Israel Greenfeld

Local Mechanical Properties of Electrospun Fibers Correlate to Their Internal Nanostructure

The properties of polymeric nanofibers can be tailored and enhanced by properly managing the structure of the polymer 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 model 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 of the network toward the jet center with a local increase of the polymer density, as observed in the solid structure. The found complex internal structure opens an interesting perspective for improving and tailoring the molecular morphology and multifunctional electronic and optical properties of polymer fibers.

Conformational Evolution of Elongated Polymer Solutions Tailors the Polarization of Light-Emission from Organic Nanofibers.

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.

Controlling the Nanostructure of Electrospun Polymeric Fibers

The high strain rate extensional flow of a semi-dilute polymer solution can cause substantial stretching and disentanglement of the polymer network. In this study, we conducted a theoretical and experimental investigation of the effects of electrospinning, a flow governed by high strain rate and rapid evaporation, on the polymer matrix of the resulting nanofibers. Modeling of the dynamic evolution of the entangled polymer network in an electrospinning jet predicted substantial longitudinal stretching and radial contraction of the network, a transformation from an equilibrium state to an almost fully-stretched state. This prediction was verified by X-ray phase-contrast imaging of electrospinning jets, which revealed a noticeable increase in polymer concentration at the jet center, within a short distance from the jet start. Additionally, polymer entanglement loss in consequence of stretching was evidenced in jet fragmentation and appearance of short nanofibers, affecting the entanglements density and molecular orientation of as-spun fibers. The stretching model was expanded to semi-flexible conjugated polymer chains, and scanning near field optical microscopy of electrospun nanofibers of such optically active polymers revealed that the network’s dense elongated conformation effectively remains after jet solidification. By tuning the electrospinning conditions, the unique size-dependent properties of nanofibers can be controlled and improved, potentially leading to novel applications in engineering and life sciences.

Chapter 5:Polymer Network Dynamics during Electrospinning: Random Walk Simulation

High strain rate extensional flow of a semi-dilute polymer solution can cause substantial stretching of the polymer network. Random walk simulations of the polymer network in an electrospinning jet demonstrate substantial longitudinal stretching and radial contraction of the network, a transformation from an equilibrium state to an almost fully stretched state. The simulation also allows visualization of the polymer network and individual chains in various stages of the electrospinning jet. An effective potential, arising from the viscous and elastic forces and from boundary constraints, is applied to calculate the random walk stepping probabilities and to describe chain conformations and statistics. The simulation tool is described, and various flow conditions and polymer conformations are simulated and discussed. The difference between the dynamics of a free chain and a network in a flow field is clarified. The analysis contributes to understanding how electrospinning affects the polymer nanostructure, and how the unique properties of electrospun nanofibers might be improved.