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Ozone-induceddegradationAtmosphericozoneusuallycausesthedegradationofpolymersundercon...
Ozone-induced degradation
Atmospheric ozone usually causes the degradation of polymers under conditions that may be considered as normal;when other oxidative aging processes are very slow and the polymer retains its properties for a rather longer time.The presence of ozone in the air, even in very small concentrations,markedly accelerates the aging of polymeric materials.This process in saturated polymers is accompanied by the intensive formation of oxygen-containing compounds, by a change in the molecular weight and by impairment of the mechanical and electrical properties of the specimens.Exposure of polymers to ozone results in the rapid and consistent formation of a variety of carbonyl and unsaturated carbonyl products based on aliphatic esters, ketones, and lactones as well as aromatic carbonyl associated with the styrene phase. This follows by a more gradual formation of ether, hydroxyl and terminal vinyl groups with time and concentration.These reactions of ozone with polymers occur with main chains containing C]C bonds, aromatic rings or saturated hydrocarbon links. The reaction proceeds through unstable intermediates such as the bipolar ion or peroxy radicals, which can isomerize or degrade, cause decomposition of macromolecules.Under the influence of external load, the rate of ozone oxidation and hydrogen atom abstraction from the polymer chain increases.This is due to the change in hybridization of the reaction centre from the state sp3 to the state sp2.In poly(vinyl alcohol) (PVAL) the chain scission is based on the ozone oxidation of the alcoholic groups of PVAL with formation of ketone groups which in turn are the source of a ketoeenol tautomerism which leads to random chain scission by further ozone attack.PVAL forms a strong hydrogen-bond complex with ozone for which the interaction energy is of the order of 47.3 kJ/mol and it is slowly degraded by ozone.The analysis with FTIR spectra indicates that the final product is a PVAL oligomer with numerous ketone groups along the main oligomer backbone and with carboxylic end groups.
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Atmospheric ozone usually causes the degradation of polymers under conditions that may be considered as normal;when other oxidative aging processes are very slow and the polymer retains its properties for a rather longer time.The presence of ozone in the air, even in very small concentrations,markedly accelerates the aging of polymeric materials.This process in saturated polymers is accompanied by the intensive formation of oxygen-containing compounds, by a change in the molecular weight and by impairment of the mechanical and electrical properties of the specimens.Exposure of polymers to ozone results in the rapid and consistent formation of a variety of carbonyl and unsaturated carbonyl products based on aliphatic esters, ketones, and lactones as well as aromatic carbonyl associated with the styrene phase. This follows by a more gradual formation of ether, hydroxyl and terminal vinyl groups with time and concentration.These reactions of ozone with polymers occur with main chains containing C]C bonds, aromatic rings or saturated hydrocarbon links. The reaction proceeds through unstable intermediates such as the bipolar ion or peroxy radicals, which can isomerize or degrade, cause decomposition of macromolecules.Under the influence of external load, the rate of ozone oxidation and hydrogen atom abstraction from the polymer chain increases.This is due to the change in hybridization of the reaction centre from the state sp3 to the state sp2.In poly(vinyl alcohol) (PVAL) the chain scission is based on the ozone oxidation of the alcoholic groups of PVAL with formation of ketone groups which in turn are the source of a ketoeenol tautomerism which leads to random chain scission by further ozone attack.PVAL forms a strong hydrogen-bond complex with ozone for which the interaction energy is of the order of 47.3 kJ/mol and it is slowly degraded by ozone.The analysis with FTIR spectra indicates that the final product is a PVAL oligomer with numerous ketone groups along the main oligomer backbone and with carboxylic end groups.
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臭氧引起的降解
正常情况下,当其他的氧化、老化过程非常慢,聚合物可以在相当长的时期内保持自身的特性,而大气中的臭氧则可诱发聚合物的降解。大气中存在的臭氧,即使很低的浓度,也将显著地加速聚合材料的老化。饱和聚合物的这一过程伴随着大量的含氧化合物的形成,并改变分子的质量、破坏材料的机械和导电性能。暴露在臭氧环境中的聚合物将迅速地相继形成大量含有羰基和基于脂肪脂、酮类和内脂以及带有苯乙烯键的芳香羰基的不饱和羰基产物。此前,随着时间和浓度,逐渐形成更多的醚类、羟基和乙烯基作为终端的产物。臭氧与聚合物产生这些反应,与主链上含有含C C键、苯环或饱和烃键有关。这一反应过程中,双极离子或过氧自由基等不稳定介质能引发异构或降解,从而导致高分子的分解。在这种外部环境的影响下,臭氧的氧化速率和聚合物链上氢原子的转移速率均有所增加。这是由于反应中心的杂化轨道从sp3变到了sp2。聚乙烯醇(PVAL)的乙醇基在臭氧的氧化作用下形成酮基,并依次成为酮烯醇互变异构现象的源头,在臭氧的进一步攻击下导致链无规则断裂,从而产生断链作用。PVAL形成了强大的氢键与臭氧的联合体,其交互作用的能量达到47.3 kJ/mol,可以慢慢通过臭氧的氧化而降解。红外光谱的分析表明,最终产品是低聚物主链上带有大量酮基和羧基作为终端的PVAL低聚物。
正常情况下,当其他的氧化、老化过程非常慢,聚合物可以在相当长的时期内保持自身的特性,而大气中的臭氧则可诱发聚合物的降解。大气中存在的臭氧,即使很低的浓度,也将显著地加速聚合材料的老化。饱和聚合物的这一过程伴随着大量的含氧化合物的形成,并改变分子的质量、破坏材料的机械和导电性能。暴露在臭氧环境中的聚合物将迅速地相继形成大量含有羰基和基于脂肪脂、酮类和内脂以及带有苯乙烯键的芳香羰基的不饱和羰基产物。此前,随着时间和浓度,逐渐形成更多的醚类、羟基和乙烯基作为终端的产物。臭氧与聚合物产生这些反应,与主链上含有含C C键、苯环或饱和烃键有关。这一反应过程中,双极离子或过氧自由基等不稳定介质能引发异构或降解,从而导致高分子的分解。在这种外部环境的影响下,臭氧的氧化速率和聚合物链上氢原子的转移速率均有所增加。这是由于反应中心的杂化轨道从sp3变到了sp2。聚乙烯醇(PVAL)的乙醇基在臭氧的氧化作用下形成酮基,并依次成为酮烯醇互变异构现象的源头,在臭氧的进一步攻击下导致链无规则断裂,从而产生断链作用。PVAL形成了强大的氢键与臭氧的联合体,其交互作用的能量达到47.3 kJ/mol,可以慢慢通过臭氧的氧化而降解。红外光谱的分析表明,最终产品是低聚物主链上带有大量酮基和羧基作为终端的PVAL低聚物。
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臭氧导致退化 通常造成大气臭氧降解的高分子聚合物的条件下可以被看作是正常的;当其它氧化老化过程是非常慢,保持它的性质的聚合物相当长的时间在场的空气中的臭氧浓度,甚至在很小,显著加速老化的高分子材料这个过程是伴随着饱和聚合形成含氧化合物,密集而改变的分子量及损伤力学和电学性能的标本暴露在高分子材料的快速、臭氧一致的各种化合物构成的基础上,饱和脂肪族酯羰基产品、酮、和lactones芳香族化合物以及与苯乙烯的阶段。这是由一个更加渐进形成醚、羟基和终端乙烯基组与时间和浓度这些反应的发生主要与聚合物臭氧的枷锁
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