Effect of surface pattern on interfacial strength between graphite layers and PP/PE: Molecular dynamics study

Abstract

Various amorphous/crystalline polypropylene (PP) and polyethylene (PE) are pressed under multi layer graphite (MLG) rigid walls and then detached or scratched to evaluate the interfacial strength by molecular dynamics simulation. Not only perfect flat MLG surface, but also periodic sine wave surfaces of stacked graphite flakes are considered to discuss the effect of surface pattern. PP always show higher strength than PE both in the debonding and shear. The debonding stress shows the highest for the flat MLG surface and decreases with narrower surface pattern. The local density at the polymer/MLG interface reveals that the adhesiveness decreases with the smaller surface pattern and leads this tendency. However, crystalline polymers, oriented normal direction to MLG, always show lower strength than the amorphous ones despite of their high density at the interface. This result emphasizes the fact that the interfacial strength is not uniquely decided by the van der Waals energy but also by the morphology of the molecular chains attached to the interface. Observations of chain morphology shows that the random coil PP chains tend to form clusters while the PE chains widely spread in the amorphous block. Thus longer PE chains bridge the surface convex and prevent filling of other chains to the surface concave, resulting in the lower interfacial density and debonding stress. On the other hand, the PP chain clusters cling to the interface with same ratio and the longer chains have massive "anchor part" in the amorphous, showing same interfacial density and higher debonding stress. Contrary to the debonding strength, the shear strength is always zero for the perfect surface and extremely increases with the narrower surface pattern for the crystalline polymers. This result can be simply explained with the difficulty of the conformation change or chain rotation in the straight chain bundle of crystal polymers, and with the gradient of the surface convex that work as barrier against the flow deformation of sandwiched polymers.