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Photoelectric Properties of ABA-Type Triblock Copolymers Designed Using Fluorine-Containing Polyimide Macroinitiators with Polyhedral Oligomeric Silsesquioxane.

INTRODUCTION

There are numerous studies of organic-inorganic nanohybrid materials [1], which are critical components of passive optical waveguides. For example, many studies have reported polyimide-silica hybrid materials that combine silica, which has high transparency, and fluorine-containing polyimides, which have high water repellency, high transparency, and a low refractive index [2], When the refractive index of the clad component is lower than that of the core in the optical waveguide, light is completely reflected on the surface according to Snell's law. Therefore, the passive optical waveguide has a structure that enables light to propagate through only the core. Precise control of the refractive index is required to manufacture optical waveguides. For optimum performance, the differences between the refractive indices of the clad and core components should be less than 1%. The application of polyimide-silica hybrid materials to control the refractive index is important for next generation optical waveguides.

Conventional methods for preparing polyimide-silica hybrid materials include sol solution mixing [3], intercalation [4], and in situ methods [5], which are the most widely used. However, transmission loss generated by scattering occurs on the surface, lowering the affinity between the polyimides and the silica, because physical blending of a polymer with two incompatible components results in a microphase-separated structure.

In this study, 4,4'-(hexafluoro-isopropylidene) diphthalic anhydride 2,3,5,6-tetramethyl-l,4-phenylenediamine (6FDA-TeMPD) was synthesized as a polyimide macroinitiator, with high heat resistance (glass transition temperature = 427[degrees]C) and transparency. Moreover, this polyimide macroinitiator was reacted at both polymer ends with polyhedral oligomeric silsesquioxane (POSS), which contains silica domains, to form an ABA-type triblock copolymer (POSS/6FDA-TeMPD/POSS) via atom transfer radical polymerization (ATRP). ATRP is an in situ method that allows good control of both the molecular weight and molecular distribution. Additionally, ATRP is the free-living radical polymerization method most commonly used for polymerizing a wide range of monomers under mild reaction conditions [6-9]. Therefore, the ABA-type triblock copolymer was expected to possess a structure with nanoscale phase separation and to produce a transparent film for high dispersion over the visible light region. Good mixing of both domains in the ABA-type polyimide-silica hybrid polymer can be obtained by ATRP, which is very important for designing the core and clad parts of polymer materials for optical waveguides. However, the optical properties of such ABA-type 6FDA-TeMPD/POSS triblock copolymers have yet to be studied.

We systematically investigated the solid and optical properties of various ABA-type 6FDA-TeMPD/POSS triblock copolymers. A poly(methacryl ethyl-POSS) (poly(MEPOSS)), which was shown to have good moldability in our previous study, was used as the POSS domain [10]. In that previous study, we analyzed only the gas transport and physical properties of the membranes. In this study, we focused on the optical properties of the thin films. To our knowledge, this is the first article that demonstrates precise control of the refractive index by using ABAtype polyimide-silica hybrid polymers. Additionally, the effect of the MEPOSS domain concentration on refractive index, absorption, fluorescence, and electronic structure was examined.

EXPERIMENTAL

Preparation of Polymer Films

6FDA-TeMPD (MEPOSS content 0 mol%) [11, 12], poly (MEPOSS) (MEPOSS content 100 mol%) [10], and various 6FDA-TeMPD/MEPOSS triblock copolymers (MEPOSS content 17, 43, and 79 mol%) [6-9] were synthesized according to the literature procedure. [sup.1]H NMR (JNM-ECA500; JEOL Ltd., Tokyo, Japan) and Fourier transform infrared (FTIR) spectroscopy (FTIR 460+; JASCO Co., Tokyo, Japan) analyses confirmed the chemical structures of these polymers, as shown in Fig. 1. Molecular weights were determined by gel permeation chromatography (GPC) (HLC-8220; Tosoh Co., Tokyo, Japan) with a refractive index detector. TSK-gel columns were connected in succession, and tetrahydrofuran was used as the eluent. Calibration was performed with linear polystyrene standards.

The isotropic dense polymer films were prepared by spin coating at 3,000 rpm a filtered 1 wt% solution of the polymer in chloroform onto quartz substrates. These polymers, with 100 nm film thickness, were then baked at room temperature for 7 days under vacuum.

Optical Property Analysis

The ultraviolet (UV) absorption spectra were determined in the film state on the quartz substrates. The UV absorption spectra were obtained using a UV-3100 spectrometer (Shimadzu Co., Kyoto, Japan) from 0.75 to 6.50 eV (254- 1,650 nm) at 23 [+ or -] 1[degrees]C. The fluorescence spectra were obtained using an FP-6500 spectrofluorometer (JASCO Co., Tokyo, Japan). The excitation wavelengths were 325 and 350 nm. and the measurement range was 300-700 nm at 23 [+ or -] 1[degrees]C. Refractive index measurements were performed on a UVISEL ellipsometer (Jobin Yvon S. A. S., Longjumeau, France) from 0.75 to 3.10 eV (400-1,650 nm) at 23 [+ or -] 1[degrees]C. All measurements were performed on the films on quartz substrates with at least three replicates to confirm the reproducibility of the experimental results.

Molecular Orbital (MO) Calculations

The structures of the 6FDA-TeMPD and poly(MEPOSS) were optimized using MOPAC (PM3 parameter) or molecular mechanics. Each structure was determined using one polymer segment unit with hydrogen termination. The energy of the lowest unoccupied molecular orbital (LUMO), [[epsilon].sub.LUMO], 'hat of the highest occupied molecular orbital (HOMO), [[epsilon].sub.HOMO], and the theoretical UV absorption spectra of the optimized structures were determined using the molecular mechanics simulator Gaussian 09W (Gaussian Inc., Wallingford, CT).

RESULTS AND DISCUSSION

Synthesis of 6FDA-TeMPD/POSS Triblock Copolymer

The chemical structures of 6FDA-TeMPD and poly(MEPOSS) shown in Fig. 1 were confirmed by FTIR and [sup.1]H NMR analyses. The spectral peak assignments were as follows: they were in good agreement with those from our previous study [10, 11].

6FDA-TeMPD: FT-IR. 1,784 and 1,724 [cm.sup.-1] (C=0 stretching), 1,354 [cm.sup.-1] (C-N stretching), 723 [cm.sup.-1] (C-N deformation); [sup.1]H NMR (in CDC13 with TMS), 2.13 ppm (12H, methyl group of TeMPD), 7.99-8.10 ppm (6H, aromatic ring of 6FDA).

Poly(MEPOSS): FT-IR, 2,965 and 2,882 [cm.sup.-1] (C-H stretching), 1,734 [cm.sup.-1] (C=0 stretching), 1,256 (C-0 stretching), 1,110 [cm.sup.-1] (Si-O-Si bending), 761 [cm.sup.-1] (Si-C stretching); [sup.1]H NMR (in CD[Cl.sub.3] with TMS), 0.59 ppm (2H, methylene group near Si of propyl group), 0.60-0.62 ppm (14H, methylene groups of heptaethyl substituent), 0.89-1.11 ppm (21H, methyl groups of heptaethyl substituent), 0.89-1.11 ppm (5H, methacrylate group), 1.72 ppm (2H, middle methylene group of propyl group), 3.87 ppm (2H, methylene group near O of propyl group).

The 6FDA-TeMPD/MEPOSS triblock copolymer was analyzed using both FTIR and [sup.1]H NMR spectroscopy, as shown in Figs. 2 and 3, respectively. The spectral peak assignments are as follows:

6FDA-TeMPD/MEPOSS triblock copolymer: FT-IR, 2,965 and 2,882 [cm.sup.-1] (C-H stretching), 1,788 and 1,728 [cm.sup.-1] (C=0 stretching), 1,516 [cm.sup.-1] (aromatic ring C=C stretching), 1,355 [cm.sup.-1] (C-N stretching), 1,107 [cm.sup.-1] (Si-O-Si bending), 761 [cm.sup.-1] (Si-C stretching), 723 [cm.sup.-1] (C-N deformation); [sup.1]H NMR (in CD[Cl.sub.3] with TMS), 0.59 ppm (2H, i), 0.60-0.62 ppm (14H, h), 0.85-1.11 ppm (26H, e-g), 1.72 ppm (2H, j), 2.13 ppm (12H, a), 3.88 ppm (2H, k), 4.57 ppm (2H, 1), 7.39-7.71 ppm (6H, m-o), 7.85 ppm (2H, p), 7.96-8.00 ppm (4H, b, c), 8.08-8.10 ppm (2H, d).

The FTIR spectrum of the obtained triblock copolymer contains bands around 1,788 and 1,728 [cm.sup.-1] (C=0 stretching), 1,355 [cm.sup.-1] (C-N stretching), and 723 [cm.sup.-1] (C-N deformation), which are the characteristic absorption bands of the 6FDA-TeMPD components, as shown in Fig. 2. Moreover, the characteristic bands of poly(MEPOSS) around 2,955 and 2,872 [cm.sup.-1] (C--H stretching) were observed in the triblock copolymer. The characteristic band for the junction of the 6FDA-TeMPD and poly(MEPOSS) components was also observed around 1,516 [cm.sup.-1] (aromatic ring C=C stretching).

The [sup.1]H NMR spectrum of the triblock copolymer is shown in Fig. 3. The spectrum includes characteristic peaks at 2.13 ppm (12H, a), 7.96-8.00 ppm (4H, b, c), and 8.08-8.10 ppm (2H, d), that are related to the 6FDA-TeMPD component. The peak characteristics of poly(MEPOSS) were observed at 0.59 ppm (2H, i), 0.60-0.62 ppm (14H, h), 0.85-1.11 ppm (26H, e-g), 1.72 ppm (2H, j), and 3.88 ppm (2H, k). Those indicative of the -C[H.sub.2]-O-between the 6FDA-TeMPD and poly(MEPOSS) components were observed around 4.57 ppm (2H, 1), 7.39-7.71 ppm (6H, m-o), and 7.85 ppm (2H, p) in the triblock copolymer.

The chemical structure of the 6FDA-TeMPD/POSS ABA-type triblock copolymer is shown in Fig. 1. The number-average molecular weights of 6FDA-TeMPD and poly(MEPOSS) estimated using GPC were 89,000 and 172,000 g [mol.sup.-1], respectively. Those of the ABA-type triblock copolymers with MEPOSS concentrations of 17, 43, and 79 mol% were estimated to be 24,000, 25,000, and 34,000 g [mol.sup.-1], respectively, by [sup.1]H NMR in our previous study [8].

Optical Properties

The UV-visible-near infrared spectra of the films are shown in Fig. 4. All the ABA-type triblock copolymers composed of 6FDA-TeMPD and poly(MEPOSS) in the molecular-size order displayed >99% transmittance within the range of 1,300 - 1,550 nm (0.80 - 0.95 eV), which is ideal for a communication network. Three absorbance peaks near 4.2, 5.5-5.7, and 6.2 - 6.4 eV were observed within the UV range. All the absorbance peak intensities decreased as the MEPOSS content increased. The triblock copolymer film maintained a high transmittance, as it was composed of two components. The POSS domains served to increase the films' transparency across the entire wavelength range. The poly(MEPOSS) film displayed a broad peak near 5.5 - 6.5 eV. For all the ABA-type block copolymers, the peak position of the 6FDA-TeMPD component remained constant, despite the POSS component. However, a weak peak near 3.5 eV was observed in the ABA-type triblock copolymer film that contained 43 mol% MEPOSS. These absorbance peaks were assigned based on the results of the MO calculations.

The 6FDA-TeMPD and poly(MEPOSS) orbital compositions of the frontier MO are shown in Fig. 5. Additionally, the calculated absorption spectra from 2.0 to 6.5 eV are shown in Fig. 6. When the measured and calculated spectra are compared, the weak peak near 4.2 eV is attributed to the excitation of one electron from the HOMO to the LUMO in 6FDA-TeMPD. It is noteworthy that the HOMO and LUMO charges are localized on the TeMPD and 6FDA site residues, respectively. This means that photo absorption depends on the charge transfer transitions from the TeMPD site to the 6FDA site because of the phenyl group [pi]-[pi]* transitions. On the other hand, the spectrum has two distinct peaks around 5.5 - 5.7 and 6.2 - 6.4 eV. Although the two peaks are broad and appear not to have fine electronic structures, the spectrum can be deconvoluted with various components considering the possible electronic transitions, as shown in Fig. 6a. The high-energy band is assignable to the n-[pi]* transition from the HOMO of the carbonyl groups to the LUMO of the 6FDA site.

The broad peak around 5.5 - 6.5 eV is described as the n - [pi]* charge transfer transition from the HOMO in the Si-0 site to the LUMO of the carbonyl group in poly(MEPOSS). Generally, in the case of silica glass, the valence band is formed by the 0(2p) anion, while the conduction band is formed by the Si([sp.sup.3]) cation. Silica glass has high transparency because the band gap is >9 eV. However, the new band, which has lower energy than the conduction band of the Si([sp.sup.3]) cation, is found in the LUMO of the carbonyl group in poly(MEPOSS). The transition whose energy is 5.5 eV occurs easily because the LUMO energy of the carbonyl group is lower than that of the Si([sp.sup.3]) cation.

According to the MO calculations, the Shomo and [[epsilon].sub.LUMO] of 6FDA-TeMPD were -9.216 and -1.806 eV, respectively, while those of poly(MEPOSS) were -8.037 and 1.445 eV, respectively, as shown in Fig. 5. The [[epsilon].sub.HOMO] of poly(MEPOSS) was higher than that of 6FDA-TeMPD, while the eLUMO of 6FDA-TeMPD was lower than that of poly(MEPOSS). Thus, the ABA-type triblock copolymers composed of 6FDA-TeMPD and poly(MEPOSS) had a new band between the HOMO of poly(MEPOSS) and the LUMO of 6FDA-TeMPD. The weak peak around 3.5 eV, which occurred in the ABA-type triblock copolymer with 43 mol% POSS content, affected that new band. This result indicates that the mixture ratio of poly(MEPOSS) and 6FDATeMPD affects the aggregation state.

The cohesiveness of the 6FDA-TeMPD-based polymers was confirmed by the fluorescence spectra. This behavior is well known with regard to the formation of charge transfer complexes (CTCs) or charge transfer (CT) interactions in 6FDATeMPD-based polymers [13], which are attributed to the presence of 7i electrons between the ring structures in polyimides. The CTC structure is similar to a sandwich structure between imides and the neighboring benzene ring. The CT interaction in relation to the extent of CTC production, strongly depends on the fluorescence spectrum. Figure 7 shows the fluorescence spectra of the ABA-type triblock copolymers. These polymers exhibited an emission spectra band at 470 nm after excitation at 325 nm, which is attributed to the interactions of the aromatic polyimide that contains an alternating sequence of electron-rich donor and electron-deficient acceptor molecules. Therefore, the CTC formation and polymer cohesiveness can be determined via the emission peak intensity.

The maximum peak emission wavelength was approximately 470 nm. Figure 7a shows that the maximum peak intensities of the 6FDA-TeMPD-based triblock copolymer decreased in relation to the poly(MEPOSS) amount. A weak emission peak around 400 nm was observed in poly(MEPOSS). It was estimated that this emission reflects the n-[pi]* charge transfer transition from the HOMO in the Si-0 site to the LUMO of the carbonyl group (Fig. 5). On the other hand, Fig. 7b shows the fluorescence spectra excited at 350 nm. This band is attributed to the weak absorption peak around 3.5 eV (Fig. 4). An emission spectra band at 470 nm, which affects the CT interaction, was observed in all 6FDA-TeMPD-based triblock copolymers (Fig. 7a). However, the 6FDA-TeMPD-based triblock copolymer with 43 mol% MEPOSS resulted in the most intense peak. The peak position slightly shifted to 480 nm, and the energy was lower than other 6FDA-TeMPD-based triblock copolymers. These changes are dependent on the transition between the HOMO in the Si-O site and the LUMO of the carbonyl group. A peak around 400 nm was also observed (Fig. 7a) in all ABA-type triblock copolymers, as well as in poly(MEPOSS). The CT interaction in MEPOSS is responsible affects this peak, indicating that MEPOSS is uniformly dispersed in the 6FDA-TeMPDbased triblock copolymers.

The wavelength dispersions of the refractive indices of the 6FDA-TeMPD-based triblock copolymers are shown in Fig. 8. As the wavelength increased, the refractive index decreased. Over the entire wavelength range, the ranking of the refractive index was 6FDA-TeMPD > 17 mol% POSS > 43 mol% POSS > 79 mol% POSS > poly(MEPOSS). These results indicate that the refractive index used in a communication network wavelength can be controlled by the MEPOSS content in all ABA-type triblock copolymers.

Figure 9 presents the MEPOSS content as a function of the refractive index at 589 nm ([n.sub.D]), which mainly uses a general refractive index. As shown in Fig. 9, the refractive index of the 6FDA-TeMPD-based triblock copolymers decreased linearly with an increase of the MEPOSS content, maintaining high transparency across the entire wavelength range. The refractive index ("), a macroscopic optical property and a function of microscopic polarizability ([alpha]), is based on the Lorentz-Lorenz equation and is represented as follows:

[n.sup.2]-1/[n.sup.2] + 2 = 4[pi]/3 [rho][N.sub.A]/M [alpha] = [R.sub.LL]/V. (1)

where [N.sub.A] is Avogadro's number, [rho] is the density, M is the molecular weight, [R.sub.LL] is the molar refraction ([cm.sup.3] [mol.sup.-1]), and V is the polymer molar volume ([cm.sup.3] [mol.sup.-1]). The refractive index depends on the molar polarization per polymer molar volume. Polarizability is a parameter that reflects the aeolotropies of the polymer chains. Generally, polyimides with an aromatic ring, such as 6FDA-TeMPD, have a high refractive index because of their high polarizability [14]. However, the refractive index of silica glass is 1.46 [15], which is lower than that of 6FDA-TeMPD because of its lower polarizability. The [n.sub.D] of the poly(MEPOSS) domain decreased linearly from 1.530 to 1.450 due to the presence of the Si[O.sub.2] group. These results indicate that the 6FDA-TeMPD-based ABA-type triblock copolymers prepared by the ATRP method are effective for controlling the refractive index while maintaining high transparency across the entire wavelength range.

CONCLUSIONS

ABA-type triblock copolymers with 6FDA-TeMPD and poly(MEPOSS) components were synthesized using ATRP. The effect of the structure on the solid and optical properties was systematically investigated in the polymer thin films, and their electronic states were also analyzed. We revealed that the refractive index of the 6FDA-TeMPD-based triblock copolymers could be easily controlled and decreased linearly with the increase of poly(MEPOSS) content. Additionally, the films mixed with 6FDA-TeMPD and poly(MEPOSS) maintained high transparency across the entire wavelength range because the poly(MEPOSS) domains had excellent dispersion into the 6FDATeMPD domains. However, the ABA-type triblock copolymer with 43 mol% poly(MEPOSS) displayed a new absorption band between the HOMO of poly(MEPOSS) and the LUMO of 6FDA-TeMPD, indicating that the mixture ratio of poly(MEPOSS) to 6FDA-TeMPD affected the electronic state in the ABA-type triblock copolymers.

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Shuichi Sato, (1) Masato Ichikawa, (2) Erika Suzuki, (2) Hironaga Matsumoto, (3) Kazukiyo Nagai (2)

(1) Department of Electrical and Electronic Engineering, Tokyo Denki University, 5 Senju-Asahi-cho, Adachi-ku, Tokyo 120-8551, Japan

(2) Department of Applied Chemistry, Meiji University, 1-1-1 Higashi-mita, Tama-ku, Kawasaki 214-8571, Japan

(3) Department of Electronics and Bioinformatics, Meiji University, 1-1-1 Higashi-mita, Tama-ku, Kawasaki 214-8571, Japan

Correspondence to: S. Sato, e-mail: s.sato@mail.dendai.ac.jp; or K. Nagai, e-mail: nagai@meiji.ac.jp nagai@shikon.meiji.ac.jp DOI 10.1002/pen.24498

Published online in Wiley Online Library (wileyonlinelibrary.com).

Caption: FIG. 1. Chemical structures of 6FDA-TeMPD, poly(MEPOSS). and 6FDA-TeMPD/MEPOSS ABA-type triblock copolymer.

Caption: FIG. 2. FT-IR spectra of 6FDA-TeMPD/MEPOSS ABA-triblock copolymer.

Caption: FIG. 3. [sup.1]H NMR spectra of 6FDA-TeMPD/MEPOSS ABA-triblock copolymer.

Caption: FIG. 4. Transmission spectra of 6FDA-TeMPD. poly(MEPOSS). 6FDA-TeMPD/MEPOSS ABA-triblock copolymer.

Caption: FIG. 5. The HOMO and LUMO compositions of the frontier molecular orbital for 6FDA-TeMPD (top) and poly(MEPOSS) (bottom). [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 6. Absorption spectra of (a) 6FDA-TeMPD and (b) poly(MEPOSS) calculated by PM3.

Caption: FIG. 7. Fluorescence spectra of 6FDA-TeMPD, poly(MEPOSS), and 6FDA-TeMPD/MEPOSS ABA-triblock copolymer at excitation wavelengths of (a) 325 nm and (b) 350 nm.

Caption: FIG. 8. Wavelength dispersion of the refractive index of 6FDA-TeMPD. poly(MEPOSS), and 6FDA-TeMPD/MEPOSS ABA-triblock copolymer.

Caption: FIG. 9. Refractive index as a function of POSS content. Data: 6FDATeMPD. poly(MEPOSS) and 6FDA-TeMPD/MEPOSS ABA-triblock copolymer.
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Author:Sato, Shuichi; Ichikawa, Masato; Suzuki, Erika; Matsumoto, Hironaga; Nagai, Kazukiyo
Publication:Polymer Engineering and Science
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Date:Nov 1, 2017
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