178
9 Molecular Flexibility and Material Properties
them at room temperature. Some studies [6, 7] suggest that this orientational disorder
is brought about not by an overall reorientation of a flat molecule but by a crankshaft
motion (or, alternately, pedal motion), which keeps the orientations of two benzene
rings mostly while the central moiety is reversed. Since this dynamics resembles an
elementary dynamics in linear polymers, the glass transition can be regarded as a
model of freezing of such local motion in (locally) disordered polymers.
The glass transition temperature is the lowest for crystalline trans-azo-benzene
among them. For trans-stilbene and its charge-transfer salts, those are well correlated
with a volume available to each trans-stilbene molecule in crystals. These facts reflect
that such molecular motion is involved in glass transitions.
In the case of the charge-transfer salt of trans-stilbene and tetracyanoquinodimethane (often abbreviated as TCNQ), the two states (orientations) of each
trans-stilbene molecule are crystallographically equivalent above the glass transition temperature. This equivalence implies that the glass transition occurs in a highsymmetry, i.e., disordered phase. On the other hand, two orientations are inequivalent
in crystalline trans-stilbene, indicating the glass transition due to the freezing-in of
residual disorder in a low-symmetry, i.e., symmetry-broken ordered phase. These
exemplify that a glass transition is possible in either a disordered or ordered phase.
9.3 Phase Transitions Related to Molecular Deformation
Although the examples identified so far are limited, the acquiring entropy by molecular deformation (internal structural degrees of freedom) can bring about a phase
transition in crystalline states, as discussed in Sect. 6.3, where the described were
the twist transition in crystalline p-polyphenyls. This section briefly describes other
examples.
The crystal of bis(4-chlorophenyl)sulfone (BCPS) mentioned in Sect. 5.5.3 undergoes a structural phase transition of the displacive type caused by the softening of
the lowest branch of lattice vibration, in which the twisting degrees of freedom of
phenyl groups are involved (Fig. 5.3). The situation is somewhat similar to the case
of crystalline biphenyl, the first member of p-polyphenyls. Crystals of molecules
with similar structures to BCPS have also been studied within this context [8–11].
Crystalline trichloroacetamide (CCl 3 CONH 2 , abbreviated as TCAA) is a wellknown organic ferroelectric [12, 13]. In the crystal, sheets formed by N–H/O
hydrogen-bonds (H-bonds) are stacked [14]. Each layer consists of dimers. The dimer
is cyclic and also formed by two H-bonds between two molecules (A and B), which
have two opposing orientations and are not related by any symmetry operations. Due
to the non-equivalence of molecules A and B, the dimer bears a small electric dipole
(ca. 10% of that of an isolated molecule), the orientational order of which brings
about the ferroelectricity. An incommensurate phase appears in a small temperature
interval of ca. 2 K between the two successive transitions. Through the successive
phase transitions on cooling, the size of the unit cell remains essentially the same.
However, the space group loses the inversion symmetry: P2 1 /c of the paraelectric
9 Molecular Flexibility and Material Properties
them at room temperature. Some studies [6, 7] suggest that this orientational disorder
is brought about not by an overall reorientation of a flat molecule but by a crankshaft
motion (or, alternately, pedal motion), which keeps the orientations of two benzene
rings mostly while the central moiety is reversed. Since this dynamics resembles an
elementary dynamics in linear polymers, the glass transition can be regarded as a
model of freezing of such local motion in (locally) disordered polymers.
The glass transition temperature is the lowest for crystalline trans-azo-benzene
among them. For trans-stilbene and its charge-transfer salts, those are well correlated
with a volume available to each trans-stilbene molecule in crystals. These facts reflect
that such molecular motion is involved in glass transitions.
In the case of the charge-transfer salt of trans-stilbene and tetracyanoquinodimethane (often abbreviated as TCNQ), the two states (orientations) of each
trans-stilbene molecule are crystallographically equivalent above the glass transition temperature. This equivalence implies that the glass transition occurs in a highsymmetry, i.e., disordered phase. On the other hand, two orientations are inequivalent
in crystalline trans-stilbene, indicating the glass transition due to the freezing-in of
residual disorder in a low-symmetry, i.e., symmetry-broken ordered phase. These
exemplify that a glass transition is possible in either a disordered or ordered phase.
9.3 Phase Transitions Related to Molecular Deformation
Although the examples identified so far are limited, the acquiring entropy by molecular deformation (internal structural degrees of freedom) can bring about a phase
transition in crystalline states, as discussed in Sect. 6.3, where the described were
the twist transition in crystalline p-polyphenyls. This section briefly describes other
examples.
The crystal of bis(4-chlorophenyl)sulfone (BCPS) mentioned in Sect. 5.5.3 undergoes a structural phase transition of the displacive type caused by the softening of
the lowest branch of lattice vibration, in which the twisting degrees of freedom of
phenyl groups are involved (Fig. 5.3). The situation is somewhat similar to the case
of crystalline biphenyl, the first member of p-polyphenyls. Crystals of molecules
with similar structures to BCPS have also been studied within this context [8–11].
Crystalline trichloroacetamide (CCl 3 CONH 2 , abbreviated as TCAA) is a wellknown organic ferroelectric [12, 13]. In the crystal, sheets formed by N–H/O
hydrogen-bonds (H-bonds) are stacked [14]. Each layer consists of dimers. The dimer
is cyclic and also formed by two H-bonds between two molecules (A and B), which
have two opposing orientations and are not related by any symmetry operations. Due
to the non-equivalence of molecules A and B, the dimer bears a small electric dipole
(ca. 10% of that of an isolated molecule), the orientational order of which brings
about the ferroelectricity. An incommensurate phase appears in a small temperature
interval of ca. 2 K between the two successive transitions. Through the successive
phase transitions on cooling, the size of the unit cell remains essentially the same.
However, the space group loses the inversion symmetry: P2 1 /c of the paraelectric