Chemistry Reference
In-Depth Information
Beyond the question of miscibility and phase separation, the individual noncovalent
interactions between components are also of interest in amorphous solid dispersions.
These interactions can include those observed in molecular crystals or in solution phase,
such as hydrogen bonding,
π
-stacking involving aromatic rings or other
π
-electron
systems,
-cation interactions, halogen bonds, and dispersive (van der Waals) forces [6].
These interactions, which are often readily observed in crystalline materials by SCXRD
and spectroscopic techniques, can be a challenge to detect in the inherently disordered
environments that occur in amorphous solid dispersions. Sensitive spectroscopic tech-
niques based on vibrational and solid-state nuclear magnetic resonance (SSNMR)
spectroscopy are particularly useful in detecting such interactions.
Characterization techniques can be used to study changes in the physical state of
amorphous solid dispersions, such as structural relaxation toward an equilibrium
amorphous state. Structural relaxation refers to the relaxation of the amorphous state
toward a lower energy, higher density state [7
π
9]. Subtle relaxation effects often occur
with aging of amorphous solid dispersions, resulting in an increase in density and a
decrease in free volume, and endothermic events corresponding to the enthalpy of
relaxation can be observed by calorimetric methods and other methods discussed in
this chapter. However, the major temperature-dependent state change that often occurs
in amorphous materials, including amorphous solid dispersions, is the glass transition
in the vicinity of the temperature
T
g
, where the mobility, viscosity, and other properties
of the amorphous solid dispersion transition from those of a glass to those of a rubber
(or supercooled liquid) [7
-
9]. A number of theoretical models of
T
g
are available,
although none fully explains all of the properties of the glassy state [10,11]. Most
importantly, the
T
g
can vary depending on the glass preparation conditions, and in the
case of a melt preparation,
T
g
typically decreases as the cooling rate of the melt is
reduced because the amorphous phase can relax to a more energetically favorable
state [11]. Below the
T
g
, the glass can still exhibit mobility and structural relaxation on
a short timescale until it is further cooled to the temperature
T
K
(known as the
Kauzmann temperature), where the entropy curve of the glass intersects that of a
reference crystalline phase [11].
T
K
values can often range well below the
T
g
, with
values of 40
-
Clowerthan
T
g
determined in some cases [12]. The
T
K
is often
found to be near to the theoretical Vogel
-
190
°
Fulcher temperature, where the relaxation
time associated with conformational molecular motions begins to deviate from that
associated with vibrational motions [11]. In a polymer, the passage through
T
g
is often
pictured as a cooperative, non-Arrhenius process wherein the polymer chains over-
come internal resistance and tangling, become less viscous, and begin to slide past one
another more freely. Far above the glass transition temperature, the components of the
dispersion move rapidly as in a liquid phase and the material exhibits Arrhenius-type
behavior [11]. The glass transition is not a true thermodynamic transition, but is
instead referred to as a kinetic transition [11].
The glass transition is often referred to as a primary or
-
α
-relaxation process or a
series of
-relaxation processes [9]. Other secondary processes are also observed and
are commonly referred to as
α
β
-processes [9,10,13]. The
β
-processes are described as
being speci
c molecular interactions that may occur in spatial regions of a dispersion with
different densities and exhibit Arrhenius-type behavior, and are sometimes referred to
