Synergistic Effect of Sulfur and Chalcogen Atoms on the Enhanced

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Synergistic Effect of Sulfur and Chalcogen Atoms on the Enhanced Refractive Indices of Polyimides in the Visible and Near-Infrared Regions Hyeonil Kim,†,‡ Bon-Cheol Ku,† Munju Goh,† Heung Cho Ko,‡ Shinji Ando,§ and Nam-Ho You*,†

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†

Carbon Composite Materials Research Center, Institute of Advanced Composite Materials, Korea Institute of Science and Technology, Chudong-ro 92, Bondong-eup, Wanju-gun, Jeollabuk-do 565-905, Republic of Korea ‡ School of Materials Science and Engineering, Gwangju Institute of Science and Technology (GIST), 261 Cheomdan-gwagiro, Buk-Gu, Gwangju 500-712, Republic of Korea § Department of Chemical Science and Engineering, Tokyo Institute of Technology, 2-12-1-E4-5, Ookayama, Meguro-ku, Tokyo 152-8552, Japan S Supporting Information *

ABSTRACT: To develop thermally stable optical polymers for visible and near-infrared sensor applications, a series of sulfurcontaining polyimides (PIs) with chalcogen atoms were successfully synthesized via conventional two-step polycondensation of 4,4′-[p-thiobis(phenylenesulfanyl)]diphthalic anhydride (3SDEA) with four diamines: 4,4′-oxidianiline (ODA), 4,4′-thiodianiline (TDA), 4,4′-selenodianiline (SEDA), and 4,4′-tellurodianiline (TEDA). Because of the large atomic polarizabilities of the sulfur, selenium, and tellurium atoms, the resultant PIs exhibited significantly high refractive indices in the 1.738−1.778 range at 637 nm along with a transmittance >90% at 650−1500 nm. In addition, the PIs exhibited good thermal stability, with high thermal decomposition and glass transition temperatures (Td5% > 390 °C and Tg = 183−217 °C, respectively). Owing to the good affinity of SEDA−3SDEA for TiO2 nanoparticles, a nanocomposite film with a 3.0 wt % loading of TiO2 nanoparticles exhibited a refractive index of 1.774 at 637 nm.

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INTRODUCTION

A general strategy to increase the refractive index of a material involves the incorporation of atoms or substituents with high atomic polarizability or molar refraction and small atomic/molar volumes.19−23 Various atoms or substituents, such as sulfur, phosphorus, heavy halogens (bromine and iodine), and aromatic and π-conjugated rings, are typical versatile candidates for this purpose because of their high atomic polarizability or molar refraction and relatively small atomic or molar volumes.24−26 Recently, highly transparent sulfur-containing polymers, such as polyimide (PI), poly(methyl methacrylate), and polycarbonate, have been widely investigated for optical applications.27−30 In fact, PIs have been attracting considerable attention because of their outstanding properties, such as high intrinsic refractive index, low dielectric constant, and excellent mechanical properties. For these reasons, PIs are promising candidates for application in optical, optoelectronic, and telecommunication systems.31,32 Several studies have investigated sulfur-containing polymers to achieve high refractive indices via thioether (−S−) linkages.

In recent years, the development of polymeric materials with high refractive index (n) and low birefringence (Δn) has been an emerging area in the optical engineering field. The applications of these materials include waveguides, organic light-emitting diodes (OLEDs), arrays of prisms, chargecoupled devices (CCDs) for digital cameras, high-resolution complementary metal-oxide semiconductor (CMOS) image sensors, and antireflective coatings.1−11 In particular, high-n polymeric materials transparent in the near-infrared (NIR) region have been developed for use in windows, integrated optics, and lenses for NIR imaging technologies. These devices have been widely utilized in a number of night vision, defense, and optical tomography monitoring devices.12−15 Recently, inorganic materials transparent in the NIR region, such as chalcogenide glasses and germanium semiconductors, have attracted attention for their potential application in NIR imaging devices. The chalcogenide glasses possess remarkable properties for NIR imaging, i.e., high NIR transmittance and refractive index (n > 2.0).16−18 However, the chalcogenide materials not only require a very high temperature (T > 300− 1000 °C) for their synthesis but also are toxic and expensive. © XXXX American Chemical Society

Received: October 18, 2018 Revised: December 26, 2018

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DOI: 10.1021/acs.macromol.8b02139 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Synthesis of PIs and SEDA−3SDEA/TiO2 Nanocomposites

IR thermal imaging. However, such chalcogenide hybrid polymers have some limitations in optical applications, including low optical transmittance in the visible region and low thermal stability, due to the relatively weak and highly flexible sulfur−sulfur and sulfur−selenium covalent bonds. On the other hand, tellurium is widely used in industrial applications, for example in resistive devices, because of the lack of centrosymmetry.46 In addition, Te-containing materials, such as TeO2 and TeO3, are known to have high dielectric constants,47 which also leads to a high refractive index. For this reason, Se- and Te-containing polymeric materials could be promising candidates for advanced optical applications, especially sensor applications in the visible and NIR regions. Furthermore, inorganic/polymer nanocomposites (NCs) have been widely used to develop high-n hybrid materials, i.e., by blending inorganic nanoparticles (NPs) of materials with high refractive indices, such as ZrO2 (n = 2.10),48 TiO2 (n = 2.7 for rutile and 2.48 for anatase),49 ZnS (n = 2.36),50 and PbS (n = 4.20),51 with a transparent polymer matrix. However, inorganic nanoparticles generally reduce the transmittance of hybrid materials due to the light scattering caused by their aggregation.52 To obtain a high-n material possessing both high transmittance and high refractive index, it is necessary to develop polymers with an intrinsically high refractive index and a good nanoparticle dispersion capability. In this paper, we report the syntheses of diamines containing heavy chalcogen atoms (Se and Te) as well as the preparation and physical properties of fully aromatic sulfur-containing polyimides incorporating four kinds of chalcogen atoms (O, S, Se, and Te) in the main chain. 4,4′-Selenodianiline (SEDA)

Recently, Ueda et al. thoroughly investigated sulfur-containing polymers.33−39 They found that polymers with high aromatic contents and low molecular volume resulted not only in low birefringence values but also in high refractive indices, highlighting the potential of these systems for various optical applications. On the other hand, incorporating heavy chalcogen atoms such as selenium (Se) and tellurium (Te) into polymers can result in a considerable enhancement of the refractive index. This is because the atomic polarizabilities (αv) of tellurium (5.5 Å3) and selenium (3.77 Å3) are significantly larger than that of sulfur (2.90 Å3). However, the van der Waals radii (rvdw) of Te (2.06 Å) and Se (1.90 Å) are slightly larger than that of S (1.80 Å).40,41 According to the Lorentz−Lorenz equation, the refractive index of polymers is approximately proportional to the α/Vvdw ratio, where α and Vvdw are the polarizability and van der Waals volume of the repeating unit, respectively, under the condition of a constant molecular packing coefficient (Kp).29 Hence, the contribution of heavy atoms to the refractive indices of polymers can be roughly estimated as αv/rvdw3. Because the ratios between the αv/rvdw3 values for Te, Se, and S are 1.27:1.11:1.0, the incorporation of the heavy chalcogen atoms can effectively enhance the refractive indices of the polymers. Ueda and co-workers42 developed highly sulfurated and selenophene-containing PIs for manufacturing high-refractive index polymer films (n = 1.759). Very recently, Pyun and coworkers43−45 extensively investigated inorganic/organic polymer hybrid materials containing selenium atoms, which exhibited a very high refractive index (>2.0), for use in midB

DOI: 10.1021/acs.macromol.8b02139 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

SEDA−3SDEA, and 0.44 dL/g for TEDA−3SDEA. The thermal imidization of the PAAs to PIs was characterized by FT-IR spectroscopy. As shown in Figure S3 (Supporting Information), the completion of the imidization reaction was confirmed by the appearance of the absorption peaks around 1774 cm −1 (CO, symmetric), 1715 cm −1 (CO, symmetric), and 1362 cm−1 (C−N, stretching). Thermal Properties of the PIs. The thermal stability and thermomechanical properties of optical materials are critical for their applications. The results of thermogravimetric analysis (TGA), dynamic mechanical analysis (DMA), and differential scanning calorimetry (DSC) measurements of the PI films are summarized in Table 1. The TGA measurements were

and 4,4′-tellurodianiline (TEDA) were synthesized by crosscoupling of aryl halides, with elemental Te and Se as the chalcogen source. All PIs were prepared from an aromatic dianhydride containing three sulfur atoms in the backbone, 4,4′-[p-thiobis(phenylenesulfanyl)]diphthalic anhydride (3SDEA), and the aromatic diamines 4,4′-oxidianiline (ODA), 4,4′-thiodianiline (TDA), SEDA, and TEDA by a conventional two-step polycondensation procedure.

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EXPERIMENTAL SECTION

Preparation of PI Films. PI films were prepared by thermal imidization of PAA films using a conventional method. PAA solutions were prepared by polycondensation of 3SDEA with four types of diamines: ODA, TDA, SEDA, and TEDA. First, 3SDEA (1.0 g, 0.001843 mol) was added and dissolved in NMP (3.66 g, solid content in 30 wt % solution) into a 20 mL vial, with continuous magnetic stirring under argon gas. Thereafter, a stoichiometric amount of TEDA (0.574 g, 0.001843 mol) was added into the vial. The solution was then mixed by magnetic stirring for 24 h at room temperature (RT) to prepare a PAA solution. The other PAA precursors were prepared by the same procedure. In addition, a control PI film of PMDA−ODA derived from pyromellitic dianhydride and 4,4′-oxidianiline was prepared using the same method to compare it with the chalcogen-containing PI films. Second, the PAA precursors were spin-coated on several substrates, including silicon wafer, glass, and silica substrates, to form films; thermal imidization of the PAA precursors was then performed under curing conditions of 100, 150, 200, and 250 °C for 1 h each, followed by treatment at 300 °C for 30 min under argon gas flow. After imidization, the PI films were peeled from the substrates by soaking in water and drying. The resulting PI films were labeled ODA−3SDEA, TDA−3SDEA, SEDA−3SDEA, and TEDA−3SDEA. The completion of the imidization was confirmed by FT-IR spectroscopy, as shown in Figure S4. All PI films exhibited comparable characteristic peaks originating from their chemical structure, without peaks corresponding to their PAA precursors. Preparation of SEDA−3SDEA/TiO2 NCs. First, TiO2 NP powders ( SEDA−3SDEA > TEDA−3SDEA, which coincides with the order of inherent viscosities (η) of the PAA solutions (TDA−3SDEA > ODA−3SDEA > TEDA−3SDEA > SEDA− 3SDEA, Table 1). Lee53 reported that the Tg of semirigid polymers is essentially a function of the chain rigidity/ flexibility of the repeating units and of van der Waals forces and can be explained without considering particular intermolecular interactions such as charge-transfer or dipolar carbonyl−carbonyl attractions. For the quantitative determination of the chain rigidity/ flexibility of the chalcogen-containing diamine moieties, the energy barriers (EB) for internal rotation around the Φ−X−Φ linkages (where Φ stands for a benzene ring, and X = O, S, Se, and Te) were calculated for four types of model compounds. Figure 2 shows the dihedral angle dependence of the conformational energies of the models, in which the two dihedral angles (ϕ, ψ) were independently varied in the range of 0° ≤ ϕ, ψ ≤ 180° at a 10° interval. The total energy of the most stable conformation was used as a standard in each contour map. As we have previously reported,54,55 both the

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RESULTS AND DISCUSSION Fabrication and Characterization PIs. In this study, a new synthetic method was adopted to incorporate Te atoms into a sulfur-containing PI to enhance its refractive index. As illustrated in Scheme 1, PIs containing different chalcogen atoms were prepared through two steps of polycondensation and thermal imidization. 3SDEA, containing flexible thioether linkages that also enhance the refractive index of the PIs, was chosen as the common dianhydride. The preparation of poly(amic acid) (PAA) solutions was performed via ringopening of 3SDEA, using equivalent amounts of diamines containing chalcogen atoms at a concentration of 30 wt % in anhydrous N-methylpyrrolidone (NMP). The reactions of 3SDEA with these diamines led to the following inherent viscosities at 30 °C in 0.5 g/NMP solutions: 0.45 dL/g for ODA−3SDEA, 0.49 dL/g for TDA−3SDEA, 0.40 dL/g for C

DOI: 10.1021/acs.macromol.8b02139 Macromolecules XXXX, XXX, XXX−XXX

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TiO2 contents. Figure S6a−c shows 3D X-ray CT images illustrating the overall state of TiO2 dispersion in the PI matrix, with the dark gray spots corresponding to TiO2 particles dispersed in the PI matrix. For all composite films, TiO2 NPs were homogeneously dispersed for TiO2 contents up to 3 wt %. No densely aggregated TiO2 NPs were observed, showing that the NPs were well incorporated in the PI matrix. To confirm the interfacial interaction between PI matrix and TiO2 NPs, cross-sectional scanning electron microscopy (SEM) images of the pristine SEDA−3SDEA and SEDA−3SDEA/ TiO2 NCs were obtained, as shown in Figure S5d−g. Most of the TiO2 NPs were well dispersed in the PI matrix, even though the roughness of cross section became significant with increasing TiO2 content. The size of the TiO2-rich domains in the SEM images obviously increased with increasing TiO2 content. Optical Properties. The optical properties of the four kinds of PIs containing chalcogen atoms and of SEDA3SDEA/TiO2 NCs are summarized in Table 2. The cutoff wavelengths (λcutoff) were determined from UV−vis absorption spectra, whereas the in-plane and out-of-plane refractive indices (nTE and nTM, respectively) and the birefringence values (Δn = nTE − nTM) were measured by a prism coupler (PC-2000, Metricon, USA) at wavelengths of 637, 829, 1310, and 1550 nm. The optical transmission in the visible and NIR regions is one of the most important parameters for optical materials. Fully aromatic polyimide films commonly exhibit inherent yellowish colors, which can be explained by the intra- and intermolecular charge-transfer (CT) interactions of the PI chains.56,57 The UV−vis optical transmission spectra of the pristine PI films with a thickness of ca. 10 μm are shown in Figure 4a. The λcutoff values varied from 405 to 417 nm, while the transmittance was in the range of 80−90% at 550 nm and ∼92% above 650 nm; these values are sufficiently high for visible and NIR sensor applications. As displayed in Figure S8, the experimental absorption edges are well reproduced in the calculated absorption spectra of the model compounds of the PIs (Scheme S2). Taking into account the Fresnel reflections at the surface of films (7.6% for n = 1.76), all PI films exhibited very high transmittance in the visible (400−780 nm) and NIR regions (>780 nm), although only TEDA−3SDEA presented slightly lower transmittance in the visible region. Because no distinct absorption peaks were detected in the whole visible and NIR regions, the lower transmittance of TEDA−3SDEA is attributable to partial oxidation of the diamine moiety or of the amino termini. In the cases of TiO2 NCs, a gradual decrease in transmittance in the visible region was observed for SEDA− 3SDEA/TiO2 NCs as the TiO2 content increased, which originated from the light scattering induced by the slight aggregation of TiO2 NPs with high refractive index, even at low content. The wavelength dispersions of the average refractive indices (nav, estimated as nav = [(2nTE2 + nTM2)/3]1/2) in the visible and NIR regions are displayed in Figure 4b. All PI films exhibited high nav values, higher than 1.7 at 637 nm; it is worth noting that the highest nav obtained for TEDA-3SDEA (1.7783) is higher than those of the other PIs containing 19.2−30.1 wt % sulfur atoms (nav = 1.746−1.768 at 633 nm).32,39 The nav values increased in the order ODA−3SDEA (1.7128) < TDA−3SDEA (1.7384) < SEDA−3SDEA (1.7618) < TEDA−3SDEA (1.7783), which agrees well with the order of the atomic polarizabilities per unit atomic volume

Figure 1. (a) DMA curves of PIs containing chalcogenide atoms (1 Hz, 3 °C/min): (i) storage modulus (E′) and (ii) tan δ. (b) DSC curves of PIs, measured at a heating rate of 10 °C/min under nitrogen gas.

highest- and lowest-energy conformations of Φ−Y−Φ linkages (Y = −O−, ⟩CO, −S−, ⟩SO2, and ⟩CH2) in the twodimensional surface plots exist on the condition that the two dihedral angles are identical. Thereby, Figure 3 shows the dihedral angle dependence of the conformational energies of the models, in which the two dihedral angles (ϕ, ψ) were identical (= φ). The most stable conformations and the energy barriers for internal symmetrical rotation of Φ−X−Φ linkages with X = −O−, −S−, −Se−, and −Te− were estimated as φ = 40.5°, 45.5°, 47.4°, and 51.4° as well as EB = 17.3, 7.2, 4.3, and 1.4 kJ/mol, respectively. The significant decrease in the EB values in the order O > S > Se > Te is consistent with the characteristic trends of the Tg values of the PIs. These findings indicate that the diamine moieties containing heavy chalcogen atoms (Se and Te) are more flexible and rotatable than those with light chalcogen species (O and S) in solution and in the solid state. Furthermore, as all Tg values of the chalcogen-containing PIs were >180 °C, these PIs have sufficient thermal stability for application in optical sensing devices. Characterization of PI NCs. For preparing PI NC films containing TiO2, 0.5, 1.0, and 3.0 wt % TiO2 NPs were dispersed in NMP solutions of SEDA−3SDEA PAA, followed by the same imidization procedures employed for the pristine PI films. To examine the actual dispersion of TiO2 NPs, threedimensional (3D) X-ray computed tomography (CT) analysis was performed for SEDA−3SDEA/TiO2 NCs with different D

DOI: 10.1021/acs.macromol.8b02139 Macromolecules XXXX, XXX, XXX−XXX

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Figure 2. Conformational energy surfaces (contour maps) of (a) diphenyl ether, (b) diphenyl thioether, (c) diphenylselane, and (d) diphenyltellane calculated using the B3LYP/Def2TZVPP level of theory. The interval of lines in the energy contour maps is 1 kJ/mol. The most energetically stable conformation in each map is depicted by the × symbols. The left and right halves of the energy surface are point-symmetrical with respect to the point of (90°, 90°) which is depicted by the + symbol. The conformations at (90°, 90°) correspond to the saddle points for symmetrical rotation of all the structures (a)−(d).

3SDEA and the lower nav values of ODA−3SDEA were well reproduced along with their wavelength dispersion. It should be noted that the replacement of only one heavy chalcogen atom for an oxygen atom in the main chain has significant effects on the refractive indices of the PIs. The wavelengthdependent refractive indices (nλ) plotted in Figure 4b were fitted by the simplified Cauchy’s equation: nλ = n∞ + D/λ2, where n∞ is the refractive index at infinite wavelength and D is the coefficient of dispersion, and the obtained n∞ and D values are also listed in Table 2. The PI films containing heavy chalcogen atoms exhibited high n∞ values (>1.7), and TEDA3SDEA showed the largest wavelength dispersion (D = 2.22 × 104) in the NIR region. This can be attributed to its highest nav value and lowest transmittance in the visible region. In contrast, it is interesting to note that TDA−3SDEA showed the smallest dispersion (D = 1.13 × 104), which is due to the highest transmittance at shorter wavelengths in the visible region. Such a small dispersion of the refractive index is desirable for optical devices using a wide range of wavelengths. In addition, all PI films showed relatively low birefringence, with Δn < 0.015. In particular, TEDA−3SDEA exhibited not only the highest refractive index but also a very low

Figure 3. Calculated conformational energies of the core structures of chalcogen-containing diamines. Three typical conformations are displayed on the right.

(O < S < Se < Te) described above. This trend is also consistent with the nav values calculated by time-dependent density functional theory (TD-DFT), as shown in Figure 5. The methods and model compounds used for the calculations are described in the Supporting Information. Although the calculated refractive indices of SEDA−3SDEA are very close to those of TDA−3SDEA, the higher nav values of TEDA− E

DOI: 10.1021/acs.macromol.8b02139 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Table 2. Optical Properties of the PI Films refractive indices and birefringenceb polyimide ODA−3SDEA TDA−3SDEA SEDA−3SDEA TEDA−3SDEA

λcutoffa

(nm)

411 405 417 408

nTE

c

1.7145 1.7420 1.7667 1.7806

nTMd

nave

Δnf

VNIRg

n∞h

Di (×104)

1.7095 1.7313 1.7521 1.7738

1.7128 1.7384 1.7618 1.7783

0.0049 0.0106 0.0146 0.0068

29.3 29.4 19.6 18.0

1.6782 1.7103 1.7135 1.7235

1.3967 1.1319 1.9569 2.2229

a

Cutoff wavelength determined by UV−vis transmission spectra. bMeasured at 637 nm. cIn-plane refractive index (transverse electric mode). dOutof-plane refractive index (transverse magnetic mode). eAverage refractive index calculated as nav = [(2nTE2 + nTM2)/3]1/2. fIn-plane/out-of-plane birefringence calculated as Δn = nTE − nTM. gAbbe’s number is given by as VNIR = (n829 − 1)/(n637 − n1306). hRefractive index at infinite wavelength determined by fitting with the simplified Cauchy’s formula (nλ = n∞ + D/λ2), where nλ is the nav at wavelength λ. iCoefficient of wavelength dispersion determined by the fitting.

Figure 5. Wavelength dispersion of refractive indices obtained from TD-DFT calculations. The refractive indices of TDA−3SDEA nearly overlap with those of SEDA−3SDEA.

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CONCLUSION In this work, we demonstrated the influence of heavy chalcogen atoms (S, Se, and Te) on the refractive indices of sulfur-containing PIs, as summarized below. We designed and successfully synthesized chalcogen-containing PI films. As expected, all PI films exhibited high nav values (>1.7). In particular, TEDA−3SDEA exhibited a refractive index of 1.778 at 637 nm, which is one of the highest nav values ever reported for PIs. This value was higher than that obtained for the SEDA−3SDEA/TiO2 nanocomposite (3 wt %). Moreover, the PI films showed high glass transition temperatures (Tg > 180 °C), high weight loss temperatures (Td10% > 400 °C), and low birefringence (Δn < 0.015). Our study indicates that the introduction of chalcogen atoms in the polyimide backbone is a promising method to improve the refractive index of thermally stable optical materials, specifically employed in advanced sensor and image applications.

Figure 4. (a) UV−vis absorption spectra of PI films and (b) average refractive index curves of PI films measured at different wavelengths (film thickness: ca. 10 μm).

birefringence (0.0068), which is beneficial for applications in microlenses and short waveguides for sensor devices. The nav values of 3SDEA/TiO2 NCs and their wavelength dispersion curves are also displayed in Figure S7. The nav values of the SEDA−3SDEA/TiO2 NCs increased in the following order: pristine (0%) (1.7618) < 0.5 wt % (1.7624) < 1.0 wt % (1.7688) < 3.0 wt % (1.7738), indicating that the nav values tended to increase with the loading of TiO2 NPs, because the latter have a high refractive index. It should be noted that the nav value of TEDA−3SDEA is even higher than that of SEDA−3SDEA/TiO2 NC with 3.0 wt % TiO2, which highlights the synergistic effect of combining three S and one Te atoms in the main chain of TEDA−3SDEA to enhance the refractive index.

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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.8b02139. Materials, instruments, experimental details, NMR spectra, FT-IR spectra, calculation of absorption spectra (PDF) F

DOI: 10.1021/acs.macromol.8b02139 Macromolecules XXXX, XXX, XXX−XXX

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Bon-Cheol Ku: 0000-0003-0048-5856 Munju Goh: 0000-0002-6061-8625 Shinji Ando: 0000-0002-3508-035X Nam-Ho You: 0000-0001-8886-226X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was partly supported by a grant from the Korea Institute of Science and Technology Institutional (KIST) Program & Open Research Program and project “The development of stiffened panel and C, Z channel using thermoplastic unidirectional tape made with PPS, PEEK resin and carbon fibre contents of more than 60 wt %” funded by the Ministry of Trade, Industry and Energy (MOTIE, Korea) [No. 10076849] and Space Core Technology Development Program through the National Research Foundation of Korea funded by the Ministry of Science, ICT and Future Planning (NRF-2017M1A3A3A02016310), and Nano Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT. This work was partly supported by Grants-in-Aid for Scientific Research, Japan Society for the Promotion of Science (17H03112).

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DOI: 10.1021/acs.macromol.8b02139 Macromolecules XXXX, XXX, XXX−XXX