Size effects in continuous polyacrylonitrile-based polymer, composite, and carbon nanofibers

Nanotechnology is expected to produce the next revolution in structural materials and composites. However, despite significant efforts worldwide, a supernanocomposite reinforced by carbon nanotubes or graphene is yet to be demonstrated. The problems are well documented and stem from the discontinuous nature of nanoreinforcement. Continuous nanofibers produced by electrospinning represent an emerging class of nanomaterials with critical advantages for structural and functional applications. However, until recently they were considered weak, and their mechanical properties and structure have not yet been sufficiently studied. The goal of this dissertation was systematic analysis of size effects on mechanical properties and structure of polyacrylonitrile-based polymer and carbon nanofibers. Control of nanofiber morphology, average diameter and diameter distribution was achieved through process and solution parameters. Individual polyacrylonitrile nanofibers were fabricated in a wide range of diameters and tested through failure. Simultaneous increases of two orders of magnitude in strength and modulus as well as three orders of magnitude in toughness (area under the stress/strain curve) were observed for ultrafine (\u3c250nm) nanofibers for the first time. Structural investigation and comparison with mechanical behavior of annealed nanofibers allowed attribution of the observed simultaneous size effects to improved polymer chain alignment coupled with low crystallinity due to rapid solvent evaporation from fine jets in electrospinning process. This structural hypothesis was further verified and supported by modification of the nanofiber structure and properties through change in solvent and addition of plasticizer. Examination of the structure of polyacrylonitrile-based carbon fibers (CNFs) revealed generally poor graphitic structure (compared to commercial carbon fibers) with some improvements in the structure and crystal orientation for thinnest nanofilaments. Graphitic structure and crystal orientation were successfully improved through addition of small amounts of carbon nanotubes and graphene oxide. These structural modifications hold the potential for improvements in mechanical and transport properties of CNFs. Results of this work and the proposed structural explanations constitute a possible paradigm shifting approach in the fiber science and technology for the development of the next generation of advanced fibers

Polarized raman analysis of polymer chain orientation in ultrafine individual nanofibers with variable low crystallinity

Recent mechanical studies of ultrafine electrospun polymer nanofibers showed they can simultaneously possess high strength and toughness attributed to high degree of chain alignment coupled with low crystallinity. Quantitative analysis of macromolecular alignment in nanofibers is needed for better understanding of processing/structure/properties relationships and optimization. However, quantification of structural features in nanofibers with ultrafine diameters and low variable crystallinity is highly challenging. Here, we show that application of standard orientation analysis protocols developed for polarized Raman microscopy of bulk polymers and films can lead to severe errors in subwavelength diameter samples. A modified polarized Raman method is proposed and implemented for study of size-dependent orientation in individual nanofibers as small as 140 nm. Macromolecular alignment improved significantly with the reduction of nanofiber diameter, correlating with nanofiber modulus increase. Applicability of the proposed method for quantitative comparative studies of nanofiber systems fabricated from solutions with different solvents is demonstrated

Dielectric and calorimetric signatures of chain orientation in strong and tough ultrafine electrospun polyacrylonitrile

International audienc

Dielectric and calorimetric signatures of chain orientation in strong and tough ultrafine electrospun polyacrylonitrile

International audienc

Hierarchical Mechanisms of Lateral Interactions in High- Performance Fibers

The processing conditions used in the production of advanced polymer fibers facilitate the formation of an oriented fibrillar network that consists of structures spanning multiple length scales. The irregular nature of fiber tensile fracture surfaces suggests that their structural integrity is defined by the degree of lateral (interfacial) interactions that exist within the fiber microstructure. To date, experimental studies have quantified interfacial adhesion between nanoscale fibrils measuring 10−50 nm in width, and the global fracture energy through applying peel loads to fiber halves. However, a more in-depth evaluation of tensile fracture indicates that fiber failure typically occurs at an intermediate length scale, involving fibrillation along interfaces between fibril bundles of a few 100s of nanometers in width. Interaction mechanisms at this length scale have not yet been studied, due in part to a lack of established experimental techniques. Here, a new focused ion beam-based sample preparation protocol is combined with nanoindentation to probe interfaces at the intermediate length scale in two high-performance fibers, a rigid-rod poly(p-phenylene terephthalamide) and a flexible chain ultrahigh molecular weight polyethylene fiber. Higher interfacial separation energy recorded in the rigid-rod fiber correlated with less intensive fibrillation during failure and is discussed in the context of fiber chemistry and processing. Power law scaling of the total absorbed interfacial separation energy at three different scales in the polyethylene fiber is observed and analyzed, and distinct energy absorption mechanisms, featuring a degree of self-similarity, are identified. The contribution of these mechanisms to the overall integrity of the fiber is discussed, and the importance of the intermediate scale is elucidated. Results from this study provide new insights into the mechanical implications of hierarchical lateral interactions and will aid in the development of novel fibers with further improved mechanical performance

Quantifying polymer chain orientation in strong and tough nanofibers with low crystallinity : towards next generation nanostructured superfibers

Advanced fibers revolutionized structural materials in the second half of the 20th 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 the 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 sub-wavelength-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

Simultaneously Strong and Tough Ultrafine Continuous Nanofibers

Strength of structural materials and fibers is usually increased at the expense of strain at failure and toughness. Recent experimental studies have demonstrated improvements in modulus and strength of electrospun polymer nanofibers with reduction of their diameter. Nanofiber toughness has not been analyzed; however, from the classical materials property trade-off, one can expect it to decrease. Here, on the basis of a comprehensive analysis of long (5–10 mm) individual polyacrylonitrile nanofibers, we show that nanofiber toughness also dramatically improves. Reduction of fiber diameter from 2.8 μm to ∼100 nm resulted in simultaneous increases in elastic modulus from 0.36 to 48 GPa, true strength from 15 to 1750 MPa, and toughness from 0.25 to 605 MPa with the largest increases recorded for the ultrafine nanofibers smaller than 250 nm. The observed size effects showed no sign of saturation. Structural investigations and comparisons with mechanical behavior of annealed nanofibers allowed us to attribute ultrahigh ductility (average failure strain stayed over 50%) and toughness to low nanofiber crystallinity resulting from rapid solidification of ultrafine electrospun jets. Demonstrated superior mechanical performance coupled with the unique macro-nano nature of continuous nanofibers makes them readily available for macroscopic materials and composites that can be used in safety-critical applications. The proposed mechanism of simultaneously high strength, modulus, and toughness challenges the prevailing 50 year old paradigm of high-performance polymer fiber development calling for high polymer crystallinity and may have broad implications in fiber science and technology