4.2 Validity of the Assumption Adopted for Mesogenic
Core Axis
As discussed above, the odd-even oscillation should be sensitive to the relative
magnitude of the conformer fraction in the nematic state. In a recent work, Centore
[23] has investigated the odd-even trend for a series of carbonate dimers containing
dimethylbenzalazine mesogens. He suggested that the ΔS NI values of the n ¼ even
members may be relatively more suppressed. The odd-even trend may be even
reversed by modifying the chemical structure of mesogens. In practice, when the
terminal group is replaced by a dibenzoxy terephthalate-type mesogen in the carbonate series (named 3MP-n), the odd members were found to exhibit higher latent
entropies than those of n ¼ even [24]. A possible answer to such a delicate variation
may be found in the integrated distribution curves of the disorientation angle θ.
A larger tilting angle ϕ affects both odd and even distribution curves: While
conformers of lower θ values tend to increase in the former curve, those in the
range θ ¼ 0 À 30
are appreciably suppressed in the latter. This suggests that the
population of conformers most suited for the nematic order may eventually become
comparable. The profiles of the two distribution curves (P(θ) À θ) are also similar in
the intermediate region of θ, indicating that the thermodynamic stabilities of the
nematic state are rather close between the n ¼ odd and even in the carbonate series.
Under such conditions, some slight variation in the tilting angle ϕ between the
first bond of the spacer and the mesogenic core axis (Fig. 4) might cause a large
effect on the odd-even trend in the transition behavior. Experimental observations
mentioned above are consistent with what is suggested by the conformational
analysis of the carbonate system. In our simplified calculation (see Sect. 5), the
biaxiality inherent to the chemical structure is not rigorously taken into account.
Admittedly, therefore, the tilting angle ϕ may be affected somewhat by the stereochemical constitution of the mesogenic unit. In effect, the reversed odd-even trend
mentioned above (3MP-n) was found to be reproduced by assuming a slightly larger
angle for the carbonate linkage in our RIS/
2 H NMR simulation scheme [24].
4.3 Rotational Characteristics of the Bonds Constituting
the Spacer
The distinct odd-even alternation in ΔS NI is a character peculiar to the
polymethylene (PM)-type spacers. A comparison between the two different types
of spacer, BCBOn (designated as CBA-n in this paper) [25] and MBBE-x with the
oxyethylene (OE)-type spacer [26, 27] is shown in Fig. 5, where the latent entropy
ΔS NI /R is plotted against the number of constituent atoms (n) of the spacer. For
MBBE-x with the OE spacer, irrespective of the parity of n, the ΔS NI versus n plot
tends to decrease monotonically with some tiny bumps. The odd-even trend characteristic of the tetrahedrally bonded chain system is rapidly smeared out by the
Nematic Conformation of Chain Molecules Predominating in the Ordered Mesophase
115
Core Axis
As discussed above, the odd-even oscillation should be sensitive to the relative
magnitude of the conformer fraction in the nematic state. In a recent work, Centore
[23] has investigated the odd-even trend for a series of carbonate dimers containing
dimethylbenzalazine mesogens. He suggested that the ΔS NI values of the n ¼ even
members may be relatively more suppressed. The odd-even trend may be even
reversed by modifying the chemical structure of mesogens. In practice, when the
terminal group is replaced by a dibenzoxy terephthalate-type mesogen in the carbonate series (named 3MP-n), the odd members were found to exhibit higher latent
entropies than those of n ¼ even [24]. A possible answer to such a delicate variation
may be found in the integrated distribution curves of the disorientation angle θ.
A larger tilting angle ϕ affects both odd and even distribution curves: While
conformers of lower θ values tend to increase in the former curve, those in the
range θ ¼ 0 À 30
are appreciably suppressed in the latter. This suggests that the
population of conformers most suited for the nematic order may eventually become
comparable. The profiles of the two distribution curves (P(θ) À θ) are also similar in
the intermediate region of θ, indicating that the thermodynamic stabilities of the
nematic state are rather close between the n ¼ odd and even in the carbonate series.
Under such conditions, some slight variation in the tilting angle ϕ between the
first bond of the spacer and the mesogenic core axis (Fig. 4) might cause a large
effect on the odd-even trend in the transition behavior. Experimental observations
mentioned above are consistent with what is suggested by the conformational
analysis of the carbonate system. In our simplified calculation (see Sect. 5), the
biaxiality inherent to the chemical structure is not rigorously taken into account.
Admittedly, therefore, the tilting angle ϕ may be affected somewhat by the stereochemical constitution of the mesogenic unit. In effect, the reversed odd-even trend
mentioned above (3MP-n) was found to be reproduced by assuming a slightly larger
angle for the carbonate linkage in our RIS/
2 H NMR simulation scheme [24].
4.3 Rotational Characteristics of the Bonds Constituting
the Spacer
The distinct odd-even alternation in ΔS NI is a character peculiar to the
polymethylene (PM)-type spacers. A comparison between the two different types
of spacer, BCBOn (designated as CBA-n in this paper) [25] and MBBE-x with the
oxyethylene (OE)-type spacer [26, 27] is shown in Fig. 5, where the latent entropy
ΔS NI /R is plotted against the number of constituent atoms (n) of the spacer. For
MBBE-x with the OE spacer, irrespective of the parity of n, the ΔS NI versus n plot
tends to decrease monotonically with some tiny bumps. The odd-even trend characteristic of the tetrahedrally bonded chain system is rapidly smeared out by the
Nematic Conformation of Chain Molecules Predominating in the Ordered Mesophase
115