S. I. J. Am. Chem. Soc. 1999, 121, 7302. (d) Stupp, S. I.; LeBonheur, V.; Walker, K.; Li, L. S.; Huggins, K. E.; Keser, M.; Amstutz, A. Science 1997, 276, 384. (4) Tessler, N.; Denton, G.; Friend, R. Nature 1996, 382, 695. (5) (a) Günes, S.; Neugebauer, H.; Sariciftci, N. S. Chem. Rev. 2007, 107, 1324. (b) Thompson, B. C.; Fréchet, J. M. J. Angew. Chem., Int. Ed. 2008, 47, 58. (c) Sariciftci, N. S.; Smilowitz, L.; Heeger, A. J.; Wudl, F. Science 1992, 258, 1474. (d) Campoy-Quiles, M.; Ferénczi, T.; Agostinelli, T.; Etchegoin, P. G.; Kim, Y.; Anthopoulos, T. D.; Stavrinou, P. N.; Bradley, D. C.; Nelson, J. Nature Mater. 2008, 7, 158. (e) Park, S. H.; Roy, A.; Beaupre, S.; Cho, S.; Coates, N.; Moon, J. S.; Moses, D.; Leclerc, M.; Lee, K.; Heeger, A. J. Nature Photonics 2009, 3, 297. (6) (a) Burroughes, J.; Bradley, D.; Brown, A.; Marks, R.; Mackay, K.; Friend, R.; Burns, P.; Holmes, A. Nature 1990, 347, 539. (b) Rhee, J.; Wang, J.; Cha, S.; Chung, J.; Lee, D.; Hong, S.; Choi, B.; Goh, J.; Jung, K.; Kim, S. J. Soc. Inf. Display 2006, 14, 895. (c) Choi, M. C.; Kim, Y.; 2170

dx.doi.org/10.1021/ma300039d | Macromolecules 2012, 45, 2166−2170