Atomic scale features of polyvinylidene fluoride molecules (PVDF) were observed with aberration corrected transmission electron microscopy. Thin, self-supporting PVDF nanofibers were used to create images that show conformations and relative locations of atoms in segments of polymer molecules, particularly segments near the surface of the nanofiber. Rows of CF2 atomic groups, at 0.25 nm intervals, which marked the paths of segments of the PVDF molecules, were seen. The fact that an electron microscope image of a segment of a PVDF molecule depended upon the particular azimuthal direction, along which the segment was viewed, enabled observation of twist around the molecular axis. The 0.2 nm side-by-side distance between the two fluorine atoms attached to the same carbon atom was clearly resolved. Morphological and chemical changes produced by energetic electrons, ranging from no change to fiber scission, over many orders of magnitude of electrons per unit area, promise quantitative new insights into radiation chemistry. Relative movements of segments of molecules were observed. Promising synergism between high resolution electron microscopy and molecular dynamic modeling was demonstrated. This paper is at the threshold of growing usefulness of electron microscopy to the science and engineering of polymer and other molecules.

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.Figure optionsDownload full-size imageDownload as PowerPoint slide

Electrospun nanofibers were captured directly between two steel rods that functioned as the “grips” of the tensile testing apparatus. Tension was applied to the selected nanofiber by displacing one of the grips at controlled rates or in steps. The stress was revealed by the deflection of a nanofiber, caused by the drag force from a broad stream of air, which flowed perpendicular to the fiber at a known velocity. The deflected position and shape of the nanofiber was observed with a light arrangement optimized to produce bright glints that were photographed with a camcorder. Image analysis of the catenary shapes of the nanofibers was combined with scanning electron microscopy measurements of the diameter of the ends of the tested fibers to evaluate the mechanical properties.Measurements of properties, including tensile strength, tensile modulus and elongation-to-break, of thin electrospun fibers were obtained on six chemically different polymers: nylon 6, poly(ethylene oxide), polyvinylpyrrolidone, poly(2-ethyl-2-oxazoline), Tecoflex® and Tecophilic® polyurethanes. To the best of our knowledge, this is the first report of tensile data on single polyvinylpyrrolidone and poly(2-ethyl-2-oxazoline) nanofibers. These soft nanofibers with low strain to break rarely survive the sample loading procedures where single fiber manipulation is involved. This method complements difficult mechanical measurements of polymer nanofibers and low strength microfibers made on miniature mechanical testing devices. Mechanical hysteresis curves were attained that show the recoverable and non-recoverable tensile deformation of PEO, nylon and Tecophilic® polyurethane fibers.

When a 25 wt% polystyrene solution was electrospun at relatively low applied voltage, the bending instability initiated, grew and disappeared, repeatedly. Upon further lowering of the applied voltage, the bending instability died immediately after it started and only a straight electrified jet was observed. By manipulating the bending instability of the jet, uniform polystyrene hierarchical patterns were produced by a buckling process.The observed buckling patterns are similar to those of a gravity-driven jet. The conditions for producing buckled polystyrene patterns are reproducible. These small scale buckled polymer patterns can be made very long and uniform. The size of the buckling coil was adjusted by changing the distance between tip and collector. Different large scale patterns, that consist of tiny buckled polymer patterns, can be written by a programmed machine. Fluffy fiber balls, composed of buckling coils, were produced.Graphical abstract

Optical observations of jets provide information useful for control of electrospinning of polymer solutions. Combinations of videography, stereography, and methods for illumination of the multiple coils of an electrospinning jet path, that depend on bright glints of reflected light from the jet, recorded quantitative information about the location, vector velocity, and rotation of selected segments of the jet. New bending coils were observed to form at rates of 200–1560 turns per second. This bending frequency decreased as the capillary number of the solutions increased. The polarization of light, in glints reflected at Brewster's angle, allows measurement of the index of refraction of the fluid jet, in flight. Asymmetric illumination of an electrospinning jet made both the handedness and changes in handedness of the electrical bending coils apparent to visual observation and in 2-dimensional images. Evidence was found, in the form of polarized ribbon-like glint traces, for the occurrence of undulations on the surface of some jets.Graphical abstract

The electrospinning technique has become a widely used method to produce nanofibers. Much experimental and theoretical work has been done to investigate the electrospinning process. A typical path of a single jet of polymer solution begins with a straight segment, and then a bending instability generates coils inside a cone shaped envelope. The jet elongates and becomes thinner, and dries into a nanofiber.Polystyrene solutions with different salt concentrations were electrospun to investigate the effect of changing the electrical conductivity on the fibers that were formed. Salt increased the conductivity of the solution and smooth fibers formed. Electrospinning a polystyrene solution with salt, at relatively high voltage, caused the jet to first form bending coils with small and slowly increasing diameters. These small diameter coils were subsequently incorporated into a larger diameter bending instability coil. That is, a coil with a slender envelope cone developed first and this slender coil formed a larger bending coil with a more rapidly increasing diameter. This unusual situation produced a complicated jet path, which is called a “garland”. The coils entangled and conglutinated in flight to form a fluffy network of fibers. The conditions for the formation of garlands also provide examples of jet splitting and branching during the electrospinning process.