A molecular dynamics study of bond exchange reactions in covalent adaptable networks

Abstract

a Covalent adaptable networks are polymers that can alter the arrangement of network connections by bond exchange reactions where an active unit attaches to an existing bond then kicks off its pre-existing peer to form a new bond. When the polymer is stretched, bond exchange reactions lead to stress relaxation and plastic deformation, or the so-called reforming. In addition, two pieces of polymers can be rejoined together without introducing additional monomers or chemicals on the interface, enabling welding and reprocessing. Although covalent adaptable networks have been researched extensively in the past, knowledge about the macromolecular level network alternations is limited. In this study, molecular dynamics simulations are used to investigate the macromolecular details of bond exchange reactions in a recently reported epoxy system. 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 the active units is tracked to show the migration of these units within the network. Network properties, such as the distance between two neighboring crosslink sites, the chain angle, and the initial modulus, are examined after each iteration of the bond exchange reactions to provide detailed information about how material behaviors and macromolecular structure evolve. Stress relaxation simulations are also conducted. It is found that even though bond exchange reactions change the macroscopic shape of the network, microscopic network characteristic features, such as the distance between two neighboring crosslink sites and the chain angle, relax back to the unstretched isotropic state. Comparison with a recent scaling theory also shows good agreement.

Molecular dynamics studying on welding behavior in thermosetting polymers due to bond exchange reactions

“…Yu et al (2015) recently developed a lattice model to capture the evolution of chain density across the interface, which depended on the BER kinetics in the bulk material. There, N lattice layers with a distance of Dx on each side were considered, and the evolution of normalized chain density on the interface was expressed as:…”

“…The structure is rst equilibrated at a temperature T ¼ 450 K and the atmosphere pressure. The crosslinking procedure uses the methodology we previously developed, 28 which combines the methods by Wu and Xu 33 and by Varshney et al 34 Briey, the distances between two active units are measured in each iteration; any two active atoms whose distance is less than the cutoff distance (4.5 A) are bonded together; then an energy minimization procedure and an MD procedure are performed to relax the structure. Aer 32 iterations of the crosslinking simulation, 97.22% percent of the active atoms are crosslinked.…”

“…We use the same method to simulate the BER process in bulk materials in our previous work. 28 If the distance of the active atoms is below the cutoff distance we dened, we connect the active atom to the existing bond; we then check the bond energy in both the new and the old bonds. If the new bond has lower bond energy, the BER occurs.…”

“…There, the diffusion of polymer chains across the interface is correlated with their pervaded volume during the Rouse-type motion, which is subsequently used to scale the interfacial chain density and network penetration. A similar approach is employed in the recent work of Yu et al (Yu et al, 2015), where the crosslinking feature of CANs is generalized into a lattice geometry. The diffusivity of polymer chains is based on the BER kinetics, and an explicit expression of chain density evolution is derived.…”

“…To account for the detailed macromolecular structures of CANs with BERs, we recently developed a new MD simulation approach for the epoxy system by Leibler and his coworkers, 15 where the transesterication reaction was used as the exchange reaction. 28 Our MD simulations in the bulk material conrmed the diffusion nature of BER in the crosslinked network. In this paper, we study the surface welding behavior of thermally activated CANs by using the MD simulation method developed in our previous work, with the goal of describing how the active atoms cross the interface via BERs and of understanding how different processing conditions can affect the efficiency of surface welding.…”