Electron micrograph images of CF2 groups, in electrospun nanofibers of polyvinylidene fluoride, appeared as rows of dots, separated by about 0.24 nanometers, along segments of molecules.  The dots arise from the three times greater electron density of the CF2 groups than the CH2 groups.  The polymer nanofibers contained crystals with the polymer chains aligned predominately along the axis of the fiber.  A significant degree of long-range translational symmetry was maintained by extended segments of the polymer molecules.  The molecules in the fibers contained a variety of non-planar zigzag conformations. A cross section of a nanofiber typically contained one or more crystals, each with its own polarization direction.  The observed segments were comparable in scale with those considered in molecular dynamics calculations and crystal defect models. The nanofibers supported themselves over open holes.  Confusion, from structures within a substrate, was thereby eliminated.  Significant observations of polymer morphology were recorded before modification to chemical bonds by the electron beam of the microscope affected the morphology of the nanofibers.  Modification of bonds by secondary radiation was negligible, since the only nearby material where secondary radiation could be generated was in the very thin fiber.The nanofibers, as made, contained from 50 to about 500 molecules in a cross section.  Irradiation with electrons caused loss of fluorine atoms, cross-linking, and chain scission. During prolonged irradiation, the nanofibers gradually became thinner.  A short segment, as thin as one molecule, was observed just before a fiber broke.

Inside nanofibers: Molecular scale images and electron beam induced changes

247th National Meeting (2014)

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