Vuković Filip, Walsh Tiffany R
Institute for Frontier Materials, Deakin University, Geelong, VIC 3216, Australia.
ACS Appl Mater Interfaces. 2020 Dec 9;12(49):55278-55289. doi: 10.1021/acsami.0c17027. Epub 2020 Nov 23.
Almost all applications of carbon fiber reinforced composites are susceptible to water aging, either via ambient humidity or through direct exposure to liquid water environments. Although the impacts of water aging in composites can be readily quantified via experimental efforts, details regarding the mechanisms of moisture ingress and aging, particularly at the incipient stages of aging under hygrothermal conditions, have proven challenging to resolve using experimental techniques alone. A deeper understanding of the factors that drive incipient moisture ingress during aging is required for more targeted approaches to combat water aging. Here, molecular dynamics simulations of a novel epoxy/carbon fiber interface exposed to liquid water under hygrothermal conditions are used to elucidate molecular details of the moisture ingress mechanisms at the incipient stages of the aging process. Remarkably, the simulations show that the fiber-matrix interface is not vulnerable to a moisture-wicking type of incipient water ingress and does not readily flood in these early stages of water aging. Instead, water is preferentially absorbed via the matrix-water interface, an ingress pathway that is facilitated by the dynamic mobility of polymer chains at this interface. These chains present electronegative sites that can capture water molecules and provide a conduit to transiently exposed pores and channels on the polymer surface, which creates a presoaked staging reservoir for subsequent deeper ingress into the composite. Characterization of the absorbed water is according to hydrogen bonding to the matrix, and the distributions and transport behavior of these waters are consistent with experimental observations. This work introduces new insights regarding the molecular-level details of moisture ingress and spatial distribution of water in these materials during hygrothermal aging, informing future design directions for extending both the service life and shelf life of next-generation composites.
几乎所有碳纤维增强复合材料的应用都易受水老化影响，无论是通过环境湿度还是直接暴露于液态水环境。尽管通过实验可以很容易地量化复合材料中水老化的影响，但仅靠实验技术来解析水分进入和老化机制的细节，尤其是在湿热条件下老化初期的细节，已被证明具有挑战性。为了采取更有针对性的方法来对抗水老化，需要更深入地了解在老化过程中驱动初期水分进入的因素。在此，利用分子动力学模拟研究一种新型环氧/碳纤维界面在湿热条件下暴露于液态水的情况，以阐明老化过程初期水分进入机制的分子细节。值得注意的是，模拟结果表明，纤维 - 基体界面在水老化初期不易受到吸湿型初期水分进入的影响，并且不会轻易被水淹没。相反，水优先通过基体 - 水界面被吸收，这种进入途径是由该界面处聚合物链的动态流动性促成的。这些链呈现出带负电的位点，可以捕获水分子，并为聚合物表面暂时暴露的孔隙和通道提供一条通道，从而形成一个预浸泡的暂存库，以便后续更深地进入复合材料内部。根据与基体的氢键作用对吸收的水进行表征，这些水的分布和传输行为与实验观察结果一致。这项工作为湿热老化过程中这些材料的水分进入分子层面细节和水的空间分布引入了新的见解，为延长下一代复合材料的使用寿命和保质期提供了未来的设计方向。
