  The properties of polymeric nanofibers can be tailored and enhanced by
properly managing the structure of the polymeric molecules at the nanoscale.
Although electrospun polymer fibers are increasingly exploited in many
technological applications, their internal nanostructure, determining their
improved physical properties, is still poorly investigated and understood.
Here, we unravel the internal structure of electrospun functional nanofibers
made by prototype conjugated polymers. The unique features of near field
optical measurements are exploited to investigate the nanoscale spatial
variation of the polymer density, evidencing the presence of a dense internal
core embedded in a less dense polymeric shell. Interestingly, nanoscale mapping
the fiber Young's modulus demonstrates that the dense core is stiffer than the
polymeric, less dense shell. These findings are rationalized by developing a
theoretical modeling and simulations of the polymer molecular structural
evolution during the electrospinning process. This model predicts that the
stretching of the polymer network induces a contraction towards the jet center
of the network with a local increase of the polymer density, as observed in the
solid structure. The found complex internal structure opens interesting
perspective for improving and tailoring the molecular morphology and
multifunctional electronic and optical properties of polymer fibers.
