Permeation of Penetrants in Nanocomposites: A Test of the Theory of Composites

这里提出的项目将对新兴的聚合物纳米复合材料领域产生广泛的影响，因为它将决定当前的复合材料理论是否适用于预测填充颗粒尺寸接近分子尺寸时的性能。此外，该项目将为各级学生提供一个工具，让他们学习如何回答新技术领域的关键问题，在这些领域，所有概念都必须受到质疑，使用结构良好的研究。正如这里所使用的，术语纳米复合材料是指含有分散良好的填充颗粒（球体、棒状或通常由无机材料组成的片状）的聚合物基体，其中至少有一个维度在一到几十纳米的范围内。这些颗粒的表面积非常大，它们之间的距离很小，这就提出了一个问题，即这些颗粒是否会通过干扰链的构象、片段的堆积或这些片段移动的力场来影响聚合物基体的性质或性能。成熟的数学理论用于预测常规复合材料的性能（刚度、渗透性等），保证了每一相的性能都是相同的，就像其他相不存在一样。我们必须质疑纳米复合材料中基体的局部性质是否真的与体态相同。我们建议通过测量各种气体和蒸汽渗透剂在精心制备和表征的纳米复合材料中的渗透率来探测这些局部特性来回答这个关键问题。任何给定渗透剂的绝对渗透率将取决于复合材料的详细形态（即，长宽比，体积分数和这些颗粒在基体中的排列）。然而，根据复合理论，一种渗透剂的渗透率与另一种渗透剂的渗透率之比在复合材料中应与在纯基体中相同。因此，系统准确地测量大范围不同尺寸渗透剂的渗透成为探索复合材料理论在纳米复合材料中的适用性的有力工具，即，微小的、高表面积的纳米颗粒是否会干扰局部性能。此外，这项工作将解决与开发纳米复合材料相关的问题，这些纳米复合材料可以用作屏障材料、分离膜和燃料电池。感兴趣的纳米颗粒将包括粘土的铝硅酸盐薄片（用于屏障材料）、二氧化硅和其他材料的球形颗粒（用于分离膜）、传导质子的颗粒（用于燃料电池）等。基质将是晶体或玻璃状聚合物，具有不同的链刚度和与纳米颗粒的相互作用。该研究将确定材料问题，确定何时基体性能受到填料的显著影响，何时不受影响。

The project proposed here will have broad impact on the emerging field of polymer nanocomposites since it will determine whether current theories of composites are applicable or not for predicting performance as the size of the filler particles approach molecular dimensions. Further, this project will provide a vehicle for students at all levels to learn how to answer key questions in new technical areas, where all concepts have to be questioned, using well-structured research.As used here, the term nanocomposites refers to a polymer matrix containing well-dispersed filler particles (spheres, rods, or platelets usually comprised of an inorganic material) where at least one dimension is in the range of one to tens of nanometers. The very high surface area of these particles and small distances between them raises the question of whether the particles may influence the nature or properties of the polymer matrix by interfering with the chain conformation, segmental packing, or the force field in which these segments move. The well-developed mathematical theories for predicting performance (stiffness, permeability, etc.) of conventional composites assure the properties for each phase are the same as if the other phase were not there. We must question whether the local properties of the matrix in nanocomposites are really the same as in the bulk state. We propose to answer this key question by probing these local properties by measuring the permeability of various gas and vapor penetrants in well-prepared and characterized nanocomposites. The absolute permeability of any given penetrant will depend on the detailed morphology of the composite (i.e., the aspect ratio, volume fraction, and arrangement of these particles in the matrix). However, according to composite theory, the ratio of the permeability of one penetrant to that of another should be the same in the composite as in the pure matrix. Thus, systematic and accurate measurement of the permeation of a wide range of penetrants of varying size becomes a powerful tool for exploring the applicability of composite theory to nanocomposites, i.e., whether local properties are perturbed by tiny, high surface area nanoparticles. Further, this work will address issues related to developing nanocomposites for applications as barrier materials and as membranes for separations and for fuel cells. The nanoparticles of interest will include the aluminosilicate platelets of clays (for barrier materials), spherical particles of silica and other materials (for separation membranes), particles that conduct protons (for fuel cells,), etc. The matrices will be either crystalline or glassy polymers of varying chain stiffness and interaction with the nanoparticles. The research will identify the materials issues that determine when the matrix properties are significantly affected by the filler and when they are not.