Cellulose nanocrystals (CNCs) are highly polar, negatively-charged hydrophilic nanoparticles extracted from biomass through acid hydrolysis. They have high crystallinity and stiffness, as well as exceptional capability in reinforcing some polymer nanocomposites. While these remarkable improvements are limited to a certain class of polymers under strict conditions, CNCs can contribute to improving nanocomposite performance via other means, such as nucleating effect or cross-linking capacity. In this review, we offer critical insights into the reinforcing mechanism(s) of CNC-polymer nanocomposites and the factors that influence their performance. We aim to provide answers to the following questions: (1) how do CNCs contribute to nanocomposite reinforcement via percolation theory; (2) how to form a percolated CNC network in a CNC-polymer nanocomposite; (3) what are the factors affecting CNC performance and how do they work; and (4) how can we take advantage of CNC’s attributes to create high-performance nanocomposites?

纤维素纳米晶体（CNC）是高极性，带负电荷的亲水性纳米颗粒，通过酸水解从生物质中提取。它们具有高结晶度和刚度，以及增强某些聚合物纳米复合材料的出色能力。尽管这些显着的改进仅限于在严格条件下用于特定种类的聚合物，但CNCs可以通过其他方法（例如成核作用或交联能力）有助于提高纳米复合材料的性能。在这篇综述中，我们对CNC聚合物纳米复合材料的增强机理以及影响其性能的因素提供了重要的见解。我们旨在为以下问题提供答案：（1）CNCs如何通过渗流理论对纳米复合材料的增强做出贡献；（2）如何在CNC聚合物纳米复合材料中形成渗滤的CNC网络；（3）影响CNC性能的因素是什么以及它们如何工作？（4）我们如何利用CNC的属性来创建高性能的纳米复合材料？