Influence of the Processes Parameters on the Properties of The ...

Chapter 5.
Characterization <strong>of</strong> Scaffolds for Connective Tissue Engineering
5.21, we have observed an opposite behaviour for a high saturation pressure value <strong>of</strong> 200 bars. This can be
attributed to <strong>the</strong> high CO 2 sorption <strong>of</strong> PLGA 85:15 at high pressure, which leads to high glass transition
depression.
(A)- PLGA 50:50 (B)- PLGA 85:15
Figure 5.21: Micrographs <strong>of</strong> scaffolds processed at P sat = 200 bars;
T sat = 36.5°C, t sat = 20 min. and dP/dt = 20 bar /s.
4.3 Effect <strong>of</strong> Saturation Pressure (P sat )
We have observed that <strong>the</strong> pressure <strong>of</strong> saturation is an important parameter to understand <strong>the</strong>
foaming phenomena. Indeed, <strong>the</strong> saturation pressure determines <strong>the</strong> amount <strong>of</strong> CO 2 sorbed into <strong>the</strong> polymer
matrix. At low pressure range, <strong>the</strong> amount <strong>of</strong> CO 2 sorbed by <strong>the</strong> polymer is inferior to that <strong>of</strong> <strong>the</strong> high
pressure ranges. It must have been noticed that <strong>the</strong> plasticization effect <strong>of</strong> <strong>the</strong> CO 2 for <strong>the</strong> polymer (and <strong>the</strong>
depression <strong>of</strong> <strong>the</strong> glass transition point) increases with <strong>the</strong> amount <strong>of</strong> CO 2 sorbed. As explained previously,
<strong>the</strong> vitrification point is found to be earlier with a P sat <strong>of</strong> 200 bars than 100 bar (cf. Figure 5.9). Since <strong>the</strong>
vitrification point determines <strong>the</strong> end <strong>of</strong> <strong>the</strong> pore growth, a polymer saturated at 100 bars has greater time for
<strong>the</strong> pore growth during depressurization and desorption. This behaviour can be explained by <strong>the</strong> diffusion
coefficient <strong>of</strong> CO 2 which increases with <strong>the</strong> increasing concentration <strong>of</strong> CO 2 (<strong>the</strong> sorption) in <strong>the</strong> polymer
matrix. So, one can say that since <strong>the</strong> diffusion coefficient and <strong>the</strong> plasticization is greater at higher
pressures, during depressurization and <strong>the</strong> following desorption, <strong>the</strong> CO 2 will desorb faster than for <strong>the</strong>
lower saturation pressures. At lower saturation pressures, <strong>the</strong> time for CO 2 to provide <strong>the</strong> expansion <strong>of</strong> CO 2
and <strong>the</strong> desorption-diffusion induced growth <strong>of</strong> <strong>the</strong> pores is greater and yields to greater pores. The results
presented in Table 5.8 and Figure 5.9, are confirming our hypo<strong>the</strong>sis.
On <strong>the</strong> o<strong>the</strong>r hand, nucleation <strong>the</strong>ory that we have used to model <strong>the</strong> pore size as a function <strong>of</strong> <strong>the</strong>
saturation pressure, must not have been forgotten. Nucleation <strong>the</strong>ory includes <strong>the</strong> energy barrier for a
generation <strong>of</strong> new nuclei. This energy barrier is given by Eq. 2.23 in Chapter 2 and according to that when
<strong>the</strong> pressure increases, <strong>the</strong> energy barrier and consequently <strong>the</strong> number <strong>of</strong> pores that can be generated by unit
volume, increases. We can look to <strong>the</strong> phenomena from <strong>the</strong> window <strong>of</strong> Gibbs free energy. According to
which ΔG = ΔH - TΔS, ΔG decreases with <strong>the</strong> increase <strong>of</strong> change in entropy ΔS, which is true for all
systems available in <strong>the</strong> universe. In our case, when we increase <strong>the</strong> pressure, we increase <strong>the</strong> entropy
change <strong>of</strong> <strong>the</strong> system (we increase greater than that we could increase with lower pressures), and
consequently decrease <strong>the</strong> ΔG greater than that we could decrease with lower pressures. Consequently, <strong>the</strong>
energy barrier that determines <strong>the</strong> capacity to generate new nuclei decreases, which means that we can create
more pores per unit volume at elevated pressures.
On <strong>the</strong> o<strong>the</strong>r hand, <strong>the</strong> difference found between <strong>the</strong> vitrification times were not significantly
different (2.64 and 2.72 s. for 100 and 200 bars, respectively). Thus, we can conclude that generation <strong>of</strong>
nuclei is <strong>the</strong> dominant factor which affects <strong>the</strong> increase <strong>of</strong> <strong>the</strong> number <strong>of</strong> pores with <strong>the</strong> increasing saturation
pressure and <strong>the</strong> coalescence phenomenon may be effective at lower saturation pressures. We finally have to
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