A Molecular Dynamics Study On Thermomechanical Behaviors Of Active Network Polymers

"Smart materials" is regarded as one of the important modern new materials due to its capacity of recognizing a stimulus as a signal, judging the degree of this signal, and consequently changing their conformation or properties as a direct response. As an important component of smart materials, active materials become hot topics recently. It is very significant to investigate the physical properties and mechanical properties of active polymers at macromolecular level. Molecular dynamics (MD) is now a well-established and important tool that allows practical problems to be explored in detail on the macromolecular scale for polymers to obtain information that cannot be extracted directly from experiments. In this work, two types of active polymers, including shape memory polymers and covalent adaptable networks polymers, are selected. Molecular dynamics simulation is used to investigate the network properties and deformation mechanism of these two types of materials. The simulation results are also compared with theoretical predictions and experimental results. The main research content and results are listed as follows:1. The thermomechanical properties and shape memory behavior of three types of epoxy SMPs with varying curing agent contents are investigated using a molecular dynamics (MD) method. The mechanical properties under uniaxial tension at different temperatures were obtained, and the simulation results compared reasonably with experimental data. In addition, in a thermomechanical cycle, ideal shape memory effects for the three types of SMPs were revealed through the shape frozen and shape recovery responses at low and high temperatures, respectively, indicating that the recovery time is strongly influenced by the ratio of epoxy resin to curing agent.2. Molecular dynamics simulation is used to investigate the macromolecular details of bond exchange reactions by using a recently reported epoxy system, a notable example of covalent adaptable network polymer. An algorithm for bond exchange reactions is first developed and applied to study a crosslinking network formed by epoxy resin DGEBA with the crosslinking agent tricarballylic acid. The trace of an active unit is tracked to show the migration of active units within the network. Network properties, such as the distance between two neighboring crosslink sites and initial modulus are examined after each iteration of the bond exchange reactions to provide detailed information about how material behaviors and molecular structure evolve. Stress relaxation simulations were also conducted. It was found that even though bond exchange reactions change the macroscopic shape of the network, microscopic network characteristic features, such as chain angles, relax back to the unstretched state, indicating an isotropic network after shape reforming.3. Molecular dynamics simulations are used to investigate surface welding due to bond exchange reactions in an epoxy system. Two epoxy networks are constructed and brought into contact, and bond exchange reactions are then activated. The trajectory of active atoms is tracked, which shows how the active atoms cross the interface. Based on simulation results, this work analyzes the influence of welding conditions, such as welding time, welding temperature, degree of polymerization and crosslinking density of original networks, on the mechanical properties of welded materials. These parameters determine the number of connected bonds on the interface, which consequently affect the welding performance. Eventually, with a sufficiently long welding time, the system can fully recover the same modulus and yielding stress as those of a fresh network. Finally, the active atoms’ penetration depth, obtained from MD simulation, shows good agreement with the predictions of some existing theories.