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641.
Lei Shi Hideki Saitoh Yoshio Shibasaki 《Journal of polymer science. Part A, Polymer chemistry》2000,38(15):2794-2803
Thermal degradation of two series of polyacrylates containing long fluorocarbon chains [abbr.: PFnA {HCF2(CF2)n−1 CH2 O C(O) , n = 4, 6, 8, 10} and abbr.: PFFnEA {CF3(CF2)n−1 CH2CH2 O C(O) , n = 6, 8, 10}] was investigated by TG /FTIR. Thermal degradation behavior of polymers changed depending on the type of tie groups, which link the fluorocarbon chains to the main chain, and also on the length of fluorocarbon chains. It was clarified that the apparent activation energies (ΔEa ) of PFnA series obtained by Ozawa's method varied in the order of PF4A > PF6A > PF8A > PF10A, while those of PFFnEA series having tie group of CH2 CH2 O C(O) were almost constant. The results for PFnA series (tie group: CH2 O C(O) ) are attributable to the shield effect of long fluorocarbon chains on the back‐biting reaction in the thermal degradation of comb polymers rather than the change of C C bond dissociation energy in the main chain. It was found that TG curves of PFFnEA series were shifted to the lower temperature region than those of PFnA. This result can be attributable to the scission of side groups followed by the evaporation of fluorocarbon compounds and carbon dioxide. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 2794–2803, 2000 相似文献
642.
Hans R. Kricheldorf Andrea Stricker Martina Lossin 《Journal of polymer science. Part A, Polymer chemistry》1999,37(13):2179-2189
2,2-Dibutyl-2-stanna-1,3-dioxepane (DSDOP) was used as cyclic initiator for the polymerization of trimethylene carbonate (TMC). The polymerizations were either conducted in concentrated chlorobenzene solution at 50 and 80°C or in bulk at 60 and 120°C. With monomer/initiator ratios ≤100 the conversion was complete within 2 h at 80°C and within 12 h at 50°C. Variation of the reaction time revealed that the rapid polymerization is followed by a relatively rapid (backbiting) degradation even at 80°C. The polymerizations in bulk at 60°C were somewhat slower than those at 80°C in solution, but the influence of degradation reactions was less pronounced. With optimized reaction time the number average molecular weight (Mn) roughly parallels the monomer/initiator ratio and Mn's up to 100,000 were obtained. In contrast to a classical living polymerization broader polydispersities (1.5–1.7) were found. In the case of 5,5-dimethyltrimethylene carbonate rapid degradation and chain transfer reactions prevented the formation of high molecular weight polymers. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 2179–2189, 1999 相似文献
643.
北京谱仪中各探测器电子学系统向触发系统传输大量并行信号,需要采用并串结合型光纤传输方法,以达到断开地回路,提高系统及线路抗干扰能力,延长传输距离的目的。对此种传输方法的误码率、报错与恢复、延时一致性和相位稳定性等关键问题进行了深入研究,得到确切参数,并提出了单路出错自恢复同步的解决方法,增强了系统的耐用性。同时发现各线路间传输延迟有半个周期的不一致,因此在接收端应把采样时钟上升沿调整在所有解出数据都稳定的时刻。此方法可满足北京谱仪工程的实际需要。 相似文献
644.