Beyond the ring flip: A molecular signature of the glass–rubber transition in tetrafunctional epoxy resins

Rapid, Accurate and Reproducible Prediction of the Glass Transition Temperature Using Ensemble-Based Molecular Dynamics Simulation

For the computational design of new polymeric materials, accurate methods for determining the glass transition temperature (Tg) are required. We apply an ensemble approach in molecular dynamics (MD) and examine its predictions of Tg and their associated uncertainty. We separate the uncertainty into the aleatoric contributions arising from dynamical chaos and that due to the computational scenarios chosen to compute Tg. We propose a new scenario for computing Tg, where the density-temperature behavior is computed by running all temperatures concurrently, rather than invoking a sequential approach, thereby significantly reducing wall-clock time from days to several hours without increasing the aleatoric uncertainty. On comparing concurrent and sequential scenarios on six highly cross-linked epoxy resins cured with aromatic amines, we find excellent agreement with our experimentally determined Tg using dynamical mechanical analysis for both scenarios. The confidence intervals are found to scale as N-0.5, where N is the number of members in the ensemble, implying that ensembles comprised of at least ten replicas are required to predict Tg using MD with 95% confidence intervals of less than 20 K. The optimal MD simulation protocol is 4 ns of burn-in time followed by 2 ns of production run time.

Effect of stoichiometry on crosslinked epoxy resin characteristics: structural heterogeneities, topological defects, properties, free volume and segmental mobility

Experimental studies have shown that changes in stoichiometry (R, ratio of amine groups to epoxy groups) cause considerable variations in the properties of epoxy-amine systems. Rationales based on free volume concepts have been routinely used to address these variations in properties but have hardly been satisfactorily substantiated. Many of these rationales remain as unverified conjectures to date. Substantiating these rationales will certainly bolster our understanding of the structure-stoichiometry-property relationship, but is difficult, due to inherent challenges involved in unambiguously characterizing the structural heterogeneities induced by changes in stoichiometry (structural heterogeneities include compositional distribution in the functionality of monomers, non-uniform dispersion of elastic chains and topological defects). The aim of the present work is to gain molecular-level insights into this relationship and to verify the rationales that rely on free volume concepts used for addressing the variations in properties with stoichiometry, with the help of all-atom molecular dynamics (MD) simulations. Five epoxy-amine systems with varying R ranging from 0.4 to 3, including the stoichiometric system (R = 1), were considered for these purposes. The properties of interest namely density, glass transition temperature (T_g) and thermal expansion coefficient in the rubbery state (α_rl) of these systems were predicted. The local structure, fractional free volume and segmental mobility of these systems were then subsequently characterized as a function of stoichiometry and the results were analysed in detail. The role played by defects in properties and fractional free volume was then investigated. The results revealed significant insights into the compositional distribution of monomers with different functionalities as well as offered insights into the dispersion state and mobility of dangling chains, sols and elastic chains in the systems. Further, strong correlations were found between defect composition, fractional free volume at an elevated temperature (600 K) and thermomechanical properties (T_g and α_rl) and it was established that the key mechanism underlying these correlations was the plasticization caused by defects. Analysis based on the rule of mixture models showed that these correlations were found to be in good agreement with the interpretations based on free volume concepts. The results also revealed a strong negative correlation between fractional free volume at room temperature and defect composition, a phenomenon typically associated with the antiplasticization effect.

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

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.