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

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Chapter 5. Characterization <strong>of</strong> Scaffolds for Connective Tissue Engineering Figure 5.7: Desorption <strong>of</strong> CO 2 from PLGA 50:50 after 100 and 200 bars, at T sat = 36.5°C and t sat = 120 min. Table 5.7 shows that <strong>the</strong> desorption-diffusion coefficient <strong>of</strong> CO 2 is increasing, in <strong>the</strong> range between 10 -11 and 10 -9 , with <strong>the</strong> increase <strong>of</strong> <strong>the</strong> saturation time. The desorption-diffusion coefficient, D dp , has an increasing trend and finally reaches a plateau after 60 minutes for P sat = 125 bar and T sat = 36.5°C with a value <strong>of</strong> 2.10 -9 m 2 /s. These data proves <strong>the</strong> change in <strong>the</strong> diffusion coefficient with <strong>the</strong> variation <strong>of</strong> CO 2 sorption inside <strong>the</strong> polymer matrix. We can observe from Table 5.7 that <strong>the</strong> sorption <strong>of</strong> CO 2 into <strong>the</strong> polymer increases with increasing time until it reaches a plateau after 60 minutes. One can say that <strong>the</strong> capacity <strong>of</strong> sorption increases with <strong>the</strong> sorption <strong>of</strong> CO 2 into <strong>the</strong> polymer. Table 5.7: Desorption-diffusion coefficients and sorption <strong>of</strong> CO 2 after different saturation times at saturation pressure 125 bars and saturation temperature 36.5°C. t sat (min) D dp (m 2 Sorption <strong>of</strong> CO / s) 2 into <strong>the</strong> PLGA 50:50 (g CO 2 / g Polymer) 10 7.29 x 10 -11 0.153 20 1.62 x 10 -10 0.195 60 2.01 x 10 -9 0.270 120 2.05 x 10 -9 0.281 240 2.06 x 10 -9 0.281 Also, as shown in Table 5.8, <strong>the</strong> desorption-diffusion coefficient increases with increasing saturation pressure. Since <strong>the</strong> density <strong>of</strong> CO 2 increases with <strong>the</strong> pressure, it is expected that <strong>the</strong> sorption capacity increases with <strong>the</strong> increasing pressure. On <strong>the</strong> o<strong>the</strong>r hand, <strong>the</strong> desorption-diffusion coefficients calculated for rubbery states, D dp , are always greater than <strong>the</strong> diffusion coefficients after vitrification, D dg . Table 5.8: Desorption-diffusion coefficients <strong>of</strong> CO 2 from PLGA 50:50 for plasticized and glassy states, after different saturation pressures at 36.5°C for 120 min <strong>of</strong> saturation time. P sat (bar) D dp (m 2 / s) D dg (m 2 / s) 55 1.437 x 10 -10 - 80 1.734 x 10 -10 - 100 2.140 x 10 -9 2.321 x 10 -11 125 2.050 x 10 -9 7.727 x 10 -11 150 2.606 x 10 -9 1.753 x 10 -10 200 3.321 x 10 -9 2.854 x 10 -10 - 122 -
Chapter 5. Characterization <strong>of</strong> Scaffolds for Connective Tissue Engineering 2.3 The Sorption Iso<strong>the</strong>rm By using desorption kinetics, <strong>the</strong> initial points <strong>of</strong> desorption have been extrapolated to t = 0 s and <strong>the</strong> amount <strong>of</strong> CO 2 sorbed by <strong>the</strong> polymer is analyzed. We underline that here, t = 0 s is <strong>the</strong> end <strong>of</strong> <strong>the</strong> saturation period. One can remember that <strong>the</strong> nucleation rate is related to <strong>the</strong> CO 2 at <strong>the</strong> end <strong>of</strong> <strong>the</strong> saturation period. Thus, we must know how much CO 2 is sorbed by <strong>the</strong> polymer to calculate <strong>the</strong> nucleation rate. All experiments have been carried out at 36.5°C during 120 minutes with different pressure conditions. The pressure conditions and <strong>the</strong> corresponding sorption data is presented in Table 5.9. It has been observed that <strong>the</strong> sorption increases monotonically with <strong>the</strong> pressure. Table 5.9: Sorption data for PLGA 50:50 at 36.5°C. P (bar) Average Sorption Data (gCO 2 /g Polymer) 55 0.112 80 0.207 100 0.258 125 0.281 150 0.296 200 0.333 Experimental results and model predictions are presented with <strong>the</strong> literature data in Figure 5.8. A significant change in slope is appeared within <strong>the</strong> interval <strong>of</strong> 70-80 bars. The change in <strong>the</strong> slope can be explained by <strong>the</strong> transition <strong>of</strong> <strong>the</strong> CO 2 rich phase from gas to <strong>the</strong> much denser supercritical state. Figure 5.8: Sorption iso<strong>the</strong>rm <strong>of</strong> CO 2 into PLGA 50:50. [Pini et al., 2008] Our sorption iso<strong>the</strong>rm and modelling with SL-EOS, is in agreement with those reported by Pini et al. [2008]. Our experimental points were always approximately 5% lower than <strong>the</strong> literature data which can be attributed to <strong>the</strong> small temperature difference. Indeed, <strong>the</strong> density <strong>of</strong> CO 2 , so <strong>the</strong> sorption decreases with <strong>the</strong> temperature. Additionally, under <strong>the</strong> critical point <strong>of</strong> CO 2 , SL-EOS diverges from <strong>the</strong> experimental data. But, it has good estimations above critical point for <strong>the</strong> sorption. Fur<strong>the</strong>rmore, since we have investigated <strong>the</strong> sorption kinetics earlier, we have also a sorption kinetic data. This evaluation was presented in Table 5.8 and found to increase with time until it reaches a plateau after 60 minutes. If we interpolate <strong>the</strong> iso<strong>the</strong>rmal sorption data for 120 minutes (Figure 5.8 and Table 5.9) between 55 and 80 bars, a sorption value <strong>of</strong> 0.195 can be found for 76.2 bars. This value is equal to <strong>the</strong> - 123 -
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