Chemistry Reference
In-Depth Information
and stronger when the concentration increases to about 4 wt% when a
gel is formed. The scattering of the gels is significantly weaker than the
lyotropic solutions at concentrations of about 0.5 wt% and higher. Upon
centrifugation, the solutions separate into two phases. The top phase is
isotropic with no observable light scattering while the bottom phase scat-
ters light strongly. The lyotropic phase also has thermotropic properties.
The temperature at which the liquid crystal to the isotropic liquid phase
transition occurs for the solution of 0.20 wt% is 75
◦
C, that for the solution
of 0.51 wt% is 90
◦
C, showing a very strong dependence on the concentration
of the solutions. The transitions are reversible.
PE-R solutions were also made to study the lyotropic phase. No solu-
tions with concentrations up to 50 wt % have been observed to scatter lights.
However, both the powder samples (obtained from solutions by the addi-
tion of precipitant methanol and drying in a vacuum) and the film samples
(casted from solutions by the evaporation of the solvent at ambient temper-
atures) are strongly birefringent when observed on a polarizing microscope.
This is true not only for PE-R but also true for PA-R polymers. Because
both PE-n and PA-n polymers are non-crystalline, the birefringence must
be a result of mesophase ordering formed during the precipitation or sol-
vent evaporation. In other words, a mesophase is formed in the course of
the preparation of the samples. Careful studies have found that with the
process of concentration of solutions of MJLCPs, a nematic phase with typ-
ical threaded and schlierene defects is developed (Zhou
et al
., 1996). With
such evidence for formation of lyotropic liquid crystals, the similarity in
the property of MJLCPs with rigid and semi-rigid main chain type liquid
crystal polymers is further demonstrated.
Experimental supports for the concept of MJLCPs were also given by
other research groups. Side chain type liquid crystal polymers with meso-
genic units laterally substituted on the main chains have also been studied
by Keller
et al
. (1988), Gray
et al
. (1991), Pugh and Schrock (1993). Meso-
genic units used by Finkelmann (polymer
3
.
61
) and by Keller (polymer
) were the same as or very similar to what we have used in poly-
mers PE-R. However, flexible spacers with different length and structure
were used in their polymers. Finkelmann's polymers are polymethacrylates,
while Keller's are polysiloxanes. Gray also used polysiloxanes as polymer
backbones and polymethylene segments as flexible spacers, but the attach-
ment of the mesogens to the backbone is not at the waist but at the shoulder
position of the mesogens (polymer
3
.
62
3
.
63
). In spite of these differences in
