分子動力学によるポリブタジエンの繰り返し変形シミュレーション : 負のbending応力によるひずみ軟化

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Cyclic deformation is applied on an amorphous nano-block of cis-1,4 polybutadiene (PB), which has 1000 random coil chains and the average length of [-CH_2-CH=CH-CH_2-]^<75>, by molecular dynamics simulation. As same as the previous polyethylene (PE) simulation, the bond stretch (BS) and van der Waals (VDW) dominate the hysteresis of the stress-strain curve; however, the bending (BE) shows negative stress despite of the elongation, leading subtle "strain softening" in the later stage of PB loading. We have carefully analyzed BS and BE stresses by focusing on the conformation of -CH=CH- and -CH_2-CH_2- dihedral angles, revealing that the negative BE stress emerges on the trans -CH=CH- segments normal to the loading axis. Here, the BE angles of the segments does not close but open by the tensile loading; the angles open in the lateral direction due to the tension in each molecular chain. But this generates the negative stress in the loading axis since the lateral contraction of BE is parallel to the loading axis. We have also revealed that there is a remarkable polarization in the BS stress; the cis -CH=CH- feels compression while the trans -CH=CH- does tension. On the other hand, the BS in the -CH_2-CH_2- shows tension in the gauche conformation and compression in the trans one, as if they balanced out the polarization of BS in the -CH=CH-. The connection of cis -CH=CH- and gauche -CH_2-CH_2- leads curled structure, while the other combination does straight one. Finally we have visualized the chain morphology by the conformation and evaluated the local density at each nodes, revealing that the former combination aggregates in the amorphous structure and doesn't dissolve under cyclic loading.
2009-07-25

分子動力学によるポリブタジエンの繰り返し変形シミュレーション : 負のbending応力によるひずみ軟化

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