Understanding the mechanical properties of polymer nanocomposite materials is essential for industrial use. Particularly, the determination of the polymer modulus at the nanofiller–polymer interphase is important for optimizing the interfacial mechanical properties. Nanoindentation via Atomic Force Microscopy (AFM) is well-established for measuring the modulus of the interphase region with nanoscale spatial resolution. However, indentation into heterogeneous materials presents a confounding issue often referred to as the “substrate effect”, i.e., the structural stress field caused by the rigid body is convoluted with the actual modulus of the interphase region. While finite element analysis (FEA)-based methods can be used to deconvolute the interphase modulus from measured apparent modulus–distance profiles, the experimental validation of this method is still needed. Here, we provide this validation using AFM nanoindentation on a layered model composite that consists of three layers with different moduli to recapitulate the properties of the matrix, the filler, and the interphase of real polymer nanocomposites. By systematically varying the thickness of the “artificial” interphase layer and the AFM probe radius, we obtain modulus–distance profiles over a wide range of indentation conditions. We validate a method to deconvolute the substrate effect using an empirically derived master curve obtained from FEA analysis. Furthermore, we showed that the effect of the artificial interphase on modulus– distance profiles can be distinguished only if the interphase layer is thick enough compared to the contact radius of the probe. Finally, we established an innovative and quantitative framework to predict the interphase thickness from mechanical nanoindentation measurements and discussed the lower, practical limit for interphase thickness determination. In summary, we provide a broadly applicable method to extract interphase mechanical properties of multiphase soft materials and practical guidelines for choosing optimal characterization conditions.

中文翻译：

---

将 AFM 推向极限：刚体附近的相间机械性能测量


了解聚合物纳米复合材料的机械性能对于工业应用至关重要。特别是，纳米填料-聚合物界面处聚合物模量的测定对于优化界面力学性能非常重要。通过原子力显微镜 （AFM） 进行纳米压痕对于以纳米级空间分辨率测量界面区域的模量是公认的。然而，压痕进入异质材料会带来一个混杂问题，通常被称为“基底效应”，即由刚体引起的结构应力场与相间区域的实际模量复杂化。虽然可以使用基于有限元分析 （FEA） 的方法从测量的视模量-距离分布中去卷积相模量，但仍需要对该方法进行实验验证。在这里，我们在分层模型复合材料上使用 AFM 纳米压痕提供这种验证，该复合材料由具有不同模量的三层组成，以概括真实聚合物纳米复合材料的基质、填料和界面的特性。通过系统地改变“人工”界面层的厚度和 AFM 探针半径，我们获得了各种压痕条件下的模量-距离分布。我们验证了一种使用从 FEA 分析中获得的经验得出的主曲线对衬底效应进行去卷积的方法。此外，我们表明，只有当相间层与探针的接触半径相比足够厚时，才能区分人工界面对模量-距离分布的影响。 最后，我们建立了一个创新的定量框架，从机械纳米压痕测量中预测相间厚度，并讨论了相间厚度测定的实用下限。总之，我们提供了一种广泛适用的方法来提取多相软材料的相间力学特性，并为选择最佳表征条件提供了实用指南。