Quantifying Polymer Chain Orientation in Strong and Tough Nanofibers with Low Crystallinity: Toward 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.

The role of RhoA kinase (ROCK) in cell alignment on nanofibers.

While the potential of nanofibers as tissue engineering scaffolds has been demonstrated, very little has been revealed as regards the molecular mechanism by which cells sense and respond to nanofibers. It was hypothesized that RhoA kinase (ROCK), one of the vital cell tension signaling cascades, plays a role in regulating cell alignment on nanofibers. To test this, unidirectionally aligned and randomly distributed nanofibers, both with an average diameter of ∼130nm, were fabricated with poly(l-lactic acid) (PLLA). A flat PLLA film was used as the control. Mesenchymal stem cells (MSCs, C3H10T1/2) displayed high fidelity in cell orientation along aligned nanofibers, and showed an increased cell spreading area on random nanofibers. Interestingly, cells cultured on aligned nanofibers displayed significantly greater ROCK expression relative to cells on a flat surface, as assessed by immunoblotting. To further test the role of ROCK, MSCs with ROCK small hairpin RNA (shRNA) were established. It is notable that, even when ROCK was stably knocked down via shRNA, cells could still display preferred orientation along aligned nanofibers. However, MSCs with shRNA-ROCK displayed a significantly decreased cell major axis length following aligned nanofibers compared with shRNA vector control, suggesting that ROCK may be involved in cell elongation on aligned nanofibers. Along with the reduction in cell length, cell area was decreased with ROCK silencing. These cell morphological changes induced by shRNA-ROCK were generally maintained on a flat surface and random nanofibers. A pharmacological ROCK inhibitor, Y-27632, produced results similar to those of shRNA-ROCK. The data on the role of ROCK in regulating cell alignment on nanofibers may provide a new mechanistic insight into nanofiber control of cells.

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.