Ishiyama Masanari, Yasuoka Kenji, Asai Makoto
Department of Mechanical Engineering, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan.
Keio University Global Research Institute, Keio University, 2-15-45, Mita, Minato-ku, Tokyo 108-8345, Japan.
Soft Matter. 2022 Aug 31;18(34):6318-6325. doi: 10.1039/d2sm00311b.
Colloidal crystals have gathered wide attention as a model material for optical applications because of their feasibility in controlling the propagation of light by their crystal structure and lattice spacing as well as the simplicity of their fabrication. However, due to the simple interaction between colloids, the colloidal crystal structures that can be formed are limited. It is also difficult to adjust the lattice spacing. Furthermore, colloidal crystals are fragile compared to other crystals. In this study, we focused on polymer-grafted nanoparticles (PGNP) as a possible solution to these unresolved issues. We expected that PGNPs, composed of two distinct layers (the hard core of a nanoparticle and the soft corona of grafted polymers on the surface), will demonstrate similar behaviors as star polymers and hard spheres. We also predicted that PGNPs may exhibit polymorphism because the interaction between PGNPs strongly depends upon their grafting density and the length of the grafted polymer chains. Moreover, we expected that crystals made from PGNPs will be structurally tough due to the entanglement of grafted polymers. From exploration of crystal polymorphs of PGNPs by molecular dynamics simulations, we found face-centered cubic (FCC)/hexagonal close-packed (HCP) and body-centered cubic (BCC) crystals, depending on the length of the grafted polymer chains. When the chains were short, PGNPs behaved like hard spheres and crystals were arranged in FCC/HCP structure, much like the phase transition observed in an Alder transition. When the chains were long enough, the increase in the free energy of grafted polymers was no longer negligible and crystals were arranged in BCC structure, which has a lower density than FCC/HCP. When the chains were not too short or long, FCC/HCP structures were first observed when the volume fraction of system was small, but a phase transition occurred when the system was further compressed and the crystals arranged themselves in a BCC structure. These results most likely have laid strong foundations for future simulations and experimental studies of PGNP crystals.
胶体晶体因其能够通过晶体结构和晶格间距控制光的传播，以及制备过程简单，作为光学应用的模型材料受到了广泛关注。然而，由于胶体之间相互作用简单，可形成的胶体晶体结构有限。调整晶格间距也很困难。此外，与其他晶体相比，胶体晶体较为脆弱。在本研究中，我们将重点放在聚合物接枝纳米粒子（PGNP）上，以期解决这些尚未解决的问题。我们预期，由两个不同层组成的PGNP（纳米粒子的硬核和表面接枝聚合物的软冠层）将表现出与星型聚合物和硬球类似的行为。我们还预测，PGNP可能会表现出多晶型现象，因为PGNP之间的相互作用强烈依赖于它们的接枝密度和接枝聚合物链的长度。此外，我们预期由PGNP制成的晶体由于接枝聚合物的缠结在结构上会很坚韧。通过分子动力学模拟探索PGNP的晶体多晶型，我们发现了面心立方（FCC）/六方密堆积（HCP）和体心立方（BCC）晶体，这取决于接枝聚合物链的长度。当链较短时，PGNP的行为类似于硬球，晶体以FCC/HCP结构排列，很像在阿尔德转变中观察到的相变。当链足够长时，接枝聚合物自由能的增加不再可忽略不计，晶体以BCC结构排列，其密度低于FCC/HCP。当链既不太短也不太长时，当系统体积分数较小时首先观察到FCC/HCP结构，但当系统进一步压缩且晶体排列成BCC结构时会发生相变。这些结果很可能为未来PGNP晶体的模拟和实验研究奠定了坚实基础。
