4種の非晶ポリアミド(PA6, PA11, PA10T, PAMXD10)の引張応答の分子動力学シミュレーション

抄録

Polyamide (PA) is an engineering polymer the main chain is composed of repeating units of amide bonds (-NHCO-). In this study, we performed tensile simulations on four types of amorphous polyamides (PA6, PA11, PA10T, and PAMXD10) using the all-atom molecular dynamics simulation based on the AMBER force field. The monomers of PA6 and PA11 have 6 and 11 CH₂ straight nodes connected with amide bonds, while those of PA10T and PAMXD10 have aromatic rings with 10 CH₂ + amide bonds. PA10T shows the highest stress increase and PA11 does two-thirds of PA10T, while PA6 and PAMXD10 don’t show stress increase. Thus, the aromatic ring doesn’t directly contribute to the stress increase. We defined the straight segment ratio (SSR), which represents the ratio against the perfect straight chain (SSR=1 is the maximum length while SSR=0 is a circular shape). The SSR is 0.72 and 0.57 for PA10T and PA11, respectively, while that is 0.11 and 0.26 for PAMXD10 and PA6 in the initial equilibrium in this simulation. Investigation on the change in the radius of gyration, SSR, and the distribution of the start-to-end vector of each chain against periodic cell revealed that the PA10T and PA11 are composed of radial bundles of long straight chains, octuple (PA10T) and quadruple (PA11) against cell length, and elongate in the tensile direction and shrink in the lateral direction (Poisson contraction). On the other hand, the start-to-end chain length of PAMXD10 and PA6 is close to the cell length (curly chains), so they deform as fluid under tension without stress increase. We also investigated the change in the bond stretch and bending angle in each node and revealed that the C-H and C=O bonds are elongated while the backbone C-C bonds are not. The decrease in bending angles for these -H and =O nodes also suggests that the forces between these -H and =O atoms is the key interaction for stress increase, contrary to the direct entanglements of backbone chains in coarse-grained MD.