Biomedical Engineering Reference
In-Depth Information
5.2
The Formation of PEM on Wood Fibres
The large efficiency of PEM treatment of fibres for preparing strong papers and its
promise as a new tool for fibre engineering naturally raised questions as to whether and
how multilayers were formed on the fibres. It is possible that PEMs are adsorbed only to
the external fibre surface or to both the external surface and other parts of the fibre wall,
or that the added polyelectrolytes form other types of structure on the fibre surface that
are advantageous for the fibre/fibre adhesion. Considering the dimensions of a typical
wood fibre, i.e.
cylindrical body with a length of about 2 mm, a diameter of 20
µ
m
and a wall thickness of about 4
m, and the difference in morphology between different
fibres, it is very difficult to sample fibres from batches of treated and untreated fibres and
determine the properties of the layers with the aid of e.g. Scanning Electron Microscopy
(SEM). Such a study is further complicated by the fact that the thickness of the layers
of polyelectrolytes formed on the fibres surface are of the order of nm, depending on
the types of polyelectrolyte used and the conditions used during their preparation (17).
Nevertheless, a solution to this problem was essential in order to create a scientific base
for the engineering of fibres with PEM for different products.
In order to overcome the problem of the difference in morphology between different
fibres, it was decided to use single fibres with the aid of a Cahn balance where only a part
of the fibre was treated with the multilayers (29). This set-up is schematically shown
in Figure 5.1 where one end of the fibre is fastened between two layers of adhesive
tape, attached to the load-cell of the microbalance and the other end is consecutively
dipped into solutions of cationic and anionic polyelectrolyte solutions with the aid of
the moving stage of the microbalance.
µ
A careful washing is conducted between each
polyelectrolyte adsorption step.
After the treatment with a predetermined number of polyelectrolyte layers, the fibre
was removed and both the treated and nontreated part were studied with Environmental
SEM (ESEM) (29). In this way the same fibre can be analysed before and after treatment.
A typical result from such a measurement is shown in Figure 5.2 where an untreated
section of a bleached chemical softwood fibre is shown together with a section that has
been treated with 5.5 bilayers of polyallylamine hydrochloride (PAH) and polyacrylic
acid (PAA) (29).
As can be seen, the PEM treatment creates a much smoother surface where the small-
scale variations have been reduced to a large extent. From these types of images, it
was concluded that the consecutive treatment with oppositely charged polyelectrolytes
with intermediate washing steps created thin films of PEM on the surface of the fibres.
Similar results have also been presented by Lvov (30) using fluorescently labeled PAH
and fluorescently labeled polystyrene sulphonate (PSS).
Having established this, the formation of the multilayers can be simply monitored by
adsorption measurements on batches of fibres using standard titration techniques and/or
standard chemical analysis where, for example, the concentration of sulphur in the fibres
after treatment can be used to determine the amount of PSS adsorbed (29). However,
the establishment of adsorption isotherms for the fibres is time-consuming and it gives
little molecular detail about either the kinetics of the adsorption or the properties of the
adsorbed layers.
Therefore the PEM-treatment of fibres, in our laboratory, is always
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