ROAMing in mutable voids: Polymer free volumes from wobbling vibrational probes
ROAMing in mutable voids: Polymer free volumes from wobbling vibrational probes
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在可变空隙中漫游：来自摆动振动探针的聚合物自由体积
DOI：
10.1073/pnas.2009604117
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发表时间：
2020
期刊：
影响因子：
--
通讯作者：
Castner, Jr, Edward W.
中科院分区：
文献类型：
--
作者：
Castner, Jr, Edward W.
聚合物和高分子材料在其分子链的相邻组分之间表现出灵活性，允许相对于晶体状态的无序构象。无序的链构象导致相邻链之间的缠结，聚合物链的局部相交导致材料中的空洞。这样的空隙或孔洞被称为聚合物自由体积元（FVEs）。Budd， McKeown和Fritsch(1)在研究聚合物材料中的FVEs与它们固有的微孔隙和气体传输之间的关系时，从卢克莱修那里获得了灵感。套用卢克莱修对希腊原子论的教导，“自然……有两种形式……：身体，以及放置身体的空间，并沿着不同的路径行进。1, 1977页)。最近，我们发现FVE尺寸是聚合物、聚合物材料和玻璃分子理论的关键描述符（1-5）。在这些材料的自组装两亲体(6)、聚合物复合材料(7)和热选择性膜(8)的应用中，FVE分布至关重要。因此，人们对fve的特征给予了极大的关注；下面将讨论这些测量中使用的主要方法。结构塑料工程需要聚合物材料中FVE分布的知识。高强度产品需要处理以控制和最小化FVEs。一个例子是聚乙烯链的加工（包括CH2基团的重复）。当长聚乙烯链被挤压以最大化其排列时，结果是高密度聚乙烯（HDPE），这是一种广泛使用的塑料材料，我们大量消费。HDPE中的空洞是由较短的链、较差的链排列和较高的温度造成的，因此fve数量和尺寸的增加与结构完整性的降低有关。在其他应用中，例如使用聚合物设计用于气体分离的透选择膜，可能需要增加材料中FVEs的数量和尺寸（1,8）。FVEs在聚合物中的性质可以用软糖来说明。在家制作软性蠕虫的一种方法是使用一盒吸管作为凝胶材料的模具。这一箱稻草的意义在于，它们的局部组织类似于棒状分子的六边形最紧密排列，这是聚合物结晶的一种方式。当蠕虫胶材料在稻草模具中凝固后，蠕虫胶就可以被挤进碗里了。碗里的一堆黏糊糊的蠕虫在彼此相交的地方会有扭曲的构象，就像熔融聚合物中的无序链一样。如果我们考虑三个或更多相邻链的局部交点，我们会发现它们之间存在空隙，如图1中的二维投影所示。聚合物链上的扭结是随机发生的
Polymers and polymeric materials exhibit flexibility between neighboring components of their molecular chains, permitting disordered conformations relative to the crystalline state. The disordered chain conformations lead to entanglements between neighboring chains, and the local intersections of polymer chains lead to voids in the material. Such voids or holes are named polymer free volume elements (FVEs). In their study of how FVEs in polymeric materials are related to their intrinsic microporosity and transport of gases, Budd, McKeown, and Fritsch (1) found inspiration from Lucretius. To paraphrase Lucretius’ teachings about atomistic theories from the Greeks,“Nature... has two forms...: the bodies, and the void where the bodies are placed and travel their varied paths”(ref. 1, p. 1977). More recently, we find that FVE sizes are key descriptors in molecular theories of polymers, polymeric materials, and glasses (1–5). FVE distributions are critically important in applications of such materials to self-assembled amphiphiles (6), polymer composites (7), and perm-selective membranes (8). Thus, a great deal of attention has been paid to characterizing FVEs; the major methods used in these measurements are discussed below. Knowledge of FVE distributions in polymeric materials is needed for engineering of structural plastics. High-strength products require treatment to control and minimize the FVEs. One example is the processing of polyethylene chains (comprising repeats of the CH2 group). When long polyethylene chains are extruded to maximize their alignment, the result is highdensity polyethylene (HDPE), a widely used plastic material that we consume in vast quantities. Voids in HDPE result from shorter chains, poor chain alignment, and exposure to higher temperatures, so an increase in the number and size of FVEs correlates with a reduction in structural integrity. In other applications, such as using polymers to engineer permselective membranes for gas separations, increasing the number and size of FVEs in the material may be required (1, 8).The properties of FVEs in polymers can be illustrated using gummy worm candies. One way to make gummy worms at home uses a box of straws as molds for the gelatinous material. The significance of the box of straws is that their local organization is analogous to hexagonal closest packing of rod-like molecules, one of the ways polymers may crystallize. After the gummy worm material has set in the straw molds, the gummy worms can be extruded into a bowl. The pile of gummy worms in the bowl will have twisted conformations where they intersect with each other, much as the disordered chains in a molten polymer will do. If we consider the local intersections of three or more adjacent chains, we would find voids between them, as shown in the two-dimensional (2D) projection in Fig. 1. The kinks in the polymer chains occur in random
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