Jhalaria Mayank, Cang Yu, Huang Yucheng, Benicewicz Brian, Kumar Sanat K, Fytas George
Department of Chemical Engineering, Columbia University, New York 10027, New York, USA.
School of Aerospace Engineering and Applied Mechanics, Tongji University, 100 Zhangwu Road, Shanghai 200092, China.
Phys Rev Lett. 2022 May 6;128(18):187801. doi: 10.1103/PhysRevLett.128.187801.
Brillouin light spectroscopy is used to measure the elastic moduli of spherical polymer-grafted nanoparticle (GNP) melts as a function of chain length at fixed grafting density (0.47 chains/nm^{2}) and nanoparticle radius (8 nm). While the moduli follow a rule of mixtures (Wood's law) for long chains, they display enhanced elasticity and anomalous dissipation for graft chains <100 kDa. GNP melts with long polymers at high σ have a dry zone near the GNP core, surrounded by a region where the grafts can interpenetrate with chain fragments from adjacent GNPs. We propose that the departures from Wood's law for short chains are due to the effectively larger silica volume fraction in the region where sound propagates-this is caused by the short, interpenetrated chain fragments being pushed out of the way. We thus conclude that transport mechanisms (of gas, ions, sound, thermal phonons) in GNP melts are radically different if interpenetrated chain segments can be "pushed out of the way" or not. This provides a facile new means for manipulating the properties of these materials.
布里渊光光谱法用于测量球形聚合物接枝纳米颗粒（GNP）熔体的弹性模量，该弹性模量是固定接枝密度（0.47链/nm²）和纳米颗粒半径（8纳米）下链长的函数。虽然对于长链，模量遵循混合法则（伍德定律），但对于接枝链分子量小于100 kDa的情况，它们表现出增强的弹性和异常耗散。在高接枝密度下，含有长聚合物的GNP熔体在GNP核附近有一个干燥区，周围是一个接枝可以与相邻GNP的链段相互渗透的区域。我们认为，短链偏离伍德定律是由于声音传播区域中二氧化硅的有效体积分数更大——这是由短的、相互渗透的链段被推开导致的。因此我们得出结论，如果相互渗透的链段能够或不能够被“推开”，GNP熔体中的传输机制（气体、离子、声音、热声子）会有根本不同。这为操纵这些材料的性能提供了一种简便的新方法。
