Highly optical transparency and thermally stable polyimides containing pyridine and phenyl pendant

Abstract In order to obtain highly optical transparency polyimides, two novel aromatic diamine monomers containing pyridine and kinky structures, 1,1-bis[4-(5-amino-2-pyridinoxy)phenyl]diphenylmethane (BAPDBP) and 1,1-bis[4-(5-amino-2-pyridinoxy)phenyl]-1-phenylethane (BAPDAP), were designed and synthesized. Polyimides based on BAPDBP, BAPDAP, 2,2-bis[4-(5-amino-2-pyridinoxy)phenyl]propane (BAPDP) with various commercial dianhydrides were prepared for comparison and structure-property relationships study. The structures of the polyimides were characterized by Fourier transform infrared (FT-IR) spectrometer, wide-angle X-ray diffractograms (XRD) and elemental analysis. Film properties including solubility, optical transparency, water uptake, thermal and mechanical properties were also evaluated. The introduction of pyridine and kinky structure into the backbones that polyimides presented good optical properties with 91–97% transparent at 500 nm and a low cut-off wavelength at 353–398 nm. Moreover, phenyl pendant groups of the polyimides showed high glass transition temperatures (T g) in the range of 257–281 °C. These results suggest that the incorporating pyridine, kinky and bulky substituents to polymer backbone can improve the optical transparency effectively without sacrificing the thermal properties.


Introduction
Polyimides are well known for their excellent thermal stability, mechanical properties, chemical resistance, and electrical properties and have been used in the fields of adhesives, composites, fibres, films, and electronics [1][2][3][4][5][6][7]. Fully aromatic polyimides have rigid chains and strong interactions derived from intra and interchain charge transfer complex (CTC), which lead to their poor solubility and low transmittance [8][9][10]. Thus, new polyimides with aliphatic, asymmetrical and flexible linkages, bulky and kinky substituents incorporated into the backbone have been developed to improve solubility, processability and optical transparency [11][12][13][14][15][16][17][18]. However, the introduction of these groups often leads to the loss of thermal stability to some extent.
To overcome these problems, we designed and synthesized a series of novel polyimides based on pyridine. Pyridine are a class of n-type heterocyclic compounds with high thermal stabilities, and because of this, they have been a key molecule in constructing functional materials [19]. Moreover, pyridine groups possess relatively high mole refraction as compared to phenyl unit which leads to the polyimides containing pyridine showed high optical transparency [20]. The polarizability derived from the nitrogen of the pyridine ring can improve the polyimides solubility in organic solvents too [21]. It has been reported that polyimides synthesized with commercial dianhydrides and diamines containing pyridine units have improved solubility [22][23][24]. In the previous work, we have studied on the structure-property relationships of pyridine-polyimides containing -(CF 3 ) 2 , -O-, -SO 2 -, -S-, -CO-, cyclohexane and biphenyl groups. These polyimides showed highly optical transparency, low dielectric constants, good thermal stability, excellent mechanical properties, respectively [25][26][27][28][29].
In this work, we have synthesized the diamines with pyridine and kinky structures derived from phenyl pendants. The introduction of pyridine can improve the optical transparency, and the phenlic pendant as kinky structure disrupt the formation of CTC without the sacrificing of thermal stability [9]. Herein, two novel diamine monomers, 1,1′-bis[4-(5-amino-2-pyridinoxy)

Monomer synthesis
The procedure to synthesize BAPDBP, BAPDAP, BAPDP were performed according to the literature [25]. The route of diamine monomers were shown in Scheme 1.

1,1′-bis[4-(5-amino-2-pyridinoxy)phenyl]diphenylmethane (BAPDBP)
Under nitrogen protection, a mixture of 7.5 g (12.57 mmol) of BNPDBP, 3 g of Pd/C catalyst, and 150 mL of dioxane was placed into a 250 mL three-necked flask equipped with a dropping funnel, and reflux condenser. The mixture was phenyl] diphenylmethane (BAPDBP) and 1,1′-bis[4-(5amino-2-pyridinoxy)phenyl]-1-phenylethane (BAPDAP) were synthesized and characterized. In addition, 2,2′-bis[4-(5-amino-2-pyridinoxy)phenyl] propane (BAPDP) was prepared for comparison [25]. We prepared the polyimides using different diamines (BAPDBP, BAPDAP, BAPDP and 6FDA) as substituents in the backbone to investigate their effect on thermal stability, optical transparency, solubility, water uptake and mechanical properties. A series of polyimides were also prepared from BAPDBP and three commercially available dianhydrides, and their properties were investigated for promising potential application. stirred under reflux for 30 min, and then 25 mL of hydrazine hydrate was added dropwise over 2 h, followed by 6 h of reflux. The resulting mixture was filtered while hot to remove the catalyst and the filtrate was subsequently concentrated and poured into 500 mL of deionized water to produce a precipitate, which was washed with water. After recrystallization from dioxane/water, the product was dried under vacuum at 80 °C for 10 h to yield 5.8 g (77%).

Preparation of polyimide films
PI films were prepared via a traditional two-step method, as shown in Scheme 2. For example, a typical polymerization procedure for the synthesis of poly(amic acid) (PAA) precursors based on 6FDA (PI-1) is as follows. 0.6619 g 6FDA (1.49 mmol) was gradually added to a solution of 0.8000 g BAPDBP (1.49 mmol) in 6 g DMAc. Additional 2.28 g DMAc was then added to adjust the solid concentration of the reaction system to 15 wt% by weight. The mixture was stirred for 12 h at room temperature to give a homogeneous PAA solution. PI films were prepared by casting PAA onto glass plates and then heated in an air oven with a programmed temperature procedure (60 °C/2 h, 80 °C/2 h, 100 °C/1 h, 120 °C/1 h) to remove the solvent. This was followed by an imidization step under vacuum (200 °C/1 h, 250 °C/0.5 h, 300 °C/0.5 h) to produce fully imidized materials. The films were stripped from the plate by soaking in distilled water after they were cooled to room temperature. PI-(2-5) were prepared using a process similar to that described above.

Characterization of structures
Hydrogen nuclear magnetic resonance ( 1 H NMR) spectra were determined using a BRUKER-300 spectrometer (Massachusetts, U.S.A.) at 300 MHz in CDCl 3 or DMSO-d 6 .
Scheme 2. synthesis route of the polyimides.
with a load of 1 kN at a speed of 5 mm/min. Measurements were performed at 25 °C with film specimens of approximately 30-40 μm thick, 3-5 mm wide and 60 mm long, and an average of at least five individual determinations was used.

Water uptake
Water uptake (WU) of the films was determined by the weight differences before and after immersion in deionized water at room temperature for 24 h, and calculated by the following equation: WU = (W wet −W dry )/W dry × 100%; where W wet is the weight of the film samples after immersion in deionized water, and W dry is the initial weight of the samples.

Synthesis of monomers
As shown in Scheme 1, BAPDBP, BAPDAP, BAPDP were synthesized by two-step procedures. Firstly, dihydroxy compounds were reacted with 2-chloro-5-nitropyridine using a nucleophilic substitution reaction in the presence of K 2 CO 3 in the DMF to produce dinitro compounds. Secondly, the dinitro compounds were reduced by Pd/C and NH 2  FT-IR measurements were performed using a Bruker Vector 22 spectrometer (Massachusetts, U.S.A.) at a resolution of 2 cm −1 in the range of 400-4000 cm −1 , with the samples in the form of powders (monomers) and thin films (PIs).
Elemental analysis was run on a Vario EL cube CHN recorder analysis instrument (Langenselbold, Germany).

Analysis of optical properties
Ultraviolet-visible (UV-vis) spectra of the films were recorded on a Shimadzu UV-vis 2501 spectrometer (Kyoto, Japan) in transmittance mode at room temperature.

Solubility
Solubility was measured by 10 mg of polyimides in 1 mL of solvent at room temperature for 24 h.

Morphology study
Wide-angle X-ray diffraction (WAXD) analysis was conducted using a Rigaku Wide-angle X-ray diffractometer (Tokyo, Japan) (D/max rA, using Cu Kα radiation at wavelength λ = 1.541 Å) to determine the morphology structures. Data were collected at 0.02° intervals over the range of 5-50°, and the scan speed was 0.5° (2θ)/min.

Analysis of thermal properties
Dynamic mechanical analysis (DMA) was measured with a TA instrument, DMA Q800 (Delaware, U.S.A.), at a heating rate of 5 °C/min from 50 to 400 °C and a load frequency of 1 Hz in film tension geometry. T g was regarded as the onset temperature of the storage modulus (E′).
Differential scanning calorimetric (DSC) analyses were performed using a TA instrument, DSC Q100 (Delaware, U.S.A.), at a scanning rate of 10 °C/min under a nitrogen flow of 50 mL/min. To investigate the glass transition temperature, the polyimides samples were heated to a temperature higher than the glass transition temperature to eliminate thermal and stress history.
Thermo gravimetric analysis (TGA) was measured by a TA 2050 (Delaware, U.S.A.), with a heating rate of 10 °C/min under nitrogen and air atmosphere, respectively.

Mechanical measurements
Mechanical properties of the films were measured by a Shimadzu AG-I universal testing apparatus (Tokyo, Japan)

Synthesis of polyimides
Polyimides based on various diamine monomers (i.e., BAPDBP, BAPDAP, BAPDP) and three commercially available aromatic dianhydrides (i.e., 6FDA, ODPA and s-BPDA) were synthesized using a conventional two-step method, as shown in Scheme 2. The inherent viscosities of the PAA samples measured at 0.38-1.79 dL/g in DMAc at 25 °C are summarized in Table 1. The values of the inherent viscosities (η inh ) tended to be lower than those of highly polymerized PAAs; however, tough and flexible polyimides have been prepared.
The polymer structure is proved with FT-IR ( Figure 3) and elemental analysis (Table 1). FT-IR spectra showed that PAA characteristic absorption bands around 3320-3460 cm −1 and 1560-1680 cm −1 had disappeared after thermal imidization. The absorptions of the imide ring appeared at 1775-1780 cm −1 (asymmetrical C=O stretching), 1720-1730 cm −1 (symmetrical C=O stretching), and 1385-1390 cm −1 (C-N stretching), which indicated the success of imidization. The elemental analysis data confirmed with the calculated values based on the polymer repeating units.

Optical transparency
Polyimide films based on pyridine and kinky structure showed high optical transparency as presented in  the range of 353-398 nm. The highly optical transparent are directly related to the kinky substituents which can improve the free volume and inhibited the formation of CTC.
PI films with the same diamine, their optical properties depend on the chemical structures of the dianhydrides. As shown in Table 2, PI-1 showed a relatively higher optical transmittance than PI-4 and PI-5 due to the contribution of -CF 3 groups in the dianhydrides, which can reduce CT interactions [32][33][34]. P1-1, PI-2, PI-3 derived from the same dianhydrides showed similarly values of λ cut-off and transmittance at 500 nm which should attribute to the long repeat units decreased the influence of phenyl or methyl groups.
PI-3(BAPDP/6FDA) and PI-6(BAPP/6FDA) were synthesized to investigate the effects of pyridine in the chain. PI-3 showed slightly higher transmittance and lower wavelength than PI-6 as listed in Table 2. These results should attribute to the presence of pyridine groups which possess relatively high mole refraction as compared to phenyl unit hence to impact on the λ cut-off and transmittance [20].

Solubility
All the fluorinated polyimides showed good solubility in high boiling point polar aprotic solvents, such as NMP, DMAc, DMF etc. and even in low boiling point solvents, such as THF, CHCl 3 etc. (Table 3) This could attribute to the presence of bulky -CF 3 groups, which increased disorder in the chains and inhibited dense chain packing, therefore, 91-97% at 500 nm. It was comparable to the commercial CP films at a nearly thickness which was developed at NASA Langley Research Center [30,31] (Table 2). All the polyimides exhibited lower cut-off wavelength (λ cut-off ) in    Cut-off wavelength.
no melting peak in the DSC curves indicates that the amorphous nature of these polyimides which is in accordance with the XRD study. T g values for BAPDBP based polyimides depending on the structure of the dianhydride component [9] and the stiffness of the polymer chain. The highest T g was observed for the PI-5 obtained from s-BPDA because of the presence of a rigid chain in the backbone. The reducing the interchain interactions to enhance solubility [35]. The kink linkages derived from phenyl pendant can reduce the effects of CTC to some extent. Moreover, the polarizability of nitrogen atom in pyridine in the backbone can improve the solubility [36]. However, PI-4, PI-5 showed a relatively poor solubility than the fluorinated polyimides. PI-4 derived from ODPA containing ether groups are, of course, flexible structure, but has little effect on the chain flexibility or configuration and, in turn, the solubility. As to PI-5, the poor solubility should ascribe to the rigid structure of s-BPDA. The solubility profiles of the polyimides correlated well with the optical transparency data.

Morphology study
The wide-angle X-ray diffractograms of polyimides are shown in Figure 5. There is no crystallization feature as observed from the wider diffraction peaks, indicating that all of the polyimides showed an amorphous pattern. This correlates well with the thermal analysis. The amorphous behavior of the polyimides is due to the kinky diphenylmethylene linkage, which significantly increased the disorder in the chains and decreased chain packing. In addition, the pendent phenyl groups also decreased the intermolecular forces between the polymer chains, subsequently causing a decrease in crystallinity [37]. Meanwhile, the existence of the pyridyl ether linkage units twist the polymer backbone structure, leading to the formation of amorphous polymer. [38]

Thermal properties
The thermal behavior of the PI films is shown in Table 4. DSC and DMA results revealed the T g of the polyimides in the range of 257-281 °C by DSC and 254-275 °C by DMA, as shown in Figures 6 and 7, respectively. There is     Pi-1  278  273  525  490  545  523  56  Pi-2  272  271  516  494  535  521  53  Pi-3  267  266  521  464  537  490  56  Pi-4  257  254  519  495  531  527  55  Pi-5  281  275  530  489  547  526  49 absorption rate of DuPont Kapton (2.50%) under the same conditions [39]. The results implied that the introduction of pyridine and phenyl pendant in the polymer backbone did not deteriorate the water absorption of the polyimides.

Conclusions
Two novel diamines containing kinky structure and pyridine were synthesized, characterized, and used for the preparation of a series of polyimides, via a traditional twostep method. All the PI-films showed high thermal stability with the T d5% at 516-530 °C and T d10% at 531-547 °C in nitrogen and high optical transparency which can be compared to the commercial colorless polyimides. The synergistic effects of kinky structure and pyridine of polyimides leading to high optical transparency without the sacrificing of thermal stability. This is, because the introduction of kinky and bulky substituents which increases the free volume and the pyridine which possess relatively high mole refraction as compared to phenyl unit gave a benefit to improve the optical properties. These properties of polyimides are desirable for application on space solar cells and thermal control coating systems.
lowest T g of PI-4 can be correlated with the flexibility of ether groups. For the structure-property relationship comparison studies, PI-1, PI-2, and PI-3 were prepared from reacting 6FDA with BAPDBP, BAPDAP and BAPDP, respectively. The resulting polyimide film samples gave T g values in the order of PI-1 > PI-2 > PI-3. This is, because the phenyl substituents are stiffer than the methyl substituents on the backbone. Thus the polyimides with a diphenylmethylene linkage have a higher T g than polyimides with a phenylethane or methyl linkage.
The thermal stabilities of the PIs were evaluated by TGA under nitrogen and air atmosphere ( Table 4). All PI films showed good thermal stability with 5% weight loss temperature above 510 °C and 10% weight loss temperature above 530 °C in N 2 . TGA curves for the 6FDA based polyimides PI-1, PI-2, and PI-3 are shown in Figure 8. While insignificant difference in T 5% and T 10% was observed from the TGA curves in N 2 , significant differences were observed from those acquired under air atmosphere. It can be argued that the methyl structure had poor thermal oxidative stability than phenyl pendant.

Mechanical properties and water uptake
The tensile properties of the PI films are summarized in Table 5. All of the polyimides displayed good mechanical properties with tensile strengths of 80-105 MPa, tensile modulus of 1.4-2.6 GPa and elongations at break of 4.3-9.6%. In contrast, PI-1, PI-2, and PI-3, with the introduction of methyl substituents showed lower tensile modulus, which coincided with the thermal analysis results.
Water uptake of the PI films was in the range of 0.56-0.76% at room temperature for 24 h (Table 5). All the polyimides showed very low water uptake compared to the water