Pal Subhadeep, Keten Sinan
Department of Civil and Environmental Engineering, Northwestern University, Evanston, IL, USA.
Department of Mechanical Engineering, Northwestern University, Evanston, IL, USA.
Soft Matter. 2024 Oct 9;20(39):7926-7935. doi: 10.1039/d4sm00718b.
Self-assembled polymer grafted nanoparticles (PGNs) are of great interest for their potential to enhance mechanical properties compared to neat polymers and nanocomposites. Apart from volume fraction of nanoparticles, recent experiments have suggested that nanoscale phenomena such as nanoconfinement of grafted chains, altered dynamics and relaxation behavior at the segmental and colloidal scales, and cohesive energy between neighboring coronas are important factors that influence mechanical and rheological properties. How these factors influence the mechanics of thin films subject to micro-ballistic impact remains to be fully understood. Here we examine the micro-ballistic impact resistance of PGN thin films with polymethyl methacrylate (PMMA) grafts using coarse-grained molecular dynamics simulations. The grafted chain length and nanoparticle core densities are systematically varied to understand the influences of interparticle spacing, cohesion, and momentum transfer effects under high-velocity impact. Our findings show that the inter-PGN cohesive energy density () is an important parameter for energy absorption. Cohesion energy density is low for short grafts but quickly saturates around entanglement length as adjacent coronas interpenetrate fully. The response of positively influences specific penetration energy, , which peaks before chain entanglement starts (<). We further divide the ballistic response into three regimes based on grafted chain length: short graft, intermediate graft, and entangled graft. The short grafted PGNs show fragmentation due to almost no cohesion between particles, and the rigid body motion of the nanoparticles absorbs most of the energy. When chains are in the intermediate graft length regime, the film fails by chain pull-out, and unraveling of grafts is the primary dissipation mechanism. The Ashby plot of penetration energy, , indicates ballistic processes are inelastic collisions when grafted chains are short and vary with density in a power law fashion as expected from momentum transfer. The response indicates that a lower nanoparticle weight fraction, wtNP, leads to higher energy absorption per mass, that is, the added mass of nanoparticles does not warrant proportionate increases in energy absorption in the parametric range studied. However, the peak deceleration, , shows a clear positive effect of adding NPs. Finally, PGNs with intermediate chain lengths simultaneously show relatively higher and .
自组装聚合物接枝纳米粒子（PGNs）因其与纯聚合物和纳米复合材料相比具有增强机械性能的潜力而备受关注。除了纳米粒子的体积分数外，最近的实验表明，诸如接枝链的纳米限域、链段和胶体尺度上改变的动力学和弛豫行为以及相邻冠层之间的内聚能等纳米尺度现象是影响机械和流变性能的重要因素。这些因素如何影响受到微弹道冲击的薄膜力学性能仍有待充分了解。在此，我们使用粗粒化分子动力学模拟研究了带有聚甲基丙烯酸甲酯（PMMA）接枝的PGN薄膜的微弹道抗冲击性。系统地改变接枝链长度和纳米粒子核心密度，以了解高速冲击下粒子间间距、内聚性和动量传递效应的影响。我们的研究结果表明，PGN之间的内聚能密度（）是能量吸收的一个重要参数。短接枝的内聚能密度较低，但随着相邻冠层完全互穿，在缠结长度附近迅速饱和。的响应积极影响特定穿透能，，其在链缠结开始之前（<）达到峰值。我们根据接枝链长度进一步将弹道响应分为三个区域：短接枝、中间接枝和缠结接枝。短接枝的PGN由于粒子之间几乎没有内聚性而表现出破碎，纳米粒子的刚体运动吸收了大部分能量。当链处于中间接枝长度区域时，薄膜因链拔出而失效，接枝的解开是主要的耗散机制。穿透能的阿什比图表明，当接枝链短时，弹道过程是非弹性碰撞，并且如动量传递所预期的那样，随密度呈幂律变化。该响应表明，较低的纳米粒子重量分数wtNP会导致单位质量的能量吸收更高，即在所研究的参数范围内，纳米粒子的附加质量并不能保证能量吸收成比例增加。然而，峰值减速度显示出添加纳米粒子的明显积极效果。最后，具有中间链长度的PGN同时显示出相对较高的和。
