Glass transition dynamics and boiling temperatures of molecular liquids and their isomers.
Glass transition dynamics and boiling temperatures of molecular liquids and their isomers.
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DOI：
10.1021/jp0688254
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发表时间：
2007-02
期刊：
影响因子：
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通讯作者：
Li-Min Wang;R. Richert
中科院分区：
文献类型：
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作者：
Li-Min Wang;R. Richert
研究了各种玻璃化液体的动态温度和热力学温度、玻璃化转变温度Tg和沸点Tb之间的关系，重点研究了单羟基醇。众所周知，Tb和Tg在多种液体中呈正相关。然而，我们发现相同的量在同分异构体系列中呈负相关，即相同化学式的不同同分异构体，Tb随Tg的增加而减小。对于3 <或= n <或= 10的CnH2n+1OH醇系，如果将温度归一化到相应的正烷醇值，则得到Tg - Tb负相关的主曲线。这种同分异构体液体的Tg - Tb依赖性与熵效应有关，并负责对大量45 < Tg < 250k的有机玻璃分子形成物观察到的相关性的散射。测量三组异构体的介电弛豫：(a) 3-甲氧基-1-丁醇和2-异丙氧基乙醇，(b) 1,4-, 1,2-和2,4-戊二醇，以及(c)二-和二-异邻苯二甲酸丁酯。两个关键的动力学参数，脆性m和拉伸指数β，在适度不同的Tgs异构体中是无法区分的。易碎性差异越大，结构变化越明显，但在同分异构体系列中没有观察到系统的趋势。结果提供了一个有用的工具来评估Tg， m和β的边缘玻璃形成的基础上，他们的异构体。
The relation between a dynamic and a thermodynamic temperature, glass transition Tg and boiling point Tb, is investigated for various glass-forming liquids, with emphasis on monohydroxy alcohols. As is well known, Tb and Tg are positively correlated across a large variety of liquids. However, we found that the same quantities show a negative correlation within an isomeric series, i.e., Tb decreases with increasing Tg for different isomers of the same chemical formula. For the alcohol series, CnH2n+1OH with 3 < or = n < or = 10, a master curve of the negative Tg - Tb correlation is obtained if the temperatures are normalized to the respective values of the n-alkanols. This Tg - Tb dependence of isomeric liquids is linked to entropic effects and responsible for much of the scatter of the correlation observed for a large number of molecular organic glass-formers with 45 < Tg < 250 K. Dielectric relaxation is measured for three groups of isomers: (a) 3-methoxyl-1-butanol and 2-iso-propoxyethanol, (b) 1,4-, 1,2-, and 2,4-pentanediol, and (c) di-n- and di-iso-butyl phthalate. Two key parameters of the dynamics, fragility m and stretching exponent beta, are found to be indistinguishable within isomers of moderately different Tgs. Larger fragility differences are readily expected with pronounced structural change, but no systematic trend is observed within an isomer series. The results provide a useful tool for assessing Tg, m, and beta for marginal glass formers on the basis of their isomers.