extension (Fig. 6), giving rise to a moderate value of the orientational order
parameter. As revealed by the studies of magnetic susceptibility [39, 40] as well
as optical anisotropy [41], the molecular anisotropy of dimer compounds CBA-n
(n ¼ 9, 10) increases on going from the isotropic to the nematic LC state. Although
the flexible spacer takes more extended conformation in the LC state, contribution
of the spacer to the orientation-dependent intermolecular (attractive) interactions
seems to be small [32]. The nematic conformation of the spacer remains nearly
invariant over the entire range of the LC state [42].
The conformational analyses of mainchain LCs have been reported from various
laboratories. Although the results seem to vary somewhat depending on the models
adopted in the treatment of experimental data, all suggest that flexible spacers
prefer to take extended conformations in the nematic state. Efforts to formulate
molecular theories to describe the NI transition characteristics of the mainchain
LCs in terms of the molecular parameters have also been reported [7, 43, 44]. In an
ideal crystalline state, molecules are aligned in a perfect order, often only the most
preferred conformation being permitted for the spacer [45]. The structural characteristics of chain molecules in the crystalline, liquid crystalline, and isotropic fluid
states must manifest themselves in the conformational entropy of the system upon
phase transitions. As the DP of the mainchain LC sample increases, however, the
degree of crystallinity tends to be lower, and accordingly the CN transition becomes
less sharp [11].
7 Concluding Remarks
In this example, we have attempted to reveal the true nature of nematic conformation characteristic of flexible spacers incorporated in the LC state. The nematic
conformation predominates in the individual –[Ms–X–(CH 2 ) n –X] x – units constituting a given polymeric sequence. In an independent work [26], PVT studies on the
mainchain LCs carrying OE-type spacers have been carried out. It is interesting that
the expansivity of the nematic LC phase was found to be larger than that of the
isotropic melt. According to the conventional thermodynamic theories of polymeric
fluids, the expansivity is closely related to the free volume of the liquid state
[46–49].
The order–disorder transition of the mesogenic molecules has been well
described by the Maier–Saupe expression in terms of the attractive dispersion
interaction [50, 51]. For mesogens of low axial ratios, contribution from the steric
repulsion should be relatively minor [52, 53]. In the mainchain LC systems,
however, the nematic alignment of the mesogenic cores is largely restricted by
the geometrical requirement from the intervening spacers. In compensation, spacers
adopt the nematic conformation to cope with the LC formation induced by the
anisotropic interactions of mesogenic groups. In this manner, amazingly long
flexible chains (n ~ 14) can be accommodated, and they seem to enjoy being in
the nematic order. Thermodynamic consideration strongly suggests that the loss of
Nematic Conformation of Chain Molecules Predominating in the Ordered Mesophase
119
parameter. As revealed by the studies of magnetic susceptibility [39, 40] as well
as optical anisotropy [41], the molecular anisotropy of dimer compounds CBA-n
(n ¼ 9, 10) increases on going from the isotropic to the nematic LC state. Although
the flexible spacer takes more extended conformation in the LC state, contribution
of the spacer to the orientation-dependent intermolecular (attractive) interactions
seems to be small [32]. The nematic conformation of the spacer remains nearly
invariant over the entire range of the LC state [42].
The conformational analyses of mainchain LCs have been reported from various
laboratories. Although the results seem to vary somewhat depending on the models
adopted in the treatment of experimental data, all suggest that flexible spacers
prefer to take extended conformations in the nematic state. Efforts to formulate
molecular theories to describe the NI transition characteristics of the mainchain
LCs in terms of the molecular parameters have also been reported [7, 43, 44]. In an
ideal crystalline state, molecules are aligned in a perfect order, often only the most
preferred conformation being permitted for the spacer [45]. The structural characteristics of chain molecules in the crystalline, liquid crystalline, and isotropic fluid
states must manifest themselves in the conformational entropy of the system upon
phase transitions. As the DP of the mainchain LC sample increases, however, the
degree of crystallinity tends to be lower, and accordingly the CN transition becomes
less sharp [11].
7 Concluding Remarks
In this example, we have attempted to reveal the true nature of nematic conformation characteristic of flexible spacers incorporated in the LC state. The nematic
conformation predominates in the individual –[Ms–X–(CH 2 ) n –X] x – units constituting a given polymeric sequence. In an independent work [26], PVT studies on the
mainchain LCs carrying OE-type spacers have been carried out. It is interesting that
the expansivity of the nematic LC phase was found to be larger than that of the
isotropic melt. According to the conventional thermodynamic theories of polymeric
fluids, the expansivity is closely related to the free volume of the liquid state
[46–49].
The order–disorder transition of the mesogenic molecules has been well
described by the Maier–Saupe expression in terms of the attractive dispersion
interaction [50, 51]. For mesogens of low axial ratios, contribution from the steric
repulsion should be relatively minor [52, 53]. In the mainchain LC systems,
however, the nematic alignment of the mesogenic cores is largely restricted by
the geometrical requirement from the intervening spacers. In compensation, spacers
adopt the nematic conformation to cope with the LC formation induced by the
anisotropic interactions of mesogenic groups. In this manner, amazingly long
flexible chains (n ~ 14) can be accommodated, and they seem to enjoy being in
the nematic order. Thermodynamic consideration strongly suggests that the loss of
Nematic Conformation of Chain Molecules Predominating in the Ordered Mesophase
119