Effect of pressure. Figs. 4 and 5 indicate that the cell size
decreases while the cell population is increased, in an orderly fashion
from pure PDLLA to PDLLA nanocomposites for low clay loading
until they are finally stabilized with further addition of the filler.
The above mentioned effect of the clay loading on the foam features
is amplified when the saturation pressure is increased from
120 bar to 150 bar. At the same saturation temperature, when the
pressure is increased, the CO2 solubility in the polymer matrix is
increased too, leading to a viscosity decrease. This, in effect, facilitates
the pore growth and tends to result in increased pore size
[6]. However, the results suggest that this phenomenon is not decisive
for the final pore size. According to the nucleation theory, an
increase in pressure can cause a reduction in the energy barrier
that has to be overcome in order to create a stable nucleus [6]. In
other words, the generation of the initial bubble nuclei that will
form the final pores is favored by higher pressures [5,6]. This is the
reason for the increased cell density at higher pressures observed
in Fig. 5b. Additionally, an increase in pressure is accompanied with
an increase in CO2 solubility, which means that more CO2 is available
for pore nucleation and growth inside the polymer matrix.
However, because more nuclei are formed in the system, the gas
is distributed in much more cells and, consequently, smaller pores
are obtained. Finally, the slight reduction in the bulk foam density,
observed with pressure increase, can be attributed to the increase
of gas solubility in the polymer matrix [6].