Modulus Of Cellulose Research Articles

Uncertainty quantification in nanomechanical measurements using the atomic forcemicroscope**This work is a partial contribution of the USDA Forest Service and NIST, agencies of theUS government, and is not subject to copyright.

Quantifying uncertainty in measured properties of nanomaterials is a prerequisite for themanufacture of reliable nanoengineered materials and products. Yet, rigorous uncertaintyquantification (UQ) is rarely applied for material property measurements with the atomicforce microscope (AFM), a widely used instrument that can measure properties atnanometer scale resolution of both inorganic and biological surfaces and nanomaterials. Wepresent a framework to ascribe uncertainty to local nanomechanical properties of anynanoparticle or surface measured with the AFM by taking into account the mainuncertainty sources inherent in such measurements. We demonstrate the framework byquantifying uncertainty in AFM-based measurements of the transverse elastic modulus ofcellulose nanocrystals (CNCs), an abundant, plant-derived nanomaterial whosemechanical properties are comparable to Kevlar fibers. For a single, isolated CNC thetransverse elastic modulus was found to have a mean of 8.1 GPa and a 95% confidenceinterval of 2.7–20 GPa. A key result is that multiple replicates of force–distancecurves do not sample the important sources of uncertainty, which are systematic innature. The dominant source of uncertainty is the nondimensional photodiodesensitivity calibration rather than the cantilever stiffness or Z-piezo calibrations.The results underscore the great need for, and open a path towards, quantifyingand minimizing uncertainty in AFM-based material property measurements ofnanoparticles, nanostructured surfaces, thin films, polymers and biomaterials.

NpH-MD-Simulations of the Elastic Moduli of Cellulose II at Room Temperatue

We have used molecular dynamics modeling to investigate the stucture and mechanical properties of regenerated cellulose fibres. This work is motivated by continued interest in replacing the environmentally hazardous viscose process by alternative spinning methods. An important input parameter for any realistic model of the elastic properties is the stiffness tensor of the crystalline constituent, cellulose II. Conventional molecular mechanics techniques can be used to estimate the elastic reaction of a material with respect to small external stresses or strains, i.e. the compliance and stiffness tensors, and the elastic moduli derived therefrom, at zero temperature. In order to access non-zero temperatures, it is necessary to use either the quasi-harmonic approximation for the vibrational free energy or molecular dynamics (MD) simulations. In the present work, Parrinello-Rahman constant-stress MD was performed to generate trajectories in constant particle number (N), constant external stress tensor (p or t) and constant enthalpy H (NpH or HtN) ensemble. This was found to be less time consuming than working with isothermal conditions, as done by other authors. The fluctuations in kinetic energy and MD cell vectors were then used to calculate adiabatic elastic constants, thermal expansion coefficients and heat capacity. The isothermal elastic constants were found by applying a standard thermodynamic relation. The Young′s modulus along the chain direction, El, was determined to be 155 GPa, whereas the values in the perpendicular directions vary between 51 and 24 GPa. These results are of the same order of magnitude as those obtained by Tashiro and Kobayashi [1] with the static (T = 0K) method, but our value of El is 5% lower and, unexpectedly, the lateral values are up to six times higher. A strong anisotropy is found for shear along the chains in planes containing the chain axis, the shear modulus ranging from 5 to 20 GPa. Convergence was achieved in the simulations, to the extend that the elastic constants become stationary, but significant internal stresses remain, pointing to shortcomings in the software used. Further work is necessary to resolve these problems, although the major conclusions should be unaffected.

Mechanical properties of LDPE/granular starch composites

AbstractThe mechanical properties of composites of granular starch and low density polyethylene (PE) have been studied as functions of starch volume fraction ϕ, granule size, and presence of compatibilizer. Property–volume fraction relationships were interpreted using various theories of composite properties. The dependence of elongation (ϵ ∼ ϕ1/3) and tensile strength (σ ∼ ϕ2/3) agree with theoretical predictions, although the proportionality constants are less negative than theoretical values. The addition of compatibilzer (ethylene‐co‐acrylic acid copolymer, EAA) did not significantly affect the elongation or tensile strength, but significantly increased the composite tensile modulus. The cornstarch/PE moduli could be described by the Kerner or Halpin‐Tsai equations. Analysis of the composite moduli data using the Halpin‐Tsai equation allowed the estimation of the modulus of granular starch. The value obtained, 15 GPa, is considerably greater than most unfilled synthetic polymers of commercial importance, but significantly lower than the modulus of cellulose. It is also greater than a previously reported value of 2.7 GPa. © 1994 John Wiley & Sons, Inc.