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Two organo-soluble polyimide resins containing phenyl-substituted quinoxaline units (PIPQ) were successfully prepared via the two-step chemical imidization procedure from an aromatic diamine, 6(7)-amino-2-(4-aminophenyl)-3-phenylquinoxaline (PQDA) and aromatic dianhydrides with flexible molecular structures. The derived PIPQ-1 resin from PQDA and 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) and the PIPQ-2 resin from PQDA and 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride (BPADA) were all soluble in polar aprotic solvents, such as N-methyl-2-pyrrolidone (NMP) and N,N-dimethylacetamide (DMAc). The PIPQ/DMAc solutions with the solid contents of 25–35 wt% were used as the starting mediums and the PIPQ ultrafine non-woven fibrous membranes (UFMs) were successfully prepared by the one-step electrospinning (ES) procedure. For comparison, the standard polyimide reference electrospun UFM (PI-ref) was also fabricated via the two-step ES procedure from the soluble poly(amic acid) (PAA) precursor based on pyromellitic anhydride (PMDA) and 4,4′-oxydianiline (ODA), followed by thermal imidization at elevated temperatures from 80–300 °C. The fabricated PIPQ UFMs exhibited good thermal stability with the 5 % weight loss temperatures (T5%) of 537 °C for PIPQ-1 (6FDA-PQDA) and 517 °C for PIPQ-2 (BPADA-PQDA) in nitrogen, respectively. In addition, the PIPQ UFMs exhibited glass transition temperatures (Tg) higher than 252 °C. Meanwhile, the PIPQ UFMs exhibited improved optical properties with the reflectance values over 75 % at the wavelength of 457 nm and high whiteness. At last, the PIPQ UFMs showed greatly enhanced hydrolysis stability in highly alkaline aqueous solution (sodium hydroxide in water, 20 wt%) either at room temperature or at elevated temperature. The PIPQ UFMs survived even during the aging in boiling aqueous sodium hydroxide solution (concentration: 20 wt%) for 3 h while the PI-ref UFM totally hydrolyzed during the test.

Electrospun polyimide (PI) ultrafine non-woven fibrous mats (UFMs) with good heat-fusibility via hot press procedure were successfully fabricated. For this purpose, two organo-soluble PI resins were prepared via a two-step chemical imidization procedure from two isomeric oxydiphthalic dianhydrides, including the asymmetrical ODPA (2,3,3′,4′-oxydiphthalic dianhydride, or aODPA) and symmetrical ODPA (4,4′-oxydiphthalic dianhydride, or ODPA) with an aromatic diamine containing phenolphthalein unit in the main chain, 3,3-bis[4-(4-aminophenoxy)phenyl]phthalide (BAPPT), respectively. The derived PI-1 (aODPA-BAPPT) and PI-2 (ODPA-BAPPT) resins showed high numerical average molecular weights (Mn) over 105 g/mol and were easily soluble in polar aprotic solvents, such as N-methyl-2-pyrrolidinone (NMP) and N, N-dimethylacetamide (DMAc). PI-1 and PI-2 UFMs were successfully prepared from the PI solutions in DMAc via the one-step electrospinning procedure. Different rotating speeds of 500–2500 round per minute (rpm) for the cylindrical collector were set to investigate the effects of the fiber morphologies on the properties of the PI UFMs. The derived PI UFMs showed good thermal stability with the glass transition temperatures (Tg) over 280 °C and 5 % weight loss temperatures (T5%) higher than 510 °C in nitrogen. The PI UFMs could be successfully heat-sealed via the hot-press temperature at 320 °C (about 40 °C higher than Tg) for 60s with the pressure of 0.5 MPa.

A series of semi-alicyclic poly(amic acid) varnishes designed for potential applications as the alignment layers for thin-film transistor driven liquid crystal display devices (TFT-LCDs) were prepared from an alicyclic dianhydride, norbornane-2-spiro-α- cyclopentanone-α′-spiro-2″-norbornane-5,5″,6,6″-tetracarboxylic dianhydride (CpODA) and aromatic diamines. Then, the liquid crystal (LC) minicells were fabricated by using the polyimide (PI) as the alignment layers, which were prepared by thermally dehydrating the PAA varnishes at elevated temperatures. The derived PI alignment layers exhibited good thermal stability with the 5% weight loss temperatures higher than 470 °C. In addition, the PI layers showed excellent optical transparency with the transmittances higher than 94.0% at the wavelength of 550 nm with a thickness around 100 nm. The LC cells fabricated with the CpODA-PI alignment layers exhibited good optoelectronic features. The rubbing PI layers could induce the effective alignment of the LC molecules with the pretilt angles in the range of 1.62–2.14°. The voltage holding ratios of the LC cells were higher than 92.00%, and the residual direct voltage values were all lower than 300 mV.

This work describes polyimide (PI) ultrafine fibrous membranes (UFMs) with aligned fibrous structures, fabricated via the high-speed electrospinning procedure. Organo-soluble intrinsically photosensitive PI is utilized as the fiber-forming agent. The effects of different rotating speeds on the fiber morphology and properties are studied. The aligned UFMs possess hydrophobicity, favorable optical properties, and improved deformation durability. The extension strength of the UFMs reinforces obviously with the increased rotating speed and reaches the maximum of 9.18 MPa at 2500 rpm. In addition, due to the photo-cross-link nature of the PI resin, the UFMs present lithography capability, which can obtain micro-sized patterns on aluminum substrates, and even part of the fibrous structure was retained after development. This work shows promise in manufacturing fiber-based photolithographic hierarchical structures on flexible substrates, which exhibit potential in achieving multiple functions on fiber-based electronic devices.

Two semi-alicyclic polyimide (PI) films with light colors, good thermal stability, and enhanced flame retardancy were successfully developed from an aromatic diamine containing phenolphthalein unit, 3,3-bis[4-(4-aminophenoxy)phenyl]phthalide (BAPPT) and alicyclic dianhydrides, including 1,2,4,5-cyclohexanetetracarboxylic acid dianhydride (HPMDA) and 3,3',4,4'-bicyclohexanetetracarboxylic acid dianhydride (HBPDA). For this target, two organo-soluble PI resins, including PI-1 (HPMDA-BAPPT) and PI-2 (HBPDA-BAPPT) were first synthesized by a high-temperature polymerization route. The derived preimidized PI resins were soluble in N-methyl-2-pyrrolidinone (NMP), N,N-dimethylacetamide (DMAc), and other polar aprotic solvents. Corresponding PI films with pale-yellow appearances at the thickness of 25 μm were prepared by casting the PI solutions on glass substrates, followed by thermally baking at elevated temperatures up to 280 °C in nitrogen. The obtained PI films exhibited the cutoff wavelengths (λcut) lower than 300 nm, the optical transmittances as high as 84.5% at the wavelength of 450 nm, the yellow indices (b*) as low as 3.10, and the haze values as low as 3.71%. Both of the PI films exhibited good thermal stability with the 5% weight loss temperatures (T5%) higher than 480 °C in nitrogen, char yields higher than 40% at 700 °C, and glass transition temperatures (Tg) as high as 318.1 °C. In addition, the semi-alicyclic PI films showed acceptable high-temperature dimensional stability with the linear coefficients of thermal expansion (CTE) values of 52.3 × 10–6/K for PI-1 and 60.4 × 10–6/K for PI-2 in the temperature range of 50–200 °C, respectively. At last, the PI-1 film showed good flame retardancy with the limited oxygen index (LOI) of 32.8%, the flame retardancy class of UL94 VTM-0, and good smoke suppression properties.

Optically transparent polyimide (PI) films with good dielectric properties and long-term sustainability in atomic-oxygen (AO) environments have been highly desired as antenna substrates in low earth orbit (LEO) aerospace applications. However, PI substrates with low dielectric constant (low-Dk), low dielectric dissipation factor (low-Df) and high AO resistance have rarely been reported due to the difficulties in achieving both high AO survivability and good dielectric parameters simultaneously. In the present work, an intrinsically low-Dk and low-Df optically transparent PI film matrix, poly[4,4′-(hexafluoroisopropylidene)diphthalic anhydride-co-2,2-bis(4-(4-aminophenoxy)phenyl)hexafluoropropane] (6FPI) was combined with a nanocage trisilanolphenyl polyhedral oligomeric silsesquioxane (TSP-POSS) additive in order to afford novel organic–inorganic nanocomposite films with enhanced AO-resistant properties and reduced dielectric parameters. The derived 6FPI/POSS films exhibited the Dk and Df values as low as 2.52 and 0.006 at the frequency of 1 MHz, respectively. Meanwhile, the composite films showed good AO resistance with the erosion yield as low as 4.0 × 10−25 cm3/atom at the exposure flux of 4.02 × 1020 atom/cm2, which decreased by nearly one order of magnitude compared with the value of 3.0 × 10−24 cm3/atom of the standard PI-ref Kapton® film.

High-temperature-resistant polymeric adhesives with high servicing temperatures and high adhesion strengths are highly desired in aerospace, aviation, microelectronic and other high-tech areas. The currently used high-temperature resistant polymeric adhesives, such as polyamic acid (PAA), are usually made from the high contents of solvents in the composition, which might cause adhesion failure due to the undesirable voids caused by the evaporation of the solvents. In the current work, electrospun preimidized polyimide (PI) nano-fibrous membranes (NFMs) were proposed to be used as solvent-free or solvent-less adhesives for stainless steel adhesion. In order to enhance the adhesion reliability of the PI NFMs, thermally crosslinkable phenylethynyl end-cappers were incorporated into the PIs derived from 3,3’,4,4’-oxydiphthalic anhydride (ODPA) and 3,3-bis[4-(4-aminophenoxy)phenyl]phthalide (BAPPT). The derived phenylethynyl-terminated PETI-10K and PETI-20K with the controlled molecular weights of 10,000 g mol−1 and 20,000 g mol−1, respectively, showed good solubility in polar aprotic solvents, such as N-methyl-2-pyrrolidinone (NMP) and N,N-dimethylacetamide (DMAc). The PI NFMs were successfully fabricated by electrospinning with the PETI/DMAc solutions. The ultrafine PETI NFMs showed the average fiber diameters (dav) of 627 nm for PETI-10K 695 nm for PETI-20K, respectively. The PETI NFMs showed good thermal resistance, which is reflected in the glass transition temperatures (Tgs) above 270 °C. The PETI NFMs exhibited excellent thermoplasticity at elevated temperatures. The stainless steel adherends were successfully adhered using the PETI NFMs as the adhesives. The PI NFMs provided good adhesion to the stainless steels with the single lap shear strengths (LSS) higher than 20.0 MPa either at room temperature (25 °C) or at an elevated temperature (200 °C).

Novel preimidized semi-alicyclic polyimide (PI) alignment layers with low curing temperature and high voltage holding ratio (VHR) have been successfully developed in order to meet the demands for l...

A series of solution-processable and photosensitive polyimide (PSPI) coatings with enhanced high-temperature dimensional stability were designed, synthesized and characterized. For this purpose, the rigid-rod benzanilide (Ph − CONH − Ph) units with methyl substituents were incorporated into the molecular structures of an intrinsically auto-photosensitive PSPI system via the two-step chemical imidization copolymerization of a specially designed aromatic diamine, 2,3'-dimethyl-4,4'- diaminobenzanilide (MMDABA) and an aromatic dianhydride, 3,3',4,4'-benzophenonetetracarboxylic dianhydride (BTDA), together with an aromatic diamine, 3,3',5,5'-tetramethyl-4,4'-diaminodiphenylmethane (TMMDA). The ortho-substituted methyl group in MMDABA, on one hand, was designed to guarantee the solubility of the derived PSPI resins in the organic solvents; and on the other hand, maintain the intrinsic photosensitivity of the pristine PSPI (BTDA-TMMDA, PSPI-1). Meanwhile, the rigid benzanilide unit was applied to reduce the coefficients of thermal expansions (CTE) of the derived PSPI coatings. Four PSPI copolymers with the TMMDA/MMDABA (molar ratio) of 95/5 (PSPI-2), 90/10 (PSPI-3), 85/15 (PSPI-4), and 80/20 (PSPI-5) were prepared. The derived PSPI resins were all soluble in polar aprotic solvents of N-methyl-2-pyrrolidone (NMP) and N,N-dimethylacetamide (DMAc). All the PSPI coatings maintained good photosensitivity to the ultraviolet (UV) light (UV-A, 320 ~ 460 nm) except PSPI-5, indicating the deterioration of the photosensitivity of the polymers by incorporation of MMDABA units. Incorporation of MMDABA units into the PSPI systems basically maintained the thermal stability of the polymers, while apparently increased the dimensional stability of the coatings at elevated temperatures. PSPI-5 possessed the CTE value of 42 × 10–6/K in the temperature range of 50 to 300 °C, which was obviously lower than that of the pristine PSPI-1 system (CTE = 56 × 10–6/K).

Uniform alignment of rigid-rod liquid crystal (LC) molecules under applied voltage is critical for achievement of high-quality display for thin-film transistor-driven liquid crystal display devices (TFT-LCDs). The polymeric components that can induce the alignment of randomly aligned LC molecules are called alignment layers (ALs). In the current work, a series of organo-soluble polyimide (SPI) ALs were designed and prepared from an alicyclic dianhydride, hydrogenated 3,3&prime, 4,4&prime, biphenyltetracarboxylic dianhydride (HBPDA), and various aromatic diamines, including 4,4&prime, methylenedianiline (MDA) for SPI-1, 4,4&prime, aminodianiline (NDA) for SPI-2, 3,3&prime, 5,5&prime, tetramethyl-4,4&prime, diaminodiphenylmethane (TMMDA) for SPI-3, and 3,3&prime, diethyl-5,5&prime, dimethyl-4,4&prime, diaminodiphenylmethane (DMDEDA) for SPI-4. The derived SPI resins were all soluble in N-methyl-2-pyrrolidone (NMP). Four SPI alignment agents with the solid content of 6 wt.% were prepared by dissolving the SPI resins in the mixed solvent of NMP and butyl cellulose (BC) (NMP/BC = 80:20, weight ratio). Liquid crystal minicells were successfully fabricated using the developed SPI varnishes as the LC molecule alignment components. The SPI ALs showed good alignment ability for the LC molecules with the pretilt angles in the range of 1.58&deg, &ndash, 1.97&deg, The LC minicells exhibited good optoelectronic characteristics with voltage holding ratio (VHR) values higher than 96%. The good alignment ability of the SPI ALs is mainly attributed to the good comprehensive properties of the SPI layers, including high volume resistivity, high degree of imidization at the processing temperature (230 &deg, C), good rubbing resistance, good thermal stability with glass transition temperatures (Tgs) higher than 260 &deg, C, and excellent optical transparency with the transmittance higher than 97% at the wavelength of 550 nm.

Polymeric nanofibrous membranes (NFMs) with both high whiteness and high thermal and ultraviolet (UV) stability are highly desired as reflectors for ultraviolet light-emitting diodes (UV-LEDs) devices. In the current work, a semi-alicyclic and fluoro-containing polyimide (PI) NFM with potential application in such kinds of circumstances was successfully fabricated from the organo-soluble PI resin solution via a one-step electrospinning procedure. In order to achieve the target, a semi-alicyclic PI resin was first designed and synthesized from an alicyclic dianhydride, 3,4-dicarboxy-1,2,3,4,5,6,7,8-decahydro-1-naphthalenesuccinic dianhydride (or hydrogenated tetralin dianhydride, HTDA), and a fluoro-containing diamine, 2,2-bis[4-(4-amino-phenoxy)phenyl]hexafluoropropane (BDAF), via an imidization procedure. The derived PI (HTDA-BDAF) resin possessed a number-average molecular weight (Mn) higher than 33,000 g/mol and was highly soluble in polar aprotic solvents, such as N,N-dimethylacetamide (DMAc). The electrospinning solution was prepared by dissolving the PI resin in DMAc at a solid content of 25–35 wt%. For comparison, the conventional high-whiteness polystyrene (PS) NFM was prepared according to a similar electrospinning procedure. The thermal and UV stability of the derived PI and PS NFMs were investigated by exposure under the UV-LED (wavelength: 365 nm) irradiation. Various thermal evaluation results indicated that the developed PI (HTDA-BDAF) NFM could maintain both the high reflectance and high whiteness at elevated temperatures. For example, after thermal treatment at 200 °C for 1 h in air, the PI (HTDA-BDAF) NFM exhibited a reflectance at a wavelength of 457 nm (R457) of 89.0%, which was comparable to that of the pristine PI NMF (R457 = 90.2%). The PI (HTDA-BDAF) NFM exhibited a whiteness index (WI) of 90.88, which was also close to that of the pristine sample (WI = 91.22). However, for the PS NFM counterpart, the R457 value decreased from the pristine 88.4% to 18.1% after thermal treatment at 150 °C for 1 h, and the sample became transparent. The PI NFM maintained good optical and mechanical properties during the high dose (2670 J/cm2) of UV exposure, while the properties of the PS NFM apparently deteriorated under the same UV aging.

A series of intrinsically black polyimide (PI) films have been designed and synthesized by the copolymerization of pyromellitic dianhydride (PMDA), 4,4′-oxydianiline (ODA) and 4,4′-diaminodiphenylamine (NDA) via a two-step polycondensation procedure, followed by the high temperature imidization reaction of the poly(amic acid) (PAA) intermediates. Incorporation of the electron-rich NDA diamine apparently enhanced the formation of charge transfer complex (CTC) between the electron-donating diamine units and the electron-accepting dianhydride moiety; thus greatly absorbing the visible light and endowing the derived PI films deep colors. PI-5 with the 100% molar ratio of NDA in the diamines exhibited a cutoff wavelength value of 555 nm, which was 127 nm higher than that of PI-1 without NDA component. The PI films were nearly totally opaque at the wavelength of 500 nm when the molar ratio of NDA exceeded 80% in the diamines. PI-5 showed International Commission on Illumination (CIE) lab color parameters of 23.36 for L*, 46.43 for a*, and 40.26 for b*, indicating the essentially black plus red color instead of the yellow plus blue color for PI-1 film (L* = 89.93, a* = −11.09, and b* = 84.23). In addition, introduction of NDA into the copolymer films increased the high temperature dimensional stability of the films. PI-5 exhibited a glass transition temperature (Tg) and coefficient of thermal expansion (CTE) of 431.6 °C and 18.8 × 10−6 /K, respectively, which were superior to those of the PI-1 film (Tg = 418.8 °C, CTE = 29.5 × 10−6 /K). At last, incorporation of NDA components somewhat decreased the tensile properties and thermal decomposition temperatures of the PI films.

A series of organo-soluble polyimide (PI) resins were prepared from a newly designed and synthesized aromatic thianthrene-containing diamine, 2,7-bis(3-aminophenoxy)thianthrene (APOT) and various aromatic dianhydrides via a one-step high temperature polycondensation procedure with N-methyl-2-pyrrolidone (NMP) as the solvent. The derived PI resins were soluble in polar aprotic solvents, including NMP and N,N- dimethylacetamide (DMAc). For comparison, several unsoluble sulfur-containing PIs were prepared from anotherpara-substituted thianthrene-containing diamine, 2,7-bis(4-aminophenylenesulfanyl)thianthrene (APTT) and aromatic dianhydrides by a two-step polycondensation procedure via poly(amic acid) (PAA) precursors at elevated temperatures as high as 300 °C. Flexible and tough PI films with the tensile strength higher than 88 MPa were successfully cast from the APOT-PI resin solution at a relatively low curing temperature of 250 °C. The meta-substituted molecular skeleton in the APOT- PIs endowed the films good optical transparency with the transmittance higher than 83% at a thickness around 10 um, which was apparently higher than those of the para-substituted APTT-PI analogues. The sulfur-rich thianthrene moiety with high molar refraction endowed the APOT-PI films high refractive indices up to 1.7147 at 632.8 nm and low birefringence in the range of 0.0064–0.0133. In addition, the PI films showed good thermal stability with the glass transition temperatures (Tg) higher than 198.3 °C and 5% weight loss temperatures higher than 510 °C in nitrogen.

The rapid development of advanced high-frequency mobile communication techniques has advanced urgent requirements for polymer materials with high-temperature resistance and good dielectric properties, including low dielectric constants (low-Dk) and low dielectric dissipation factors (low-Df). The relatively poor dielectric properties of common polymer candidates, such as standard polyimides (PIs) greatly limited their application in high-frequency areas. In the current work, benzoxazole units were successfully incorporated into the molecular structures of the fluoro-containing PIs to afford the poly(imide-benzoxazole) (PIBO) nano-fibrous membranes (NFMs) via electrospinning fabrication. First, the PI NFMs were prepared by the electrospinning procedure from organo-soluble PI resins derived from 2,2’-bis(3,4-dicarboxy-phenyl)hexafluoropropane dianhydride (6FDA) and aromatic diamines containing ortho-hydroxy-substituted benzamide units, including 2,2-bis[3-(4-aminobenzamide)-4-hydroxylphenyl]hexafluoropropane (p6FAHP) and 2,2-bis[3-(3-aminobenzamide)-4-hydroxyphenyl]hexafluoropropane (m6FAHP). Then, the PI NFMs were thermally dehydrated at 350 °C in nitrogen to afford the PIBO NFMs. The average fiber diameters (dav) for the PIBO NFMs were 1225 nm for PIBO-1 derived from PI-1 (6FDA-p6FAHP) precursor and 816 nm for PIBO-2 derived from PI-2 (6FDA-m6FAHP). The derived PIBO NFMs showed good thermal stability with the glass transition temperatures (Tgs) over 310 °C and the 5% weight loss temperatures (T5%) higher than 500 °C in nitrogen. The PIBO NFMs showed low dielectric features with the Dk value of 1.64 for PIBO-1 and 1.82 for PIBO-2 at the frequency of 1 MHz, respectively. The Df values were in the range of 0.010~0.018 for the PIBO NFMs.