Molecular dynamics study on the influence of water on curing kinetics of biobased epoxy resin
Topic(s) : Material and Structural Behavior - Simulation & Testing
Co-authors :
Jean-Baptiste JOUENNE (FRANCE), Delphine BARBIER (FRANCE), Viwanou HOUNKPATI , Laurent CAURET (FRANCE), Alexandre VIVETAbstract :
Composites with bio-sourced matrices reinforced with natural fibers are a solution envisaged to meet the environmental constraints of the industry [1][2]. However, the interactions between these two bio-components remain poorly controlled, which can have significant consequences on the final properties of the material. Our work is particularly interested in the influence of flax fibers on the crosslinking of a partially bio-sourced epoxy resin. The understanding of physical and chemical mechanisms involved in the crosslinking reaction aims to improve the large-scale industrialization of natural fiber composites. The crosslinking of an epoxy-amine system lies in the opening of the epoxy functions by nucleophilic attack of the amine groups provided by the hardener. It has been shown that nucleophilic groups such as ROH or H2O accelerate crosslinking [3]. Other studies showed that water can escape from the flax fibers and influence the curing kinetics [4]. In general, it is attributed to a chemical action that is the intervention of the proton from the water molecule to assist the hardener nucleophilic attack. However, this effect isn’t limited to the chemical aspect. In this study we propose to focus on physical interaction of water molecules on the network formation of epoxy-amine system using molecular dynamics simulations.
Molecular dynamics simulations are a powerful tool for understanding and predicting chemical phenomenon at the atomic scale. However, the crosslinking of thermosetting polymers can last from a few minutes to an hour, which would make atomic simulations on a realistic scale inaccessible. An accelerated simulation method was therefore used to address this issue. The bond boost method developed by Vashisth et al. [5], based on the ReaxFF force fields [6], follows the movement of the atoms involved in the crosslinking reaction until they reach a configuration conducive to the opening of the epoxide rings. When this configuration is reached, an additional energy potential is added to the reactants allowing them to overcome the energy barrier of the crosslinking reaction thus forming the desired result [5].
With this numerical tool, it is thus possible to monitor the evolution of the crosslinking by the rate of carbon atoms forming a continuous network. Results show that the presence of water molecules in the simulation box affect the network formation. In one hand, it increases the macromolecular chain mobility by repulsing effect between molecules. It thus enhances the frequency of encounters between reactive sites leading to an accelerated growth of the network. On the other hand, the network formation in presence of water molecules is more unstable resulting in variability in the crosslinking ratio value along simulation. The use of atomic scale simulations brings new insights on the curing kinetics of epoxy-amine systems that could be useful to master the manufacturing of biobased composites.
Molecular dynamics simulations are a powerful tool for understanding and predicting chemical phenomenon at the atomic scale. However, the crosslinking of thermosetting polymers can last from a few minutes to an hour, which would make atomic simulations on a realistic scale inaccessible. An accelerated simulation method was therefore used to address this issue. The bond boost method developed by Vashisth et al. [5], based on the ReaxFF force fields [6], follows the movement of the atoms involved in the crosslinking reaction until they reach a configuration conducive to the opening of the epoxide rings. When this configuration is reached, an additional energy potential is added to the reactants allowing them to overcome the energy barrier of the crosslinking reaction thus forming the desired result [5].
With this numerical tool, it is thus possible to monitor the evolution of the crosslinking by the rate of carbon atoms forming a continuous network. Results show that the presence of water molecules in the simulation box affect the network formation. In one hand, it increases the macromolecular chain mobility by repulsing effect between molecules. It thus enhances the frequency of encounters between reactive sites leading to an accelerated growth of the network. On the other hand, the network formation in presence of water molecules is more unstable resulting in variability in the crosslinking ratio value along simulation. The use of atomic scale simulations brings new insights on the curing kinetics of epoxy-amine systems that could be useful to master the manufacturing of biobased composites.