Focus on the deformation mechanism at the interfacial layer in nano-reinforced polymers: A molecular dynamics study of silica - poly(methyl methacrylate) nano-composite

The effects of nanoparticle size on the “macroscopic” mechanical response and interfacial interaction in the case of model nano-reinforced polymers were investigated using molecular dynamics simulations. Different ensembles, of homogeneous polymer matrices, amorphous silica particle, and their binary mixtures were prepared. The binary mixture was made with silica nano-particle 3 nm in size, embedded in poly (methyl methacrylate) or PMMA polymeric matrix. At the macroscopic scale, the mechanical response of the matrix and nano-composite was evaluated using simulated tensile tests. Interfacial interaction between the NP and the PMMA matrix was qualitatively evaluated using the thermodynamic analysis of nanocomposite systems. Entropy (S) and internal energy (E) were derived from relatively short molecular dynamics trajectories, using the two-phase thermodynamic method (2-PT). The PMMA matrix was decomposed into concentric layers composed of atoms from different polymer chains but located at an equal distance from the center of mass of the silica NP. For both nanocomposite systems, the interface layer of the polymer closest to the silica NP surface exhibited both the lowest entropy and a well-organized structure. Entropy and internal energy patterns were derived from tensile stretched samples. Entropy and internal energy variation on stretched samples revealed the existence of two distinct domains. The first domain deformation was a mixture of internal energy increase and entropy decrease. In the second domain, the deformation mechanism was mostly governed by variations in entropy. These observations will be discussed about polymer – nanoparticle attractivity.