Yuris A. Dzenis

Ultrafine continuous nanofibers (NFs) are at the forefront of structural materials and advanced composites research and development due to their recently discovered unique properties, including simultaneously ultrahigh strength and stiffness. Their continuity differentiates them from other discontinuous nanomaterials, and their capacity to exhibit simultaneous strength *and

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 nanometers 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 (FIB)-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, analyzed and distinct energy absorption mechanisms, featuring a degree of self-similarity, are identified. 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 the development of novel fibers with further improved mechanical performance.

Graphene and graphene oxide attract rapidly growing interest as prospective building blocks for nanotechnology applications and composites. Recently, we showed that a small amount of graphene oxide produced significant templating effects on the structure of continuous carbon nanofibers (CNFs). However, the produced nanofibers had significant nonuniformities that could be detrimental to their mechanical properties. Controlled nanofabrication is critical for obtaining uniform, high-quality nanofibers with tunable diameters and properties. Here, we analyze the effects of graphene oxide type, concentration, and processing parameters on the morphology of continuous graphene oxide/polyacrylonitrile nanofibers produced by electrospinning. Four types of graphene oxides with different average nanoparticle sizes were examined, and the effects of electric field and polymer concentration on nanofiber diameters were analyzed. Good-quality nanofibers were produced with up to 2 wt % graphene oxide in polyacrylonitrile. Uniform nanofibers were obtained for solid content above 9 wt % in dimethylformamide (DMF). Composite nanofibers containing graphene oxide nanoparticles exhibited reduced diameters throughout the polyacrylonitrile concentration range before and after carbonization compared to nanofibers prepared from neat polymer. The obtained results open up a pathway for controlled nanofabrication of uniform CNFs with improved structure for a variety of structural and functional applications.

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 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.

An explicit three‐dimensional (3D) finite‐element model for dilute nanofibre networks is presented. The model takes into account 3D structure of networks by mimicking nanomanufacturing process. Realistic elasto‐plastic behaviour of individual nanofibres with failure and frictional interfibre contacts are incorporated. The model is capable of predicting through failure mechanical behaviour of nanofibre networks with large fibre reorientation and fibre breaks. Comparison of simulated force–strain behaviour with experimental data showed that predicted and experimental curves exhibited similar shapes consisting of an elastic stage, a strain‐hardening stage, and a softening stage, with abrupt drops coincident with fibre breaks. The numerically predicted maximum tensile force and total failure strain were of the same magnitude as experimental results. The developed explicit model can be used to study the effects of fibre diameter and mechanical properties, network density, fibre orientation distribution, and contact conditions on mechanical behaviour of nanofibre networks. Such studies can shed light on the mechanisms of complex nonlinear deformation and failure of networks and can be used for networks design and optimisation for applications.

Summary: Stimuli‐responsive polymerized hydrogels represent a promising class of materials for biomedical and other applications such as muscle‐type sensors/actuators due to substantial change in dimensions or shape in response to minute change in their environment, resemblance of soft biological tissues, ability to operate in wet environments, and chemical and physical tailorability. However, despite extensive fundamental research, slow response rate and poor mechanical robustness and durability of gels remain the major unresolved barriers for wider application of these materials. Nanostructured responsive polymer gels based on continuous electrospun nanofilaments are expected to have fast response and enhanced strength, modulus, and toughness, thus resolving both main current shortcomings. In this article, poly(vinyl alcohol)–poly(acrylic acid) and Dextran–poly(acrylic acid) pH‐responsive nanofilamentary gels were prepared and compared to bulk gels. In addition to fast response rate, the nanofilamentary gels demonstrated unusual mechanical properties such as cross‐over of load vs. length curves obtained at different pH and anisotropy of response.

Extracellular mechanophysical signals from both static substrate cue and dynamic mechanical loading have strong potential to regulate cell functions. Most of the studies have adopted either static or dynamic cue and shown that each cue can regulate cell adhesion, spreading, migration, proliferation, lineage commitment, and differentiation. However, there is limited information on the integrative control of cell functions by the static and dynamic mechanophysical signals. For example, a majority of dynamic loading studies have tested mechanical stimulation of cells utilizing cultures on flat surfaces without any surface modification. While these approaches have provided significant information on cell mechanotransduction, obtained outcomes may not correctly recapitulate complex cellular mechanosensing milieus in vivo. Several pioneering studies documented cellular response to mechanical stimulations upon cultures with biomimetic substrate modifications. In this min-review, we will highlight key findings on the integrative role of substrate cue (topographic, geometric, etc.) and mechanical stimulation (stretch, fluid shear) in modulating cell function and fate. The integrative approaches, though not fully established yet, will help properly understand cell mechanotransduction under biomimetic mechanophysical environments. This may further lead to advanced functional tissue engineering and regenerative medicine protocols.