Orientation and Partial Disentanglement in Individual Electrospun Fibers: Diameter Dependence and Correlation with Mechanical Properties

Electrospun fibers are versatile materials that exhibit unusual and tunable properties when studied at the single fiber level, including an exponential increase in modulus with a decreasing diameter toward the nanoscale. Understanding the detailed molecular organization giving rise to this behavior is a key for reaching the ultimate goal of controlling their properties and realizing their full potential as 1D materials. In particular, molecular orientation and chain disentanglement are thought to play a critical role, but their study in individual fibers has proven extremely challenging. Here, we quantify molecular orientation in more than 100 individual fibers of atactic polystyrene (PS) from the micro- to nanoscale by polarized Raman spectroscopy, and we probe for the first time a disentanglement-related conformation in the same spectra. We observe an exponential increase of both parameters when decreasing the fiber diameter below an impressively large onset of 2.5 mu m, much larger than the typical onset values. The orientation quantified for 500 nm fibers is among the highest values ever reported for PS samples. A clear correlation is found with the previously published diameter dependence of modulus measured in individual PS fibers. Our results also highlight the longitudinal and radial structural heterogeneity of electrospun fibers and suggest separate mechanisms for molecular orientation and disentanglement, which is shown to be mainly situated in the fiber shell. Finally, we combine our observations in a model describing the evolution of chain organization with fiber diameter.