probably due to their higher solubility, which might depress the formation of
spontaneously aligned self-assemblies that is a prerequisite to the formation of
domains of lyotropic LC. In other words, polymers with high solubility are freely
dissolved in a solvent with randomly oriented conformation and form neither a
regularly stacked nor an associated structure. This situation is far from what is
necessary for the formation of the lyotropic LC. In the case of the PDPA main
chain structure of PA12, the stilbene fragment might be suitable for the formation of
an interchain π-electron overlapped association through van der Waals interactions,
which enables the polymers to exhibit the lyotropic LC.
PA12 demonstrates lyotropic liquid crystallinity at a critical concentration range
from 10 to 15 wt% in toluene. Analogous to thermotropic LCs which show an
isotropic phase above a critical temperature, PA12 becomes an isotropic liquid
below 10 wt% concentration in toluene. At the isotropic phase, the polymer main
chains (which act as LC mesogens) are dispersed randomly without any ordering. At
the lyotropic LC concentration range, there are weak π-interactions between the
aromatic stilbene structure and toluene. In such a situation, the toluene enhances the
attraction between polymer main chains for spontaneous self-assembly to occur and
also provide enough fluidity in the system for liquid crystallinity to take place.
The lyotropic LC phase of PA12 was prepared from a 10 wt% solution using
toluene as a solvent. Figure 4e shows the POM image of the polymer solution,
depicting an optical texture characteristic of the N-LC phase. The XRD analysis of
PA12 shows a single broad diffraction peak at 17.6
in 2θ. The peak corresponds to
5.0 Å, which is assigned to the distance between the mesogenic groups (Fig. 4f).
Linearly Polarized Luminescence of the Di-LCPAs
The macroscopic alignment of the Group 1 and Group 2 di-LCPA films was
achieved using rubbing technique. Polymers on quartz substrates were heated to
the LC temperature region and rubbed with a glass rod along the long axis of the
quartz substrate before cooling to room temperature. In the case of PA12, lyotropic
LC solution in 10–15 wt% toluene was prepared, then applied to a quartz substrate,
and then rubbed with a glass rod along the long axis. The PL intensity was measured
as a function of the polarizer angle with respect to rubbing direction. The dichroic
ratio (DR) of the di-LCPAs is summarized in Table 2 and is defined as the ratio of
parallel to perpendicular polarized PL intensity (DR = I // / I ⊥ ) or vice versa
(DR = I ⊥ / I // ).
Three patterns of alignment and emission behavior were observed on the aligned
PA films (see Fig. 6 later). The alignment behavior of the PA films can be related to
the structure of the polymer main chain and the LC moieties. In the di-LCPA
derivatives, the main chain and the side chain can both act as LC mesogens. The
interactions between the main chain and the LC moieties determine the alignment
behavior of the PA films.
On the other hand, the emission behavior of the PA films can be related to the diPAs exciton confinement (localization) - deconfinement (delocalization) mechanism
354
K. Akagi
spontaneously aligned self-assemblies that is a prerequisite to the formation of
domains of lyotropic LC. In other words, polymers with high solubility are freely
dissolved in a solvent with randomly oriented conformation and form neither a
regularly stacked nor an associated structure. This situation is far from what is
necessary for the formation of the lyotropic LC. In the case of the PDPA main
chain structure of PA12, the stilbene fragment might be suitable for the formation of
an interchain π-electron overlapped association through van der Waals interactions,
which enables the polymers to exhibit the lyotropic LC.
PA12 demonstrates lyotropic liquid crystallinity at a critical concentration range
from 10 to 15 wt% in toluene. Analogous to thermotropic LCs which show an
isotropic phase above a critical temperature, PA12 becomes an isotropic liquid
below 10 wt% concentration in toluene. At the isotropic phase, the polymer main
chains (which act as LC mesogens) are dispersed randomly without any ordering. At
the lyotropic LC concentration range, there are weak π-interactions between the
aromatic stilbene structure and toluene. In such a situation, the toluene enhances the
attraction between polymer main chains for spontaneous self-assembly to occur and
also provide enough fluidity in the system for liquid crystallinity to take place.
The lyotropic LC phase of PA12 was prepared from a 10 wt% solution using
toluene as a solvent. Figure 4e shows the POM image of the polymer solution,
depicting an optical texture characteristic of the N-LC phase. The XRD analysis of
PA12 shows a single broad diffraction peak at 17.6
in 2θ. The peak corresponds to
5.0 Å, which is assigned to the distance between the mesogenic groups (Fig. 4f).
Linearly Polarized Luminescence of the Di-LCPAs
The macroscopic alignment of the Group 1 and Group 2 di-LCPA films was
achieved using rubbing technique. Polymers on quartz substrates were heated to
the LC temperature region and rubbed with a glass rod along the long axis of the
quartz substrate before cooling to room temperature. In the case of PA12, lyotropic
LC solution in 10–15 wt% toluene was prepared, then applied to a quartz substrate,
and then rubbed with a glass rod along the long axis. The PL intensity was measured
as a function of the polarizer angle with respect to rubbing direction. The dichroic
ratio (DR) of the di-LCPAs is summarized in Table 2 and is defined as the ratio of
parallel to perpendicular polarized PL intensity (DR = I // / I ⊥ ) or vice versa
(DR = I ⊥ / I // ).
Three patterns of alignment and emission behavior were observed on the aligned
PA films (see Fig. 6 later). The alignment behavior of the PA films can be related to
the structure of the polymer main chain and the LC moieties. In the di-LCPA
derivatives, the main chain and the side chain can both act as LC mesogens. The
interactions between the main chain and the LC moieties determine the alignment
behavior of the PA films.
On the other hand, the emission behavior of the PA films can be related to the diPAs exciton confinement (localization) - deconfinement (delocalization) mechanism
354
K. Akagi