Tools for the Characterization of Biomass at the Nanometer Scale - The Nanoscience and Technology of Renewable Biomaterials - page 75

However, nanoindenter probes are on the order of 50 nm in tip radius, compared with
approximately 10 nm for an AFM. A review of the relevant contact mechanics has been
published (118), emphasizing the assumptions underlying and restricting the application
of most commonly used models and their implications for nanoscale force measurements.
Nanoindentation offers an excellent way to transition between the micron scale of
optical microscopy and the nanometer scale of AFM. It has been used to quantitatively
measure static (119, 120) and dynamic (121) mechanical properties on the nanometer
scale. One of the more difficult aspects of nanoindentation experiments is preparing a
relatively smooth surface free of artifacts.
The earliest applications of nanoindentation in biomass compared the hardness and
Young's modulus of spruce tracheid secondary walls (122) with the lignin-rich cell corner
middle lamella (123). Since then, nanoindentation has been used to characterize cell wall
development (124), the effect of anisotropy (125), microfibril angle (126), and pyrolysis
(127) on mechanical properties. Nanoindentation has also been useful in understanding
adhesive-wood interactions (128-130).
The original nanoindentation theory, which was used in the previously mentioned
studies, was derived assuming the indentation of an isotropic elastic half-space. Almost
all biological specimens will violate this assumption. m-diameter

nanoindent placed on the transverse plane of a cell wall of wood will be in relatively
close proximity to structural heterogeneities, such as the empty lumen or middle lamella,
because the cell wall is typically only 5

For example, a 1- µ

m wide. Previous researchers minimized this
effect by filling the lumina with epoxy to support the cell walls during testing, but whether
epoxy changes the properties of the cell walls is not certain. Methods to prepare wood
specimens without any embedment and to account for structural compliances that result
from nearby structural heterogeneities have since been developed (131, 132).

µ

3.4.2.6 Scanning Thermal Microscopy

AFM and nanoindenter tips can also be used to measure thermal properties of materials.
An electrically heated AFM probe with a tip radius of about 20 nm can be used to
simultaneously heat and image the specimen surface. When the material below the tip
reaches a phase transition, it softens and the probe penetrates the specimen. This provides
the nanoscale equivalent of a bulk thermomechanical analysis experiment, where phase
transition temperatures T g or T m are measured (133, 134). The thermal conductivity and
surface temperature of the specimen can be measured as well. Nanothermal analysis has
been used to investigate adhesive penetration and modification in wood cell walls (135).

3.4.3 Focused Beam Microscopies

Focused beam microscopies differ from SPM in that a beam of particles or rays, rather
than solid objects, are scanned across the specimen. In most cases, the absorption
or scattering of the incident beam is monitored, but particles or radiation generated
by the specimen can also be analyzed. The major experimental artifact generated by
focused beams is specimen damage. Scientists must be aware that the beams used almost
always have enough energy to break chemical bonds in biological specimens, commonly
resulting in mass loss and chemical modification.