Quantifying Polymer Chain Orientation in Strong and Tough Nanofibers with Low Crystallinity: Toward Next Generation Nanostructured Superfibers

Abstract

Advanced fibers revolutionized structural materials in the second half of the 20 th Century.However, all high-strength fibers developed to date are brittle. Recently, pioneering simultaneous ultrahigh strength and toughness were discovered in fine (<250 nm) individual electrospun polymer nanofibers (NFs). This highly desirable combination of properties was attributed to high macromolecular chain alignment coupled with low crystallinity. Quantitative analysis of the degree of preferred chain orientation will be crucial for control of NF mechanical properties. However, quantification of supramolecular nanoarchitecture in NFs with low crystallinity in the ultrafine diameter range is highly challenging. Here, we discuss applicability of traditional as well as emerging methods for quantification of polymer chain orientation in nanoscale one-dimensional samples. Advantages and limitations of different techniques are critically evaluated on experimental examples. It is shown that straightforward application of some of the techniques to subwavelength-diameter NFs can lead to severe quantitative and even qualitative artifacts. Sources of such size-related artifacts, stemming from instrumental, materials, and geometric phenomena at the nanoscale, are analyzed on the example of polarized Raman method, but are relevant to other spectroscopic techniques. A proposed modified, artifact-free method is demonstrated. Outstanding issues and their proposed solutions are discussed. The results provide guidance for accurate nanofiber characterization to improve fundamental understanding and accelerate development of nanofibers and related nanostructured materials produced by electrospinning or other methods. We expect that the discussion in this review will also be useful to studies of many biological systems that exhibit nanofilamentary architectures and combinations of high strength and toughness.

On the Importance of Noncrystalline Phases in Semicrystalline Electrospun Nanofibers

“…1,2 To make the most of electrospun (ES) fabrics, a thorough knowledge of their constituents' characteristics is essential. 3 An important, yet intricate characteristic of ES fibers is the nature and extent of their molecular orientation, which directly affects the microstructure and properties of individual fibers and their mats. 4 Many reports have outlined enhanced molecular orientation with a decrease in fiber diameter due to higher draw ratios, and to the lesser extent, confinement effects.…”

“…4 Many reports have outlined enhanced molecular orientation with a decrease in fiber diameter due to higher draw ratios, and to the lesser extent, confinement effects. [3][4][5][6] However, many details of the structure/property relationships remain unclear and are sometimes controversial for various systems. 3-5, 7, 8 Such discrepancies may arise from different intrinsic properties of polymers, fabrication parameters, or even from the characterization methods utilized at the fiber or at the mat scales.…”

WO3Nanowires Enhance Molecular Alignment and Optical Anisotropy in Electrospun Nanocomposite Fibers: Implications for Hybrid Light-Emitting Systems

“…If a concomitant study of the PVP and MEH-PPV molecular orientation is desired, it could be probed by using polarized infrared scanning microscopy techniques, such as scattering-type SNOM and atomic force microscopy-infrared spectroscopy, through spectroscopic investigation of vibrational modes characteristic of either PVP or MEH-PPV. 58 , 59 …”

Raman Investigation of the Processing Structure Relations in Individual Poly(ethylene terephthalate) Electrospun Fibers

“…24,25 However, our partial understanding of the processing-structure-properties relationships in electrospun fibers still limits their optimization and widespread application. 1,2,26 Confocal Raman spectromicroscopy is a technique of choice for investigating the structure of electrospun fibers because it allows quantification of molecular and supramolecular characteristics, such as molecular orientation, conformation, and crystallinity, at the individual fiber level. 1,26À31 This structural information can then be correlated to fiber diameter, processing parameters, and to the ultimate properties.…”

“…43À51 However, structural information obtained at the mat level depends not only on the orientation of the polymer chains but also on the alignment of fibers in the mat, leading to clear discrepancies between these studies and those conducted at the single fiber level. 1,26 Although strategies were developed to deconvolute the molecular orientation, such as maximizing the alignment or taking it into account using the Legendre addition theorem, 43,47 measurements at the mat level remain intrinsically affected by the presence of defects and by the fiber diameter polydispersity, both inherent in the electrospinning process. 1,26 The few published studies on the effect of collection on the structure of individual fibers stand out by revealing a strong impact of the nature of the electrospun polymer that appears consistent with the above-mentioned effect of crystallinity on the diameter-dependance of orientation.…”