6.2.1 Thermoplastic Polyimides Based on BEPA

ULTEM is one of the typical bisphenol A bisether-4-diphthalic anhydride (BEPA)–based thermoplastic polyimides; it was commercialized by General Electric Company in 1982. ULTEM can be processed by conventional techniques such as injection molding at 360–380 °C. However, the thermos-oxidative stability of ULTEM is relatively poor owing to the presence of the thermally unstable. Takekoshi et al., [1] reported that ULTEM is produced via nitrodisplacement of bisnitroimides (scheme.6.1) with bisphenol A in polar aprotic solvents.

Shi et al., [2] synthesized another several BEPA-based TPI with bisphenol A and a variety of diamines Table 6.1.

Abbreviations	Structures
PDA
m-PDA
4,4’-ODA
3,4’-ODA
p-BAPS
m-BAPS
TPER
TPEQ
BAPB
PTPEQ
o-TOL
m-TOL

TPIs based on BEPA were prepared with various aromatic diamines in the conventional two-step process via poly(amic acid) (PAA) polymerization and successive thermal imidization. A typical procedure of PAA polymerization is as follows: to a solution of 8 mmol 1,3-phenylenediamine in 40 ml of DMAc, 8 mmol BEPA powder was gradually added. The solution was stirred at room temperature for 24 h. The viscous PAA solution obtained was cast with a doctor blade and dried in an oven at 70 °C for 3 h. The dried PAA films were then thermally imidized upon a stepwise heating process of 150 °C/1h + 200 °C/1h + 250 °C/1h + 280 °C/2h under vacuum in a free-standing state. The thickness of PI films ranged from 20 to 40 µm. To obtain the homopolyimide powder, the PAA solutions were refluxed at 170 °C for 4 h and then poured into a large excess of methanol for precipitation. The obtained powder was postcured under vacuum at 250 °C/h or 280 °C/h to ensure complete imidization.

6.2.2 Thermoplastic Polyimides based on PMDA

Pyromellitic dianhydride (PMDA) is one of the massive production and cheapest aromatic dianhydrides. Developing TPI based on PMDA could effectively lower the fabrication cost. But there are few TPIs based on PMDA due to their strong rigid structures. In the late 1980s, Mitsui Chemicals, Inc. began investigating and developing a thermoplastic polyimide to meet industry needs. As a result of these efforts, Aurum was launched. This material is synthesized from pyromellitic dianhydride (PMDA) and 4,4-bis (3-aminophenoxy)biphenyl and has a high heat resistance with glass transition temperature Tg = 250 °C. AURUM is an injection-moldable semicrystalline polyimide, but it has a very slow crystallization rate. The part obtained through injection molding is amorphous, not crystalline, although Aurum is a semicrystalline polymer. The postcuring after injection molding enabled crystallization, but control of the tight dimension was not sufficient [3].

AURUM has better heat resisting property than ULTEM, but the raw material 4,4-bis (3-aminophenoxy)biphenyl is much more cost which limited its yield and applications. E.I. du Pont de Nemours and Company developed another PMDA-based TPI called VESPEL. Its T_g is up to 385 °C. S. Tarmai and A. Yamaguchi used PMDA and ether diamines to synthesize a series of TPI. The typical polymerization is as follows: A mixture of 0.0582 mol of PMDA, 0.0600 mol of diamines, 0.0036 mol of phthalic anhydride, which was a termination of a polymer chain end, 0.009 mol of 7-picoline in 123 g of m-cresol was stirred at 150 °C for 4 h, and then after cooling down, the reaction mixture was poured into methanol. The precipitated polyimide was collected by filtration, followed by thorough washing with methanol and dried in a forced air oven to a temperature around the Tg of the polyimide. The other polyimide powders were prepared by the same method as mentioned above.

Scheme 6.3 shows the typical synthesis route of PMDA based TPI. The structures of the ether diamines can be found in Table 6.2.

Abbreviations	Structures
4d-m
4d-p
4f-m
4f-p
4g-m
4g-p
4e-m
4e-p
4b-m
4b-p
4c-m
4c-p

6.2.3 Thermoplastic Polyimides Based on BTDA

V. Ratta et al., used 3,3’,4,4’-biphenonetetracarboxylic dianhydride (BTDA) and an 1,3-bis(4-aminophenoxy) benzene (TPER diamine) to synthesize one kind of high-temperature semicrystalline thermoplastic polyimides. The polyimide displays a T_g at ca. 230 °C and two prominent melting endosperms at 360 °C and 460 °C, respectively, with a sharp recrystallization exotherm following the lower melting endotherm.

The reaction vessel was a three-neck round bottom flask equipped with a mechanical stirrer, nitrogen inlet, and a drying tube. Sufficient N-methylpyrrolidinone (NMP) was added to achieve a 10% solid concentration, and the solution was allowed to stir for 24 h, to afford a homogenous poly(amic acid) solution as shown in Scheme 6.4. A stepwise thermal imidization procedure was utilized. The first step was the casting of the poly(amic acid) precursor on the Pyrex glass plates. These plates were placed in the vacuum oven overnight until smooth nontacky films were obtained. Thermal imidization was achieved by raising the temperature to 100, 200, and 300 °C and holding at each of these temperatures for 1 h. The time to go from one temperature to the next was ca. 1 h each at the fastest heating rate available with the oven. After the completion of the cycle, the plates were allowed to cool to room temperature before being removed from the oven. The films were stripped off the glass plates in hot water and stored in a desiccator before use. Tapas koley et al., [6]. used a diamine monomer, 1, 4-bis-[{2’-trifluromethyl 4′-(4″-aminophenyl)phenoxy}] benzene (HQA), and BTDA to synthesize one kind of BTDA-based TPI (Figure 6.2). The polyimides showed reasonably high glass-transition temperature (Tg280 °C) and high thermal stability (Td, 558 °C).

The bis(ether amine) monomer was reacted with BTDA according to two-step conventional procedure to obtain poly(ether imide) membranes. A representative polymerization procedure is as follows: BPADA (0.338 g, 0.65 mmol) was added in portions into the diamine (HQA) (0.377 g, 0.65 mmol) solution in dry DMF (7.0 mL) containing in a 25 mL round bottom flask equipped with a guard tube. The mixture was left for stirring at room temperature while it is transformed to a highly viscous solution of polyamic acid (PAA) within 45–60 min. In the second step, viscous PAA solution was spread onto a clean and dry flat-bottomed Petri dish and kept in oven initially at 80 °C for overnight for slow removal of the solvent. Finally, the thermal cyclization of the PAA to PI was achieved by sequential heating at 100, 150, 200, 250, and 300 °C, each for 1 h and at 350 °C for 30 min in an oven under nitrogen atmosphere. The resulting polyimide membranes were removed from the Petri dishes by immersing in hot water after that the membranes were dried under vacuum at 140 °C for 26 h.

6.2.4 Thermoplastic Polyimides Based on ODPA

Shanghai Research Institute of synthetic resins developed one TPI with 4,4’-oxy-diphthalic anhydride (ODPA) and 4,4’-oxybisbenzenamine (4,4’-ODA) called Ratem-YS-20 in 1970s [7]. NASA used ODPA and 3, 4’-oxybisbenzenamine (3, 4’-ODA) to synthesize another TPI called LaRC-IA. The structures of these two TPI based on ODPA could be found in Figure 6.3 [8, 9].

Shi et al., [2]. also synthesized and characterized a serial TPI with ODPA and a series of diamines (4,4’-ODA and 3, 4’-ODA included). The synthesis route of ODPA-based TPI is just the same as the BEPA-based ones as Shi et al., reported.

6.2.5 Thermoplastic Polyimides Based on BPDA

In 2001, S. Tamai et al., synthesized 3,3’,4,4’-biphenyltetracarboxylic dianhydride (BPDA)-based TPI. The synthesis procedure is as follows: 3,4’-oxydianiline (3,4’-ODA) polyimide was synthesized with 28.10 g (95.5 mmol) of BPDA, 20.03 g (100 mmol) of 3,4’-ODA, 1.33 g (9 mmol) of PA, which was used for terminating the polymer chain end. A four-neck round-bottom flask equipped with a mechanical stirrer, nitrogen pad, thermometer, and a condenser with a Dean–Stark trap was used as the reaction vessel. All monomers were added to a reaction vessel and then 3-methylphenol was added to achieve a 10% solids concentration. This solution was stirred and heated under nitrogen atmosphere for 12 h at 202 °C, to afford the PA end-capped polyimide. During the reaction, the by-product, water, was removed by nitrogen flow. After cooling down, methylethyl ketone was added to the reaction mixture. The precipitated polyimide was collected by filtration, followed by thorough washing with methylethyl ketone and dried in a forced air oven at 300 °C for 4 h under nitrogen atmosphere. The other polyimide obtained from 1,3-bis(4-aminophenoxy)benzene (TPER), and BPDA was also prepared by almost the same method as mentioned above.

6.2.6 Thermoplastic Copolyimides

E.I. du Pont de Nemours and Company published a patent which described the synthesis of thermoplastic copolyimides in 2001. The polymers were the reaction products of components comprising an aromatic dianhydride component, an aromatic diamine component, and an end-capping component. The aromatic dianhydride component is selected from the group consisting of BPDA and BTDA, BPDA is preferred.

The aromatic diamine component consists of a first aromatic diamine and a second aromatic diamine. The first aromatic diamine is selected from the group consisting of APB-134 and 3,4’-ODA. The second aromatic diamine is selected from the group consisting of APB-133, 4,4’-ODA, MPD, APB-144,BAPS, BAPB, BAPE, BAPP, and 4,4’-ODA, MPD in combination, and 4,4’-ODA and PPD in combination; wherein APB-133,APB-144, 4,4’-ODA,BAPS, and 4,4’-ODA and MPD in combination are preferred. 4,4’-ODA, and 4,4’-ODA and MPD in combination are most preferred.

Suitable end-capping components when diamines are in excess include, but are not limited to, phthalic anhydride and naphthalic anhydride. These copolyimides have a stoichiometry in the range from 93% to 98 %, exhibit a melting point in the range of 330 °C to 385 °C, and exhibit recoverable crystallinity as determined by differential scanning calorimetry analysis.

A typical synthesis procedure is as follows:

Preparation of Polyimide Based on BPDA//3,4’-ODA/APB-134//PA 95//70/30//10—(95% of stoichiometric dianhydride)

Into a 250 mL round bottom flask equipped with a mechanical stirrer and nitrogen purge were charged 5.3695 g (0.02681 mole) of diamine 3,4’-ODA, 3.3596 g (0.01149 mole) of diamine APB-134 and 60 ml of NMP. After dissolution of the diamines, 10.7073 g (0.03639 mole) of dianhydride BPDA and 0.5674 g (0.00383 mole) phthalic anhydride were added with stirring under nitrogen and rinsed in with 20 ml NMP. The following day, 14.46 ml (0.153 mole) of acetic anhydride (4 × moles of diamine) and 21.36 ml (1.53 mole) of triethylamine (4 × moles of diamine) were added to the poly(amic acid) solution to effect imidization. After about 10 minutes the polymer precipitated, any clumps were broken up by manual manipulation of the mechanical stirrer, and stirring was continued for about 6 hours. The resulting polymer slurry was then added to methanol in a blender to complete precipitation and remove NMP. The polymer was separated by filtration, washed with methanol, and then dried at ca. 200 °C overnight under vacuum with a nitrogen bleed. DSC analysis (10 °C/min.) of the resulting polyimide showed a melting point of 345 °C during the first heating scan, a crystallization exotherm upon the subsequent cooling scan at 296 °C, and a melting point at 346 °C during the subsequent reheat scan, indicating recoverable crystallinity from the melt.

The synthesis of other compolyimides was similar to the typical manner.

6.3.1 TPI Based on BEPA

ULTEM (Scheme 6.1) is one of the typical bisphenol A bisether-4-diphthalic anhydride (BEPA)-based thermoplastic polyimides; it was commercialized by General Electric Company in 1982. The properties of ULTEM are the worst of the TPI family. But the advantages of lower cost, easy to process made it popular in the market. Its global yield achieved ten thousand tons these years. The properties of ULTEM are listed in Table 6.4

Shi et al., synthesized and characterized a serial TPI based on BEPA as we mentioned in Section 6.2.1. The properties of the TPI are as shown in Table 6.4.

Table 6.5 shows the properties of BEPA-based homopolyimides without any additives. The reduced viscosities of those PAAs ranged from 0.86 to 2.65 dl g^–1, indicating the corresponding PIs probably had sufficiently high molecular weights. Generally speaking, the processing condition for the crystalline PIs is often limited in the range from T_m +α to T_d. In many cases, aromatic PIs do not become a free-flowing liquid just above the T_m‘s in contrast to common semicrystalline polymers such as poly(ethylene terephthalate). This is probably due to surviving intermolecular interactions even just above the T_m‘s. Therefore, the thermo-processing temperatures for semicrystalline PIs are generally several tens of degrees higher than the T_m‘s. For example, semicrystalline PI AURUM (T_m = 388 °C) can be processed at 410 °C (α = 22 °C) [12]. However, in most cases, the upper temperature limit for the processing of aromatic PIs should be established below 400 °C in order to avoid undesirable thermal degradation such as crosslinking and also to use common molding machines. Thus, for designing TPI, the limited processing temperature range is a critical factor.

The T_g‘s of BEPA-based PIs ranged from 189 to 253 °C depending on the diamine structures, and they are 17–93 °C lower than those of the ODPA systems. The melt flowability in BEPA-based PIs varied significantly depending on the diamine structures. As shown in 16.5, m-diamines usually have good melt flowability, whereas p-diamines have poor melt flowability. It is worth noticing that all of the BEPA polyimides in this case have no T_m points. They are completely amorphous even in the PI powder prepared via reflux of the PAA solutions. This is most likely attributed to the presence of the bent and bulky isopropylidene groups in the backbone.

Among the BEPA-based PIs, only BEPA/m-PDA (ULTEM type) simultaneously satisfied the demands of a high T_g (>200 °C) and good melt flowability. This PI was the earliest developed by the General Electric Company, as an injection-moldable PI.

6.3.2 Thermoplastic Polyimides Based on PMDA

In the late 1980s, Mitsui Chemicals, Inc. developed a TPI called AURUM which is based on PMDA. Its synthesis and structure could be found in Scheme 6.2.

S. Tarmai and A. Yamaguchi used PMDA and ether diamines to synthesize a series of TPI. They tested the inherent viscosity (η) and T_g of the PMDA-based TPI [4].

6.3.3 TPI Based on ODPA

Shanghai Research Institute of synthetic resins developed a series TPI called Ratem-YS-20 and its properties were shown in Table 6.11.

NASA used ODPA and 3,4’-oxybisbenzenamine (3, 4’-ODA) to synthesize another TPI called LaRC-IA. This TPI possesses good agglutinating property and thermal oxidation stability. Its properties are shown in Tables 6.12 and 6.13 [7–9].

Shi et al., synthesized and characterized a serial TPI based on ODPA as we mentioned in Section 6.2.4. The properties of the TPI are as shown in Table 6.13.

Table 6.13 shows the properties of ODPA-based homopolyimides without any additives. Compared to BEPA polyimides, ODPA-based homopolyimides possess higher T_m, T_g, T_d, and the viscosities. For ODPA/4,4’-ODA, ODPA/3,4’-ODA, ODPA/TPEQ, ODPA/TPER, and ODPA/BAPB, the PI powder prepared via reflux of the PAA solutions displayed melting peaks in the DSC scan, indicating that these PIs are semi-crystalline. However, one notices that the T_m‘s of some ODPA-derived PIs are too high for processing, for example T_m = 415 °C for ODPA/TPEQ and 390°C for ODPA/4,4’-ODA. In ODPA/PDA and ODPA/m-PDA, both without flexible ether linkages, no T_m‘s in the DSC heating process were exhibited up to 450 °C. Therefore, we estimated their T_m‘s from a T_m/T_g relation. As illustrated in Table 6.13, the ratios T_m/T_g range from 1.17 to 1.35 for semicrystalline ODPA-based PIs, clearly in agreement with an empirical equation for a series of PI systems: T_m≈4.3T_g (in kelvin) [15]. The calculated T_m‘s from this relation are 506 °C for ODPA/PDA, 423 °C for ODPA/m-PDA, and 418 °C for ODPA/p-BAPS, respectively. These T_m‘s are much higher than 400 °C, implying that these PIs are also removed from the TPI candidate list. However, according to the T_m/T_g relationship, the molecular design toward lowering T_m inevitably brings about an undesirable T_g decrease. This is the main problem in obtaining high-performance TPIs possessing both high T_g‘s and low T_m‘s for semicrystalline PIs. On the other hand, noncrystalline PIs are expected to have a much wider thermoprocessing window than semicrystalline PIs; ODPA/PTPEQ and ODPA/m-BAPS are classified in this category.

6.3.4 Thermoplastic Polyimides Based on BPDA

S Tarmai et al., reported the synthesis and characterization of high thermally stable semicrystalline polyimide based on 3,4’-oxydianiline and 3,3’,4,4’-biphenyltetracarboxylic dianhydride, end-capped with phthalic anhydride. The quenched film of this polyimide displayed a glass transition temperature and a melting temperature at 251 °C and 402 °C, respectively, by differential scanning colorimeter measurement. Isothermal differential scanning colorimeter studies ascertained this polyimide to be a polymer having first crystallization kinetics. Thermogravimetric analyses and isothermal melt viscosity studies evidenced excellent thermal stability (thermo-oxidative stability and melt viscosity stability) for this polyimide. The η_inh of the two TPI were 0.62 and 1.65, respectively. The weight loss temperature could be found in Table 6.14.

6.3.5 Thermoplastic Copolyimides

E.I. du Pont de Nemours and Company published a patent which described the synthesis of thermoplastic copolyimides in 2001. The structures and properties could be found in Table 6.14.

6.4.1 Hydrolytic Stability

Because of their excellent thermal stability, polyimides are considered as strong candidates for use as interlevel dielectrics in a variety of advanced packaging applications. It is important to determine whether or not degradative reactions that might occur in processing or during use could cause any significant deterioration in these dielectric properties. One such question is whether or not polyimides are susceptible to any hydrolytic reactions during long-term use.

The reported research of the TPI hydrolysis properties was most focused on the PMDA- and ODA-based TPI, such as Kapton series. Kapton is prepared via the polycondensation of pyromellitic dianhydride (PMDA) and oxydianiline (ODA) in, N.,N-dimethylacetamide (DMAc), forming the corresponding polyamic acid. The polymer solution is chemically imidized using the appropriate chemicals, followed by a heat treatment to remove solvent and conversion chemicals and to also complete the imidization. DeIasi et al., [15] reported the effect of an aqueous environment on the properties of Kapton polyimide film. Immersion of specimens in distilled water at 25 °C to 100 °C for time periods ranging from one hour to several hundred hours resulted in a decrease in the ultimate tensile strength of the polymer from 157.32 MPa to approximately 95.76 MPa, and a corresponding decrease in elongation to failure from 38% to approximately 5%. The kinetics of this decrease in mechanical properties is second order and yields activation energy of approximately 15.6 kcal/mole. The reaction is slightly dependent on pH in the range 2.0 to 12.0 but is highly dependent on the pH in the range 0.4 to 2.0. The decrease in mechanical properties at pH 2.0 to 6.0 appears to be due to hydrolysis of either uncyclized amic acid linkages or diamide functional groups present in the polyimide, whereas that at pH below 2.0 is probably the result of hydrolysis of both imide and amide bonds. Prolonged reflux of the polyimide in water resulted in the extraction of a water-soluble, amide-containing material.

DeIasi [16] also researched the thermal regeneration of the tensile properties of hydrolytically degraded polyimide film. Prior to degradation, the film had an ultimate tensile strength of 182 MPa and an elongation to failure of 62.0%. The regenerated polyimide exhibits a much improved hydrolytic stability over the untreated material and specimens heat treated directly without prior aqueous degradation. Pawlowsk [17] found that a 6.7 M/1.9 M KOH/ K₂CO₃ solution had a 40–50 % faster etch rate at 5- and 30-sec exposures compared to a 6.7 M KOH solution. The faster etch rate, along with the visual observation of a thinner gel layer. and less flocculant etch by-product in solution, suggested that the K₂CO₃ additive enhances the dissolution of the hydrolyzed polyimide. As the K₂CO₃ concentration was increased, the etch rate increases when the KOH concentration was maintained at 2.7 M. The maximum etch rate observed at 1.9 M K₂CO₃ when the KOH concentration was held at 6.7 M probably results from solubility limits. Pryde [18]used IR to research the hydrolytic stability of TPI membranes. Hydrolyses were carried out at 90 °C / 95% relative humidity by placing the samples over a saturated solution of K₂SO₄ at 90 ± 2 °C. Table 6.16 shows the hydrolytic stability of TPI on different substrates.

Stephans [19]studied the alkaline hydrolysis of a polyimide (PMDA-ODA) surface as a function of time, temperature, and hydroxide ion concentration. Quantification of the number of carboxylic acid groups formed on the modified polyimide surface was accomplished by analysis of data from contact angle titration experiments. Using a large excess of NaOH, pseudo-first-order kinetics were found, yielding k_obs ≈ 0.1–0.9 min^-1 for conversion of polyimide to poly(amic acid) depending on [OH^-]. Conversion of the polyimide surface to one of poly(amic acid) was found to reach a limiting value with a formation constant, K, in the range 2–10 L/moL.

6.4.2 Oxidative Stability

The low Earth orbit, 200–700 km above the Earth surface, sees a harsh space environment with hazards such as atomic oxygen, ultraviolet radiation, ionizing radiation, high vacuum, micrometeoroids, and detritus as well as severe temperature cycling, which may cause considerable damages to spacecraft materials. Atomic oxygen (OA) is predominant among those hazards. OA is formed by solar ultraviolet radiation, which dissociates oxygen molecules into free atomic oxygen in the outer ionosphere at an altitude higher than 100 km. The concentration of OA is approximately from 10⁶ to 10⁹ atoms/cm³. When the aircraft travels at 8 km/s, the OA flux is 10¹²–10¹⁵ atoms/(cm².s). The energy release of OA crash is about 5 eV, which is adequate to break the C-O and C-C bond; this is so called OA attack [22]. It causes micrometer roughness when the surface of the PI attacked by OA. Inorganic coats are usually used to improve the OA attack resistance, such as ITO, silicon resin. Li et al., [23] researched the nano-SiO₂-doped PI; it was found that the OA attack resistance was greatly improved when the contents of SiO₂ were 20%.

6.5.1 Chloromethylation

Chloromethylation of PEI chains [24] was carried out to introduce the ATRP initiators onto their backbones and the surface, including the pore surface, of the resulting microporous membranes. Amphiphilic poly-(poly(ethylene glycol) methyl ether methacrylate)-block-poly-(2,2,2-trifluoroethyl methacrylate), or P(PEGMA)-b-P(TFEMA), copolymer brushes were incorporated via consecutive surface-initiated ATRPs from the alkyl halide sites on the PEI microporous membrane. The chloromethylation of PEI is as follows.

In a 100 mL, three-necked, round-bottom flask equipped with a condenser (with a drying tube attached at the top) and an argon inlet, 5 g of PEI (equivalent to 8.4 mmol of the repeat units) was dissolved in 40 mL of dichloroethane under stirring at 60 °C. Paraformaldehyde (1.5 g), phosphorus trichloride (10 g), and zinc chloride (5 g) were added slowly to the reaction mixture under an argon atmosphere. The mixture was stirred at 60 °C for 48 h and then poured into 400 mL of methanol. The precipitate was filtered, redissolved, and reprecipitated into a water/methanol mixture (50/50, v/v) and then dried under reduced pressure at 80 °C for at least 24 h until a constant weight was obtained. The chloromethylated PEI, or PEICl, so-obtained was further purified by dissolving in N,N-dimethylformamide and then reprecipitating in methanol.

Chloromethylation of 6FDA-ODA PI [25–26]

A typical chloromethylation procedure was as follows. In a 100-mL, three-necked, round-bottom flask equipped with a condenser (with a drying tube at the top) and a nitrogen inlet, 6FDA-ODA PI (0.290 g, 0.5 mmol of the repeating unit) was dissolved in 15 mL of chloroform. The solution was heated to 60 °C, and 0.20 mL of chloromethyl methyl ether (1.5 mmol) and 0.18 mL of tin(IV) chloride (1.5 mmol) were slowly added to the reaction mixture under a nitrogen atmosphere. The mixture was stirred at different allocated time intervals and then poured into 300 mL of methanol. The precipitate was filtered, reprecipitated into a water/methanol mixture (50/50 v/v), and dried in a vacuum oven at 60 °C for 2 days. The maximum degree of chloromethylation was 1.81. To attempt further functionalization, Zhang et al., explored the esterification after chloromethylation of 6FDA-ODA PI with cinnamic acid [28].

The esterification was performed in the presence of K₂CO₃ and TBAB, which acted as a reaction catalyst and a phase-transfer catalyst, respectively. From the NMR spectra, it was confirmed that the reaction was almost quantitative. The esterification procedure is as follows: To a completely dissolved dimethylformamide (DMF) solution (5 mL) of chloromethylated 6FDA-ODA PI (0.25 g) and cinnamic acid (1.2 equiv to OCH₂Cl), one equiv (to OCH₂Cl) of K₂CO₃ and TBAB were added. The reaction mixture was stirred for 24 h at 40 °C under a nitrogen atmosphere. The solution was poured into water, and the precipitate was collected by filtration. Reprecipitation from THF into methanol was repeated three times, and the resulting polymer was dried in a vacuum oven for 48 h at 50 °C. In addition, after chloromethylation of TPI, quaternization reaction could be introduced onto the chloromethanol group [27].

6.5.2 Sulfonation

Sulfonation of PEI by chlorosulfonic acid [27, 29].

PEI (20 g) was dissolved in 100 mL of 1,2-dichloroethane at 60 °C, and subsequently the PEI solution was kept at 30 °C. Chlorosulfonic acid mixed with 75 mL of 1,2-dichloroethane was added to the PEI solution within 1 h with vigorous stirring. After being reacted for a definite period, the reaction product, which precipitated in the reaction medium, was dissolved in DMAc at 50 °C, coagulated with excess isopropanol, filtered, washed with isopropanol, and dried at 40 °C in a vacuum oven. The sodium salt form of the product was obtained by soaking it in excess 0.1 mol/L NaOH aqueous solution for 2 days. Moreover, TPI also can be sulfonated by concentrated H₂SO₄ and SO₃ under differential conditions [30, 31].The gas phase concentration of SO₃ can be calculated according to the literature [32].

6.5.3 Phosphorylation

In a nitrogen atmosphere, 1 g of PIA, 10 g of dichlorophenylphosphine, and 1 g aluminum chloride were stirred at 130 °C for 6 h. After cooling, the reaction mixture was poured into 400 g of crushed ice. The solid was filtered and washed with icy 10% sodium hydroxide aqueous solution and water. The product (PIA-P) was then dried under vacuum. PIA-P was then dissolved in THF and treated with aqueous H₂O₂. The product (PIA-PO) was obtained by reprecipitation from methanol. The introduction of phosphorus-containing groups was found to slightly reduce the thermal stability of the polyimides and conspicuously increase their flame retardance.

6.5.4 Bromination

Brominated Matrimid was prepared as shown in Scheme 16.9. The bromination was conducted by the addition of bromine (6 mL, 18.72 g, 117.13 mmol) to a yellowish solution of PI (2.00 g, 3.62 mmol) dissolved in chloroform (13.3 mL, 15% w/v) in a 100-mL, round-bottom flask equipped with a condenser and a magnetic stirrer. The dark red solution was vigorously stirred for 6 h, during which time a dark gum was precipitated from the reaction mixture. Methanol (70 mL) was poured into the flask under vigorous stirring to precipitate the yellow, modified polymer. The precipitated polymer was filtered and stirred vigorously again in fresh methanol. This was repeated until no more bromine could be extracted, this being indicated by colorless methanol. The brominated PI was purified by redissolution in chloroform and precipitation from 95% ethanol. After vacuum drying, a yield of 90% (2.06 g) was obtained.

The brominated polymer retained the good solubility and thermal properties that it possessed before modification. Compared with unmodified PI, brominated Matrimid had a lower decomposition temperature but an increased Tg, most likely attributable to decreased chain mobility in the presence of bulky bromine atoms. The brominated polymer exhibited approximately 1.6-fold increases in gas permeabilities for O₂ and CO₂, with some concurrent decreases in permselectivity of approximately 7–8% for the O₂ /N₂ and CO₂ /CH₄ gas pairs.

6.5.5 Arylation

High molecular weight polymers containing triarylsulfonium salt groups as part of the main chain could be prepared by arylation of sulfur-containing aromatic polyimides. The experimental procedure is as follows:

Sulfur-containing TPI (10.64 g repeating units) was dissolved in 50 mL hot chlorobenzene and then arylated using 0.03 g cupric benzoate and 1.278 g diphenyliodonium hexafluorophosphate at 130 °C for 3 h under an inert atmosphere of N₂. The resulting polymer solution was poured into methanol and the precipitated polyimide–sulfonium salt, the product was isolated by filtration, washed with methanol, and dried in vacuum. Such polymers were photosensitive, undergoing photodegradation when irradiated in the presence of perylene as a photosensitizer.

6.6.1 Injection Molding

Injection molding is a manufacturing process for producing parts by injecting material into a mold. Injection molding can be performed with a host of materials mainly including metals (for which the process is called die casting), glasses, elastomers, confections, and most commonly thermoplastic and thermosetting polymers. Material for the part is fed into a heated barrel, mixed, and forced into a mold cavity, where it cools and hardens to the configuration of the cavity.

When thermoplastics are molded, typically pelletized raw material is fed through a hopper into a heated barrel with a reciprocating screw. Upon entrance to the barrel, the temperature increases and the Van der Waals forces that resist relative flow of individual chains are weakened as a result of increased space between molecules at higher thermal energy states. This process reduces its viscosity, which enables the polymer to flow with the driving force of the injection unit. The screw delivers the raw material forward, mixes and homogenizes the thermal and viscous distributions of the polymer, and reduces the required heating time by mechanically shearing the material and adding a significant amount of frictional heating to the polymer. The material feeds forward through a check valve and collects at the front of the screw into a volume known as a shot. A shot is the volume of material that is used to fill the mold cavity, compensate for shrinkage, and provide a cushion (approximately 10% of the total shot volume, which remains in the barrel and prevents the screw from bottoming out) to transfer pressure from the screw to the mold cavity. When enough material has gathered, the material is forced at high pressure and velocity into the part forming cavity. To prevent spikes in pressure, the process normally uses a transfer position corresponding to a 95–98% full cavity where the screw shifts from a constant velocity to a constant pressure control. Often injection times are well under 1 second. Once the screw reaches the transfer position, the packing pressure is applied, which completes mold filling and compensates for thermal shrinkage, which is quite high for thermoplastics relative to many other materials. The packing pressure is applied until the gate (cavity entrance) solidifies. Due to its small size, the gate is normally the first place to solidify through its entire thickness [37]. Once the gate solidifies, no more material can enter the cavity; accordingly, the screw reciprocates and acquires material for the next cycle while the material within the mold cools so that it can be ejected and be dimensionally stable. This cooling duration is dramatically reduced by the use of cooling lines circulating water or oil from an external temperature controller. Once the required temperature has been achieved, the mold opens and an array of pins, sleeves, strippers, etc. are driven forward to demold the article. Then, the mold closes and the process is repeated.

Briefly speaking, the injection molding includes the following steps:

1. Drying the pellets;
2. Loading into moulding machine;
3. Heating and shearing material into a melt;
4. Injecting the melt into the mold cavity;
5. Part cooling, solidification;
6. Ejecting the finished part from the mold.

6.6.2 Compression Molding

Compression molding is a method of molding in which the molding material, generally preheated, is first placed in an open, heated mold cavity. The mold is closed with a top force or plug member, pressure is applied to force the material into contact with all mold areas, and heat and pressure are maintained until the molding material has cured. The process employs thermosetting resins in a partially cured stage, either in the form of granules, putty-like masses, or preforms. Compression molding is a high-volume, high-pressure method suitable for molding complex, high-strength fiber glass reinforcements. Advanced composite thermoplastics can also be compression molded with unidirectional tapes, woven fabrics, randomly orientated fiber mat, or chopped strand. The advantage of compression molding is its ability to mold large, fairly intricate parts. Compression molding produces fewer knit lines and less fiber-length degradation than injection molding.

The compression molding starts, with an allotted amount of plastic or gelatin placed over or inserted into a mold. Afterward the material is heated to a pliable state in and by the mold. Shortly thereafter the hydraulic press compresses the pliable plastic against the mold, resulting in a perfectly molded piece, retaining the shape of the inside surface of the mold. After the hydraulic press releases, an ejector pin in the bottom of the mold quickly ejects the finished piece out of the mold and then the process is finished. Depending on the type of plunger used in the press, there will or won’t be excess material on the mold [38].

6.6.3 Extrusion Molding

Extrusion is a manufacturing process used to make pipes, hoses, drinking straws, curtain tracks, rods, and fibers. The granules melt into a liquid which is forced through a die, forming a long “tube-like” shape. The shape of the die determines the shape of the tube. The extrusion is then cooled and forms a solid shape. The tube may be printed upon and cut at equal intervals. The pieces may be rolled for storage or packed together. Shapes that can result from extrusion include T-sections, U-sections, square sections, I-sections, L-sections, and circular sections. One of the most famous products of extrusion molding is the optical fiber cable. Extrusion is similar to injection molding except that a long continuous shape is produced.

6.6.4 Coating

A coating is a covering that is applied to the surface of an object, usually referred to as the substrate. The purpose of applying the coating may be decorative, functional, or both. The coating itself may be an all-over coating, completely covering the substrate, or it may only cover parts of the substrate.

Coating processes may be classified as follows:

Chemical vapor deposition;

Physical vapor deposition;

Chemical and electrochemical techniques;

Spraying;

Roll-to-roll coating processes.

Among these processes, chemical vapor deposition is preferred for TPI. Masayuki Iijima et al., prepared polyimide thin films which were on the whole surface of a substrate using a high-temperature vapor deposition polymerization method (VDP-H). In this method, pyromellitic dianhydride (PMDA) and oxydiamine (ODA), which are monomers of polyimides, are introduced into a process chamber from separate evaporation sources through inlet tubes.

The apparatus for the VDP-H (ULVAC Japan model VEP-3040) is shown in Figure 6.7. The temperature of the process chamber 1 (450 mm × 650 mm) was kept at about 200 °C with a mantle heater by a temperature controller. A substrate set up on the substrate holder 12 was rotated at 1 rpm to ensure a uniform distribution of film thickness. The evaporation source ovens for PMDA and ODA joined to the side of the chamber were kept at about 200 °C and 181 °C, respectively. A reflection board was employed to extend the time that the monomers remained in the chamber. The pressure during deposition was kept at about 0.1 Pa. Under these conditions, the deposition rate of the polymer films was 54 nm min^-1. The monomer inlet tubes six were designed to supply the monomers uniformly. Film thickness was controlled by changing the amounts of monomers in the source chamber. For example, 10 g of each monomer afforded films of 4 µm thickness. Deposition rate and film thickness were monitored by an optical monitor.

As the process chamber was kept at about 200 °C, neither monomer could deposit on the substrate. Because the temperature of the substrate was higher than the evaporation temperature of the monomers, only polymerized films were deposited on the substrate. Infrared spectroscopy indicated that the VDP-H films were about 50% imidized. When the film thickness was less than 1 µm, uniform step coverage was observed, and above 2.5µm a patterned surface was reduced to a planar surface. Film thickness was controlled by changing the amounts of monomers in the evaporation sources. The dielectric constant (ε = 3.5), dissipation factor (tanδ=0.002), thermal characteristics (5% weight loss temperature = 551 °C, and tensile strength (170 MPa) of the VDP-H films cured for 60 min at 300 °C in vacuum were the same as those reported for Kapton^® films. The modulus of elasticity (4.0 GPa) was larger than that of Kapton^®, and it was found that the VDP-H films showed less elongation than Kapton^® [39].

6.6.5 Spinning

Spinning is a manufacturing process for creating polymer fibers. It is a specialized form of extrusion that uses a spinneret to form multiple continuous filaments. There are many types of spinning: wet, dry, dry jet-wet, melt, gel, and electrospinning. First, the polymer being spun must be converted into a fluid state. If the polymer is a thermoplastic, then it can be simply melted; otherwise it is dissolved in a solvent or chemically treated to form soluble or thermoplastic derivatives. The molten polymer is then forced through the spinneret, and then it cools to a rubbery state, and then a solidified state. If a polymer solution is used, then the solvent is removed after being forced through the spinneret.

6.7.1 Membranes

Dupont developed one kind of TPI membranes called IMIDEX which is based on AURUM. Its thickness is 25 µm–1mm, and wideness is 660 mm. The properties of IMIDEX were shown in Table 6.18 [41].

Affix Technology Co., Ltd. developed ESD Polyimide Tapes. This Tape is constructed to fill the requirements of a high-performance thermoplastic (Polyimide Backing Materials). It is manufactured from polyimide film backing with high-efficient silicone adhesive. The ESD additive in the tape acts to reduce the electrostatic discharge that occurs upon tape removal. The product’s low static properties can eliminate circuit board degradation due to electrostatic discharge.

The typical product is Low Static Kapton^® Tapes; these tapes are made of 1 millimeter Kapton^® film backed by silicone adhesive. This tape can withstand temperatures of up to 500 °F/260 °C. Adhesive is 1.6 millimeter thick. Total thickness of this product is 2.6 ± 1 mil. Color is Amber and is packaged in a 36 yds roll. This tape is ideal for environment where static discharge is a concern [42].

6.7.2 Adhesives

As the adhesives, TPI could strongly cohere with several substrates, such as metals, nonmetals, and polymers. TPI-based adhesives were widely used in aerospace and electronics industries. Among the different kinds of TPI, BTDA-based TPI is the best adhesive material and mostly used these years. Strong contributions to this area from the workers of NASA Largely Research Center have led to a variety of TPI adhesives. One typical adhesive is the LaRC-TPI (Largely Research Center Thermoplastic Polyimides), which is prepared with BTDA and 3,3’-DABP.

Another typical BTDA-based TPI adhesive is the LaRC-CPI (Largely Research Center Crystalline Polyimides). As a crystalline thermoplastic, LaRC-CPI could be used as a high-temperature adhesive.

Titanium alloys are widely used in aerospace; they have very high tensile strength and toughness. They are light in weight and have extraordinary corrosion resistance and the ability to withstand extreme temperatures. Hergenrother et al., [43] used LaRC-CPI for the adhesion of the titanium alloys. They have researched the influences of relative molecular weight and different adhesive conditions including pressure on the adhesive performance. The results suggest that the adhesive performance is no good when the molecular weight is too higher or lower. Commonly, low temperature and high pressure will lead to good adhesive performance.

Hergenrother also studied the adhesion of titanium with LaRC-CPI2. LaRC-CPI2 is synthesized from with 4,4’-oxy-diphthalic anhydride (ODPA). Different from LaRC-CPI, in the range of the researched relative molecular weight, the original adhesive performance of LaRC-CPI2 was not changed a lot. After the aging test at 204 °C, the lower molecular weight LaRC-CPI2 possesses better adhesive performance. In contrast, after the aging test at 300 °C, the higher molecular weight LaRC-CPI2 would possess better adhesive performance. Higher adhesive pressure (1.38 Mpa) would lower the performance of the LaRC-CPI2 [44]. Other LaRC adhesive series like LaRC-8515 [45], LaRC-MPEI [46] (modified phenylethynyl terminated polyimide), and their applications could be seen according to the literature.

6.7.3 Composites

Whether it is bismaleimide (BMI) resin or polymerization of monomer reactants (PMRs) resin, the fracture toughness cannot meet the needs of some applications, although the fracture toughness is increased by using of linear polymers, but usually, T_g, the important property would be decreased in the same time. This is unacceptable in the application of advanced composite materials. In order to obtain high fracture toughness composite materials, researchers have spent great efforts to explore the high-performance thermoplastic resin for advanced composites in the 1980s and 1990s. The most typical one is based on the poly-etherketone resin composites; however, the T_g of this kind of polymer is hardly to excess above 200 °C. So, even the required temperature is 177 °C, this kind of resin-based composite materials could not be used in HSCT project (High Speed Civil Transport, NASA). During this period, NASA led the development of advanced composite materials based on the thermoplastic polyimide resin. Some of the main products and its applications are introduced below.

6.7.3.1 Skybond

Skybond is developed for composite resin of aircrafts polyimide varnish. The Skybond^® product line encompasses a number of products usable as matrix resins for manufacture of high thermal stability structural composites. Composites based on the Skybond^® family of resins have continuous use temperatures in excess of 600° F with short-term capability greater than 1000° F. Prepreg has been successfully manufactured from all common high-performance substrates, such as E-glass, high modulus carbon, and quartz. Two new classes of matrix resins have been developed in the Skybond^® family. The first can be processed by RTM molding techniques to bring the outstanding thermal resistance of polyimide resins to RTM molded composites. The second new class of Skybond^® matrix resins is the two-stage cure resins that when used in prepreg applications allow the manufacture of very low void level composites by autoclave processing (<0.5% voids) combined with excellent thermal stability of polyimide resins. More specific properties and applications could be found in Skybond^® 700 Technical Bullitin [47].

6.7.4 Engineering Plastics

High-performance engineering plastics are a kind of plastic materials that have better mechanical and/or thermal properties than the more widely used commodity plastics (such as polystyrene, PVC, polypropylene, and polyethylene). Because of their high thermal stability and easy machinability, TPIs were also wildly used in high-performance engineering plastics, such as VESPEL, ULTEM, AURUM, and Ratem.

6.7.4.1 VESPEL Plastics

VESPEL is a range of durable high-performance polyimide-based plastics which are manufactured by DuPont. It is synthesized from PMDA and ODA. The basic structure is shown in Figure 6.11.

This high-performance polymer is mostly used in aerospace, semiconductor, and transportation technology. It combines heat resistance, lubricity, dimensional stability, chemical resistance, and creep resistance, to be used in hostile and extreme environmental conditions.

Unlike most plastics, it does not produce significant outgassing even at high temperatures, which makes it useful for lightweight heat shields and crucible support. It also performs well in vacuum applications, down to extremely low cryogenic temperatures. However, VESPEL tends to absorb a small amount of water, resulting in longer pump time while placed in a vacuum. For different applications, special formulations are blended/compounded. The product number and its applications are shown in Table 6.19.

Product no.	Instructions	Application area
SP-1	Unfilled base polyimide resin	Aerospace, electronics, sealing
SP-21	15% graphite filled	Bearings, thrust pad
SP-22	40% graphite filled	Bearings, wear strip
SP-211	15% graphite and 10% PTFE filled	Bearings, wear strip, gasket
SP-3	15% MoS2 filled	Vacuum and dry circumstances sealing, bearings

6.8.1 TPI Blends with TPI

Ma et al., [52], reported the blends of New-TPI(AURUM) and ULTEM 1000 (PEI). New-TPI with a weight-average molecular weight (Mw) of ca. 30000 and PEI (ULTEM 1000, Mw, ca. 30000) was used as a raw material for the blends. Seven melt-quenched New-TPI/PEI blends (New-TPI/PEI = 100/0; 80/20. 60/40, 50/50, 40/60, 20/80, 0/100) were supplied by Mitsui-Toatsu Chemicals, Inc. The blend samples were prepared by using a single screw extruder. The phase behavior and semicrystalline morphology of the blends were studied by using DSC and SAXS. As their results: melt-quenched amorphous New-TPI/PEI blends are miscible over the whole composition range. Phase separation occurs when melt-quenched New-TPI/PEI blends are heat treated at 260 °C; crystallization does not occur in this case. Semicrystalline structures with phase-separated amorphous regions develop during heat treatment of the New-TPI/PEI blends at temperatures of 290–340 °C. Semicrystalline structures with miscible amorphous regions develop when the melt-quenched New-TPI/PEI blends are heat treated at temperatures above 350 °C. Similar structures are also found in the melt-crystallized New-TPI/PEI blends. PEI segments are incorporated in the amorphous layer between the New-TPI crystals, regardless of phase separation (Table 6.20). The structures of two TPIs can be found in scheme 6.1 and scheme 6.2.

Goodwin [53] also studied the properties of New-TPI/PEI blends. It was found that the New-TPI/PEI blends were single-phase blends by mixing and quenching the two polyimides according to the variation in amorphous T_g, as monitored by DSC and DMA. Annealing of the blends above the T_g did not induce shifts in the T_g of any composition, nor did it produce any changes in the magnitude and location of the endothermic overshoot arising from physical ageing. Thus, we concluded that the blends did not phase separate.

6.8.2 Polyamic Acid Blending

Mazinani et al., [54] reported the intermolecular interactions and miscibility behavior of two polyimide blend systems, Extem/Matrimid and Extem/U-Varnish. DSC results for the Extem/U-Varnish system showed the existence of a single glass transition temperature (Tg) in each composition, suggesting the miscibility of the blends, whereas DSC analysis of Extem/Matrimid system indicated immiscibility but compatibility between two polymers. Interactions between studied polymer systems and four aprotic solvents including N-methyl-2-pyrrolidone (NMP), dimethylacetamide (DMAc), dimethylformamide (DMF), and dimethyl sulfoxide (DMSO) were assessed on the basis of the difference between their solubility parameters. Among the selected solvents, DMAc showed the highest affinity with both blend systems. XRD patterns and rheological behavior of Extem/U-Varnish system revealed that the crystalline nature and viscosity of the blend polymers decreases as the ratio of Extem/U-Varnish increases. In conclusion, Extem and U-Varnish were found to constitute a miscible pair at a molecular level over the entire composition range whereas Extem and Matrimid could not form a miscible blend.

In order to study the specific interactions between Extem and U-Varnish polymers, the Tgs of the polymer blends were estimated by theoretical equations and compared with experimental data. The empirical Tg values formed a concave curve as a function of composition and exhibited a positive deviation from the linearity, indicating the presence of specific interactions between Extem and U-Varnish polymer chain.

Tong et al., [55] reported the preparation of rodlike/flexible polyimide blends is feasible by utilizing poly(amic acid) amine salt precursors, which are free from the intermolecular transamidation reaction. Compared to poly(amic ester)s, the preparation of poly(amic acid) amine salts is much more straightforward and easier. In addition, poly(amic acid) amine salts as PI precursors are used more and more in practice. Typical preparation method of poly(amic acid) amine salt precursors is as follows: a mixture of a dianhydride and a respective diamine in NMP was stirred for 6 h at room temperature under nitrogen, and an appropriate amount of TEA was added to the above poly(amic acid) solution to neutralize all the carboxylic acid groups.

6.9.1 LaRC Composites

LaRC thermoplastic polyamides products series are researched and developed by Langley Research Center of NASA. LaRC is the oldest of NASA’s field centers, established in 1917, located in Hampton, Virginia, United States. A series of TPI composites named after Langley Research Center of NASA will be introduced as follows.

LaRC-TPI/AS4 composites: LaRC-TPI is prepared with BTDA and 3,3’-DABP, its composites properties with AS4 glass cloth are shown in Table 6.23

LaRC-8515/IM7: LARC-8515 (Langley Research Center-8515) is an aromatic polyimide based on 3,3’,4,4’-biphenyltetracarboxylic dianhydride (BPDA) and an 85:15 molar ratio of 3,4’-oxydianiline (3,4’-ODA) and 1,3-bis(3-amnophenoxy)benzene (APB) [56]. Its composite properties with IM7 carbon fiber are shown in Table 6.24.

LARC-IAX-3 [57] (Langley Research Center™-improved adhesive experimental resin-3) aromatic polyimide, based on oxydiphthalic anhydride, 3,’4-oxydianiline (3,’4-ODA), and 1,4-phenylenediamine (p-PDA), was evaluated as a matrix for high-performance composites. T H HouT and L St Clair characterized the composite mechanical properties such as short-beam shear strength, longitudinal and transverse flexural strength and flexural modulus, longitudinal tensile strength and notched and unnotched compression strengths measured at room temperature (RT) and elevated temperatures. Similar properties were obtained independent of the carrier solvent used during matrix resin synthesis. These mechanical properties were superior to those previously measured for IM7/LARC-IA and IM7/LARC-IAX composites. The enhanced mechanical properties were attributed to the substitution of 25% 3,’4-ODA by p-PDA in the LARC-IA imide backbones. The properties of the LARC-IAX composites are shown in Table 6.25.

In continuation of an effort to develop TPI for use in aerospace applications, P M Hergenrothert and S J Havens [44] developed a new polyimide. LARC-CPI 2, This semicrystalline polyimide was prepared by the reaction of 4.4’-oxydiphthalic anhydride with 1,4-bis(4-aminophenoxy-4’-benzoyl)benzene. In high-molecular-weight form, this material has a Tg of 223 °C and a crystalline melt temperature of approximately 350 °C. Controlled-molecular-weight end-capped versions of LARC-CPI two were used to fabricate adhesive panels and composites that exhibited good mechanical properties at temperatures as high as 200 °C. The properties of the LARC-CPI two composites are shown in Table 6.26.

LARC-SCI [58] (Langley Research Center—Semi-Crystalline polyImide) is an aromatic polyimide based on 3,4’-oxydianiline and 3,3’,4,4’-biphenyl tetracarboxylic dianhydride. This polyimide was synthesized and evaluated for use as a neat resin and a matrix resin for advanced composites. Three 30% w/w solids polyamic acid/N-methypyrrolidone solutions were prepared using 2, 3, and 4% stoichiometric imbalances end-capped with phthalic anhydride to provide polyimides with theoretical number average molecular weights of approximately 22,800, 15,100, and 11,400 g mol^–1, respectively. The composites properties of IM7 carbon fiber with these three resins are shown in Table 6.27.

LARC-SI (NASA Langley Research Center-Soluble Imide) is an aromatic thermoplastic polyimide. LARC-SI is synthesized from equimolar amounts of oxydiphthalic anhydride (ODPA), 3, 3’, 4, 4’-biphenyltetracarboxylic dianhydride (BPDA) and the equivalent amount of 3, 4’-oxydianiline (3, 4’-ODA). Phthalic anhydride (PA) was used as an end-capper to control molecular weight. The composites properties of IM7/LaRC SI are shown in Table 6.28.

LaRC-ITPI [59] is an isomeric variation of the better known LaRC-TPI which is based on 4,4’-isophthaloydiphthalic anhydride and 1,3-phenylenediamine. It was evaluated as a matrix for high-performance composites. Its structure is shown in Figure 6.14. The composite properties of LaRC-ITPI/IM7 are shown in Table 6.29. As seen in the table, 80% of the room temperature OHC strength was retained at 177/ °C, indicating that the LaRC-ITPI is an excellent high-temperature matrix material for selected future aerospace applications where solvent resistance is not a key requirement.

6.9.2 Skybond

Skybond [60] is developed for composite resin of aircrafts polyimide varnish. The Skybond^® product line encompasses a number of products usable as matrix resins for manufacture of high thermal stability structural composites. Composites based on the Skybond^® family of resins have continuous use temperatures in excess of 600° F with short-term capability greater than 1000° F. Prepreg has been successfully manufactured from all common high-performance substrates, such as E-glass, high modulus carbon, and quartz. Two new classes of matrix resins have been developed in the Skybond^® family. The first can be processed by RTM molding techniques to bring the outstanding thermal resistance of polyimide resins to RTM molded composites. The second new class of Skybond^® matrix resins is the two-stage cure resins that when used in prepreg applications allow the manufacture of very low void level composites by autoclave processing (<0.5% voids) combined with excellent thermal stability of polyimide resins. The properties of Skybond/glass cloth composites are shown in Table 6.30. The results shown in Tables 6-30 are representative of laboratory data on 1/8 inch thick laminates. Laminates were made using 181 glass cloth with A-1100 soft finish. The thermal aging was in air. The testing temperatures were the same as the exposure condition temperatures in most cases. Testing temperatures are noted in parentheses.

6.9.3 PAI Polyamide–Imide Composites

PAI: Polyamide-imide [61] is a kind of new engineering plastics containing amide group. Polyamide-imides display a combination of properties from both polyamides and polyimides, such as high strength, melt processibility, exceptional high heat capability, and broad chemical resistance. Polyamide-imide polymers can be processed into a wide variety of forms, from injection or compression molded parts and ingots, to coatings, films, fibers and adhesives. Generally these articles reach their maximum properties with a subsequent thermal cure process.

Solvay Company developed a series of PAI-based composites called TORLON. Table 6.31 shows the simple description and their processing methods. The specific properties of these composites could be found on the Solvay Company websites.

These composites could be used in the production of high-tech equipment, precision parts, and requiring high wear resistance, such as no lubrication bearing, seal and bearing retainer and reciprocating compressor parts, electronics and semiconductor industries. Used in high temperature, high vacuum, strong radiation, ultralow temperature conditions, work products, mold used for aircraft parts, reciprocating compressor impeller, the bearing retainer, the jet engine parts for combustion system Chipset and socket welding pedestal, cup body. Used for production of precision parts, cases such as electronics and semiconductor industry, oil drilling equipment and so on.

6.10.1 TPI/silver Nanocomposite

Khalil Faghihi and Meisam Shabanian [62] reported a TPI/silver nanocomposite. The soluble polyimide–silver nanocomposite containing chalcone moieties as a photosensitive group was synthesized by a convenient ultraviolet irradiation technique. A precursor such as AgNO₃ was used as the source of the silver particles. The soluble polyimide was synthesized by the one-step synthesis of polyimide from polycondensation reaction of 4, 4’- diamino chalcone with pyromellitic anhydride in the presence of isoquinoline solution.

The temperatures of 5% and 10% wt losses and also the char yield at 800 °C of PISN were higher than the pure PI. The higher thermal stability of nanocomposite can be attributed to the presence of inorganic silver nanoparticles in the PI matrix. Also, the Tg of nanocomposite was higher than pure PI. The increase in the Tg can be attributed to the strong interaction between the silver nanoparticles and polar imide chain.

The silver nanoparticles were homogeneously dispersed in the PI matrix. In the UV–vis absorption spectra of the PISN, the absorption peak due to the SPR of silver particles was observed at 418 nm. Due to the presence chalcone moieties in polymer backbone and good thermal properties, these silver/PI nanocomposites can be photosensitive and have the potential for the use in microfabrication of conductive components in microelectronic industry.

Thanh Hoai Lucie Nguyen etc. [63] reported another TPI/silver nanocomposite, which is based on silver nanowires and thermoplastic polyamide. The typical fabrication procedure of the composite is as follows: A volume of Ag NW suspension was added to the PI/NEP solution under sonification. This suspension was film coated on a glass plate and placed in an oven at 80 °C during 30 min to evaporate the solvent. The film was removed from the glass plate and heated at 300 °C during 2 h in order to evaporate the residual traces of NEP. The formed composite was a film with a thickness around 50 µm. PI/Ag NW composites were elaborated with a volume fraction varying from 0 to 3 vol%.

For the neat PI, the Tg is near 330 °C. The glass transition temperature exhibits a very slight modification with the filler content. This variation is not significant. It is suggested that the addition of Ag NWs does not have impact on PI physical structure.

The addition of Ag NWs into the PI matrix increased the conductivity by 16 orders of magnitude and reached a conductivity value of 102 S·m^–1.

6.10.2 TPI/Fe-FeO Nanocomposite

Jiahua Zhu et al., [64] used the commercialized TPI Matrimid 5218 to fabricate the TPI-Fe-FeO nanocomposite. Pure PI and Fe-FeO/PI nanocomposite fibers with various Fe@FeO nanoparticle loadings (5, 10, 20, and 30 wt %) are fabricated by the electrospinning process.

Higher voltage is required for the fabrication of nanocomposite fibers at high nanoparticle loadings owing to the increased viscosity of the nanocomposite solutions. The nanoparticles in the nanocomposite fibers become magnetically harder with a significant increase of coercivity from 62.3 Oe (as-received Fe@FeO nanoparticles) to 188.2 Oe. Meanwhile, an increase of about 7.4% in the nanoparticle shell thickness is deduced during the high-voltage electrospinning fiber fabrication process.

TGA and DSC results indicate an enhanced thermal stability with an increased glass transition (Tg) temperature of the nanocomposite fibers as compared to that of the pure PI fibers. The Tg and Tm of the nanocomposite does not much increase when the loading amount increased from 10wt% to 30wt%.

6.10.3 TPI/Carbon Nanocomposites

A kind of TPI/carbon nanocomposites [65] were reported by M Lebróncolón in 2010. These nanocomposites were based on the SWCNT, pyrene derivative, and the BPADA thermoplastic polyimide.

(1-pyrene-N-(4-N’-(5-norbornene-2,3-dicarboxyimido)phenyl butanamide)

It has been found that polyimide films prepared with the SWCNT/1 complexes showed higher tensile strengths and storage moduli than those of the neat polyimide film and polyimide nanocomposite films prepared with pristine SWCNTs at the same loading levels. The films fabricated with the TPI nanocomposites also had higher Tg and better high-temperature stability than both the neat polyimide films and corresponding nanocomposite films prepared with pristine SWCNTs. Furthermore, DC volume conductivities increased significantly with addition of these complexes and were higher than nanocomposite films prepared with uncomplexed SWCNTs. These materials could be suitable for use as multifunctional materials in applications where the combination of good mechanical properties and electrical conductivity is required.

VE Smirnova et al., [66] reported TPI/carbon nanocomposite. They have studied the effects of additives of single-walled carbon nanotubes prepared via electric-arc synthesis and carbon nanofibers produced via gas-phase synthesis on the crystallization capacities and mechanical and electric properties of composite films. The thermoplastic polyimide (PI R–ODFO) matrix was synthesized from 1,3-bis-(3,3’,4,4’-dicarboxyphenoxy) benzene and 4,4’-bis-(4-aminophenoxy)biphenyl.

The typical fabrication of TPI/carbon nanocomposite is as follows: A homogeneous suspension of a calculated amount of CNTs and CNFs in NMP was prepared with an IL 100–6 ultrasonic dispenser (a generator capacity of 0.5 kW, ultrasonic treatment times of 2 h and 30 min for dispersing the CNFs and single-walled CNTs, respectively). The dispersion was poured into a solution of PAA in NMP, and the resulting mixture was stirred with a mechanical stirrer (1500 rpm) for 4 h. The solutions were poured on a glass substrate and dried at 60 °C for 20 h. Thermal cyclization was performed in a stage heating mode at 100, 200, and 300°C for 1 h at each temperature. To standardize the initial state of the samples, they were amorphized via annealing at 360 °C for 10 min followed by cooling to room temperature at a rate of 10 K/min.

The introduction of CNFs into the matrix of the thermoplastic PI R–ODFO increases the degree of crystallinity achieved during annealing. An increase in the filler concentration leads to an increase in the crystallinity of the material. Sharp decreases in the volume resistivities of the composite films are observed at concentrations of 0.1–0.5 wt % single-walled CNTs and carboxylated single-walled CNTs and 3 wt % CNFs because of the formation of a percolation cluster.

Zhang and Du [67] introduced another CNT-CF/TPI nanocomposite. Carbon nanotubes (CNTs) were introduced into carbon fiber (CF) by chemical vapor deposition and then were incorporated in the GCTP^TM thermoplastic polyimide composites. CNT-coated CF was shown to be significant with elevated tensile properties of CNT-CF/TPI hybrid composite. In contrast with the neat CF/TPI composite, CNT-CF/TPI composite has shown enhanced tensile strength increased to approximately 30%. The justification of supreme tensile properties of hybrid composite was explained to be dependent on fiber–matrix adhesion incorporated by interfacial CNT. In 2015, S. V. Larin et al., [68] also studied and reported structural properties of TPI/carbon nanocomposites based on thermoplastic polyimides filled with surface-modified carbon nanotubes (CNT) by means of fully atomistic molecular dynamics simulations.

It is shown that the CNT surface area available for contacts with TPI matrix influences significantly the polymer matrix structure. Increase of CNT surface modification degree leads to reduced surface area of the filler available for the interaction with polymer chains. Thus, in the case of uniform distribution of functional groups on the CNT surface, no orientation ordering of the flat fragments of polyimide chains has observed even near the filler surface.

6.10.4 TPI/CF/TiO₂ Nanocomposite

Li [69] reported a TPI/CF/TiO₂ nanocomposite which was prepared through compression molding as milled CF/TPI mixtures without further melt mixing. The incorporation of TiO₂ leads to a significant improvement in friction and wear properties of the CF/TPI composite. The scratches on the worn surface of the CF/TPI composite filled with TiO₂ are considerably reduced. When the TiO₂ particle content was less than 10 vol%, the wear rates of TiO₂ filled composites decreased with increasing TiO₂ filler content, which were lower than that of composites filled with the same content of CF. With further increase in TiO₂ filler content, the wear rate of TiO₂-filled composites sharply increased close to that of pure PI and higher than that of composites filled with CF.