EXPLORING POLY (ETHYLENE GLYCOL) AND MONTMORILLONITE CLAY INTERACTIONS BY MOLECULAR DYNAMICS SIMULATION

Abstract

Computer-aided materials science and engineering provides novel opportunities to hasten the exploration in many areas of human life. One of the directions of computational multi-scale modeling and simulation is atomistic level classical molecular dynamics, which helps to look at the material's structure in detail and develop a sense of interaction mechanisms between different nanostructured materials. In this thesis, the interaction mechanisms between non-ionic poly (ethylene glycol) (PEG) with montmorillonite (MMT) clay have been examined by atomic-level molecular dynamics simulations. The simulations run in either NVT or NPT ensembles. Within the interlayer region of anhydrous sodium montmorillonite (Na-MMT) clay, PEG demonstrates orientational and conformational uniformity and loss of translational and rotational degrees of freedom during NVT simulation. PEG chain becomes along and parallel to the basal surface of the Na-MMT clay or is positioned in the way that it has contacts with two parallel layers of clay. This happens due to the Na ions remaining at the surface of the clay, and PEG interacts with the dry Na-MMT by forming an ion-dipole type of interaction between polar oxygen atoms of the polar polymer and Na ions of the clay. This type of interface may be significant in nanocomposite production when the presence of water is undesirable. In opposite, during NVT simulations PEG in the interlayer region of hydrated MMT clay remains flexible, highly mobile, and explores its translational and rotational degrees of freedom as it was observed for the PEG behavior in aqueous solution. In this system, PEG doesn’t contact the surface of the clay even if the concentration of the PEG is increased. This happens due to the volume of the system being fixed and layers of the clay constrained. The sodium ions are separated from the basal surface of the MMT clay and are attracted by PEG and water oxygen atoms that are facilitated by entropic forces. It is observed that PEG is attracted by negatively charged hydrated MMT surface during NPT simulation for the model with unconstrained layers at ambient conditions. PEG allocates itself parallel to the clay surface and loses its translational degrees of freedom. NPT simulation results show more realistic results that match the experimental observations as the volume is adjusted to the equilibrium state to give a more realistic density of the system. In addition, the results obtained from molecular dynamics simulation in NVT ensemble for interactions of anionic PAA and CaCl2 that is used to mimic the cationic polymer with the surface of the Na-MMT clay are included.