Synthesis and properties of polyimides containing hexaisobutyl-substituted T₈ cages in their main chains

Abstract

Polymerization of a para-substituted bis(3-aminopropyl)hexaisobutyl-substituted T₈ cage with pyromellitic dianhydride (PMDA) and 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) resulted in corresponding polyimides containing T₈ cages in the main chains, denoted as T₈/PMDA polyimide and T₈/6FDA polyimide, respectively. The optical transmittance of the yellow-colored freestanding film of T₈/PMDA polyimide was over 90% in the visible region between 780 and 475 nm with a film thickness of 0.1 mm and the absorption edge was observed at approximately 590 nm. On the other hand, the film of T₈/6FDA polyimide showed excellent transparency in the visible region with an excellent optical transparency of over 90% even at 360 nm. DSC analysis of both the polyimides showed no glass transition and melt behavior between room temperature and 400 °C. Contact angles of water for the films of T₈/PMDA polyimide and T₈/6FDA polyimide were 97 ± 2° and 96 ± 5°, respectively, and both are significantly higher than that of poly(pyromellitic dianhydride-oxydianiline) (PMDA-ODA) polyimide and comparable to that of polyethylene. Martens' hardness and coefficient of thermal expansion (CTE) of T₈/PMDA polyimide were significantly lower and higher than that of PMDA-ODA polyimide, respectively, and these values are comparable to those of polyethylene. The replacement of PMDA with 6FDA significantly improved Martens' hardness and CTE. The XRD analysis for the T₈-containing polyimides indicates a highly packed nanostructure of the hexaisobutyl-T₈ unit in the polymer films and the isobutyl-T₈ unit in T₈/6FDA polyimide has a more packed structure than that in T₈/PMDA polyimide.

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Introduction

Transparent polymer films with high thermal and mechanical stability have been widely sought for applications in prospective flexible substrates in electronic devices and micro-optical devices in the fields of displays, memory, lighting, solar cells, sensors, and waveguides. Among the available candidates, polyimide films are one of the most promising, owing to their outstanding thermal stability, good mechanical properties, flexibility, and chemical inertness.^1–3 Polyimide films, however, usually show yellow or brown color due to absorption characteristics in the visible region, caused by intra- and inter-molecular charge transfer (CT) interactions of the polymer backbones.^4–7 The CT interactions occur between electron-rich nitrogen atoms and electron-poor carbonyl groups, which are characteristic of the imide group. To resolve this color problem, alicyclic or fluorinated polyimides are usually employed to diminish the CT interaction.^2,8–14 Another disadvantage of polyimides is that the electronegativity of the carbonyl groups influences the susceptibility of hydrogen bonding with capable low-molecular-weight species, most pervasively environmental water.^15–18 The use of constituent monomers possessing aliphatic and fluorinated groups also results minimal affinity toward water.^8,13,19–21

Recently, “polymeric materials based on element-blocks” have been proposed as a new concept of hybrid polymeric materials, and we expect this to promote new research and to present new ideas for material design involving all elements in the periodic table.²² In particular, the use of caged silsesquioxanes, denoted as (RSiO_1.5)_n or labeled T_n cages as element-blocks has been demonstrated to be an efficient method for designing of element-block polymers.^23–39 Specifically, polymers incorporating the cage silsesquioxanes in the main chain are expected to possess significantly improved mechanical and thermal stability, because the volume fraction of the caged silsesquioxane units was highest.^32–37 Partially condensed disilanol silsesquioxanes are obtained by hydrolytic condensation of cyclohexyltrichlorosilane or partially hydrolyzed T₈-cages and used as monomers for polymerization.^40–44 Di- and tri-functional T₁₀ and T₁₂ cages are prepared by rearrangement of T₈ cages caused by F⁻ and employed to obtain soluble and processable polymers incorporating the caged silsesquioxanes in the main chain.^45–48 A soluble polymer containing caged silsesquioxnaes in its main chain is prepared in one-step by hydrolytic condensation of amino group containing organotrialkoxysilanes.⁴⁹ The use of double-decker-shaped phenyl-substituted silsesquioxanes (DDSQs) possessing precisely two reactive hydrosilane groups is the most successful approach to making linear hybrid polymers.^32–37 Introduction of DDSQ in the main chain of polymer backbones showed outstanding thermal stability and significantly increased in glass transition temperatures. However, the limitation of the phenyl substituent and the resulting mixture of cis and trans isomers in DDSQ inhibit the further development of hybrid polymer materials. Development of well-defined caged silsesquioxane monomers having two polymerizable functional groups would enable us to develop various types of polymeric materials based on T_n cages as element-blocks.

The alkyl-substituted T₈ cages are cube-octameric molecules with an inner inorganic silicon and oxygen framework, which is externally covered by flexible organic substituents. The isobutyl-substituted T₈ cages are regarded as organic–inorganic hybrid units, in which the isobutyl substitutes act as molecular level-softened segments and the inner T₈ caged framework provide improved mechanical and thermal properties to suppress their movement. In contrast to large number of reports on polymers containing the caged silsesquioxanes in the side chain, few papers have reported the synthesis of polymers with the caged silsesquioxanes in the main chain. These are prepared by a step polymerization system using well-defined caged silsesquioxane monomers having two polymerizable functional groups.^38,39 We have recently reported that a para-substituted bis(3-aminopropyl)hexaisobutyl-substituted T₈ cage was successfully synthesized via a selective corner-opening reaction of 3-aminopropylheptaisobutyl-substituted T₈ cage and a subsequent corner-capping reaction.³⁸ The key stage of this route is the corner-opening of the completely condensed 3-aminopropyl heptaisobutyl-substituted T₈ cage with aqueous tetraethylammonium hydroxide (TEAOH), affording trisilanol aminopropyl hexaisobutyl-substituted T₈ cage as the predominant product.^50,51 Our previous letter reported initial studies about polymerization of the T₈ monomer with pyromellitic dianhydride (PMDA) to obtain a yellow, free-standing film (Scheme 1).³⁸ In the present manuscript, polymerization of the T₈ monomer and 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) was performed to test the effect of a different imide linker structure (Scheme 2). We found that the replacement of PMDA with 6FDA resulted in colorless transparent films with high thermal stability, and their mechanical, thermal, and optical properties can be tuned by changing the structure of the linking imide segments.

Scheme 1 Synthesis of T₈/PMDA polyimide.

Scheme 2 Synthesis of T₈/6FDA polyimide.

Experimental

Materials

All the solvents and chemicals used were of reagent-grade quality and were used without further purification. All reactions were performed under a nitrogen atmosphere. Pyromellitic dianhydride (PMDA) was purchased from Nacalai Tesque (Kyoto, Japan). 4,4′-(Hexafluoroisopropylidene)diphthalic anhydride (6FDA) was purchased from Wako Pure Chemical Industries, Ltd. Low-density polyethylene (LDPE) with a melt index of 25 g/10 min (190 °C/2.16 kg) and high density polyethylene (HDPE) with a melt index of 12 g/10 min (190 °C/2.16 kg) were purchased from Sigma-Aldrich Japan (Tokyo, Japan). Poly(pyromellitic dianhydride-oxydianiline) (PMDA-ODA polyimide) was prepared by heated of a cast film of poly(pyromellitic dianhydride-co-4,4′-oxydianiline), amic acid solution (12.0 wt% ± 0.5 wt% (80% NMP/20% xylene)), purchased from Sigma-Aldrich Japan (Tokyo, Japan), at 220 °C for 30 min on a Teflon sheet. A para-substituted bis(3-aminopropyl)hexaisobutyl-substituted T₈ cage (1) was prepared according to our previous report.³⁸

Polyimides synthesis

T₈/PMDA polyimide. To prepare the T₈/PMDA polyimide precursor solution, a mixture of 1 (0.500 g, 0.571 mmol) and PMDA (0.126 g, 0.578 mmol) in 1-methyl-2-pyrrolidione (NMP) (5 mL) was stirred at room temperature for 24 h. FT-IR (KBr): ν = 1112, 1230, 1638, 1719, 2955 cm⁻¹. ¹H-NMR (CDCl₃): δ 0.55–0.64 (m, CH₂–Si, 16H), 0.89–1.02 (m, –CH₃, 36H), 1.82–1.90 (m, –CH₂CH(CH₃)₂, CH₂CH₂CH₂NH₂, 10H), 8.31–8.44 (b, ArH, 2H).

The T₈/PMDA polyimide precursor solution was cast on a Teflon sheet and heated at 220 °C for 30 min to result in a yellow transparent film. FT-IR (KBr): ν = 1090, 1364, 1391, 1721, 1773, 2953 cm⁻¹.

T₈/6FDA polyimide. To prepare the T₈/6FDA polyimide precursor solution, a mixture of 1 (0.119 g, 0.136 mmol) and 6FDA (0.0615 g, 0.136 mmol) in a mixed solvent (toluene/N,N-dimethylacetamide (DMAc), 1/2, v/v) (3 mL) was stirred at room temperature for 1 h. FT-IR (KBr): ν = 1119, 1229, 1655, 1719, 2955 cm⁻¹. ¹H-NMR (CDCl₃): δ 0.52–0.73 (m, CH₂–Si, 16H), 0.80–1.08 (m, –CH₃, 36H), 1.72–1.96 (m, –CH₂CH(CH₃)₂, CH₂CH₂CH₂NH₂, 10H), 3.39–3.57 (b, –CH₂NH, 4H), 7.39–8.19 (b, ArH, 6H).

The reaction solution was cast on a Teflon sheet and heated at 220 °C for 30 min to result in a colorless transparent film. FT-IR (KBr): ν = 1094, 1209, 1366, 1392, 1716, 1778, 2953 cm⁻¹.

Instrumentation and characterization details

¹H-(400 MHz), ¹³C-(100 MHz), and ²⁹Si-(80 MHz) NMR spectra were obtained using a BRUKER DPX-400 instrument (Bruker Biospin, Rheinstetten, Germany). Fourier transform infrared (FTIR) spectra were obtained using a Shimadzu IRAffinity-IS with ATR MIRacle 10. Gel permeation chromatography (GPC) was performed using a Shimadzu LC-6AD (Shimadzu, Kyoto, Japan) with a Shodex KF-803 column using THF as an eluent. UV-vis spectra were obtained using a JASCO spectrophotometer V-670 KKN (JASCO, Tokyo, Japan). Differential scanning calorimetry (DSC) data were obtained using a Shimadzu DSC-60 calorimeter (Shimadzu, Kyoto, Japan) at a heating rate of 10 °C min⁻¹ in N₂ flow for the whole temperature range. DSC curves were recorded upon the second heating scan. Thermogravimetric analysis (TGA) was performed using a Shimadzu TGA-50H thermogravimetric analyzer (Shimadzu, Kyoto, Japan) at a heating rate of 10 °C min⁻¹ in N₂ flow. The samples were heating at 300 °C for 10 min by an oven in N₂ flow before the TGA measurements. Coefficient of thermal expansion (CTE) measurement of the film was performed on Shimadzu TMA-60 using an expansion probe. These samples had a size of 13 × 5 mm. The samples were mounted on the TMA and heated from room temperature to 350 °C at a heating rate of 10 °C min⁻¹ under 2 g load. After the heating, sample was cooled to room temperature and we measured the CTE. Martens' hardness measurement was performed on Shimadzu DUH-211. A load of 10 mN was applied for 5 seconds. The crosshead speed of the indenter during loading was 1 mN s⁻¹. Density data were obtained using an AccuPyc II 1340 gas displacement pycnometry system (Micromeritics Instrument Corporation, Norcross, USA). Sample (0.0645 g) was placed in 0.1 cm³ chamber. Measurement pressure of He at 135 kPa at room temperature was applied for measurement. Powder X-ray diffractometry (XRD) studies were performed on a Rigaku Smartlab X-ray diffractometer with Cu Kα radiation (λ = 1.5406 Å) in the 2θ/θ mode at room temperature. The 2θ scan data were collected at 0.01° intervals and the scan speed was 5° (2θ) min⁻¹. Water contact angles were measured with a high-resolution digital camera (DS-Fi7, Nikon), at 20 °C using water as the probe fluid (1.5 μL). Each contact angle reported was an average value of at least four independent measurements. Dynamic viscoelasticity of the specimens was measured with a RSA-G2 (TA instrument) using tension mode. Imposed strain amplitude was 0.04% within the linear elasticity regime, and the frequency was 1 Hz. Frequency was varied from 0.1 to 10 Hz, but no appreciable dependence of dynamic modulus on frequency was observed (in at least 25 °C).

Results and discussion

Synthesis and characterization of polyimides

The polymerization and film formation of two types of polyimides are depicted in Schemes 1 and 2. When PMDA was used as a co-monomer, 1 was polymerized with PMDA at room temperature in NMP for 24 h (Scheme 1). Although 1 was dispersed in NMP, the polymerization solution became a homogeneous solution during the reaction. To confirm the formation of T₈/PMDA poly(amic acid), one drop of the reaction mixture was collected and the solvent was removed under reduced pressure. The FT-IR analysis of the resulting residues shows peaks characteristic of the amide structure at 1638 cm⁻¹ (C=O stretching) and the carboxylic acid at 1719 cm⁻¹ (C=O stretching), as well as the asymmetric stretching of the Si–O–Si of the cage framework at 1112 cm⁻¹ (ESI†). GPC analysis of the soluble part of T₈/PMDA poly(amic acid) in THF suggests polymer formation, with number average and weight average molecular weights (polystyrene standards) of 12 100 and 29 700, respectively (see ESI†). One drop of the reaction mixture was added to CDCl₃ and the ¹H-NMR spectrum was measured to confirm the reaction. The ¹H-NMR spectrum of the reaction mixture showed the disappearance of α-methylene and β-methylene protons of the aminopropyl groups in 1 at 2.67 and 1.56 ppm and the appearance of new peaks corresponding to the α-methylene and β-methylene protons at 3.44 and 1.84 ppm, suggesting the formation of T₈/PMDA poly(amic acid) (see ESI†).

The polymerization solution of T₈/PMDA poly(amic acid) was then cast on a Teflon sheet and heated at 220 °C for 30 min to obtain a yellow, free-standing film with high transparency. The obtained sample was insoluble in common organic solvents such as chloroform, THF, acetone, and DMF. FT-IR analysis of the film sample shows peaks characteristic of the imide structure at 1364 cm⁻¹ (C–N stretching) and 1721 and 1773 cm⁻¹ (C=O stretching) as well as the asymmetric stretching of the Si–O–Si of the cage framework at 1090 cm⁻¹, suggesting that no decomposition of the T₈ caged structure occurred and T₈/PMDA polyimide was obtained (Fig. 1a). No peaks characteristic of the primary amine at 3433 cm⁻¹ and the carboxylic acid or anhydride structures suggest formation of a relatively high-molecular-weight polymer.

Fig. 1 FT-IR spectra of T₈/PMDA polyimide and T₈/6FDA polyimide.

Because the polymerization solution of 1 with 6FDA was hardly soluble in NMP, 1 was polymerized with 6FDA at room temperature in a mixed solvent (toluene/DMAc, 1/2, v/v) for 1 h. To confirm the formation of T₈/6FDA poly(amic acid), one drop of the reaction mixture was collected and the solvent was removed under reduced pressure. The FT-IR analysis of the resulting residues shows peaks characteristic of the amide structure at 1655 cm⁻¹ (C=O stretching) and the carboxylic acid at 1719 cm⁻¹ (C=O stretching), as well as the asymmetric stretching of the Si–O–Si of the cage framework at 1119 cm⁻¹ (ESI†). The ¹H-NMR spectrum of the soluble part of the resulting residues in CDCl₃ showed the disappearance of α-methylene and β-methylene protons of the aminopropyl groups in 1 at 2.67 and 1.56 ppm and the appearance of new peaks corresponding to the α-methylene protons at 3.44, indicating the formation of T₈/6FDA poly(amic acid) (see ESI†). GPC analysis of the soluble part of T₈/6FDA poly(amic acid) in THF suggests polymer formation, with number average and weight average molecular weights (polystyrene standards) of 3100 and 52 000, respectively (see ESI†).

The solution of T₈/6FDA poly(amic acid) was then cast on a Teflon sheet and heated at 220 °C for 30 min to obtain a colorless, free-standing film with high transparency. The obtained sample was insoluble in common organic solvents such as chloroform, THF, acetone, and DMF. FT-IR analysis of the film shows peaks characteristic of the imide structure at 1368 cm⁻¹ (C–N stretching), 1718 and 1778 cm⁻¹ (C=O stretching), and CF₃ at around 1200 cm⁻¹, as well as the asymmetric stretching of the Si–O–Si of the cage framework at 1093 cm⁻¹, suggesting that there was no decomposition of the T₈ caged structure and the polyimide with the T₈ caged unit in the main chain was obtained (Fig. 1b). Again no peaks characteristic of the primary amine at 3433 cm⁻¹ and the carboxylic acid or anhydride structures suggest the formation of a relatively high-molecular-weight polymer.

Properties of the polyimides

The optical transmittance of the yellow-colored film of T₈/PMDA polyimide was over 90% in the visible region between 780 and 475 nm with a film thickness of 0.1 mm (Fig. 2).³⁸ The absorption edge was observed at approximately 590 nm. This may be due to the intermolecular charge-transfer (CT) of the imide units.^4–7,52 On the other hand, the film of T₈/6FDA polyimide showed excellent transparency in the visible region, due to avoid CT interaction of the 6FDA units.^8–14 The polymer exhibits excellent optical transparency of over 80% transmittance even at 360 nm, with low cutoff wavelength (319 nm).

Fig. 2 UV-vis transmittance spectra of the films obtained from T₈/PMDA polyimide³⁸ and T₈/6FDA polyimide on soda lime glasses. The thickness was about 0.1 mm. The insert shows the appearance of the films.

The samples were heating at 300 °C for 10 min by an oven in N₂ flow before the TGA measurements. T₈/PMDA polyimide³⁸ and T₈/6FDA polyimide under N₂ showed 5% weight losses at 505 °C and 483 °C, respectively (Fig. 3). The weight loss of T₈/PMDA polyimide and T₈/6FDA polyimide between 400 and 600 °C was 26 wt% and 29 wt%, respectively. DSC analysis of both the polyimides showed no glass transition and melt behavior between room temperature and 400 °C (Fig. 4). The dynamic Young's modulus (E′) of T₈/PMDA polyimide film at 25 °C was 2.4 × 10⁹ Pa, which is comparable to that of typical amorphous polymers in the glassy state.⁵³ This result suggests that T₈/PMDA polyimide film was in glassy state at between room temperature and 400 °C. Generally, relative lower glass transition temperatures around 300 °C or less are observed for the aliphatic polyimides, especially using 6FDA.^54,55 However, both the present T₈-containing polyimides significantly increase glass transition temperature.

Fig. 3 TGA thermograms of the films of T₈/PMDA polyimide and T₈/6FDA polyimide at a heating rate of 10 °C min⁻¹ in N₂ flow.

Fig. 4 DSC curves of T₈/PMDA polyimide and T₈/6FDA polyimide at a heating rate of 10 °C min⁻¹ in N₂ flow for the whole temperature range.

Thermal properties of the polyimide films were also estimated by thermomechanical analysis (TMA) (Fig. 5). The TMA slopes for both the polyimide films were proportion to the temperature in the whole range of the experiments. In the case of T₈/PMDA polyimide, no inflection point was observed between room temperature and 350 °C, suggesting no glass transition. This observation agrees with the DSC analysis, which showed no glass transition and melt behavior between room temperature and 400 °C. The decrease in the TMA value for T₈/6FDA polyimide observed over 300 °C, which may be due to volumetric shrinkage of the film. The coefficient of thermal expansion (CTE) was determined from the slopes of the plot between thermal expansion and temperature. CTE for T₈/PMDA polyimide is 18.2 × 10⁻⁵ K⁻¹, which is higher than that of PMDA/ODA polyimide and comparable to that of HDPE (Table 1). CTE for T₈/6FDA polyimide is 8.13 × 10⁻⁵ K⁻¹, which is lower than that of T₈/PMDA polyimide.

Fig. 5 TMA measurements of T₈/PMDA polyimide and T₈/6FDA polyimide at a heating rate of 10 °C min⁻¹ under 2 g load.

Table 1 Hardness, CTE, density, and contact angle of the polyimides

	Martens' hardness (N mm^−2)	CTE (K)	Density (d) (g cm^−3)	Contact angle (°)
T8/PMDA polyimide	37.8 ± 0.2	18.2 × 10^−5	1.24 ± 0.00	97 ± 2
T8/6FDA polyimide	140.5 ± 19.1	8.13 × 10^−5	1.12 ± 0.00	96 ± 5
PMDA-ODA polyimide	219.7 ± 0.5	5.0 × 10^−5	1.48 ± 0.01	76 ± 2
HDPE	42.1 ± 6.2	22.2 × 10^−5	0.946 ± 0.002	—
LDPE	22.1 ± 1.0	45.8 × 10^−5	0.919 ± 0.001	—

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Martens' hardness measurement was performed for the T₈-containing polyimides, PMDA-ODA polyimide, HDPE, and LDPE (Table 1). The Martens' hardness of T₈/PMDA polyimide was significantly lower than that of PMDA-ODA polyimide and comparable to that of HDPE. On the other hand, that of T₈/6FDA polyimide was approximately 4 times higher than that of T₈/PMDA polyimide. However, the value was still lower than that of PMDA-ODA polyimide.

Martens' hardness and CTE value of T₈/PMDA polyimide were comparable to those of HDPE. However, T_g of T₈/PMDA polyimide is significantly higher than that of HDPE. Polyethylene is a thermoplastic material, and its elastomeric properties depend on the crystallinity of the polymer. The crystallinity and its structure of the thermoplastic material significantly affect its physical property. In addition, the relationship between the macroscopic mechanical behavior and structure on the micro- and nanometer scale is still a topic of debate. Introduction of the T₈-unit in the polymer main chain is promising way to secure amorphous character as well as good thermal properties. The isobutyl-substituted T₈ cages are types of organic–inorganic hybrid units, in which the isobutyl substitutes act as molecular level-softened segments and the T₈ cage provide improvement of mechanical and thermal properties due to suppress their movements by their bulky and relative high-molecular-weight frameworks.

Contact angles of water for the films of T₈/PMDA polyimide and T₈/6FDA polyimide were 97 ± 2° and 96 ± 5°, respectively (Fig. 6). These are significantly higher than that of PMDA-ODA polyimide (76 ± 2°). Both the T₈-containing polyimides exhibited the same contact angles, suggesting hydrophobic character of the isobutyl-substituted T₈ cages determines the high contact angles. These observations suggest that introduction of the T₈-unit in the main chain significantly improved hydrophobicity.

Fig. 6 Photographs of 1.5 μL water droplets on the films of T₈/PMDA polyimide, T₈/6FDA polyimide, and PMDA-ODA polyimide.

Density data were obtained by gas displacement pycnometry. Densities (d) of T₈/PMDA polyimide and T₈/6FDA polyimide were 1.24 g cm⁻³ and 1.12 g cm⁻³, respectively, which are lower than that of PMDA-ODA polyimide (1.48 g cm⁻³). Density of octaisobutyl-substituted T₈ cage is 1.248 g cm⁻³ according to crystal packing.⁵⁶ High volume fraction of the T₈ cages in the present polyimides decreased their densities in comparison with common polyimides.

The XRD patterns of both the T₈-containing polyimides show broad peaks centered near a 2θ value of 20°, indicating amorphous polymers (Fig. 7). T₈/PMDA polyimide³⁸ and T₈/6FDA polyimide also show broad diffraction peaks at 2θ values of 7.4 and 8.3°, respectively. The spacings for T₈/PMDA polyimide and T₈/6FDA polyimide evaluated from the peak positions were 1.2 nm and 1.1 nm, respectively. Diffraction peaks of mono-functional heptaisobutyl-T₈ cages appeared at 2θ = 8.2, 11.0, 12.2, and 19.1°, and were associated with the hexagonal crystalline structure of the T₈ cage.⁵⁷ The present diffraction peaks at 2θ values of 7.4 and 8.3° for the T₈-containing polyimides indicate that regular repeating distances are consistent with the dimensions of the isobutyl-T₈ cage. The lower 2θ value for T₈/6FDA polyimide suggests that the isobutyl-T₈ unit in T₈/6FDA polyimide has more packed structure than that in T₈/PMDA polyimide. Martens' hardness of T₈/6FDA polyimide was approximately 4 times higher than that of T₈/PMDA polyimide and CTE of the former was more than 2 times lower than that of the later. Replacement of the imide linker units significantly affects the mechanical properties of the T₈-containing polyimides.

Fig. 7 XRD traces of T₈/PMDA polyimide³⁸ and T₈/6FDA polyimide at room temperature.

Conclusions

We prepared two types of polyimides containing T₈ cages in the main chains, denoted as T₈/PMDA polyimide and T₈/6FDA polyimide, by polymerization of para-substituted bis(3-aminopropyl)hexaisobutyl-substituted T₈ cage (1) with PMDA and 6FDA, respectively. Both the polyimides show no glass transition and melt behavior between room temperature and 400 °C and highly hydrophobic character. Although the transparent free-standing film of T₈/PMDA polyimide showed yellow color, the film of T₈/6FDA polyimide showed excellent transparency in the visible region. Generally, introducing fluorinated substituents such as 6FDA has been recognized as one of the most promising methods to synthesize colorless and hydrophobic polyimides. However, these types of polyimides often reduce T_g.^54,55 Our observations suggest that introduction of the T₈-unit in the main chain significantly improved hydrophobicity as well as increased the T_g.

Martens' hardness and coefficient of thermal expansion (CTE) of T₈/PMDA polyimide were significantly lower and higher than that of PMDA-ODA polyimide, respectively, and these values are comparable to those of HDPE. The replacement of PMDA with 6FDA significantly improved Martens' hardness and CTE, which may be due to higher packed structure of the isobutyl-T₈ unit in T₈/6FDA polyimide than that in T₈/PMDA polyimide. These results suggest that physical properties of polyimides containing hexaisobutyl-substituted T₈ cages in their main chains can be tuned by changing their linkage structures without decreasing their T_g. We also found that the di-functional T₈ cages significantly reduce their crystallinity in comparison with those of mono-functionalized T₈ cages. Polymers incorporating T₈ cages in the main chain are expected to develop various types of polymeric materials based on T₈ cages as element-blocks.

Acknowledgements

This study is a part of a Grant-in-Aid for Scientific Research on Innovative Areas “New Polymeric Materials Based on Element-Blocks (No. 2401)” (24102003) of The Ministry of Education, Culture, Sports, Science, and Technology, Japan. We thank Shimadzu co. for measuring thermal and mechanical analyses. We also thank Professor Q. Tran-Cong-Miyata of Kyoto Institute of Technology for measurement of contact angles. We also thank Professor Kenji Urayama of Kyoto Institute of Technology for measurement of dynamic viscoelasticity.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Characterization data for T₈/PMDA poly(amic acid) and T₈/6FDA poly(amic acid). See DOI: 10.1039/c6ra04860a

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