The quantum efficiencies of photoluminescence (PL) and electro‐luminescence (EL) of poly[2‐methoxy‐5‐(2′‐ethyl‐hexyloxy)‐1,4‐phenylenevinylene] (MEH‐PPV) were significantly increased by heat treatments under vacuum with further removing the undissolved portion. The UV–vis absorption was found to decrease with heating time, while PL intensity increased. The maximum PL quantum yield was 6.5 times that of the untreated MEH‐PPV, which was attributed to the reduction of chain aggregations and the interruption of conjugation length. The maximum EL quantum yield of their prepared ITO/PANI/MEH‐PPV/Ca/AL light emitting diodes (PLED) was 46 (at 3 V) times that of the untreated sample. A typical turn‐on voltage of 2.5 V for MEH‐PPV PLED was able to decrease to 1 V after heat treatments, which was believed to result from the decrease of cis linkages in the polymer chains as revealed by the 1H NMR spectroscopy. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1705–1711, 2005

Single rodlike poly͓2-methoxy, 5-͑2Ј-ethyl-hexyloxy͒-p-phenylene-vinylene͔ ͑MEH-PPV͒, in which the chain length supports only two or three chromophores, was studied using optical spectroscopy in order to elucidate exciton dynamics within a single conjugated polymer. Emission at one to three discrete intensity levels coupled with on/off blinking was seen in the fluorescence time traces. Drops in emission intensity were accompanied in the majority of cases by an abrupt spectral blueshift and a corresponding increase in the fluorescence lifetime. Photon correlation measurements indicated that at any point in time there was only one emitter active. This suggests that the two or three chromophores in a short-chain polymer act as a typical donor-acceptor system with energy absorbed by the donors ͑shorter chromophores͒ transferring to the acceptor ͑longer chromophore͒ along the polymer backbone before emission. Once the acceptor is quenched, a donor takes turns to emit.

The presence of cis-vinylene bonds in Gilch-polymerized poly[2-methoxy-5-(2 0 -ethyl-hexyloxy)-1,4-phenylene vinylene] is reported. Through fractionation, species with a weight-average molecular weight of less than 37,000 exhibited an abnormal blueshift of photoluminescence spectra in toluene solutions, and this was attributed to the presence of cis-vinylene bonds, as verified by NMR spectroscopy. Surprisingly, the fractionated species ($1 wt %) with a weight-average molecular weight of 5000 were mostly linked by the cis-vinylene bonds. The concentration decreased with the molecular weight until a molecular weight of 37,000 was reached; at that point, the polymer chains contained mainly trans-vinylene bonds. Obviously, the formation of cis-vinylene bonds strongly inhibited the growth of polymer chains during Gilch polymerization.

Unsatisfactory dimensional control, distortion, and defects are frequently observed in powderinjection-molded parts, particularly after the solvent and thermal-debinding processing steps. One of the reasons is that the amount of soluble binder removed during the first step, solvent debinding, is not great enough to form interconnected pores throughout the compact, particularly in the core region. Thus, blistering, cracking, and bubbles can form easily during the subsequent thermal debinding. To determine the minimum debinding fraction required for solvent debinding, at which point interconnected pore channels are formed at the center, modeling of the distribution of the remaining soluble binder in the compact was established. The actual distribution, which was obtained by measuring the binder content layer by layer with the soxhelt extraction method, is in good agreement with the model. The modeling, bubble test, and fluorescence dye-penetration analysis show that, regardless of the compact thickness, the minimum bulk debinding fraction needed is consistently approximately 59 pct, yielding a local debinding fraction of 37 pct and a porosity of 8.5 pct at the center. This porosity is close to the value at which pores in a sintered compact transform from open to closed at the beginning of the final stage of sintering.

Impurities containing methylene bridges between 2‐((2′‐ethylhexyl)oxy)‐5‐methoxy‐benzene molecules are inevitably formed during the synthesis of 1,4‐bis(chloromethyl)‐2‐((2′‐ethylhexyl)oxy)‐5‐methoxy‐benzene, the monomer used in the preparation of poly[2‐methoxy‐5‐(2′‐ethyl‐hexyloxy)‐1,4‐phenylenevinylene] (MEH‐PPV), but they can be removed by double recrystallization of the monomer prior to polymerization. When impurities containing methylene bridges participate in a Gilch polymerization, the methylene bonds formed in the main chains are prone to break at 200 °C, that is, at least 150 °C below the major degradation temperature of defect‐free MEH‐PPV. Interestingly, the thermal treatment used to break the methylene bonds present reduces the chain aggregation of MEH‐PPV during film formation and induces its blends with poly(2,3‐diphenyl‐5‐octyl‐p‐phenylene‐vinylene) (DPO‐PPV) to form a morphology similar to that of block copolymers. Both significantly enhance the luminescence properties. Copyright © 2006 Society of Chemical Industry