Materials Science and Engineering Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA.
J Chem Phys. 2017 Oct 21;147(15):154902. doi: 10.1063/1.5006949.
We investigate the relaxation dynamics of thin polymer films at temperatures below the bulk glass transition T by first compressing polystyrene films supported on a polydimethylsiloxane substrate to create wrinkling patterns and then observing the slow relaxation of the wrinkled films back to their final equilibrium flat state by small angle light scattering. As with recent relaxation measurements on thin glassy films reported by Fakhraai and co-workers, we find the relaxation time of our wrinkled films to be strongly dependent on film thickness below an onset thickness on the order of 100 nm. By varying the temperature between room temperature and T (≈100 °C), we find that the relaxation time follows an Arrhenius-type temperature dependence to a good approximation at all film thicknesses investigated, where both the activation energy and the relaxation time pre-factor depend appreciably on film thickness. The wrinkling relaxation curves tend to cross at a common temperature somewhat below T, indicating an entropy-enthalpy compensation relation between the activation free energy parameters. This compensation effect has also been observed recently in simulated supported polymer films in the high temperature Arrhenius relaxation regime rather than the glassy state. In addition, we find that the film stress relaxation function, as well as the height of the wrinkle ridges, follows a stretched exponential time dependence and the short-time effective Young's modulus derived from our modeling decreases sigmoidally with increasing temperature-both characteristic features of glassy materials. The relatively facile nature of the wrinkling-based measurements in comparison to other film relaxation measurements makes our method attractive for practical materials development, as well as fundamental studies of glass formation.
我们通过在聚二甲基硅氧烷基底上支撑的聚苯乙烯薄膜中首先压缩产生褶皱图案，然后通过小角度光散射观察褶皱薄膜缓慢恢复到最终的平衡平面状态，从而研究低于本体玻璃化转变温度 T 的薄膜的松弛动力学。与 Fakhraai 及其同事最近报道的关于薄玻璃态薄膜的松弛测量结果类似，我们发现我们的褶皱薄膜的松弛时间强烈依赖于薄膜厚度，在约 100nm 的起始厚度以下。通过在室温至 T（≈100°C）之间改变温度，我们发现松弛时间几乎在所有研究的薄膜厚度下都很好地遵循了 Arrhenius 型温度依赖性，其中激活能和松弛时间前因子都明显依赖于薄膜厚度。褶皱松弛曲线在略低于 T 的共同温度下趋于交叉，表明激活自由能参数之间存在熵焓补偿关系。这种补偿效应最近也在高温 Arrhenius 松弛区而不是玻璃态的模拟支撑聚合物薄膜中观察到。此外，我们发现薄膜应力松弛函数以及褶皱脊的高度遵循扩展指数时间依赖性，并且我们的建模得出的短时间有效杨氏模量随着温度的升高呈指数衰减——这都是玻璃态材料的特征。与其他薄膜松弛测量相比，基于褶皱的测量相对容易，这使得我们的方法对于实际材料开发以及玻璃形成的基础研究都具有吸引力。
