Molecular scale imaging and observation of electron beam-induced changes of polyvinylidene fluoride molecules in electrospun nanofibers

Transmission electron micrograph images, made at high magnification, of electrospun nanofibers of polyvinylidene fluoride showed rows of dark dots, separated by about 0.24 nm, along segments of molecules. The thin fibers supported themselves across tiny holes, so there was no support material in the field of view. The dots were seen because the electron density of the CF2 groups is three times that of the intervening 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, associated with the planar zigzag of backbone carbon atoms and the average lateral separation of the molecules, was maintained as the radiation gradually modified the polymer molecules. These high magnification images showed surprising persistence of the chain-like morphology and segmental motion. Primary radiation damage events were dominant. Many more numerous and damaging secondary radiation events that are encountered in thicker samples, or in support films were almost completely avoided, since the only nearby material where secondary radiation could be generated was in the very thin fiber. The nanofibers contained from 50 to a few hundred molecules in a typical cross section. Irradiation severed the molecules at slow rates until only two or three molecules remained in the fiber, and finally the fiber broke. Evidence was noted that irradiation with electrons also caused loss of fluorine atoms, cross-linking, and chain scission. The entire observed segments of the nanofibers were small enough for detailed comparison of images with calculated molecular models. (C) 2013 Elsevier Ltd. All rights reserved.