Reversibility of Semicrystalline Polymers in Creep Testing by Coarse-Grained Molecular Dynamics Simulations

Unraveling the deformation processes of semicrystalline polymers is essential for improving their durability. Owing to their hierarchical structures composed of lamellae and spherulites, many aspects of these deformation processes remain unclear at the molecular scale, such as the differences in molecular structure changes in the elastic and plastic regions and the molecular-scale structural changes during reversible and irreversible processes. Herein, simulated creep tests of the lamellar structure of polyethylene under a constant load are performed using the coarse-grained molecular dynamics method. Typical creep curves are observed under various constant loads. During the recovery process after stretching, the reversible and irreversible processes are distinguished by a strain of approximately 0.4 at the boundary of the elastic and plastic regions. Interestingly, during recovery, the interfaces between the amorphous and crystalline layers are highly oriented, which may inhibit strain relaxation. In terms of the molecular structure changes in the plastic region, the number of tie chains remains constant, whereas the numbers of chain ends and loops in the amorphous layers decrease. These simulation results advance current understanding of the molecular-scale deformation processes of semicrystalline polymers, which contribute to the improvement of long-term durability and reliability. Coarse-grained molecular dynamics simulation on creep test and recovery test of a lamellar structure reveals the molecular structural change of orientation at the interface between the amorphous and crystalline layer; the movement of loops and chain ends in the amorphous layer; and the role of tie chains. image