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Highly transparent triethoxysilane-terminated copolyimide and its SiO2 composite with enhanced thermal stability and reduced thermal expansion
European Polymer Journal 64 (2015) 206–214
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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Highly transparent triethoxysilane-terminated copolyimide and its SiO2 composite with enhanced thermal stability and reduced thermal expansion Seong-Ku Kim a, Xingyuan Wang a, Shinji Ando b,⇑, Xiaogong Wang a,⇑ a b
Department of Chemical Engineering, Laboratory of Advanced Materials (MOE), Tsinghua University, Beijing 100084, PR China Department of Chemistry & Materials Science, Tokyo Institute of Technology, Ookayama 2-12-1-E4-5, Meguro-ku, Tokyo 152-8552, Japan
a r t i c l e
i n f o
Article history: Received 30 October 2014 Received in revised form 30 December 2014 Accepted 11 January 2015 Available online 17 January 2015 Keywords: Transparent polyimide Thermal stable polyimide Hybrid PI/silica composite Sol–gel method Organic–inorganic composite
a b s t r a c t A highly transparent and colorless copolyimide was synthesized to contain electron withdrawing sulfone and fluorine groups. Then, the triethoxysilane terminal groups were introduced by reacting the anhydride end groups of the polymer with (3-aminopropyl)triethoxysilane. Based on the triethoxysilane-terminated copolyimide, a series of composites of the copolyimide (PI6FDA-TFDB-mBAPS(Si)) with inorganic SiO2 network was successfully fabricated in order to satisfy the requirements for transparent insulators with high thermal stability. Because the hybrid with inorganic SiO2 networks, the composite (PI6FDA-TFDB-mBAPS(Si)_SiO2) can complementarily improve the thermal resistant drawbacks of organic systems. As a result, the composite films exhibit small values of the coefficient of thermal expansion (CTE) with the increasing SiO2 content. The coefficient of thermal expansion (CTE) of the hybrid PI6FDA-TFDB-mBAPS(Si)_SiO2-40% film is 14.4 ppm °C1 in the temperature range from 80 to 170 °C. Compared to a value of 34.7 ppm °C1 for the pristine PI, a 57% reduction is achieved for the composite. The composite possesses the glass transition temperature (Tg) of 276.0 °C and shows good optical transmittance of 94% at the wavelength of 450 nm. The incorporation of cross-linked SiO2 networks by using the triethoxysilane-terminated PI preserves the high transparency and enhances the thermal stability for the composite films. Due to these optimized properties, the composite can show potential applications in the micro-flexible transparent electronic devices such as transparent insulating panel of advanced transparent display devices and transparent flexible circuit board. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Since it was reported by Borgert and Renshaw [1], and Sroog et al. [2], polyimides (PIs) have received considerable attention for applications in many fields, such as gas separation membrane [3–5], and nano-material technology [6,7]. PIs show some unique and outstanding properties compared with conventional polymers, such as their inherent toughness and flexibility, low density, remarkable ther⇑ Corresponding authors. Tel.: +86 10 62784561; fax: +86 10 62770304. E-mail address: [email protected] (X. Wang). http://dx.doi.org/10.1016/j.eurpolymj.2015.01.012 0014-3057/Ó 2015 Elsevier Ltd. All rights reserved.
mal stability, radiation resistance and mechanical strength [8,9]. With the rapid development of electronic industry, PIs have become necessary key materials in this area for fabricating integrated circuits and other devices. Some recently emerging applications of PIs include the advanced flexible display [9,10] and flexible electronic devices [11– 13]. However, some obstacles for these new applications have also been noticed at the same time. A relatively higher value of the coefficient of thermal expansion (CTE) of PIs can cause serious reliability problems, such as bending, curling, cracking and delamination, when used in multi-layer-structured devices [14,15]. In some cases,
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thermal stability of PIs also needs to be improved for the applications. Inorganic materials exhibit excellent thermal stability and high modulus [16]. Therefore, forming hybrid polyimide composites with inorganic materials is a promising approach to obtain balanced properties by exploiting the advantages of both organic and inorganic materials [17– 21]. In recent years, researches have been carried out with great interest on the organic–inorganic PI composites [21], such as organic-silica or other composite materials [22–24,26–32]. Since a large amount of thin films are required in microelectronics and photonic applications, an efficient process to obtain high-quality composite films are always required. In addition, the thermodynamic immiscibility between organic and inorganic materials may lead to phase separation in composite films [26– 30]. Generally speaking, compatibility can be enhanced by a variety of methods, such as functionalizing polymer chains at their ends, selecting appropriate groups of polymers within the repeat units, adding a coupling agent to bond PI chains and inorganic network [17,25]. However, it is still a challenge to homogeneously disperse inorganic phase in polymer matrices in a highly efficient and controllable way. Most of the PI/silica composites are prepared by a two-stage process [22–24,26–30]. In the process, polyamic acids (PAAs) as precursors of PIs are first synthesized; then hydrolyzed tetraethoxysilane (TEOS) is added to perform so-called ‘sol–gel’ process. The advantage of this procedure is that uncyclized carboxyl groups can form hydrogen bonds with the acid hydrolyzed Si(OH)4 solution to disperse the inorganic phase homogeneously within the polymer matrix. However, a large amount of water released from the cyclization may scissor molecular chains and deteriorate the properties of films. In addition, most of the PI/silica composites are opaque because of the light scattering and opacity caused by aggregated inorganic silica phase in wavelength scale or above [33]. Therefore, there are some limitations for their applications as transparent insulator materials for flexible display or transparent electronic device fields. As an ultimate goal, ideal transparent insulator materials should not only possess high transparent properties, but also show good mechanical properties and high thermal stability. In this study, we synthesized a highly transparent and colorless copolyimide terminated with triethoxysilane functional groups (PI6FDA-TFDB-mBAPS(Si)). The PI was used as a precursor to prepare composites with inorganic silica via ‘sol–gel’ method. In order to improve the optical transparency, we chose three kinds of functional monomers containing ASO2A, ACF3, kink and distorted structures for the copolymer backbone, which were used to prevent the formation of the charge-transfer (CT) complex. The triethoxysilane-terminated PI was designed to react with Si(OH)4 to achieve a homogeneous dispersion of the inorganic phase in PI matrix [34,35]. It was proved to be an efficient way for this purpose and showed ability to optimize other properties at the same time. The preparation, structures and properties of the series of hybrid composites are presented as follows.
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2. Experimental 2.1. Materials 4,40 -(Hexafluoroisopropylidene)diphthalic anhydride (6FDA, 95%), (3-amino-propyl)triethoxysilane (APTEOS, 98%), and 2,20 -bis(trifluoromethyl)benzidine (TFDB, 98%) were purchased from Adamas-Reagent Co., Ltd. 4,40 -Bis(3amino-phenoxy)diphenylsulfone (mBAPS, 98%) was purchased from Tokyo Chemical Industry Co., Ltd. (TCI). The solvent N-methyl-2-pyrrolidone (NMP, 97%) was purchased from Beijing Modern East Fine Chemical. Hydrochloric acid (HCl) and tetraethoxysilane (TEOS, 98%) were purchased from Alfa Aesar. If it is not mentioned specifically, the reactants and solvents were used as received without further purification. 2.2. Measurements Fourier transform infrared (FTIR) spectroscopic measurements were performed using a Magna-IR Nicolet 560 FTIR spectrophotometer by incorporating samples in KBr disks. The spectra were recorded in the range of 4000– 450 cm1 at a resolution of 0.35 cm1. The phase transitions and thermal properties were characterized by differential scanning calorimeter (DSC), thermal gravimetric analysis (TGA) and dynamic mechanical analysis (DMA). Thermal phase transitions of the polymers were scanned by DSC 2910 (TA Instrument Co.) with a heating rate of 20 °C min1 under nitrogen atmosphere flow. Thermal stability was measured with a TGA 2050 thermo gravimetric analyzer (TA instrument Co.) at a heating rate of 20 °C from room temperature to 900 °C under a continuous flow of nitrogen. Dynamic mechanical analysis (DMA) measurements were performed using a DMA Q800-Dynamic Mechanical Analyzer (TA Instrument Co.) in the tension mode over a temperature range from 25 °C. Data acquisition and analysis of the storage modulus (E0 ) and loss tangent (tan d) were recorded automatically by the system. The samples used for the measurements were 14 mm in length, 5 mm in width, and 22– 45 lm in thickness. The heating rate and frequency were fixed at 2 °C min1 and 1 Hz, respectively. The coefficients of thermal expansion (CTE) parallel to the film surfaces were measured using a DMA Q800 dynamic mechanical analyzer (TA Instrument Co.) in extension mode over a temperature range from 25 to 320 °C with a force of 0.01 N. The samples used for the measurements were 14 mm in length, 5 mm in width, and 21–43 lm in thickness. UV–visible absorption spectra of films were measured on a Lambda Bio-40 spectrometer (Perkin–Elmer). The samples used for the measurements were solid thin films with thickness about the 10 lm. Cross-sectional images of the polyimide composite films were studied by scanning electron microscopy (SEM). The SEM images were obtained using a S4700I microscope (Hitachi) operating at an acceleration voltage of 15.0 kV. 2.3. Synthesis process The synthetic routes of the materials are shown in Schemes 1 and 2, which include the synthesis of
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PAA6FDA-TFDB-mBAPS(Si) (Scheme 1) and its composite with silica prepared by a typical ‘sol–gel’ method (Scheme 2). The polyamic acid (PAA) was firstly synthesized at 0 °C. Then, APTEOS was added into the PAA solution to obtain the PAA with triethoxysilane-terminal groups. The composites were prepared to contain SiO2-10–40% (TEOS) and heated through several stages to the required high temperature and dehydrated to obtain the composite films. A PI without the triethoxysilane-terminal groups was synthesized for comparison, which is referred to as the pristine PI (PI6FDA-TFDB-mBAPS). 2.3.1. Polyamic acid and pristine PI 2,20 -Bis(trifluoromethyl)benzidine (TFDB, 3.76 g, 11.5 mmol) and 4,40 -bis(3-amino-phenoxy)diphenylsulfone (mBAPS, 3.68 g, 11.5 mmol) as the diamine monomers
were dissolved in NMP (20 wt%) and cooled with the ice water bath. After completely dissolved, 4,40 -(hexafluoroisopropylidene)diphthalic anhydride (6FDA, 10 g, 23 mmol) as the anhydride monomer was added into the above solution under stirring for 12 h. The molar ratio of 6FDA:TFDB:mBAPS in above reaction was 5:2.5:2.5. The reaction composite was stirred at room temperature for 6 h to obtain the pristine PAA (PAA6FDA-TFDB-mBAPS). In the second stage, the PAA6FDA-TFDB-mBAPS precursor solution was casted on the glass substrate and heated at 80 °C for 2 h. The film was then thermally imidized by step-wise heating at 150 °C (1 h), 200 °C (1 h), and 300 °C (1 h) to afford the PI6FDA-TFDB-mBAPS film. The film was separated from substrate by ultrasonication in the hot deionized water, and the delaminated film was dried at 60 °C for 3 h under vacuum condition.
Scheme 1. Synthetic route for the PAA6FDA-TFDB-mBAPS(Si).
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A series of composite films with SiO2 contents of 10%, 20%, 30%, 40% was obtained by similar procedure. The composites are named as PI6FDA-TFDB-mBAPS(Si)_SiO2-10–40%, where the percentage given in the genetic abbreviations is the weight percentage. The FTIR results of one representative composite are given below. FTIR of PI6FDA-TFDB-mBAPS(Si)_SiO2-40% (KBr, cm1): 3600– 3400 cm1 (SiAOH); 2966 cm1 (CACH2AC); 1782 cm1 (C@O sym. str.); 1720 cm1 (C@O asym. str.); 1587 cm1 (C@C str. Ar.); 1475 cm1 (CACH2AC); 1445 cm1 (C@C str. Ar.); 1364 cm1 (CANAC str. imide.); 1302 cm1 (CASO2AC str. imide.); 1232 cm1 (Ar.AOAAr.); 1125 cm1 (ACF3); 1096–1067 cm1 (SiAOASi); 964, 835 cm1 (Ar.AH); 778, 745 cm1 (subst. Ar.); and 1660– 1670 cm1 (non PAA structure band).
3. Results and discussion Scheme 2. Synthetic route for the PI6FDA-TFDB-mBAPS(Si)_SiO2 composite film.
1
1
FTIR of PI6FDA-TFDB-mBAPS (KBr, cm ): 1782 cm (C@O sym. str.); 1720 cm1 (C@O asym. str.); 1587 cm1 (C@C str. Ar.); 1445 cm1 (C@C str. Ar.); 1364 cm1 (CANAC str. imide.); 1302 cm1 (CASO2AC str. imide.); 1232 cm1 (Ar.AOAAr.); 1125 cm1 (ACF3); 964, 835 cm1 (Ar.AH); 778, 745 cm1 (subst. Ar.); and 1660–1670 cm1 (non PAA structure band). 2.3.2. Polyimide/SiO2 composite film (PI6FDA-TFDB-mBAPS(Si)_SiO2) 2,20 -Bis(trifluoromethyl)benzidine (TFDB, 2.88 g, 9 mmol) and 4,40 -bis(3-amino-phenoxy)diphenylsulfone (mBAPS, 3.89 g, 9 mmol) as the diamine monomers were dissolved in NMP (20 wt%) and cooled with the ice water bath. After completely dissolved, 4,40 -(hexafluoroisopropylidene)diphthalic anhydride (6FDA, 10 g, 22.5 mmol) as the anhydride monomer was added into the above solution under stirring for 12 h. Then, (3-aminopropyl)triethoxysilane (APTEOS, 1.99 g, 9 mmol) was slowly dropped into the mixture, the molar ratio of 6FDA: TFDB:mBAPS:APTEOS was 5:2:2:2. The reaction composite was stirred at room temperature for 6 h to obtain the PAA with triethoxysilane-terminated groups (PAA6FDA-TFDB-mBAPS(Si)). The composite film containing 40 wt% SiO2 was given here as a typical example. Stoichiometric quantities of tetraethoxysilane (TEOS, 1.39 g), and hydrochloric acid in deionized water (0.1 N HCl, 0.48 g) were added into the PAA6FDA-TFDB-mBAPS(Si) solution (40 wt%) by droplets under stirring. The mixture was then stirred at room temperature for 6 h to obtain the PAA6FDA-TFDB-mBAPS(Si)_SiO2-40% precursor. In the second stage, the PAA6FDA-TFDB-mBAPS(Si)_SiO240% precursor solution was casted on the glass substrate, heated to 80 °C and kept at the temperature for 2 h. The film was then thermally imidized by stepwise heating at 150 °C (1 h), 200 °C (1 h), and 300 °C (1 h) to afford the PI6FDA-TFDB-mBAPS(Si)_SiO2-40% composite film. The composite film was separated from substrate by ultrasonication in the hot deionized water, and the delaminated film was dried at 60 °C for 3 h under vacuum condition.
3.1. Synthesis and characterization A copolyimide and series of composite films were prepared in this study. To facilitate the discussion, a genetic abbreviation of PIHA-0–40 is used to denote the pristine PI (PI6FDA-TFDB-mBAPS) and hybrid composites PI6FDA-TFDB-mBAPS(Si)_SiO2-10–40%, where the percentage is the weight percentage. The chemical structure and synthetic routes of the materials are shown in Schemes 1 and 2. The preparation details are described in the experimental section. The copolyimide (PI6FDA-TFDB-mBAPS(Si)), which is a key component for preparing the highly transparent composites, was synthesized by the two step polycondensation. The composite films were obtained by using the PI6FDA-TFDB-mBAPS(Si) and TEOS through the ‘sol–gel’ method. The structures of the PIs and hybrid composites were characterized by FTIR. The FT-IR spectra of PIHA-0 (PI6FDA-TFDB-mBAPS) and PIHA-40 (PI6FDA-TFDB-mBAPS(Si)_SiO240%) composite films are shown in Fig. 1 as typical examples. In the spectra of PIHA-0 and PIHA-40, absorption bands of the imide groups appear at 1782 and 1720 cm1 for the symmetric and anti-symmetric stretching vibrations of the carbonyl groups. The absorption bands of the sulfone of mBAPS appear at 1302 cm1, and those of trifluoromethyl of 6FDA and TFDB can be seen at
Fig. 1. FTIR spectra of PIHA-0 (PI6FDA-TFDB-mBAPS) and composite film PIHA40 (PI6FDA-TFDB-mBAPS(Si)_SiO2-40%).
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Fig. 2. UV–Vis spectra of PIHA-0 (PI6FDA-TFDB-mBAPS) and composite films PIHA-10–40 (PI6FDA-TFDB-mBAPS(Si)_SiO2-10–40%).
1125 cm1. The absorption bands of poly(amic acid)s (PAAs) can no longer be observed in the range between 1660 and 1670 cm1. In addition, the absorption bands of CACH2AC linkage of PIHA-40 are observed at 2966 and 1475 cm1. It verifies that the amine groups of APTEOS agent are successfully introduced into the anhydride terminal groups of the PAA6FDA-TFDB-mBAPS. These spectral characteristics indicate that the PIs with proposed structures have been obtained. For the composites containing inorganic cross-linked component obtained from TEOS (PIHA-40), the FT-IR spectra show absorption bands at 3600–3400 cm1, 890 cm1 (SiAOH), and around 1000–1100 cm1 (SiAOASi symmetric stretching vibrations). It verifies that inorganic silica networks were successfully formed in the hybrid copolyimide composites by the ‘sol–gel’ process. 3.2. Optical properties of PI and composites The optical transmittance spectra of the copolyimide (PI6FDA-TFDB-mBAPS) and a series of composite films (PI6FDA-TFDB-mBAPS(Si)_SiO2-10–40%) are shown in Fig. 2. The cutoff wavelengths (absorption edge, kcutoff) and the transmittance at 450 nm and 400 nm obtained from these spectra are listed in Table 1. PIHA-0 (PI6FDA-TFDB-mBAPS) shows the highest transmittance of 97% at 450 nm, and the film is entirely colorless. It can be attributed to the meta-linked diphenylsulfone linkages in the mBAPS
moiety, low polarizable fluorine substituent in 6FDA and the distorted TFDB moieties, which are all beneficial to the high optical transparency of the films. The rigid, kinked, and distorted molecular structure inhibits dense packing of the polymer chains and p-electrons transfer between benzene and imide rings in conventional PIs. These factors effectively reduce the formation of intermolecular charge transfer complex (CT-complex) between polymer backbones through inductive effect and steric hindrance. The colorless feature can also be attributed to the substantial reduction of the electron-donating property of diamine moieties and electron-accepting property of anhydride moieties. After incorporating SiO2 by the sol–gel reactions with weight percentage from 10% to 40% into PI6FDA-TFDB-mBAPS(Si), the composite films kept transparent and uniform appearance, and their transmittance was only slightly decreased. The light scattering and opacity caused by aggregated inorganic silica phases in the wavelength scale were effectively avoided for the films, which is attributed to the reaction between triethoxysilane terminal groups and TEOS. Even for the high SiO2 concentration, the strong phase separation of silica can be prevented for PI6FDA-TFDB-mBAPS(Si)_SiO2-40%, which leads to a homogenous distribution of SiO2 in the composite. Compared with typical PI/Silica composite system [36,37], the composites prepared in this study show significantly improved optical properties. The improvement can be attributed to the effect of triethoxysilane groups to form covalent linkage and hydrogen bonding between the organic and inorganic phase through the hydrolysis and condensation reaction [38,39]. The hydrolysis and condensation are completed and the sizes of silica phase particles decrease through enhanced interaction between the components. This point was confirmed by scanning electron microscopy as it will be shown in Section 3.3. As a consequence, inhomogeneous phase separation is effectively inhibited to avoid the strong aggregation and poor dispersion of silica phases in the PI matrices, which reduces the light scattering in a substantial degree.
3.3. Morphology of PI and composites The morphology of the copolyimide (PI6FDA-TFDB-mBAPS) and composite film (PI6FDA-TFDB-mBAPS(Si)_SiO2-40%) was monitored with scanning electron microscopy (SEM).
Table 1 Optical and thermal properties of PIHA-0 (PI6FDA-TFDB-mBAPS) and PIHA-10–40 (PI6FDA-TFDB-mBAPS(Si)_SiO2-10–40%) composite films properties. Sample
PI6FDA-TFDB-mBAPS PI6FDA-TFDB-mBAPS(Si)_SiO2-10% PI6FDA-TFDB-mBAPS(Si)_SiO2-20% PI6FDA-TFDB-mBAPS(Si)_SiO2-30% PI6FDA-TFDB-mBAPS(Si)_SiO2-40% a b c
kcutoff (nm)
300 301 310 315 327
Transmittance
DSC
TGA
CTEc (ppm °C1)
400 nm (%)
450 nm (%)
Tg (°C)
T5%a (°C) d
T10%a (°C) d
Rw800b (%)
94 91 90 88 84
97 95 94 94 94
263.1 264.4 267.7 268.5 276.0
494 505 508 517 525
523 529 532 541 548
39 45 47 49 51
34.7 32.5 30.8 28.6 14.4
Weight loss temperatures at 5 wt% and 10 wt% were recorded by TGA at a heating rate of 20 °C min1 and a N2 gas flow rate of 25 cm3 min1. Weight residue at 800 °C. The temperature range from 80 to 170 °C with a force of 0.01 N.
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nal groups introduced by the reaction with APTEOS are important to counter-balance the phase separation tendency when the SiO2 content is too high. 3.4. Thermal properties of PI and composites The phase transitions of PI6FDA-TFDB-mBAPS and composite films (PI6FDA-TFDB-mBAPS(Si)_SiO2-10–40%) were investigated by DSC. The results are shown in Fig. 4 and summarized in Table 1. The PI and composites all show glass transitions related with the amorphous phase of the copolyimide. Compared with Tg of 263.1 °C for PIHA-0 (PI6FDA-TFDB-mBAPS), the composite films PIHA-10–40 show the Tg values ranging from 264.4 to 276.0 °C. After introducing the inorganic phase, the Tgs of composite films become higher than that of PIHA-0. This can be attributed to the cross-linking through the covalent linkage and hydrogen bonding between the two phases, which causes the increased rigidity of the matrix and higher Tg [41]. The thermal decomposition temperatures of the composites and related materials were measured by TGA analysis. The results are shown in Fig. 5 and summarized in Table 1. It can be seen that for the copolyimide (PI6FDA-TFDB-mBAPS) and composite films
Fig. 3. Typical SEM images of PIHA-0 (PI6FDA-TFDB-mBAPS) and composite film PIHA-40 (PI6FDA-TFDB-mBAPS(Si)_SiO2-40%). Scale bar: 1 lm.
Fig. 3 shows typical SEM images of the copolyimide and selected composite films. The silica loading and reinforcement binding with PI6FDA-TFDB-mBAPS(Si) matrix are confirmed by the morphology. Compared with other PI_SiO2 composites [26–35,40], Fig. 3(a) and (b) show homogeneous and improved phase dispersion morphology between organic and inorganic phase. As discussed above, the terminal groups of PI6FDA-TFDB-mBAPS(Si) can inhibit strong phase separation tendency between organic PI and inorganic silica. Even for the high SiO2 concentration, PI6FDA-TFDB-mBAPS(Si)_SiO2-40% still can prevent the significant phase separation of silica and lead to a homogenous distribution of SiO2. The sizes of SiO2 inorganic particles formed by sol–gel method are smaller than 100 nm. The experimental results also showed that when the amount of APTEOS is reduced in the reaction, as shown for PIHC40 in supporting information, unbounded silica particles and the interfacial voids were revealed by in the SEM images, which were caused by the weak interactions between the organic phase and inorganic silica phase. The above observation confirms that triethoxysilane termi-
Fig. 4. DSC curves of PIHA-0 (PI6FDA-TFDB-mBAPS) and composite films PIHA10–40 (PI6FDA-TFDB-mBAPS(Si)_SiO2-10–40%).
Fig. 5. TGA curves of PIHA-0 (PI6FDA-TFDB-mBAPS) and composite films PIHA10–40 (PI6FDA-TFDB-mBAPS(Si)_SiO2-10–40%).
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(PI6FDA-TFDB-mBAPS(Si)_SiO2-10–40%), the weight losses are negligible below 490 °C. The composite films (PIHA-10–40) show the 5% weight loss at temperatures ranging from 505 to 525 °C, and 10% weight loss at temperatures ranging from 529 to 548 °C. The thermal stability is improved compared with those of PIHA-0, which shows the 5% weight loss at 494 °C and the 10% weight loss at 523 °C, respectively. When the SiO2 content is increased, the residual weights at 800 °C obviously increase. The residual weight percentage of PIHA-40 is 51% compared to 39% for PIHA-0 owing to the existence of the inorganic phase. The effectively reduced CTE values were also observed for the composite films compared with the pristine PI. The results are shown in Fig. 6 and summarized in Table 1. It can be ascribed to the formation of the networks of welldispersed silica nano-particles, which obstructs the expansion of copolyimide chains at elevated temperatures. The inorganic silica networks show a significant effect to reduce the CTE value. This tendency is apparent when comparing the CTE values among the PIHA-10–40. The CTE value of 34.7 ppm °C1 for PI6FDA-TFDB-mBAPS is reduced to 14.4 ppm °C1 for PI6FDA-TFDB-mBAPS(Si)_SiO2-40% in the temperature range from 80 °C to 170 °C. 3.5. Mechanical properties of PI and composites The temperature variation of storage modulus (E0 ) and phase angle (tan d, the ratio of loss modulus (E00 ) to storage modulus (E0 )) for the PIHA-0 and PIHA-10–40, which were measured by DMA Analysis, are listed in Table 2 and shown in Fig. 7. As compared with PIHA-0, the PIHA-10–
Fig. 7. Storage modulus (MPa) PIHA-0 (PI6FDA-TFDB-mBAPS) and (PI6FDA-TFDB-mBAPS(Si)_SiO2-10–40%).
(a) and tan d (b) spectra of composite films PIHA-10–40
40 exhibit enhanced storage modulus (E0 ), higher Tg and reduced tan d with the increase of the amount of inorganic phase. The increase in E0 is indicative of enhanced strong interfacial interaction by covalent bonding between PI and the inorganic phase. It is caused by cross-linking reactions between the triethoxysilane-terminated PI and TEOS in the two-step network forming process as the hydrolysis and polycondensation reaction occur at elevated temperature. The PIHA-40 exhibits the largest storage modulus (E0 ) of 1806 MPa at 260 °C. The values of tan d for the composite films decrease with increasing inorganic phase. It also evidences the enhanced elastic feature of the composite films (PIHA-10–40) than PI matrix, which is attributed to the strong interfacial interaction of covalent linkage between the organic–inorganic phases. Fig. 6. CTE curves of PIHA-0 (PI6FDA-TFDB-mBAPS) and composite films PIHA10–40 (PI6FDA-TFDB-mBAPS(Si)_SiO2-10–40%).
Table 2 Storage modulus (MPa) at 260 °C, tan d and Tgs of PIHA-0 (PI6FDA-TFDB-mBAPS) and hybrid films for PIHA-10–40 (PI6FDA-TFDB-mBAPS(Si)_SiO2-10–40%). Sample
Storage modulus E0 at 260 °C (MPa)
tan d
Tg (°C)
PI6FDA-TFDB-mBAPS PI6FDA-TFDB-mBAPS(Si)_SiO2-10% PI6FDA-TFDB-mBAPS(Si)_SiO2-20% PI6FDA-TFDB-mBAPS(Si)_SiO2-30% PI6FDA-TFDB-mBAPS(Si)_SiO2-40%
480 642 885 1200 1806
1.00 0.89 0.84 0.69 0.53
266.1 269.7 272.8 279.0 284.3
3.6. Discussion The above results all indicate that the significant improvements in thermal stability have been achieved by incorporation of the inorganic silica phase. At the same time, optical transparency is not deteriorated by using the triethoxysilane-terminated copolyimide (PI6FDA-TFDB-mBAPS(Si)). Therefore, the composite films can be a very promising candidate for transparent insulated materials with optimized properties. ITO glass substrates have been widely used in displays and other devices, which show Tg over 1200 °C, high transparency, and a
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low CTE of 9 ppm °C1. However, glass substrates show the limitation for their applications in the flexible devices. For common polymeric materials with flexibility, polyurethane (PU) has Tg in a range from 60.15 to 138.82 °C and CTE of 30–70 ppm °C1; polyethylene terephthalate (PET) has Tg in a range from 67 to 81 °C and CTE of 70 ppm °C1; polydimethylsiloxane (PDMS) has an extremely low Tg (111 to 120 °C) and large CTE of 310 ppm °C1. Although crystallization in some of the polymeric materials can significantly improve their thermal stability, the optical properties become poor at the same time due to the semi-crystalline nature. Compared with these materials, the hybrid composites developed based on the well-designed PIs are promising to meet the requirements for flexible transparent interlayer and other advanced electronic devices. 4. Conclusions A highly transparent and colorless copolyimide (PI6FDA-TFDB-mBAPS(Si)) and a series of composites (PI6FDA-TFDB-mBAPS(Si)_SiO2-10–40%) were synthesized in this study. PI6FDA-TFDB-mBAPS(Si) was designed to contain electron withdrawing sulfone and fluorine groups in the polymeric structure and triethoxysilane groups in termini, which was designed for the ‘sol–gel’ reaction. Based on PI6FDA-TFDB-mBAPS(Si), a series of composites PI6FDA-TFDB-mBAPS(Si)_SiO2-10–40% was fabricated through the ‘sol–gel’ method with TEOS cross linkage agent. The composite films exhibited significantly improved optical transparency and thermal stability, which are required for transparent insulator applications. The best thermal properties were observed for PI6FDA-TFDB-mBAPS(Si)_SiO240%, which also exhibited good optical transmittance of 94% at 450 nm. Incorporation of PI6FDA-TFDB-mBAPS(Si) with reactive terminal groups showed ability to prevent the phase separation of SiO2 in PI matrices due to the strong covalent and partial hydrogen bonding networks formed during the sol–gel process. The composite films exhibited the far smaller CTE value with the increase of SiO2. Compared to the value of 34.7 ppm °C1 for PI6FDA-TFDB-mBAPS, a much smaller CTE value of 14.4 ppm °C1 was observe for PI6FDA-TFDB-mBAPS(Si)_SiO2-40%, where a reduction of 57% was attained. From these results, we conclude that the triethoxysilane-terminal groups in PI6FDA-TFDB-mBAPS(Si) has an effect to enhance the interaction between organic polyimide and inorganic silica network through the ‘sol–gel’ reaction. Because of this advantage, the incorporation of the SiO2 phase can effectively enhance the thermal stability without negative effects on the optical transparency, which is also effective to improve comprehensive properties. The components and interaction in PI6FDA-TFDB-mBAPS(Si)_SiO2-40% are complementary to improve the thermal resistant drawbacks of PI6FDA-TFDB-mBAPS and optical drawbacks of typical PI/SiO2 composite system. Acknowledgement The financial support from National Basic Research Program of China (973 Program) under Project 2011CB606102 is gratefully acknowledged.
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Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.eurpolymj.2015.01.012.
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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Highly transparent triethoxysilane-terminated copolyimide and its SiO2 composite with enhanced thermal stability and reduced thermal expansion Seong-Ku Kim a, Xingyuan Wang a, Shinji Ando b,⇑, Xiaogong Wang a,⇑ a b
Department of Chemical Engineering, Laboratory of Advanced Materials (MOE), Tsinghua University, Beijing 100084, PR China Department of Chemistry & Materials Science, Tokyo Institute of Technology, Ookayama 2-12-1-E4-5, Meguro-ku, Tokyo 152-8552, Japan
a r t i c l e
i n f o
Article history: Received 30 October 2014 Received in revised form 30 December 2014 Accepted 11 January 2015 Available online 17 January 2015 Keywords: Transparent polyimide Thermal stable polyimide Hybrid PI/silica composite Sol–gel method Organic–inorganic composite
a b s t r a c t A highly transparent and colorless copolyimide was synthesized to contain electron withdrawing sulfone and fluorine groups. Then, the triethoxysilane terminal groups were introduced by reacting the anhydride end groups of the polymer with (3-aminopropyl)triethoxysilane. Based on the triethoxysilane-terminated copolyimide, a series of composites of the copolyimide (PI6FDA-TFDB-mBAPS(Si)) with inorganic SiO2 network was successfully fabricated in order to satisfy the requirements for transparent insulators with high thermal stability. Because the hybrid with inorganic SiO2 networks, the composite (PI6FDA-TFDB-mBAPS(Si)_SiO2) can complementarily improve the thermal resistant drawbacks of organic systems. As a result, the composite films exhibit small values of the coefficient of thermal expansion (CTE) with the increasing SiO2 content. The coefficient of thermal expansion (CTE) of the hybrid PI6FDA-TFDB-mBAPS(Si)_SiO2-40% film is 14.4 ppm °C1 in the temperature range from 80 to 170 °C. Compared to a value of 34.7 ppm °C1 for the pristine PI, a 57% reduction is achieved for the composite. The composite possesses the glass transition temperature (Tg) of 276.0 °C and shows good optical transmittance of 94% at the wavelength of 450 nm. The incorporation of cross-linked SiO2 networks by using the triethoxysilane-terminated PI preserves the high transparency and enhances the thermal stability for the composite films. Due to these optimized properties, the composite can show potential applications in the micro-flexible transparent electronic devices such as transparent insulating panel of advanced transparent display devices and transparent flexible circuit board. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Since it was reported by Borgert and Renshaw [1], and Sroog et al. [2], polyimides (PIs) have received considerable attention for applications in many fields, such as gas separation membrane [3–5], and nano-material technology [6,7]. PIs show some unique and outstanding properties compared with conventional polymers, such as their inherent toughness and flexibility, low density, remarkable ther⇑ Corresponding authors. Tel.: +86 10 62784561; fax: +86 10 62770304. E-mail address: [email protected] (X. Wang). http://dx.doi.org/10.1016/j.eurpolymj.2015.01.012 0014-3057/Ó 2015 Elsevier Ltd. All rights reserved.
mal stability, radiation resistance and mechanical strength [8,9]. With the rapid development of electronic industry, PIs have become necessary key materials in this area for fabricating integrated circuits and other devices. Some recently emerging applications of PIs include the advanced flexible display [9,10] and flexible electronic devices [11– 13]. However, some obstacles for these new applications have also been noticed at the same time. A relatively higher value of the coefficient of thermal expansion (CTE) of PIs can cause serious reliability problems, such as bending, curling, cracking and delamination, when used in multi-layer-structured devices [14,15]. In some cases,
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thermal stability of PIs also needs to be improved for the applications. Inorganic materials exhibit excellent thermal stability and high modulus [16]. Therefore, forming hybrid polyimide composites with inorganic materials is a promising approach to obtain balanced properties by exploiting the advantages of both organic and inorganic materials [17– 21]. In recent years, researches have been carried out with great interest on the organic–inorganic PI composites [21], such as organic-silica or other composite materials [22–24,26–32]. Since a large amount of thin films are required in microelectronics and photonic applications, an efficient process to obtain high-quality composite films are always required. In addition, the thermodynamic immiscibility between organic and inorganic materials may lead to phase separation in composite films [26– 30]. Generally speaking, compatibility can be enhanced by a variety of methods, such as functionalizing polymer chains at their ends, selecting appropriate groups of polymers within the repeat units, adding a coupling agent to bond PI chains and inorganic network [17,25]. However, it is still a challenge to homogeneously disperse inorganic phase in polymer matrices in a highly efficient and controllable way. Most of the PI/silica composites are prepared by a two-stage process [22–24,26–30]. In the process, polyamic acids (PAAs) as precursors of PIs are first synthesized; then hydrolyzed tetraethoxysilane (TEOS) is added to perform so-called ‘sol–gel’ process. The advantage of this procedure is that uncyclized carboxyl groups can form hydrogen bonds with the acid hydrolyzed Si(OH)4 solution to disperse the inorganic phase homogeneously within the polymer matrix. However, a large amount of water released from the cyclization may scissor molecular chains and deteriorate the properties of films. In addition, most of the PI/silica composites are opaque because of the light scattering and opacity caused by aggregated inorganic silica phase in wavelength scale or above [33]. Therefore, there are some limitations for their applications as transparent insulator materials for flexible display or transparent electronic device fields. As an ultimate goal, ideal transparent insulator materials should not only possess high transparent properties, but also show good mechanical properties and high thermal stability. In this study, we synthesized a highly transparent and colorless copolyimide terminated with triethoxysilane functional groups (PI6FDA-TFDB-mBAPS(Si)). The PI was used as a precursor to prepare composites with inorganic silica via ‘sol–gel’ method. In order to improve the optical transparency, we chose three kinds of functional monomers containing ASO2A, ACF3, kink and distorted structures for the copolymer backbone, which were used to prevent the formation of the charge-transfer (CT) complex. The triethoxysilane-terminated PI was designed to react with Si(OH)4 to achieve a homogeneous dispersion of the inorganic phase in PI matrix [34,35]. It was proved to be an efficient way for this purpose and showed ability to optimize other properties at the same time. The preparation, structures and properties of the series of hybrid composites are presented as follows.
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2. Experimental 2.1. Materials 4,40 -(Hexafluoroisopropylidene)diphthalic anhydride (6FDA, 95%), (3-amino-propyl)triethoxysilane (APTEOS, 98%), and 2,20 -bis(trifluoromethyl)benzidine (TFDB, 98%) were purchased from Adamas-Reagent Co., Ltd. 4,40 -Bis(3amino-phenoxy)diphenylsulfone (mBAPS, 98%) was purchased from Tokyo Chemical Industry Co., Ltd. (TCI). The solvent N-methyl-2-pyrrolidone (NMP, 97%) was purchased from Beijing Modern East Fine Chemical. Hydrochloric acid (HCl) and tetraethoxysilane (TEOS, 98%) were purchased from Alfa Aesar. If it is not mentioned specifically, the reactants and solvents were used as received without further purification. 2.2. Measurements Fourier transform infrared (FTIR) spectroscopic measurements were performed using a Magna-IR Nicolet 560 FTIR spectrophotometer by incorporating samples in KBr disks. The spectra were recorded in the range of 4000– 450 cm1 at a resolution of 0.35 cm1. The phase transitions and thermal properties were characterized by differential scanning calorimeter (DSC), thermal gravimetric analysis (TGA) and dynamic mechanical analysis (DMA). Thermal phase transitions of the polymers were scanned by DSC 2910 (TA Instrument Co.) with a heating rate of 20 °C min1 under nitrogen atmosphere flow. Thermal stability was measured with a TGA 2050 thermo gravimetric analyzer (TA instrument Co.) at a heating rate of 20 °C from room temperature to 900 °C under a continuous flow of nitrogen. Dynamic mechanical analysis (DMA) measurements were performed using a DMA Q800-Dynamic Mechanical Analyzer (TA Instrument Co.) in the tension mode over a temperature range from 25 °C. Data acquisition and analysis of the storage modulus (E0 ) and loss tangent (tan d) were recorded automatically by the system. The samples used for the measurements were 14 mm in length, 5 mm in width, and 22– 45 lm in thickness. The heating rate and frequency were fixed at 2 °C min1 and 1 Hz, respectively. The coefficients of thermal expansion (CTE) parallel to the film surfaces were measured using a DMA Q800 dynamic mechanical analyzer (TA Instrument Co.) in extension mode over a temperature range from 25 to 320 °C with a force of 0.01 N. The samples used for the measurements were 14 mm in length, 5 mm in width, and 21–43 lm in thickness. UV–visible absorption spectra of films were measured on a Lambda Bio-40 spectrometer (Perkin–Elmer). The samples used for the measurements were solid thin films with thickness about the 10 lm. Cross-sectional images of the polyimide composite films were studied by scanning electron microscopy (SEM). The SEM images were obtained using a S4700I microscope (Hitachi) operating at an acceleration voltage of 15.0 kV. 2.3. Synthesis process The synthetic routes of the materials are shown in Schemes 1 and 2, which include the synthesis of
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PAA6FDA-TFDB-mBAPS(Si) (Scheme 1) and its composite with silica prepared by a typical ‘sol–gel’ method (Scheme 2). The polyamic acid (PAA) was firstly synthesized at 0 °C. Then, APTEOS was added into the PAA solution to obtain the PAA with triethoxysilane-terminal groups. The composites were prepared to contain SiO2-10–40% (TEOS) and heated through several stages to the required high temperature and dehydrated to obtain the composite films. A PI without the triethoxysilane-terminal groups was synthesized for comparison, which is referred to as the pristine PI (PI6FDA-TFDB-mBAPS). 2.3.1. Polyamic acid and pristine PI 2,20 -Bis(trifluoromethyl)benzidine (TFDB, 3.76 g, 11.5 mmol) and 4,40 -bis(3-amino-phenoxy)diphenylsulfone (mBAPS, 3.68 g, 11.5 mmol) as the diamine monomers
were dissolved in NMP (20 wt%) and cooled with the ice water bath. After completely dissolved, 4,40 -(hexafluoroisopropylidene)diphthalic anhydride (6FDA, 10 g, 23 mmol) as the anhydride monomer was added into the above solution under stirring for 12 h. The molar ratio of 6FDA:TFDB:mBAPS in above reaction was 5:2.5:2.5. The reaction composite was stirred at room temperature for 6 h to obtain the pristine PAA (PAA6FDA-TFDB-mBAPS). In the second stage, the PAA6FDA-TFDB-mBAPS precursor solution was casted on the glass substrate and heated at 80 °C for 2 h. The film was then thermally imidized by step-wise heating at 150 °C (1 h), 200 °C (1 h), and 300 °C (1 h) to afford the PI6FDA-TFDB-mBAPS film. The film was separated from substrate by ultrasonication in the hot deionized water, and the delaminated film was dried at 60 °C for 3 h under vacuum condition.
Scheme 1. Synthetic route for the PAA6FDA-TFDB-mBAPS(Si).
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A series of composite films with SiO2 contents of 10%, 20%, 30%, 40% was obtained by similar procedure. The composites are named as PI6FDA-TFDB-mBAPS(Si)_SiO2-10–40%, where the percentage given in the genetic abbreviations is the weight percentage. The FTIR results of one representative composite are given below. FTIR of PI6FDA-TFDB-mBAPS(Si)_SiO2-40% (KBr, cm1): 3600– 3400 cm1 (SiAOH); 2966 cm1 (CACH2AC); 1782 cm1 (C@O sym. str.); 1720 cm1 (C@O asym. str.); 1587 cm1 (C@C str. Ar.); 1475 cm1 (CACH2AC); 1445 cm1 (C@C str. Ar.); 1364 cm1 (CANAC str. imide.); 1302 cm1 (CASO2AC str. imide.); 1232 cm1 (Ar.AOAAr.); 1125 cm1 (ACF3); 1096–1067 cm1 (SiAOASi); 964, 835 cm1 (Ar.AH); 778, 745 cm1 (subst. Ar.); and 1660– 1670 cm1 (non PAA structure band).
3. Results and discussion Scheme 2. Synthetic route for the PI6FDA-TFDB-mBAPS(Si)_SiO2 composite film.
1
1
FTIR of PI6FDA-TFDB-mBAPS (KBr, cm ): 1782 cm (C@O sym. str.); 1720 cm1 (C@O asym. str.); 1587 cm1 (C@C str. Ar.); 1445 cm1 (C@C str. Ar.); 1364 cm1 (CANAC str. imide.); 1302 cm1 (CASO2AC str. imide.); 1232 cm1 (Ar.AOAAr.); 1125 cm1 (ACF3); 964, 835 cm1 (Ar.AH); 778, 745 cm1 (subst. Ar.); and 1660–1670 cm1 (non PAA structure band). 2.3.2. Polyimide/SiO2 composite film (PI6FDA-TFDB-mBAPS(Si)_SiO2) 2,20 -Bis(trifluoromethyl)benzidine (TFDB, 2.88 g, 9 mmol) and 4,40 -bis(3-amino-phenoxy)diphenylsulfone (mBAPS, 3.89 g, 9 mmol) as the diamine monomers were dissolved in NMP (20 wt%) and cooled with the ice water bath. After completely dissolved, 4,40 -(hexafluoroisopropylidene)diphthalic anhydride (6FDA, 10 g, 22.5 mmol) as the anhydride monomer was added into the above solution under stirring for 12 h. Then, (3-aminopropyl)triethoxysilane (APTEOS, 1.99 g, 9 mmol) was slowly dropped into the mixture, the molar ratio of 6FDA: TFDB:mBAPS:APTEOS was 5:2:2:2. The reaction composite was stirred at room temperature for 6 h to obtain the PAA with triethoxysilane-terminated groups (PAA6FDA-TFDB-mBAPS(Si)). The composite film containing 40 wt% SiO2 was given here as a typical example. Stoichiometric quantities of tetraethoxysilane (TEOS, 1.39 g), and hydrochloric acid in deionized water (0.1 N HCl, 0.48 g) were added into the PAA6FDA-TFDB-mBAPS(Si) solution (40 wt%) by droplets under stirring. The mixture was then stirred at room temperature for 6 h to obtain the PAA6FDA-TFDB-mBAPS(Si)_SiO2-40% precursor. In the second stage, the PAA6FDA-TFDB-mBAPS(Si)_SiO240% precursor solution was casted on the glass substrate, heated to 80 °C and kept at the temperature for 2 h. The film was then thermally imidized by stepwise heating at 150 °C (1 h), 200 °C (1 h), and 300 °C (1 h) to afford the PI6FDA-TFDB-mBAPS(Si)_SiO2-40% composite film. The composite film was separated from substrate by ultrasonication in the hot deionized water, and the delaminated film was dried at 60 °C for 3 h under vacuum condition.
3.1. Synthesis and characterization A copolyimide and series of composite films were prepared in this study. To facilitate the discussion, a genetic abbreviation of PIHA-0–40 is used to denote the pristine PI (PI6FDA-TFDB-mBAPS) and hybrid composites PI6FDA-TFDB-mBAPS(Si)_SiO2-10–40%, where the percentage is the weight percentage. The chemical structure and synthetic routes of the materials are shown in Schemes 1 and 2. The preparation details are described in the experimental section. The copolyimide (PI6FDA-TFDB-mBAPS(Si)), which is a key component for preparing the highly transparent composites, was synthesized by the two step polycondensation. The composite films were obtained by using the PI6FDA-TFDB-mBAPS(Si) and TEOS through the ‘sol–gel’ method. The structures of the PIs and hybrid composites were characterized by FTIR. The FT-IR spectra of PIHA-0 (PI6FDA-TFDB-mBAPS) and PIHA-40 (PI6FDA-TFDB-mBAPS(Si)_SiO240%) composite films are shown in Fig. 1 as typical examples. In the spectra of PIHA-0 and PIHA-40, absorption bands of the imide groups appear at 1782 and 1720 cm1 for the symmetric and anti-symmetric stretching vibrations of the carbonyl groups. The absorption bands of the sulfone of mBAPS appear at 1302 cm1, and those of trifluoromethyl of 6FDA and TFDB can be seen at
Fig. 1. FTIR spectra of PIHA-0 (PI6FDA-TFDB-mBAPS) and composite film PIHA40 (PI6FDA-TFDB-mBAPS(Si)_SiO2-40%).
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Fig. 2. UV–Vis spectra of PIHA-0 (PI6FDA-TFDB-mBAPS) and composite films PIHA-10–40 (PI6FDA-TFDB-mBAPS(Si)_SiO2-10–40%).
1125 cm1. The absorption bands of poly(amic acid)s (PAAs) can no longer be observed in the range between 1660 and 1670 cm1. In addition, the absorption bands of CACH2AC linkage of PIHA-40 are observed at 2966 and 1475 cm1. It verifies that the amine groups of APTEOS agent are successfully introduced into the anhydride terminal groups of the PAA6FDA-TFDB-mBAPS. These spectral characteristics indicate that the PIs with proposed structures have been obtained. For the composites containing inorganic cross-linked component obtained from TEOS (PIHA-40), the FT-IR spectra show absorption bands at 3600–3400 cm1, 890 cm1 (SiAOH), and around 1000–1100 cm1 (SiAOASi symmetric stretching vibrations). It verifies that inorganic silica networks were successfully formed in the hybrid copolyimide composites by the ‘sol–gel’ process. 3.2. Optical properties of PI and composites The optical transmittance spectra of the copolyimide (PI6FDA-TFDB-mBAPS) and a series of composite films (PI6FDA-TFDB-mBAPS(Si)_SiO2-10–40%) are shown in Fig. 2. The cutoff wavelengths (absorption edge, kcutoff) and the transmittance at 450 nm and 400 nm obtained from these spectra are listed in Table 1. PIHA-0 (PI6FDA-TFDB-mBAPS) shows the highest transmittance of 97% at 450 nm, and the film is entirely colorless. It can be attributed to the meta-linked diphenylsulfone linkages in the mBAPS
moiety, low polarizable fluorine substituent in 6FDA and the distorted TFDB moieties, which are all beneficial to the high optical transparency of the films. The rigid, kinked, and distorted molecular structure inhibits dense packing of the polymer chains and p-electrons transfer between benzene and imide rings in conventional PIs. These factors effectively reduce the formation of intermolecular charge transfer complex (CT-complex) between polymer backbones through inductive effect and steric hindrance. The colorless feature can also be attributed to the substantial reduction of the electron-donating property of diamine moieties and electron-accepting property of anhydride moieties. After incorporating SiO2 by the sol–gel reactions with weight percentage from 10% to 40% into PI6FDA-TFDB-mBAPS(Si), the composite films kept transparent and uniform appearance, and their transmittance was only slightly decreased. The light scattering and opacity caused by aggregated inorganic silica phases in the wavelength scale were effectively avoided for the films, which is attributed to the reaction between triethoxysilane terminal groups and TEOS. Even for the high SiO2 concentration, the strong phase separation of silica can be prevented for PI6FDA-TFDB-mBAPS(Si)_SiO2-40%, which leads to a homogenous distribution of SiO2 in the composite. Compared with typical PI/Silica composite system [36,37], the composites prepared in this study show significantly improved optical properties. The improvement can be attributed to the effect of triethoxysilane groups to form covalent linkage and hydrogen bonding between the organic and inorganic phase through the hydrolysis and condensation reaction [38,39]. The hydrolysis and condensation are completed and the sizes of silica phase particles decrease through enhanced interaction between the components. This point was confirmed by scanning electron microscopy as it will be shown in Section 3.3. As a consequence, inhomogeneous phase separation is effectively inhibited to avoid the strong aggregation and poor dispersion of silica phases in the PI matrices, which reduces the light scattering in a substantial degree.
3.3. Morphology of PI and composites The morphology of the copolyimide (PI6FDA-TFDB-mBAPS) and composite film (PI6FDA-TFDB-mBAPS(Si)_SiO2-40%) was monitored with scanning electron microscopy (SEM).
Table 1 Optical and thermal properties of PIHA-0 (PI6FDA-TFDB-mBAPS) and PIHA-10–40 (PI6FDA-TFDB-mBAPS(Si)_SiO2-10–40%) composite films properties. Sample
PI6FDA-TFDB-mBAPS PI6FDA-TFDB-mBAPS(Si)_SiO2-10% PI6FDA-TFDB-mBAPS(Si)_SiO2-20% PI6FDA-TFDB-mBAPS(Si)_SiO2-30% PI6FDA-TFDB-mBAPS(Si)_SiO2-40% a b c
kcutoff (nm)
300 301 310 315 327
Transmittance
DSC
TGA
CTEc (ppm °C1)
400 nm (%)
450 nm (%)
Tg (°C)
T5%a (°C) d
T10%a (°C) d
Rw800b (%)
94 91 90 88 84
97 95 94 94 94
263.1 264.4 267.7 268.5 276.0
494 505 508 517 525
523 529 532 541 548
39 45 47 49 51
34.7 32.5 30.8 28.6 14.4
Weight loss temperatures at 5 wt% and 10 wt% were recorded by TGA at a heating rate of 20 °C min1 and a N2 gas flow rate of 25 cm3 min1. Weight residue at 800 °C. The temperature range from 80 to 170 °C with a force of 0.01 N.
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nal groups introduced by the reaction with APTEOS are important to counter-balance the phase separation tendency when the SiO2 content is too high. 3.4. Thermal properties of PI and composites The phase transitions of PI6FDA-TFDB-mBAPS and composite films (PI6FDA-TFDB-mBAPS(Si)_SiO2-10–40%) were investigated by DSC. The results are shown in Fig. 4 and summarized in Table 1. The PI and composites all show glass transitions related with the amorphous phase of the copolyimide. Compared with Tg of 263.1 °C for PIHA-0 (PI6FDA-TFDB-mBAPS), the composite films PIHA-10–40 show the Tg values ranging from 264.4 to 276.0 °C. After introducing the inorganic phase, the Tgs of composite films become higher than that of PIHA-0. This can be attributed to the cross-linking through the covalent linkage and hydrogen bonding between the two phases, which causes the increased rigidity of the matrix and higher Tg [41]. The thermal decomposition temperatures of the composites and related materials were measured by TGA analysis. The results are shown in Fig. 5 and summarized in Table 1. It can be seen that for the copolyimide (PI6FDA-TFDB-mBAPS) and composite films
Fig. 3. Typical SEM images of PIHA-0 (PI6FDA-TFDB-mBAPS) and composite film PIHA-40 (PI6FDA-TFDB-mBAPS(Si)_SiO2-40%). Scale bar: 1 lm.
Fig. 3 shows typical SEM images of the copolyimide and selected composite films. The silica loading and reinforcement binding with PI6FDA-TFDB-mBAPS(Si) matrix are confirmed by the morphology. Compared with other PI_SiO2 composites [26–35,40], Fig. 3(a) and (b) show homogeneous and improved phase dispersion morphology between organic and inorganic phase. As discussed above, the terminal groups of PI6FDA-TFDB-mBAPS(Si) can inhibit strong phase separation tendency between organic PI and inorganic silica. Even for the high SiO2 concentration, PI6FDA-TFDB-mBAPS(Si)_SiO2-40% still can prevent the significant phase separation of silica and lead to a homogenous distribution of SiO2. The sizes of SiO2 inorganic particles formed by sol–gel method are smaller than 100 nm. The experimental results also showed that when the amount of APTEOS is reduced in the reaction, as shown for PIHC40 in supporting information, unbounded silica particles and the interfacial voids were revealed by in the SEM images, which were caused by the weak interactions between the organic phase and inorganic silica phase. The above observation confirms that triethoxysilane termi-
Fig. 4. DSC curves of PIHA-0 (PI6FDA-TFDB-mBAPS) and composite films PIHA10–40 (PI6FDA-TFDB-mBAPS(Si)_SiO2-10–40%).
Fig. 5. TGA curves of PIHA-0 (PI6FDA-TFDB-mBAPS) and composite films PIHA10–40 (PI6FDA-TFDB-mBAPS(Si)_SiO2-10–40%).
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(PI6FDA-TFDB-mBAPS(Si)_SiO2-10–40%), the weight losses are negligible below 490 °C. The composite films (PIHA-10–40) show the 5% weight loss at temperatures ranging from 505 to 525 °C, and 10% weight loss at temperatures ranging from 529 to 548 °C. The thermal stability is improved compared with those of PIHA-0, which shows the 5% weight loss at 494 °C and the 10% weight loss at 523 °C, respectively. When the SiO2 content is increased, the residual weights at 800 °C obviously increase. The residual weight percentage of PIHA-40 is 51% compared to 39% for PIHA-0 owing to the existence of the inorganic phase. The effectively reduced CTE values were also observed for the composite films compared with the pristine PI. The results are shown in Fig. 6 and summarized in Table 1. It can be ascribed to the formation of the networks of welldispersed silica nano-particles, which obstructs the expansion of copolyimide chains at elevated temperatures. The inorganic silica networks show a significant effect to reduce the CTE value. This tendency is apparent when comparing the CTE values among the PIHA-10–40. The CTE value of 34.7 ppm °C1 for PI6FDA-TFDB-mBAPS is reduced to 14.4 ppm °C1 for PI6FDA-TFDB-mBAPS(Si)_SiO2-40% in the temperature range from 80 °C to 170 °C. 3.5. Mechanical properties of PI and composites The temperature variation of storage modulus (E0 ) and phase angle (tan d, the ratio of loss modulus (E00 ) to storage modulus (E0 )) for the PIHA-0 and PIHA-10–40, which were measured by DMA Analysis, are listed in Table 2 and shown in Fig. 7. As compared with PIHA-0, the PIHA-10–
Fig. 7. Storage modulus (MPa) PIHA-0 (PI6FDA-TFDB-mBAPS) and (PI6FDA-TFDB-mBAPS(Si)_SiO2-10–40%).
(a) and tan d (b) spectra of composite films PIHA-10–40
40 exhibit enhanced storage modulus (E0 ), higher Tg and reduced tan d with the increase of the amount of inorganic phase. The increase in E0 is indicative of enhanced strong interfacial interaction by covalent bonding between PI and the inorganic phase. It is caused by cross-linking reactions between the triethoxysilane-terminated PI and TEOS in the two-step network forming process as the hydrolysis and polycondensation reaction occur at elevated temperature. The PIHA-40 exhibits the largest storage modulus (E0 ) of 1806 MPa at 260 °C. The values of tan d for the composite films decrease with increasing inorganic phase. It also evidences the enhanced elastic feature of the composite films (PIHA-10–40) than PI matrix, which is attributed to the strong interfacial interaction of covalent linkage between the organic–inorganic phases. Fig. 6. CTE curves of PIHA-0 (PI6FDA-TFDB-mBAPS) and composite films PIHA10–40 (PI6FDA-TFDB-mBAPS(Si)_SiO2-10–40%).
Table 2 Storage modulus (MPa) at 260 °C, tan d and Tgs of PIHA-0 (PI6FDA-TFDB-mBAPS) and hybrid films for PIHA-10–40 (PI6FDA-TFDB-mBAPS(Si)_SiO2-10–40%). Sample
Storage modulus E0 at 260 °C (MPa)
tan d
Tg (°C)
PI6FDA-TFDB-mBAPS PI6FDA-TFDB-mBAPS(Si)_SiO2-10% PI6FDA-TFDB-mBAPS(Si)_SiO2-20% PI6FDA-TFDB-mBAPS(Si)_SiO2-30% PI6FDA-TFDB-mBAPS(Si)_SiO2-40%
480 642 885 1200 1806
1.00 0.89 0.84 0.69 0.53
266.1 269.7 272.8 279.0 284.3
3.6. Discussion The above results all indicate that the significant improvements in thermal stability have been achieved by incorporation of the inorganic silica phase. At the same time, optical transparency is not deteriorated by using the triethoxysilane-terminated copolyimide (PI6FDA-TFDB-mBAPS(Si)). Therefore, the composite films can be a very promising candidate for transparent insulated materials with optimized properties. ITO glass substrates have been widely used in displays and other devices, which show Tg over 1200 °C, high transparency, and a
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low CTE of 9 ppm °C1. However, glass substrates show the limitation for their applications in the flexible devices. For common polymeric materials with flexibility, polyurethane (PU) has Tg in a range from 60.15 to 138.82 °C and CTE of 30–70 ppm °C1; polyethylene terephthalate (PET) has Tg in a range from 67 to 81 °C and CTE of 70 ppm °C1; polydimethylsiloxane (PDMS) has an extremely low Tg (111 to 120 °C) and large CTE of 310 ppm °C1. Although crystallization in some of the polymeric materials can significantly improve their thermal stability, the optical properties become poor at the same time due to the semi-crystalline nature. Compared with these materials, the hybrid composites developed based on the well-designed PIs are promising to meet the requirements for flexible transparent interlayer and other advanced electronic devices. 4. Conclusions A highly transparent and colorless copolyimide (PI6FDA-TFDB-mBAPS(Si)) and a series of composites (PI6FDA-TFDB-mBAPS(Si)_SiO2-10–40%) were synthesized in this study. PI6FDA-TFDB-mBAPS(Si) was designed to contain electron withdrawing sulfone and fluorine groups in the polymeric structure and triethoxysilane groups in termini, which was designed for the ‘sol–gel’ reaction. Based on PI6FDA-TFDB-mBAPS(Si), a series of composites PI6FDA-TFDB-mBAPS(Si)_SiO2-10–40% was fabricated through the ‘sol–gel’ method with TEOS cross linkage agent. The composite films exhibited significantly improved optical transparency and thermal stability, which are required for transparent insulator applications. The best thermal properties were observed for PI6FDA-TFDB-mBAPS(Si)_SiO240%, which also exhibited good optical transmittance of 94% at 450 nm. Incorporation of PI6FDA-TFDB-mBAPS(Si) with reactive terminal groups showed ability to prevent the phase separation of SiO2 in PI matrices due to the strong covalent and partial hydrogen bonding networks formed during the sol–gel process. The composite films exhibited the far smaller CTE value with the increase of SiO2. Compared to the value of 34.7 ppm °C1 for PI6FDA-TFDB-mBAPS, a much smaller CTE value of 14.4 ppm °C1 was observe for PI6FDA-TFDB-mBAPS(Si)_SiO2-40%, where a reduction of 57% was attained. From these results, we conclude that the triethoxysilane-terminal groups in PI6FDA-TFDB-mBAPS(Si) has an effect to enhance the interaction between organic polyimide and inorganic silica network through the ‘sol–gel’ reaction. Because of this advantage, the incorporation of the SiO2 phase can effectively enhance the thermal stability without negative effects on the optical transparency, which is also effective to improve comprehensive properties. The components and interaction in PI6FDA-TFDB-mBAPS(Si)_SiO2-40% are complementary to improve the thermal resistant drawbacks of PI6FDA-TFDB-mBAPS and optical drawbacks of typical PI/SiO2 composite system. Acknowledgement The financial support from National Basic Research Program of China (973 Program) under Project 2011CB606102 is gratefully acknowledged.
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Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.eurpolymj.2015.01.012.
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