A molecular dynamics study of a model nanoparticle embedded in a polymer matrix

Molecular dynamics (MD) simulations have been used to examine the structure and dynamics of a system containing an inorganic nanoparticle embedded in a polymer matrix. This paper represents a preliminary investigation into the feasibility of examining such relatively large systems using atomistic modeling techniques. No attempt is made here to model any specific system. A generic linear polymer "united-atom" model is first created in an amorphous phase before the insertion of an atomistically detailed silica nanoparticle of diameter similar to4.4 nm. A novel method to insert a nanoparticle into a polymer matrix is given. The entire system is contained in a standard periodic simulation cell of side length similar to10 nm. The volume fraction of silica corresponded to similar to4.5%. The composite system was subsequently relaxed at 300 K and at two different pressures using standard MD techniques, the gmq suite of programs being used for this purpose. Results are presented regarding the variation of the structure and dynamics of the system with respect to the distance from the polymer-nanoparticle interface and as a function of pressure. A clear structuring of the polymer chains around the nanoparticle is seen with prominent first and second peaks in the radial density function and a concurrent development of preferred chain orientation. The probability of trans conformers is also higher close to the interface and shows a distinct gradient. In contrast, evidence for chain immobilization is less obvious overall although dynamic properties are more sensitive to changes in the pressure. Comparisons are also made between the bulk moduli of the pure polymer and composite systems.