Laminated composites mostly suffer from layer separation and/or delamination, which may affect the stiffness, strength and lifetime of structures. In this study, we aim to produce micronscale thin carbon nanotubes (CNTs) reinforced adhesive nanofibrous interleaves and to explore their effectiveness when incorporated into structural composites. Neat polyvinyl butyral (PVB) and solutions containing low fractions of CNTs from 0.5 to 2 wt.% were electrospun directly onto carbon fiber prepregs. These interlayered laminates were cured above the glass transition temperature (Tg) of PVB to achieve strong interlaminar binding and also to resist crack reinitiation. The effect of CNTs presence and their mass fractions both on total Mixed-Mode I+II fracture toughness (GC) and crack length was investigated under Mixed-Mode I+II loading. Almost 2-fold increase in GC was reported in interlayered composites compared to noninterlayered laminates, associated to toughening effect of adhesive PVB/CNTs nanofibrous interlayers. Furthermore, the post-fracture analysis revealed the aid of CNTs interleaves in retarding delamination and afterward stabilization of crack propagation.

Interphase effects have been studied for their effect on composite properties for many decades, and it is well documented that an interphase can exist in polymer composites comprised of nanofibers as well. We present a first study of interphase effects on the basic elastic response of wavy aligned carbon nanotube (A-CNT) polymer nanocomposties (PNCs). Waviness is characterized by ex situ pre-fabrication imaging of A-CNT forests and used as an input to finite element analyses of the PNCs containing an interphase region in the thermoset polymer defined using molecular dynamics (MD) simulations. The interphase thickness of ~1nm is found to be independent of crosslink density and contain regions of both higher and lower mass density than the bulk polymer. Finite element analyses of wavy single and double-wall A-CNT PNCs incorporating this interphase, allow the effective stiffness based on a representative volume element to be calculated. Waviness of the A-CNTs dominates the effective axial stiffness of the PNCs, with the interphase having a negligible effect. The interphase changes the stress and strain distribution local to the CNT 'fiber' and this is expected to play an important role in failure, such as CNT pullout, of the CNTpolymer system. These findings, in addition to the relatively high volume fraction (V f ) of the interphase in PNCs with high CNT V f , suggests that the interphase may play a more important role in PNCs than in micron-scale (typical) collimated fiber composites. Future work in this area includes inelastic polymer response during nanofiber pullout. Nomenclature A = waviness amplitude of CNT A-PNC = aligned-CNT polymer nanocomposite Case-A = DWCNT outer wall contribution to stiffness Case-B = DWCNT all walls contribution to stiffness CNT = carbon nanotube CFRP = carbon fiber reinforced plastic DWCNT = double walled carbon nanotube d i = inner diameter of CNT d o = outer diameter of CNT 2 E RVE = axial modulus of representative volume element E w = wall modulus FEA = finite element analysis FRP = filament fiber reinforced plastic HR-SEM = high resolution scanning electron microscope L = length of wavy CNT MD = molecular dynamics PNC = polymer nanocomposite RVE = representative volume element SWCNT = single walled carbon nanotube t w = wall thickness w = waviness ratio of CNT

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