In-situ X-ray scattering studies of a unique toughening mechanism in surface-modified carbon nanofiber/UHMWPE nanocomposite films

The toughening mechanism of nanocomposite films comprising ultrahigh molecular weight polyethylene (UHMWPE) and modified carbon nanofiber (MCNF) was investigated by in-situ synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) techniques during uniaxial stretching. Surface modification of carbon nanofibers included oxidation and subsequent chemical reaction with octadecylamine. At room temperature, the toughness of melt-pressed nanocomposite films was found to increase over 10 times by addition of 0.2 and 5 wt% of MCNF compared to that of pure UHMWPE. WAXD and SAXS results indicated that MCNF acted as a solvent carrier in the stiff UHMWPE matrix, whereby the grafted short hydrocarbon chains (n = 18) plasticized the surrounding UHMWPE chains in the nanoscale vicinity (10-20 nm) and induced interfacial flow under stretching, resulting in a large elongation-to-break ratio (>500%). A martensitic crystal transformation in UHMWPE was detected in all samples during deformation, where the transformation mode could be assigned as T ₁₂. At high temperature (118°C), the toughness of the MCNF/UHMWPE composite films was still about 2 times higher than that of pure UHMWPE. The mobile hydrocarbon layers at the UHMWPE/MCNF interface appeared to be the key to overcome the barrier of high chain entanglements in the solid UHMWPE matrix and to induce the significantly toughened performance.