Mechanical properties of the multilayer polymer films: Molecular dynamics simulations

The effect of several key factors on the mechanical properties of multilayer polymer films under tensile and shear deformations was thoroughly investigated using atomistic molecular dynamics simulations. These factors included the composition of the layers, the compatibility of polymers in the layers, the crystallinity of polymers, and the thickness of the layers, especially when it decreases to values close to the radius of gyration (R_g) of the polymers. Three types of multilayer systems were considered: polylactide/poly(3-hydroxybutyrate) (PLA/PHB) based on polymers compatible for the selected chain lengths, polylactide/polyethylene (PLA/PE) with incompatible polymers in layers, and polylactide/polylactide (PLA/PLA). It was shown that reducing the layer thickness to the value close to R_g led to an increase in Young’s modulus for both types of systems with compatible polymers in layers PLA/PHB and with incompatible polymers PLA/PE. This effect was found for the systems composed of amorphous polymers. The influence of the layer thickness on shear modulus, yield stress under tensile and shear deformations was also analyzed. Young’s modulus and yield stress under tensile deformation were in line with the “rule of mixture” for all types of the systems. Both the shear modulus and yield stress under shear for PLA/PE tended to the values for bulk PE. Analysis of local atomic shear strain was employed to quantify local plastic deformations at the atomic level during the shear deformation. The pattern of local atomic shear strain distribution for the PLA/PHB and PLA/PLA systems was found to be significantly different from that for PLA/PE.