Lignin: Functional Biomaterial with Potential in Surface Chemistry and Nanoscience - The Nanoscience and Technology of Renewable Biomaterials - page 188

is to closely mimic the naturally occurring material, however, with the reduction or
elimination of the chemical and structural heterogeneity. This may be achieved through
the use of a well characterised sample of lignin (either isolated from naturally occurring
materials or through synthetic means) and modern techniques in the preparation of thin,
smooth and continuous films. In much the same way as model films of the predominant
carbohydrate structure in wood fibres, cellulose, have been prepared and used in the
past decade for the measurement of fundament physical and chemical properties (Notley
et al . 2004, Notley and Wagberg 2005, Notley et al . 2006, Kontturi et al . 2006, Eriks-
son et al . 2007, Notley 2008), so too have lignin films been prepared recently by a
number of research groups using slightly different methodology, bearing in mind the
underlying goals of achieving chemical and structural uniformity.

7.5.1 Preparation and Properties of Lignin Thin Films

Many different types of model lignin surfaces have been prepared for use in the measure-
ment of fundamental properties of lignin in the solid state. Table 7.4 summarises these
recently published studies. The first model films were prepared by Lee and Luner (1972)
from a commercial sample for the use in investigating the wettability and interfacial prop-
erties of lignin. It has long been speculated that lignin provides a hydrophobising means
for the wood cell wall. The contact angle of water on these lignin model surfaces was
measured and showed a rapid decrease with time from about 60 ◦ to 0 ◦ indicating the
highly porous nature of the films and that the lignin polymer has a strong affinity for
water. Whilst providing the first experimental data of the surface properties of lignin
and in particular, a relative measure of the hydrophilicity of lignin, films prepared in this
manner are far from ideal for the calculation of surface energy. The surfaces in this study
were prepared by either evaporation of a drop of lignin solution on a microscope slide,
resulting in extensive cracking, or through heat moulding of the lignin powder under
pressure to give smoother surfaces. A key point is worth reinforcing here; a model
surface must closely mimic the naturally occurring material. Severe chemical methods
for the isolation of lignin will significantly alter the natural structure limiting the utility
of the 'model' surface.
Subsequent studies have attempted to improve the quality of the lignin surfaces in
order to overcome these initial limitations. However, a number of problems have per-
sisted including high surface roughness, nonuniformity including discontinuous films and
instability, particularly in aqueous solution conditions. Constantino et al . (1996, 1998,
2000) have used the Langmuir-Blodgett technique to prepare model lignin films for over
a decade now and have extended their use to sense heavy metals (Martins et al . 2008).
The group of Micic et al . have investigated the use of model lignin macromolecules in
determining the interactions between lignin globules thus not requiring a smooth, con-
tinuous film over a large area (Micic et al . 2001a,b). Spin-coating seems particularly
promising for the reproducible preparation of lignin model films which are both smooth
and continuous. Norgren and co-workers have prepared smooth, continuous lignin films
by the spin-coating of softwood kraft lignin dissolved in ammonium hydroxide solu-
tion onto oxidised silicon wafers (Norgren et al . 2006, 2007, Notley and Norgren 2006
and 2008). These lignin surfaces have been optimised in terms of their thickness with
reproducible films prepared in the range of 30-150 nm with minimum roughness as