The mean IFSS of non-functionalized PR-24-HHT-LD grade nanofibers to EPON epoxy was 66±10 MPa based on the nominal interfacial area without accounting for the surface roughness of each nanofiber. Much higher IFSS values of 215±12 MPa were measured for oxidatively surface treated nanofibers (PR-24-HHT-OX) that also had larger peak-to-valley surface roughness (~5 nm) compared to the non-functionalized nanofibers. Consequently, surface functionalization and chemical modification of VGCNFs dramatically increased their IFSS by more than a factor of three compared to the non-functionalized primitive (PR-24-HHT-LD grade). 0 50 100 150 200 250 0.0 0.4 0.7 1.1 1.4 Embedded length (μm) IFSS (MPa) HHT HHT-OX (a) (b) Figure 4. (a) Force vs. crosshead displacement (dominated by the loadcell extension) of a pull-out experiment. (b) IFSS of high temperature heat treated (HHT) and oxidized HHT (HHT-OX) VGCNFs. The average IFSS values for both nanofiber grades indicate that the shear strength of the matrix near the nanofiber is much higher than the typical bulk value of about 40 MPa, which implies that the very existence of the carbon nanofiber could alter the shear properties of the polymer at the interface. The nanometer-level surface roughness of the VGCNFs potentially creates an undulating fracture surface increasing the apparent critical energy release rate. Compared to literature results on the IFSS of CNTs in a polymer matrix obtained in references [3,4], the IFSSs obtained in this study were characterized by much smaller data scatter. Literature data on the IFSS between CNTs and various polymeric matrices varied in the wide range of 20-376 MPa although specimens were taken from the same manufacturing batch [3,4,10]. The IFSS values reported in this study are larger than the corresponding experimental pull-out values for micron sized carbon fibers, which can be as low as 28 MPa for specimens without any surface treatment and can attain values as high as 64.6 MPa following chemical functionalization or surface treatment [9,11]. The average IFSS of around 66 MPa for the PR-24-HHT-LD grade VGCNF-epoxy interface indicates that even the adhesion of the heat-treated, non-functionalized carbon nanofiber grade is almost as good as its chemically functionalized and oxidized microscale counterparts. Such improved adhesion could arise from nanoscale details, such as preferable molecular orientation of polymeric matrix chains along the nanoscale fiber-matrix interface or the presence of denser covalent bonding due to intrinsic defects in the nanofiber exterior surface and its increased surface roughness. Furthermore, such size dependence in IFSS could as well arise from the significantly smaller singular stress region where failure initiates in a nanofiber compared to a microscale fiber. The IFSS values reported in this study also correlate reasonably well with trends in composite fracture toughness measurements performed with the same material systems, indicating the direct relationship between local and effective nanocomposite properties [12,13]. 78
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