Amorphous polymers are considered promising materials for separation applications due to their excellent transport properties and low fabrication costs. The separation performance of a polymer membrane is characterized by its permeability and selectivity. Both permeability and selectivity are controlled by the diffusion of penetrants through the matrix, which is strongly influenced by the distribution and morphology of the free volume elements (FVEs). FVEs are void spaces in the polymer matrix that result from the inefficient packing of bulky and rigid groups on the polymer backbone. Thus, FVEs dictate the efficiency of membrane polymers, and it is imperative to understand how processing conditions such as high pressures and temperatures influence their structure. In this work, we apply uniaxial tensile deformation on three polymers, polystyrene (PS), polymethylpentene (PMP), and HAB-6FDA thermally rearranged polymer (TRP), at varying temperatures and strain rates. We characterize the stress strain behavior, tensile modulus, and free volume element evolution at these conditions. We find that PMP and PS with low and moderate glass transition temperature, respectively, exhibit the most change in mechanical properties as a function of strain rate and temperature. The properties of TRP, however, do not vary as much, which we attribute to the rigidity of the chains. We also find that FVEs shift to broader distributions with deformation, and the extent of this change is in line with the overall change of mechanical properties of the material.

中文翻译：

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经历单轴变形的无定形聚合物中自由体积元素的演变：分子动力学模拟研究

无定形聚合物由于其优异的传输性能和较低的制造成本而被认为是用于分离应用的有前途的材料。聚合物膜的分离性能通过其渗透性和选择性来表征。渗透率和选择性均由渗透剂在基质中的扩散控制，而渗透剂的扩散受自由体积元素 (FVE) 的分布和形态的强烈影响。FVE 是聚合物基质中的空隙空间，是由于聚合物主链上大体积刚性基团的低效堆积而产生的。因此，FVE 决定了膜聚合物的效率，因此必须了解高压和温度等加工条件如何影响其结构。在这项工作中，我们在不同的温度和应变率下对聚苯乙烯 (PS)、聚甲基戊烯 (PMP) 和 HAB-6FDA 热重排聚合物 (TRP) 三种聚合物应用单轴拉伸变形。我们表征了这些条件下的应力应变行为、拉伸模量和自由体积单元演化。我们发现，分别具有低玻璃化转变温度和中等玻璃化转变温度的 PMP 和 PS，随着应变速率和温度的变化，机械性能变化最大。然而，TRP 的特性变化不大，我们将其归因于链的刚性。我们还发现，FVE 随着变形而向更广泛的分布变化，并且这种变化的程度与材料机械性能的整体变化一致。