Civil Engineering Reference
In-Depth Information
h e glucose molecules that make up cellulose each have three free hydroxyl groups.
h is provides cellulose with high ai nity to hydroxyl containing materials, including
itself [123] . h e hydrophilic nature of cellulose can weaken blends with other materials
as water can become contained in the matrix [124], however the large number of free
hydroxyl groups does make it possible for chemical modii cations to be carried out to
make the cellulose more hydrophobic, and some authors have attempted to do this in
order to increase the interfacial adhesion between potential matrices and i bers [125] .
h ere have been mixed successes in these processes, which are described below. In
addition, physical changes to bacterial cellulose can result as a consequence of treating
the cellulose in dif erent ways, prior to its inclusion in blends.
4.3.3.1
Physical Modii cations
Bacterial cellulose can be used in various forms in composites as a reinforcing phase in
blends (Section 4.4), but any prior treatment to inclusion in such mixtures can af ect its
structure and properties. It can be necessary to remove moisture prior to further treat-
ment, and there are various ways to dry bacterial cellulose, including freeze-drying,
heat or air drying, or it can be used in a never-dried state. However it has been found
that air-dried and never-dried cellulose exhibit dif erences in crystallite size [126], so
simply selecting a method of drying, or choosing not to dry it at all could result in a
change in properties.
Chemical modii cations are described below, however some chemical treatments
that are carried out specii cally to achieve physical changes, such as dissolution, are
listed here.
4.3.3.1.1 Dissolution
h e use of cellulose is limited due to dii culties dissolving it and the limited number of
appropriate solvents [127]. Cellulose is a long chain polymer composed of glucose units
and is extremely hydrophilic, however it is insoluble in water and most organic solvents
due to its extensive intra- and intermolecular hydrogen bonding [128]. h ere have
been some reports of cellulose solvent systems including
N, N
-dimethylacetamide/lith-
ium chloride (DMAc/LiCl) [129], dimethyl sulfoxide-paraformaldehyde (DMSO-PF)
[130] ,
N
-methylmorpholine-
N
-oxide [131] and NaOH/urea aqueous solution [132].
Whilst the dissolution of bacterial cellulose has been dii cult in the past due its hydro-
gen bonds and high degree of polymerization, the determination of ionic liquids that
allow such dissolution of bacterial cellulose to occur may of er further possibilities to
modify and process this material.
Ionic liquids consist entirely of ions, and are made up of at least two components,
an anion and a cation, which create an enormous number of potential combinations
simply by varying these components [133]. Ionic liquids are ot en referred to as “green
solvents” as they have the capability of dissolving many substances, including many
organic molecules such as enzymes [134], DNA [135] and collagen [136], and have
desirable properties, for instance chemical stability, thermal stability, low vapor pres-
sure and high ionic conductivity [137]. h ey are media that can af ect various kinds
of polymerization and have been used to synthesize a variety of molecules, including
proteins. Ionic liquids can be water soluble and, as such, have previously been included
in growth media for bacteria [138, 139].
