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 ( T g ) of polymer glass is lower near the free surface. However, the experimental T g -gradients showed a linear variation with depth ( z ) from the free surface, while the simulated equilibrium T g -gradients exhibited a double exponential z -dependence. In typical simulations, T g is determined based on the relaxation time of the system reaching a prescribed threshold value at equilibrium. Conversely, the experiments determined T g 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 T g -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 z -dependence consistent with a linear T g -gradient near the free surface. We further validate the impact of nonequilibrium effects by studying the dependence of the T g on the heating/cooling rate for polymer films of varying thickness ( h ). Our experimental results reveal significant variations in the T g -heating/cooling rate dependence with h below the bulk T g , 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 T g of a polymer when placed next to a softer material.

中文翻译：

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混合平衡/非平衡效应控制聚合物玻璃的表面迁移率


使用角度分辨 X 射线光电子能谱、和频产生振动光谱、接触角测量和分子动力学模拟，我们验证了聚合物玻璃的玻璃化转变温度 （Tg） 在自由表面附近较低。然而，实验 Tg 梯度显示出与自由表面的深度 （ z ） 的线性变化，而模拟的平衡 Tg 梯度表现出双指数 z 依赖性。在典型的模拟中，Tg 是根据系统的弛豫时间在平衡时达到规定的阈值来确定的。相反，实验通过观察分子迁移率在加热过程中从动力学停滞、非平衡玻璃态的解冻来确定 Tg。为了研究非平衡效应对 Tg 梯度的影响，我们缩短了仿真中的热退火时间，使系统失去平衡。我们观察到弛豫时间的减少和修正的 z 依赖性的出现，这与自由表面附近的线性 Tg 梯度一致。我们通过研究 Tg 对不同厚度 （ h 的聚合物薄膜的加热/冷却速率的依赖性） 进一步验证了非平衡效应的影响。我们的实验结果揭示了 h 低于本体 Tg 的 Tg 加热/冷却速率依赖性存在显着变化，当模拟系统不平衡时，在模拟中也可以观察到这种变化。我们通过在模拟中观察到的非平衡条件下系统内部区域内质量密度的降低来解释我们的发现，最近的研究表明，当将聚合物放置在较软的材料旁边时，聚合物的局部 Tg 会降低。