Enhancement of Solvent Resistance of Polyimide Electrospun Mat via the UV-Assisted Electrospinning and Photosensitive Varnish

Electrospun Nanofibers Based on Polymer Blends with Tunable High-Performance Properties for Innovative Fire-Resistant Materials

The main concern of materials designed for firefighting protective clothing applications is heat protection, which can be experienced from any uncomfortably hot objects or inner spaces, as well as direct contact with flame. While textile fibers are one of the most important components of clothing, there is a constant need for the development of innovative fire-retardant textile fibers with improved thermal characteristics. Lately, inherently fire-resistant fibers have become very popular to provide better protection for firefighters. In the current study, the electrospinning technique was applied as a versatile method to produce micro-/nano-scaled non-woven fibrous membranes based on various ratios of a poly(ether-ether-ketone) (PEEK) and a phosphorus-containing polyimide. Rheological measurements have been performed on solutions of certain ratios of these components in order to optimize the electrospinning process. FTIR spectroscopy and scanning electron microscopy were used to investigate the chemical structure and morphology of electrospun nanofiber membranes, while thermogravimetric analysis, heat transfer measurements and differential scanning calorimetry were used to determine their thermal properties. The water vapor sorption behavior and mechanical properties of the optimized electrospun nanofiber membranes were also evaluated.

Methylethynyl-Terminated Polyimide Nanofibrous Membranes: High-Temperature-Resistant Adhesives with Low-Temperature Processability

As an alternative to traditional riveting and welding materials, high-temperature-resistant adhesives, with their unique advantages, have been widely used in aviation, aerospace, and other fields. Among them, polyimide (PI) adhesives have been one of the most studied species both from basic and practical application aspects. However, in the main applications of solvent-type PI adhesives, pinholes or bubbles often exist in the cured PI adhesive layers due to the solvent volatilization and dehydration reaction, which directly affect the adhesive performance. To address this issue, electrospun PI nanofibrous membranes (NFMs) were employed as solvent-free or solvent-less adhesives for stainless steel in the current work. To enhance the adhesion of PI adhesives to the metal substrates, phenolphthalein groups and flexible ether bonds were introduced into the main chain of PIs via the monomers of 4,4′-oxydiphthalic anhydride (ODPA) and 3,3-bis[4-(4-aminophenoxy)phenyl] phthalide (BAPPT). At the same time, the methylethynyl group was used as the end-capping component, and the crosslinking reaction of the alkynyl group at high temperature further increased the adhesive strength of the PI adhesives. Three kinds of methylethynyl-terminated PI (METI) NFMs with the set molecular weights of 5000, 10,000, and 20,000 g/mol were first prepared via the one-step high-temperature polycondensation procedure. Then, the PI NFMs were fabricated via the standard electrospinning procedure from the soluble METI solutions. The afforded METI NFMs showed excellent melt-flowing behaviors at high temperature. Incorporation of the methylethynyl end-capping achieved a crosslinking reaction at 280−310 °C for the NFMs, which was about 70 °C lower than those of the phenylacetylene end-capping counterparts. Using the METI NFMs as adhesive, stainless steel adherends were successfully bonded, and the single-lap shear strength (LSS) was higher than 20.0 MPa at both room temperature (25 °C) and high temperature (200 °C).

Formation of Nano-Fibrous Patterns on Aluminum Substrates via Photolithographic Fabrication of Electrospun Photosensitive Polyimide Fibrous Membranes

The formation of polymeric micro-patterns on various substrates via a photolithography procedure has been widely used in semiconductor fabrication. Standard polymer patterns are usually fabricated via photosensitive polymer varnishes, in which large amounts of potentially harmful solvents with weight ratios over 50 wt% have to be removed. In the current work, a novel pattern-formation methodology via solvent-free electrospun photosensitive polymeric fibrous membranes (NFMs) instead of the conventional photosensitive solutions as the starting photoresists was proposed and practiced. For this purpose, a series of preimidized negative auto-photosensitive polyimide (PSPI) resins were first prepared via the two-step chemical imidization procedure from the copolymerization reactions of 3,3',4,4'-benzophenonetetracarboxylic- dianhydride (BTDA) and two ortho-methyl-substituted aromatic diamines, including 3,3',5,5'-tetramethyl-4,4'-diaminodiphenylmethane (TMMDA) and 3,7-diamino-2,8-dimethyl- dibenzothiophene sulfone (TSN). The derived homopolymer PI-1 (BTDA-TMMDA) and the copolymers, including SPI-2~SPI-6, with the molar ratio of 5~25% for TSN in the diamine units, showed good solubility in polar solvents. Then, a series of PSPI NFMs were fabricated via standard electrospinning procedure with the developed PSPI solutions in N,N-dimethylacetamide (DMAc) with a solid content of 25 wt% as the starting materials. The derived PSPI NFMs showed good thermal stability with 5% weight loss temperatures higher than 500 °C in nitrogen. Meanwhile, the derived PSPIs showed good photosensitivity to the ultraviolet (UV) emitting wavelengths of i-line (365 nm), g-line (405 nm) and h-line (436 nm) of the high-pressure mercury lamps in both forms of transparent films and opaque NFMs. Fine micro-patterns with a line width of around 100 μm were directly obtained from the representative SPI-4 NFM via standard photolithography procedure.

Electrospun Semi-Alicyclic Polyimide Nanofibrous Membrane: High-Reflectance and High-Whiteness with Superior Thermal and Ultraviolet Radiation Stability for Potential Applications in High-Power UV-LEDs

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.

Characterization of the cylindrical electrospun nanofibrous polysulfone membrane for hemodialysis with modelling approach

Electrospun nanofibrous membrane (ENM) is a membrane fabricated using electrospinning technique which has considerable characteristics such as high porosity, nanometer pore size, and simple process. Although ENMs are being evaluated in various medical applications, the effectiveness for hemodialysis (HD) has not been evaluated carefully. Thus, in this study, the cylindrical electrospun nanofibrous polysulfone (CENP) membrane was fabricated and its performance in the dialysis adequacy in HD patients was evaluated.The CENP membrane was fabricated in a tabular shape. The physical characteristics of the membrane are examined using scanning electron microscope (SEM) images and the permporometry technique. Then, its efficiency in urea and creatinine removal from the blood serum of 21 HD patients was evaluated at a low blood flow rate (BFR) of 200 ml min^-1 and dialysate fluid rate (DFR) of 300 ml min^-1. Afterwards, the results were modeled and optimized using artificial neural network (ANN) and genetic algorithm (GA), respectively. Finally, sensitive analysis was performed via Spearman's rank correlation coefficient. The highest dialysis adequacy was observed in membranes with an inner diameter of 3 mm. The CENP membrane belongs to the super high-flux membrane and it could be replaced with existing commercial hollow fiber membranes.

Preparation and Properties of Electrospun Phenylethynyl-Terminated Polyimide Nano-Fibrous Membranes with Potential Applications as Solvent-Free and High-Temperature Resistant Adhesives for Harsh Environments

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).

Applications of polyimide coatings: a review

Polyimides, high-performance polymers with superior properties such as high temperature stability, resistance to solvents and high strength, can be used in high-tech applications of the aerospace and aviation, medical or electronics industry in different forms (film, fiber, nanofiber, membrane, foam, adhesive or coating). Among these applications, coating has a special place and is used to develop advanced structures having high temperature resistance, flame retardancy and etc. for high tech industries via an economical and feasible way. Therefore, in this review, we aimed to report the broad application status of polyimide coatings by reviewing publications, patents and commercial products. Thus, this study can assist in selecting suitable polyimide types and production methods for polyimide coating applications and in understanding their applicability for future products.

Preparation and Characterization of Semi-alicyclic Polyimides Containing Trifluoromethyl Groups for Optoelectronic Application

Transparent polyimides (PI) films with outstanding overall performance are attractive for next generation optoelectronic and microelectronic applications. Semi-alicyclic PIs derived from alicyclic dianhydrides and aromatic diamines have proved effective to prepare transparent PIs with high transmittance. To optimize the combined properties of semi-alicyclic PIs, incorporating bulky trifluoromethyl groups into the backbones is regarded as a powerful tool. However, the lack of fundamental understanding of structure–property relationships of fluorinated semi-alicyclic PIs constrains the design and engineering of advanced films for such challenging applications. Herein, a series of semi-alicyclic PIs derived from alicyclic dianhydrides and trifluoromethyl-containing aromatic diamines was synthesized by solution polycondensation at high temperature. The effects of alicyclic structures and bulky trifluoromethyl groups on thermal, dielectric and optical properties of PIs were investigated systematically. These PI films had excellent solubility, low water absorption and good mechanical property. They showed high heat resistance with T_g in the range of 294–390 °C. It is noted that tensile strength and thermal stability were greatly affected by the rigid linkages and alicyclic moieties, respectively. These films exhibited obviously low refractive indices and significantly reduced dielectric constants from 2.61 to 2.76, together with low optical birefringence and dielectric anisotropy. Highly transparent films exhibited cutoff wavelength even as low as 298 nm and transmittance at 500 nm over 85%, displaying almost colorless appearance with yellowness index (b*) below 4.2. The remarkable optical improvement should be mainly ascribed to both weak electron-accepting alicyclic units and bulky electron-withdrawing trifluoromethyl or sulfone groups. The present work provides an effective strategy to design molecular structures of optically transparent PIs for a trade-off between solution-processability, low water uptake, good toughness, high heat resistance, low dielectric constant and excellent optical transparency.

Preparation and Characterization of Semi-Alicyclic Polyimide Resins and the Derived Alignment Layers for Liquid Crystal Display Technology

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',4,4'-biphenyltetracarboxylic dianhydride (HBPDA), and various aromatic diamines, including 4,4'-methylenedianiline (MDA) for SPI-1, 4,4'-aminodianiline (NDA) for SPI-2, 3,3',5,5'-tetramethyl-4,4'-diaminodiphenylmethane (TMMDA) for SPI-3, and 3,3'-diethyl-5,5'-dimethyl-4,4'-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°-1.97°. 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 °C), good rubbing resistance, good thermal stability with glass transition temperatures (T_gs) higher than 260 °C, and excellent optical transparency with the transmittance higher than 97% at the wavelength of 550 nm.