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The goal of this project is to investigate the interfacial stress transfer between PMMA and embedded individual carbon nanotubes (CNT) in CNT-reinforced PMMA polymers. Our approach is to first engineer doubly-clamped CNT bridges with both ends fully embedded into the polymer (PMMA). The interfacial stress transfer between PMMA and the embedded CNT will be obtained by atomic force microscopy (AFM)-based pull-out experiments. The key challenge of this research is the preparation of free-standing CNT bridges that are suited for the AFM pull-out measurements. We have made significant progress in the preparation of free-standing CNT bridges, as exemplified by the AFM image shown in Figure 1. The CNT bridge was formed by three spin-coating processes: (1) a polymer layer; (2) a diluted CNT layer; (3) another polymer layer, which was followed by patterning of the polymer layers using photolithograph. Our next step research work is to fine-tune our fabrication process to significantly increase the yield of the free-standing CNT bridges and to perform the AFM pull-out measurements. In addition to the CNT bridge preparation, we were also making progress on the fabrication of the focused-ion beam (FIB) modified AFM probes, which are needed for the AFM-based pull-out test. For the time-being, two FIB-modified AFM probes have been manufactured and are available for our pull-out tests.  

Our research progress from this PRF support also includes the mechanical testing of carbon nanotubes and the interfacial binding interaction among tubes in the bundle structure and between nanotube and substrate.  Our research results reveal that the interfacial interaction between nanotubes can have a significant impact on its mechanical deformation, including the adhesion-driven buckling. Our research also reveals that the interfacial interaction between the CNT and the substrate is strongly modulated by the deformation of the CNT. These results, which were reported in two peer-reviewed journal articles, are very useful to the understanding of the interfacial binding interaction in 1D nanostructures and the pursuit of adhesion-involved applications, including polymer composites.

In addition to the study of the interfacial stress transfer between PMMA and CNT, we are pursuing new research direction to study the interfacial stress transfer between polymer and graphene sheet, which is a key and challenging issue for the emerging graphene-based polymer composite. This new research direction is a direct extension of the research work supported by this PRF grant.  The manufacturing process that we demonstrated for engineering free-standing CNT bridges is currently being applied to fabricate free-standing graphene sheet with one end embedded into the polymer. The interfacial shear strength between the graphene and the polymer will be characterized by in-situ SEM nanomechanical testing which utilizes a flexible AFM cantilever as force sensor.  Our experimental measurements will be interpreted by molecular dynamical simulation through an established collaboration with Dr. Rajesh Khare at Texas Technology University. The effect of the surface functionalization of the graphene on the interfacial strength of graphene/polymer interface will be investigated through an established collaboration with Dr. Shiren Wang at Texas Technology University. Currently our three research groups are preparing a collaborative research proposal to seek research funding from National Science Foundation to support this research work.