3.1. The Energy Balance in the Evolving Polymer Foams and the Equation of Foam State
In the foaming of the plasticized D,L-polylactide, the plasticizing/foaming agent plays the role of an energy depot for the foam incipience and expansion. Correspondingly, the internal energy decrement of the agent, which occurs during the transition between the initial and final states of the expanding foam, is distributed among the various energy-consuming processes, such as the expansion work, development of fluid-polymer interfaces, heat exchange with the environment, dissipative losses due to viscous friction in the plasticized polymer, etc. In general, the energy balance equation can be considered as:
where
is the expansion work for the foam volume,
is the increment of the total surface energy of polymer-fluid interfaces,
is the heat exchange between the foam and the environment,
are the total dissipative losses due to viscous friction in the expanding foam. The term
is related to the inertial forces occurring in the case of the rapid accelerated expansion of the foam volume. The subscripts
and
correspond to the initial and final states of the system. We assume that these terms on the right side of Equation (2) provide, under certain conditions, the dominating contributions to the consumption of the energy depot associated with the total internal energy of CO
2 in the initial state.
We also use the equation of state for equilibrium liquid foams pioneered by S. Ross [
39]. This equation can be written as ([
39,
40]):
where
is the surface tension at the liquid–gas interfaces,
is the total area of these interfaces,
is the total number of gas molecules in the foam volume, and
is the absolute temperature. It should be noted that Equation (3) is somewhat controversial due to different dimensions of the right and left sides (J against K). This contradiction is circumvented by considering the absolute temperature in units of energy [
40].
Further consideration should be preceded by some clarifying comments. Equation (4a) describes the thermodynamically stable liquid foams with negligible contributions of the internal energy of the liquid phase to the energy balance. The foam expansion due to the external pressure drop is obviously a non-equilibrium process. Note that under the remarkable contribution of the second term on the left side of Equation (4a) (this means that a foamed liquid is characterized by a developed system of liquid–gas interfaces and the surface tension is large), the expansion of the foam volume evidently leads to the saturation of the
dependence. This saturation is manifested for the experimentally obtained quasi-adiabats and quasi-isotherms of the foamed polylactide (see
Figure 4,
Figure 5 and
Figure 6).
It is seen that, under 0, Equation (4a) is reduced to a form similar to the equation of state of an ideal gas. Therefore, we can compare the experimentally observed evolution of the polymer foam in the coordinates with the modeled dependencies for carbon dioxide. This comparison allows for the interpretation of the occurring discrepancies of the experimental and modeled data in terms of dominating energy-consuming processes.
The mentioned above differences in experimental conditions of the foam growth (confined expansion between the cuvette walls in the quasi-adiabatic mode and free expansion in the case of quasi-isothermal depressurization) can be considered in terms of Equation (4a) by taking into account the topological features of expanding foam. Thus, in particular, the extended version of Equation (4a) has the following form [
40]:
where
is the topological dimension of a space in which the foam exists. Consequently, we can consider the equation of foam state as
, where
is the renormalized surface tension equal to
in the 2D case and
in the 3D case. Therefore, we can assume that the confinement effect (quasi-2D expansion) will lead to a slight decrease in the apparent surface tension in comparison with the 3D expansion into a half-space. It should be noted, however, that this effect is rather subtle and does not provide any remarkable influence on the
dependencies discussed due to the unchangeable space dimensions
during the quasi-adiabatic and quasi-isothermal experiments.
3.2. Characteristics of Adiabatic Expansion of Sub- and Supercritical Carbon Dioxide: Modeling and Analysis
Let us consider the adiabatic expansion of the foaming CO
2 in the absence of heat exchange with the environment. This leads to the main property of any adiabatically evolving system, the isentropicity (
0). The constant entropy of an expanding volume of carbon dioxide will be a starting point in our consideration. Another point follows from the basic interrelation between a unique adiabatic curve and a family of isotherms for the considered fluidic system at the “pressure-volume” coordinate plane (
Figure 8). Each cross-section of the adiabatic curve with the given isotherm (point I in
Figure 8) corresponds to the system state defined by the parameters (
). Consequently, the adiabatic transition to a new state (point II) is accompanied by the changes in the parameters, except for the entropy (
).
Another feature is that the dewpoint condition (
) can be overcome (
) for certain system states along the adiabatic curve. For these states, the coexistence of the liquid and gas phases takes place in the expanding volume and the problem of the adiabatic curve recovery
is required to account for the coexistence. Following on from the isentropicity and the additivity of the system entropy, we can introduce the coexistence rule:
where
and
are the current values of the specific entropy of the gas and liquid phases,
is the current mass fraction of the gas phase, and
is the specific entropy of a single-phase system at the initial state. The term
corresponds to contribution of the surface entropy; consequently,
is the mass fraction of the interfaces between the gas and liquid phases. In further consideration, we will neglect this term, assuming its smallness. Correspondingly, Equation (5) is reduced to:
Our consideration is based on the application of the datasets on the CO
2 isothermal properties obtained using the online NIST (National Institute of Standard and Technology, USA) calculator [
37]. This calculator does not allow for the modeling of the adiabatic regime, and the generated isothermal datasets were applied as arrays of the input parameters in our modeling, provided using the developed MatLab software. The algorithm was based on the following procedure: at the first step, the initial specific values of the entropy
, internal energy
, and volume
were interpolated from the isothermal dataset for the initial conditions (
and
). Next, a new value of the system temperature was set using the predetermined decrement
and the values of the pressure, specific volume, and specific internal energy were recovered under the isentropicity condition using the interpolation along the isotherm (
). This procedure was repeated until the current temperature became equal or lower than the final temperature, set slightly above the temperature of CO
2 transition from the liquid to solid state (the melting point, ≈ 216.6 K). Correspondingly, the final temperature in modeling was chosen as equal to
216.8 K.
If gas–liquid coexistence occurs at the current step, the value is between the values of specific entropy for the gas and liquid phases, and (i.e., the found isentropic state is below the dewpoint). In this case, the current mass fraction is calculated using Equation (5), and the specific values of the system volume and internal energy are calculated as and (here, and are the current phase densities).
The modeling results are presented in
Figure 9 and
Figure 10.
Figure 9 displays a family of curves characterizing the behavior of the normalized specific volume
of CO
2 under the decreasing pressure. The initial temperature
corresponds to the value applied in our quasi-adiabatic experiments (309.15 K).
The bold dashed line corresponds to the adiabatic expansion of an ideal three-atomic gas with a polytropic index equal to
, where
is the number of degrees of freedom for a gas molecule [
41]. In the case of carbon dioxide molecules with the linear geometry,
5, we have set
1.4 as the reference curve in
Figure 9. Note that the modeled curves exhibit behavior similar to that of an ideal gas in a relatively narrow range of pressures (0.2 MPa
1.0 MPa). At larger pressures, the power-law decay of
with an increasing pressure (
) is corrupted due to a non-ideal gas behavior, a coexistence of phases, and, finally, the transition to the supercritical state.
Figure 10 displays the decrement values of the specific internal energy
, the expansion factor
, and the mass fraction of the liquid phase in the system
at the final stage against the initial pressure
. Note the increase in the absolute value of
and the abrupt growth of
in the region of initial pressure around the critical pressure (
7.38 MPa). In addition, the dependence of the expansion factor
on
rapidly saturates in the supercritical domain.
3.3. Interpretation of the Experimental Data on Quasi-Adiabatic Foaming
Comparing the data on
(
Figure 6) and the modeled dependence
(
Figure 10a), we can see that the maximal
value occurs under the condition
. Similarly, the same condition is valid for the maximal decrement
. On the other hand, theoretical values of the pressure-dependent expansion factor
for CO
2 (
Figure 10b; curve 1) exhibit a rapid growth with further saturation in the supercritical domain. In contrast, the values
demonstrate a rapid decay with a further saturation. The presumable reason for this discrepancy between the empirical data for the polylactide foam and the modeling results for carbon dioxide is a significant increase in the dissipative losses (
). Indeed, based on the modeling results (see
Figure 10b; curve 1), we can assume that there is a tendency to a dramatic increase in the mass transfer rate of the polymeric component within the range of 7.0 MPa
9.0 MPa due to the jump-like behavior of
. This parameter exhibits more than a two-fold increase in the abovementioned interval of the initial pressures. In turn, this must cause a significant increase in the viscous friction in the polymeric component of the expanding foam. Thus, a rapid decay of
with a further saturation above
can be interpreted in terms of competition between the increasing expansion rate of the foaming agent and increasing dissipative losses due to a viscous friction. Note that the dissipative losses under the pressure
above 9.0 MPa must stabilize or even decrease due to the saturation of
and the decrease in the CO
2-impregnated polymer viscosity. Additionally, the dependence
also tends to saturate with increasing
(
Figure 10a). These factors cause the asymptotic behavior of
within the range of initial pressures above 9.0 MPa (
Figure 6). It should be noted that the maximal expansion under the condition
is accompanied by the previously established largest degree of structure fragmentation of the foamed matrices [
38]. The largest degree of structure fragmentation corresponds to the sufficiently smaller average size of the pores in the foamed polylactide compared to foaming with the initial pressures chosen (far from the critical pressure of carbon dioxide).
In the case of large initial pressures, the expected remarkable amounts of the liquid phase in the expanding gas volume at the final stage of adiabatic expansion (
Figure 10b, curve 2) cause the effect of residual expansion for the quasi-adiabatically expanding foams (
Figure 3a and
Figure 4). The residual expansion occurs due to the final transition of the liquid phase in the bubbles to the gaseous phase, because of the relatively small surface tension and viscosity of a foamed polymer. Typically, in the cases of high initial pressures, the additional contribution of the residual expansion to
does not exceed 5%.
A suitable approach for the identification of the various processes controlling the incipience, expansion, and stabilization of the polymer foams can be based on estimations of the instantaneous value of the polytropic index depending on the current pressure
The reasonability of this approach results from a high sensitivity of to the relationship between the expansion rate and the depressurization rate. In particular, if the volume of the foamed system insignificantly increases under a remarkable decrease in the pressure, . A similar behavior is associated with the stabilization of the foam structure at the final stage, when a contribution of becomes significant. In addition, such “quasi-isochoric” behavior will manifest itself at the foam incipience stages, when the formation of the bubble nuclei is due to the phase separation between the polymeric, liquid or supercritical phases. The large values of will occur when the nucleation is suppressed due to the high viscosity and surface tension of the polymeric matrix. Note that an increase in the dissipative losses in the course of foam extension must also cause an increase in .
In case of insignificant contributions of the energy-consuming processes to the energy balance, the polytropic index is expected to be close to a similar value of the foaming agent.
Figure 11 displays theoretical dependencies
for carbon dioxide, which were obtained using the differentiation procedure (Equation (7)) for the recovered adiabatic curves (
Figure 9). When depressurization begins from the initial pressures equal to or exceeding the critical pressure (curves 1–3 in
Figure 9), the polytropic indices rapidly decay together with the decreasing pressure until the gaseous phase appears in addition to the liquid phase in the expanding volume. The moments of the initial stage of coexistence are marked by the arrows (I); with further depressurization, the
values slowly increase due to the increasing volume fraction of the gaseous phase. In the case of depressurization from the initial pressure below
(curve 4), only the gaseous phase exists until the dewpoint condition is reached (arrow II). This moment is accompanied by a jump-like decay in
; a further expansion leads to a slow increase in the polytropic index; however, the absolute value of the increase rate is sufficiently smaller compared to the above-considered cases (curves 1–3). This is due to different trends in the behavior of the liquid volume fraction, which decreases under depressurization modes with
and increases in the case of depressurization under the condition
. Curve 5 in
Figure 11 corresponds to an extreme case of expansion; when the initial pressure is small, the dewpoint condition is unreachable within a whole range of the applied pressures, and the polytropic index has a constant value.
In the case of a quasi-adiabatic expansion of the polylactide foam, the pressure-dependent polytropic indices exhibit a much more diverse behavior.
Figure 12 displays the smoothed dependencies
recovered from the experimental quasi-adiabatic curves (
Figure 4). The smoothing was carried out using the 11-point-window Savitzsky–Golay filter to suppress the short-range ripple-like behavior of the output values in the numerical logarithmic differentiation. A remarkable feature is an abrupt increase in
in the region of small
values. This feature, marked as II, is related to saturation of the quasi-adiabatic curves (
Figure 4) due to the increasing role of the energy-consuming channel associated with
(Equation (3)). Note that this feature occurs at sufficiently larger values of
for the foaming modes with the initial pressures chosen in the vicinity of
(curves 2 and 3) compared to the foaming modes with
significantly detuned from
(curves 1, 4, 5). Typically, in the former case, an abrupt growth of
begins when the external pressure drops down to ≈2.5 MPa, whereas a similar behavior in the latter case occurs under the condition 0.5 MPa
1.0 MPa.
This difference can be interpreted in terms of the foam structure fragmentation in the course of expansion. In particular, analysis of the structure of the rapidly foamed polylactides using the low-coherence reflectometry [
38] showed a significant increase in the structure fragmentation if the initial pressure is set near the critical pressure. It can be shown that the surface energy of the polymer–gas interfaces in the expanding foam is approximately proportional to the cube root from the number of fragments (bubbles) in the foam volume (
). A remarkable increase in
correlates with the maximal values of
of carbon dioxide (see
Figure 10a).
The occurrence of the two-phase coexistence is manifested for curves 3, 4, and 5 (see the markers I). Curve 1, corresponding to the low initial pressure (
4.0 MPa), exhibits a peculiar behavior, which differs from the behavior of other dependencies. Note that this peculiarity does not relate to the two-phase coexistence (only the gaseous phase is expected), and is presumably caused by a heavily hindered nucleation and foam incipience in a partially plasticized polymer with large viscosity and surface tension. Under these conditions, the rate of foam expansion with the pressure decrease is small (
Figure 4) and becomes comparable to the rate for other data series (e.g., curves 4, 5) when the pressure decreases below 2.5 MPa.
The oscillating behavior of
at the stage of foam expansion is caused by intermittent instabilities in the expansion due to the random structure of the evolving foam. At this stage, the average
values acceptably agree with the modeling results for CO
2 (
Figure 11). Note that these averages for curves 2 and 3 are systematically lesser, and the corresponding values for curves 1, 4, and 5 are larger than the corresponding predictions for carbon dioxide. The marginal theoretical values of
for carbon dioxide at the stage of the expressed adiabatic expansion are marked in
Figure 12 by the horizontal dotted lines. The above mentioned systematic deviations of
for the expanding foam at the stage of expressed expansion from the modeled values for carbon dioxide can be interpreted in terms of the competition between two factors, such as the dissipative losses due to viscous friction in the expanding polymer matrix (the term
in Equation (3)) and the action of inertial forces associated with the term
. The domination of
must cause a decrease in the expansion rate and, correspondingly, an increase in
. On the contrary, a rapid accelerated expansion of the foam volume must cause a decreasing value of
. Therefore, we can assume that larger dissipation losses are typical for data series 1 (due to a large viscosity of the polymer matrix at the low initial pressure) and 4 and 5 (due to the tendency of the foaming agent to expand with high rates, see
Figure 10b). In contrast, curves 2 and 3 correspond to an intermediate expansion mode, when the dissipative losses are not very high but the expansion rates are sufficient.
3.4. Features of the Quasi-Isothermal Expansion of the Polylactide Foam
In contrast with the quasi-adiabatic expansion, the quasi-isothermal expansion is free from the influence of certain factors. In particular, thecoexistence of the liquid and gaseous phases in the growing bubbles is absent under
313.15 K. In addition, the factors associated with a rapid expansion of the foam (
and
in Equation (2)) can be neglected. At the same time, the heat transfer
from the environment to the expanding foam is necessary to provide an entropy increase during depressurization. Following from the relationship between
and
(
) and based on a set of the isothermal data [
37], we can roughly estimate the rate
of heat transfer to the expanding foam. For the used depressurization rates, these estimations give the values of the order of a few hundredths of W. Taking into account typical values of the thermal conductivity and thermal diffusivity of the polymeric and gaseous components and characteristic sizes of the expanding foam, we can conclude that the temperature differences inside the foam, and between the foam and the environment, do not exceed a few tenths of a degree. Consequently, we can state applicability of the concept of the quasi-isothermal behavior of slowly expanding polylactide foams.
The features of quasi-isothermal foam expansion can also be interpreted in terms of a comparison between the empirically obtained pressure-dependent polytropic index for the polylactide foam and a corresponding theoretical value for carbon dioxide.
Figure 13 displays the theoretical isothermal dependence
for carbon dioxide compared to the typical
datasets for the expanding foam. These datasets were obtained using a logarithmic differentiation of the experimental curves (
Figure 5) and correspond to a remarkable detuning of the initial pressures from
(
6.0 MPa and
12.0 MPa).
By analyzing the behavior of for the curve 2 (the initial pressure is below (), we can identify an abrupt decay of in the narrow interval of around 3.0 MPa as a result of a transition from the nucleation stage in the plasticized polymer to the stage of foam formation and expansion. The polytropic index rapidly decreases to the value of the order of 0.5, which is approximately two times less than the polytropic index of carbon dioxide, and increases with a decreasing pressure, gradually approaching the expected theoretical value for CO2. At this stage, this behavior indicates the high expansion rates of the polylactide foam and is presumably caused by the additional influence of a rapid decrease in the polymer volume fraction in the expanding foam volume. This feature can be interpreted as the “burst-like” foam incipience immediately after the nucleation. A dramatic increase in for the pressures below 0.5 MPa is associated with the stage of foam structure stabilization, when the contribution of the surface energy term in Equation (2) becomes significant.
In the case of polylactide foaming from a supercritical domain (curve 3), the burst-like foam incipience stage is absent. The behavior of for the evolving foam can be adequately described by the dependence of the polytropic index on the current pressure for CO2 except for the final stage, with a strong influence of the surface energy (below 2.0 MPa). Note that the foam growth begins in the supercritical domain; this means that the nucleation is due to a separation of supercritical and polymeric phases into the plasticized polymer and the bubble embryos are voids in the polymer matrix filled with scCO2.
Another feature related to the foaming from a supercritical domain is the non-monotonic alternating-sign behavior of at the stage of foam stabilization, which is associated with an expansion-to-shrinkage transition. The transition is indicated by a change in the sign of the derivative from negative to positive values. The criterion of expansion-to-shrinkage transition can be obtained using Equation (4a) to account for the dependence of the surface tension of polymer–gas interfaces on . This dependence occurs due to the release of CO2 from the polymeric matrix to the environment in the course of slow depressurization. Taking into account the conditions applied, we can consider the evolution of the expanding foam as a sequence of transitions between equilibrium states, with each state described by Equation (3).
Considering the relationships between the foam volume
, the average size of bubbles
, the volume fraction of gas phase
, and number of bubbles
in the foam volume, we can write
. Thus, the total area of the gas–polymer interfaces is approximately proportional to
. Consequently, Equation (4a) can be rewritten to the following form
where the dimensionless factor
collects various coefficients including
. By differentiating both sides of Equation (8) and assuming strong changes of
and weak dependencies of
and
on the external pressure at the final stages of expansion, we obtain:
Note that Equation (9) reduces to a simpler form:
where
0; the negative sign of the right side occurs in all the possible intervals of
and
variations. In this case, the foam volume increases with a decreasing external pressure, asymptotically approaching its extreme value, which can be obtained by setting
0 in Equation (8). This extreme value is approximately equal to
.
The expansion-to-shrinkage transition occurs when the right side of Equation (9) changes its sign, and the criterion for this change is the zero value of the numerator on the right side. Consequently,
This condition is reachable if the surface tension at the polymer–gas interfaces during the quasi-isothermal expansion is a monotonically decaying function of
. The validity of this assumption is supported by the reported data on the properties of polylactides plasticized using supercritical carbon dioxide [
42]. An evident reason for the negative values of
in the course of the quasi-isothermal expansion is the gradual release of CO
2 from polylactide to the environment. Another factor influencing occurrence of the expansion-to-shrinkage condition is the number of bubbles in the expanding foam (and, correspondingly, the dependence of the average bubble size on the time lapse). The criterion of the expansion-to-shrinkage condition corresponds to an intersection of the dependencies
and
, where
is a critical bubble size defined as
(
is a dimensionless coefficient differing from
). The rate
is dependent on the nucleation conditions at the stage preceding the incipience of the expanding foam. That is why the expansion-to-shrinkage transition occurs under the condition of depressurization from the supercritical domain, when the nucleation proceeds with sufficient ease (see
Figure 13).
The characteristic value of the average bubble radius corresponding to the expansion-to-shrinkage transition in the expanding polylactide foam and defined by Equation (11) was roughly evaluated using an estimate of the average radius of bubble embryos at the final stage of nucleation preceding the foam growth. The typical state of the depressurized polylactide at this stage is shown in
Figure 7e. The following assumption was applied for this evaluation: changes in the total number of bubbles in the foam evolving between the final stage of nucleation and the moment of the expansion-to-shrinkage transition are not significant. In this case, the following relationship between the average radius
of bubble embryos at the final stage of nucleation and the average radius of bubbles
in the expanded foam at the moment of the expansion-to-shrinkage transition is valid:
where
are the corresponding expansion factors. Equation (12) follows from the condition of equality of polymer-filled volume in the expanding foam at any stage of expansion. This approach was applied to the case of polylactide foaming with the initial pressure
11.0 MPa, when the expansion-to-shrinkage transition was manifested with
10.2 under the current external pressure
, approximately equal to 2.8 MPa. The value of
(0.21±0.02) mm was estimated using image processing similar to that shown in
Figure 7e, which corresponds to the final stage of nucleation at
8.1 MPa. The expansion factor
was estimated for this state of the depressurized system as ≈ 1.057 using the above described side image analysis (the “shadowgramm” technique in the transillumination mode). Consequently, the value of the average radius of bubbles corresponding to the condition of the expansion-to-shrinkage transition under the quasi-isothermal depressurization from 11 MPa is approximately equal to 1.14 mm.
An analysis of the obtained experimental data allows us to conclude that the initial external pressure is a key parameter, mainly affecting such macroscopic properties of the polylactide foam at the final stage as the expansion factor
(see
Figure 6). The influence of the depressurization rate on
is much less pronounced; in particular, an increase in the average pressure drop rate by more than 100 times (the transition from the quasi-isothermal to quasi-adiabatic mode) leads to a relatively insignificant decrease in the expansion factor (approximately in 1.5–2 times, see
Figure 6). On the contrary, such an increase in
leads to remarkable changes in the foam structure (in particular, to a significant decrease in the characteristic cell size). Typically, quasi-isothermally expanding foams consist of cells with an average size of the order of several millimeters, while “quasi-adiabatic” foams exhibit the average size of cells (bubbles) ranging from tens to hundreds of micrometers (depending on the initial pressure). The evidence of such a difference in the foam structures is clearly seen in
Figure 2, where the small-scale cellular structure of the quasi-adiabatically expanding foam is not resolved by the optical system used (
Figure 2a). At the same time, the quasi-isothermally expanding foams (
Figure 2b and
Figure 7c,f) are composed of millimeter-sized “macro-cells”. The previously reported results on the structure of rapidly foamed polylactides also show strong structure fragmentation with characteristic cell sizes in the sub-millimeter domain [
38]. In addition, the tendency of the the average cell size of polylactide foams to increase with a decrease in the depressurization rate was also mentioned in [
22].
The pressure drop rate must play an important role in the course of the nucleation stage preceding foam formation and expansion. The number of bubble embryos appearing during this stage affects the structural properties of expanding foam. We can assume that for each value of the initial pressure there is a critical pressure drop rate, below which the depressurization does not lead to foaming. The reason for this assumption is that at low pressure drop rates, the diffusion outflow of the plasticizing/foaming agent from the bulk of the material can suppress nucleation. In other words, at any moment of such slow depressurization, the external pressure corresponds to the thermodynamically equilibrium mass fraction of the agent in the polymer. In our case, this is manifested when the external pressure drops from the initial values below approximately 4.5 MPa with pressure drops of the order of 0.005 MPa/s or less. In this case, such pressure drop rates for the existing mass fractions of carbon dioxide in polylactide are insufficient for effective nucleation.
This item can also be considered in terms of the nucleation rate
governed by the free (Gibbs) energy of the bubble embryo birth
and the concentration of carbon dioxide molecules in polylactide
(see, e.g., [
29,
30,
31]:
Consequently, the number of bubble embryos appearing during the nucleation stage with the duration of
can be expressed as:
In accordance with [
31], the free energy of homogeneous nucleation is defined as:
where
, as before, is the surface tension of the polymer and
is the difference between the saturation pressure for the current mass fraction of carbon dioxide in polylactide and the external pressure. Introducing a characteristic time scale
for depressurization, we can assume that
. Thus,
asymptotically falls to zero with the decreasing pressure drop rate and approaches a certain extreme value with the increase in
. This approximate model allows for a qualitative interpretation of the observed features, such as a non-significant decrease in the foam expansion factor during the transition from the quasi-isothermal expansion to the quasi-adiabatic mode and the practical absence of quasi-isothermal foaming at the initial pressures below 4.5 MPa. In the former case, the abrupt increase in
leads to the saturation of the dependence of
on the pressure drop rate. Additionally, fast depressurization causes a shorter nucleation stage
. The joint competing influence of these factors causes a slight decrease in the foam expansion factor and remarkable decrease in the average cell size in the transition from quasi-isothermal foaming to the quasi-adiabatic mode.
In the latter case, large values of in combination with the relatively small concentrations of carbon dioxide in polylactide under the initial external pressures below 4.5 MPa do not provide the number of bubble embryos sufficient for foaming. Of course, the considered approach gives only general outlines of the physical picture of the transition from nucleation to polylactide foam evolution; however, it can be useful for developing more detailed models of polymer foam formation.
The observed features in the behavior of the quasi-isothermally foamed polylactide at the stage of nucleation (the dominating heterogeneous nucleation in the case of low initial pressures, the increasing contribution of the homogeneous (bulk) nucleation and the increasing nucleation rate with the increase in the initial external pressure up to 11–12 MPa, see
Figure 7) can be qualitatively interpreted in terms of the reduction of the Gibbs energy
necessary for birth of a bubble embryo. Equation (15) gives the value of
characteristic for homogeneous (bulk) nucleation; near the container wall (the case of heterogeneous nucleation), the Gibbs energy
is reduced [
30,
31] by the factor
dependent on the contact angle
at the boundary between the container wall and plasticized polymer:
In the case of the partial wetting of the container wall by the polymer and
(the latter condition occurs for low external pressures and small mass fractions of the plasticizing/foaming agent in the polymer; see, e.g., [
30]), the rate of heterogeneous nucleation significantly exceeds the rate of homogeneous nucleation due to a remarkable difference between
and
(see Equation (16)). This situation is clearly seen in
Figure 7a.
With the increasing initial external pressure and, correspondingly, the mass fraction of carbon dioxide in polylactide, the surface tension
of polylactide gradually decreases [
42]. In turn, this leads to a decrease in
due to the strong (cubic) dependence of the Gibbs energy on
(see Equation (15)). The difference between
and
diminishes and the rates of both types of nucleation increase (see
Figure 7d). It is necessary to note that further increase in
above the value equal to 12 MPa can lead to small values of
equal to or less than
. Under these conditions, the more probable mechanism of carbon dioxide–polylactide separation is spinodal decomposition, but not nucleation [
30]. Consequently, the foaming efficiency is expected to decrease with a significant increase in the external initial pressure. This issue is the subject of further research.