Zhang Ruibin, Fall William S, Hall Kyle Wm, Gehring Gillian A, Zeng Xiangbing, Ungar Goran
State Key Laboratory for Mechanical Behavior of Materials, Shaanxi International Research Centre for Soft Materials, Xi'an Jiaotong University, Xi'an, China.
Department of Materials Science and Engineering, University of Sheffield, Sheffield, UK.
Nat Commun. 2021 Mar 17;12(1):1710. doi: 10.1038/s41467-021-21799-9.
Condensed matter textbooks teach us that melting cannot be continuous and indeed experience, including with polymers and other long-chain compounds, tells us that it is a strongly first-order transition. However, here we report nearly continuous melting of monolayers of ultralong n-alkane CH on graphite, observed by AFM and reproduced by mean-field theory and MD simulation. On heating, the crystal-melt interface moves steadily and reversibly from chain ends inward. Remarkably, the final melting point is 80 K above that of the bulk, and equilibrium crystallinity decreases continuously from ~100% to <50% prior to final melting. We show that the similarity in melting behavior of polymers and non-polymers is coincidental. In the bulk, the intermediate melting stages of long-chain crystals are forbidden by steric overcrowding at the crystal-liquid interface. However, there is no crowding in a monolayer as chain segments can escape to the third dimension.
凝聚态物质教材告诉我们，熔化不可能是连续的，而实际经验，包括对聚合物和其他长链化合物的研究，也表明这是一个强烈的一级相变。然而，在此我们报告了通过原子力显微镜（AFM）观察到并由平均场理论和分子动力学（MD）模拟重现的石墨上超长正构烷烃CH单层几乎连续的熔化过程。加热时，晶体 - 熔体界面从链端向内稳定且可逆地移动。值得注意的是，最终熔点比体相高80 K，并且在最终熔化之前，平衡结晶度从约100%连续降至<50%。我们表明，聚合物和非聚合物熔化行为的相似性是偶然的。在体相中，长链晶体的中间熔化阶段因晶体 - 液体界面处的空间拥挤而被禁止。然而，在单层中不存在拥挤现象，因为链段可以逃逸到三维空间。
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