Zhang Aixi, Moore Alex R, Zhao Haoqiang, Govind Shivajee, Wolf Sarah E, Jin Yi, Walsh Patrick J, Riggleman Robert A, Fakhraai Zahra
Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA.
Department of Chemical and Biomolecular Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA.
J Chem Phys. 2022 Jun 28;156(24):244703. doi: 10.1063/5.0087600.
Stable glasses (SGs) are formed through surface-mediated equilibration (SME) during physical vapor deposition (PVD). Unlike intermolecular interactions, the role of intramolecular degrees of freedom in this process remains unexplored. Here, using experiments and coarse-grained molecular dynamics simulations, we demonstrate that varying dihedral rotation barriers of even a single bond, in otherwise isomeric molecules, can strongly influence the structure and stability of PVD glasses. These effects arise from variations in the degree of surface mobility, mobility gradients, and mobility anisotropy, at a given deposition temperature (T). At high T, flexible molecules have access to more configurations, which enhances the rate of SME, forming isotropic SGs. At low T, stability is achieved by out of equilibrium aging of the surface layer. Here, the poor packing of rigid molecules enhances the rate of surface-mediated aging, producing stable glasses with layered structures in a broad range of T. In contrast, the dynamics of flexible molecules couple more efficiently to the glass layers underneath, resulting in reduced mobility and weaker mobility gradients, producing unstable glasses. Independent of stability, the flattened shape of flexible molecules can also promote in-plane orientational order at low T. These results indicate that small changes in intramolecular relaxation barriers can be used as an approach to independently tune the structure and mobility profiles of the surface layer and, thus, the stability and structure of PVD glasses.
稳定玻璃态（SGs）是在物理气相沉积（PVD）过程中通过表面介导平衡（SME）形成的。与分子间相互作用不同，分子内自由度在这一过程中的作用仍未得到探索。在这里，我们通过实验和粗粒度分子动力学模拟表明，即使在其他方面为同分异构体的分子中，改变单键的二面角旋转势垒，也会强烈影响PVD玻璃态的结构和稳定性。这些效应源于在给定沉积温度（T）下表面迁移率、迁移率梯度和迁移率各向异性的变化。在高温下，柔性分子可以获得更多构型，这提高了SME的速率，形成各向同性的SGs。在低温下，通过表面层的非平衡老化实现稳定性。在这里，刚性分子的堆积不佳提高了表面介导老化的速率，在很宽的温度范围内产生具有层状结构的稳定玻璃态。相比之下，柔性分子的动力学更有效地与下面的玻璃层耦合，导致迁移率降低和迁移率梯度减弱，产生不稳定的玻璃态。与稳定性无关，柔性分子的扁平形状在低温下也可以促进面内取向有序。这些结果表明，分子内弛豫势垒的微小变化可以作为一种方法来独立调节表面层的结构和迁移率分布，从而调节PVD玻璃态的稳定性和结构。
