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A fundamental understanding of cellulose nanoparticle morphology, structure, and properties as well as their role in composite property enhancement is not available. The absence of such fundamental metrology seriously impedes the advancement and economic viability of cellulose nanoparticle in composites. Critically needed are quantitative, validated measurement techniques, combined with the necessary protocols and understood in terms of fundamental modeling. To address this, the metrology research program is developing the necessary protocol for a series of measurement techniques to quantitatively characterize isolated cellulose nanoparticles, in terms of, morphology, terminating surfaces, crystal structure, properties (electrical, thermal, mechanical, etc), surface chemistry, and assess the uniformity of properties along the length of a single particle and between different particles.

The long term goal of this program will be to provide the most reliable/systematic estimates available of the intrinsic and interfacial nanomechanical properties of cellulose nanoparticles.

Atomic Force Microscopy (AFM) of Cellulose Nanoparticles

The objective of this research is to predict and validate the intrinsic and interfacial nanomechanical properties of CNCs. The research is being conducted through a closely coordinated experimental and modeling effort to determine the following:

Atomic Force Microscopy (AFM)

Atomic force microscopy

Atomic force microscopy (AFM) is a instrument that is capable of measuring nanometer scale features on surfaces for a variety of materials. Typically, an AFM works by bringing a sharp tip mounted on a mircocantilever into contact with a sample and monitoring response of the cantilever with a laser photodiode system. The sample is mounted on a piezoelectric device providing precise displacement control in three dimensions. An AFM is capable of operating in several modes (tapping, contact, force displacement), and under many different environments (vacuum, vapor, fluid). AFM can measure many different surface properties, such as: topography, elasticity, adhesion, thermal, and electrical.

Elastic Modulus Prediction with AFM

Our program uses displacement AFM or measuring elastic properties of surfaces because it is the simplest mode to implement and it allows for extensive model considerations to be made with respect to AFM tip-surface geometry. In force-displacement mode, the force (F) is recorded as a function of the sample displacement (Z). This process results in a force-displacement curve (FZ curve). Below are a series of images showing the process and our capabilities. a) Schematic of AFM indentation of a cellulose nanoparticles b) topography AFM image of tunicate CNCs. c) high resolution force-displacement map of a region on a CNC, every point in this map has a F-Z curve, the blue crosses are the location of the F-Z data used for modulus calculation. d) resulting F-Z curves.

Elastic modulus prediction

To relate the resulting elastic response within the FZ curve to the elastic properties of the sample the FZ curve must be converted into a force vs. tip-sample distance curve (Fd curve). This is accomplished by subtracting the cantilever defection (D) from the sample displacement (Z). This Fd curve describes the AFM tip indentation of the sample surface. The sample elastic modulus can be extracted from a Fd curve by fitting an appropriate material-mechanics model to the curve This is a standard AFM method that has been used in many previous studies. Below are images of the three types of modeling techniques we are using to extract elastic modulus from the Fd curves; Hertz contact mechanics, finite elemental modeling, molecular dynamic modeling.

Elastic modulus prediction

Uncertainty Quantification with AFM Elastic Modulus Predictions

Uncertainty Quantification

The AFM is used extensively for measuring the material properties of nanomaterials with nanometer resolution, unfortunately there is a lack of standards and uncertainty quantification (UQ) in these measurements. Other fields, such as six sigma standards in industry and beam corrections in scanning electron microscopy, have developed thorough methods for quantifying the uncertainty in a given measurement, model, or system. Broadly speaking these methods can be classified as UQ. Without applying the methods of UQ to AFM measurements it is impossible to say if the measurements are accurate within 5% or 100%. Furthermore, AFM based material property measurements are inevitably model based so the fundamental physics and model assumptions of tip-sample interactions must be taken into account in the describing the uncertainties. The research in the program will develop the necessary verification and validation (V&V) and UQ protocols necessary to provide a solid statistical foundation for the metrology and modeling thrusts. Beyond the applications to cellulose nanoparticles the effort will lay down a UQ and V&V foundation for the nanomechanical measurements of many types of nanoparticles and nanomaterials.

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