Poly(phenylene thioether)s with Fluorene-Based Cardo Structure

Aug 5, 2016 - Our designed poly(phenylene thioether)s with the fluorene-based cardo structure achieved excellent transparency, high refractive indices...
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Poly(phenylene thioether)s with Fluorene-Based Cardo Structure toward High Transparency, High Refractive Index, and Low Birefringence Kazuhiro Nakabayashi,† Takayuki Imai,† Mao-Chun Fu,† Shinji Ando,‡ Tomoya Higashihara,† and Mitsuru Ueda*,† †

Graduate School of Organic Materials Science, Yamagata University, 4-3-16 Jonan, Yonezawa 992-8510 Japan Department of Chemistry and Materials, Tokyo Institute of Technology, 2-12-1-E4-5 Ookayama, Meguro-ku, Tokyo 152-8552 Japan

‡

S Supporting Information *

ABSTRACT: To realize high-performance thermoplastic camera lenses for compensating chromatic aberration in which lenses with large and small Abbe numbers were combined, novel poly(phenylene thioether)s with a fluorenebased cardo structure were developed with the potential for simultaneously realizing high transparency, a high refractive index, low birefringence, and small Abbe number. Excellent transmittance was observed in all polymer films because the interchain packing was effectively suppressed by the cardo structure (e.g., transmittance was as high as 90% at 400 nm), and furthermore, high refractive index values (1.6553−1.6762) were attained. The polymer with the highest content of cardo structure exhibited a low birefringence of 0.0014. The efficient cancellation of polarization anisotropy between the polymer backbone and the fluorene units directed perpendicular to the polymer backbone contributed to the low birefringence. These results indicate that promising materials for high-performance optical applications can be developed based on the well-suited incorporation of the cardo structure into the polymer backbone.

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range of 1.6565−1.6780.17 Thus, a precise polymer design is essential to realizing both a high refractive index and low Δn values simultaneously. A cardo structure consisting of a 9,9-diaryl-substituted fluorene skeleton is significant for realizing a small Δn value.18−22 Δn is defined as Δn = n∥ − n⊥, where n∥ and n⊥ are respectively the refractive indices for linearly polarized light parallel and perpendicular to the orientation direction.23 As for a polymer with a fluorene-based cardo structure in the main chain, the polymer backbone and fluorene skeleton are mutually orthogonal. That is the polarization anisotropy between the polymer backbone and fluorene skeleton can be significantly reduced, which potentially leads to a small Δn value. Furthermore, the fluorene-based aromatic cardo structure can enhance the refractive index simultaneously due to the plural benzene rings bearing high polarizability. To date, this concept has been utilized to develop transparent polymers with relatively high refractive indices (1.607−1.635) and small Δn values (ca. 1.0 × 10−3), such as polyesters and polycarbonate with a fluorene-based cardo structure.24,25 To use thermoplastics camera lenses for compensating chromatic aberration, however, the polymers with much higher refractive index values of 1.7 are required. Furthermore, in terms of processability, a

INTRODUCTION Much attention has been paid to materials having high thermal stability, high transparency, high refractive index, and low birefringence to develop high-performance components for advanced display devices, various lenses, optical waveguides, and diffractive gratings.1−3 Recently, in an image pickup optical system of a camera, thermoplastics camera lenses used for compensating chromatic aberration instead of optical inorganic glasses, which were composed by lenses with large and small Abbe numbers, require a high refractive index and low birefringence (Δn). According to the Lorentz−Lorenz equation, introduction of substituents with a high molar refraction and a small molar volume efficiently increases the refractive indices of polymers. In other words, aromatic rings, halogen atoms except fluorine, sulfur atoms, and metal atoms are elemental components that achieve a high refractive index.4−6 To date, efficient incorporation of such components, particularly sulfur atoms and aromatic rings, has afforded various polymers with high refractive index values reaching up to 1.7, such as high-refractive-index polyimides,7−12 poly(phenylene sulfide)s,13−15 and poly(arylene thioether).16 On the other hand, the incorporation of aromatic components into the polymer backbone enhances the rigidity of the polymer, which increases Δn due to interchain packing. In our previous work, a series of poly(phenylene thioether)s with heteroaromatic pyrimidine units exhibited relatively high Δn values of ca. 1.0 × 10−2 while achieving a high refractive index in the © XXXX American Chemical Society

Received: June 2, 2016 Revised: August 1, 2016

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DOI: 10.1021/acs.macromol.6b01182 Macromolecules XXXX, XXX, XXX−XXX

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dependent DFT (TD-DFT) theory.30 All the calculations were performed using the software package of Gaussian 09 (Rev. D01). The wavelength-dependent refractive indices of monomeric models for polymers were calculated according to eq 1. Synthesis of P1. To the dry chloroform solution (5.60 mL) of FDE (0.690 g, 1.50 mmol) and 9,9-bis(4-sulfanylphenyl)fluorene (0.570 g, 1.50 mmol), triethylamine (TEA) (0.200 mL) was added dropwise at room temperature (RT) under the nitrogen atmosphere. Them the reaction was continued at RT for 12 h. After the reaction, the reaction mixture was poured into water to yield the precipitate. The precipitate was dissolved in DMAc, and the solution was poured into methanol to yield the white solid. The obtained polymer was collected with filtration and then dried in vacuo at 80 °C (1.14 g, 90.5%). 1H NMR (CDCl3, δ, ppm): 7.71−7.74 (4H, m), 7.09−7.34 (24H, m), 6.88 (4H, d), 3.16 (4H, t), 2.79 (4H, t). 13C NMR (CDCl3, δ, ppm): 170.3, 150.8, 150.6, 149.4, 144.4, 143.4, 140.1, 133.4, 130.1, 129.2, 128.9, 128.0, 127.8, 126.2, 126.1, 121.2, 120.4, 64.8, 64.6, 34.7, 29.1. Anal. Calcd for C56H40O4S2O·38H2O: C, 79.33; H, 4.85. Found: C, 79.39; H, 4.91. Mn = 17 000 (Mw/Mn = 1.76). Synthesis of P2. To the dry THF solution (5.30 mL) of FDE (0.460 g, 1.00 mmol) and 4,4′-thiobisbenzenethiol (0.250 g, 1.00 mmol), TEA (0.200 mL) was added dropwise at RT under the nitrogen atmosphere. Them the reaction was continued at RT for 12 h. After the reaction, the reaction mixture was poured into water to yield the white precipitate. The obtained polymer was collected with filtration and then dried in vacuo at 80 °C (0.630 g, 88.7%). 1H NMR (CDCl3, δ, ppm): 7.74 (2H, d), 7.17−7.36 (18H, m), 6.91 (4H, d), 3.21 (4H, t), 2.82 (4H, t). 13C NMR (CDCl3, δ, ppm): 170.1, 150.6, 149.2, 143.3, 140.0, 134.3, 133.9, 131.5, 130.7, 129.1, 127.9, 127.7, 126.1, 121.1, 120.3, 64.5, 34.3, 29.0. Anal. Calcd for C43H32O4S3O· 50H2O: C, 71.94; H, 4.63. Found: C, 72.27; H, 5.00. Mn = 37 000 (Mw/Mn = 1.84). Synthesis of P3. To the dry chloroform solution (3.10 mL) of FDE (0.460 g, 1.00 mmol) and phenoxybenzene-4,4′-dithiol (0.230 g, 1.00 mmol), TEA (0.200 mL) was added dropwise at RT under the nitrogen atmosphere. Then the reaction was continued at RT for 12 h. After the reaction, the reaction mixture was poured into water to yield the precipitate. The precipitate was dissolved in DMAc, and the solution was poured into methanol to yield the white solid. The obtained polymer was collected with filtration and then dried in vacuo at 80 °C (0.524 g, 75.6%). 1H NMR (CDCl3, δ, ppm): 7.75 (2H, d), 7.17−7.40 (14H, m), 6.92 (8H, d), 3.17 (4H, t), 2.80 (4H, t). 13C NMR (CDCl3, δ, ppm): 170.3, 156.5, 150.8, 149.4, 143.4, 140.1, 133.6, 129.2, 128.9, 128.0, 127.8, 126.2, 121.3, 120.4, 119.7, 64.5, 34.7, 30.6. Anal. Calcd for C43H32O5S2O·30H2O: C, 73.97; H, 4.71. Found: C, 73.99; H, 4.80. Mn = 16 000 (Mw/Mn = 1.70). Synthesis of P4. To the dry chloroform solution (3.10 mL) of FDE (0.460 g, 1.00 mmol) and bis(4-mercaptophenyl)sulfone (0.280 g, 1.000 mmol), TEA (0.200 mL) was added dropwise at RT under the nitrogen atmosphere. Them the reaction was continued at RT for 12 h. After the reaction, the reaction mixture was poured into water to yield the precipitate. The precipitate was dissolved in DMAc, and the solution was poured into methanol to yield the white solid. The obtained polymer was collected with filtration and then dried in vacuo at 80 °C (0.557 g, 75.1%). 1H NMR (CDCl3, δ, ppm): 7.72−7.75 (6H, m), 7.15−7.38 (14H, m), 6.91 (4H, d), 3.21 (4H, t), 2.81 (4H, t). 13C NMR (CDCl3, δ, ppm): 170.2, 150.8, 149.4, 143.4, 140.1, 134.4, 134.0, 131.6, 130.9, 129.2, 128.0, 127.8, 126.2, 121.3, 120.4, 64.6, 34.5, 29.1. Anal. Calcd for C43H32O6S3O·40H2O: C, 69.54; H, 4.37. Found: C, 69.34; H, 4.44. Mn = 23 000 (Mw/Mn = 1.91). Theoretical Background. As stated above, the refractive index of a polymer can be estimated based on the Lorentz−Lorenz formula:4−6

medium range of the glass transition temperature (Tg) at ca. 150 °C has to be achieved in addition to the above optical properties. We herein developed novel poly(phenylene thioether)s with a fluorene-based cardo structure by the facile and atomeconomically polyaddition reaction between 9,9-bis((acryloyloxy)phenyl)fluorene (FDE) and various dithiol monomers (9,9-bis(4-sulfanylphenyl)fluorene, 4,4′-thiobisbenzenethiol, phenoxybenzene-4,4′-dithiol, and bis(4mercaptophenyl)sulfone). The proposed polymer architecture potentially achieves the essential properties for thermoplastics camera lenses for compensating chromatic aberration: (1) high transparency due to the poly(phenylene thioether) backbone, (2) a high refractive index due to the high aromatic and sulfur contents, (3) small Δn due to the cardo structure, (4) small Abbe number due to aromatic units, and (5) relatively low Tg (ca. 150 °C) due to the flexible ester linkage. The polyaddition reactions under room temperature gave colorless poly(phenylene thioether)s in high yields. The polymers exhibited excellent optical transparency as high as 90% over a wavelength of 400 nm and suitable Tg values in the range of 120−194 °C. Furthermore, the polymers achieved a high refractive index of 1.6553−1.6762 and small Δn value of ca. 1.0 × 10 −3 simultaneously. The well-suited incorporation of the fluorenebased cardo structure into the polymer backbone resulted in the excellent optical and thermal properties required for highperformance thermoplastic lenses.

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EXPERIMENTAL SECTION

Materials. All reagents and solvents were used as received unless otherwise stated. 9,9-Bis((acryloyloxy)phenyl)fluorene (FDE)26 and phenoxybenzene-4,4′-dithiol27 were synthesized according to the previous literatures. 9,9-Bis(4-sulfanylphenyl)fluorene28 and bis(4mercaptophenyl)sulfone29 were synthesized by the modified the procedure of previous literatures. Instrumentation. The 1H and 13C NMR spectra were recorded with a JEOL JNM-ECX400. Elemental analysis was carried out on a PerkinElmer 2400 II CHNS/O analyzer. The UV−vis spectra were recorded on a JASCO V-630BIO UV−vis spectrophotometer. The number-average molecular weight (Mn) and molecular weight distribution (Mw/Mn) were estimated by size exclusion chromatography (SEC) using a HLC-8320 system. The column set was as follows: a guard column (TSK guard column HHR-H) and three consecutive columns (TSKgel GMHHR-M, TSKgel GMHHR-M, and TSKgel superH-RC) eluted with chloroform at a flow rate of 1.0 mL/ min. Polystyrene standards were employed for calibration. Thermal gravimetric (TG) analysis was performed on a SEIKO TG/DTA6200 at a heating rate of 10 °C/min under a nitrogen atmosphere. Differential scanning calorimetry (DSC) analysis was performed on a SEIKO DSC6200 at a heating rate of 10 °C/min under a nitrogen atmosphere. The in-plane (nTE) and out-of-plane (nTM) refractive indexes of polyimide films were carried out using a prism coupler (Metricon, model PC-2000) equipped with a He−Ne laser (wavelength: 633 nm) and a half-waveplate in the light path. The in-plane/ out-of-plane Δn values were estimated as a difference between nTE and nTM, and the average n values were calculated according tothe equation nav = [(2nTE2 + nTM2)/3]1/2. DFT Calculation. The density functional theory (DFT) with the three-parameter Becke-style hybrid functional (B3LYP), which employs the Becke exchange and LYP correlation functions, was adopted for the calculation of molecular polarizabilities in conjunction with the Gaussian basis sets. The 6-31G(d,p) basis set was used for geometry optimizations under no constraints, and the 6-311+G(2d,p) basis set was used for calculating frequency-dependent linear polarizabilities at a wavelength of 633 nm. The 6-311++G(d,p) basis set was used for calculating optical absorption spectra based on time-

nav 2 − 1 nav 2 + 2

=

4π ρNA αav 3 M

(1)

where nav is the average refractive index, ρ is the density of the material, NA is Avogadro’s number, M is the molecular weight per a repeating unit, and αav is the average molecular polarizability. Here, the intrinsic volume (Vint) is expressed by B

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Vint =

Scheme 1. Model Compounds Used for Calculations of Molecular Polarizability, van der Waals Volume, and Polarizability Anisotropy

(2)

and the molecular packing coefficient Kp is defined as Kp =

Vvdw Vint

(3)

where Vvdw is the van der Waals volume. Thus, eq 1 can be rewritten as

nav 2 − 1 2

nav + 2

=

α 4π K p av 3 Vvdw

(4)

Note that the left-hand side is nearly proportional to nav, and the value of Kp has been reported to be nearly constant (= 0.681) for amorphous polymers.31 Thus, eq 4 indicates that nav is nearly proportional to αav/Vvdw. On the other hand, the principal values of the refractive index ellipsoid are approximated by the Vuks equation:32

nii 2 − 1 2

nav + 2

=

α 4π ρNA 4π αii = K p ii 3 M 3 Vvdw

equivalent measures for estimation of Δn0. The large η values obtained for the unit structures of DT-O, DT-S, DE-S, and DEO, for which the linear sequences consisted of p-phenylene groups with ether (−O−) or thioether (−S−) linkages, were assumed to generate a large intrinsic birefringence. Interestingly, the η value of DT-O was larger than DT-S, whereas those of DE-O and DE-S were nearly the same. The large van der Waals radii and spatially spreading 3p-orbitals of the three sulfur (S) atoms in DT-S may reduce the polarizability anisotropy. Moreover, the η values of DT-SO2 and DE-SO2 were significantly smaller than those of DT-O, DT-S, DE-S, and DE-O, which indicates that the substitution of −SO2− linkage for −O− and −S− linkages effectively reduces the birefringence, though the substitution of −SO2− for −S− lowers the refractive index, as described below. In contrast, the exceptionally small or nearly zero η values obtained for DPF, DT-F, and DE-F, all of which includes “aromatic cardo” structures, reflect the highly isotropic nature of the polarizability tensor of the cardo structure and strongly suggest the overwhelming reducing effects of birefringence. Note that the η values of DT-F and DE-F were slightly smaller than that of DPF, which indicates that the −O− and −S− linkages attached to the cardo structure do not enhance the polarizability anisotropy of the local structure. Figure 1b displays the plots of α/Vvdw against η. As explained in the Theoretical Background section, if we can assume that Kp is constant, the calculated values of αav/Vvdw and η should be nearly proportional to nav and Δn0, respectively. As shown in Figure 1b, an increase in αav/Vvdw enhances the refractive index, and a decrease in η lowers the intrinsic birefringence. Hence, the DT-F structure, which is located in the lowest and the rightmost section of the figure, was the best structure inducing the highest refractive index and the lowest birefringence. Additionally, DE-F structure is the second best with a medium refractive index and the lowest birefringence. On the other hand, DT-S, DT-O, and DT-SO2 structures containing two −S− linkages on the both sides effectively enhanced the refractive indices compared with those containing two −O− linkages, but they did not reduce the birefringence as mentioned above. In addition, the substitution of −SO2− for the −S− linkage considerably reduced α/Vvdw values, which lowered the refractive indices, whereas that for the −O− linkage effectively lowered the birefringence without lowering the refractive index. The calculated optical absorption spectra of the model compounds based on the TD-DFT theory (Figure

(5)

where nii and αii (ii = x, y, z) are the directional components of the refractive index and macroscopic polarizability tensors, respectively. For polymer films formed on isotropic substrates, the z-axis is defined as perpendicular to the film surface, and the x- and y-axes lie on the film plane. Because the refractive index is isotropic in the film plane (nxx = nyy), nzz is denoted as η0⊥, and both of nxx and nyy are denoted as η0∥. In general, the polarizability tensor of polymer chains including plural aromatic groups in the main chain can be treated as axially symmetric along the chain axis. Thus, the components of the polarizability of a repeating unit parallel and perpendicular to the main chain are denoted as α∥ and α⊥, respectively, where α∥ corresponds to the largest component α11, and α⊥ can be approximated by the average of α22 and α33. Using eq 5, the difference between η0∥ and η0⊥ can be written as

(n 0)2 − (n⊥0)2 2

nav + 2

=

4π α − α⊥ Kp 3 Vvdw

(6)

In case that Kp is a constant, eq 6 readily indicates the following relation because η0∥ + η0⊥ and nav are almost constant.

Δn0 = n 0 − n⊥0 ∝

α − α⊥ Vvdw

=

Δα Vvdw

(7)

where Δα = α∥ − α⊥, and Δn is the intrinsic birefringence that corresponds to the fully extended main chain structure. Because the average polarizability αav (= (2α∥ + α⊥)/3) has the dimensions of volume, and αav is nearly proportional to Vvdw, “the coefficient of polarizability anisotropy η” can be defined as follows: α − α⊥ η= 2α + α⊥ (8) 0

Based on eqs 7 and 8, the values of Δα/Vvdw and η calculated for a repeating unit can be treated as good measures for the Δn0 of the corresponding polymer.

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RESULTS AND DISCUSSION Polarizability and Its Anisotropy in Model Compounds. To confirm the advantages of the sulfur-substituted fluorene-based aromatic cardo structure to enhance the refractive index and reduce birefringence, α/Vvdw, Δα/Vvdw, and η were calculated for the model compounds as depicted in Scheme 1, and the values thus obtained are listed in Table S1. In Figure 1a, the calculated Δα/Vvdw values are plotted against those of η, in which the highly linear relationship demonstrates that both parameters of Δα/Vvdw and η can be used as C

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Figure 1. Relationships between calculated values of (a) Δα/Vvdw and η as well as (b) α/Vvdw and η.

Scheme 2. Synthesis of P1−P4 by Polyaddition Reactions

S5) predicted that none of these unit structures exhibit absorption peaks in the visible region (wavelength, λ = 400− 800 nm). Consequently, the introduction of the unit structures of DT-F and DE-F into the polymer main chain could be the best choice for designing novel polymer materials for highperformance optical components. Synthesis and Characterization of Polymers. Diacrylate and dithiol monomers, 9,9-bis((acryloyloxy)phenyl)fluorene (FDE), 9,9-bis(4-sulfanylphenyl)fluorene, phenoxybenzene4,4′-dithiol, and bis(4-mercaptophenyl)sulfone were synthesized by reference to previous literatures.26−29 The structures of all monomers were confirmed by the 1H NMR spectroscopy to determine that the desired monomers were obtained. Then polyaddition reactions between FDE and each dithiol monomer were carried out in the presence of a catalytic amount of triethylamine to prepare poly(phenylene thioether)s with a fluorene-based cardo structure (Scheme 2 and Table 1). The reaction between FDE and 9,9-bis(4-sulfanylphenyl)fluorene was carried out in the presence of triethylamine at room temperature for 12 h in chloroform (Table S2). After the reaction, the precipitation into methanol was conducted twice to remove triethylamine completely from the polymers because

Table 1. Molecular Information and Thermal Properties of P1−P4

a b

polymer

Mna

Mw/Mna

yield (%)

T5db (°C)

Tgb (°C)

P1 P2 P3 P4

17 000 37 000 17 000 23 000

1.76 1.84 1.70 1.91

91 89 76 75

359 354 340 359

194 125 120 174

Measured by SEC using polystyrene standards in chloroform. Measured under a nitrogen atmosphere.

the residual triethylamine may cause the decomposition of polymer backbones. After the repeated precipitation, colorless P1 with a number-average molecular weight (Mn) of 17 000 (Mw/Mn = 1.76) was obtained in a high yield (90%). The structure of P1 was characterized by the 1H and 13C NMR spectroscopies and elemental analysis (Figure 2). The peaks of protons corresponding to vinyl group were observed at 5.98, 6.29, and 6.57 ppm in the 1H NMR spectrum of FDE (Figure S1), whereas the peaks of protons corresponding to vinyl groups completely disappeared and the peaks of protons corresponding to methylene groups were clearly observed at D

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Figure 2. 1H (a) and 13C (b) NMR spectra of P1 in CDCl3. The residual CHCl3 peak is marked.

2.79 and 3.16 ppm in the 1H NMR spectrum of P1 (Figure 2a). The characteristic peaks of carbons corresponding to the carbonyl group (170.3 ppm) and C-9 in the fluorene (64.6 and 64.8 ppm) were also clearly observed (Figure 2b). The obtained data indicated that the polyaddition reaction successfully proceeded to yield P1 with the desired structure. Furthermore, P1 exhibited high solubility for common organic solvents, such as THF, DMAc, DMF, chloroform, and dichloromethane (Table S3). For comparison, P2−P4 were synthesized by polyaddition reactions using 4,4′-thiobisbenzenethiol, phenoxybenzene-4,4′dithiol, and bis(4-mercaptophenyl)sulfone, respectively, as a dithiol monomer (Scheme 2). Colorless P2 with Mn = 37 000 (Mw/Mn = 1.84) was obtained in a yield of 89% from FDE and 4,4′-thiobisbenzenethiol in the presence of triethylamine at room temperature for 12 h in THF. Colorless P3 with Mn = 16 000 (Mw/Mn = 1.70) was obtained in a yield of 76% from FDE and phenoxybenzene-4,4′-dithiol in the presence of triethylamine at room temperature for 12 h in chloroform. The polyaddition reaction between FDE and bis(4-mercaptophenyl)sulfone, which was carried out in the presence of triethylamine at room temperature for 12 h in chloroform, also gave colorless P4 with Mn = 23 000 (Mw/Mn = 1.91) in a yield of 75%. The polyaddition reactions of P1−P4 strongly depended on the reaction solvent and temperature, and all results are summarized in Table S2. P2−P4 were characterized by the 1H and 13C NMR spectroscopies (Figure S2) and exhibited high solubility for common organic solvents. Herein, a series of colorless poly(phenylene thioether)s with a fluorenebased cardo structure (P1−P4) were successfully synthesized under the moderate polyaddition reaction conditions. Thermal Properties. The thermal decomposition temperature and Tg of polymers are quite important properties for the optical device fabrication process. The requirements for the thermal properties originate from the servicing conditions for optical devices and problems caused by miniaturization of the opto-integrated assembly.33,34 The thermal properties of P1−

P4 investigated by TG and DSC are summarized in Table 1. As shown in Figure 3, P1−P4 exhibited high thermal stability, i.e.,

Figure 3. TG curves of P1−P4 under a nitrogen atmosphere.

5% weight loss temperature (T5d) above 340 °C under a nitrogen atmosphere. In the DSC profiles (Figure 4), the Tg values of P1−P4 were relatively low at 120−194 °C due to the ester linkage. In particular, the low Tg values of P2 and P3 were due to the flexible ether and thioether linkages of the polymer backbones in addition to the ester linkage. On the other hand, the relatively high Tg of P1 resulted from the highest content of aromatic units. As noted earlier, thermoplastic polymers with Tg at ca. 150 °C are suitable for injection molding. In that regard, the thermal properties of P1−P4 in the range of 120−194 °C are suitable for the injection molding process. Optical Properties. The UV−vis transmittance spectra of the P1−P4 films (film thickness; ca. 30−40 μm) are shown in Figure 5. The P1−P4 chloroform solutions were drop-casted onto glass substrates and dried under the ambient condition to yield the P1−P4 films. The cutoff wavelengths of P1−P4 films were in the range of 320−340 nm, and the transmittance was as high as 90% at 400 nm. In particular, the P1, P3, and P4 films E

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backbone to be parallel to the film plane. The average refractive index (nav) was in the range of 1.6553−1.6741. The nav values are generally dependent on the sulfur content in the polymer repeating unis. Thus, P2 with the highest sulfur content (13.57 wt %) exhibited the highest nav value of 1.6741. As for P1, a high nav value of 1.6605 was achieved even with the lowest sulfur content (7.62 wt %) due to the contribution of the high content of aromatic units to nav. P1−P4 exhibited the low Δn values in the range of 0.0014−0.0030 due to the flexible polymer backbone structure and incorporation of the fluorenebased cardo structure. Considering that the lowest Δn value of 0.0014 was observed in P1 with the highest content of the cardo structure in the polymer repeating units, a significant reduction in the polarization anisotropy between the polymer backbone and fluorene units perpendicular to the polymer backbone was truly achieved as expected. The calculated intrinsic birefringence (Δn0) of P1 was estimated to be lowest (0.0755) among P1−P4; this also agrees with the measured Δn value. Figure 6 shows the wavelength-dependent calculated refractive indices (ncalc) of the monomeric models for P1−

Figure 4. DSC profiles of P1−P4 under a nitrogen atmosphere.

Figure 5. UV−vis transmittance spectra of P1−P4 thin films.

exhibited the excellent transparency of 95% at 400 nm. Among transparent polymers with a fluorene-based cardo structure, P1−P4 showed some of the highest transparency values.19−22 The flexible linkage of the polymer backbone (ester, ether, and thioether linkages) and/or high contents of bulky fluorene units in the polymer repeating units may prevent interchain packing, contributing to the excellent transparency of P1−P4. Furthermore, it should be noted that the excellent transparency was maintained even after a harsh thermal treatment (at 200 °C for 6 h under a nitrogen atmosphere) due to the high thermal stability (Figures S3 and S4). The high transparency in the visible region observed for P1−P4 was also supported by the calculated optical absorption spectra based on the TD-DFT theory (Figures S6). The in-plane (nTE) and out-of-plane (nTM) refractive indices of the P1−P4 films (film thickness; ca. 4 μm) were 1.6558− 1.6751 and 1.6543−1.6721, respectively. The nTE values were slightly higher than those of nTM for all samples (Table 2). This fact indicates that the preferential orientation of the polymer

Figure 6. Wavelength-dependent calculated refractive indices of monomeric models for P1−P4.

P4. The ncalc values were in good agreement with the observed nav values of P1−P4. That is, the lowest nav value of P3 in P1− P4, which is attributable to the low sulfur and cardo structures contents, is well reproduced by the calculation. On the other hand, the observed nav value of P1 was lower than that of P4, which is contrary to their ncalc values (Figure 6). This may result from the lower packing coefficient of P1 chains due to the sequential fluorene-based cardo structures in the repeating unit. The calculated Abbe numbers for P1−P4 were 19.6, 19.6, 18.8, and 17.5, respectively. These results indicate that a poly(phenylene thioether) containing a fluorene-based cardo structure has high potential as a target material that simultaneously exhibits a high refractive index with low birefringence, high transparency, small Abbe number, and excellent glass transition temperature.

Table 2. Optical Properties of P1−P4

a

polymer

sulfur content (wt %)

cutoff wavelength (nm)

nTEa

nTMa

nava

Δna

Δn0 b

P1 P2 P3 P4

7.62 13.57 9.25 12.98

317 340 320 335

1.6610 1.6751 1.6558 1.6734

1.6595 1.6721 1.6543 1.6717

1.6605 1.6741 1.6553 1.6730

0.0014 0.0030 0.0015 0.0020

0.076 0.208 0.222 0.142

Measured at 633 nm. bCalculated at 633 nm with fully extended polymer structures. F

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CONCLUSION A series of colorless poly(phenylene thioether)s containing the fluorene-based cardo structure (P1−P4) were successfully prepared by facile and atom-economically polyaddition reactions. P1−P4 exhibited high thermal stability (T5d > 340 °C), and the observed Tg was in the range of 120−194 °C. The optical transmittance of the P1−P4 films was as high as 90% at 400 nm. In particular, the transparency of the P1, P3, and P4 films was as high as 95% at 400 nm. The nav and Δn values of P1−P4 were in the range of 1.6553−1.6741 and 0.0014− 0.0030 at 633 nm, respectively. The small Δn values resulted from the efficient cancellation of polarization anisotropy between the polymer backbone and fluorene units perpendicular to the polymer backbone. In particular, P1 with the highest content of fluorene units in the polymer repeating units achieved the lowest Δn value of 0.0014 while maintaining a high nav value of 1.6605. Our designed poly(phenylene thioether)s with the fluorene-based cardo structure achieved excellent transparency, high refractive indices, low birefringence, small Abbe number, and suitable glass transition temperatures for the injection molding process simultaneously, which can be promising thermoplastic candidate materials for compensating chromatic aberration.

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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.6b01182. Synthetic procedure of monomers, polymerization results, and 1H NMR spectra of polymers, thermal stability and solubility tests of polymers, and calculated optical properties of polymers (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(M.U.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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DOI: 10.1021/acs.macromol.6b01182 Macromolecules XXXX, XXX, XXX−XXX

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NOTE ADDED AFTER ASAP PUBLICATION This paper was published ASAP on August 5, 2016 with errors in the Results and Discussion section. The corrected version was reposted on August 11, 2016.

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DOI: 10.1021/acs.macromol.6b01182 Macromolecules XXXX, XXX, XXX−XXX