Xu Jianquan, Ghanekarade Asieh, Li Li, Zhu Huifeng, Yuan Hailin, Yan Jinsong, Simmons David S, Tsui Ophelia K C, Wang Xinping
School of Chemistry and Chemical Engineering, Key Laboratory of Surface & Interface Science of Polymer Materials of Zhejiang Province, Zhejiang Sci-Tech University, Hangzhou 310018, China.
Department of Chemical, Biological, and Materials Engineering, University of South Florida, Tampa, FL 33620.
Proc Natl Acad Sci U S A. 2024 Oct 8;121(41):e2406262121. doi: 10.1073/pnas.2406262121. Epub 2024 Oct 3.
Using angle-resolved X-ray photoelectron spectroscopy, sum-frequency generation vibrational spectroscopy, contact angle measurements, and molecular dynamics simulations, we verify that the glass transition temperature () of polymer glass is lower near the free surface. However, the experimental -gradients showed a linear variation with depth () from the free surface, while the simulated equilibrium -gradients exhibited a double exponential -dependence. In typical simulations, is determined based on the relaxation time of the system reaching a prescribed threshold value at equilibrium. Conversely, the experiments determined by observing the unfreezing of molecular mobility during heating from a kinetically arrested, nonequilibrium glassy state. To investigate the impact of nonequilibrium effects on the -gradient, we reduced the thermal annealing time in simulations, allowing the system to fall out of equilibrium. We observe a decrease in the relaxation time and the emergence of a modified -dependence consistent with a linear -gradient near the free surface. We further validate the impact of nonequilibrium effects by studying the dependence of the on the heating/cooling rate for polymer films of varying thickness (). Our experimental results reveal significant variations in the -heating/cooling rate dependence with below the bulk , which are also observed in simulation when the simulated system is not equilibrated. We explain our findings by the reduction in mass density within the inner region of the system under nonequilibrium conditions, as observed in simulation, and recent research indicating a decrease in the local of a polymer when placed next to a softer material.
我们使用角分辨X射线光电子能谱、和频振动光谱、接触角测量以及分子动力学模拟，证实了聚合物玻璃的玻璃化转变温度（）在自由表面附近较低。然而，实验测得的 - 梯度随距自由表面深度（）呈线性变化，而模拟得到的平衡 - 梯度则呈现双指数 - 依赖性。在典型模拟中，基于系统在平衡时达到规定阈值的弛豫时间来确定。相反，实验是通过观察从动力学受阻的非平衡玻璃态加热过程中分子迁移率的解冻来确定。为了研究非平衡效应对 - 梯度的影响，我们在模拟中缩短了热退火时间，使系统偏离平衡。我们观察到弛豫时间减少，并且出现了与自由表面附近线性 - 梯度一致的修正 - 依赖性。我们通过研究不同厚度（）聚合物薄膜的 对加热/冷却速率的依赖性，进一步验证了非平衡效应的影响。我们的实验结果表明，在本体 以下， - 加热/冷却速率依赖性随 有显著变化，在模拟未达到平衡的系统时也观察到了这种现象。我们通过模拟观察到的非平衡条件下系统内部区域质量密度的降低，以及近期研究表明聚合物与较软材料相邻时局部 的降低，来解释我们的发现。
