Khairy Yasmin, Alvarez Fernando, Arbe Arantxa, Colmenero Juan
Physics Department, Faculty of Science, Zagazig University, Zagazig 44519, Egypt.
Centro de Física de Materiales (CSIC, UPV/EHU), Paseo Manuel de Lardizabal 5, E-20018 San Sebastián, Spain.
Polymers (Basel). 2021 Feb 23;13(4):670. doi: 10.3390/polym13040670.
We present fully atomistic molecular dynamics simulations on polyisobutylene (PIB) in a wide temperature range above the glass transition. The cell is validated by direct comparison of magnitudes computed from the simulation and measured by neutron scattering on protonated samples reported in previous works. Once the reliability of the simulation is assured, we exploit the information in the atomic trajectories to characterize the dynamics of the different kinds of atoms in PIB. All of them, including main-chain carbons, show a crossover from Gaussian to non-Gaussian behavior in the intermediate scattering function that can be described in terms of the anomalous jump diffusion model. The full characterization of the methyl-group hydrogen motions requires accounting for rotational motions. We show that the usually assumed statistically independence of rotational and segmental motions fails in this case. We apply the rotational rate distribution model to correlation functions calculated for the relative positions of methyl-group hydrogens with respect to the carbon atom at which they are linked. The contributions to the vibrational density of states are also discussed. We conclude that methyl-group rotations are coupled with the main-chain dynamics. Finally, we revise in the light of the simulations the hypothesis and conclusions made in previously reported neutron scattering investigations on protonated samples trying to address the origin of the dielectric β-process.
我们展示了在高于玻璃化转变温度的宽温度范围内对聚异丁烯（PIB）进行的全原子分子动力学模拟。通过将模拟计算得到的幅度与先前工作中报道的质子化样品的中子散射测量值进行直接比较，验证了模拟单元。一旦确保了模拟的可靠性，我们就利用原子轨迹中的信息来表征PIB中不同种类原子的动力学。所有原子，包括主链碳，在中间散射函数中都表现出从高斯行为到非高斯行为的转变，这可以用反常跳跃扩散模型来描述。甲基氢运动的完整表征需要考虑旋转运动。我们表明，通常假设的旋转运动和链段运动的统计独立性在这种情况下不成立。我们将旋转速率分布模型应用于为甲基氢相对于它们所连接的碳原子的相对位置计算的相关函数。还讨论了对振动态密度的贡献。我们得出结论，甲基旋转与主链动力学耦合。最后，根据模拟结果，我们对先前报道的关于质子化样品的中子散射研究中提出的假设和结论进行了修订，试图解决介电β过程的起源问题。
