Civil Engineering Reference
In-Depth Information
Figure 8.1
Schematic representation of the structure of cellulose. h e hydroxyl groups are responsible
for inter- and intra-hydrogen bonds, which strengthen the structure by also promoting ties between
neighboring cellulosic chains.
the i rst time [4]. h e structure of cellulose (represented in Figure 8.1) was determined
late, in 1920, by Hermann Staudinger.
h e several hydroxyl groups that are attached to the cellulose chain form hydrogen
bonds with oxygen atoms of adjacent polymer chains, linking the cellulose molecules
together side-by-side and forming monocrystalline microi brils. h ese self-assembled
objects are insoluble in water and form a network inside the plant cell walls [5].
Although cellulose represents a good raw material for many i elds of application, the
presence of hydroxyl groups in the main chain allows for chemical reactions of substi-
tution where the hydrogen atom of each hydroxyl group can be substituted by dif erent
chemical groups. h e addition of lateral chains to the cellulose backbone opens up the
opportunity for the production of many dif erent cellulose derivatives, depending on
the added chain and on the degree of substitution. h is allows for the production of
many cellulose-based materials with dif erent properties [6].
One of the main properties of cellulose derivatives is the fact that they can originate,
under suitable conditions, liquid crystalline phases (mesophases). For each derivative,
the solvent used and the critical concentration needed for the formation of a lyotropic
phase depend on the type of lateral chain; the interaction solvent/lateral chain is a key
factor in the formation of a mesophase. Some cellulose derivatives never form a meso-
phase with certain solvents and, in some cases, the liquid crystalline phase only forms
at er shearing [7-9] due to the alignment promoted by the l ow of the molecules [10].
Besides being at the origin of lyotropic phases, cellulose derivatives can also origi-
nate thermotropic liquid crystalline phases without solvent. h is behavior is an indi-
cation that lateral chains act as solvent, or plasticizer, increasing the mobility of the
polymer backbone.
Liquid crystalline properties of cellulose and its derivatives can be exploited to pro-
duce biomimetic materials or all-cellulosic-based composites with enhanced mechani-
cal properties. h ese materials will be the focus of this chapter.
8.2
Liquid Crystalline Phases of Cellulose and Its Derivatives
h e structure of the cellulose crystallites was assumed to be chiral [11] with a screw-
like shape, and when the packing of two microi brils takes place they are shit ed by a
few degrees (see Figure 8.2). When all the layers are considered a helical structure is
formed. h e schematic of the described structure is shown in Figure 8.2.
S u g i y a m a
et al.
[12] observed, in 1990, the formation of chiral nematic (or cho-
lesteric) phases in suspensions of cellulose microi brils that could have their origin
in the way the packing of cellulose microi brils takes place [5]. In 2001, Fleming
et
