Hemicelluloses at Interfaces: Some Aspects of the Interactions - The Nanoscience and Technology of Renewable Biomaterials - page 162

µ f ( × 10 5 )/Nm − 2

h f (10 − 3 )/Nm − 2

2.5

2.0

Ubl TMP
high I

Ubl TMP
high I

1.8

2

Bl TMP
low I

Xylan
low I

1.6

Ubl TMP
low I

1.5

Bl TMP
low I

1.4

Xylan
low I

Ubl TMP
low I

1

1.2

1.0 0.5

50 100 150 200 250 50 100 150 200 250

Time (min) Time (min)

(b)

(a)

Figure 6.5 Variations in (a) shear viscosity and (b) shear elasticmodulus as a function of
timecorrespondingtothebestfittingsbasedonVoigtmodel foraviscoelasticsolid.

unbleached TMP at low (a) and high ionic strength (b) on cellulose. Similar fittings
were carried out for dissolved hemicelluloses isolated from peroxide bleached TMP at
low ionic strength (I 10 mM) and xylan at low and high ionic strength at pH 10

(fitting data not shown). In spite of the simplicity of the Voigt model and a very strong
approximation of its contributions to the elastic properties of an adsorbed polymer layer,
the fitting to the frequency and dissipation data was reasonable. Only the modeling of
xylan layer properties at high ionic strength failed.
The development of the hemicellulose layers shear viscosities ( η f ) and shear modulus
( µ f ) values during the adsorption process estimated by the application of the Voigt
model is shown in Figure 6.5.
The shear viscosity of the formed hemicellulose films stayed more or less at the same
level showing a slightly decreasing trend as the film formation proceeded for all the
systems except for the dissolved hemicelluloses isolated from unbleached TMP at high
ionic strength, which stayed constant. The shear elastic modulus was highest for the
layer of dissolved hemicelluloses from unbleached TMP at high ionic strength and lowest
for xylan layer at low ionic strength. The layer formed by adsorbing peroxide bleached
TMP hemicelluloses showed relatively high shear elastic modulus values. According to
these results, the hemicelluloses isolated from unbleached TMP at high ionic strength
seemed to be most strongly bound on cellulose. Surprisingly the xylan layer at low ionic
strength appeared to be rather loosely bound on cellulose. Furthermore, it can be noted
that the Voigt model failed when the xylan film was modeled at high ionic strength. The
reasons for the low shear viscosity and low shear elastic modulus are discussed later.
The estimated hydrodynamic thickness of the hemicellulose films as the adsorption
proceeds is plotted in Figure 6.6. At the end of the adsorption process the film thickness
of dissolved hemicelluloses at low ionic is approximately 4 nm and at high ionic strength
the final thickness is 9 nm. The hemicellulose film isolated from peroxide bleached
TMP has a final thickness of roughly 2 nm.

=

The xylan film reaches a final thickness

value of 4 nm.