Donostia International Physics Center, Paseo Manuel de Lardizabal 4, 20018 San Sebastián, Spain.
J Chem Phys. 2011 May 28;134(20):204906. doi: 10.1063/1.3592560.
We present a quasielastic neutron scattering (QENS) investigation of the component dynamics in an aqueous Poly(vinyl methyl ether) (PVME) solution (30% water content in weight). In the glassy state, an important shift in the Boson peak of PVME is found upon hydration. At higher temperatures, the diffusive-like motions of the components take place with very different characteristic times, revealing a strong dynamic asymmetry that increases with decreasing T. For both components, we observe stretching of the scattering functions with respect to those in the bulk and non-Gaussian behavior in the whole momentum transfer range investigated. To explain these observations we invoke a distribution of mobilities for both components, probably originated from structural heterogeneities. The diffusive-like motion of PVME in solution takes place faster and apparently in a more continuous way than in bulk. We find that the T-dependence of the characteristic relaxation time of water changes at T ≲ 225 K, near the temperature where a crossover from a low temperature Arrhenius to a high temperature cooperative behavior has been observed by broadband dielectric spectroscopy (BDS) [S. Cerveny, J. Colmenero and A. Alegría, Macromolecules, 38, 7056 (2005)]. This observation might be a signature of the onset of confined dynamics of water due to the freezing of the PVME dynamics, that has been selectively followed by these QENS experiments. On the other hand, revisiting the BDS results on this system we could identify an additional "fast" process that can be attributed to water motions coupled with PVME local relaxations that could strongly affect the QENS results. Both kinds of interpretations, confinement effects due to the increasing dynamic asymmetry and influence of localized motions, could provide alternative scenarios to the invoked "strong-to-fragile" transition.
我们提出了一个准弹性中子散射（QENS）研究的组件动力学在一个含水的聚（甲基乙烯基醚）（PVME）溶液（30 ％的水含量，重量）。在玻璃态，一个重要的转变玻峰在 PVME 水化后发现。在较高的温度下，组件的扩散状运动发生与非常不同的特征时间，揭示了一个强烈的动态不对称，随着 T 的降低而增加。对于两个组件，我们观察到散射函数相对于那些在体积和非高斯行为的拉伸在整个动量转移范围的研究。为了解释这些观察结果，我们调用了两个组件的迁移率分布，可能源于结构不均匀性。 PVME 在溶液中的扩散状运动发生得更快，显然在更连续的方式比在体积。我们发现，水的特征松弛时间的 T 依赖性在 T ≲ 225 K 时发生变化，在温度附近，已经通过宽带介电光谱（BDS）观察到从低温 Arrhenius 到高温协同行为的转变[ S. Cerveny，J. Colmenero 和 A. Alegría，Macromolecules，38，7056（2005）]。这种观察可能是由于 PVME 动力学的冻结而导致水的受限动力学的开始的特征，这已经被这些 QENS 实验选择性地跟踪。另一方面，重新审视该系统的 BDS 结果，我们可以识别出一个额外的“快”过程，它可以归因于与 PVME 局部松弛耦合的水运动，这可能强烈影响 QENS 结果。这两种解释，由于动态不对称性的增加而导致的限制效应和局部运动的影响，可以为所提出的“强到脆弱”转变提供替代方案。
