Search

The factors affecting the bound acrylonitrile content of NBR product during the polyme-rization process were investigated, including monomer reactivity ratio, monomer conversion, monomer concentration, and polymerization temperature and pressure, the structural composition of the product was analyzed, and the polymerization reaction kinetics and repeatability experiment were carried out. The results showed that under the polymerization temperature of 5.0 ℃ and polymerization pressure of 0.15-0.20 MPa, NBR products with uniform distribution of acrylonitrile structure units and mass fraction of bound acrylonitrile of 17%-19% could be obtained by using multi-stage addition of acrylonitrile and controlling the monomer conversion of 70%-75%. The polymerization formula and process had good repeatability.

Butadiene-acrylonitrile-butylacrylate terpolymer (BNBR) was synthesized by emulsion polymerization method. The effects of amount of initiator, emulsifier and regulator of relative molecular weight on the emulsion polymerization rate were investigated. And the microstructure and dynamic mechanical properties were characterized. The results showed that the polymerization rate increased and the reaction induction period shortened with increasing polymerization temperature. The increasing amount of initiator and emulsifier could increase the reaction rate, while the change in the amount of relative molecular weight regulator had little effect on the reaction rate. The apparent activation energy of the BNBR was 27.97 kJ/mol, and the particle size distribution of BNBR latex was uniform. The glass transition temperature of BNBR was lower than that of nitrile rubber（NBR）with a difference of about 10 ℃. BNBR had better dynamic mechanical pro-perties at low temperature than NBR.

Star shaped miktoarm isoprene-butadiene-styrene rubber with high wet skid resistance and low rolling resistance was prepared by living anionic solution polymerization with n-butyl lithium as initiator, cyclohexane as solvent, sodium dodecylbenzene sulfonate/tetramethylethylenediamine as complex regulator and divinylbenzene as coupling agent, the effects of polymerization temperature and amount of complex structure regulator and coupling agent on microstructure, dynamic mechanical pro-perties and mechanical properties of the polymer were investigated. The results showed that the content of non-1,4-unit in the polymer increased gradually with increase of the amount of complex regulator, and decreased with rise of polymerization temperature. The loss factor of the polymer at 0 ℃ increased gradually with increase of the amount of complex regulator and dropping of polymerization temperature, which improved wet resistance of the polymer; The loss factor at 60 ℃ decreased gradually with increase of the amount of coupling agent, which reduced rolling resistance of the polymer. In addition, molecular weight distribution of the polymer became wider after coupled and its processability was improved.

Canopy scale fluxes of isoprene and monoterpenes were investigated in both wet and dry seasons above a rubber tree (Hevea brasiliensis)/secondary tropical forest in the Yunnan province of southwestern China. Drought conditions were unusually high during the dry season experiment. The eddy covariance measurement technique was used to measure isoprene fluxes, while monoterpene fluxes were modeled based on leaf level emission measurements. Maximum observed isoprene fluxes occurred during the wet season and daytime average fluxes were about 1 mg C m -2 h -1. Dry season fluxes were much lower with a daytime average of 0.15 mg C m -2 h -1. Wet season isoprene fluxes compare quite well with isoprene fluxes observed from other tropical forests. Monoterpene fluxes came, almost entirely, from Hevea brasiliensis, which is a light-dependent monoterpene emitter. Modeled wet season total monoterpene fluxes were about 2 mg C m -2 h -1 (average for the daytime), and in the dry season were undetectable. Extreme drought conditions, and the drought deciduous nature of Hevea brasiliensis may be the cause of the low dry season fluxes. © 2004 Elsevier Ltd. All rights reserved.

The SrWO4:RE3+ (RE = Eu, Sm, Pr) fluorescent powders are synthesized using a high-temperature solid-state method. The phase structures and morphologies of the fluorescent powders are examined using X-ray diffraction and field-emission scanning electron microscopy, respectively. The results reveal that pure phase SrWO4 can be prepared via calcination at 900oC for 4 h and that doping with rare-earth ions has no effect on the crystal phase of the fluorescent powders; SrWO4 powders doped with three rare-earth ions all form spherical-like structures. The luminescence properties of the samples are evaluated using a fluorescence spectrometer, and the excitation and emission spectra of the SrWO4:RE3+ (RE = Eu, Sm, Pr) fluorescent powders are analyzed. The results indicate that SrWO4:Eu3+ fluorescent powder can be effectively excited by ultraviolet light at 395 nm and emit red light at 615 nm when a doping concentration of Eu3+ is 7%; Sm3+- and Pr3+-doped SrWO4 can emit red light at 650 and 687 nm when excited at 406 and 460 nm, respectively, with a doping concentration of 5% each. A W-LED device with SrWO4:Eu3+ pink fluorescent powder and a 395-nm UV chip is successfully assembled. The chromaticity coordinates of the device are (0.3055, 0.3532), exhibiting prominent white light. SrWO4:RE3+ (RE = Eu, Sm, Pr) fluorescent powders are expected to be red suorescent powders for UV-excited white LEDs. [ABSTRACT FROM AUTHOR]