Paper Technology International 2020 - Journal - Page 84
PAPERTECHNOLOGYINTERNATIONAL
Figure 5:
AFM images of plain fibres. The RH was
varied from 2 % to a wet condition. (a-d)
Topography images, (e-h) DMT modulus
maps and (i-l) adhesion maps. The scale
bar is 800 nm.
The colour bar is -1 – 1.3 µm for
topography images, 0 – 3 GPa for DMT
modulus maps and 0 – 2 nN for adhesion
maps.
In the white framed topography images, the
colour scale was modified to 0 – 250 nm.
The white boxed images in a) and d) show
the topography of the swollen microfibrils.
An amorphous region of cellulose is
highlighted in green, while the crystalline
region is marked in red, and a transition
between crystalline and amorphous
regions is shown in blue.
(Auernhammer et al. 2021a)
In figure 5, the topography as well as the mechanical properties of the plain
fibres as a function of the RH are shown (Auernhammer et al. 2021a). As shown in
figure 5, the plain fibre exhibited crystalline and amorphous regions in the topography
image, but especially in the mechanical property maps, such as the DMT modulus
and adhesion, the differences in the crystal structure of cellulose were clearly
distinguishable. In the mechanical property maps, the adhesion increased with
increasing RH, while the DMT modulus decreased. Overall, swelling of microfibrils was
observed in the AFM topography images as the RH was increased. Figure 5a shows
the topography image with the microfibrils at 2 % RH. Comparing the microfibrils in
figure 5a to those in figure 5d, which shows a fully hydrated fibre, the microfibrils
appear to be swollen under wet conditions. The white box (figure 5a, d) indicates in
detail the swelling of microfibrils due to absorbed water inside the cellulose network.
The insolubility of cellulose results in swelling of the microfibrils (Lindman et al.
2010). Water infiltrates the network and diffuses through the pore space system by
capillary condensation. Therefore, the volume increases, which leads to an expansion
in the crystal structure and eventually breaks the hydrogen bonds in the cellulose
network (Cabrera et al. 2011; Gumuskaya et al. 2003; John and Thomas 2008). Thus,
variations in the RH can induce changes in the shape of a microfibril due to the not
entirely connected cellulose molecules, as shown in figure 5d, in the marked areas.
Crystalline and amorphous regions showed different values for the DMT modulus,
which is in agreement with Cabrera et al. (Cabrera et al. 2011).
For the polymer-coated fibres, fluorescence microscopy was
applied before the AFM measurements to identify homogenously
coated regions of interest on the paper fibre.
Figure 6 shows the AFM images of a polymer coated
fibre. As seen in figure 6a, for 2 % RH, and in figure 6b, for the
wet condition, there was no significant swelling of the microfibrils
of the fibre in the topography images. This is interpreted as the
first indicator of a hydrophobic polymer coating. By checking the
mechanical properties such as the DMT modulus and adhesion in
figure 6d,f for 2 % RH and figure 6e,g for the wet condition, they
also revealed no significant changes when increasing the RH.
Cross sections of the red framed areas in figure 6a-g are shown
in figure 6h and i. This is interpreted as a strong indicator of the
hydrophobicity of the polymer coating. Thus, it is concluded that the
presence of a homogenous and stable hydrophobic polymer coating
can be inferred from AFM measurements. Unmodified fibres exhibit
large changes in mechanical properties, such as the DMT modulus
82
Figure 6:
AFM images of a polymer coated fibre with a homogeneously
coated area. a) Topography image at 2 % RH with the
corresponding mechanical properties of d) DMT modulus and f)
adhesion. b) The topography image under wet conditions with
the associated mechanical properties of e) DMT modulus and
g) adhesion. The scale bar represents 2 µm. The colour scale
ranges from -550 – 550 nm for topography images, from 0 – 300
MPa for DMT modulus maps and from 0 – 10 nN for adhesion
maps (Auernhammer et al. 2021a).
and adhesion, as well as the swelling of microfibrils in topographic
images. Hydrophobic coated paper fibres show small changes in
mechanical properties and almost no swelling in topographical
images when the RH was increased.
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