Molecular Dynamics Study on Directional Orientation of Molecular Chains and Nucleation of Chain Entanglements in Amorphous Polymer under Uniaxial Tension

摘要

A nanoscopic specimen of amorphous polyethylene, involving 3,542 random coil molecular chains composed of 500-1500 methylene monomers with about 2 million methylene groups, is subjected to uniaxial tension by means of molecular dynamics simulation. After showing a linear elastic relationship at the initial stage of epsilon_(zz) <= 0.03 at d epsilon/dt_(zz)=5.0 X 10~(11)/s, the material "yields" by elongating without stress increase up to the strain of 1.5, where strain hardening appears. Careful investigation on change in dihedral angle and morphology of all molecular chains reveals that the gauche -> trans transition takes place during yielding, generating a new network-like structure composed of entangled molecular clusters and oriented chains bridging them. The strain hardening is caused by the directional orientation and stretching of molecular chains between entanglements in the nucleated structure.