The effect of carbon nanotube aspect ratio and loading on the elastic modulus of electrospun poly(vinyl alcohol)-carbon nanotube hybrid fibers

The reinforcement effect of carbon nanotubes (CNTs) has been examined as a function of their loading and aspect ratio in poly(vinyl alcohol) (PVA) based hybird fibers. Lignosulfonic acid sodium salt (LSA) was used to disperse CNTs to produce consistently high CNT loaded PVA-LSA-CNT hybrid fibers using an electrospinning process. The elastic modulus of individual fibers was measured using atomic force microscopy. The presence of CNTs significantly increased the average elastic modulus of PVA-LSA-CNT fibers compared to PVA-LSA fibers. The elastic modulus, however, exhibited no fiber diameter dependency. Transmission electron microscopy (TEM) was used to determine the loading and the aspect ratio of CNTs in each hybrid fiber. The CNT loading in PVA-LSA-CNT fibers varied widely due to non-uniform CNT dispersion and displayed no relationship with the elastic modulus. Our results also demonstrated that the average value of CNT aspect ratio significantly affected the elastic modulus of the hybrid fibers. Such a result was in agreement with theoretical prediction in which the stress transfer efficiency in a composite matrix is strongly dependent on the CNT aspect ratio.NRC publication: Ye

Importance of Axial Stretch on the Determination of Young’s Modulus of Electrospun Poly(ε-caprolactone) Nanofibres by Atomic Force Microscopy

With increasing interest in the use of polymeric nanofibres for biomedical applications such as composite materials and tissue scaffolding, accurate determination of their mechanical properties is essential. Fibre orientation and the stiffness of individual fibres determine the overall elastic modulus of nanofibrous materials. However, accurate measurements of the elastic properties of single fibres are challenging at the nanoscale, and distinguishing between results arising from competing models can be difficult. We report here on investigations of the Young’s modulus of single poly(ε-caprolactone) (PCL) electrospun nanofibres by measuring the deflection of fibres due to a loading force applied by an atomic force microscope (AFM). Although such testing is often performed with the tacit assumption that bending resistance alone is responsible for the fibre response, we found that consistent results could only be obtained if the overall fibre stretch is taken into account. The Young’s modulus we measured for electrospun PCL fibres with diameters ranging from 100 to 400 nm was 0.480.03 GPa, which is similar to the modulus of bulk PCL, with no apparent dependence on diameter. Our findings highlight the importance of the assumptions used in the analysis of bending data, as discounting the effect of axial stretch and pre-existing tension typically lead to an overestimate of the Young’s modulus.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author

SANS Characterization of an anisotropic poly(vinyl alcohol) hydrogel with vascular applications

Poly(vinyl alcohol) (PVA) hydrogels are formed by physical cross-linking through freeze/thaw cycles. By controlling the stress applied during the freeze/thaw process, anisotropic PVA hydrogels can be produced. An anisotropic PVA hydrogel conduit that mimics the nonlinear and anisotropic mechanical properties displayed by porcine aorta was developed. Preliminary structural characterization of isotropic and anisotropic PVA samples using small-angle neutron scattering reveals a polymer mesh cross-linked by crystallites spaced by about 18 nm and, most importantly, that the anisotropic properties are due to large-scale (>100 nm) structures alone; the geometry of the polymer mesh and crystallites remains largely unaltered. Controlling the properties of these anisotropic PVA hydrogels promises a broad range of potential applications in biomedical devices, such as coronary bypass grafts, where compliance mismatch between the implanted synthetic graft and the surrounding tissue has been identified as a major cause of failure.NRC publication: Ye

Microbially Mediated Mineral Carbonation: Roles of Phototrophy and Heterotrophy

Ultramafic mine tailings from the Diavik Diamond Mine, Canada and the Mount Keith Nickel Mine, Western Australia are valuable feedstocks for sequestering CO via mineral carbonation. In microcosm experiments, tailings were leached using various dilute acids to produce subsaline solutions at circumneutral pH that were inoculated with a phototrophic consortium that is able to induce carbonate precipitation. Geochemical modeling of the experimental solutions indicates that up to 2.5% and 16.7% of the annual emissions for Diavik and Mount Keith mines, respectively, could be sequestered as carbonate minerals and phototrophic biomass. CO sequestration rates are mainly limited by cation availability and the uptake of CO . Abundant carbonate mineral precipitation occurred when heterotrophic oxidation of acetate acted as an alternative pathway for CO delivery. These experiments highlight the importance of heterotrophy in producing sufficient DIC concentrations while phototrophy causes alkalinization of waters and produces biomass (fatty acids = 7.6 wt.%), a potential feedstock for biofuel production. Tailings storage facilities could be redesigned to promote CO sequestration by directing leachate waters from tailings piles into specially designed ponds where carbonate precipitation would be mediated by both chemical and biological processes, thereby storing carbon in stable carbonate minerals and potentially valuable biomass

Growth kinetics of lipid-based nanodiscs to unilamellar vesicles\u2014A time-resolved small angle neutron scattering (SANS) study

Mixtures of dimyristoyl-phosphatidylcholine (DMPC), dimyristoyl-phosphatidylglycerol (DMPG) and dihexanoylphosphatidylcholine (DHPC) in aqueous solutions spontaneously form monodisperse, bilayered nanodiscs (also known as \u201cbicelles\u201d) at or below the melting transition temperature of DMPC (TM ~23 \ub0C). In dilute systems above the main transition temperature TM of DMPC, bicelles coalesce (increasing their diameter) and eventually self-fold into unilamellar vesicles (ULVs). Time-resolved small angle neutron scattering was used to study the growth kinetics of nanodiscs below and equal to TM over a period of hours as a function of temperature at two lipid concentrations in presence or absence of NaCl salt. Bicelles seem to undergo a sudden initial growth phase with increased temperature, which is then followed by a slower reaction-limited growth phase that depends on ionic strength, lipid concentration and temperature. The bicelle interaction energy was derived from the colloidal theory of Derjaguin and Landau, and Verwey and Overbeek (DLVO). While the calculated total energy between discs is attractive and proportional to their growth rate, a more detailed mechanism is proposed to describe the mechanism of disc coalescence. After annealing at low temperature (low-T), samples were heated to 50 \ub0C in order to promote the formation of ULVs. Although the low-T annealing of samples has only a marginal effect on the mean size of end-state ULVs, it does affect their polydispersity, which increases with increased T, presumably driven by the entropy of the system.Peer reviewed: YesNRC publication: Ye