Thermodynamic mechanism based on thermoplastic epoxy resin/polyethylene glycol composites: An experimental and molecular dynamics simulation study

• MD simulation of representative TEP/PEG molecular structure • Preparation and mechanical analyses of films with different PEG contents • Reference for the design of high-performance TEP matrix composites • Comprehensive analysis of the mechanism on the addition of PEG decreases the T g of TEP/PEG composites without loss of Young's modulus of the material Thermoplastic epoxy resins (TEP) are widely utilized in shape memory materials. To enhance the spinnability of thermoplastic epoxy resins, researchers have chosen to add polyethylene glycol (PEG) with the aim of reducing the glass transition temperature of the system. There are related studies reported currently; however, the internal mechanisms of this process have not been clearly explained. Thus, this study investigated the mechanism of the intrinsic changes in the mechanical and thermomechanical properties of thermoplastic epoxy resin (TEP) and polyethylene glycol (PEG) composites using a combination of experimental and simulation methods. Molecular dynamics (MD) simulations were used to explore the ideal structural design of the TEP/PEG composites. A TEP matrix was packed with different ratios of PEG, and the cohesive energy density, mean-square displacement, radial distribution function (RDF), fractional free volume (FFV), and thermal and mechanical properties of the TEP/PEG composites with different structures were calculated. With increase in the proportion of PEG molecular chains, the FFV within the system was enhanced, which facilitated the movement of the TEP molecular chains; therefore, the T g of the composites decreased. In addition, the RDF results revealed that the addition of PEG did not affect the structure of TEP itself, and the interaction energy improved with increase PEG content. However, with the addition of flexible short-chain PEG, the close stacking of epoxy molecular chains was hindered, which prevented the formation of hydrogen bonds (H-bonds) between the TEP molecules, thus promoting their movement. In addition, the presence of PEG reduced the intermolecular forces of TEP and increased the tensile strain of the composite. Moreover, it was shown experimentally that an increase in PEG did not decrease the modulus of the composites, while decreasing the T g of the system, which facilitates the use of the TEP/PEG composite film in more applications. Interestingly, PEG reduced the hydrophilicity of the surface for epoxy/PEG film to a certain extent, and the developed epoxy/PEG film maintained high stability in water. This study provides insights into the rational design of TEP/PEG composite structures to optimize the performance of PEG in enhancing thermoplastic epoxy matrix composites, leveraging the synergy of experiments and simulations to deepen our understanding of underlying mechanisms, improve the reliability of findings, and bridge the gap between theoretical modeling and experimental observations.