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TEM images of pure PAN fibers after stretching. (a) Straight and smooth electrospun PAN fibers. The crazing areas are noted by arrows. (b) Enlarged image of the frame in (a), showing the crazing deformation. The arrow indicates the direction of crazing extension in the fiber. (c) High resolution TEM image of a crazing fibril, showing amorphous structure with texture. The black arrow indicates the orientation of the texture. The inset is FFT of the image which confirms the texture. (d) Fracture surface of a pure PAN fiber.
TEM images of MWNT-PAN fibers after stretching. (a) MWNT-PAN fibers with a rough surface. (b) Enlarged image of the frame in (a), showing the crazing. (c) High resolution TEM image of MWNTs embedded in crazing area. (d) A protruding MWNT at the fracture surface of the composite fiber.
TEM images of SWNT-PAN fibers after stretching. (a) Regularly arranged crazing along the SWNT-PAN fiber. (b) Protruding SWNTs at the fracture surface of a broken fiber. Enlarged image of a SWNT is shown in the inset. (c) A SWNT bridging a broken fiber. No polymer is found to be attached to the surface of the SWNTs pulled out of the fiber.
Schematic illustration of the crazing and rupture of a CNT-PAN composite fiber under tension. (a) Voids form in front of notch as a result of tensile forces. (b) Voids are stretched and fibrils form at the crazing area. CNTs are aligned to be parallel to crazing fibrils. (c) Crazing extends across the fiber with CNTs well aligned. (d) Crazing fibrils are broken. CNTs reinforce the fiber by pull-out and bridging mechanism.
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