Molecular Structure, Interfacial Chain Topology, Electronic Structure and Fracture Toughness of Polyethylene: A Multiscale Computational Study

Sammanfattning: The structure of semicrystalline polyethylene (PE) strongly affects its properties. Two important structural features, namely the concentrations of tie chains and entanglements cannot be directly assessed using experimental techniques. These parameters have a major impact on mechanical properties of the material, especially on its fracture toughness. The present study has therefore focused on developing methods based on computer simulation in order to determine the concentrations of tie chains and entanglements as a function of molecular structure in unimodal and bimodal PE systems.An off-lattice Monte Carlo (MC) method was developed to simulate the semicrystalline PE. The code was able to input molar mass distribution, short-chain branch distribution, and crystallinity data and model the crystalline-amorphous lamellar structure with the focus on determining the concentrations of tie chains and entanglements. Introduction of the short-chain branches significantly increased the tie chain and entanglement concentrations. The method was then used to simulate a typical semicrystalline structure, and this structure as well as other simulated variations of the PE structure were equilibrated using molecular dynamics (MD) simulations. A linear-scaling DFT (density functional theory) method was then used in order to determine the electronic structure of the materials. Bandgap of the semicrystalline model was found to be smaller than both pure crystalline or amorphous systems. This could indicate the preference for electrons to reside in the interfacial regions rather than in crystalline or bulk amorphous regions. Low effective activation energies obtained indicated a high mobility of holes, excess electrons, and charge carriers at room temperature.Coarse-grained (CG) potentials were derived using the iterative Boltzmann inversion (IBI) method to describe linear and branched PE. The potentials were then used in CG-MD simulations to crystallize and draw blends of low and high molar mass PE. The purpose was to determine the concentrations of tie chains and entanglements as well as their effect on the fracture toughness. Addition of a linear high molar mass component (only 25 % by weight) significantly increased the concentration of entanglements and thus the fracture toughness of the material. The introduction of a butyl-branched high molar mass fraction had an even stronger effect on the concentration of entanglements and, in particular, on the tie chain concentration. These latter systems exhibited the highest fracture toughness values of all systems studied.