Lim Seong Y, Sahimi Muhammad, Tsotsis Theodore T, Kim Nayong
Mork Family Department of Chemical Engineering & Materials Science, University of Southern California, Los Angeles, California 90089-1211, USA.
Phys Rev E Stat Nonlin Soft Matter Phys. 2007 Jul;76(1 Pt 1):011810. doi: 10.1103/PhysRevE.76.011810. Epub 2007 Jul 31.
Extensive molecular dynamics (MD) simulations were carried out to compute the solubilities and self-diffusivities of CO2 and CH4 in amorphous polyetherimide (PEI) and mixed-matrix PEI generated by inserting single-walled carbon nanotubes into the polymer. Atomistic models of PEI and its composites were generated using energy minimizations, MD simulations, and the polymer-consistent force field. Two types of polymer composite were generated by inserting (7,0) and (12,0) zigzag carbon nanotubes into the PEI structure. The morphologies of PEI and its composites were characterized by their densities, radial distribution functions, and the accessible free volumes, which were computed with probe molecules of different sizes. The distributions of the cavity volumes were computed using the Voronoi tessellation method. The computed self-diffusivities of the gases in the polymer composites are much larger than those in pure PEI. We find, however, that the increase is not due to diffusion of the gases through the nanotubes which have smooth energy surfaces and, therefore, provide fast transport paths. Instead, the MD simulations indicate a squeezing effect of the nanotubes on the polymer matrix that changes the composite polymers' free-volume distributions and makes them more sharply peaked. The presence of nanotubes also creates several cavities with large volumes that give rise to larger diffusivities in the polymer composites. This effect is due to the repulsive interactions between the polymer and the nanotubes. The solubilities of the gases in the polymer composites are also larger than those in pure PEI, hence indicating larger gas permeabilities for mixed-matrix PEI than PEI itself.
进行了广泛的分子动力学（MD）模拟，以计算二氧化碳（CO₂）和甲烷（CH₄）在非晶态聚醚酰亚胺（PEI）以及通过将单壁碳纳米管插入聚合物中生成的混合基质PEI中的溶解度和自扩散系数。使用能量最小化、MD模拟和聚合物一致力场生成了PEI及其复合材料的原子模型。通过将（7,0）和（12,0）锯齿形碳纳米管插入PEI结构中生成了两种类型的聚合物复合材料。PEI及其复合材料的形态通过其密度、径向分布函数和可及自由体积来表征，这些是用不同尺寸的探针分子计算得出的。使用Voronoi镶嵌方法计算了空腔体积的分布。计算得出的聚合物复合材料中气体的自扩散系数比纯PEI中的大得多。然而，我们发现这种增加并非由于气体通过具有光滑能量表面、因而提供快速传输路径的纳米管扩散所致。相反，MD模拟表明纳米管对聚合物基质有挤压作用，这改变了复合聚合物的自由体积分布并使其峰值更尖锐。纳米管的存在还产生了几个大体积的空腔，这导致聚合物复合材料中具有更大的扩散系数。这种效应是由于聚合物与纳米管之间的排斥相互作用。气体在聚合物复合材料中的溶解度也比在纯PEI中的大，因此表明混合基质PEI比PEI本身具有更大的气体渗透率。
