Abstract
A series of molecular-weight-controlled aromatic copolyimides based on anhydrides 4,4′-oxydiphthalic anhydride (ODPA) or/and 2,3,3′,4′-biphenyltetracarboxylic dianhydride (α-BPDA) were prepared by polycondensation reaction with mixture of diamines 1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene (6FAPB) and 4,4′-diaminodiphenylsulphone (DDS) in the presence of phthalic anhydride (PA) as end-capping agent. The effect of the chemical structure of the copolyimides on solubility, rheological behavior, thermal stabilities and adhesive properties was investigated. Experimental results suggested that the solubility and thermal properties of copolyimides could be improved by incorporation of asymmetric α-BPDA. The copolyimide TPI-B (α-BPDA/6FAPB-DDS/PA) exhibited better solubility with the solid content as high as 45 wt.% in N,N-dimethylacetamide and high thermal stability with the T g of 306 °C. TPI-A (ODPA/6FAPB-DDS/PA) and TPI-C (α-BPDA-ODPA/6FAPB-DDS/PA) could be produced into glass-cloth-supported adhesive films. TPI-A adhesive film gave better adhesive properties at room temperature with lap shear strength of 16.41 MPa and T-peel strength of 2.1 kN m−1, which attributed to its flexible structure and good melt processability. TPI-C adhesive film exhibited higher adhesive properties at elevated temperature with the lap shear strength exceeding 9 MPa at 280 °C. The copolyimides with combination of rigid and flexible structures could afford good melt flow behavior and high thermal stability, which yielded good adhesive properties at high temperature.
Introduction
During the past decade, more attention has been paid to aromatic polyimides (PI) in many fields, such as aerospace, advanced microelectronics and the printed circuit industries.1–7 Polyimides have been extensively promoted as one of the candidates for high temperature adhesives due to their outstanding combinations of mechanical and thermal properties.8–14 However, the rigid polymer backbones and the strong chain interactions make the most of the insolubility and infusibility of the linear aromatic polyimides. Therefore, the early commercially available polyimide adhesives were used in a soluble poly(amic acid) precursor, which was followed by imidization into the polyimide. Unfortunately, the evolution of volatiles, such as water during the procedure of thermal imidization, is harmful for bonding and the environment.15–17 Therefore, many efforts have focused on thermoplastic polyimides which are amenable to melt processing.18–20 In addition, polyimide adhesive films have attracted the interest of polymer chemists because of their advantages in large-area bonding and convenience in application.21–23 In recent years, various thermoplastic polyimides and copolyimides have been developed and applied as film adhesives, including some in self-supported and glass-cloth-supported forms.24–27
One of the most successful thermoplastic polyimide adhesives is LARC-TPI, which was developed by NASA and which prepared from 3,3′,4,4′-benzophenonetetracarboxylic anhydride (BTDA) and 3,3′-diaminobenzophenone (3,3′-DABP) in bis(2-methoxyethyl)ether. 28 It exhibited good thermal stability with a glass transition temperature of 260 °C and could be produced into glass-cloth-supported adhesive film, which showed a high adhesive property with lap shear strength of 13.1 MPa at 232 °C when bonded to titanium. Saeed et al. have presented their work on adhesive properties of partially imidized thermoplastic polyimides films. 29 Two kinds of polyimides based on 4,4′-oxydiphthalic anhydride (ODPA) or 3,3′,4,4′-biphenyltetracarboxylic dianhydride (s-BPDA) were prepared by polycondensation reaction with 3,4′-oxydianiline (3,4′-ODA) in the presence of phthalic anhydride (PA) as end-capping agent. They found that the self-supported PI films in partially imidized form used as the adhesive in a bonded joint improved wetting of the adherent surface. The lap shear strength of polyimide films bonded to stainless steel at 350–375 °C was 5–12 MPa at 250 °C. Although these polyimide adhesive films are well established, their further utility to applications at temperatures above 250 °C is restricted. Therefore, many efforts have been made to develop adhesive films with better adhesive properties at high temperature. However, in general, this has led to poor rheological behavior by enhancing the thermal properties of polymers.30–32 Hence, a method of improving the thermal properties of polyimides without sacrificing their processability is an urgent problem that is still to be resolved. Since the 1980s, polyimides based on asymmetric α-BPDA have been developed to produce polyimides with relatively lower melt viscosities and higher glass transition temperatures (T g) that surpass the corresponding polyimides derived from the symmetric BPDA.33–35 This behavior is presumably due to the non-flexible structure and the significantly distorted and bent chain structure in α-BPDA. This provides a promising way to prepare the polyimides with high thermal stability without detracting from the processing characteristics.
In the present study, a series of aromatic copolyimides based on anhydrides 4,4′-oxydiphthalic anhydride (ODPA) or/and 2,3,3′,4′-biphenyltetracarboxylic dianhydride (α-BPDA) were prepared by polycondensation reaction with mixture of diamines 1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene (6FAPB) and 4,4′-diaminodiphenylsulphone (DDS) in the presence of phthalic anhydride (PA) as end-capping agent and with the controlled molecular weight of 30 000 g mol−1 were synthesized. They were defined as TPI-A (ODPA/6FAPB-DDS/PA), TPI-B (α-BPDA/6FAPB-DDS/PA) and TPI-C (α-BPDA-ODPA/6FAPB-DDS/PA), respectively. The employment of α-BPDA was expected to enhance the thermal properties of the copolyimides while retaining their melt processability. The correlations in detail between the molecular structure and properties of copolyimides, such as rheological behavior, thermo-mechanical properties and adhesive properties, are discussed.
Experimental
Materials
The 4,4′-oxydiphthalic anhydride (ODPA) were purchased from Shanghai Research Institute of Synthetic Resins and asymmetric 2,3,3′,4′-biphenyltetracarboxylic dianhydride (α-BPDA) was supplied by POME Sci-tech Co., China. They were all used after heating at 160 °C for 12 h in a vacuum oven. The 1,4-bis(4-amino-2-trifluoromethylphenoxy) benzene (6FAPB) was synthesized in the authors' laboratory according to the method previously reported. 36 The 4,4′-diaminodiphenylsulphone (Shanghai Research Institute of Synthetic Resins) were dried at 80 °C for 12 h in a vacuum oven before use. Phthalic anhydride were purchased from Beihua Fine Chemicals Co. China and purified by vacuum sublimation prior to use. N-methyl-2-pyrrolidinone (NMP) and m-cresol were purified by vacuum distillation before use. Commercially available N,N′-dimethylacetamide (DMAc), N,N-dimethylformamide (DMF), toluene, isoquinoline, and other solvents were used as received.
Copolyimide synthesis
The copolyimides were synthesized via a high-temperature polycondensation procedure by the reaction of a diamine mixture of 6FAPB and DDS at a mole ratio of 1 : 1 with ODPA or/and α-BPDA. In a typical experiment, copolyimide (TPI-A) was prepared with the following procedure: 6FAPB (21.42 g, 50.0 mmol), DDS (12.42 g, 50.0 mmol) and m-cresol (300 mL) were placed in a 1000 mL three-necked flask equipped with a mechanical stirrer, a thermometer, and a Dean–Stark trap topped by a condenser. The mixture was stirred at ambient temperature for 1 h until the aromatic diamines were completely dissolved to give a homogeneous solution. Then, ODPA (30.39 g, 98.0 mmol) and end-capping agent PA (0.60 g, 0.04 mmol) were added followed by the addition of m-cresol (233 mL) to yield a reaction mixture with 10 wt.% of solid content. After the mixture had been stirred in nitrogen at ambient temperature for 1 h, the azeotropic agent toluene (25 mL) and six drops of isoquinoline to act as a catalyst were added. The reaction solution was gradually heated to 180 °C and maintained at that temperature for 12 h with stirring, during which the water evolved in the procedure of imidization was removed simultaneously by azeotropic distillation. After the thermal imidization reaction was completed, the reaction solution was cooled to room temperature and then poured into an excess of ethanol with vigorous stirring to precipitate the copolyimide resin. The white precipitate was collected by filtration, washed thoroughly with ethanol and water, and dried at 100 °C for 2 h to remove most of the ethanol. The copolyimide resin was completely dried at 200 °C in a vacuum overnight to provide a yellowish white powder of TPI-A (57.25 g, 94.0% yield). The TPI-B and TPI-C were prepared by a similar procedure to that described above, with the exception that the α-BPDA and the mixture of α-BPDA and ODPA at a mole ratio of 1 : 1, respectively, were used instead of ODPA. The number-average molecular weight (M n) for all the copolymers was controlled to 30 000 g mol−1.
Preparation of adhesive films
Glass-cloth-supported copolyimide films were prepared by brush coating the polymer solution (35 wt.%) in DMAc onto an EW-100 glass cloth with a thickness of 0.1 mm. The glass cloth was tightly mounted on a metal frame and placed in a forced-air oven for 30 min at 280 °C. Then several coats of the originally prepared solution were applied until the thickness of the film reached about 0.24 mm. After each coat was applied, the film was placed in an oven and then heated using the following procedure: 80 °C/2 h, 120 °C/1 h, 150 °C/1 h, 180 °C/1 h, and 200 °C/1 h, successively. The prepared films were stiff and boardy with no tack or drape.
Preparation of lap shear strength test specimens
The lap shear strength test specimens were prepared in accordance with GB 7124-08. The stainless steel plates (AISI 321, Fe/Cr18/Ni9/Ti) of size 100 mm × 25 mm × 1.6 mm were used as adherend in this study. The steel plates were polished with a 100 grade emery paper to expose a new chemically active surface and then cleaned with acetone prior to bonding. Adhesive joints were prepared using all copolyimide solutions as well as glass-cloth-supported TPI-A and TPI-C films, respectively.
Preparation of lap shear strength test specimens using PI solution
The copolyimide powder was dissolved in DMAc to get a homogenous solution at a concentration of 30–35 wt.%, which was then applied to the steel plate surface using a spatula. The plates were then placed in a forced-air oven and heated using the procedure of: 80 °C/2 h, 120 °C/1 h, 150 °C/1 h, 180 °C/1 h, and 200 °C/1 h, successively. After the drying process, the other half was overlapped in the horizontal direction with an overlap of 12.5 mm. The bonded specimens were placed in a press that had been preheated to 320 °C and 2 MPa pressure was applied and held for 1 h. Then, the press was cooled and the bonded specimens were removed from the press at room temperature.
Preparation of lap shear strength test specimens using PI films
The glass-cloth-supported adhesive films were cut into 25 mm × 12.5 mm pieces and placed between the two steel plates with thickness of 0.24 mm. The bonded specimens were then placed in a press that had been preheated to 320 °C followed by applying a pressure of 2 MPa which was held for 1 h. The press was then cooled and the bonded specimens were removed from the press at room temperature.
Preparation of peel strength test specimens
The T-peel strength test specimens were prepared by bonding aluminum foils to aluminum alloy plates (LY 12-CZ) with a size of 200 mm × 25 mm × 2 mm. The aluminum foils and aluminum alloy plates were immersed in sodium dichromate/sulfuric acid solution at 60–70 °C to deoxidize, followed by rinsing with deionized water and drying in an oven prior to bonding. The peel strength test specimens were coated with copolyimide solution (30 wt.% in DMAc) and dried in an vacuum oven at 200 °C. The specimens were then bonded in the horizontal direction with the overlap area of 180 mm × 25 mm. The bonded specimens were placed in a hot press that had been preheated to 320 °C and kept for 1 h under contact pressure.
Characterization and measurements
Infrared spectroscopy spectra (FT-IR) were recorded on a Bruker Tensor 27 Fourier transform spectrophotometer. Wide-angle X-ray diffraction (XRD) measurements were conducted on a Rigaku D/max-2500 X-ray diffractometer with Cu/Kα radiation, operated at 40 kV and 200 mA. The number average molecular weights (M n) and polydispersities (M w/M n) were determined by gel permeation chromatography (GPC) on a Waters GPC system equipped with a Waters 1515 HPLC pump, a Waters 2414 differential refractometer, and three Styragel columns of HT-3, HT-4, and HT-5, using THF as the eluent at a flow rate of 1.0 mL min−1 at 35 °C. The system was calibrated using a series of monodisperse linear polystyrene standards.
The solubility of the copolyimides was determined by dissolving the copolyimide powders in different solvents with 15 wt.% of solid content at room temperature. The absolute viscosity of the copolyimide solutions in DMAc was measured on a Brookfield programmable DV-II+ Viscometer at 25 °C.
Differential scanning calorimetry (DSC) was performed using a TA Q100 thermal analyzer in nitrogen. The T g values were determined from the endothermic peak in the second heating run of the DSC curves at a heating rate of 10 °C min−1. Thermogravimetric analysis (TGA) was performed on a TA Q50 thermal analyzer at a heating rate of 20 °C min−1 in a nitrogen atmosphere.
Rheological measurements were conducted on a TA AR2000 rheometer. The specimens were prepared by the press molding of vacuum-dried copolyimide powder at room temperature into compacted discs of 25 mm diameter and 1.2 mm thickness. The measurements for the compacted resin discs were performed using a parallel-plate fixture by flow mode at a fixed shear stress of 10 000 Pa. The melt viscosity was measured as a function of temperature from 260 to 450 °C at a heating rate of 4 °C min−1.
The lap shear strength and T-peel strength of the adhesive joints were measured on an Instron 5567 Tensile Apparatus according to GB/T 7124-2008 and GB/T 7122-1996 of SAC, respectively. Lap shear strength specimens for testing at elevated temperatures were allowed to equilibrate for 5 min at the temperature prior to testing. The crosshead speed was set to travel at a rate of 5 mm min−1. The T-peel strength was tested at room temperature with a crosshead speed of 100 mm min−1. These measurement results were determined by the average value of five specimens for each test condition.
Results and discussion
Synthesis of copolyimides
A series of aromatic copolyimides based on anhydrides ODPA or/and α-BPDA were prepared by polycondensation reaction with mixture of diamines 6FAPB and DDS in the presence of PA as end-capping agent with a controlled molecular weight of 30 000 g mol−1 via a high-temperature polycondensation procedure as illustrated in Scheme 1. The reaction was carried out in a homogeneous solution with m-cresol as the solvent and isoquinoline as the catalyst. Toluene was employed as the azeotropic reagent to remove the water by-product to ensure the thermal cyclization was being processed completely.
Synthesis of the molecular-weight-controlled copolyimides.
It has been reported that incorporation of flexible linkages into the polymer backbones, such as ether linking groups, 32 and fluorine-containing groups36,37 provided higher adhesive strengths because of enhanced melt flow which, in turn, provided better wetting of substrate surface. The introduction of a flexible molecular structure was also beneficial to improve the polymer toughness and consequently enhance the peel resistance of polymers. In addition, the fluorine-containing polyimides displayed good thermal stability, as has been confirmed in the authors' previous work. 38 Furthermore, DDS with a sulfonyl group was considered to provide high adhesive strength together with high thermo-mechanical properties. 39 On the other hand, copolymers generally have weak intermolecular interaction because of the addition of the third component disrupts the symmetry of the polymer chains; hence, they display a good melt flow behavior. 40 Therefore, the dianhydride ODPA and a mixture of diamines of 6FAPB-DDS were selected for preparation in order to get copolyimides with good melt flow behavior without sacrificing their thermo-mechanical properties. Moreover, it is well known that the polyimides based on α-BPDA exhibited high thermal stability combined with good melt processability due to its asymmetric structure. In order to clarify the relationship between the molecular structure and rheological behavior in detail, the thermo-mechanical and adhesive properties of polymers, the copolyimides derived from dianhydrides α-BPDA and the mixture of ODPA with α-BPDA in mole ratio of 1 : 1, instead of ODPA, were also prepared.
The melt flow behavior of polyimides is also significantly affected by the molecular weight of the polyimide. Although the thermoplastic polyimides with relatively lower molecular weight exhibit better melt flow behavior, they often display a decline in thermo-mechanical properties. On the contrary, polyimides with high molecular weight will have good thermal stability. However, they are difficult to flow even at high temperature and therefore, it is necessary to control the molecular weight to within a reasonable range to achieve desirable melt flow behavior without sacrificing thermo-mechanical properties. 29 In the present study, the molecular weight of these copolyimides was controlled to 30 000 g mol−1 by endcapping with PA.
The chemical structure of the copolyimides was confirmed by FT-IR spectra as illustrated in Figure 1 . All the copolyimides exhibited absorptions around 1780 and 1726 cm−1 (imide C=O asymmetric and symmetric stretching), 1600 cm−1 (aromatic C=C stretching) and 1370 cm−1 (imide C–N stretching). Moreover, the absorptions around 1319 and 1140 cm−1 (sulfonyl S=O stretching) and at 1240 cm−1 (trifluoromethyl C–F stretching) were also detected in the FT-IR spectra of these polymers. The results demonstrated that the copolyimides had the expected chemical structures. The X-ray diffraction patterns of these copolyimides are shown in Figure 2 , in which no crystalline peaks were observed, thus revealing the amorphous nature of these copolyimides. This may be related to the decreasing intra- and inter-molecular interaction because of the existence of bulky trifluoromethyl groups of 6FAPB and the irregular structure of α-BPDA, which resulted in loose polymer packaging and aggregates.
FT-IR spectra of copolyimides.
Wide-angle X-ray diffraction curves of copolyimides.
The molecular weight of the copolyimides determined by GPC is summarized in Table 1 . There was no clear trend in the polydispersity indices (M w/M n), which were in the range of 1.44 to 1.60. Moreover, the GPC curves of these copolyimides as shown in Figure 3 exhibited a Gaussian-type distribution. Therefore, the effect of the molecular weight distribution on the polymer properties could be ignored in this work. It has also been found that the measured M n values for all the copolyimides were lower than the theoretical ones (M n T), implying the molecular weights of the copolyimides did not grow as expected. This may be related to the high electronegative sulfonyl substituent of DDS, which could have caused the low nucleophilic reactivity of amine. Furthermore, the M n values of TPI-B and TPI-C were 10 512 and 12 057 g mol−1, respectively, and were even lower than TPI-A. This could be interpreted as due to the low reactivity of α-BPDA. Therefore, the length of polymer chains were shorter than expected, especially for the polymer with the structure of α-BPDA.
Molecular weights of the molecular-weight-controlled copolyimides.
a Mn T = 30 000 g mol−1
GPC curves of the copolyimides in THF.
Solubility
The solubility of copolyimides was qualitatively determined by the dissolution of solid powder of the copolyimides in different solvents with 15 wt.% of solid content, which was stirred and placed for 24 h at room temperature. Table 2 summarizes the solubility of the copolyimides. It was found that all of the copolyimides could easily be dissolved in strong aprotic solvents, such as NMP, DMF, and DMAc, to afford homogeneous solutions. Additionally, the copolyimides were also soluble in less polar solvents, such as DMSO, and partially soluble in THF and chloroform. Figure 4 depicts the dependence of the absolute viscosity of copolyimide solutions in DMAc on the solid content. The copolyimides in DMAc with 25 wt.% of solid content gave absolute viscosities of no more than 500 mPa s, and exhibited only a slight increase in values as the solid content was increased to 35 wt.%. When comparing these polymer solutions at the same concentrations, it was found that TPI-B and TPI-C showed relatively lower absolute viscosity values than TPI-A. Moreover, homogeneous and stable TPI-B and TPI-C solutions in DMAc with solid content as high as 45 wt.% could be achieved. The good solubility of TPI-B and TPI-C is associated with the introduction of α-BPDA. It is known that the steric repulsion of the substituents in the 2- and 2′-positions of the biphenyl moieties of α-BPDA apparently forces the two phenyl rings into adopting a non-coplanar conformation, which, in turn, inhibits chain packing and crystallization, and thus increases solubility. 41 The good solubility of copolyimides provided them with a distinct advantage in processing when they were used as adhesives.
Solubility of the copolyimides. a
a ++, Soluble at room temperature; +, partially soluble; −, insoluble.
The absolute viscosity of copolyimides in DMAc solution with different solid contents.
Thermal stability
The thermal properties of these copolyimides were evaluated by DSC and TGA and the results are summarized in Table 3 . From the DSC curves shown in Figure 5 , it was found that the glass transition temperatures (T g) of these copolyimides were in the range 263 to 306 °C, which displayed a increasing trend with the incorporation of α-BPDA. TPI-B exhibited the highest T g value of 306 °C, which was 40 °C higher than that for TPI-A. This can be explained by the asymmetric structure of α-BPDA, which requires a larger sweep volume for the crank shaft motion and accordingly needs more energy (higher temperature) for the motion. 42
Thermal properties of the copolyimides determined by DSC and TGA.
DSC curves of copolyimides.
Figure 6 compares the TGA curves of the copolyimides, which exhibited excellent thermal stability with the onset decomposition temperatures and temperatures at 5 wt.% weight loss in the range of 568–571 °C and 568–574 °C in nitrogen, respectively. The copolyimides did not show obvious weight loss before the scanning temperature reached 500 °C in nitrogen, indicating that no thermal decomposition occurred. The residual weight retentions at 700 °C for all the copolyimides were higher than 60%. When comparing the TGA data of these copolyimides, it was also found that TPI-B exhibited a much higher decomposition temperature than the others. From the results it was concluded that the introduction of α-BPDA is beneficial for enhancing the thermal properties of copolyimides.
TGA curves of copolyimides.
Melt processability
For polymers applied as adhesives, proper melt viscosities are required to allow good flow and wetting in a bond. The melt processability of the copolyimides was investigated by melt rheology. The melt viscosity of these copolyimides as a function of temperature from 290 to 400 °C is shown in Figure 7 . The onset melt temperature (T onset) and the melt viscosity of copolyimides at 320 and 360 °C are summarized in Table 4 . It can be seen that all the copolymers exhibited good melt processability with onset melt temperatures in the range of 290 to 300 °C and that their melt viscosities declined as the temperature increased. TPI-A showed better melt flow behavior at the onset melt temperature of 290 °C and a melt viscosity of 0.9 × 106 Pa s at 320 °C. The good melt processability of TPI-A was related to its relatively flexible polymer structure. The incorporation of α-BPDA in TPI-B and TPI-C afford copolymers with melt viscosities higher than TPI-A at a temperature of no more than 360 °C. TPI-B and TPI-C exhibited melt viscosities of 2.9 × 106 and 1.9 × 106 Pa s at 320 °C, respectively. The unexpected enhancement in melt viscosity of copolyimides containing α-BPDA is considered to be related to there being insufficient molecular mobility at 320 °C. It was also noticed that the melt viscosities of TPI-B and TPI-C decreased abruptly as the temperature increased. They showed a melt viscosity of 104 Pa s at 360 °C, which was quite similar to that of TPI-A but as the temperature increased continuously, the melt viscosities of TPI-B and TPI-C were even lower than that of TPI-A.
Melt viscosity of copolyimides as a function of temperature.
The initial melt temperature and melt viscosity of copolyimides.
Adhesive properties
The adhesive joints were prepared with copolyimide solutions and films adhered to stainless steel at 320 °C, respectively. The adhesive properties of the copolyimides were evaluated by lap shear strength testing at different temperatures. The results for the joints prepared with copolyimide solutions are shown in Figure 8 . It was found that TPI-A exhibited the highest lap shear strength of 19.88 MPa at room temperature. The good adhesive properties of TPI-A were considered to be related to its flexible structure, which allowed good surface wetting and spreading. On the contrary, TPI-B gave the lowest lap shear strength value of 10.07 MPa, indicating that the joints lacked surface wetting because of the insufficient fluidity of TPI-B at the bonding temperature. When tested at 250 °C, the lap shear strength values for these copolyimide solutions showed decreasing values in the range of 7.93 to 16.10 MPa. It is known that the degradation of polymeric adhesive or the interface may lead to failure at the temperature at which it is used. 43 In addition, the lap shear strength of joints prepared with these copolyimide solutions were still in the order of TPI-A > TPI-C > TPI-B when tested at 250 °C. The corresponding retention ratios were in the range of 78.7 to 87.9% as listed in Table 5 . The lap shear strength of TPI-A dropped abruptly to 1.33 MPa with a retention ratio of 6.7% as the testing temperature increased to 280 °C. In the investigation of the thermal properties of these copolyimides, it is known that the glass transition temperature of TPI-A determined by DSC was 263 °C. Therefore, the dramatic decreasing of lap shear strength of TPI-A can be explained in terms of its flexible structure and insufficient thermal stability. The lap shear strengths of TPI-B and TPI-C exhibited decreases of different amounts as the testing temperature increased. It should be mentioned that TPI-C gave the highest lap shear strength of 9.66 MPa and a retention ratio of 63.9% when tested at 280 °C. The good adhesive properties of TPI-C were associated with the combination of rigid and flexible structures, which allowed moderate melt fluidity of the polymer in the bonding process and provided high thermal stability. Although TPI-B showed good thermal stability with the highest T g value of 306 °C, it exhibited a relatively lower lap shear strength than TPI-C because of its poor melt fluidity in the bonding process.
Lap shear strength of copolyimides prepared in form of solution.
Lap shear strength of copolyimides prepared in form of solutions and films tested at different temperatures.
TPI-A and TPI-C had good flexibility and could be produced as glass-cloth-supported adhesive films. However, TPI-B was too brittle to form an adhesive film. The lap shear strength of joints prepared with TPI-A and TPI-C adhesive films were also measured at different temperatures and the results are shown in Figure 9 . The lap shear strengths of joints prepared with TPI-A film were 16.41 MPa at room temperature and 12.16 MPa at 250 °C, respectively. As the testing temperature increased to 280 °C, the lap shear strength for TPI-A film declined abruptly to 1.27 MPa with a retention ratio of 7.7%. The lap shear strength of joints prepared from TPI-A in the form of solution and of film showed similar decreasing trends as the testing temperature increased. On the other hand, the lap shear strength of joints prepared with TPI-C film was 12.28 MPa when tested at room temperature, and this was maintained at 9.23 MPa with a retention ratio of 75.2% even when tested at 280 °C. When comparing the adhesive properties of TPI-A and TPI-C in the form of film and solution, it was found that the former gave lap shear strength values that were slightly lower than the latter when tested at the same temperature. This may be affected by the weak interface of the glass cloth in the joints prepared with supported film.
Lap shear strength of TPI-A and TPI-C prepared in form of glass-cloth-supported film.
The T-peel strengths of copolyimides in the form of solution adhered to aluminum foils and aluminum alloy plates are shown in Figure 10 . TPI-A exhibited the highest peel strength of 2.1 kN m−1, whereas TPI-B gave the lowest value of 0.9 kN m−1. The results showed good agreement with their flexibility and melt processability behavior.
Peel strength of copolyimides processed at 320 °C for 1 h.
Conclusions
A series of aromatic copolyimides based on anhydrides ODPA or/and α-BPDA were prepared by polycondensation reaction with mixture of diamines 6FAPB and DDS in the presence of PA as end-capping agent with the controlled molecular weight of 30 000 g mol−1. These copolyimides had good solubility not only in most of the polar aprotic solvents but also in some common organic solvents and afforded homogeneous and stable copolyimide solutions with solid content as high as 40–45 wt.%. The solubility and thermal properties of the copolyimides could be improved by incorporation of asymmetric α-BPDA. The copolyimides TPI-B and TPI-C exhibited outstanding thermal properties with T g values of 306 and 288 °C, respectively. The rheological behavior of these copolyimides was affected by their molecular structure. TPI-A and TPI-C could be produced as glass-cloth-supported adhesive films. When comparing the adhesive properties of the copolyimides, it was found that TPI-A adhesive film gave better adhesive properties at room temperature with lap shear strength of 16.41 MPa and T-peel strength of 2.1 kN m−1, which were attributed to its flexible structure and good melt processability. TPI-C adhesive film exhibited higher adhesive properties at elevated temperatures with the lap shear strength exceeding 9 MPa at 280 °C. It was concluded that copolyimides with a combination of rigid and flexible structures could afford good melt flow behavior and high thermal stability, which yielded good adhesive properties at high temperature.