Lin Guoxing, Jones Alan A
Department of Chemistry, The Carlson School of Chemistry and Biochemistry, Clark University, Worcester, MA 01610-1400, USA.
Solid State Nucl Magn Reson. 2004 Sep;26(2):87-98. doi: 10.1016/j.ssnmr.2004.06.001.
Xenon-129 spectra in some heterogeneous polymer systems consist of two resonances which collapse to a single resonance as a function of temperature. Two different resonances arise from spatially separated, distinct sorption environments and spectral collapse occurs when xenon atoms diffuse from one environment to the other at a sufficiently fast rate. This exchange mechanism involves a distribution of time constants and a two domain lattice model is used to generate a realistic distribution of correlation times resulting from diffusion in a heterogeneous matrix. The distribution of correlation times is inhomogeneous in the sense that different xenon atoms would exchange between the two domains or environments with a variety of time constants and the resulting spectrum is a superposition of spectra associated with each of the time constants. To demonstrate the nature of exchange according to this model, diffusion out of a sphere is simulated which corresponds to a progressive saturation experiment used to determine the diffusion constant of xenon in polystyrene. Then the model is used to demonstrate the difference between homogeneous and heterogeneous spectral collapse in one- and two-dimensional examples. Lastly, the simulation model is used to interpret one- and two- dimensional xenon-129 line shape changes for xenon sorbed into poly(2,6-dimethyl-1,4-phenylene oxide) as a function of temperature. Two broad resonances are observed at low temperatures in this polymer corresponding to xenon-129 sorbed in high free volume and low free volume domains. Exchange between the two main resonances collapses the spectrum to a single peak at higher temperatures. Both the collapse in one dimension and exchange in two dimensions as a function of mixing time can be simulated using the distribution from the lattice model. An average domain size of 70 nm is estimated by combining the simulation of the exchange experiment with the results of a one-dimensional progressive saturation experiment. The size of the sites sorbing individual xenon atoms has been reported from positron annihilation lifetime spectroscopy as 1.4 nm for the high free volume sites and 0.3 nm for the low free volume sites. The domain size is more than an order of magnitude larger than the individual sorption site indicating that domains consist of many sites as assumed in the lattice model description.
一些非均相聚合物体系中的氙-129光谱由两个共振峰组成，这两个共振峰随温度变化会合并为一个共振峰。两种不同的共振峰源于空间上分离的、不同的吸附环境，当氙原子以足够快的速率从一种环境扩散到另一种环境时，就会发生光谱合并。这种交换机制涉及时间常数的分布，并且使用双域晶格模型来生成由非均相基质中的扩散导致的相关时间的实际分布。相关时间的分布是不均匀的，因为不同的氙原子会以各种时间常数在两个域或环境之间交换，并且所得光谱是与每个时间常数相关的光谱的叠加。为了根据该模型证明交换的性质，模拟了从球体中的扩散，这对应于用于确定氙在聚苯乙烯中扩散常数的渐进饱和实验。然后该模型用于在一维和二维示例中证明均匀和非均匀光谱合并之间的差异。最后，模拟模型用于解释吸附在聚（2,6-二甲基-1,4-苯醚）中的氙-129的一维和二维线形随温度的变化。在该聚合物中，在低温下观察到两个宽共振峰，对应于吸附在高自由体积和低自由体积域中的氙-129。两个主要共振峰之间的交换在较高温度下使光谱合并为一个单峰。使用晶格模型的分布可以模拟一维中的合并和二维中作为混合时间函数的交换。通过将交换实验的模拟与一维渐进饱和实验的结果相结合，估计平均域大小为70nm。正电子湮没寿命光谱法报道，吸附单个氙原子的位点大小对于高自由体积位点为1.4nm，对于低自由体积位点为0.3nm。域大小比单个吸附位点大一个数量级以上，这表明域由晶格模型描述中假设的许多位点组成。
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