9.4 Conformational Disordering of Alkyl Groups
189
Fig. 9.8 Mechanism of
stabilization of liquid
crystalline mesophases for
mesogens having alkyl
group(s) at its end(s)
T•Schain
G
= H – T•S
T
crystal
LC
with chain
IL without chain
LC without chain
IL with chain
9.4.4.2 Chains as Intramolecular Solvent
The analyses described in Sect. 9.4.4.1 indicate that, in liquid crystals, the alkyl chains
attached to the rigid core of mesogenic molecules may behave differently from the
core. The inspection over the variation in their “composition” yields another fruitful
insight. Figure 9.9 compares phase diagrams of the neat (pure) ANBC(n) against the
chain length and of ANBC(16)–n-tetradecane binary system [68]. The axis for the
binary system is against n
∗ , which is the number of paraffinic (or aliphatic) carbon
atoms per core in the system and alternately regarded as an effective chain length
of the mesogen. The cubic phase of the binary system covers both of the I a3d
and cub* phases in the neat series. The resemblance between them is evident if we
omit some structural details of cubic phases. Similar behaviors were obtained for
other mixtures of ANBC(n) (n = 14, 16, 18) and long alkanes (n-tetradecane and
n-hexadecane) [68, 69]. It is noteworthy that the cubic phase appears n
∗
15 in the
mixture despite the absence of cubic phases in neat ANBC(14). Kutsumizu et al. [69]
also confirmed the change in symmetry of the cubic phases between n
∗
∼ 17 and
18 in the ANBC(14)–n-hexadecane system. Since long alkanes are real solvents, it
is reasonable to conclude that the intramolecular chain serves as the solvent (selfsolvent) in a neat substance. These resemblances in the phase behavior lead to the
concept, “Quasi-Binary (QB) picture of Thermotropics” [70, 71].
Another example that contributed to the establishment of the QB picture lies
in a simpler, even most famous, mesogenic series, nCB (R–(C 6 H 4 ) 2 –CN, R = alkyl
group) [71]. nCB is one of the most representative series of rodlike mesogens exhibiting much simpler liquid crystalline phases than those in ANBC(n). Short-chain nCBs
(n ≤ 7) exhibit only the N phase in between the ordered crystal and the isotropic
liquid while nCBs with a long chain (n ≥ 10) exhibit only the SmA phase. Both SmA
and N phases appear in 8CB and 9CB in this order upon heating. Phase diagrams
have been reported for binary systems of 7CB and n-heptane [71] and n-octane [72].
Although the structural characterization in the latter is insufficient, the phase dia-
189
Fig. 9.8 Mechanism of
stabilization of liquid
crystalline mesophases for
mesogens having alkyl
group(s) at its end(s)
T•Schain
G
= H – T•S
T
crystal
LC
with chain
IL without chain
LC without chain
IL with chain
9.4.4.2 Chains as Intramolecular Solvent
The analyses described in Sect. 9.4.4.1 indicate that, in liquid crystals, the alkyl chains
attached to the rigid core of mesogenic molecules may behave differently from the
core. The inspection over the variation in their “composition” yields another fruitful
insight. Figure 9.9 compares phase diagrams of the neat (pure) ANBC(n) against the
chain length and of ANBC(16)–n-tetradecane binary system [68]. The axis for the
binary system is against n
∗ , which is the number of paraffinic (or aliphatic) carbon
atoms per core in the system and alternately regarded as an effective chain length
of the mesogen. The cubic phase of the binary system covers both of the I a3d
and cub* phases in the neat series. The resemblance between them is evident if we
omit some structural details of cubic phases. Similar behaviors were obtained for
other mixtures of ANBC(n) (n = 14, 16, 18) and long alkanes (n-tetradecane and
n-hexadecane) [68, 69]. It is noteworthy that the cubic phase appears n
∗
15 in the
mixture despite the absence of cubic phases in neat ANBC(14). Kutsumizu et al. [69]
also confirmed the change in symmetry of the cubic phases between n
∗
∼ 17 and
18 in the ANBC(14)–n-hexadecane system. Since long alkanes are real solvents, it
is reasonable to conclude that the intramolecular chain serves as the solvent (selfsolvent) in a neat substance. These resemblances in the phase behavior lead to the
concept, “Quasi-Binary (QB) picture of Thermotropics” [70, 71].
Another example that contributed to the establishment of the QB picture lies
in a simpler, even most famous, mesogenic series, nCB (R–(C 6 H 4 ) 2 –CN, R = alkyl
group) [71]. nCB is one of the most representative series of rodlike mesogens exhibiting much simpler liquid crystalline phases than those in ANBC(n). Short-chain nCBs
(n ≤ 7) exhibit only the N phase in between the ordered crystal and the isotropic
liquid while nCBs with a long chain (n ≥ 10) exhibit only the SmA phase. Both SmA
and N phases appear in 8CB and 9CB in this order upon heating. Phase diagrams
have been reported for binary systems of 7CB and n-heptane [71] and n-octane [72].
Although the structural characterization in the latter is insufficient, the phase dia-