124
M. Jasiurkowska-Delaporte
contrast to the non-chiral 5P-EtFLEt-P5 compound substituted by diethyl chains, the
asymmetric 9,9-dimethylbutyl substitution in 5P-Am*FLAm*P5 molecules induces
chirality in the nematic phase. The differential scanning calorimetric (DSC) thermograms and the phase sequences obtained by means of polarizing optical microscopy
(POM) are shown in Fig. 2.
It can be seen that in the nematic phase (N), 5P-EtFLEt-P5 displays vitrification for
even very slow cooling rates, such as 0.1 K/min. This inhibition of crystal formation
is presumably associated with energetic flustration between locally favored nematic
ordering and the thermodynamic stability of the crystalline state [29]. Upon heating,
the nematic glass softened and cold crystallization to the Cr1 phase was observed,
followed by transformation from Cr1 to Cr2 phase. A particularly complex phase
diagram strongly dependent on cooling/heating rate was seen for 5P-Am*FLAm*P5:
(i) At higher cooling rates (φ ≥ 5 K/min), vitrification of chiral nematic phase (N*)
was observed with cold crystallization to the Cr2 phase occurring upon heating; in
contrast, (ii) slow cooling (φ < 5 K/min) results in the formation of the glass-forming
Cr1 phase.
4 Molecular Dynamics in 5P-EtFLEt-P5
and 5P-Am*FLAm*P5
The dielectric properties of the investigated compounds are determined by the low
molecular dipole moment (Table 1) associated with the fluorene unites. Figure 3
presents the dielectric loss spectra ε
(f ) of 5P-EtFLEt-P5 and 5P-Am*FLAm*P5,
respectively, in the glass of N and N* phases, as well as in the metastable nematic
phases. The structural α-relaxation process is generally ascribed to the reorientation
of molecules around the short axis. For 5P-EtFLEt-P5, the temperature dependence
of the shape parameter n = a H b H indicates a substantial increase in local cooperativity on approaching T g whereas the long-range correlation parameter, measured as
1 − m (m = a H ), is temperature insensitive. In the case of 5P-Am*FLAm*P5, both
the short- and long-range correlations are temperature independent. Two secondary
β and γ processes are ascribed, respectively, to the small-angle rotational diffusion
of molecules around the short axis and to librations around the long axis [20]. Interestingly, the relaxation processes were also detected in the Cr1 and Cr2 crystalline
forms of 5P-Am*FLAm*P5. Examples of dielectric loss spectra are given in Fig. 4.
As ionic conductivity represented such a strong contribution in the dielectric
spectra of Cr1 in 5P-Am*FLAm*P5 (see Fig. 4a), the relaxation times of the hightemperature I-process were determined by fitting Eq. 6 to the derivative dielectric
spectra (Fig. 4b). To expose the second relaxation process (II), observed in Cr1 at the
lower temperatures, the results are presented in the temperature domain (Fig. 4a).
The relaxation rates of the II-process were determined according to the maximum
dielectric loss versus temperature E
(T ) at a fixed frequency. The value of E
(T )
maximum was designated by fitting a Gaussian function to the data as reported
M. Jasiurkowska-Delaporte
contrast to the non-chiral 5P-EtFLEt-P5 compound substituted by diethyl chains, the
asymmetric 9,9-dimethylbutyl substitution in 5P-Am*FLAm*P5 molecules induces
chirality in the nematic phase. The differential scanning calorimetric (DSC) thermograms and the phase sequences obtained by means of polarizing optical microscopy
(POM) are shown in Fig. 2.
It can be seen that in the nematic phase (N), 5P-EtFLEt-P5 displays vitrification for
even very slow cooling rates, such as 0.1 K/min. This inhibition of crystal formation
is presumably associated with energetic flustration between locally favored nematic
ordering and the thermodynamic stability of the crystalline state [29]. Upon heating,
the nematic glass softened and cold crystallization to the Cr1 phase was observed,
followed by transformation from Cr1 to Cr2 phase. A particularly complex phase
diagram strongly dependent on cooling/heating rate was seen for 5P-Am*FLAm*P5:
(i) At higher cooling rates (φ ≥ 5 K/min), vitrification of chiral nematic phase (N*)
was observed with cold crystallization to the Cr2 phase occurring upon heating; in
contrast, (ii) slow cooling (φ < 5 K/min) results in the formation of the glass-forming
Cr1 phase.
4 Molecular Dynamics in 5P-EtFLEt-P5
and 5P-Am*FLAm*P5
The dielectric properties of the investigated compounds are determined by the low
molecular dipole moment (Table 1) associated with the fluorene unites. Figure 3
presents the dielectric loss spectra ε
(f ) of 5P-EtFLEt-P5 and 5P-Am*FLAm*P5,
respectively, in the glass of N and N* phases, as well as in the metastable nematic
phases. The structural α-relaxation process is generally ascribed to the reorientation
of molecules around the short axis. For 5P-EtFLEt-P5, the temperature dependence
of the shape parameter n = a H b H indicates a substantial increase in local cooperativity on approaching T g whereas the long-range correlation parameter, measured as
1 − m (m = a H ), is temperature insensitive. In the case of 5P-Am*FLAm*P5, both
the short- and long-range correlations are temperature independent. Two secondary
β and γ processes are ascribed, respectively, to the small-angle rotational diffusion
of molecules around the short axis and to librations around the long axis [20]. Interestingly, the relaxation processes were also detected in the Cr1 and Cr2 crystalline
forms of 5P-Am*FLAm*P5. Examples of dielectric loss spectra are given in Fig. 4.
As ionic conductivity represented such a strong contribution in the dielectric
spectra of Cr1 in 5P-Am*FLAm*P5 (see Fig. 4a), the relaxation times of the hightemperature I-process were determined by fitting Eq. 6 to the derivative dielectric
spectra (Fig. 4b). To expose the second relaxation process (II), observed in Cr1 at the
lower temperatures, the results are presented in the temperature domain (Fig. 4a).
The relaxation rates of the II-process were determined according to the maximum
dielectric loss versus temperature E
(T ) at a fixed frequency. The value of E
(T )
maximum was designated by fitting a Gaussian function to the data as reported