Performance improvement of polybenzoxazine by alloying with polyimide: effect of preparation method on the properties

Polymer alloys of polyimide and polybenzoxazine were prepared from the combination of a bifunctional benzoxazine monomer, 6,6′-(1-methylethylidene)bis(3,4-dihydro-3-2H-1,3-benzoxazine) (B-a) and a soluble polyimide (PI) or its precursor, poly(amide acid) (PAA), that was synthesized from oxydianiline (ODA) and bisphenol A di(phthalic anhydride) ether (BPADA). It was observed from DSC that the onset temperature for the curing exotherm of B-a in the blend with PAA was remarkably lowered compared with that of pristine B-a, while the curing temperature of B-a in the blend with PI was almost the same as that of pristine B-a. The cast films of PI/B-a or PAA/B-a were thermally treated at 150, 200 and 240 °C for 2 h each, affording deep wine, transparent films. The PI component in the polymer alloy films from PI/B-a dissolved thoroughly. On the other hand, the polymer alloy films from PAA/B-a were not soluble at all in NMP. The polymer alloy films from both PI/B-a and PAA/B-a showed only one glass transition temperature (T_g) from their viscoelastic analyses. The T_g values remarkably increased as the content of PI increased. The thermal stabilities of both films from PI/B-a and PAA/B-a increased as the PI component increased in the similar manner.     

Orange to black electrochromic behaviour in poly(2-(2-thienyl)-1H-pyrrole) thin films

We have studied an electrochromic precursor, 2-(2-thienyl)-1H-pyrrole (1), using two improved procedures of the Trofimov reaction. Optimised stereochemical calculations at the B3LYP/6-311G* level showed almost equal s-cis and s-trans conformational populations in 1 with marked out-of-plane deviations of ca. 30°. Model calculations suggest that the predominant rotational conformation in undoped poly(1) would be s-trans with the essential out-of-plane deviations around the all three interheterocyclic bonds of ca. 25-30°. Monomer 1 exhibited two irreversible oxidation processes at +0.86 and +1.3 V corresponding to the oxidation of the pyrrole and thiophene rings, respectively. Orange to black electrochromic behaviour was found in ClO₄⁻ doped poly(1) thin films with colouring and bleaching times of 1.8 and 1.3 s, respectively. The colouration efficiency during the bleaching process was 233 cm²/C. The optical contrast at 450 nm was 19% and in the near-IR was 36%. The band-gap of poly(1) (1.6-1.7 eV) was found to be significantly lower than that of polypyrrole (2.85 eV) and polythiophene (2.3 eV) as a consequence of increased electron delocalisation in the system. Important differences in the morphology of doped and dedoped poly(1) films were observed by atomic-force microscopy (AFM). Doped poly(1) films showed a granular morphology with primary particles of 45-60 nm in size and an average surface roughness of 3.5 nm. On the other hand, dedoped poly(1) films showed interconnected aggregates of 65-90 nm in size as a consequence of particle fusion, with a surface roughness of 9.2 nm. In summary, poly(1) is a promising material for emerging flexible electrochromic devices such as displays and variable optical attenuators.     

Highly optically transparent/low color polyimide films prepared from hydroquinone- or resorcinol-based bis(ether anhydride) and trifluoromethyl-containing bis(ether amine)s

Two series of novel polyimides (5a-g and 6a-g) containing flexible ether linkages and pendent trifluoromethyl (CF₃) groups were synthesized from 1,4-bis(3,4-dicarboxyphenoxy)benzene dianhydride (3a) and 1,3-bis(3,4-dicarboxyphenoxy)benzene dianhydride (3b) with various CF₃-substituted aromatic bis(ether amine)s (4a-g) via ring-opening polyaddition to poly(amic acid)s, followed by thermal or chemical imidization. These polyimides were readily soluble in a variety of organic solvents and could be solution-cast into flexible and tough films. The cast films exhibited high optical transparency and almost no color, with a UV-vis absorption edge of 368-382 nm and a very low b^∗ value (a yellowness index) of 6.2-15.5. They had good thermal stability with glass-transition temperatures of 186-288 °C, and most of them did not show significant decomposition before 500 °C. Moreover, these polyimide films also possessed low dielectric constants of 2.79-3.49 (at 1 MHz) and low water uptakes (<0.65 wt%).     

Properties of organosoluble aromatic polyimides from 3′-trifluoromethyl-3,4′-oxydianiline

A colorless fluorinated diamine, 3′-trifluoromethyl-3,4′-oxydianiline (3′-CF₃-3,4′-ODA) (II) was prepared through the nucleophilic substitution reaction of 3-nitrophenol and 2-chloro-5-nitrobenzotrifluoride by catalytic reduction with hydrazine and Pd/C. A series of Polyimides V were synthesized from the diamine II with various aromatic dianhydrides III_a-f via thermal and chemical imidization. These polyimides had inherent viscosities ranging from 0.88 to 1.12 dl/g. A comparison of V, VI to analogous polyimides VII, VIII. VI, VII and VIII was based on 3′4-ODA, 3-CF₃-4,4′-ODA, 4,4′-ODA, respectively. In terms of the color of PI revealed that the color intensity of phenoxy-containing amine of the meta-structure and the para-structure with the CF₃ group would fell off color intensity. The color intensity of the four polyimide series was lessened in the following order: V>VII>VI>VIII. The solubility of V is better than VI, VII and VII. The polyimide V films had a tensile strength ranging from 124 to 147 MPa, elongation at break from 9 to 65%, and initial modulus from 2.3 to 2.8 GPa. The glass transition temperature of polymers was recorded at 234-313 °C. They had 10% weight loss at a temperature above 515 °C and left more than 50% residue even at 800 °C in nitrogen. Compared with polyimides VI, V showed the lower dielectric constants of 2.80-3.50 (40 MHz), and moisture absorptions in the range of 0.44-1.02 wt%.     

Structure-property relationships for a series of polyimide copolymers with sulfonated pendant groups

A series of high molecular weight, sulfonated polyimide copolymers (8a-f) with controlled acid contents have been obtained using 2,2′-bis(4-sulfobenzyloxy)benzidine (14) prepared via a flexible synthetic route. This series of novel sulfonated polyimide membranes were found to possess higher hydrolytic stability than polyimides in which the sulfonic acid groups are bound directly to the polymer main chain. An in-depth analysis of conductivity data was also performed for 8 and compared to the results for Nafion^® (1), sulfonated poly(ether ether ketone) (2) and a main-chain sulfonated polyimide (3). In order to remove the influence of acid strength, the proton mobility value for 8 at infinite dilution was calculated and found to be 1.2(±0.6) × 10⁻³ cm² s⁻¹ V⁻¹. A catalyst-coated membrane (CCM)-MEA based on a polyimide incorporating 60% sulfonated monomer (8d) was found to exhibit comparable beginning-of-life fuel cell performance as a Nafion^®-based CCM MEA at 50 °C.     

Colorless and high organosoluble polyimides from 2,5-bis(3,4-dicarboxyphenoxy)-t-butylbenzene dianhydride and aromatic bis(ether amine)s bearing pendent trifluoromethyl groups

A series of polyimides III_a-h characterized by colorlessness, high transparency, high solubility, and good mechanical property, was synthesized from the aromatic dianhydride, 2,5-bis(3,4-dicarboxyphenoxy)-t-butylbenzene dianhydride (I), and various aromatic diamines (II_a-h) with pendent trifluoromethyl group via polyaddition, chemical imidization, and direct cast films. The III series showed more colorless than the polyimides (V and VI series) of 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) contained, the VI series was synthesized from the II with 6FDA. These films III had cut-off wavelengths between 371 and 376 nm, as well as b* value (a yellowness index) ranging from 3.0 to 4.7. In fact, it is so far the most colorless aromatic polyimide in our systematical researches. The III series had inherent viscosity ranging from 0.72 to 1.33 dL/g and showed excellent solubility in a variety of organic solvents. They were soluble in a concentration of 5-10% in the amide polar solvent, ether solvent, and chlorinated solvent. These films showed strength tensile of 97-123 MPa, dielectric constants of 2.78-3.28 (1 MHz), and moisture absorptions of 0.11-0.36 wt%. The glass transition temperature of the III series was recorded at 214-259 °C, the 10% weight loss temperature was over 468 °C, and the residue was more than 47% at 800 °C in nitrogen.     

Near-infrared emissive BODIPY polymeric and copolymeric dyes

Novel near-Infrared emissive BODIPY polymeric dyes (polymers A and B) were prepared by Sonogashira cross-coupling reaction of 2,6-diiodo BODIPY dyes bearing one and two styryl groups at 3,5 positions (5, 6) with 2,6-diethynyl BODIPY dye, respectively. These polymeric dyes (A and B) display absorption maxima at 697 and 738 nm, and emission maxima at 715 and 760 nm, respectively. These polymeric dyes exhibit significant red shifts in absorption and emission maxima due to their extended π-conjugation systems compared with their BODIPY monomeric dyes. The thin films of polymers A and B display further red shift with emission maxima 764 and 810 nm, respectively. Near-infrared BODIPY copolymeric dye (C) was prepared by Sonogashira polymerization of 2,6-diiodo BODIPY dye bearing two styryl groups with 2,5-diethynyl-3-decylthiophene. For comparison in optical properties, deep-red and red emissive BODIPY copolymeric dyes (D and E) were prepared by Sonogashira polymerization of 2,6-diiodo BODIPY dye bearing monostyryl group and 2,6-diiodo BODIPY dye with 2,5-diethynyl-3-decylthiothene, respectively. These polymers display their absorption maxima at 649 nm and 634 nm, and emission maxima at 694 nm and 669 nm, respectively. All the polymers displayed good thermal stability and solubility in dichloromethane, and their lifetimes ranged from 0.7 to 3.4 ns.     

Electrochemical behaviour of tetraazamacrocyclic nickel(II) complexes functionalized with crown ethers

Tetraazamacrocyclic nickel(II) complexes linked by the system of conjugated double bonds with one or two benzo-15-crown-5 moieties (2Ni and 3Ni) were synthesized and characterized. The CV of these complexes in acetonitrile (ACN) solution exhibited reversible Ni^II/III redox process. At more positive potentials oxidation of the bezno-15-crown-5 ether (B15C5) moiety was found. In complexes 2Ni and 3Ni redox potential of Ni^II/III did not change after the interactions of alkali and alkaline earth metal cations with B15C5 unit. However, the presence of guest cations caused a significant change of B15C5 oxidation potential. Complexation of two Cl⁻ anions to the coordinated Ni^II in 2Ni and 3Ni resulted in an appreciable shift of both Ni^II/III and B15C5 redox processes after addition of metal cations. Behaviour of complexes 2Ni and 3Ni was compared with behaviour of similar Cu^II complexes.     

Random vs. alternating donor-acceptor copolymers: A comparative study of absorption and field effect mobility

The influence of the arrangement of donor and acceptor units in a conjugated copolymer chain on the absorption and field effect mobilities was studied. We synthesized a target random copolymer and compared it with two structurally relevant alternating copolymers, all consisting of 2,1,3-Benzothiadiazole (BT) as acceptor and 3-Hexylthiophene (Th) as donor units. Especially, bifunctional AB-type monomers were developed to obtain the desired randomly linked copolymer r-BT-2Th. We chose AA/BB-type monomers as well to obtain relevant alternating copolymers a-BT-2Th and a-BT-1Th. The systematic structural variation enables us to compare the copolymers in a precise manner. In dilute solutions r-BT-2Th and a-BT-2Th resemble closely in absorption spectra and have similar oxidation potentials regardless of random or alternating arrangement of donor and acceptor. In thin films, a-BT-2Th shows the lowest optical gap and depicts the highest field effect hole mobility of 1.5 × 10⁻³ cm² V⁻¹ s⁻¹.     

Synthesis and electrochromic properties of aromatic polyimides bearing pendent triphenylamine units

A series of aromatic polyimides with pendent triphenylamine group were synthesized from equimolar mixtures of 4,4′-oxydianiline (ODA) and 4-(3,5-diaminobenzamido)triphenylamine (4), 4-(3,5-diaminobenzamido)-4′,4″-di-tert-butyltriphenylamine (t-Bu-4) or 4-(3,5-diaminobenzamido)-4′,4″-dimethoxytriphenylamine (MeO-4) with two aromatic tetracarboxylic dianhydrides (DSDA or 6FDA) via a conventional two-step procedure that included a ring-opening polyaddition to give poly(amic acid)s, followed by chemical imidization. These polyimides exhibited good solubility in polar organic solvents and could be solution-cast into flexible and strong films. They showed excellent thermal stability, with T_g values in the range of 284–309 °C. The polyimides derived from diamines t-Bu-4 and MeO-4 exhibited reversible electrochemical oxidation, accompanied by strong color changes with high contrast ratio and electrochromic stability. For the polyimides derived from diamine 4, the coupling reaction between the triphenylamine radical cations occurred during the oxidative process forming a tetraphenylbenzidine structure, which resulted in an additional oxidation state and color change together with enhanced near-IR absorption at fully oxidized state.     

Anodic carbon-boron bond cleavage through the intermediacy of electrogenerated bromonium ion

The electrochemical properties of a series of cyclic arylboronic esters, XC₆H₄B(OR)₂ [RR = CH₂CH₂; X = H (1a); p-Me (1b); p-OMe (1c); p-Cl (1d); p-Ph (1e); m-Cl (1f); m-OMe (1g); CF₃ (1h); OMe (1i); 2,6-dimethyl (1j); 1b with RR = (CH₂)₃, (1k); 1b with RR = CMe₂CMe₂, (1m)] has been studied in acetonitrile by cyclic voltammetry (CV) and controlled-potential electrolysis (CPE). The CV of representative examples of aryl borates with different substituents show one irreversible oxidation wave on a Pt cathode, at 1.8-1.9 V (vs. Ag/AgCl), with a negligible substituent effect. The cathodic CPE process led to small amounts of biaryls only, whereas the direct anodic CPE could not be carried out practically due to low currents. However, in the presence of electrogenerated bromonium (or iodonium) ions a C-B bond cleavage does take place to yield the corresponding bromoaryls, brominated phenols, and arylboronic acids as the major products.     

Colorless polyimides from 2,3,3′,4′-biphenyltetracarboxylic dianhydride (α-BPDA) and various aromatic bis(ether amine)s bearing pendent trifluoromethyl groups

A series of organosoluble and light-colored polyimides (III) was prepared from 2,3,3′,4′-biphenyltetracarboxylic dianhydride (α-BPDA) with various fluorinated aromatic bis(ether amine)s via two-step method with thermal or chemical imidization of poly(amic acid)s yielded polyimides. The III series had inherent viscosity of 0.74-1.01 dl/g and showed excellent solubility in a variety of organic solvents. They were soluble in the amide polar solvent, ether-type solvent, and chlorinated solvent. These polyimide films also showed a high optical transparency and less color intensity, with an ultraviolet-visible absorption edge of 369-382 nm and low b^* values (a yellowness index) of 5.0-11.7. Glass-transition temperature of the III series was recorded at 244-319 °C and higher than the isomeric polyimides V series. Compared with the nonfluorinated polyimides IV, the III series showed lighter-colored and lower dielectric constants and moisture absorptions. The good tensile properties and excellent thermal properties of the III series were also observed.     

Thermally cross-linkable hyperbranched polymers containing triphenylamine moieties: Synthesis, curing and application in light-emitting diodes

This paper demonstrates synthesis of hyperbranched polymers (HTP and HTPOCH₃), containing triphenylamine moieties in main chain and thermally cross-linkable periphery or terminal vinyl groups, and application as hole-transporting layer (HTL) in multilayer light-emitting diodes. Absorption and photoluminescence (PL) spectroscopy, cyclic voltammetry (CV) and differential scanning calorimetry (DSC) were employed to investigate their photophysical, electrochemical properties and thermal curing behaviors, respectively. The hyperbranched HTP and HTPOCH₃ were readily cross-linked by heating scan, with the exothermic peaks being at 221 and 210 °C respectively. The glass-transition temperatures (T_g) of the hyperbranched polymers were higher than 140 °C after thermal cross-linking at 210 °C for 30 min. Multilayer light-emitting diodes (ITO/PEDOT:PSS/HTL/MEH-PPV/Ca/Al), using HTP and HTPOCH₃ as HTL, were readily fabricated by successive spin-coating. The performance of MEH-PPV device (maximum luminance: 9310 cd/m², luminance efficiency: 0.26 cd/A) was effectively enhanced by inserting the thermally cross-linked HTP or HTPOCH₃ as HTL (HTP: 12610 cd/m², 0.32 cd/A; HTPOCH₃: 14060 cd/m², 0.33 cd/A). This indicates that these thermally cross-linkable hyperbranched HTP and HTPOCH₃ are very suitable for the fabrication of multilayer PLEDs using solution processes.     

Investigations on the electrochemical properties of new conjugated polymers containing benzo[c]cinnoline and oxadiazole moieties

A four-step route was designed to synthesize 3,8-benzo[c]cinnoline dicarboxylic acid (4). New conjugated polymers, POXD (I) and POXD (T), containing benzo[c]cinnoline and oxadiazole moieties, were obtained by thermal cyclodehydration of their soluble polyhydrazide precursors PHA (I) and PHA (T), respectively. Two reduction peaks were observed for these new conjugated polymers during CV cathodic scan. From the CV voltammograms combined with the results from molecular simulation, we concluded that the first reduction occurred at oxadiazole moiety and benzo[c]cinnoline moiety was responsible for the second reduction. It indicates that oxadiazole has stronger electron affinity than benzo[c]cinnoline. We proposed a mechanism to explain this two-stage reduction process. Due to the planar and electron-accepting ability of benzo[c]cinnoline and oxadiazole moieties, POXD (I) and POXD (T) exhibited very low LUMO (−3.42 and −3.45 eV) and HOMO (−6.23 and −6.27 eV) energy levels. They can be used as hole-blocking or electron-injection layers for OLED applications.     

Non-catalytic Oppenauer oxidation of alcohols in supercritical water

Non-catalytic Oppenauer oxidation was applied for alcohols, such as benzyl alcohol (4) and benzhydrol (1), in the presence of an excess amount of carbonyl compound, formaldehyde (5a), as an oxidant with and without water. Oppenauer oxidation took place in both reactions of 4 and 1 to afford the oxidation products, benzaldehyde (6) (95%) and benzophenone (2) (64%), concomitant with relatively small amounts of reduction products, toluene (7) (1%) and diphenylmethane (3) (13%), respectively, at 400 °C for 10 min without water in an SUS 316 batch-type tubular reactor. Lower yields of oxidation products 6 (68%) and 2 (30%) were obtained in supercritical water under the conditions of 400 °C, 10 min, and 0.35 g/mL water density, while the formation of the reduction products 7 and 3 was completely suppressed. Thus, water was indispensable for the clean and highly selective Oppenauer oxidation of 4 and 1 to yield 6 and 2.     

Highly active and trans-1,4 specific polymerization of 1,3-butadiene catalyzed by 2-pyrazolyl substituted 1,10-phenanthroline ligated iron (II) complexes

A new family of iron (II) complexes (2a–h) bearing tridentate 2-pyrazolyl substituted 1,10-phenanthroline ligands were successfully prepared and characterized by IR spectroscopy, elemental analysis, and mass spectra. Complexes 2a–f and 2h were further confirmed by the X-ray crystallographic analysis. 2a, 2b, 2e, and 2f adopted distorted trigonal bipyramidal configuration. 2c displayed a distorted octahedron formed by six coordinated nitrogen atoms of the two ligands. Linked by two bridged chloride atoms, complex 2d was a centrosymmetric dimmer, and complex 2h adopted a six-coordinate distorted octahedral geometry due to the coordination of two solvent molecules. These complexes activated by alkylaluminum were examined in butadiene polymerization. In combination with AlⁱBu₃, complexes 2a–c exhibited high catalytic activity (73.5%–94.3%) at 20 °C, whereas other complexes exhibited much lower activity. Interestingly, the activity and selectivity of the complexes increased as increasing polymerization temperature. In particular, 2b and 2c displayed both high activity (99% and 80%, respectively) and trans-1,4 selectivity (95.6% and 96.2%, respectively) at 60 °C. The trans-1,4 selectivity of 2b varied as alkylaluminum used as a cocatalyst, in the following order: AlⁱBu₃ > AlOct₃ > AlEt₃ > AlMe₃, whereas much lower trans-1,4 selectivity was observed in the cases of using MAO and MMAO.     

Synthesis and property of 9,9′-spirobifluorene-containing aromatic polyesters as optical polymers with high refractive index and low birefringence

C₂-Symmetric 9,9′-spirobifluorene-containing polyesters (PEs) were synthesized by polycondensation of 2,2′-dihydroxy-9,9′-spirobifluorene (1) with bis(acyl chloride)s (2) at 230 °C in diphenylether. The molecular weights of PEs 3a-3f were sufficiently high (M_w 13,400-41,600). PEs displayed high thermal stability. The glass transition temperatures (T_g) estimated by differential scanning calorimetry analysis appeared in a range 177-352 °C depending on the spacer structure, while the 5% decomposition temperatures (T_d5) measured by thermogravimetric analysis were over 416 °C both under nitrogen atmosphere and in air. PEs showed good solubility in typical organic solvents such as CHCl₃ and THF easily to afford the tough, transparent, and flexible cast films. The transmittance of the polymer films reached over 90% in the wavelength range from ca. 410-900 nm. In addition, PEs exhibited higher refractive index rather than that of commercially available 9,9-diarylfluorene-containing PE, in addition to very low degree of birefringence presumably due to the C₂-symmetric structure.     

Novel fluorene-based light-emitting copolymers containing cyanophenyl pendants and carbazole-triphenylamines: Synthesis, characterization and their PLED application

A series of novel blue light-emitting copolymers PCC-1, PCC-2, and PCC-3, composed of different ratios of electron-withdrawing segments (spirobifluorene substituted with cyanophenyl groups) and electron-donating segments (carbazole-triphenylamines), has been synthesized and characterized. In order to investigate the effect of hole/electron charge transporting segments, two reference polymers PSF and PCF, containing only one charge transporting moiety in the polymer backbone, were also synthesized. Incorporation of the rigid spirobifluorene units substituted with cyanophenyl groups into the polymer backbone improved not only the thermal stabilities but also the photoluminescence efficiencies. The polymers except PSF possess similar hole injection barriers but different hole transporting abilities. With the device configuration of ITO/PEDOT:PSS/polymers:PBD/CsF/Ca/Al, PCC-2 showed the best performance with the lowest turn-on voltage of 3.1 V, the highest luminance of 6369 cd/m², the highest current efficiency of 1.97 cd/A, and the best power efficiency of 1.40 lm/w.     

Low bandgap EDOT-quinoxaline and EDOT-thiadiazol-quinoxaline conjugated polymers: Synthesis, redox, and photovoltaic device

Three new low bandgap conjugated copolymers with 3,4-ethylenedioxythiophene (EDOT) as donor and 2,3-bis(4-octyloxyphenyl)-quinoxaline (P1), 2,3-bis(4-octyloxyphenyl)-thiadiazol-quinoxaline (P2, P3) as acceptors were synthesized by Stille cross-coupling reaction, and their optical and electrochemical properties were studied. These polymers exhibited optical bandgap of 1.77, 1.29 and 1.13 eV, for P1, P2 and P3, respectively. Photovoltaic cells with device configuration of ITO/PEDOT: PSS/Copolymer: PCBM (1:4 w/w)/LiF/Al were fabricated. The measurements revealed an open-circuit voltage (V_oc) of 0.52 V, short-circuit current density (J_sc) of 3.24 mA/cm² and power conversion efficiency (PCE) of 0.60% for P1, and showed a V_oc of 0.33 V, J_sc of 2.11 mA/cm², PCE of 0.39% for P2.     

Conjugated copolymers based on dihexyl-benzimidazole moiety for organic photovoltaics

In this study, we report the synthesis, characterization, and photovoltaic properties of a series of four conjugated polymers based on donor-acceptor (D-A) structure. New polymers, P1, P2, P3 and P4 utilizing a new accepter, dihexyl-2H-benzimidazole, were synthesized using Stille coupling reaction. Even with two bithiophene units in the copolymers to facilitate absorption at the longer wavelength, the incorporation of dihexyl-substituent on 2H-benzimidazole enables the polymers to have good solubility. The spectra of the solid films show absorption bands with maximum peaks at about 442-479 nm and the absorption onsets at 529-575 nm, corresponding to band gaps of 2.16-2.34 eV. The device with P4:PC₇₁BM blend demonstrated an open-circuit voltage (V_OC) of 0.59 V, a short-circuit current (J_SC) of 6.43 mA/cm², and a fill factor (FF) of 0.39, leading to the efficiency of 1.46%.