Atomistic modeling of environmental aging of epoxy resins

Epoxy resins are finding widespread applications in a variety of structural applications, including demanding aerospace structures. Thus, understanding the behavior of epoxy is very important to design structures and to understand the reliability of them. Polymers undergo aging with time, diminishing their properties. Therefore understanding the effective life span of such epoxy structures, during which the material is safe, is a very important design criterion. Exhaustive experimental studies are costly and time consuming. Thus, computational approaches are attractive to at least in identifying promising candidates for further experimental studies. In this work, nanoscale epoxy resins were modeled using all atom representations in a simulation box, to understand their behavior. Tetrafunctional epoxy and corresponding multifunctional amine were chosen as model materials, since they are used widely in the industry. Algorithms of constructing interconnected network structures were developed to accurately incorporate the chemical structures and geometrical aspects of epoxy with minimum cost. Monomers were generated in a diamond lattice and crosslinked to model complex epoxy multifunctional network. These initial polymer structures were relaxed and equilibrated using molecular dynamics and a suitable potential function, known as force field, representing interactions between molecular units. Physical, thermal and mechanical properties obtained from equilibrated epoxy samples are found to be in good agreement with experimental results.;Possible impact of chemical degradation was studied by incorporating oxidation and hydrolysis in the simulation process. Mechanism of degradation was based on bond reaction probabilities and chemical structures of epoxies. Network structures have different architectural aspects, the chain length between cross-links, number of crosslinks per unit volume, the chemical structure of the chains, etc. Both oxidation and hydrolysis were found to decrease materials' performances by reducing the number of crosslinking points. Elastic modulus of materials was directly related to crosslinking density. Hydrolysis has a more severe effect on degrading materials properties since chain scission only takes place at network nodes.;Interfaces between two types of epoxies were constructed to study interactions at interfaces. Covalent bonds linking two components play an important role in interfacial strength. Free volume calculation, representing a measure of the unoccupied volume, helps to identify and monitor generation of crazes and voids within materials. It was found that voids and cracks initiate and grow at interfaces and lead to materials failures. Compatibilizer layers can improve overall composite performances by preventing void growth at interfaces.;Water absorption and diffusion in epoxy matrix was also examined. Diffusion pattern of water in epoxy resins was studied by tracking displacements of single molecules in specific time intervals. The typical jump-diffuse pattern seems to agree for epoxy system and is consistent with other properties of epoxy systems. The characteristic pattern of water diffusion in epoxies was interpreted by a free volume theory. A large number of small voids in epoxy confine water molecules’ ability of movement. Meanwhile, hydrogen bonds between polymer and water also constrain mobility of water. Formation of water clusters increases diameters of penetrants and affect the jump dynamics. Mechanical modulus of wet epoxy resin was found to be higher than neat epoxy due to the presence of hydrogen bonds.;A reactive force field was introduced to study thermal degradation behavior of epoxy resins. Variation of different types of covalent bonds and number of effective chains were tracked during heating processes and analyzed to uncover the degradation mechanism of epoxy resins. Order-of-bond scission obtained from simulation agrees well with experimental results. Effects of chemical environments were studied by adding water or oxygen molecules into the nanoscale material. It was found that oxygen reacted with epoxy continuously but did not destroy network structures. Water molecules decreased decomposition temperature of epoxy by swelling network structures.