188
9 Molecular Flexibility and Material Properties
to that of alkanes in Fig. 6.6. This coincidence indicates that the chain of nTCB is,
starting from the completely ordered state in OC, certainly molten, or disordered
in the same degree as that in the isotropic liquid of alkanes. Thus, the constant
entropic state of chains in the SmE phase shows that it is already molten in the
SmE phase. The molten state of chains in the liquid crystalline state revealed by
thermodynamic analyses has support from a spectroscopic technique [59]. Although
a remarkable change happens upon the transition from the OC to the SmE phase,
little change occurs upon the phase transitions not only from the SmA phase to IL
but also from the SmE phase to the SmA phase. Similar behaviors in the ΔS analysis
and vibrational spectroscopy appeared in another mesogenic series exhibiting the
SmE, nPA [60, 61]. In summary, alkyl chains attached to a rigid core at its end(s)
gain the conformational disorder comparable to the isotropic liquid already in the
SmE phase, which is one of the liquid crystalline mesophases with the highest order.
Since it is hard to imagine that highly disordered chains recover their order in more
disordered mesophases, we conclude that the alkyl chains are mostly molten in any
liquid crystalline phases.
It is interesting to note here that, through thermodynamic analyses, Sorai et al. [62]
had interpreted the successive phase transitions in solid states of a discotic mesogenic
series as the successive chain melting process. IR [62] and NMR [63] measurements
supported their interpretation. They concluded that the chain melting prepared liquid
crystalline states. Its active mechanism was, however, left unrevealed. Now, we can
understand the mechanism. Under the ambient condition, the thermodynamic stability of a given phase is governed by the Gibbs energy (G) that consists of enthalpy
(H ) and entropy (S) terms as G = H − T S. Molecular motions (disorder) have a
direct effect on the entropy of a substance and modulates the Gibbs energy. Unless
we take the chain entropy into account, the entropy gain of the SmE phase, which
comes from the loss of head/tail and front/rear discriminations, is merely 2R ln 2 ≈
11 J K
−1 mol
−1 . The thermodynamic analyses described in the previous paragraph
reveal that the chains are molten even in the most rigid liquid crystalline phase (SmE
phase). The large entropy reserved in the chain enhances the thermodynamic stability of liquid crystalline phases through the entropy term in the Gibbs energy. This
mechanism is summarized in Fig. 9.8. This is just predicted theoretically by Dowell
[50–54]. The statement “Chain melting prepares LC states” by Sorai [62] can also
be understood in the same context.
Although the excluded volume effect drives the formation of some liquid crystalline phases such as N, SmA, and HexB phases (as exemplified by molecular simulations [47, 48]), this fact does not guarantee its role in real mesogens. The analysis
described here indicates that, contrary to the common belief, the presence of the
alkyl chain(s) attached to the rigid core moiety in most real mesogens is not merely
accidental but vital. The rareness of the mesogenic behavior of small molecules without alkyl chain(s) like p-sexiphenyl [64] should instead be considered as reasonable
though liquid crystalline states driven by the excluded volume effect are well known
in colloidal systems [65–67].
9 Molecular Flexibility and Material Properties
to that of alkanes in Fig. 6.6. This coincidence indicates that the chain of nTCB is,
starting from the completely ordered state in OC, certainly molten, or disordered
in the same degree as that in the isotropic liquid of alkanes. Thus, the constant
entropic state of chains in the SmE phase shows that it is already molten in the
SmE phase. The molten state of chains in the liquid crystalline state revealed by
thermodynamic analyses has support from a spectroscopic technique [59]. Although
a remarkable change happens upon the transition from the OC to the SmE phase,
little change occurs upon the phase transitions not only from the SmA phase to IL
but also from the SmE phase to the SmA phase. Similar behaviors in the ΔS analysis
and vibrational spectroscopy appeared in another mesogenic series exhibiting the
SmE, nPA [60, 61]. In summary, alkyl chains attached to a rigid core at its end(s)
gain the conformational disorder comparable to the isotropic liquid already in the
SmE phase, which is one of the liquid crystalline mesophases with the highest order.
Since it is hard to imagine that highly disordered chains recover their order in more
disordered mesophases, we conclude that the alkyl chains are mostly molten in any
liquid crystalline phases.
It is interesting to note here that, through thermodynamic analyses, Sorai et al. [62]
had interpreted the successive phase transitions in solid states of a discotic mesogenic
series as the successive chain melting process. IR [62] and NMR [63] measurements
supported their interpretation. They concluded that the chain melting prepared liquid
crystalline states. Its active mechanism was, however, left unrevealed. Now, we can
understand the mechanism. Under the ambient condition, the thermodynamic stability of a given phase is governed by the Gibbs energy (G) that consists of enthalpy
(H ) and entropy (S) terms as G = H − T S. Molecular motions (disorder) have a
direct effect on the entropy of a substance and modulates the Gibbs energy. Unless
we take the chain entropy into account, the entropy gain of the SmE phase, which
comes from the loss of head/tail and front/rear discriminations, is merely 2R ln 2 ≈
11 J K
−1 mol
−1 . The thermodynamic analyses described in the previous paragraph
reveal that the chains are molten even in the most rigid liquid crystalline phase (SmE
phase). The large entropy reserved in the chain enhances the thermodynamic stability of liquid crystalline phases through the entropy term in the Gibbs energy. This
mechanism is summarized in Fig. 9.8. This is just predicted theoretically by Dowell
[50–54]. The statement “Chain melting prepares LC states” by Sorai [62] can also
be understood in the same context.
Although the excluded volume effect drives the formation of some liquid crystalline phases such as N, SmA, and HexB phases (as exemplified by molecular simulations [47, 48]), this fact does not guarantee its role in real mesogens. The analysis
described here indicates that, contrary to the common belief, the presence of the
alkyl chain(s) attached to the rigid core moiety in most real mesogens is not merely
accidental but vital. The rareness of the mesogenic behavior of small molecules without alkyl chain(s) like p-sexiphenyl [64] should instead be considered as reasonable
though liquid crystalline states driven by the excluded volume effect are well known
in colloidal systems [65–67].