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Dec 10, 2015 - photothermal properties of polybenzimidazole fibers with meta-linkage under different excitation wavelengths (λex) and powers of laser...
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Effects of Chain Orientation and Packing on the Photoluminescence and Photothermal Properties of Polybenzimidazole Fibers with Meta-Linkage Jinho Park and Young Gyu Jeong* Department of Advanced Organic Materials and Textile System Engineering, Chungnam National University, Daejeon 34134, Republic of Korea S Supporting Information *

ABSTRACT: We herein report the influences of chain orientation and packing on the photoluminescent (PL) and photothermal properties of polybenzimidazole fibers with meta-linkage under different excitation wavelengths (λex) and powers of laser sources. For the purpose, poly[2,2′-(mphenylene)-5,5′-bibenzimidazole] (PmBI) was synthesized and then processed into fibers via solution spinning and heat treatment. The chain orientation and packing of the as-spun PmBI fiber decreased and increased owing to the heat treatment, respectively, which resulted from the nonlinear chain conformation of PmBI with meta-linkage. It was found that the visible PL intensity under λex = 325 nm, which is associated with an efficient interchain photorelaxation between benzimidazole rings, was much higher for the as-spun fiber with higher chain orientation, while the infrared PL intensity under λex = 514 nm, which is dominated by an intrachain photorelaxation, was far higher for the heat-treated fiber with higher chain packing. Accordingly, the heat-treated PmBI fiber exhibited excellent photothermal heating behavior by attaining maximum temperature increments (ΔTmax) of ∼31.7 °C under λex = 405 nm and 18.4 mW, which was even higher than the as-spun fibers (ΔTmax ∼ 27.5 °C). For the PmBI fibers, the ΔTmax values were also found to increase with the decrease of λex.

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INTRODUCTION Photoluminescent (PL) and photothermal properties of polymeric materials are beneficial characteristics for a variety of applications including sensors,1−4 biomedical therapy,5 and functional fibers;6,7 PL is the spontaneous emission of light from a matter after the absorption of photons (photoexcitation) of electromagnetic radiation. The excitation energy and intensity are chosen to probe different regions and excitation concentration in the material. This PL behavior has been thus utilized to characterize the local conformational or structural changes of conventional polymeric materials containing main chain aromatic groups.8−10 The photothermal process, which is also a phenomenon associated with the photoexcitation of a material, results in the production of thermal energy (heat).11 The basis of photothermal effect is the change in thermal state of the material resulting from the absorption of radiation. Light absorbed and not lost by emission results in heating. The heat raises temperature, thereby influencing the thermodynamic properties of the material or of a suitable material adjacent to it. In general, the photothermal behavior can be analyzed by measuring the temperature changes that occur due to light absorption. This photothermal property has been well reported for inorganic materials such as GaAs nanowire,12 gold nanorod,13 Fe3O4 particles,14 graphene,15,16 and carbon nanotubes,16,17 in © 2015 American Chemical Society

addition to conductive polymers including polypyrrole, polyaniline, and polythiophene in solutions or films or particles.18−20 Polymer composites such as poly(ethylene oxide)/gold nanoparticle and polythiophene/graphene were also reported to have excellent photothermal property for biomedical therapy applications.21,22 However, the superior PL and photothermal properties have yet to be realized for macroscopic structures of fiber-forming polymeric materials. Polybenzimidazoles (PBIs) are a class of typical aromatic, heterocyclic polymeric materials with exceptionally high thermal stability, chemical resistance, and mechanical strength.23−26 Among the PBI family, poly[2,2′-(m-phenylene)-5,5′-bibenzimidazole] (PmBI) has been extensively adopted in a variety of applications such as high performance technical fibers, coating varnishes, and membranes for fuel cells.27 Although the PL behavior of PBIs in solutions or in films was investigated limitedly,28−32 the photothermal properties have not been reported. In the present study, for the first time, we have investigated the PL and photothermal behaviors of PmBI fibers as functions of the visible laser wavelength and power. For the purpose, a high molecular weight PmBI Received: October 14, 2015 Revised: December 4, 2015 Published: December 10, 2015 8823

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Figure 1. Schematic procedure to synthesize a PmBI homopolymer and to manufacture PmBI fibers via solution spinning. via a single-hole spinneret with inner diameter of 0.41 mm at a spinning rate of 5 mL/min. The fiber was taken up from the coagulation bath by using a synindrical winder at a speed of 6.28 m/ min and then dried in an oven at 100 °C for 12 h. The average diameter of the as-spun fiber was measured to be ∼80 μm. To prepare PmBI fibers with different chain orientation and packing, we have tried to manufacture a variety of PmBI fibers by changing annealing temperatures (300−500 °C) and time (1−12 h). It was found that the heat treatment at 300 °C for 1 h did not affect the chain structure of the as-spun fiber and that the heat treatment of 500 °C for 1 h or 400 °C for 12 h induced dominantly thermal decomposition of the fibers. Therefore, the heat treatment of 400 °C and 1 h was chosen to be an optimum condition to induce decreased chain orientation and increased chain packing, which can be thus compared with the asspun fiber with higher chain orientation and lower chain packing. The as-spun and heat-treated fibers were named as PmBI_a and PmBI_h, respectively. For comparison, a PmBI film with ∼70 μm thickness was also manufactured by casting a PmBI/MSA solution on a glass plate at 140 °C for 24 h, washing with a distilled water, and drying in a vacuum oven at 80 °C. Characterization. The intrinsic viscosity of the synthesized PmBI was measured by using an Ubbelohde viscometer with 95% sulfuric acid at 30 °C and associated molecular weight was evaluated by using Mark−Houwink relation. The chain orientation and packing of the asspun and heat-treated PmBI fibers were characterized with aid of a 2dimensional X-ray fiber diffractometer equipped with Cu Kα radiation (D/MAX-2500, Rigaku). The optical morphology of the PmBI fibers was characterized by using a polarized optical microscope (POM, S38, Bimience). The light absorption properties of the PmBI fibers were measured by using a diffuse reflectance spectrometer (Solidspec-3700, Shimadzu). The UV−vis absorption spectrum of the PmBI film was obtained by a UV−vis spectrometer (S-3100, Scinco). The visible and infrared PL properties of the as-spun and heat-treated PmBI fibers were characterized in the wide wavelength range of 350−1600 nm by a photoluminescence spectrometer (LabRam HR-800, Horiba) equipped with two different excitation wavelengths (λex) of 325 and 514 nm as well as two different detectors (multichannel air-cooled CCD for 220−1050 nm and InGaAs array detector for 800−1600 nm). To investigate the effects of the chain orientation and packing of the PmBI fibers on the visible and infrared PL properties, the PL intensities were measured by varying the angle (α) between the fiber direction and the polarized laser sources. The photothermal properties of the fibers were characterized with an infrared camera (SE/A325, FLIR system Inc.)

homopolymer with meta-linkage was synthesized by condensation polymerization, and it was processed into fibers via solution spinning and following heat treatment. The chain orientation and packing of the as-spun and heat-treated PmBI fibers were characterized by using 2D X-ray fiber diffraction, PL spectroscopy, and polarized optical microscopy. The PL and photothermal properties of the PmBI fibers were then analyzed systematically by taking account the differences in the chain orientation and packing.

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EXPERIMENTAL SECTION

Materials. 3,3′-Diaminobenzidine (DAB, >98.0%, TCI) and isophthaltic acid (IPA, >99%, TCI) were adopted as monomers for the synthesis of PmBI homopolymer with meta-linkage. Polyphosphoric acid (PPA, >83% P2O5, Sigma-Aldrich) was used for the polymerization solvent. Methanesulfonic acid (MSA, >99%, Samchun Chemical) was used as a solvent for manufacturing PmBI fibers. Sulfuric acid (95%, Samchun Chemical) was used as a solvent to measure the intrinsic viscosity of the synthesized PmBI. All the chemicals and materials were used as received without further purification. Synthesis of PmBI and Fabrication of PmBI Fibers. The synthesis route of PmBI homopolymer and the following fiber fabrication process are schematically presented in Figure 1. The synthesis of PmBI was carried out as follows. First, DAB of 12.00 g (56.0 mmol) was dissolved in PPA of 200 g in a three-neck round flask at 140 °C with mechanical stirring under inert nitrogen flow. Then, stoichiometrically balanced IPA of 9.31 g (56.0 mmol) was added in the solution, and the temperature was raised to 200 °C to synthesize a PmBI homopolymer. During the reaction at 200 °C for 10 h, the viscosity of the polymerization solution increased progressively, and the solution color changed from light brown to dark purple. After the polymerization, the reaction solution was poured into a distilled water bath, and the solidified product was pulverized into powders. The powder product was immersed in distilled water at 80 °C for 24 h to remove remaining PPA solvent and unreacted monomers. Finally, the PmBI powder was dried in a vacumm oven at 150 °C for 24 h. As-spun PmBI fiber was manufactured by a solution spinning process. The predetermined amount of PmBI powder (8.0 wt %) was dissolved in MSA solvent, which was heated to 80 °C until obtaining a homogeneous solution. The PmBI solution was then spun in a water coagulation bath by using a syringe pump (Regato 200, KdScientific) 8824

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Figure 2. (A, B) 2D X-ray fiber diagrams, (C) equatorial scans, and (D) azimuthal scans at 2θ = 23° for as-spun and heat-treated PmBI fibers.

Figure 3. (A) Schematic chain conformations and (B) polarized optical microscope image of as-spun and heat-treated PmBI fibers.

weight of the PmBI, the Mark−Houwink relation of ηint = 1.35326 × 10−4Mw0.7328 was applied.26 As the result, the intrinsic viscosity (ηint) and weight-average molecular weight (Mw) of the PmBI were evaluated to be ∼2.02 dL/g and ∼497 000 g/mol, respectively, which were found to be significantly higher than the values reported in the literature.33 This high molecular weight PmBI is generally effective to obtain high performance technical fibers via solution spinning. To confirm the successful synthesis of the PmBI, the intrinsic

and custom-designed power-controllable laser sources with three different visible wavelengths of 405, 520, and 635 nm. For the photothermal experiments, a bundle of PmBI fibers composed of 10 strands was prepared.

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RESULTS AND DISCUSSION Structural Characterization. The intrinsic viscosity of the synthesized PmBI homopolymer was characterized by measuring solution viscosities in 95% sulfuric acid at 30 °C using an Ubbelohde viscometer. In order to evaluate the molecular 8825

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Figure 4. (A) UV−vis absorption spectrum of an as-casted PmBI film and (B) diffuse reflectance light absorption spectra of as-spun and heat-treated PmBI fibers.

Figure 5. PL intensity spectra of as-spun and heat-treated PmBI fibers under different excitation wavelengths (λex) of (A) 325 nm and (B) 514 nm.

orientation factor of the as-spun fiber was evaluated to be Π = ∼0.51, which was somewhat higher than that of the heat-treated fiber (Π = ∼0.44). This result demonstrates that the chain orientation of the PmBI backbones aligned in the as-spun fiber decreased noticeably by the heat treatment, although the chain packing associated with the decrement of the interchain distance was enhanced, which was caused by the nonlinear chain conformation of PmBI with meta-linkage, as shown schematically in Figure 3A. The decreased chain orientation as well as the increased chain packing by the heat treatment was also supported by the POM images of the as-spun and heattreated PmBI fibers, as can be seen in Figure 3B, which reveals that the optical brightness of the as-spun PmBI fibers under crossed polarizers decreased for the heat-treated fiber with lower anisotropic chain orientation. Light Absorption and Photoluminescence Behavior. Figure 4A exhibits the UV−vis spectrum of the PmBI film in a wavelength range of 200−1100 nm, which shows four distinct light absorption peaks at ∼215, ∼240, ∼300, and ∼480 nm. On the other hand, Figure 4B displays the diffuse reflectance absorption spectra for the as-spun and heat-treated PmBI fibers in a wide wavelength range of 250−3000 nm of UV, visible, and infrared light. It was found that the light absorption spectrum of the heat-treated PmBI fiber in Figure 4B was found to be quite

viscosity measurement, FT-IR spectrum, and TGA curves were presented in the Supporting Information. Figure 2 shows 2D X-ray fiber diagrams of the as-spun and heat-treated PmBI fibers, in addition to associated equatorial and azimuthal scans. It was found that both the as-spun and heat-treated PmBI fibers are in mesomorphic or partially oriented amorphous state without any well-ordered crystalline features (Figure 2A,B), which stems from the nonlinear chain conformation of PmBI with meta-linkage. On the other hand, in the equatorial scans for the PmBI fibers of Figure 2C, the maximum scattering peak at 2θ = 22.6° (d-spacing = ∼0.393 nm) for the as-spun fiber was found to shift to 2θ = 23.0° (dspacing = ∼0.386 nm) for the heat-treated fiber, which indicates that the interchain distance of the PmBI fibers decreased slightly by the heat treatment. To identify the chain orientation of the as-spun and heat-treated PmBI fibers, the azimuthal scans at 2θ = 23° were obtained, as shown in Figure 2D. The orientation factor, Π, of the PmBI fibers was evaluated by using the expression

Π=

180 − A 180

(1)

where A denotes the azimuthal angle range at half-maximum intensity of the azimuthal scans at 2θ = 25°. The chain 8826

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Figure 6. Polar plots of the normalized PL intensity of as-spun and heat-treated PmBI fibers at different excitation wavelengths (λex) of (A) 325 nm and (B) 514 nm. The green solid lines were drawn by using equations of I = n[x sin2 a + (1 − x)] for (A) and I = n[x cos2 a + (1 − x)] for (B), where x denotes the degree of chain orientation.

similar to the UV−vis spectrum of the PmBI film with random chain orientation in Figure 4A. In addition, unlike the as-spun fiber, a strong and broad visible light absorption at higher wavelength of 500−750 nm was detected for the heat-treated PmBI fiber, which is considered to be related with the changes of the chain conformation and packing of PmBI backbones owing to the heat treatment. Consistently, it was reported that in the UV−vis absorption spectra of a dilute PmBI/PPA solution a new absorption band at ∼615 nm appeared when the solution turned to a gel state by fibrillar network formation of PmBI, which is a two-step process of conformational transformation and aggregation.34 To investigate the PL properties of the as-spun and heattreated PBI fibers, PL spectra were obtained under laser irradiation with different photoexcitation wavelengths (λex) of 325 and 514 nm, as shown in Figure 5A,B. Under the photoexcitation with λex of 325 nm, the PL spectra were observed at the visible wavelength range of 350−800 nm and also the maximum PL intensity at ∼520 nm for the as-spun PmBI fiber was much stronger than that of the as-spun fiber (Figure 5A). On the other hand, under the photoexcitation with λex of 514 nm, the PL spectra were detected at the infrared wavelength range of 800−1600 nm, and the maximum PL intensity observed at ∼1200 nm was far stronger for the heattreated PmBI fiber compared with the as-spun fiber (Figure 5B). The unexpected visible and infrared PL intensity spectra of the as-spun and heat-treated PmBI fibers, which are dependent on the laser excitation wavelengths, are believed to be associated with differences in the chain orientation and packing of PmBI fibers as well as the photorelaxation process. In order to delve the influences of chain orientation and photorelaxation mechanism on the PL spectra of the as-spun and heat-treated PmBI fibers, polar plots of the normalized PL intensity under different λex values of 325 and 514 nm were

obtained by varying the angle (α) between of the polarized laser light direction and the fiber direction, as shown in Figure 6A,B. The green solid lines of Figures 6A and 6B were drawn by using I = n[x sin2 α + (1 − x)] and I = n[x sin2 α + (1 − x)], respectively, where I is the PL intensity, x is the degree of chain orientation, α is the angle between polarized laser light direction and the fiber direction, and n is the normalization constant. In the case of λex = 325 nm of Figure 6A, the x value as an adjustable parameter for the as-spun and heat-treated PmBI fibers was evaluated to be 0.60 and 0.20, respectively. It reveals that the degree of chain orientation is much higher for the as-spun PmBI fiber compared with the heat-treated fiber. Similarly, for λex = 514 nm of Figure 6B, the x values of the asspun and heat-treated PmBI fibers were calculated to be 0.45 and 0.05, respectively. These results revealed that the chain orientation of the as-spun fiber was much higher than the heattreated fiber, which was consistent with the above 2D X-ray fiber diagram analysis. It is interesting to note that for the as-spun fiber with higher chain orientation the maximum visible PL intensity under λex = 325 nm was detected when the polarized laser light was parallel to the fiber direction (α = 90° and 270°), while the maximum infrared PL intensity under λex = 325 nm was observed when the laser light was perpendicular to the fiber direction (α = 0° and 180°), as can be seen in Figures 6A,B. It is thus conjectured that the difference in the maximum visible or infrared PL intensity distribution in the polar plots is associated with the different PL mechanisms of the PmBI fibers with different chain orientation and packing under different photoexcitation wavelengths of λex = 325 or 514 nm. As noted above (Figure 5), the PL spectra of the PmBI fibers under λex = 325 nm was detected in the visible wavelength range of 350−800 nm, whereas the PL spectra under λex = 514 nm was observed in the infrared wavelength range of 800−1600 nm. It is believed that 8827

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Figure 7. (A) Digital and (B) infrared images of a bundle of PmBI fibers under a visible laser source of λex = 405 nm.

Figure 8. Time-dependence changes of the temperature increment (ΔT) for (A) as-spun and (B) heat-treated PmBI fibers under a visible laser source with λex = 405 nm and 0.3−18.4 mW. (C) Maximum temperature increments (ΔTmax) of as-spun and heat-treated PmBI fibers under various visible laser sources of different wavelengths and powers.

heat-treated PmBI fibers with relatively low chain orientation, the ellipsoidal visible or infrared PL intensity distributions were detected with the α variation, as shown in Figure 6. Photothermal Heating Property. It is generally possible for photoexcited electrons to return to the ground state in a nonradiative manner, and thus the light energy absorbed by the PL materials is given up as heat to the surrounding atoms (called phonons in solids) or to electronic states of atoms in the vicinity (called energy transfer) during the photorelaxation

for the as-spun PmBI fiber with high chain orientation the maximum visible PL intensities at α = 90° and 270° under λex = 325 nm were associated with a photorelaxation process by an efficient interchain energy transfer owing to the π−π interactions between neighboring benzimidazole units in the fibers with higher chain orientation, while the maximum infrared PL intensities at α = 0° and 180° under λex = 514 nm were related to a photorelaxation along PmBI backbones via an effective intrachain energy transfer. On the other hand, for the 8828

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fibers and films can influence the photothermal performance; i.e., PmBI fibers with a certain chain orientation can exhibit lower photothermal property compared to as-casted films with random isotropic chain orientation. To identify the efficiency of the photothermal properties of as-spun and heat-treated PmBI fibers, the ΔTmax values attained at a visible laser source were compared with the commercially available wool, PET, and nylon fibers. It was found that under a visible laser source of λex = 405 nm and 18.4 mW the ΔTmax values of the as-spun and heat-treated PmBI fiber were ∼27.5 and ∼31.7 °C, respectivley, which were much higher than those of wool (∼8.9 °C), PET (∼2.0 °C), and nylon (∼0.8 °C). Therefore, it is reasonable to note that the remarkable photothermal property of the PmBI fibers is very effective compared with other commercial synthetic or natural polymers.

process.35 In cases of the PmBI fibers, it is conjectured that the infrared PL spectra under λex = 405 nm shown in Figure 6B is associated with the photothermal heating behavior, a conversion of light energy to heat as thermal energy. Accordingly, the photothermal heating property of the PmBI fibers was examined by monitoring the temperature change under different visible light wavelengths of 405, 520, and 635 nm. For instance, when a visible laser source of λex = 405 nm was applied to a bundle of PmBI fibers, the fiber temperature increased rapidly to a certain state value, as can be seen in digital and infrared images of Figure 7. To characterize the time-dependent photothermal behavior of the PmBI fibers, the temperature increment (ΔT = Tt − Ti where Tt is the temperature at an arbitrary time and Ti is the initial temperature of ∼25 °C) was monitored as a function of the time under visible laser sources with different wavelengths and powers. For instance, the changes of ΔT with the time for the as-spun and heat-treated PmBI fibers under a visible laser source with λex = 405 nm and different powers of 0.3−18.4 mW are shown in Figures 8A,B. When the power-controllable visible laser source with λex = 405 nm was applied to the PmBI fibers at 5 s, the ΔT increased rapidly up to a steady-state maximum value within ∼5 s, remained unchanged over the time, and decreased quickly to zero when the visible laser source was off at 65 s. It is noticeable to mention that the temperature responsiveness of the PmBI fibers was particularly rapid compared with photothermal heating systems reported in the literature.17 In addition, the maximum temperature increments (ΔTmax = Tmax − Ti, where Tmax is the maximum temperature attained at a given visible laser source) of the PmBI fibers under λex = 405 nm were dependent on visible laser powers, as can be seen in Figures 8A,B. The ΔTmax values of the fibers were found to increase with increasing the visible laser power or decreasing the visible laser wavelength, as shown in Figure 8C. These laser power- or wavelength-dependent photothermal behaviors of the PmBI fibers were consistent with the trends reported in other systems.36 On the other hand, at a given visible laser wavelength and power, the ΔTmax values of the heat-treated PmBI fiber were somewhat higher than those of the as-spun fiber. It means that the photothermal property of the heattreated PmBI fiber is more effective compared with the as-spun PmBI fiber. This result is considered to be caused by the fact that the heat evolution by the collision between photons and PmBI backbones is efficient for the heat-treated PmBI fibers with higher chain packing density, which is consistent with the fact that the infrared PL intensity of the heat-treated PmBI fiber under a visible laser source of λex = 514 nm was much more intense than the as-spun fiber, as shown in Figure 5B. To examine the effect of the material shape (fiber and film) on the photothermal properties of PmBI, the photothermal behavior of the PmBI film with ∼70 μm thickness was also characterized under λex = 405 nm with 0.3−18.4 mW (see Figure S4 of the Supporting Information). It was found that the as-casted PmBI film achieved much higher ΔTmax values compared to the as-spun or heat-treated PmBI fibers (Figures 8A,B). For instance, under a same condition of λex = 405 nm and 18.4 mW, the PmBI film attained ΔTmax ∼ 65 °C, which was far higher than that (∼32 °C) of the as-spun fiber. This result is believed to be caused by the difference in surface areas between PmBI fibers and films because the fibers with higher surface area are easy to lose their thermal energy (heat) to the surrounding air environment compared to the films. In addition, the difference in chain orientation between PmBI

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CONCLUSIONS

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ASSOCIATED CONTENT

In this study, the PL and photothermal heating properties of PmBI fibers with different chain orientation and packing, which were manufactured by using solution spinning and following heat treatment, were investigated as functions of excitation wavelength (λex) and power of visible laser sources. 2D X-ray fiber diagrams and POM images revealed that the chain orientation and packing density of the as-spun PmBI fiber was higher and lower than that of the heat-treated fiber, respectively, owing to the nonlinear chain conformation of PmBI with meta-linkage. Accordingly, the visible PL intensity under λex = 325 nm, which was dominated by with an interchain photorelaxation, was higher for the as-spun fiber with higher chain orientation, whereas the infrared PL intensity under λex = 514 nm, which was associated with an intrachain photorelaxation, was higher for the heat-treated fiber with higher chain packing. On the other hand, in cases of the photothermal heating properties, the as-spun and heat-treated PmBI fibers under a visible laser source of λex = 405 nm and 18.4 mW exhibited maximum temperature increments (ΔTmax) of ∼27.5 and ∼31.7 °C, respectively, which were markedly higher than those of conventional natural or synthetic polymers such as wool, nylon, and PET (ΔTmax = 0.8−8.9 °C). For the heat-treated PmBI fiber, the ΔTmax values increased from ∼14.7 to ∼31.7 °C with the decrease of λex from 635 to 405 nm. Overall, it is valid to contend that the PmBI fibers with exceptional PL and photothermal heating behavior under visible light sources can be utilized for photoinduced heating elements and biomedical therapy platform, in addition to high performance technical fibers and membranes.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b02266. Figures S1−S4 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (Y.G.J.). Notes

The authors declare no competing financial interest. 8829

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(33) Xiao, L.; Zhang, H.; Scanlon, E.; Ramanathan, L. S.; Choe, E.W.; Rogers, D.; Apple, T.; Benicewicz, B. C. Chem. Mater. 2005, 17, 5328−5333. (34) Sannigrahi, A.; Arunbabu, D.; Jana, T. Macromol. Rapid Commun. 2006, 27, 1962−1967. (35) Blasse, G.; Grabmaier, B. C. Luminescent Materials; Springer: Berlin, 1994; Vol. 34. (36) McNally, K. M.; Sorg, B. S.; Welch, A. J.; Dawes, J. M.; Owen, E. R. Phys. Med. Biol. 1999, 44, 983.

ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korean Government (MOE) (2013R1A1A2A10010080).

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DOI: 10.1021/acs.macromol.5b02266 Macromolecules 2015, 48, 8823−8830