This study evaluates the mechanical properties of carbon fiber/polymer interfaces. The specimens are of three types: carbon fiber/vinyl ester resin, carbon fiber/epoxy resin, and carbon fiber/polyimide resin. Microbond tests are carried out, and the fiber load at interface debonding is obtained. By performing finite element analysis, true interface strengths are determined. The strength values are in the following order: polyimide specimen>epoxy specimen>vinyl ester specimen. Molecular modeling is performed for these specimens. Three stacked graphene layers are considered to be the carbon fiber surface and the interface energy is evaluated. The interface energy of these three systems are in the same order as the strength. By performing the molecular simulation, a qualitative discussion is possible. Next, the interface debonding simulation is performed, but the interface does not fail, while the resin part fails, meaning that this is an evaluation of the resin strength and not the interface strength. For the quantitative evaluation of the interfacial debonding strength, further study is necessary.

Previously, the authors proposed a method to endow PLA with hydrolysis-controllability by introducing o-nitrobenzyl alcohol as photo-dissociable protecting groups. This study aimed to obtain the design guideline for optimum molecular structures in the long-chain PLA generated by introducing intermediate photo-dissociable protecting groups. This would help suppress decrease in the elastic modulus of hydrolysis-controllable PLA. First, the possibility of generating long-chain PLA was discussed using a molecular orbital calculation. Based on the results, it was suggested that the reaction to generate the long-chain PLA via o-nitrobenzyl alcohol could occur depending on the combination of the reaction sites. Secondly, uniaxial tensile simulation for the long-chain PLA, whose molecular structures were deemed as “generatable”by the MO calculation, was conducted using molecular dynamics. The results suggested that the jackknife-like structure of the long-chain PLA formed by the introduction of the intermediate o-nitrobenzyl alcohol decreases the elastic modulus of PLA. Therefore, it might be necessary to select photo-dissociable protecting groups, which could in turn maintain the straight molecular structures of the long-chain PLA, in order to suppress decrease in the elastic modulus.

In this study, the tensile strength at a carbon fiber/epoxy resin interface was investigated using molecular dynamics (MD) simulations. The simulated tension speed and strength were initially estimated, and a realistic tension speed was subsequently selected. The tensile strength calculated using the simulation was in good agreement with the experimental data. Based on the this study and our previous work, we analyzed the contributing factors to tensile strength. The surface energy between the graphene sheet and resin as well as the molecular structure of the resin in the vicinity of graphene significantly affected the tensile strength. In addition, MD simulations were shown to be a useful tool for composite material prediction and analysis.

Generally, polymeric materials show viscoelasticity because of their structures. It is difficult to measure the bulk creep compliance; however, it is becoming increasingly important to measure bulk properties because of increasing environments of high pressure, such as the inside of an injection molding apparatus. In this study, a molecular dynamics (MD) simulation was performed to understand the bulk creep mechanisms by investigating each potential energy during bulk creep analysis. An amorphous polycarbonate model was prepared and analyzed at three different temperatures under T_g. A hydrostatic pressure (7 conditions) was applied to the model for 10 ns to simulate the bulk creep test. The results indicate time, temperature, and pressure dependency; further, there is a high interrelationship between the hydrostatic pressure and creep speed. Master curves of the bulk creep compliance with hydrostatic pressure were obtained by shifting the bulk creep compliance curves, and the shift factors for hydrostatic pressure were also obtained. Furthermore, MD simulation could estimate the long-term bulk creep behavior for 100 s. From the viewpoint of energy change, the bulk creep behavior could be explained by the change in the dihedral angle potential energy.

While poly-L-lactic acid (PLLA) is being developed for use in various medical devices, there is still an urgent need to elucidate the correlation between molecular chain deformation behavior and mechanical properties such as stress. In addition, there have been few published studies where molecular dynamics simulations were used to compare the molecular chain deformation behavior, which is difficult to evaluate in the micro region, and the potential energy and evaluate mechanical properties like stress. Therefore, in this study, in order to study the molecular chain deformation behavior of PLLA using molecular dynamics methods, analysis models of initial orientation 0º, 45º, 90º and four randomly oriented models were prepared. In addition, tensile simulations were carried out using affine deformation, and the influence exerted by molecular chain deformation behavior on the mechanical properties was evaluated. Molecular chain deformation behavior, molecular chain length, molecular chain angle, and the distance between centers of gravity of different molecular chains were determined and compared with the potential energy and stress generated in tensile simulations to evaluate the mechanical behavior. As a result, it was found that the model showing curing behavior exhibited increased stress after reaching the lower yield point, while the model not showing curing behavior exhibited a remarkable increase after the stress reached the lower yield point. In addition, it was suggested that the elongation and angle change of the molecular chain had a large influence on the stress, and that among these factors, the angle change exerted the largest influence on the stress.