Transition between Different Diffusion Regimes and Its Relationship with Structural Properties in Nafion by High Field Diffusion NMR in Combination with Small-Angle X-ray and Neutron Scattering
Transition between Different Diffusion Regimes and Its Relationship with Structural Properties in Nafion by High Field Diffusion NMR in Combination with Small-Angle X-ray and Neutron Scattering
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DOI：
10.1021/acs.jpcb.0c07249
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发表时间：
2020-10-08
影响因子：
3.3
通讯作者：
Vasenkov, Sergey
中科院分区：
文献类型：
--
作者：
Berens, Samuel J.;Yahya, Ahmad;Vasenkov, Sergey
脉冲场梯度（PFG）NMR在高场被用来直接观察两个不同的扩散制度之间的过渡，在Nafion 117膜加载水和丙酮。虽然水的自扩散系数在小水负荷被观察到扩散时间无关的限制小和大的扩散时间，它表现出显着的下降，随着扩散时间的增加，在中间的时间对应于根均方位移的顺序为几微米。在我们的实验条件下，没有自扩散系数对扩散时间的依赖性被发现在大的水负荷和丙酮在所有研究的丙酮负荷的水。在小的水浓度的扩散时间依赖性的自扩散系数g的解释是由存在的有限域的互连水通道的尺寸在几个微米的范围内，形成在Nafion中的丙酮的存在下。根据测量的PFG NMR数据，估计了分离相邻畴的输运势垒的畴尺寸和磁导率。在大的水浓度下，水通道形成完全互连的网络，导致与时间无关的自扩散。没有这样一个类似的澄清过渡与丙酮的分子浓度增加是由于在Nafion中的水和丙酮扩散的可用区域的差异。扩散数据与使用小角X射线和中子散射技术获得的结构数据相关并得到支持。这些技术揭示了不同的水通道的径向尺寸在纳米范围内增加后加水，而丙酮似乎是在界面的全氟醚区域，减少了径向通道尺寸的大小。
Pulsed field gradient (PFG) NMR at high field was utilized to directly observe a transition between two different diffusion regimes in a Nafion 117 membrane loaded with water and acetone. Although water self-diffusivity at small water loadings was observed to be diffusion time-independent in the limit of small and large diffusion times, it showed a significant decrease with increasing diffusion time at intermediate times corresponding to root mean square displacements on the order of several microns. Under our experimental conditions, no self-diffusivity dependence on diffusion time was found for water at large water loadings and for acetone at all studied acetone loadings. The diffusion time-dependent self-diffusivity g at small water concentration is explained by the existence of finite domains of interconnected water channels with sizes in the range of several microns that form in Nafion in the presence of acetone. The domain sizes and permeance of transport barriers separating adjacent domains are estimated based on the measured PFG NMR data. At large water concentrations, the water channels form a fully interconnected network, resulting in time-independent self-diffusivity. The absence of such a percolation-like transition with increasing molecular concentration for acetone is attributed to a difference in the regions available for water and acetone diffusion in Nafion. The diffusion data are correlated with and supported by structural data obtained using small-angle X-ray and neutron scattering techniques. These techniques reveal distinct water channels with radial dimensions in the nanometer range increasing upon water addition, while acetone appears to be in an interfacial perfluoroether region, reducing the size of the radial channel dimension.