168
8 Molecular Glasses
tem, the exothermic effect is observed for T T g whereas the endothermic one for
T T g . The observation of a set of a stepped anomaly in heat capacity and enthalpy
relaxation is regarded as concrete evidence of the detection of a glass transition.
8.1.2 Structural Chemistry of Glass Transitions
Even if we restrict ourselves to motional degrees of freedom, there exist many kinds
of them in molecular systems, as discussed in previous chapters. Each of them has
an intrinsic time scale with different temperature dependence. On cooling, therefore,
the order of their time scales may vary. Dynamics slows down with the decrease in
temperature. When a relaxation time of the slowest mode surmounts the daily time
scale (ca. 10
2
− 10
3 s), a freezing-in indeed happens. If this involves inequivalent
states, a kind of glass transition takes place. Because of the presence of many degrees
of freedom, plural kinds of glass transition have been identified.
When isotropic molecular liquid, in which both positional and orientational
degrees of freedom are isotropically disordered, is subject to rapid cooling (quench),
a liquid-quenched glass (LQG) or glassy liquid is formed. Extensive studies have
been devoted to this type of glass transition in real systems. However, theoretical
and computational studies have targeted on an abstract and simpler LQG consisting
of spherical particles without rotational degrees of freedom (neglecting even kinetic
energy of rotational degrees of freedom). Most of such studies nowadays seem to
assume the existence of an ideal thermodynamic phase transition (Sect. 8.3) as a
counterpart of the kinetic glass transition defined in Sect. 8.1.1.3.
Not only isotropic liquids but also other systems possessing partial long-ranged
order(s) can exhibit glass transitions. A glassy state derived from liquid crystals is a
glassy liquid crystal [8], whereas that derived from orientationally disordered crystal
is a glassy crystal [9]. For glassy crystals, the degree of the frozen-in disorder can
be qualified through the determination of the so-called residual entropy. The most
famous example of residual entropy of a crystal and its resolution can be found for
the ice [10–13]. Note that the frozen-in disorder does not guarantee the presence of
detectable glass transition even if the residual entropy (discussed in the next section)
is undoubtedly resolved [14, 15] in contrast to the successful case, such as the ice [16].
As in the case of the melting process of molecular crystals discussed in Chap. 6,
internal motional degrees of freedom can be involved in glass transitions. Clear examples were found in organic conductors containing a half structural unit of a famous
donor molecule abbreviated as BEDT-TTF or ET, of which the molecular structure is
shown in Fig. 8.4. In agreement with the chemical common sense, the six-membered
rings are not flat. In some crystals of their charge-transfer complexes with counter
anion(s), the ring exhibits structural disorder. The freezing-in of this structural disorder causes glass transitions [17–20]. The presence of this glass transition resolved
the mysterious history-dependence of conducting properties of an organic superconductor, κ-(ET) 2 Cu[CN(CN) 2 ]Br [18, 19], which exhibited the highest transition
temperature T c = 10.8 K at that time.
8 Molecular Glasses
tem, the exothermic effect is observed for T T g whereas the endothermic one for
T T g . The observation of a set of a stepped anomaly in heat capacity and enthalpy
relaxation is regarded as concrete evidence of the detection of a glass transition.
8.1.2 Structural Chemistry of Glass Transitions
Even if we restrict ourselves to motional degrees of freedom, there exist many kinds
of them in molecular systems, as discussed in previous chapters. Each of them has
an intrinsic time scale with different temperature dependence. On cooling, therefore,
the order of their time scales may vary. Dynamics slows down with the decrease in
temperature. When a relaxation time of the slowest mode surmounts the daily time
scale (ca. 10
2
− 10
3 s), a freezing-in indeed happens. If this involves inequivalent
states, a kind of glass transition takes place. Because of the presence of many degrees
of freedom, plural kinds of glass transition have been identified.
When isotropic molecular liquid, in which both positional and orientational
degrees of freedom are isotropically disordered, is subject to rapid cooling (quench),
a liquid-quenched glass (LQG) or glassy liquid is formed. Extensive studies have
been devoted to this type of glass transition in real systems. However, theoretical
and computational studies have targeted on an abstract and simpler LQG consisting
of spherical particles without rotational degrees of freedom (neglecting even kinetic
energy of rotational degrees of freedom). Most of such studies nowadays seem to
assume the existence of an ideal thermodynamic phase transition (Sect. 8.3) as a
counterpart of the kinetic glass transition defined in Sect. 8.1.1.3.
Not only isotropic liquids but also other systems possessing partial long-ranged
order(s) can exhibit glass transitions. A glassy state derived from liquid crystals is a
glassy liquid crystal [8], whereas that derived from orientationally disordered crystal
is a glassy crystal [9]. For glassy crystals, the degree of the frozen-in disorder can
be qualified through the determination of the so-called residual entropy. The most
famous example of residual entropy of a crystal and its resolution can be found for
the ice [10–13]. Note that the frozen-in disorder does not guarantee the presence of
detectable glass transition even if the residual entropy (discussed in the next section)
is undoubtedly resolved [14, 15] in contrast to the successful case, such as the ice [16].
As in the case of the melting process of molecular crystals discussed in Chap. 6,
internal motional degrees of freedom can be involved in glass transitions. Clear examples were found in organic conductors containing a half structural unit of a famous
donor molecule abbreviated as BEDT-TTF or ET, of which the molecular structure is
shown in Fig. 8.4. In agreement with the chemical common sense, the six-membered
rings are not flat. In some crystals of their charge-transfer complexes with counter
anion(s), the ring exhibits structural disorder. The freezing-in of this structural disorder causes glass transitions [17–20]. The presence of this glass transition resolved
the mysterious history-dependence of conducting properties of an organic superconductor, κ-(ET) 2 Cu[CN(CN) 2 ]Br [18, 19], which exhibited the highest transition
temperature T c = 10.8 K at that time.