Viscoelasticity of Epoxy Nano-Composites (Invited)

Nanocomposites have been modeled in a multiscale covering from molecular scale (e.g., molecular dynamics, Monte Carlo), microscale (e.g., Brownian dynamics, dissipative particle dynamics, lattice Boltzmann, time-dependent Ginzburg–Landau method, dynamic density functional theory method) to mesoscale and macroscale (e.g., micromechanics, equivalent-continuum and self-similar approaches, finite element method) The presence of layered silicates in nonaqueous polymers changes the viscoelastic behavior of the unfilled matrix from liquid-like to solid-like because of the formation of a three-dimensional percolating network of exfoliated or intercalated stacks. This gel-like behavior is a direct consequence of the highly anisotropic nature of the nanoclays which prevents their free rotation and the dissipation of stress. Particle to particle interactions is the dominant mechanism in fumed silica nanocomposites whereas particle to polymer interaction is the dominant one in colloidal silica nanocomposites at identical filler concentrations. These interactions are balanced in each nanocomposite systems by the silica surface treatments (chain grafting, silane modification) and the molecular weight of the matrix.

Nano clays organically functionalized were mixed with Epoxy in a high shear mixer. Viscosity, normal stresses and dynamic viscoelastic measurements at various temperatures were determined. Elastic and viscoelastic modulus as function of frequency was used to get relaxation modulus by using transformation functions. Relaxation modulus as function of time at different temperatures was used to get power law fits. Relaxation modulus at different crosslinking level in the nanocomposite shows large difference from nanocomposite with uncrosslinked epoxy. These structures by X-ray diffraction and TEM showed significant exfoliation.