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An ultraviolet-assisted electrospinning (UVAES) procedure was performed using in situ photopolymerization during the electrospinning process to enhance the solvent resistance of polyimide (PI) ultrafine fibrous membranes (UFMs). The resultant PI UFMs were fabricated via solution electrospinning using photosensitive poly(amic acid ester) (PSPAE) modified with unsaturated double bond side chains as the main fiber-forming agent, diethylene glycol diacrylate as the crosslinker, and titanocene as the photoinitiator. The precursor PSPAE UFMs were prepared via both of normal electrospinning and UVAES procedures and then were converted to PI using thermal imidization at 350 °C in a nitrogen imidization. The physical and chemical properties of the obtained PI UFMs were subsequently studied. The fibers exposed to ultraviolet irradiation possessed a smooth surface morphology and maintained their fibrous structure after thermal imidization at elevated temperature as high as 350 °C in nitrogen, whereas the crosslink-free PSPAE fibers lost their fibrous structure after heating. Furthermore, as well as providing enhanced thermal decomposition stability, the introduction of a crosslinked structure via ultraviolet irradiation also protected the molecular chains from organic solvents, resulting in excellent solvent resistance of the PI UFMs.

A series of polyphenylquinoxaline (PPQ) ultrafine non-woven fibrous membranes have been first successfully prepared via the electrospinning procedure with the soluble PPQ solutions as the starting materials. For this purpose, various organo-soluble PPQ resins were synthesized via the one-step high temperature polycondensation procedure from the aromatic ether-bridged bis(α-diketone) and bis(o-diamine) monomers. Flexible ether linkages and pendant bulky phenyl substituents endowed the PPQ resins good solubility in polar aprotic solvents. The high-molecular-weight PPQ resins were dissolved in N-methyl-2-pyrrolidone (NMP) to afford the PPQ electrospinning solution except PPQ-Ia derived from 4,4′-oxydibenzil (ODB) and 3,3′-diaminobenzidine (DAB) due to the limited solubility in the solvent. All the derived PPQ ultrafine non-woven fibrous membranes maintained good structure integrity after hydrolysis aging either at room temperature (25 °C) for 72 h or at refluxing temperature (100 °C) for 24 h in an aqueous sodium hydroxide (NaOH) solution at a solid content of 20 wt%. Comparatively, the polyimide (PI) reference electrospun membrane (PI-ref) derived from 1,2,4,5-pyrromellitic anhydride (PMDA) and 4,4′-oxydianiline (ODA) lost its original structure only after boiling in the same NaOH solution within 3 h. In addition, the developed PPQ ultrafine non-woven fibrous membranes exhibited good thermal stability with the 5 % weight loss temperatures (T5%) higher than 555.0 °C in nitrogen and glass transition temperatures (Tg) in the range of 248.1–266.1 °C, respectively.

A series of semi-alicyclic polyimide (PI) resins with the number average molecular weights (Mn) in the range of 34901–74790 g/mol were prepared from a hydrogenated 3,3′,4,4′-biphenyltetracarboxylic dianhydride (HBPDA) and various aromatic diamines by a high-temperature polycondensation procedure. For comparison, the analogous PI reference resins were prepared from hydrogenated 1,2,4,5-pyromellitic dianhydride (HPMDA) and the same aromatic diamines. The prepared PI resins were all soluble in polar aprotic solvents, such as N-methyl-pyrrolidinone (NMP) and N,N-dimethylacetamide (DMAc). Various PI varnishes with the solid contents as high as 35–45 wt% were successfully prepared by dissolving the PI resins in NMP. Some of the PI varnishes, including PI-3 derived from HBPDA and 1,3-bis(3-aminophenoxy)benzene (133APB), PI-5 from HBPDA and 2,2′-bis(trifluoromethyl)benzidine (TFMB), and PI-3′ from HPMDA and 133APB exhibited desirable processing viscosities at high solid contents. For example, when the solid content was 35 wt%, the PI varnishes showed absolute viscosities of 2144 mPa s for PI-3, 9971 mPa s for PI-5, and 3156 mPa s for PI-3′, respectively. The values were 6877 mPa s for PI-3, 42370 mPa s for PI-5, and 13700 mPa s for PI-3′, respectively when the solid contents of the PI varnishes increased to 40 wt%. This unique feature made the current PI varnishes good candidates as thick-film passivation layers for chip protection. Bare light emitting diodes chips were successfully passivated via once coating by using PI-5 (HBPDA–TFMB) varnish with a solid content of 35 wt%. The HBPDA–PI passivation layers exhibited good thermal stabilities with the glass transition temperatures (Tg) in the range of 171.5–296.3 °C and 5% weight loss temperatures higher than 470 °C. In addition, they showed good optical transparency with the transmittance higher than 72% at 450 nm at a thickness of 25 µm and low yellow indices and haze values.

Electrospun polyphenylquinoxaline (PPQ) ultrafine non-woven fibrous membrane (UFM) has been successfully used as the filtering medium for the separation of highly alkaline aqueous red mud (RM) slurry (pH = 13.0 at 21.5 °C), known as the industrial waste during production of alumina. For this purpose, the PPQ solution was prepared by dissolving the PPQ resin derived from 1,4-bis(4-benzilyloxy)benzene (DBOB) and 3,3′,4,4′-tetraaminodiphenylether (TADPE) in N-methyl-2-pyrrolidone (NMP) at a solid content of 8 wt%. Then, the PPQ UFM was fabricated by a one-step ES procedure. For comparison, the polyimide (PI) UFM, poly(pyromellitic dianhydride-co-4,4′-oxydianiline) (PMDA-ODA, PI-ref) was made by a two-step ES procedure. The obtained flexible and tough PPQ UFM was used as the filter for the separation of highly alkaline aqueous RM slurry. During the separation process, the PPQ UFM exhibited excellent stability in the aqueous alkaline environments of RM slurry either at room temperature (pH = 13.0 at 21.5 °C) or at higher processing temperature (pH = 12.1 at 60.5 °C). RM powder was efficiently separated from the slurry via the PPQ UFM and easily stripped from the membrane. Water contact angle (WCA) measurements revealed the hydrophobic nature of the PPQ UFM (WCA = 113.4° for pure water and WCA = 128.4° for aqueous sodium hydroxide solution at a solid content of 20 wt%). Comparatively, PI-ref (PMDA-ODA) UFM exhibited poor stability in the high-temperature alkaline environment.

A series of flexible and tough polyimide (PI) microfibrous mats (PI-1~PI-4) have been prepared via the one-step electrospinning procedure with the organo-soluble PI resins as the starting materials. For this purpose, four PI resins were first synthesized by the chemical imidization reaction from 2,2′-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) and four aromatic diamines containing rigid-rod moieties in their molecular skeletons, respectively. The PI resins derived from 6FDA and aromatic diamines, including PI-1 from 2-(4-aminophenyl)-5-aminobenzimidazole (APBI), PI-2 from 2-(4-aminophenyl)-5-aminobenzoxazole (APBO), PI-3 from 4,4′-diaminobenzanilide (DABA), and PI-4 from 2-chloro-4,4-diaminobenzanilide (Cl-DABA) exhibited good solubility in polar aprotic solvents, such as N-methyl-2-pyrrolidone (NMP) and N,N-dimethylacetamide (DMAc). Flexible and tough microfibrous mats were successfully prepared by a one-step electrospinning procedure from the PI/DMAc solution (solid content: 15–20 wt%; absolute viscosity: 8000–10000 mPa·s). The derived PI mats exhibited good whiteness according to the CIE Lab measurements with W (whiteness) values as high as 94.31, L (lightness) values higher than 94.00, b* (yellowness) values as low as 2.98 and yellow indices (YI) as low as 4.87. In addition, the prepared PI mats exhibited excellent thermal and dimensional stability with the glass transition temperatures (Tg) higher than 345 °C and linear coefficients of thermal expansion (CTE) as low as 27.8×10-6 /K.

A new methodology for enhancing the solvent resistance of electrospun polyimide (PI) ultrafine fibrous mat (UFM) was investigated in the current work. For this purpose, a negative intrinsically photosensitive polyimide (PSPI) resin was prepared by the one-step high- temperature polycondensation procedure from 3,3&rsquo, 4,4&rsquo, benzophenonetetracarboxylic dianhydride (BTDA) and &alpha, &alpha, bis(4-amino-3,5-dimethylphenyl)phenylmethane (PTMDA). The PI varnish, by dissolving the derived PI (BTDA-PTMDA) resin in N,N-dimethylacetamide (DMAc) at a solid of 20 wt %, was used as the starting material for the standard electrospinning (ES) and ultraviolet-assisted ES (UVAES) fabrications, respectively. The 365 nm wavelength of the high-pressure mercury lamp ultraviolet (UV) irradiation induced the photocrosslinking reaction in the PSPI mat. Solubility tests indicated that the PI UFM fabricated by standard ES procedure showed poor DMAc resistance, while the one by UVAES (PI-UV) exhibited excellent resistance to DMAc.

A type of photonic crystal fiber based on Kagome lattice cladding and slot air holes in a rectangular core is investigated. Full vector finite element method is used to evaluate the modal and propagation properties of the designed fiber. High birefringence of 0.089 and low effective material loss of 0.055 cm−1 are obtained at 1 THz. The y-polarized fundamental mode of the designed fiber shows a flattened and near-zero dispersion of 0 ± 0.45 ps · THz−1 · cm−1 within a broad frequency range (0.5 THz–1.5 THz). Our results provide the theory basis for applications of the designed fiber in terahertz polarization maintaining systems.