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Electrospun nanofibers (NFs) often demonstrate an exponential increase in mechanical and other properties at reduced diameters. Molecular orientation emerges as a key parameter for the performance of amorphous and low crystallinity polymers, but single-fiber structural investigations are still lacking for highly crystalline polymers. Herein, polarized confocal Raman spectroscopy reveals that fibers of highly crystalline poly(ethylene oxide) (PEO) maintain a high orientation over a broad range of diameters, in strong contrast with the usual exponential trend. This observation stands for five electrospinning solvents of widely different properties. By comparison, poly(oxymethylene) (POM) NFs also show a high orientation at low diameters, but it decreases substantially for diameters larger than ∼1400 nm, a result attributed to the lower crystallinity of POM compared to that of PEO. The results show that the exponential orientation dependence on fiber diameter is not universal and stress the importance of polymer crystallinity on the structure and properties of electrospun nanofibers. This work guides the preparation of fibers with optimal orientationdependent properties and shows that high crystallinity can afford more robust materials whose performance is less affected by variations in experimental conditions, a valuable feature for most applications.

Heavy atom main group element-containing conjugated polymers have attracted increasing attention in recent years. The synthesis of these compounds is generally involved, and little is known about their optoelectronic device performance. Here we examine the relationship between polymer structure and optoelectronic behavior in a series of chalcogenophene homopolymers of thiophene, selenophene, and tellurophene with well-matched molecular weights, dispersity, and regioregularity. We employ fast and slow drying device preparations to study the effect of polymer–fullerene separation on charge separation and collection in canonical bulk heterojunction photovoltaic cells. In both preparations, increasing heteroatom size leads to larger proportions of finely mixed polymer–fullerene domains. Differences in polymer–fullerene separation between preparations result in the formation of optimal morphologies in selenophene and tellurophene devices with little impact on thiophene devices. We then use planar heterojunction devices to directly examine the effects of heteroatom substitution on charge transport and charge generation and find that in the absence of polymer–fullerene mixing, devices exhibit similar diode behavior. We further demonstrate that ultrafast decay pathways unique to heavy heteroatom-containing polymers are apparent in both planar and bulk heterojunctions and thus not dependent on polymer–fullerene mixing or polymer assembly. This work directly examines the role of heteroatom substitution in defining the photovoltaic performance of conjugated homopolymers. Through single-atom substitution we are able to significantly modify polymer assembly, mixing, and optoelectronic properties. Specific emphasis on tellurophene polymers reveals relationships between polymer structure and properties that are not apparent in more traditional light-atom chalcogenophenes such as thiophene and selenophene.

Electrospun fibers often exhibit enhanced properties at reduced diameters, a characteristic now widely attributed to a high molecular orientation of the polymer chains along the fiber axis. A parameter that can affect the molecular organization is the type of collector onto which fibers are electrospun. In this work, we use polarized confocal Raman spectromicroscopy to determine the incidence of the three most common types of collectors on the molecular orientation and structure in individual fibers of a broad range of diameters. Poly(ethylene terephthalate) is used as a model system for fibers of weakly crystalline polymers. A clear correlation emerges between the choice of collector, the induced molecular orientation, the fraction of trans conformers, and the degree of crystallinity within fibers. Quantitative structural information gathered by Raman contributes to a general description of the mechanism of action of the collectors based on the additional strain they exert on the forming fibers.