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which is common to all main-chain lyotropic and thermotropic LCPs. Band textures
are alternating dark and bright bands perpendicular to the shearing direction, observed
by polarizing optical microscopy (POM) [69, 70], and have been reported in the lit-
erature since 1979 [71]. h e band structure may be characterized by the band spacing
(typically 4-10 micron) [72] and is energetically very stable, but it only persists during
a characteristic time t
d
(relaxation time - typically t
d
~10 min). h e hydroxypropylcel-
lulose (HPC) lyotropic solutions are, regarding the ability to form band texture, among
the most studied systems and thoroughly described in the literature [69, 73-76]. h e
formed periodic structures can be locked within the polymer at er the solvent evapora-
tion, if the evaporation time is shorter than the relaxation time.
Borges
et al.
[58, 77] explored the liquid crystalline properties of the polymeric
matrix to produce aligned short-i ber composites. To the best of our knowledge this
was the i rst report on i ber orientation induced by the anisotropic properties of the
composite matrix. h e authors have produced solid composite i lms from liquid crys-
talline solutions of HPC (matrix) and microcrystalline cellulose (MCC), AVICEL
(reinforcement) by a shear-casting technique. h e POM images in Figure 8.5 [77] show
that solid i lms cast from solutions of 10-12 wt% HPC are isotropic (dark i eld under
cross polars (Figure 8.5a). In contrast, the i lms cast from the chiral nematic solutions
of HPC in Dimethylacetamide (DMAc) (60 wt% HPC/40 wt% DMAc) are clearly bire-
fringent (anisotropic) and present a characteristic banded texture perpendicular to
shear direction (Figure 8.5c). h e results clearly show that liquid crystalline order is
preserved in HPC/AVICEL composite i lms (i gure 8.5d). Moreover, in all-cellulosic
composite i lms the i bers (blue rods in Figure 8.5d) are oriented in an average direc-
tion that makes an angle (θ) about 14° to the shear direction. h is i ber alignment is not
observed when composite i lms are prepared from isotropic solutions of HPC (Figure
8.5b), which indicates that this is due to a competition between the alignment pro-
moted by the matrix and the shear.
Fiber alignment in short-i ber composites is very dii cult to achieve. However, the
results obtained by these authors, with anisotropic all-cellulosic-based composites,
point out a simple way of producing this type of composite with enhanced mechanical
properties.
Figure 8.5
POM photographs of isotropic (b) and anisotropic (d) all-cellulosic-based composites with
4% w/w HPC of Avicel i bers (birefringent rods in images (b) and (d), respectively). Images (a) and (c)
are the POM photographs of the isotropic (a) and anisotropic (c) HPC matrix. White arrows in images
(c) and (d) indicate the shear direction. All images were taken under crossed polars. Images were
obtained from references [12] (a and b) and [14] (c and d). Copyright 2010, with kind permission from
Springer Science and Business Media.
