(103f) Molecular Dynamics Simulations of An Oxidized Vapor-Grown Carbon Nanofiber and Vinyl Ester Resin Interactions Leading to a Possible Interphase Formation In the Cured Nanocomposite
Molecular dynamics simulations were used to study the effect of vapor-grown carbon nanofiber (VGCNF) surface oxidation on VGCNF-vinyl ester (VE) resin monomer interactions. It was anticipated that the interfacial interactions between resin monomers and the oxidized carbon nanofiber surface would result in different local molar ratios of the resin monomers compared to the bulk resin. The resin was comprised of styrene and two VE monomers with either one or two bisphenol A groups in their backbones (designated as VE1 and VE2, respectively). Differences in local molar ratios of these resin monomers may result in the formation of an interphase region in the final cured nanocomposite. The time-averaged relative concentration profiles were used to monitor the temporal and spatial distributions of the resin monomers in a simulation cell of size 60×50×60 Å3. Initially the cell was filled with monomers in a ratio consistent with a 33 wt% commercial epoxy vinyl ester resin (Derakane 441-400, Ashland Co.). The idealized VGCNF surface was represented by two overlapped (shingled) graphene sheets, which were in contact with the resin. Oxidation of the graphene sheets was represented by the introduction of functional groups on the edges and the surfaces of the graphene sheets. The edges were more highly oxidized than the surfaces, containing of hydroxyl, phenolic, lactone, quinone, hydroquinone, and anhydride functional groups. Hydroxyl, epoxide, aromatic, ketone and carboxylic acid groups were attached to the surfaces (e.g. basal planes). These functional groups have been previously identified in our laboratory . The NVT (constant number of atoms, N; constant volume, V; constant temperature, T) ensemble was used for simulations. All simulations were conducted with a simulation time of 10 ns at 1000 K for monomer distribution equilibration followed by another 5 ns simulation at 300 K. The relative concentration profiles were generated for the direction roughly perpendicular to the graphene sheet surfaces. The VE1 monomer concentration was higher near the oxidized graphene surface, compared to a simulation performed using a pristine graphene , where VE1 was depleted near the graphene surface. Styrene accumulated in the interfacial region in both cases, but more so for the pristine surface. Overall, a ~5 Å thick interfacial region was observed near each surface of the graphene sheet. In these regions, the monomer molar ratios differed from those in the bulk resin. The increased concentration of VE1 near the oxidized carbon nanofiber surface may lead to a stiffer interphase compared to the bulk matrix. In addition, favorable VGCNF-matrix polar interfacial interactions may lead to increased interfacial adhesion and subsequently, increase the interfacial shear strength in the final cured nanocomposite. The results of this study can be extended to other resins and surfaces.
 Lakshminarayanan, PV, Toghiani, H, Pittman, CU. Nitric acid oxidation of vapor grown carbon nanofibers. Carbon 2004; 42: 2422-2433.
 Nouranian, S, Jang, C, Lacy, TE, Gwaltney, SR, Toghiani, H, Pittman Jr, CU. Molecular dynamics simulations of vinyl ester resin monomer interactions with a pristine vapor-grown carbon nanofiber and their implications for composite interphase formation. Carbon (In Press).