Moisture Ingress at the Molecular Scale in Hygrothermal Aging of Fiber–Epoxy Interfaces

Abstract

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.

Experimental and Theoretical Study of Gamma Radiolysis and Dose Rate Effect of o-Cresol Formaldehyde Epoxy Composites

“…The space charge density and water diffusion coefficient of silica-filled epoxy were increased after γ radiation (4 and 8 kGy) and hydrothermal aging (90 °C) . According to our studies and other investigations, silica and water were proved to be active reactants in the γ radiolysis of polymer composites such as polysiloxane. ,, The radiation-induced aging of epoxy bulk may increase the gas and moisture permeability and deteriorate the electrical insulation and adhesive strength of epoxy materials. ,, …”

Reactive Molecular Dynamics Study of Hygrothermal Degradation of Crosslinked Epoxy Polymers

“…The difference in the hydrogen bonding distance of both systems can be related to the rigidity of the backbone. The previous MD study shows that the dynamic mobility of polymer chains facilitates the ingress pathway for moisture in the epoxy network . The flexible backbone of the Bis A–JEFFAMINE D-230 system decreases the average hydrogen bond distance between polar groups and water compared to the rigid backbone in the Bis A–DETDA system.…”

“…The previous MD study shows that the dynamic mobility of polymer chains facilitates the ingress pathway for moisture in the epoxy network. 67 The flexible backbone of the Bis A−JEFFAMINE D-230 system decreases the average hydrogen bond distance between polar groups and water compared to the rigid backbone in the Bis A−DETDA system. In both systems, the change in water content does not change the interatomic distance (peak position), but the peak height changes with the water content.…”

Achieving flame retardancy and mechanical integrity via phosphites in bio‐based resins

“…Although the experimental DOC of EP2-DDM was not reported in the literature, previous simulation studies on epoxy resins have based predictions of thermomechanical properties on a ~80% DOC 19,21,22,49 as a close comparison to the experimental DOC. Hence, we used the 80% DOC samples as a possible comparison with other computational and experimental results.…”

“…Molecular simulation allows the prediction of molecular-level structures of the resin, to provide insights into their thermal and mechanical properties. MD simulations have been used successfully in a number of recent studies regarding epoxy resins, [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36] providing molecular-level based explanations for their thermomechanical properties.…”