96
K. Kawakami
Class 1: Compounds that crystallize during cooling from the melt at 20 °C/min.
Class 2: Compounds that do not crystallize during the cooling mentioned above
but crystallize during subsequent heating at 10 °C/min.
Class 3: Compounds that do not crystallize during the cooling/heating cycle
mentioned above.
Determination of a critical cooling rate that inhibits crystallization was also introduced as an alternative way by, Rades et al., to assess the crystallization tendency
[7].
Class 1: Compounds that crystallize at a cooling rate of 750 °C/min.
Class 2: Compounds that crystallize at ca. 10–20 °C/min.
Class 3: Compounds that do not crystallize at a cooling rate of 2 °C/min.
Despite having different definitions, both these classifications provide similar
results. The difference in class 1 and 2 compounds is likely to have originated from
the degree of difference in nucleation and crystal growth temperatures, whereas the
greater stability of class 3 compounds seems to be due to the strong intermolecular
interactions [5, 8].
Isothermal crystallization is practically more significant than the non-isothermal
one for pharmaceutical applications. Under careful investigations, the initiation time
for crystallization of class 1 and 2 compounds can be summarized as a function of
T g /T, where T g and T are the glass transition and storage temperature, respectively
(Fig. 6.1) [9, 10]. Although the class 3 compounds have higher stability, their worst
case of stability agreed with that of class 1 and 2 compounds [11]. This result indicates
that crystallization is basically governed by molecular mobility. Strong molecular
interactions result in higher stability. However, it must be emphasized that this result
was obtained after very careful investigation, where the glass samples were prepared
by quenching the melts in DSC pans, and then subjected to isothermal storage as they
were. Crystallization of practical glasses, for which disturbance factors including
mechanical stress are applied during preparation, cannot be interpreted in such a
simple manner. These details will be discussed later.
Thermal history is one of the least important factors in many crystallization studies
but it has a significant effect on crystallization behavior. Due to their non-equilibrium
nature, amorphous solids undergo relaxation to obtain a more stable state. Nucleation
may proceed as well but it is difficult to detect it directly. The influence of thermal
history on glass stability is discussed below.
6.2 Effect of Cooling Rate on the Properties of Quenched
Glass
When glasses are prepared by quenching, their stability is influenced by the cooling
rate. Figure 6.2a, b show reversing and non-reversing heating curves of probucol glass
obtained by cooling the melt [12]. The effect of the cooling rate was observed at 20,
K. Kawakami
Class 1: Compounds that crystallize during cooling from the melt at 20 °C/min.
Class 2: Compounds that do not crystallize during the cooling mentioned above
but crystallize during subsequent heating at 10 °C/min.
Class 3: Compounds that do not crystallize during the cooling/heating cycle
mentioned above.
Determination of a critical cooling rate that inhibits crystallization was also introduced as an alternative way by, Rades et al., to assess the crystallization tendency
[7].
Class 1: Compounds that crystallize at a cooling rate of 750 °C/min.
Class 2: Compounds that crystallize at ca. 10–20 °C/min.
Class 3: Compounds that do not crystallize at a cooling rate of 2 °C/min.
Despite having different definitions, both these classifications provide similar
results. The difference in class 1 and 2 compounds is likely to have originated from
the degree of difference in nucleation and crystal growth temperatures, whereas the
greater stability of class 3 compounds seems to be due to the strong intermolecular
interactions [5, 8].
Isothermal crystallization is practically more significant than the non-isothermal
one for pharmaceutical applications. Under careful investigations, the initiation time
for crystallization of class 1 and 2 compounds can be summarized as a function of
T g /T, where T g and T are the glass transition and storage temperature, respectively
(Fig. 6.1) [9, 10]. Although the class 3 compounds have higher stability, their worst
case of stability agreed with that of class 1 and 2 compounds [11]. This result indicates
that crystallization is basically governed by molecular mobility. Strong molecular
interactions result in higher stability. However, it must be emphasized that this result
was obtained after very careful investigation, where the glass samples were prepared
by quenching the melts in DSC pans, and then subjected to isothermal storage as they
were. Crystallization of practical glasses, for which disturbance factors including
mechanical stress are applied during preparation, cannot be interpreted in such a
simple manner. These details will be discussed later.
Thermal history is one of the least important factors in many crystallization studies
but it has a significant effect on crystallization behavior. Due to their non-equilibrium
nature, amorphous solids undergo relaxation to obtain a more stable state. Nucleation
may proceed as well but it is difficult to detect it directly. The influence of thermal
history on glass stability is discussed below.
6.2 Effect of Cooling Rate on the Properties of Quenched
Glass
When glasses are prepared by quenching, their stability is influenced by the cooling
rate. Figure 6.2a, b show reversing and non-reversing heating curves of probucol glass
obtained by cooling the melt [12]. The effect of the cooling rate was observed at 20,