Another approach to the Gibbs-Thomson equation and the melting point of polymers and oligomers
Another approach to the Gibbs-Thomson equation and the melting point of polymers and oligomers
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DOI：
10.1016/s0032-3861(02)00305-1
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发表时间：
2002-08-01
期刊：
影响因子：
4.6
通讯作者：
Höhne, GWH
中科院分区：
文献类型：
--
作者：
Höhne, GWH
吉布斯-汤姆逊方程（英语：Gibbs-Thomson equation）是一个基于给定的熔化热、表面自由能和晶体的尺寸（厚度）的方程，被广泛用于解释层状晶体的熔化温度。与晶体的厚度相比，使用该方程不可能解释环烷烃和超高摩尔质量聚乙烯（UHMMPE）的非常高的熔融温度。另一种热力学方法的吉布斯-汤姆逊方程，从一个增量组成的焓和熵的链分子，提出。这描述了线性、折叠和环状烷烃以及UHMMPE的（片层）晶体的熔化温度，所有这些晶体都形成相同晶格类型的晶体，只有一组参数。基本变量原来是相应分子的CH 2-基团的数量，纳入微晶，而不是其厚度。这可以解释，如果我们假设熔融过程中所造成的构象动力学，这是更多地限制了更多的CH 2-基团参与链运动。在一定厚度的层状晶体中，环状烷烃“感觉”比正烷烃长，并且具有相邻或紧密折叠的线性分子感觉比无定形中具有随机分布的链和大环的线性分子长。这种方法有助于理解形成折叠链晶体的聚合物的熔融行为。它使环状和折叠的超长烷烃作为模型物质的折叠链晶体的聚乙烯，没有进一步的假设有关的表面能，并适合所有的调查结果顺利进入一个图片。(C)2002爱思唯尔科技有限公司。保留所有权利。
The common Gibbs-Thomson equation, widely used to explain the melting temperature of lamella crystals, is based on a given heat of fusion and a given surface free energy and the size (thickness) of the crystal. With this equation it is not possible to explain the, compared to the thickness of the crystals, very high melting temperature of cyclic alkanes and ultra-high molar mass polyethylene (UHMMPE). Another thermodynamic approach to the Gibbs-Thomson equation, starting from an incremental composition of enthalpy and entropy of the chain molecule, is presented. This describes the melting temperature of (lamella) crystals of linear, folded and cyclic alkanes as well as UHMMPE, all forming crystals of the same lattice type, with only one set of parameters. The essential variable turns out to be the number of CH2-groups of the respective molecule, incorporated into the crystallite, rather than its thickness. This may be explained if we assume the melting process caused by conformation dynamics which are more restricting the greater number of CH2-groups that are involved in the chain movement. In a lamella crystal of a certain thickness, a cyclic alkane 'feels' longer than an n-alkane, as well as a linear molecule with adjacent or tight folds feels longer than one with randomly distributed chains and large loops in the amorphous. This approach helps to understand the melting behavior of polymers forming folded-chain crystals. It enables the cyclic and folded ultra-long alkanes to serve as model substances for the folded-chain crystals of polyethylene without further assumptions concerning the surface energy and fits all findings smoothly into one picture. (C) 2002 Elsevier Science Ltd. All rights reserved.