  Polymer fibers are currently exploited in tremendously important
technologies. Their innovative properties are mainly determined by the behavior
of the polymer macromolecules under the elongation induced by external
mechanical or electrostatic forces, characterizing the fiber drawing process.
Although enhanced physical properties were observed in polymer fibers produced
under strong stretching conditions, studies of the process-induced nanoscale
organization of the polymer molecules are not available, and most of fiber
properties are still obtained on an empirical basis. Here we reveal the
orientational properties of semiflexible polymers in electrospun nanofibers,
which allow the polarization properties of active fibers to be finely
controlled. Modeling and simulations of the conformational evolution of the
polymer chains during electrostatic elongation of semidilute solutions
demonstrate that the molecules stretch almost fully within less than 1 mm from
jet start, increasing polymer axial orientation at the jet center. The
nanoscale mapping of the local dichroism of individual fibers by polarized
near-field optical microscopy unveils for the first time the presence of an
internal spatial variation of the molecular order, namely the presence of a
core with axially aligned molecules and a sheath with almost radially oriented
molecules. These results allow important and specific fiber properties to be
manipulated and tailored, as here demonstrated for the polarization of emitted
light.
