Synthesis and properties of low coefficient of thermal expansion copolyimides derived from biphenyltetracarboxylic dianhydride with p-phenylenediamine and 4,4′-oxydialinine

2.1 Materials

The a-BPDA, s-BPDA and 4,4′-ODA were obtained from Shandong Wanda Chemical Industries Co Ltd (China). PDA was purchased from Shanghai Jindaimao Chemical Industries Co Ltd (China). The diamines were used as received after vacuum-drying at 70°C for 24 h. The dianhydrides before synthesis were heated after vacuum-drying at 200°C for 24 h. N,N-Dimethylacetamide (99.8%, DMAC) was provided from Chengdu Chemical Reagent Factory (China) and used as-received.

2.2 Synthesis of co-poly(amic acid)

The co-poly(amic acid)s (PAAs) were synthesized by adding solid forms of a-BPDA and s-BPDA to a stirred mixture solution of 4,4′-ODA and PDA in DMAC in a nitrogen atmosphere at 0°C. The reaction mixture was stirred for 0.5 h at this 0°C, after that it was heated to room temperature and stirred for 48 h. The molar ratio of the dianhydrides and diamines, and the inherent film qualities of poly(amic acid)s dried in 145°C are presented in Table 1. The PAAs structure of obtained no crack films are shown in Figure 1. The IR and ¹³C NMR data of those PAAs are as follow.

Figure 1:

The structure of obtained flexible polyamic acids.

Co-PIs-3 PAA, IR (KBr) 3295, 2921, 1712, 1658, 1604, 1515, 1404, 1321, 1255, 1019, 838, 795, 742 and 709 cm^-1. ¹³C NMR (DMSO d₆) δ 170.07 (C-1), 167.66(C-2), 149.70(C-16), 139.77(C-7), 138.66(C-17,18), 131.06(C-6,11,13), 127.31(C-5,10), 123.58(C-3,8,9,12), 120.07(C-14), 112.47(C-15) and 99.74(C-4) ppm.

Co-PIs-5 PAA, IR (KBr) 2908, 2357, 1714, 1605, 1515, 1405, 1321, 1258, 1019, 837, 795, 744 and 708 cm^-1. ¹³C NMR (DMSO d₆) δ 170.07 (C-1), 167.66(C-2), 149.80(C-16), 140.11(C-7), 135.69(C-17, 18), 130.39(C-6, 11, 13), 128.34(C-5, 10), 121.43(C-3, 8, 9, 12) and 120.39(C-14, 15) ppm.

Co-PIs-6 PAA, IR (KBr) 3291, 2960, 2358, 1720, 1666, 1605, 1511, 1406, 1322, 1255, 1019, 839, 795 and 746 cm^-1. ¹³C NMR (DMSO d₆) δ 170.06 (C-1), 167.66(C-2), 143.50(C-7), 135.99(C-17, 18), 127.99(C-5, 10), 121.42(C-3, 8, 9, 12) and 120.07(C-14, 15) ppm.

2.3 Films of copolyimides preparation

Thin films were prepared from DMAC solutions of PAAs. The solutions were coated onto clean, dry plate-glass and then dried at 60°C for 2 h in an air-convection oven. The coatings were cured by heating under mild thermal conditions in follow stages: at 100°C for 40 min, 125°C for 30 min, 145°C for 20 min, 185°C for 10 min, 220°C for 20 min, 260°C for 20 min, 300°C for 40 min, 330°C for 20 min, 350°C for 20 min. The thickness of the films was 60–80 μm.

2.4 Characterizations

The chemical structure information of polyimide films was collected using infrared measurements (Version BM Spectrometer). ¹³C NMR spectra were recorded on a Brucker Avance 600 spectrometer with dimethylsulfoxide-d₆ (DMSO-d₆) as the solvents, and tetramethylsilane (TMS) as the internal reference. The molecular weights were determined by gel permeation chromatography (PL-GPC220). Wide angle X-ray diffract (WAXD) patterns of the films were obtained with a diffractometer (Cu Ka, 40 kV, 100 mA, Xpert MPD Pro). The surface morphologies of the films were studied by field-emission scanning electron microscopy (FE-SEM, Ultra 55). Thermogravimetric (TG) analyses of the films were performed with a TA Instrument SDT Q600 in a nitrogen atmosphere at a heating rate of 10°C/min. Mechanical properties were determined from stress-strain curves with a Toyo Instron UTM-III-500 with a load cell of 10 kg at a drawing speed of 5 cm/min. Measurement was performed at 25°C with film specimens (about 50 μm thick, 1.0 cm wide and 5 cm long) and an average of at least five individual determinations was used. Dynamic mechanical thermal analysis (DMA) was performed on thin film specimens (23.7 mm long, 5 mm wide and 0.05 mm thick) on a TA Instruments Q800 DMA at a heating rate of 3°C/min and a load frequency of 1 Hz in an air atmosphere as a function of temperature from 20°C to 400°C. Coefficients of thermal expansion (CTE) of PI specimens (10 mm long, 3 mm wide, 0.05 mm thick) were measured as the average within 50–150°C for the in-plane direction on a Perkin-Elmer Thermal Mechanical Analyzer-7 at a heating rate of 2°C/min and load of 14 g (0.3 g per a unit film thickness in μm, typically, 14 g load for 50 μm thick films) were measured.

3.1 Synthesis of copolyimides

To obtain the copolyimides, we have employed the most popular way of polyimide synthesis, the reaction of two aromatic dianhydrides with two aromatic diamines to form soluble PAA as a precursor (stage 1), which is thermally converted (stage 2) to a copolyimide (Scheme 1).

Scheme 1:

Two-stage synthesis of copolyimides.

In order to investigate the flexibility of copolyimide film, the inherent film qualities of polyamic acids dried in 145°C were observed. Because of the strong rigidity structure of monomers, the PAAs from Co-PIs-1, Co-PIs-2 and Co-PIs-4 failed to provide flexible films. The films of Co-PIs-7, Co-PIs-8 and Co-PIs-9 had some cracks and many small pieces, as results of low molecular weights (Table 1). Flexible copolyimide films (Co-PIs-3, Co-PIs-5 and Co-PIs-6) could be produced by thermal imidization using the following steps: 100°C for 40 min, 125°C for 30 min, 145°C for 20 min, 185°C for 10 min, 220°C for 20min, 260°C for 20 min, 300°C for 40 min, 340°C for 20 min. The infrared and ¹³C NMR data of obtained flexible PAAs confirmed that 4,4′-ODA, PDA, a-BPDA and s-BPDA entered into the macromolecular chain of copolyamic acids. The complete imidization of polyimide from polyamic acids was proved by Fourier transform infrared (FT-IR)spectroscopy.

The FT-IR spectra of three flexible copolyimide films showed the characteristic imide bands at 1775, 1717, 1360 and 725 cm^-1 (Figure 2). Additionally, band at 3284–3400 cm^-1 that was related to the N-H bond of amine group in the diamine was disappeared, which was also a proof of a complete imidization of those films. Furthermore, due to copolymerization with 4,4′-ODA of those films, in the FTIR spectra of those films, the =C-O-C= stretching of Aryl ether at 1229 cm^-1 could be observed.

Figure 2:

The FTIR spectra of Co-PIs-3, Co-PIs-5 and Co-PIs-6 films.

3.2 Wide-angle X-ray diffraction of copolyimides

Representative WAXDs of films of three polymers (Co-PIs-3, Co-PIs-5 and Co-PIs-6) are shown in Figure 3. Three films had non-Gaussian distribution curves that revealed a semi-crystalline structure of those polymers. Such diffraction peaks were attributed to the molecular chain orientation along the chain axis as well as to a good lateral packing order. Compared with the three films, Co-PIs-5 had lowest crystallinity. Co-PIs-6 had lower crystallinity than Co-PIs-3, which indicated that the increasing part of a-BPDA as a more bent and non-coplanar unit decreased the crystallinity of the product.

Figure 3:

The WAXD of Co-PIs-3, Co-PIs-5 and Co-PIs-6 films.

3.3 The glass transition temperature and thermal stability of copolyimides

Glass transition temperatures were determined by DMA. It can be seen from Table 2 and Figure 4, the T_gs of the prepared copolyimide films are about 358°C for the Co-PIs-3 and 369°C to 353°C for Co-PIs-5 to Co-PIs-6. The slight decrease of Co-PIs-3 to Co-PIs-6 in T_g is ascribed to a partial substitution of a-BPDA for s-BPDA, as the a-BPDA monomer has greater bend and non-coplanar structure than the s-BPDA monomer, as results from the weaker intermolecular interaction in Co-PIs-6 than in Co-PIs-3. The substitution of ODA for PDA also decreases the rigidity of the polymer chains, as well as the T_g of corresponding polyimides. The polyimides with less content of flexible ODA and higher content of rigid PDA have higher T_g. The decrease of Co-PIs-5 to Co-PIs-6 in T_g can be explained by a partial substitution of 4,4′-ODA for PDA monomer.

Figure 4:

Tan δ of Co-PIs-3, Co-PIs-5 and Co-PIs-6 films.

The thermal stability of three prepared copolyimide films was evaluated by TG (Figure 5 and Table 2). According to Table 2, when the partial substitution degree of a-BPDA for s-BPDA and 4,4′-ODA for PDA increases, the thermal stability decreases. In fact, Co-PIs-5 has the highest degradation temperature, at 602°C of T₅ and 627°C of T₁₀. This result indicates that a-BPDA moiety slightly reduces the thermal stability of the copolymers.

Figure 5:

TG curves of Co-PIs-3, Co-PIs-5 and Co-PIs-6 films.

3.4 Dynamic mechanical properties of copolyimides

The dynamic mechanical properties of copolyimides were measured. The dynamic viscoelastic curves for Co-PIs-3, Co-PIs-5 and Co-PIs-6 are shown in Figure 6. The storage modulus curves of three copolyimides displayed marked drop up to the T_g. The extent of the storage modulus drops of three copolyimides decreased with an increase of a-BPDA. Comparing the three copolyimides, the storage modulus of Co-PIs-5 was lowest from 25°C to 300°C and the extent of the storage modulus drop of Co-PIs-5 was least, reflecting that the Co-PIs-5 had the best flexibility.

Figure 6:

Dynamic storage modulus curves of Co-PIs-3, Co-PIs-5 and Co-PIs-6 films.

3.5 Mechanical properties of copolyimides

All polyimides formed flexible films by step imidization in a forced air oven. The mechanical properties of the films were measured at room temperature, and are summarized in Table 3.

One might note that generally the tensile strengths were higher when the amount of the PDA monomers were larger and the more s-BPDA monomers were in the macromolecule. These results of tensile strength might be attributed to the fact that more linear rigid structures were beneficial to the stacking of polymer chains and the enhancement of tensile strength. These analyses of tensile strength for three copolyimides can be confirmed by the SEM images of three copolyimides (Figure 7). The surface of the Co-PIs-6 sample had more and larger sized micro cracks than that of Co-PIs-3 and Co-PIs-5.

Figure 7:

The SEM of the copolyimide films: (A) Co-PIs-3, (B) Co-PIs-5 and (C) Co-PIs-6.

Due to an increase of a more kinked structure in the macromolecules and a more chain-chain entanglement, the elongation at break of copolyimides containing more s-BPDA monomers was less than that of copolyimides containing more a-BPDA monomers. Co-PIs-5 had excellent tensile strength, and its elongation at breaks the largest.

3.6 Coefficient of thermal expansion

The CTE of the copolyimide films are shown in Table 2.

The Co-PIs-5 film displayed a considerably low CTE of 13 ppm/K, which was much lower than that of Co-PIs-3 film(CTE=21 ppm/K) and that of Co-PIs-6 film(CTE=26ppm/K). Based on their structural distinction, BPDA-PDA was regarded as rigid or planar structures and had larger in-plane molecular orientation (ordered structure) which was known to be responsible for their low CTE. The flexible (ODA) and bent and non-coplanar units (a-BPDA) in BPDA-PDA made the polymers in-plane orientation decrease and the CTE increase. In addition, the CTE was related to the molecular packing degree. The polymer chains were packed more densely and the polymer had smaller free volume that was favorable to form lower CTE polymers. The lowest CTE of Co-PIs-5 film is in accordance with the greatest packing degree as shown in Figure 7.