Kalathi Jagannathan T, Kumar Sanat K, Rubinstein Michael, Grest Gary S
Department of Chemical Engineering, Columbia University, New York, NY 10027, USA.
Soft Matter. 2015 May 28;11(20):4123-32. doi: 10.1039/c5sm00754b. Epub 2015 May 5.
Large-scale molecular dynamics simulations are used to study the internal relaxations of chains in nanoparticle (NP)/polymer composites. We examine the Rouse modes of the chains, a quantity that is closest in spirit to the self-intermediate scattering function, typically determined in an (incoherent) inelastic neutron scattering experiment. Our simulations show that for weakly interacting mixtures of NPs and polymers, the effective monomeric relaxation rates are faster than in a neat melt when the NPs are smaller than the entanglement mesh size. In this case, the NPs serve to reduce both the monomeric friction and the entanglements in the polymer melt, as in the case of a polymer-solvent system. However, for NPs larger than half the entanglement mesh size, the effective monomer relaxation is essentially unaffected for low NP concentrations. Even in this case, we observe a strong reduction in chain entanglements for larger NP loadings. Thus, the role of NPs is to always reduce the number of entanglements, with this effect only becoming pronounced for small NPs or for high concentrations of large NPs. Our studies of the relaxation of single chains resonate with recent neutron spin echo (NSE) experiments, which deduce a similar entanglement dilution effect.
大规模分子动力学模拟用于研究纳米颗粒（NP）/聚合物复合材料中链的内部弛豫。我们研究了链的Rouse模式，这一量在本质上最接近通常在（非相干）非弹性中子散射实验中测定的自中间散射函数。我们的模拟表明，对于NP与聚合物的弱相互作用混合物，当NP小于缠结网尺寸时，有效单体弛豫速率比在纯熔体中更快。在这种情况下，NP起到降低聚合物熔体中单体摩擦和缠结的作用，就像聚合物 - 溶剂体系那样。然而，对于大于缠结网尺寸一半的NP，在低NP浓度下有效单体弛豫基本不受影响。即使在这种情况下，我们也观察到对于较大的NP负载量，链缠结有显著减少。因此，NP的作用总是减少缠结数量，这种效应仅在小NP或高浓度大NP时才变得明显。我们对单链弛豫的研究与最近的中子自旋回波（NSE）实验结果一致，该实验推断出类似的缠结稀释效应。
