Civil Engineering Reference
In-Depth Information
Figure 8.2
Schematic representation of the packing of two crystallites of cellulose. h is type of packing
could be the basis of the cholesteric liquid crystalline phases observed [11]. Copyright © 2001, WILEY-
VCH Verlag GmbH, Weinheim, Fed. Rep. of Germany.
al.
[5] found that the formation of a cholesteric phase from a suspension of crystal-
lites obtained by acid hydrolysis of natural i bers of cellulose is strongly dependent on
which mineral acid was used. h eir results show that with the use of sulfuric or phos-
phoric acid a cholesteric phase appeared, but not when hydrochloric acid was used.
h e authors attributed this to anionic stabilization due to the asymmetric deposition of
sulfur around the surface of the crystallites.
Liquid crystalline phases obtained from cellulose were i rst reported by Chanzy
et al.
[13] in 1980. h e authors were able to produce i lms and i bers from a solution of
cellulose in a number of organic solvents of the cyclic amine family, with a concentra-
tion in cellulose ranging from 20-55 wt% depending on the cellulose source and on
the average degree of polymerization. X-ray dif raction of the i bers obtained, from the
solution heated to 110ºC and subsequently regenerated in methanol or water, showed
a high degree of orientation of cellulose II that was not in i bers obtained from the
isotropic solution. h e fact that orientation of the cellulose i bers, produced from the
mesophase, was similar to what is observed in high-strength polymers such as rayon
and Fortisan directed researchers to the possibility of producing cellulose i bers with
enhanced mechanical properties that could rival other non-cellulosic polymers like
Kevlar.
h e i rst observation of cellulose-based liquid crystalline phases were followed in the
same year by those of Navard [14] and Patel [15]. In 1983, McCormik [16] observed a
nematic phase in a solution of cellulose, with a concentration of 14 wt%, in dimethyl-
acetamide/lithium chloride.
However, when the i rst cellulosic liquid crystalline phase was observed it was not
in cellulose itself but in a solution of a cellulose derivative. h is could be explained by
the fact that with cellulose it is much more dii cult to obtain a solution with a high
enough concentration needed for the mesophase to form [13]. h e i rst observation
of a cellulosic liquid crystalline phase was published by Gray
et al.
[17] in 1976. h e
authors observed that a drop of solution of hydroxypropylcellulose (HPC) in water
placed between two glass plates and at er the water was allowed to evaporate from the
edges to center, exhibits bright colors that changes with the viewing angle and the tem-
perature. Polarized optical microscopy (POM) observations revealed that samples with
