Quantitatively identify and understand the interphase of SiO₂/rubber nanocomposites by using nanomechanical mapping technique of AFM

The interphase between nanofillers and rubber matrix, also known as “bound rubber (BR)” composed of a tightly BR (TBR) layer that strongly interacts with the filler and a loosely BR (LBR) layer that is physically adsorbed, plays an important role in the properties of rubber nanocomposites. Up to now, there is seldom direct evidence of such interfacial double layer structure and their thickness. In this study, we quantitatively identified the interphase of a representative nano silicon dioxide (SiO₂)/rubber composites by using peak force quantitative nanomechanical mapping (PFQNM) mode of Atomic Force Microscope (AFM). The thickness of interphase was quantitatively obtained, and the double layer structure of interphase was directly identified based on PFQNM images and the corresponding force-distance curves, and was further evidenced by high resolution transmission electron microscopy. We further studied the effect of molecular polarity on interphase of SiO₂/hydrogenated nitrile butadiene rubber (HNBR) composites. Interestingly, the molecular polarity of HNBR has almost no effect on the thickness of TBR layer but has a significant effect on the LBR layer, leading to the remarkable increase in the total interfacial thickness of the composites with increasing the acrylonitrile content. The mechanism for the formation of interfacial double layer structure of BR and the effect of molecular polarity on the double layer structure was discussed. This study provides a simple method to identify and deeply understand the double layer structure of the interphase, and thus provides guidance for the design of interphase for the preparation of high performance rubber nanocomposites.