Black polymer films with high thermal stability are highly desired in flexible electrical and electronic fields. Conventional black polymer films based on high-temperature resistant polymers and black inorganic dyes are usually suffered from the poor electrical and tensile properties. In the current work, a series of intrinsically black polyimide (BPI) films with International Commission on Illumination (CIE) Lab optical parameters close to zero and high thermal stability have been designed and prepared. For this purpose, an electron-rich aromatic diamine, 4,4′-iminodianiline (NDA), was copolymerized with 1,4-phenylenediamine (PDA) and 3,3′,4,4′-biphenyltetracarboxylic dianhydride (sBPDA) to afford a series of poly(amic acid) (PAA) solutions, which were then thermally dehydrated to provide the final BPI films at elevated temperatures up to 400 °C in air. The molar fraction of NDA in the total diamine monomers was 0 for BPI-0 (sBPDA-PDA), 10% for BPI-1, 20% for BPI-2, 30% for BPI-3, 40% for BPI-4, 50% for BPI-5, and 100% for BPI-6. For comparison, two referenced polyimide (PI) films, including PI-ref1 and PI-ref2, were prepared according to a similar procedure. The former was derived from pyromellitic dianhydride (PMDA) and 4,4′-oxydianiline (ODA) and the latter was from PMDA and NDA. The BPI films exhibited an increasing degree of blackness with the increasing contents of NDA units in the polymer films. For example, the BPI-6 (sBPDA-NDA) film exhibited the optical transmittance of 1.4% at a wavelength of 650 nm (T650), which was obviously lower than those of PI-ref1 (T650 = 74.6%) and PI-ref2 (T650 = 3.6%). In addition, the BPI-6 film showed the CIE Lab parameters of 0.39 for L*, 2.65 for a*, 0.66 for b*, and haze of 1.83, which was very close to the criterion of “pure blackness” for polymer films (L* = a* = b* = 0). At last, incorporation of the NDA units in the rigid-rod BPI-0 (BPDA-PDA) film slightly deteriorated the high-temperature dimensional stability of the derived BPI films. BPI-6 film showed a linear coefficient of thermal expansion (CTE) value of 34.8 × 10−6/K in the temperature range of 50 to 250 °C, which was higher than those of the BPI-0 (CTE = 12.3 × 10−6/K), PI-ref1 (CTE = 29.5 × 10−6/K), and PI-ref2 (CTE = 18.8 × 10−6/K) films. Nevertheless, the BPI films maintained good thermal stability with the 5% weight loss temperatures (T5%) higher than 590 °C, and the glass transition temperatures (Tg) higher than 340 °C.

Wholly aromatic polyimide (PI) films with good solution processability, light colors, good optical transparency, high storage modulus, and improved heat resistance were prepared and characterized. For this purpose, a multi-component copolymerization methodology was performed from a fluoro-containing dianhydride, 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA), a rigid dianhydride, 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA), and a fluoro-containing diamine, 2,2′-bis(trifluoromethyl)-4,4′-bis [4-(4-amino-3-methyl)benzamide]biphenyl (MABTFMB). One homopolymer, FPI-1 (6FDA-MABTFMB), and five copolymers, FPI-2~FPI-6, containing the BPDA units from 10 mol% to 50 mol% in the dianhydride moieties, were prepared, respectively. The derived PI resins showed good solubility in the polar aprotic solvents, such as N-methyl-2-pyrrolidone (NMP) and N,N-dimethylacetamide (DMAc). The flexible PI films obtained by the solution casting procedure showed good optical properties with the transmittances higher than 74.0% at the wavelength of 450 nm. The PI films exhibited excellent thermal properties, including 5% weight loss temperatures (T5%) over 510 °C, together with glass transition temperatures (Tg) over 350.0 °C according to the peak temperatures of the loss modulus in dynamical mechanical analysis (DMA) measurements. The FPI-6 film also showed the lowest linear coefficient of thermal expansion (CTE) value of 23.4 × 10−6/K from 50 to 250 °C according to the thermomechanical analysis (TMA) measurements, which was obviously lower than that of FPI-1 (CTE = 30.6 × 10−6/K).

Negative photosensitive polyimides (PSPIs) with the photo-patterned ability via the photocrosslinking reactions induced by the i-line (365 nm) and h-line (426 nm) emitting wavelengths in high-pressure mercury lamps have been paid increasing attention in semiconductor fabrication, optical fiber communications, and other advanced optoelectronic areas. In the current work, in view of the optical and thermo-mechanical disadvantages of the currently used negative PSPIs, such as the intrinsically photosensitive or auto-photosensitive systems derived from 3,3’,4,4’-benzophenonetetracarboxylic dianhydride (BTDA) and the ortho-alkyl- substituted aromatic diamines, a series of modified negative PSPIs with the enhanced optical transparency in the wavelength of 365~436 nm and apparently reduced coefficients of linear thermal expansion (CTE) were developed. For this purpose, a specific aromatic diamine with both of trifluoromethyl and benzanilide units in the molecular structures, 2,2’-bis(trifluoromethyl)-4,4’-bis[4-(4-amino-3-methyl)benzamide]biphenyl (MABTFMB) was copolymerized with BTDA and the standard 3,3’,5,5’-tetramethyl-4,4’-diaminodiphenylmethane (TMMDA) diamine via a two-step chemical imidization procedure. As compared with the pristine PI-1 (BTDA-TMMDA) system, the new-developed fluoro-containing PSPI systems (FPI-2~FPI-7) exhibited the same-level solubility in polar aprotic solvents, including N-methyl-2-pyrrolidone (NMP) and N,N- dimethylacetamide (DMAc). The FPI films cast from the corresponding FPI solutions in NMP showed the optical transmittances of 78.3–81.3% at the wavelength of 436 nm (T436, h-line), which were much higher than that of the PI-1 (T436 = 60.9%). The FPI films showed the CTE values in the range of 40.7 × 10−6/K to 54.0 × 10−6/K in the temperature range of 50 to 250 °C, which were obviously lower than that of PI-1 (CTE = 56.5 × 10−6/K). At last, the photosensitivity of the FPI systems was maintained and the micro-pattern with the line width of 10 μm could be clearly obtained via the standard photolithography process of FPI-7 with the molar ratio of 50% for MABTFMB in the diamine moiety.

Light-colored and transparent polyimide (PI) films with good high-temperature dimensional stability are highly desired for advanced optoelectronic applications. However, in practice, the simultaneous achievement of good optical and thermal properties in one PI film is usually difficult due to the inter-conflicting molecular design of the polymers. In the present work, a series of PI-SiO2 nanocomposite films (ABTFCPI) were developed based on the PI matrix derived from hydrogenated pyromellitic anhydride (HPMDA) and an aromatic diamine containing benzanilide and trifluoromethyl substituents in the structure, 2,2′-bis(trifluoromethyl)-4,4′-bis [4-(4-aminobenzamide)]biphenyl (ABTFMB). The inorganic SiO2 fillers were incorporated into the nanocomposite films in the form of colloidal nanoparticles dispersed in the good solvent of N,N-dimethylacetamide (DMAc) for the PI matrix. The derived ABTFCPI nanocomposite films showed good film-forming ability, flexible and tough nature, good optical transparency, and good thermal properties with loading amounts of SiO2 up to 30 wt% in the system. The ABTFCPI-30 film with a SiO2 content of 30 wt% in the film showed an optical transmittance of 79.6% at the wavelength of 400 nm (T400) with a thickness of 25 μm, yellow index (b*) of 2.15, and 5% weight loss temperatures (T5%) of 491 °C, which are all comparable to those the pristine ABTFCPI-0 matrix without filler (T400 = 81.8%; b* = 1.77; T5% = 492 °C). Meanwhile, the ABTFCPI-30 film exhibited obviously enhanced high-temperature dimensional stability with linear coefficients of thermal expansion (CTE) of 25.4 × 10−6/K in the temperature range of 50 to 250 °C, which is much lower than that of the AMTFCPI-0 film (CTE = 32.7 × 10−6/K).

Polymeric ultrafine fibrous membranes (UFMs) with high thermal stability and high whiteness are highly desired in modern optoelectronic applications. A series of fluoro-containing polyimide (FPI) UFMs with high whiteness, good thermal stability, and good hydrophobicity were prepared via a one-step electrospinning procedure from the organo-soluble FPI resins derived from a fluoro-containing dianhydride, 4,4′-(hexafluoroisopropylidene) diphthalic anhydride (6FDA), and various diamines containing either pendant trifluoromethyl (–CF3) groups or alicyclic units in the side chains. The obtained FPI UFMs, including FPI-1 from 6FDA and 3,5-diaminobenzotrifluoride (TFMDA), FPI-2 from 6FDA and 2′-trifluoromethyl-3,4′-oxydianiline (3FODA), FPI-3 from 6FDA and 1,4-bis[(4-amino-2-trifluoromethyl)phenoxy]benzene (6FAPB), FPI-4 from 4,4′-bis[(4-amino-2-trifluoromethyl)phenoxy]biphenyl (6FBAB), and FPI-5 from 6FDA and 4’-tert-butyl-cyclohexyl-3,5-diaminobenzoate (DABC) showed whiteness indices (WI) higher than 87.00 and optical reflectance values higher than 80% at the wavelength of 457 nm (R457), respectively. The FPI-5 UFM, especially, showed the highest WI of 92.88. Meanwhile, the prepared PI UFMs exhibited good hydrophobic features with water contact angles (WCA) higher than 105°. At last, the PI UFMs exhibited good thermal stability with glass transition temperatures (Tg) higher than 255 °C, and the 5% weight-loss temperatures (T5%) higher than 510 °C in nitrogen.