Synthesis, electroluminescence, and photovoltaic properties of dendronized poly(p-phenylene vinylene) derivatives

Two dendronized poly(p-phenylene vinylene) (PPV) derivatives, ED-PPV and BB-PPV, have been successfully synthesized according to the Gilch route. The obtained polymers possess excellent solubility in common solvents, good thermal stability with 5% weight loss temperature of more than 340 °C. The weight-average molecular weight (M_w) and polydispersity index (PDI) of ED-PPV and BB-PPV are in the range of (1.26-2.34)×10⁵ and 1.37-1.45, respectively. Polymer light-emitting diodes (PLEDs) with the configuration of ITO/PEDOT:PSS/polymer/Ca/Al devices were fabricated, and the PLEDs emitted green-yellow light. The turn-on voltages of the PLEDs based on ED-PPV and BB-PPV were approximately 4.3, and 4.5 V, respectively. The PLED devices of ED-PPV exhibited the maximum luminance of about 157 cd/m² at 10.5 V. Photovoltaic cells with the configuration of ITO/PEDOT:PSS/polymer:C₆₀ (1:1)/Al were also fabricated, and the energy conversion efficiency of the devices based on ED-PPV and BB-PPV was measured to be 0.58, and 0.014%, respectively, under the white light at 75 mW/cm².     

Synthesis and gas permeability of ester substituted poly(p-phenylene)s

Polymerizations of various ester substituted 2,5-dichlorobenzoates [substituent: linear alkyl groups (1a-f), branched alkyl groups (1g-l), cyclohexyl groups (1m-o), phenyl groups (1p-r), and oxyethylene units (1s-v)] were investigated with Ni-catalyzed/Zn-mediated system in 1-methyl-2-pyrrolidone (NMP) at 80 °C. Most of monomers bearing linear and branched alkyl groups successfully polymerized to give relatively high-molecular-weight polymers (M_n = 10,000-20,800). However, the molecular weight of the polymer having eicocyl groups was low because of steric hindrance of long alkyl chain. The polymerizations of cyclohexyl 2,5-dichlorobenzoate and phenyl 2,5-dichlorobenzoate produced low-molecular-weight polymers, while the polymerizations of monomers with alkyl cyclohexyl and alkyl phenyl groups proceeded to afford polymers with relatively high-molecular-weights. The polymers possessing oxyethylene units were obtained, but the molecular weights were low when the oxyethylene chains were long. The gas permeability of membranes of poly(p-phenylene)s with alkyl chains increased as increasing the length of alkyl chain. The membranes of poly(p-phenylene)s with phenyl groups and oxyethylene units exhibited high densities and relatively low gas permeability. However, the CO₂/N₂ separation factor of membrane of poly(p-phenylene) having oxyethylene units was as large as 73.6.     

Considerations of polymerization method and molecular weight for proton-conducting poly(p-phenylene) derivatives

Ni(0)-catalyzed coupling polymerization of 2,5-dichloro-4′-phenoxybenzophenone was investigated by varying the ligand and coligand, temperature, reaction time, and solvent. The weight-average molecular weight (M_w) of poly(4-phenoxybenzoyl-1,4-phenylene)s (PPBPs) could be controlled by the polymerization conditions and reached a maximum of 4.4 × 10⁵ g mol⁻¹. Sulfonated PPBPs (S-PPBPs) with various M_ws were prepared with sulfuric acid to study the effect of molecular weight on the chemical and electrical properties of PPBP-based electrolytes. The strong molecular interactions in S-PPBP provided an ion exchange capacity of 2.9 meq g⁻¹ without loss of high mechanical properties. High molecular weight S-PPBPs had more desirable properties for fuel cell applications. While the swelling ratios and hydration numbers of S-PPBPs decreased with increasing molecular weight, the mechanical strength, proton conductivity, and fuel cell performance increased. S-PPBP also showed anisotropic behavior in the swelling and proton conductivity; such behavior is caused by the rigid-rod nature and the liquid-crystal structure.     

Solution properties and kinetics of aggregation of an alkyl-substituted poly(p-phenylene)

Freshly prepared solutions of poly(2,5-di-n-dodecyl-1,4-phenylene) (PPP 12) in toluene are metastable at room temperature with regard to a process which leads to the formation of aggregates composed of up to 100 individual macromolecules. This aggregation process has an induction period of more than 10 h at room temperature. The kinetics of aggregation was investigated by making use of a fast capillary membrane osmometer. Aggregation follows an Avrami-Evans type formalism and suggests that clusters of a lyotropic liquid crystalline phase of the polymer are formed of the same type as observed in the melt. The long induction period of aggregate formation in dilute solution in toluene allows to apply conventional techniques of molar mass determination like membrane osmometry and size-exclusion chromatography (SEC). A relationship [η] = 1.94 × 10⁻³ M^0.94 was found for PPP 12 in toluene at 20 °C and a persistence length of 15.6 nm was derived applying the Bohdanecky-formalism. This gives evidence of the worm-like nature of the non-aggregated PPP 12 in dilute solution.     

Direct fluorination of various poly(p-phenylene): Effects of the polymer synthesis and thermal post-treatment

In order to complete a preliminary work about the direct fluorination of poly(p-phenylene) (PPP), the main parameters of the process were investigated. The molecular (chain length and crystallinity) and morphological effects (B.E.T. surface, granulometry) were studied. So, two new starting samples were studied in addition to PPP synthesized by the Kovacic's method (i) a commercial PPP (Polysciences) and (ii) a pyrolyzed PPP, which is similar to an amorphous hydrogenated carbon. Moreover, an annealed PPP (Kovacic's synthesis and post-treatment at 400 °C for 36 h) was compared to the as-synthesized polymer. The reactions with F₂ gas differ significantly in accordance with the synthesis way and the post-treatment (annealing or pyrolysis). Investigations about the direct fluorination of PPPs were carried out for a better understanding of their behavior with respect to molecular fluorine. An extensive characterization was performed by complementary techniques (¹⁹F and ¹³C NMR, FT-IR, and EPR). The fluorine content in the fluorinated PPPs is evaluated by these methods and a reaction mechanism is proposed.     

Structural characterization and luminescent properties of poly(p-phenylene vinylene) and poly(ethylene glycol) blends

The luminescent properties of poly(p-phenylenevinylene) (PPV) blending with poly (ethylene glycol) (PEG) were investigated in terms of their structural formation during sample preparation. The blended systems were prepared from an aqueous solution of water-soluble poly (xylylene tetrahydrothiophenium chloride) (PPV precursor) mixed with PEG, followed by heat treatment to remove the tetrahydrothiophene groups from the PPV precursor. Structural analysis showed that PEG could react with PPV precursor to form C-O-C linkage and carbonyl groups in PPV chains, interrupting their conjugated length as suggested by their Infrared, Raman and UV/vis spectroscopes. Wide angle X-ray scattering (WARS) of blended systems also showed that PPV in blends had less packing. As to luminescent properties, the UV/vis and photoluminescent (PL) spectra show that the energy gap needed to produce the excitons increased along with the increase of PL intensity when PPV was blended with PEG. Similar results were also found for the EL properties of ITO/polyblends/Al devices. The EL light emission from blends was blue-shifted (compared to PPV) with a rather low threshold electric field strength. The EL performance of polyblends was better than that of pure PPV. Among them, the PPV-50PEG showed the highest EL intensity. The improved EL efficiency was attributed to the dilution effect, interrupted conjugated length, and lower packing of PPV chains.     

Non-isothermal elimination process in the solid state of n-alkyl-sulphinyl precursor polymers towards conjugated poly[2-(3′,7′-dimethyloctyloxy)-5-methoxy-1,4-phenylene vinylene] studied with MTDSC and TGA

The elimination process of n-alkyl-sulphinyl precursor polymers towards conjugated poly[2-(3′,7′-dimethyloctyloxy)-5-methoxy-1,4-phenylene vinylene], or OC₁C₁₀-PPV, was studied with modulated temperature differential scanning calorimetry (MTDSC) and thermogravimetric analysis (TGA), with a focus on the subsequent reactions of the elimination products. The latter reactions were monitored using the non-reversing heat flow and the heat capacity (C_p) measured in non-isothermal MTDSC experiments. The disproportionation reaction occurs in a temperature range between 85 and 135 °C and is seen as an increase in C_p. Water and elimination products released during the elimination reaction act as plasticizers and lower the T_g. TGA experiments show that the temperature, film thickness, and the eliminated group play an important role on the diffusion and evaporation of the elimination products. The elimination products can further decompose and interact with the conjugated system to form undesirable crosslinks (network formation) in a temperature range of 140-160 °C.     

Synthesis and characterization of poly(1,5-naphthylene vinylene) and its copolymers with poly(2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene vinylene)

Poly(1,5-naphthylene vinylene) (PNV) and its copolymers with poly(2-methoxy-5-(2′-ethylhexyloxy)-p-phenylenevinylene) (MEH-PPV) were synthesized via a liquid-solid two-phase reaction. The liquid phase was a tetrahydrofuran (THF) solution of the monomers, 1,4-bis(bromomethyl)-2-methoxy-5-(2-ethylhexyloxy)benzene (MOEHODCX) and 1,5-bis(bromomethyl) naphthalene (DBMN), and contained a certain amount of tetrabutylammonium bromide (TBAB) as a phase transfer catalyst. The solid phase was potassium hydroxide particles with diameters smaller than 2 mm. The structures of the polymers were studied by infrared and Raman spectroscopies. The solutions of the copolymers emitted green light. The UV-vis and photoluminescence spectral bands of the copolymers showed blue shifts with the increase in their PNV contents. A polymer light-emitting diode was fabricated with the copolymer prepared by copolymerization MOEHODCX and DBMN with feeding molar ratio of 1:1, and its luminescence efficiency was measured to be 0.069.     

Experimental verification on upper critical solution temperature (UCST) behavior in blend of poly(2,6-dimethyl p-phenylene oxide) with poly(4-methyl styrene)

Rare upper critical solution temperature (UCST) behavior was found and experimentally demonstrated in the blend comprising poly(2,6-dimethyl p-phenylene oxide) with poly(4-methyl styrene) (PPO/P4MS). Complexity of phase behavior in the PPO/P4MS system has caused puzzling analyses in the past years. This study re-investigated and clarified past mis-interpretations related to this interesting blend system. This study concluded that the PPO/P4MS blend is an immiscible system at ambient, which, however, turns into a miscible phase with UCST behavior at higher temperatures. With the finding of UCST in the PPO/P4MS blend, a critical contribution of this work was to resolve the conflicting arguments that have gone on for a long time in determination and interpretation of the thermodynamic phase behavior of PPO/P4MS. Phase behavior with UCST in the PPO/P4MS blend system and its interpretation were supported with clear experimental evidence.     

A novel biodegradable poly(p-dioxanone)-grafted poly(vinyl alcohol) copolymer with a controllable in vitro degradation

A novel biodegradable copolymer was synthesized from poly(vinyl alcohol) and poly(p-dioxanone) by ring-opening polymerization. The molecular structure of the copolymer was characterized by one- and two-dimensional NMR spectroscopy. The results of differential scanning calorimetry (DSC) show that the amphiphilic and comb grafted structure of the copolymer make its crystalline behavior different from that of the poly(p-dioxanone) homopolymer (PPDO). The in vitro degradation rate of the copolymers can be controlled via adjusting the number and length of PPDO segments of PVA-g-PPDO copolymers. The copolymer has a potential application in biomedical materials or in the controlled release of drug.     

The direct synthesis of wholly aromatic poly(p-phenylene)s bearing sulfobenzoyl side groups as proton exchange membranes

Sulfonated poly(p-phenylene)s (SPPs) containing sulfonic acid groups in their side chains had been directly synthesized by Ni(0) catalytic coupling of sodium 3-(2,5-dichlorobenzoyl)benzenesulfonate and 2,5-dichlorobenzophenone. The synthesized copolymers possessed high molecular weights revealed by their high viscosity, and the formation of tough and flexible membranes by casting from DMAc solution. The copolymers exhibited excellent oxidative stability and mechanical properties due to their fully aromatic structure extending through the backbone and pendent groups. Transmission electron microscopic (TEM) analysis revealed that these side-chain type SPP membranes have a microphase-separated structure composed of hydrophilic side-chain domains and hydrophobic polyphenylene main chain domains. The proton conductivities of copolymer membranes increased with the increase of IEC and temperature, reaching values above 3.4 × 10⁻¹ S/cm at 120 °C, which are almost 2-3 times higher than that of Nafion 117 at the same measurement conditions. Consequently, these materials proved to be promising as proton exchange membranes.     

Electrosynthesis and characterization of a poly(paraphenylene) deriving from p-fluoroanisole

The electrochemical oxidation of p-fluoroanisole (p-FA) in the solvent acetonitrile leads to oligomers and polymers of poly(paraphenylene) type. The electropolymerization process involves coupling reactions of the cation radicals intermediates. The obtained polymers are separated according to their chain length by selective precipitation in cyclohexane and ether. The corresponding structures are characterized by NMR, MS, FTIR, UV and XR diffraction. A preliminary physical study shows that the polymers are photoluminescent with a maximum emission in the near infrared.     

Short poly(phenylene vinylene) chains grafted poly(organophosphazene)

Brominated poly(bis(4-methylphenoxyphosphazene) was allowed to react with 1,4-bischloromethylbenzene or 1,4-bischloromethyl-2,5-dimethoxybenzene in solution using phase transfer catalyst or potassium t-butoxide. Poly(p-phenylene vinylene) or poly(2,5-methoxy-1,4-phenylene vinylene) grafted organophosphazene copolymers were obtained. The UV-Vis absorption, photoluminescent, and thermal properties of the copolymers were measured. The copolymers are complete soluble in common organic solvents and fluoresce in the blue color range. The copolymers were used to build a series of organic light emitting diode (OLED). Only weak to nominated intensities with emission color from blue to red were obtained. The photoluminescent and electroluminescent (EL) spectra indicated there is a distribution in the PPV conjugated length. The compositions of the copolymers before and after the graft reaction were analyzed using NMR.     

Highly efficient electroluminescent biphenylyl-substituted poly(p-phenylenevinylene)s through fine tuning the polymer structure

A series of novel biphenylyl-substituted PPV derivatives, polymers 1-4, with different substitution patterns, has been synthesized and characterized. These polymers possess excellent solubilities, good thermal stabilities, and high-photoluminescent efficiencies. ¹H NMR measurements indicated that the polymers contain negligible tolane-bisbenzyl (TBB) structural defects. Light-emitting diodes fabricated from the four polymers with the configuration of ITO/PEDOT:PSS (50 nm)/polymer (80 nm)/LiF (0.4 nm)/Ca (20 nm)/Ag emitted a saturated green light and demonstrated maximum current efficiencies of 5.1, 4.5, 4.7, and 1.4 cd/A for polymers 1-4, respectively. The much higher current efficiencies of polymers 1-3 than polymer 4 are ascribed to more balanced charge transport in the polymer layers of the three polymers, which has been confirmed by time of flight (TOF) charge mobility measurement. The hole mobilities of the polymers at the applied electric field of 2.0×10⁵ V/cm are 4.70×10⁻⁶, 3.83×10⁻⁶, 7.21×10⁻⁶, and 1.76×10⁻⁵ cm²/Vs for polymers 1-4. This research indicated that fine tuning the substitution pattern of the polymer side chains is an effective way to optimize the LED device performance by controlling the structural defects as well as balancing the charge mobility of the polymers.     

Cationic phenyl-substituted poly(p-phenylenevinylene) related copolymers with efficient photoluminescence and synthetically tunable emissive colors

A series of amino-functionalized phenyl-substituted poly(p-phenylenevinylene) (PPV) related copolymers were synthesized by Wittig reaction. Their corresponding cationic conjugated polymers were successfully obtained via a post-polymerization approach. On the basis of FT-IR and ¹H NMR spectra, it was found that phenyl-substituted PPV related copolymers containing alkoxylated benzene (neutral polymer P1 and quaternized polymer P1′), phenylated benzene (neutral polymer P2 and quaternized polymer P2′) and fluorene (neutral polymer P3 and quaternized polymer P3′) moieties are of 55, 80, and 45% cis-vinylic linkage respectively while the polymer containing thiophene moiety (neutral polymer P4 and quaternized polymer P4′) is primarily of trans-vinylic linkage. Their photoluminescence (PL) were conveniently tuned from blue color to yellow color by introducing units with different optoelectronic properties into the PPV backbones. The polymer with fluorene unit and bulky phenylene-substituted benzene unit in the backbone exhibited the highest PL efficiency among these neutral and quaternized PPVs. P4′ containing little cis-vinylic linkage showed complete quenching while P1′-P3′ containing much more cis-vinylic linkage showed incomplete quenching, indicating that the quenching behavior of these cationic PPVs may be highly influenced by the content of cis-vinylic linkage in the PPV backbones.     

Polymeric molecular shuttles: Polypseudorotaxanes & polyrotaxanes based on viologen (paraquat) urethane backbones & bis(p-phenylene)-34-crown-10

Reaction of di(p-isocyanatophenyl)methane (MDI, 4) with N,N′-di(2-hydroxyethyl)- (1b) or N,N′-di[2-(2′-hydroxyethoxy)ethyl]-4,4′-bipyridinium di(hexafluorophosphate) (1e) and other diols [oligo(ethylene glycol)s and poly(tetramethylene oxide)s] in the presence of bis(p-phenylene)-34-crown-10 (2) afforded polyurethane (pseudo)rotaxanes as statistical (7P or 7R) and segmented analogs 10P (P = pseudorotaxane, R = rotaxane). In 7R a bulky alcohol was incorporated at the chain ends and in 13R a bulky diol as in-chain units to form polyrotaxanes and preclude the possibility of dethreading. The crown ether 2 in 10P and 13R was shown by ¹H NMR spectroscopy to be shuttling between the viologen (paraquat) and urethane sites; in DMSO the crown ether prefers the urethane site, probably because of H-bonding with the N–H moieties and complexation of the pyridinium site by the dipolar solvent, while in acetone at low temperatures the viologen site is preferred by the crown ether, with ΔH = −6.91 kcal/mol and ΔS = −22.9 eu for 13R.     

Semi-empirical calculation and spectroscopic study of protonated poly(p-phenylene benzobisoxazole) models

The characters of poly(p-phenylene benzobisoxazole) (PBO) in methanesulfonic acid (MSA) are strongly influenced by the protonation effect. Due to such protonation effect, the experimental optical absorption spectrum was red shifted from the optical absorption spectrum predicted by AM1/ZINDO–CI (Austin Method 1/Zerner's intermediate neglect of differential overlap coupled to single configuration interaction) method. Further studies on AM1-optimized geometries of PBO showed that the final minimized torsion angle of benzoxazole and phenylene rings increased from 0.04° for the neutral molecule to 18.53° for protonated form at N atom. Moreover, the repulsive Coulombic interactions originated from protonation tend to stiffen the PBO chain. The AM1/ZINDO–CI calculation also indicated that the strongest UV absorption peak of PBO was blue shifted with increasing torsion angle.     

Melt processed blends of poly(styrene-co-acrylonitrile) and poly(phenylene ether) compatibilized with polystyrene-b-polybutadiene-b-poly(methyl methacrylate) triblock terpolymers

Polystyrene-b-polybutadiene-b-poly(methyl methacrylate) triblock terpolymers (SBM) with equal (symmetric) and different (asymmetric) block lengths were used to compatibilize polymer blends based on poly(2,6-dimethyl-1,4-phenylene ether) (PPE) and poly(styrene-co-acrylonitrile) (SAN). First, the rheological behavior of the individual components and their binary mixtures was investigated. Based on the results, samples of PPE, SAN and SBM in weight ratios of 32/48/20 were melt blended and the morphology development during melt processing was investigated. It was found that a raspberry morphology, i.e. dispersion of PPE in SAN with rubbery PB domains at the PPE/SAN interface, could be achieved with a symmetric SBM with under sufficiently high shear rate, while a symmetric SBM with did not yield the desired morphology. Asymmetric SBMs with long PS blocks dissolved in the PPE phase did not display the expected compatibilization effect. In order to obtain a raspberry morphology with asymmetric copolymers it is suggested to pre-blend the SBM with SAN before adding the PPE. Finally it is shown that a commercial PPE containing High Impact Polystyrene (HIPS) as a toughness modifier can be compatibilized with SAN by melt processing using a symmetric SBM triblock terpolymer with      

Time resolved study of shear-induced crystallization of poly(p-dioxanone) polymers under low-shear, nucleation-enhancing shear conditions by small angle light scattering and optical microscopy

Isothermal crystallization of the biodegradable homopolymer polyester poly(p-dioxanone) (PDS), and the p-dioxanone copolymer with glycolide PDS-copolymer were studied in situ and in real-time under quiescent or nucleation-enhancing shear conditions. It was found that the spherulitic growth rates remain unchanged with shear, while the nucleation density increases dramatically. Nucleation-enhancing shear conditions, which do not alter the general spherulitic morphology, consist of a short-duration step-shear. This is in contrast to isothermal crystallization under steady-shear conditions, where at low-shear rates, fibrillar crystalline structures form. At high crystallization temperatures, where under quiescent conditions crystal development requires several days, both PDS, and the PDS-copolymer can be made to crystallize in several hours by the imposition of a step-shear.