Thermo-Processable Polyimides with High Thermo-Oxidative Stability as Derived from Oxydiphthalic Anhydride and Bisphenol a Type Dianhydride

Colorless Polyimides Derived from 5,5'-bis(2,3-norbornanedicarboxylic anhydride): Strategies to Reduce the Linear Coefficients of Thermal Expansion and Improve the Film Toughness

In this paper, novel colorless polyimides (PIs) derived from 5,5'-bis(2,3-norbornanedicarboxylic anhydride) (BNBDA) were presented. The results of single-crystal X-ray structural analysis using a BNBDA-based model compound suggested that it had a unique steric structure with high structural linearity. Therefore, BNBDA is expected to afford new colorless PI films with an extremely high glass transition temperature (Tg) and a low linear coefficient of thermal expansion (CTE) when combined with aromatic diamines with rigid and linear structures (typically, 2,2'-bis(trifluoromethyl)benzidine (TFMB)). However, the polyaddition of BNBDA and TFMB did not form a PI precursor with a sufficiently high molecular weight; consequently, the formation of a flexible, free-standing PI film via the two-step process was inhibited because of its brittleness. One-pot polycondensation was also unsuccessful in this system because of precipitation during the reaction, probably owing to the poor solubility of the initially yielded BNBDA/TFMB imide oligomers. The combinations of (1) the structural modification of the BNBDA/TFMB system, (2) the application of a modified one-pot process, in which the conditions of the temperature-rising profile, solvents, azeotropic agent, catalysts, and reactor were refined, and (3) the optimization of the film preparation conditions overcame the trade-off between low CTE and high film toughness and afforded unprecedented PI films with well-balanced properties, simultaneously achieving excellent optical transparency, extremely high Tg, sufficiently high thermal stability, low CTE, high toughness, relatively low water uptake, and excellent solution processability.

Preparation and Properties of Modified Phenylethynyl Terminated Polyimide with Neodymium Oxide

Modified phenylethynyl terminated polyimides (PIs) were successfully prepared by using neodymium oxide (Nd₂O₃) via high-speed stirring and ultrasonic dispersion methods. In addition, the structure and properties of the Nd₂O₃-modified imide oligomers as well as the thermo-oxidative stability of the modified polyimides (PI/Nd₂O₃ hybrid) and its modification mechanism were investigated in detail. The thermogravimetric analysis (TGA) results indicated that the 5% decomposition temperature (Td5%) of the PI/Nd₂O₃ hybrids improved from 557 °C to 575 °C, which was also verified by the TGA-IR tests. Meanwhile, the weight loss rate of the PI/Nd₂O₃ hybrids significantly decreased by 28% to 31% compared to that of pure PI under isothermal aging at 350 °C for 450 h when the added content of Nd₂O₃ was between 0.4 wt% and 1 wt%, showing outstanding thermo-oxidative stability. Moreover, the mechanism of the enhanced thermo-oxidative stability for the modified PIs was analyzed by scanning electron microscopy (SEM) and X-ray diffraction (XRD).

Solution-Processable Colorless Polyimides Derived from Hydrogenated Pyromellitic Dianhydride: Strategies to Reduce the Coefficients of Thermal Expansion by Maximizing the Spontaneous Chain Orientation Behavior during Solution Casting

In this study, practically useful colorless polyimides (PIs) with low coefficients of thermal expansion (CTEs) and other desirable properties were prepared from hydrogenated pyromellitic dianhydride (1-exo,2-exo,4-exo,5-exo-cyclohexanetetracarboxylic dianhydride, H-PMDA). A modified one-pot polymerization method afforded a high-molecular-weight PI with sufficient film-forming ability from 2,2′-bis(trifluoromethyl)benzidine (TFMB) with a rod-like structure and H-PMDA. However, the PI film cast from its homogeneous solution did not have low CTEs, similar to the analogous system using meta-tolidine. To solve this problem, a series of amide- and amide-imide-containing diamines were designed and synthesized. The modified one-pot polymerization of H-PMDA and the diamines in γ-butyrolactone produced homogeneous, viscous, and stable solutions of high-molecular-weight PIs with high solid contents. The cast films of certain systems examined in this study simultaneously achieved low CTEs, high optical transparency, considerably high glass transition temperatures (T_gs), and sufficient ductility. A possible mechanism for the generation of low CTEs, which is closely related to the spontaneous in-plane orientation behavior during solution casting, was proposed. Certain H-PMDA-based PIs developed in this study are promising colorless heat-resistant plastic substrates for use in image display devices and other optical applications.

High-performance polyimides based on pyridine and xanthene pendant groups; synthesis, characterization, photoactivity, thermal, antibacterial, and Cr(VI) ion adsorption properties

A group of new polyimides (PIs) containing xanthene bulky groups and heterocyclic pyridine ring were synthesized by polycondensation reaction by using new symmetric and asymmetric diamines with available dianhydrides. The prepared diamines and PIs were characterized by Fourier transform infrared, elemental analysis, and proton nuclear magnetic resonance spectroscopy and carbon nuclear magnetic resonance spectroscopy. The PIs have desirable solubility in common aprotic solvents like dimethyl sulfoxide, N-methyl-2-pyrrolidone, dimethylacetamide, and pyridine as a less polar solvent. The PIs with inherent viscosities in the range of 0.51–0.72 dL/g were amorphous in nature, showed glass transition temperatures ( Tgs) of about 270–300°C, and 10% weight loss at temperatures 384–443°C. They showed fluorescence emission (quantum yield 7–12%) due to the presence of xanthene and pyridine groups in their structure. The prepared PIs showed tensile strength of 81–91 MPa with elongation at break of 19–23%. The antibacterial properties of the PIs against Gram negative and Gram positive bacteria and funguses were investigated. Also, a membrane of PI prepared by electrospinning method was used for the removal of different concentrations of Cr(VI) from aqueous solutions.

Synthesis and characterization of novel fluorinated pyridine-based polyimides derived from 4-(4-trifluoromethylphenyl)-2,6-bis(4-aminophenyl)pyridine and dianhydrides

A series of new fluorinated pyridine-based polyimides (PIs) was synthesized by polycondensation of 4-(4-trifluoromethylphenyl)-2,6-bis(4-aminophenyl)pyridine with various aromatic dianhydrides in N, N-dimethylacetamide (DMAc) via the conventional two-step method. The inherent viscosities of the resulting poly(amic acid)s and PIs were 0.54–0.94 and 0.58–0.88 dLg−1, respectively. The obtained PIs were predominantly amorphous and soluble in polar solvents such as N-methyl-2-pyrrolidone, N, N-dimethylformamide, and DMAc at room temperature or upon heating at 70°C. They also displayed good thermal stability with the glass transition temperature of 324–416°C, the temperature at 5% weight loss of 546–584°C, and the residue of 58–69% at 750°C under nitrogen, as well as outstanding mechanical properties with tensile strengths of 69132 MPa, tensile moduli of 1.1–2.0 GPa, and elongations at break of 7–27%. Meanwhile, these PI films had dielectric constants of 2.95–3.22 (1 MHz), low water uptake 0.44–0.71%, and high optical transparency with the ultraviolet cutoff wavelength in the 388–406 nm range and the wavelength of 80% transparency in the range of 550–680 nm.

Synthesis and properties of polyimides containing tetraphenylnaphthalene units

Aromatic polyimides (PI1x) with a tetraphenylnaphthalene unit between ether linkages were synthesized from 1,4-bis[4-(aminophenoxy)phenyl]-2,3-diphenylnaphthalene (1) and various tetracarboxylic dianhydrides by a conventional two-step procedure that included ring-opening polymerization in N-methyl-2-pyrrolidone and subsequent thermal cyclic dehydration. PI1x were characterized by X-ray diffraction, differential scanning calorimetry, thermogravimetry, and dynamic mechanical analysis. PI1x had glass transition temperatures in the range of 270–315°C and all PI1x were amorphous. The structure–property relationships of these PIs were examined and compared with those of polyimides (PI2x) from 4,4′-bis(4-aminophenoxy)biphenyl (2) and polyimides (PI3x) from 1,4-bis(4-aminophenyl)-2,3-diphenylnaphthalene (3). Water absorption and dielectric constants ( ε) of the PIs were also compared and discussed on the basis of imide content per repeating unit.

Thermo-Processable Polyimides with HighTg and High Thermo-Oxidative Stability as Derived from 2,3,3′,4′-biphenyltetracarboxylic dianhydride

Aromatic copolyimides were prepared using 2,3,3′,4′-biphenyltetracarboxylic dianhydride (asymmetric BPDA or a-BPDA) and several aromatic diamines with flexible ether linkages to obtain melt-processable polyimides (PIs) with balanced properties, i.e. high Tg, high thermo-oxidative stability, low melt viscosity, and sufficient toughness. The use of a-BPDA for the present purpose is based on our previous results which showed that a-BPDA-based PIs always show both higher Tg and much higher thermoplasticity than the corresponding symmetric BPDA-derived PIs. On the basis of the structure–property relationship, a copolyimide system was focused upon as a promising candidate. The molecular weight of the selected copolyimide was controlled using an end-capping reagent to balance between Tg, the melt viscosity, and toughness. The effect of the polymerization route (one-pot and two-step) on the properties was also discussed. An a-BPDA-based copolyimide accomplished a high Tg at 249 °C, a low melt viscosity of 8200 Poise at 400 °C, and high thermo-oxidative stability concurrently with considerably high film toughness (elongation at break = 66%).

Isomeric Biphenyl Polyimides. (I) Chemical Structure-property Relationships

A series of polyimides (PIs) were prepared from three biphenyltetracarboxylic dianhydrides (BPDA) isomers, i.e., 3,3′,4,4′-BPDA (s-BPDA), 2,3,3′,4′-BPDA (a-BPDA), and 2,2′',3,3′'-BPDA (iBPDA) with various diamines through the conventional two step process. Polymerization reactivity, solubility, and dynamic mechanical properties (T g, softening behavior, and storage modulus at glassy state) were compared between PIs derived from these BPDA isomers and para/meta diamine isomers. The solubility in NMP increased as follows: i-BPDA-PI > a-BPDA-PI > s-BPDA-PI. The order is attributed to semi-crystalline and fully amorphous morphologies of s-BPDA-PI and other isomeric BPDA-PIs, respectively. The use of meta-diamines lowered the T g whereas the use of bent/distorted BPDA isomers enhanced the T g (s-BPDA-PI < a-BPDA-PI < i-BPDA-PI). The BPDA isomer effect on T g is based on the difficulty of obtaining internal rotation around the biphenyl linkages. The width of the DMA rubbery plateau region decreased in the order of i-BPDA < a-BPDA-PI < s-BPDA-PI, depending on the crystallinity and the extent of chain entanglement. In a-BPDA and i-BPDA-PI systems, PIs from metadiamines led to higher modulus than the corresponding PIs from para-diamines.

Preparation, characterization, and properties of poly(thioether imide)s from isomeric bis(chlorophthalimide)s and bisthiophenols

A series of isomeric poly(thioether imide)s (PTIs) containing thioether linkages were prepared by aromatic nucleophilic substitution reaction of isomeric bis(chlorophthalimide)s (BCPIs) and bisthiophenols. The glass transition temperatures ( Tgs) of the isomeric PTIs were 190–264°C, the 5% weight loss temperature ( T5%) reached up to 441–508°C under nitrogen and 472–520°C in air atmospheres, respectively. It was found that the Tg values of the PTIs from three isomeric BCPIs with the same bisthiophenol increased in the order of 4,4′-BCPI < 3,4′-BCPI < 3,3′-BCPI, while the T5% values gradually decreased in the order of 4,4′-BCPI > 3,4′-BCPI > 3,3′-BCPI. Flexible films that could be cast from the polymer solutions exhibited good mechanical properties with tensile strengths of 91–121 MPa, elongations at break of 8–12%, and tensile moduli of 2.2–2.6 GPa. The minimum melt viscosity of isomeric PTIs decreased with increasing the content of asymmetric 3,4′-substituted phthalimide unit, and the PTI (2c) showed the lowest melt viscosity about 760 Pa·s at 264°C among these isomeric polymers.

Synthesis and properties of polyimides having a hexaphenylbenzene unit

Aromatic polyimides (PIs) with a hexaphenylbenzene unit were synthesized from 1,4-bis[4-(4-aminophenoxy)phenyl]-2,3,5,6-tetraphenylbenzene (1) and various tetracarboxylic dianhydrides by a conventional two-step procedure that included ring-opening polymerization in N-methyl-2-pyrrolidone and subsequent thermal cyclic dehydration. The PIs were characterized by X-ray diffraction, differential scanning calorimetry, thermogravimetry, and dynamic mechanical analysis. The PIs had glass transition temperatures in the range of 289–352°C, and all the polymers were amorphous. The structure–property relationships of these PIs were examined and compared with those of the previously prepared analogous PIs from 4,4′-bis(4-aminophenoxy)biphenyl (2) and 4,4′-bis(4-amino-2-phenylphenoxy)biphenyl (3). Water absorption and dielectric constants of the PIs were also compared and discussed on the basis of imide content per repeating unit.

Preparation, characterization, and properties of poly(thioether ether imide)s from isomeric bis(chlorophthalimide)s and bis(4-mercaptophenyl) ether

A series of isomeric poly(thioether ether imide)s (PTEIs) containing both thioether and ether linkages were prepared by aromatic nucleophilic substitution reaction of isomeric bis(chlorophthalimide)s (BCPIs) with bis(4-mercaptophenyl) ether (BMPE). The inherent viscosities of synthesized polymers were found in the range of 0.41–0.86 dL g−1 in N-methyl-2-pyrrolidone at 30°C. The glass transition temperature ( Tg) of the isomeric PTEIs were 210–242°C and increased by increasing the content of 3-substituted phthalimide unit in the polymer backbone. The 5% weight loss temperature values reached up to 525–539°C under nitrogen and 523–534°C in air atmospheres, respectively, which indicated this kind of polyimide possessed excellent thermal stability. Flexible films could be cast from the polymer solution. The PTEI films exhibited moderate mechanical properties with tensile strengths of 106–127 MPa, elongations at break of 8.6–11.5%, and tensile moduli of 2.2–2.8 GPa, respectively. Dynamic mechanical thermal analysis results illustrated that the storage moduli ( E′) of PTEI (a–e) almost completely maintained at about 2.3 GPa before reaching the corresponding Tg. It is noted that the minimum melt viscosity of isomeric PTEIs (a′–e′) decreased by increasing the content of unsymmetrical 3,4′-substituted phthalimide unit in the polymer main chain.

Comparative studies on melt processable polyimides derived from 2,3,3′,4′-oxydiphthalic anhydride and 2,3,3′,4′-thioetherdiphthalic anhydride

Two thermoplastic polyimides were controllably synthesized with similar molecular weight from 2,3,3′,4′-oxydiphthalic anhydride (3,4′-ODPA) or 2,3,3′,4′-thioetherdiphthalic anhydride (3,4′-TDPA) with 4,4′-diaminodiphenyl ether (ODA) by a one-step method of m-cresol. The solubility and mechanical properties of these polymers were investigated for comparison. The thermal and melt processable properties of these polymers were examined carefully by differential scanning calorimetry, thermogravimetric analysis (TGA), rheometer and melt index, and so on. It was found that the glass transition temperature ( Tg) of 3,4′-TDPA/ODA was about 15°C lower than that of 3,4′-ODPA/ODA, the melt indices of 3,4′-TDPA/ODA at different temperatures were higher than those of 3,4′-ODPA/ODA, while the minimum melt viscosity of 3,4′-TDPA/ODA was lower than those of 3,4′-ODPA/ODA. Furthermore, TGA results showed that 3,4′-TDPA/ODA had different degradation mechanism in air compared with 3,4′-ODPA/ODA. The long-term thermooxidative stability results showed that 3,4′-TDPA/ODA had better thermal oxidative stability than 3,4′-ODPA/ODA at 410°C for 6 h in air, which could be associated with the results of apparent activation energy ( Ea) of polymers calculated by TGA dynamics. The natural bond orbital charge of ethero atoms (S or O), and energy gap of the corresponding model compounds were also obtained by computational simulation to correlate with polymer properties.

Synthesis and properties of high performance polyimides derived from a novel diamine containing a N-octylcarbazole unit and two 3,5-diarylimidazole groups

A novel heterocyclic diamine based on carbazole and imidazole rings, 5,5′-(9-octyl-carbazole-3,6-diyl)bis[4-phenyl-2-(4-amino-phenyl)-1H-imidazole], was synthesized from readily available compounds through a multi-step procedure in high yield. The obtained diamine was fully characterized and its polycondensation reaction with different aromatic dianhydrides via the conventional two-step method, ring-opening polycondensation and chemical imidization, led to preparation of novel photoactive and heat resistant polyimides with improved solubility. The effects of the heteroaromatic rings into the main chain of the prepared polyimides were evaluated through the study of their solubility, thermal and optical properties. The inherent viscosities of the resulting polyimides were in the range of 0.39–0.51 dL g−1, and all of  them were soluble in common polar aprotic solvents. Meanwhile, they had good thermal stability with glass transition temperature ( Tg) values of 312–330 °C, the temperature at 10% weight loss of 475–495 °C, and the residue at 700 °C of 61–65% in nitrogen. These polymers exhibited strong UV-vis absorption, and their photoluminescence (PL) spectra showed maxima at 413–426 nm in solution and at 429–438 nm in the solid state. Wide-angle X-ray diffraction measurements revealed that these polyimides were predominantly amorphous.