Moore Alex R, Huang Georgia, Wolf Sarah, Walsh Patrick J, Fakhraai Zahra, Riggleman Robert A
Department of Chemical and Biomolecular Engineering, University of Pennsylvania, Philadelphia, PA 19104.
Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104.
Proc Natl Acad Sci U S A. 2019 Mar 26;116(13):5937-5942. doi: 10.1073/pnas.1821761116. Epub 2019 Mar 13.
Glasses formed by physical vapor deposition (PVD) are an interesting new class of materials, exhibiting properties thought to be equivalent to those of glasses aged for thousands of years. Exerting control over the structure and properties of PVD glasses formed with different types of glass-forming molecules is now an emerging challenge. In this work, we study coarse-grained models of organic glass formers containing fluorocarbon tails of increasing length, corresponding to an increased tendency to form microstructures. We use simulated PVD to examine how the presence of the microphase-separated domains in the supercooled liquid influences the ability to form stable glasses. This model suggests that increasing molecule tail length results in decreased thermodynamic stability of the molecules in PVD films. The reduced stability is further linked to the reduced ability of these molecules to equilibrate at the free surface during PVD. We find that, as the tail length is increased, the relaxation times near the surface of the supercooled equilibrium liquid films of these molecules are slowed and become essentially bulk-like, due to the segregation of the fluorocarbon tails to the free surface. Surface diffusion is also markedly reduced due to clustering of the molecules at the surface. Based on these results, we propose a trapping mechanism where tails are unable to move between local phase-separated domains on the relevant deposition time scales.
通过物理气相沉积（PVD）形成的玻璃是一类有趣的新型材料，其展现出的性能被认为等同于经过数千年老化的玻璃。如今，控制由不同类型玻璃形成分子构成的PVD玻璃的结构和性能成为了一个新出现的挑战。在这项工作中，我们研究了含有长度不断增加的碳氟链尾巴的有机玻璃形成剂的粗粒度模型，碳氟链尾巴长度增加对应着形成微观结构的趋势增强。我们使用模拟物理气相沉积来研究过冷液体中微相分离域的存在如何影响形成稳定玻璃的能力。该模型表明，分子尾巴长度增加会导致PVD薄膜中分子的热力学稳定性降低。稳定性降低还与这些分子在物理气相沉积过程中在自由表面达到平衡的能力下降有关。我们发现，随着尾巴长度增加，这些分子的过冷平衡液膜表面附近的弛豫时间变慢，并基本上变得类似本体，这是由于碳氟链尾巴向自由表面偏析所致。由于分子在表面聚集，表面扩散也显著降低。基于这些结果，我们提出了一种俘获机制，即在相关沉积时间尺度上，尾巴无法在局部相分离域之间移动。
