Prof. Petri P. Kärenlampi
Wood Cell Wall Structure:
How can one study wood cell wall structure?
There are many ways.
Here, we will discuss some results achieved using
Differential Scanning Calorimetry.
This technique reports thermal transitions in matter.
In other words, it shows what happens when matter is heated.
The thermal properties of water depend on pressure.
One factor inducing pressure variations is the surface tension.
The pressure within a water drop depends on the
radius of the drop, and thus the melting temperature of
water depends on the size of the water drop.
Due to the above, the pore size distribution in
water-saturated wood cell walls can be clarified through
measuring the latent heat of melting at a range of
temperatures.
What kind of pore size distributions are there?
Please have a look at the pore size distributions
of Spruce wood cell walls:
We find that water-saturated earlywood fibers have water
16...20% of their dry weight in pores of nanometer
scale (Non-Freezing Water). The cell walls of latewood
fibers have some 27% NFW.
We further find above that earlywood fibers have
a very significant amount of water in pores of
size between 100 nanometers and one micrometer.
What happens in drying?
Drying the and overnight wetting of the specimens
results as the following pore size distributions:
We find that the large pores
within earlywood
cell walls have been closed. Perhaps this is
not surprizing.
We also find that nanometer-scale pores
within latewood cell walls have been closed.
The surprizing thing is that it appears that
the Non-Freezing Water Content in earlywood
cell walls has been significantly increased.
In other words, the pore volume in pores of
nanometer scale has been significantly increased
due to drying!
Why do the cell walls behave in such a peculiar
manner?
It is obvious that the cell walls of earlywood and latewood
tracheids have very different structure.
The structure is different in the nanometer scale
as well as in the micrometer scale.
We think that the increment of earlywood pores
of nanometer scale is due to internal drying stresses,
due to anisotropic shrinkage of cell wall elements.
How do we apply the above results?
There are many applications for such fundamental results.
We have applied the results in order to study the
molecular response of wood to mechanical fatigue treatments.
Please have a look: