Philip A. Yuya

Contact-resonance atomic force microscopy for viscoelasticity

We present a quantitative method for determining the viscoelastic properties of materials with nanometer spatial resolution. The approach is based on the atomic force acoustic microscopy technique that involves the resonant frequencies of the atomic force microscopy cantilever when its tip is in contact with a sample surface. We derive expressions for the viscoelastic properties of the sample in terms of the cantilever frequency response and damping loss. We demonstrate the approach by obtaining experimental values for the storage and loss moduli of a poly(methyl methacrylate) film using a polystyrene sample as a reference material. Experimental techniques and system calibration methods to perform material property measurements are also presented.

Viscoelastic Property Mapping with Contact Resonance Force Microscopy

We demonstrate the accurate nanoscale mapping of near-surface loss and storage moduli on a polystyrene-polypropylene blend with contact resonance force microscopy (CR-FM). These viscoelastic properties are extracted from spatially resolved maps of the contact resonance frequency and quality factor of the AFM cantilever. We consider two methods of data acquisition: (i) discrete stepping between mapping points and (ii) continuous scanning. For point mapping and low-speed scanning, the values of the relative loss and storage modulus are in good agreement with the time-temperature superposition of low-frequency dynamic mechanical analysis measurements to the high frequencies probed by CR-FM.

Relationship between Q-factor and sample damping for contact resonance atomic force microscope measurement of viscoelastic properties

Contact resonance AFM characterization techniques rely on the dynamics of the cantilever as it vibrates while in contact with the sample. In this article, the dependence of the quality factor of the vibration modes on the sample properties is shown to be a complex combination of beam and sample properties as well as the applied static tip force. Here the tip-sample interaction is represented as a linear spring and viscous dashpot as a model for sample (or contact) stiffness and damping. It is shown that the quality factor alone cannot be used to infer the damping directly. Experimental results for polystyrene and polypropylene are found to be in good agreement with predictions from the model developed. These results form the basis for mapping viscoelastic properties with nanoscale resolution.

Determination of Young’s modulus of individual electrospun nanofibers by microcantilever vibration method

The authors report a technique for measuring Young’s modulus of a single electrospun nanofiber using the vibrations of two microcantilevers coupled with the nanofiber. The modulus is calculated from the resonant frequency shift resulting from the nanofiber. Polyacrylonitrile nanofibers (200nm diameter) were collected during electrospinning and wrapped on two similar microcantilevers causing a shift in first resonance from 10.0to19.4kHz. Finite element analysis was used to analyze the frequency shift using images from a scanning electron microscope giving a modulus of the as-spun polyacrylonitrile nanofiber of 26.8GPa.

Fabrication, nanomechanical characterization, and cytocompatibility of gold-reinforced chitosan bio-nanocomposites.

Chitosan, a naturally derived polymer represents one of the most technologically important classes of active materials with applications in a variety of industrial and biomedical fields. Gold nanoparticles (~32 nm) were synthesized via a citrate reduction method from chloroauric acid and incorporated in Chitosan matrix. Bio-nanocomposite films with varying concentrations of gold nanoparticles were prepared through solution casting process. Uniform distribution of gold nanoparticles was achieved throughout the chitosan matrix and was confirmed with SEM. Synthesis outcomes and prepared nanocomposites were characterized using SEM, TEM, EDX, SAED, UV-vis, XRD, DLS, and Zeta potential for their physical, morphological and structural properties. Nanoscale properties of materials under the influence of temperature were characterized through nanoindentation techniques. From quasi-static nanoindentation, it was observed that hardness and reduced modulus of the nanocomposites were increased significantly in direct proportion to the gold nanoparticle concentration. Gold nanoparticle concentration also showed positive impact on storage modulus and thermal stability of the material. The obtained films were confirmed to be biocompatible by their ability to support growth of human cells in vitro. In summary, the results show enhanced mechanical properties with increasing gold nanoparticle concentration, and provide better understanding of the structure-property relationships of such biocompatible materials for potential biomedical applications.

Analytical model for nanoscale viscoelastic properties characterization using dynamic nanoindentation

In the last few decades, nanoindentation has gained widespread acceptance as a technique for materials properties characterization at micron and submicron length scales. Accurate and precise characterization of material properties with a nanoindenter is critically dependent on the ability to correctly model the response of the test equipment in contact with the material. In dynamic nanoindention analysis, a simple Kelvin–Voigt model is commonly used to capture the viscoelastic response. However, this model oversimplifies the response of real viscoelastic materials such as polymers. A model is developed that captures the dynamic nanoindentation response of a viscoelastic material. Indenter tip-sample contact forces are modelled using a generalized Maxwell model. The results on a silicon elastomer were analysed using conventional two element Kelvin–Voigt model and contrasted to analysis done using the Maxwell model. The results show that conventional Kelvin–Voigt model overestimates the storage modulus of the silicone elastomer by ~30%. Maxwell model represents a significant improvement in capturing the viscoelastic material behaviour over the Voigt model.

Cyclic cryopreservation affects the nanoscale material properties of trabecular bone

Tissues such as bone are often stored via freezing, or cryopreservation. During an experimental protocol, bone may be frozen and thawed a number of times. For whole bone, the mechanical properties (strength and modulus) do not significantly change throughout five freeze-thaw cycles. Material properties at the trabecular and lamellar scales are distinct from whole bone properties, thus the impact of freeze-thaw cycling at this scale is unknown. To address this, the effect of repeated freezing on viscoelastic material properties of trabecular bone was quantified via dynamic nanoindentation. Vertebrae from five cervine spines (1.5-year-old, male) were semi-randomly assigned, three-to-a-cycle, to 0-10 freeze-thaw cycles. After freeze-thaw cycling, the vertebrae were dissected, prepared and tested. ANOVA (factors cycle, frequency, and donor) on storage modulus, loss modulus, and loss tangent, were conducted. Results revealed significant changes between cycles for all material properties for most cycles, no significant difference across most of the dynamic range, and significant differences between some donors. Regression analysis showed a moderate positive correlation between cycles and material property for loss modulus and loss tangent, and weak negative correlation for storage modulus, all correlations were significant. These results indicate that not only is elasticity unpredictably altered, but also that damping and viscoelasticity tend to increase with additional freeze-thaw cycling.

Effects of Freeze-Thaw Cycling on Material Properties of Cancellous Cervine Bone as Characterized by Nanoindentation

Cancellous bone is an important load-bearing component of whole bone, and due to the plate-and-rod nature of trabeculae, small-scale testing is required to measure material parameters for use in modern analytic techniques such as finite element modeling [1, 2]. These material properties are measurable via nanoindentation techniques. During nanoindentation, the indenter tip is forced into the surface of the material while the applied load and tip displacement are monitored. Using these data, along with the tip’s cross-sectional area, mechanical properties are determined. Dynamic testing quantifies viscoelastic response and can obtain material response parameters such as storage and loss moduli. During dynamic testing, a low magnitude sinusoidal force is superimposed on a constant static force. The displacement response is measured at the same frequency as the applied oscillating force, and the resulting phase lag is related to material damping [3].Copyright

Flexural vibrations of microcantilevers coupled with a nanofiber

In this study, the flexural vibrations of a microcantilever system coupled with a nanofiber offset from the free ends of the cantilevers are explored. Here, for this system, we derive the general expressions for the characteristic equation and sensitivity of vibration modes of a microcantilever system with a nanofiber wrapped offset from the free ends of the cantilevers. It is shown that each mode has a different sensitivity that depends on the nanofiber stiffness relative to the beam stiffness. We also show how the characteristic equation can be exploited with the knowledge of the resonant frequencies to determine the nanofiber stiffness and its attachment position on the microcantilevers. Plots of nanofiber position on the cantilevers versus wavenumber are used to show the effect of the position of the nanofiber on frequency measurements. The results are anticipated to provide a basic understanding between the vibration modes and the properties of the electrospun nanofibers, that may be used to optimize the experiments.