(87b) Investigation of Vinyl Ester Resin/Vapor-Grown Carbon Nanofiber Surface Interactions Using Molecular Dynamics Simulations

The role of ?interphase?, the matrix region immediately surrounding the nanoreinforcement, in polymeric nanocomposites is believed to be pivotal in determining the ultimate mechanical properties of these materials. Though this has been confirmed for thermoplastic matrix nanocomposites, no clear understanding now exists for thermoset matrix nanocomposites. Here, a molecular dynamics (MD) simulation was performed on vinyl ester (VE)/vapor-grown carbon nanofiber (VGCNF) nanocomposites, where the interactions of resin constituents (VE monomers and styrene) with the surface of two overlapping (shingled) graphene sheets representing the surface of a pristine carbon nanofiber were investigated. This could have implications during the formation of an interphase region in the resin curing step. Using the Accelrys Materials Studio? v5.0 MD simulation software, a periodic boundary system containing VE monomers and styrene in contact with both sides of the graphene sheets were constructed. The resin system was based on Derakane 441-400 epoxy VE resin (Ashland Co.) with 33 wt% styrene. This resin is a mixture of VE monomers with an average molecular weight of 690 g/mol and n = 1.62 bisphenol-A groups in the backbone. In the simulation, 38 VE monomers with n = 1, 62 VE monomers with n = 2, and 320 styrene molecules were used to represent the true monomer mole ratios. The total number of atoms was 17180. Using the COMPASS force filed, the simulation cell of liquid resin and graphene sheets was relaxed for 50000 steps (time step of 1 fs) at 300 K before the dynamics simulation using the Conjugate Gradient method. The density was adjusted to 1.18 g/cm3, which is in accordance with the experimentally measured density. Then, dynamics simulation was started using the NVT ensemble at 10 K with a time step of 1 fs. The simulation ran for 1 ps at this temperature. Next, the temperature was ramped up to 50 K and further up to 600 K in increments of 50 K. At each intermediate temperature the dynamics simulation ran for 1 ps except for 300 K where it ran for 100 ps. At 600 K, the simulation ran for a total time of 10 ns. The trajectory files were saved every 100 ps. The system was then cooled down to 300 K in the same manner and the simulation ran for another 5 ns. Time-averaged concentration profiles were obtained for styrene and VE monomers. Based on the results, styrene accumulates on the nanofiber surface yielding a higher styrene to VE oligomer ratio in the interphase region. This suggests that, in contrast to most thermoplastic matrices, a softer interphase may result in VE/VGCNF nanocomposites when the curing is completed since the crosslink density will be lower in this region due to higher styrene concentration. This assumes that the polymerization kinetics are fast enough that a growing chain end remains in this interface region long enough that the polymerization composition will reflect this local equilibrium concentration of monomers.