All-atomic and coarse-grained molecular dynamics investigation of deformation in semi-crystalline lamellar polyethylene

Abstract

In the present work we have performed classical molecular dynamics modelling to investigate the effects of different types of forcefields on the stress-strain and yielding behaviours in semi-crystalline lamellar stacked linear polyethylene. To this end, specifically the all-atomic optimized potential for liquid simulations (OPLS-AA) and the coarse-grained united-atom (UA) force-fields are used to simulate the yielding and tensile behaviour for the lamellar separation mode. Despite that the considered samples and their topologies are identical for both approaches, the results show that they predict widely different stress-strain and yielding behaviours. For all UA simulations we obtain oscillating stress-strain curves accompanied by repetitive chain transport to the amorphous region, along with substantial chain slip and crystal reorientation. For the OPLS-AA modelling primarily cavitation formation is observed, with small amounts of chain slip to reorient the crystal such that the chains align in the tensile direction. This force-field dependence is rooted in the lack of explicit H-H and C-H repulsion in the UA approach, which gives rise to underestimated ideal critical resolved shear stress. The computed critical resolved shear stress for the OPLS-AA approach is in good agreement with density functional theory calculations and the yielding mechanisms resemble those of the lamellar separation mode. The disparate energy and shear stress barriers for chain slip of the different models can be interpreted as differently predicted intrinsic activation rates for the mechanism, which ultimately are responsible for the observed diverse responses of the two modelling approaches.

Understanding flow-induced crystallization in polymers: A perspective on the role of molecular simulations

“…Of particular note in this context is that the united atom force field for polyethylene, as used in the majority of studies discussed herein, crystallises into the hexagonal phase rather than the more common orthorhombic phase [57]. All-atom simulations of polyethylene crystallisation do show orthorhombic crystals (see for example [58,59]) but the range of chain lengths that can be simulated is smaller for these more detailed simulations.…”

“…Recent studies from the groups of Rutledge [57,58,68,75,76] and Schilling [69,77] aimed to, not just observe nucleation, but to quantify its rate and elucidate its mechanism. Both groups achieved this by applying a collection of analysis tools.…”

Two-Step Heat Fusion Kinetics and Mechanical Performance of Thermoplastic Interfaces

“…Crystallization is another important process during fusion, which unfortunately cannot be well captured by the CG approach adopted here 26,27 . Interfacial entanglement established in the melt state could be anchored in chain-folded lamellae upon crystallization by using metallocene-catalyzed polymers 28 .…”

Two-step heat fusion kinetics and mechanical performance of thermoplastic interfaces

“…Crystallization is another important process during fusion, which unfortunately cannot be well captured by the CG approach adopted here 37 , 38 . Interfacial entanglement established in the melt state could be anchored in chain-folded lamellae upon crystallization by using metallocene-catalyzed polymers 39 .…”