Blue‐emitting poly[(m‐phenylene vinylene)‐alt‐(o‐phenylene vinylene)]s: The effect of regioregularity on the optical properties

Poly[(m‐phenylene vinylene)‐alt‐(o‐phenylene vinylene)]s with different contents of cis‐/trans‐CH?CH ( 3 and 6 ) have been synthesized through Wittig condensation. The polymers exhibit good solubility in common organic solvents such as toluene and tetrahydrofuran. A comparison of the optical properties has been made between 3 and its phenyl regioisomers containing either p‐phenylene or m‐phenylene units. The results show that the regiochemistry of the phenyl ring can be a useful tool for tuning the emission color of π‐conjugated polymers because the extension of π conjugation can only partially be achieved through an o‐phenylene bridge. Although both polymers 3 and 6 exhibit comparable low fluorescence quantum efficiencies (≈0.18) in solution, their films are highly luminescent, showing a broad emission band near 456 nm (blue color). Electroluminescence results show that the device of polymer 3 , which has a higher content of trans‐CH?CH linkages, is about 20 times more efficient than that of 6 . © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2650–2658, 2003     

Synthesis and electroluminescent studies of blue‐emitting copolymers containing phenylene vinylene and oxadiazole moieties in the main chain

Two statistical copolymers III and IV combining features of the two reference polymers I and II were synthesized by a Wittig reaction with the objective of raising the electron‐transport properties and fluorescence quantum yields relative to the alternating block copolymers I and II . The electroluminescent properties of single‐layer LEDs using these copolymers were studied. External quantum efficiencies of 0.035 and 0.11% were obtained from single‐layer devices on the basis of III and IV , respectively, which are higher than those of similar devices using I and II . Two single‐layer LEDs using a blend of I and II (4:1 and 1:1 wt/wt) corresponding to the compositions of copolymers III and IV , respectively, were also fabricated for comparison. Results indicated that the covalent incorporation of oxadiazole is effective in improving the efficiency of LEDs and that the molar content of oxadiazole plays an important role in the performance of the devices. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 40: 235–241, 2002     

Domain size dependence of glass transition temperature

The effect of particle size on glass transition temperatures is discussed. The phenomenon is treated in terms of Ehrenfest second-order thermodynamics and in addition related to free volume concepts. Consistent formulas are obtained and the order of magnitude of the effect is estimated.     

Relaxation properties of some segmented polyurethane-CaCO3 composites. A Dielectric study

Dielectric relaxation in three segmented polyurethane-CaCO₃ composites was investigated between ?70°C and +150°C in the 300 Hz to 100 kHz frequency range. Two of the polymers contained a polyacetal-polyether soft segment, whereas the soft component of the third polymer was polypropylene oxide. The hard segments consisted of 4,4′-diphenyl methane diisocyanate in two cases and toluene-2,4-diisocyanate in the third case. In parallel studies two calorimetric relaxations, designated α and α′, were observed for each sample and were determined to be glass transitions of the soft and hard segments, respectively. In general, the transition temperatures decrease with increasing filler content. High frequency, low temperature permittivities increase while low frequency, high temperature AC conductivities generally decrease with increasing filler content. The shift in the transition temperatures can be explained using the adsorption theory of filler-polymer interactions and the densities of the samples. The interfacial polarization mechanism becomes important only above the α′ transition temperature and below 1 kHz.     

Studies of thermally stimulated current in polypropylene/calcium carbonate/surfactant systems

Thermally stimulated polarization (TSP) and depolarization (TSD) experiments have been performed with two grades of polypropylene and some respective CaCO₃-filled composites containing small amounts of nonionic surfactant (0–2 wt%). The effects of electrode blocking, electrode materials on the thermally stimulated currents, and reproducibility of the measurements have been studied. The effect of water vapor adsorbed from the ambient air on the AC dielectric properties and on the thermally stimulated polarization behavior has been determined.The addition of either CaCO₃ or surfactant to PP decreases the intensity of the _c depolarization current peak in the pre-melting region, while the presence of both components increases the current. Partial discharges are present in poly propylene/CaCO₃ composites under high voltages if neither water vapor nor a coherent surfactant layer is present at the matrix/filler interface.A short literature survey is presented on the TSC studies of polyolefins and their composites, and various mechanisms responsible for the observed changes are discussed, including interfacial polarization, trap redistribution through nucleation, and oriented adsorption.     

Poly(alkyl α-chloroacrylates). V. Preparation and properties of methyl,ethyl, and isopropyl polymers of varied tacticity

Polymers of different tacticities, from highly isotactic to highly syndiotactic, were prepared from methyl, ethyl, and isopropyl α-chloroacrylates. These polymers were characterized for tacticity by infrared spectroscopy and 100 and 300 MHz nuclear magnetic resonance (NMR) and for thermal properties by differential scanning calorimetry (DSC). After corrections were made for molecular weight effects, the observed glass temperature-tacticity results were analyzed, and it was determined that the maximum differences in glass temperatures of the purely isotactic compared to the purely syndiotactic polymers should be 92°C for the methyl ester, 86°C for the ethyl ester, and 68°C for the isopropyl ester polymers. The highly isotactic polymers of all three esters were crystalline. Possible polymerization reaction mechanisms are discussed on the basis of the triad and tetrad tacticity values observed and the calculated propagation statistics.     

Comment on the “dielectric properties of LDPE/Ny6 blends”

Data published by La Mantia et al. [1] on dielectric dispersion and loss in polyethylene/nylon 6 blends are analyzed in terms of dielectric mixture formulae. It is shown that an ohmic interfacial polarization process can not be responsible for the unexpected increase of and values observed in these blends at high temperatures. The observed phenomena are tentatively attributed to space charge processes at the electrodes or to other defects dipole mechanisms.     

THE EFFECTS OF IRRADIATION ON THE THERMAL PROPERTIES OF SEMI-CRYSTALLINE POLYAMIDES

Irradiation crosslinking of semi-crystalline polyamides was performed by high energy electronswith various dosages. It is known that the melting behavior of the polymers after irradiation is acomplex phenomenon. In company with the wide angle X-ray diffraction and DSC data of irradiatedand unirradiated polyamides it is possible to develop the local order and perfection of the crystallinitiesslightly which resulted from introduction of intermolecular crosslinking in amorphous region, incl-uding in amorphous-crystalline interface and crystalline defect regions due to irradiation. It canbe explained that slight increase of melting temperature (T_m) and heat of fusion (△H_f) with increasingdosage for both of higher crystallinity nylon 4 and nylon 6. For irradiated lower crystallinity nylons,in contrast, the T_m and △H_f decrease obviously with increasing dosage. In this case, radiation cross-linking "freeze in" the pre-existing morphology, and then the prevention for reorganization duringheating is a dominant effect. The T_m from the second melting for all of the samples were depressed,corresponding with Flory theory. Therefore the crosslinks imposed on the molecules restrainedthe molecular mobility, and that not only depresses the crystallinity but also increases the imperfec-tion of crystallites when the radiated polymer melted and then recrystallized. These are also reflectedin the depression of heat and entropy of fusion as well as the appearance of double melting peakson the DSC thermograms.