Recent Advances in Fluorescent Polyimides

Polyimide (PI) refers to a type of high-performance polymer containing imide rings in the main chain, which has been widely used in fields of aerospace, microelectronic and photonic devices, gas separation technology, and so on. However, traditional aromatic PIs are, in general, the inefficient fluorescence or even no fluorescence, due to the strong inter- and intramolecular charge transfer (CT) interactions causing unavoidable fluorescence quenching, which greatly restricts their applications as light-emitting functional layers in the fabrication of organic light-emitting diode (OLED) devices. As such, the development of fluorescent PIs with high fluorescence quantum efficiency for their application fields in the OLED is an important research direction in the near future. In this review, we provide a comprehensive overview of fluorescent PIs as well as the methods to improve the fluorescence quantum efficiency of PIs. It is anticipated that this review will serve as a valuable reference and offer guidance for the design and development of fluorescent PIs with high fluorescence quantum efficiency, ultimately fostering further progress in OLED research.

Effects of organoclay on colorless and transparent polyimide nanocomposites: thermomechanical properties, morphology, and optical transparency

Although aromatic polyimide (PI) exhibits excellent mechanical performance and thermal stability, its dark color limits applicability in optical displays. Therefore, it is desirable to manufacture colorless, transparent PI (CPI) nanocomposite films that retain excellent physical properties. In this study, a solution intercalation method was used to disperse organoclay (Cloisite 25A; CS25A) in poly(amic acid), which was prepared using 4,4'-oxydiphthalic dianhydride and 3,4'-oxydianiline as monomers. This dispersion was then subjected to thermal imidization to synthesize CPI hybrid films. The influence of the CS25A content (0-1.00 wt%) on the thermomechanical properties, optical transmittance, and morphology of the prepared films was investigated. The hybrid film with a CS25A content of 0.50 wt% exhibited the best thermomechanical properties. However, upon further increasing the organoclay content to 1.00 wt%, the physical properties deteriorated. At 0.50 wt% CS25A, some agglomeration occurred but most of the clay was well dispersed as nano-sized particles, as revealed by transmission electron microscopy. In contrast, when the CS25A content exceeded a critical content, most of the clay was agglomerated and the physical properties were reduced. All the obtained CPI hybrid films were colorless and transparent, regardless of the organoclay content.

A Novel Diamine Containing Ester and Diphenylethane Groups for Colorless Polyimide with a Low Dielectric Constant and Low Water Absorption

In this study, a novel diamine monomer containing ester and phenyl moieties, 1,2-diphenylethane-1,2-diyl bis(4-aminobenzoate) (1,2-DPEDBA), was synthesized through a three-step reaction. Using this diamine, a novel polyimide (PI) film was prepared with 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (6-FDA) as a counter dianhydride through a typical two-step chemical imidization. For comparison, poly(pyromellitic dianhydride-co-4,4'-oxydianiline) (PMDA-ODA PI) was also synthesized via thermal imidization. The resulting 6-FDA-DPEDBA PI film was not only soluble in common polar solvents with high boiling points, such as N,N-dimethylacetamide (DMAc) and N,N-dimethylformamide (DMF), but also soluble in common low-boiling-point polar solvents, such as chloroform (CHCl₃) and dichloromethane (CH₂Cl₂), at room temperature. The resulting novel PI showed a 5% weight loss temperature (T⁵_d) at 360 °C under a nitrogen atmosphere. The resulting PI film was colorless and transparent with a transmittance of 87.1% in the visible light region ranging from 400 to 760 nm. The water absorption of the novel PI film was of 1.78%. The PI film also possessed a good moisture barrier and hydrophobicity. Furthermore, the resulting PI film displayed a low dielectric constant of 2.17 at 10⁶ Hz at room temperature. In conclusion, the novel PI film exhibited much better optical transparency, lower moisture absorption, and a lower dielectric constant as well as better solubility than the PMDA-ODA PI film, which is insoluble in any solvent, although its thermal stability is not better than that of PMDA-ODA PI.

Synthesis of a Novel Rigid Semi-Alicyclic Dianhydride and Its Copolymerized Transparent Polyimide Films' Properties

A new series of colorless polyimides (CPIs) with outstanding thermal properties and mechanical properties were fabricated by the copolymerization of a novel dianhydride and 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) with 2,2′-bistrifluoromethyl benzidine (TFDB). The novel dianhydride, 10-oxo-9-phenyl-9-(trifluoromethyl)-9,10-dihydroanthracene-2,3,6,7-tetraacid dianhydride (3FPODA), possessed a rigid semi-alicyclic structure, –CF₃ and phenyl side groups, and an active carbonyl group. Benefitting from the special structure of 3FPODA, the glass transition temperatures (T_g) of the new CPIs improved from 330 °C to 377 °C, the coefficient of thermal expansion (CTE) decreased from 46 ppm/K to 24 ppm/K, and the tensile strength (T_S), tensile modulus (T_M), and elongation at break (E_B) increased from 84 MPa to 136 MPa, 3.2 GPa to 4.4 GPa, and 2.94% to 4.13% with the increasing amount of 3FPODA, respectively. Moreover, the active carbonyl group of the 3FPODA could enhance the CPI’s adhesive properties. These results render the new dianhydride 3FPODA an ideal candidate monomer for the fabrication of high-performance CPIs.

Preparation and Properties of Intrinsically Black Polyimide Films with CIE Lab Color Parameters Close to Zero and High Thermal Stability for Potential Applications in Flexible Printed Circuit Boards

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 (T_5%) higher than 590 °C, and the glass transition temperatures (T_g) higher than 340 °C.

Preparation and Properties of Polyimide Composite Membrane with High Transmittance and Surface Hydrophobicity for Lightweight Optical System

Polyimide membranes have excellent physiochemical properties which make them valuable materials for optical area. However, common aromatic polyimide membrane trend to show low transmittance in visible region because of the charge-transfer complex (CTC) in molecular structures. Moreover, it’s trending to show high moisture uptakes because of the hydrophilic imide rings in molecular structure. In this work, a polyimide composite membrane with SiO₂ antireflective membrane on both sides was prepared. High transmittance (93% within 500~800 nm) and surface hydrophobicity was realized simultaneously. The polyimide composite membrane showed great optical homogeneity. The SiO₂ antireflective membranes on polyimide substrate were prepared through a simple and efficient sol-gel method. The surface roughness of polyimide membrane substrate on each side has been improved to 1.56 nm and 3.14 nm, respectively. Moreover, the excellent thermal stability and mechanical property of polyimide membrane has been preserved, which greatly improves the range of applications for the composite membrane. It is a good candidate for light weight optical system.

Recent advances in tailoring and improving the properties of polyimide aerogels and their application

With the rapid advancements in technology and growing aerospace applications, there is a need for effective low-weight and thermally insulating materials. Aerogels are known for their ultra-lightweight and they are highly porous materials with nanopores in a range of 2 to 50 nm with very low thermal conductivity values. However, due to hygroscopic nature and brittleness, aerogels are not used commercially and in daily life. To enhance the mechanical and hydrophobic properties, reinforcement materials such as styrene, cyanoacrylates, epoxy along with hydroxyl, amines, vinyl groups are added to the surface. The addition of organic materials resulted in lower service temperatures which reduce its potential applications. Polyimides (PI) are commonly used in engine applications due to their suitable stability at high temperatures along with excellent mechanical properties. Previous research on polyimide aerogels reported high flexibility or even foldability. However, those works' strategy was mainly limited to altering the backbone chemistry of polyimide aerogels by changing either the monomer's compositions or the chemical crosslinker. This work aims to summarize, categorize, and highlight the recent techniques for improving and tailoring properties of polyimide aerogels followed by the recent advancements in their applications.

Synthesis and characterization of an adamantane-based copolyimides with high transparency

In this work, a copolyimide (Co-PI) film with high transparency was prepared by the copolymerization of hexafluoroisopropylidene)diphthalic anhydride (6FDA), 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA), 2,2′-Bis(trifluoromethyl)benzidine (TFMB) and adamantane-1,3-diamine (DAA). The effects of DAA monomers on the optical, thermal, and mechanical properties of the co-PIs were discussed in detail. We found that the preparation of polyimide (PI) based on the combination of two dianhydrides and two diamines could obtain the co-PI film with excellent comprehensive performance due to the synergy between the -CF3 group, the aliphatic ring and the aromatic structure. Through the structure and composition optimization, the co-PI film with 1.30% DAA (Q3) has a Tg of 374oC, T5 higher than 530oC, T430 of 82% and the tensile strength higher than 145 MPa. These results indicate that the Co-PI films can be successfully utilized in the development of novel heat-resistant plastic substrates for the optoelectronic engineering applications.

A Universal Study on the Effect Thermal Imidization Has on the Physico-Chemical, Mechanical, Thermal and Electrical Properties of Polyimide for Integrated Electronics Applications

Polyimides (PI) are a class of dielectric polymer used in a wide range of electronics and electrical engineering applications from low-voltage microelectronics to high voltage isolation. They are well appreciated because of their excellent thermal, electrical, and mechanical properties, each of which need to be optimized uniquely depending on the end application. For example, for high-voltage applications, the final polymer breakdown field and dielectric properties must be optimized, both of which are dependent on the curing process and the final physico-chemical properties of PI. The majority of studies to date have focused on a limited set of properties of the polymer and have analyzed the effect of curing from a physicochemical-, mechanical- or electrical-centric viewpoint. This paper seeks to overcome this, unifying all of these characterizations in the same study to accurately describe the universal effect of the cure temperature on the properties of PI and at an industrial processing scale. This paper reports the widest-ranging study of its kind on the effect that cure temperature has on the physico-chemical, mechanical, thermal and electrical properties of polyimide, specifically poly (pyromellitic dianhydride-co-4, 4′-oxydianiline) (PMDA/ODA). The optimization of the cure temperature is accurately studied not only regarding the degree of imidization (DOI), but also considering the entire physical properties. Particularly, the analysis elucidates the key role of the charge–transfer complex (CTC) on these properties. The results show that while the thermal and mechanical properties improve with both DOI and CTC formation, the electrical properties, particularly at high field conditions, show an antagonistic behavior enhancing with increasing DOI while degrading at higher temperatures as the CTC formation increases. The electrical characterization at low field presents an enhancement of the final PI properties likely due to the DOI. On the contrary, at high electric field, the conductivity results show an improvement at an intermediate temperature emphasizing an ideal compromise between a high DOI and PI chain packing when the thermal imidization process is performed over this equilibrium. This balance enables maximum performance to be obtained for the PI film with optimized electrical properties and, overall, optimal thermal and mechanical properties are achieved.

Preparation and characterization of soluble heat-resistant polyimide films containing bis-N-phenyl-benzimidazole

To prepare soluble polyimides with high temperature resistance, two new diamine monomers, namely, 2,2′-(4,4′-oxybisphenylene)-bis(1-phenyl-5-aminobenzimidazole) (5a), and 2,2′-(4,4′-hexafluoroisopropylidene)-bis(1-phenyl-5- aminobenzimidazole) (5b), were synthesized and exploited to prepare three series of poly(benzimidazole imides)s (PBIIs) by a conventional two-stage synthesis. The resulting PI films were flexible and tough, possessing high glass-transition temperatures (Tgs = 311°C–390°C), improved optical transparency, and excellent solubility. Moreover, the effect of different configuration on performance was revealed, and these data provided a feasible method to enhance both Tg and solubility of PIs by incorporating N-phenyl benzimidazole and corresponding functional moieties.

Copolyamide-Imide Membrane with Low CTE and CME for Potential Space Optical Applications

Polyimide diffractive membrane lens can be used in space optical telescope to reduce the size and mass of an imaging system. However, traditional commercial aromatic polyimide membrane is hard to meet the challenging requirements of dimensional stability and optical homogeneity for optical use. Based on molecular structure design and the optimization of fabrication process, the prepared copolyamide-imide membrane achieved the desired performance of membrane as an optical material. It showed a very low coefficient of thermal expansion (CTE), which is 0.95 ppm/°C over a temperature range of −150–100 °C and relatively low coefficient of moisture expansion (CME), which is only 13.30 ppm/% RH (0~90% RH). For the optical use, the prepared copolyamide-imide membrane (φ200 mm) achieved good thickness uniformity with wave-front error smaller than λ/30 (λ = 632 nm) in RMS (root mean square). Besides, it simultaneously meets the optical, thermal, and mechanical requirements for space telescope use. Copolyamide-imide membranes in this research with good comprehensive performance can be used as large aperture membrane optical system architectures.

Syntheses of Colorless and Transparent Polyimide Membranes for Microfiltration

Herein, poly(amic acid) (PAA) was synthesized using 4,4’-(hexafluoroisopropylidene) diphthalic anhydride (6FDA) as a dianhydride and 2,2-bis(3-aminophenyl)hexafluoropropane (6FAm) and 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane (6FAm-OH) as diamines. Poly(vinyl alcohol) (PVA) at various contents (0–5.0 wt%) was blended with PAA to prepare a composite material. Then, colorless and transparent polyimide (CPI) composite films were prepared by applying various stages of heat treatment using the PAA/PVA blend film as a precursor. These film-type composites were immersed in water to completely dissolve PVA, a water-soluble polymer, and their pore sizes were investigated to determine their potential as a porous membrane. According to the results of scanning electronic microscopy (SEM), as the concentration of PVA increased from 0 to 5.0 wt% in the CPI/PVA composite films, the size of the pores resulting from the dissolution of water-soluble PVA increased. Further, the micrometer-sized pores were uniformly dispersed in the CPI films. The thermal properties, morphology, and optical transparency of the two types of CPI membranes synthesized using 6FAm and 6FAm-OH monomers were examined and compared.

Enhancement of Flame Retardancy of Colorless and Transparent Semi-Alicyclic Polyimide Film from Hydrogenated-BPDA and 4,4'-oxydianiline via the Incorporation of Phosphazene Oligomer

Enhancement of flame retardancy of a colorless and transparent semi-alicyclic polyimide (PI) film was carried out by the incorporation of phosphazene (PPZ) flame retardant (FR). For this purpose, PI-1 matrix was first synthesized from hydrogenated 3,3′,4,4′-biphenyltetracarboxylic dianhydride (HBPDA) and 4,4′-oxydianiline (ODA). The soluble PI-1 resin was dissolved in N,N-dimethylacetamide (DMAc) to afford the PI-1 solution, which was then physically blended with PPZ FR with the loading amounts in the range of 0–25 wt.%. The PPZ FR exhibited good miscibility with the PI-1 matrix when its proportion was lower than 10 wt.% in the composite films. PI-3 composite film with the PPZ loading of 10 wt.% showed an optical transmittance of 75% at the wavelength of 450 nm with a thickness of 50 μm. More importantly, PI-3 exhibited a flame retardancy class of UL 94 VTM-0 and reduced total heat release (THR), heat release rate (HRR), smoke production rate (SPR), and rate of smoke release (RSR) values during combustion compared with the original PI-1 film. In addition, PI-3 film had a limiting oxygen index (LOI) of 30.9%, which is much higher than that of PI-1 matrix (LOI: 20.1%). Finally, incorporation of PPZ FR decreased the thermal stability of the PI films. The 10% weight loss temperature (T_10%) and the glass transition temperature (T_g) of the PI-3 film were 411.6 °C and 227.4 °C, respectively, which were lower than those of the PI-1 matrix (T_10%: 487.3 °C; T_g: 260.6 °C)

Biomass materials derived from anethole: conversion and application

Renewable biomass has attracted much attention because of its advantages over fossil fuels. Of these biomasses, anethole has been developed as a reliable monomer or precursor for diverse materials with potential applications.

Soluble copolyimides containing 4,4′-isopropylidenedicyclohexanol (HBPA) isomer units: Synthesis, characterization, thermal, mechanical, and optical properties

In this work, 4,4′-isopropylidenedicyclohexanol (HBPA)-based dinitro isomers mixture (H″BPBN and H′BPBN) was synthesized and separated, and the structures of dinitro isomers were confirmed by differential scanning calorimetry and proton nuclear magnetic resonance spectroscopy. A series of copolyimides were prepared from diamino monomers with different percentages of novel diamine H″BPDA and 4,4′-(hexafluoroisopropylidene) diphthalic anhydride via a conventional two-step procedure. All the copolyimides could afford flexible, tough, and transparent films with transmittance no less than 73% at 450 nm, which was attributed to the fact that nonplanar alicyclic diamine and the bulky and weakly polarizable trifluoromethyl hampered the formation of charge-transfer complex. Moreover, all the copolyimides were soluble not only in polar solvents such as N,N-dimethylformamide but also in low-boiling-point solvents such as dichloromethane, which was related to the fact that the existence of alicyclic diamine and bulky trifluoromethyl decreased the intermolecular force. In addition, the conformation effects of H″BPDA and H′BPDA on the aspects of thermal, mechanical, optical, and soluble performance of copolyimides were investigated and their structure–property relationships were discussed in detail.

Colorless Semi-Alicyclic Copolyimides with High Thermal Stability and Solubility

A series of colorless copolyimide films with high thermal stability and good solubility are synthesized from (trifluoromethyl)biphenyl-4,4’-diamine (TFMB) with different 1,2,4,5-cyclohexanetetracarboxylic dianhydride (HPMDA) to 2,2-bis(3,4-dicarboxyphenyl)-hexafluoropropane (6FDA) dianhydride mole ratios through one-pot solution polycondensation. These copolyimide films exhibit excellent optical transparency (T₄₀₀ > 90% and λ₀ ~305–333 nm) with a thickness of 15 μm and good solubility in most organic solvents. The excellent optical properties are mainly attributed to the low inter- and intra-molecular charge transfer interactions due to the alicyclic structure and the strong electronegative CF₃ groups. The glass transition temperature increases from 332 to 352 °C with increasing HPMDA content in the copolymers, while the thermal decomposition temperature is improved with increasing 6FDA content. These results indicate that the copolyimide films can be successfully utilized in the development of novel heat-resistant plastic substrates for the optoelectronic engineering applications.

Soluble Polyimides Bearing (cis, trans)-Hydrogenated Bisphenol A and (trans, trans)-Hydrogenated Bisphenol A Moieties: Synthesis, Properties and the Conformational Effect

In this work, hydrogenated bisphenol A (HBPA) based dinitro mixed isomers (1a′ and 1a) were synthesized and separated via vacuum distillation under the monitor of DSC and ¹H NMR. Corresponding diamines (2a′ and 2a) were separately polycondensed with five commercial dianhydrides via a two-step thermal imidization to obtain PI-(1′-5′) and PI-(1-5). All the polyimides could afford flexible, tough, and transparent films, and most of them were readily soluble not only in common polar solvents like DMAc, but also in low boiling point solvents such as chloroform. ¹H NMR spectra of the polyimides demonstrated that HBPA moiety showed no conformation changes during the preparation of polymers. For a given dianhydride, PI-(1-5) exhibited better thermal stability than that of PI-(1′-5′), this can be attributed that the equatorial, equatorial C–O in PI-(1-5) promoted denser and more regular molecular chain stacking, as can be evidenced by the WAXD and geometric optimization results. Additionally, when the dianhydride was ODPA, BPADA or 6FDA, no apparent difference was found in either the transmittance or solubility between two series of polyimides, which could be attributed that twisted and flexible ether linkages, as well as bulky substituents, led to the “already weakened” inter- and intramolecular CT interaction and cohesive force. However, when it came to rigid and stiff dianhydride, e.g., BPDA, PI-3′ took an obvious advantage over PI-3 in transmittance and solubility, which was possibly owed to the larger molecular chain d-spacing imparted by equatorial, axial C–O. An overall investigation of PI-(1′-5′) and PI-(1-5) on aspects of thermal, mechanical, morphological, soluble and optical performance values was carried out, and the conformation effects of HBPA isomers on the properties of two series of polyimides were discussed in detail.