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

Chapter 5.
Characterization <strong>of</strong> Scaffolds for Connective Tissue Engineering
Crystallization, crystallinity degree, and <strong>the</strong>rmal properties <strong>of</strong> P L LA depend on <strong>the</strong> polymer
molecular weight, polymerization conditions, <strong>the</strong>rmal history, purity, and so on. As reported by Ikada et al.
[1987], blending <strong>of</strong> P L LA and P D LA results in <strong>the</strong> formation <strong>of</strong> a stereo-complex with a crystalline structure
different from that <strong>of</strong> each homopolymer and melting temperatures that reach 230 o C. Contradictory data are
reported about P L LA melting enthalpy, ranging in <strong>the</strong> literature from 40 to 203 J/g. To calculate <strong>the</strong> degree
<strong>of</strong> crystallinity, we have used <strong>the</strong> most common value adopted for <strong>the</strong> melting enthalpy <strong>of</strong> <strong>the</strong> totally
crystallised P L LA, [Auras et al., 2010].
c % = 100 (ΔH m - ΔH c )/93.6
Poly(lactic acid) (PLA) is a glassy, high modulus <strong>the</strong>rmoplastic polymer with properties
comparable to polystyrene (PS). The mechanical properties <strong>of</strong> <strong>the</strong> polylactides depends upon <strong>the</strong>
manufacturing method, <strong>the</strong> crystalinity factor, <strong>the</strong> T g and ratio <strong>of</strong> L-lactide, D-lactide or racemic D,L-lactide
contents. In case <strong>of</strong> poly(lactic -co-glycolic) acid <strong>the</strong> properties are intermediate between those <strong>of</strong> poly lactic
acid and polyglycolic acid. There is increase in <strong>the</strong> mechanical properties but a decrease in <strong>the</strong> T g due to <strong>the</strong>
addition <strong>of</strong> PGA structure.
2 Kinematics and Thermodynamics Experiments
The aim <strong>of</strong> <strong>the</strong> sorption-desorption studies was to be able to consider <strong>the</strong> foaming phenomenon
more precisely. Thus, in this part, we have realized experiments to achieve <strong>the</strong> sorption kinetics by using <strong>the</strong>
desorption data. As we have mentioned earlier, <strong>the</strong> sorption behaviour <strong>of</strong> CO 2 into <strong>the</strong> polymer determines
<strong>the</strong> number <strong>of</strong> nuclei generated and it deserves a proper investigation. We have also investigated <strong>the</strong>
desorption phenomenon which has influence both on <strong>the</strong> pore growth and <strong>the</strong> final structure <strong>of</strong> <strong>the</strong> polymer.
2.1 Sorption-Diffusion Kinetics
The aim to plot kinetic curves was to determine <strong>the</strong> time <strong>of</strong> saturation <strong>of</strong> <strong>the</strong> polymer by CO 2 . To
obtain kinetic curves, experiments <strong>of</strong> 10, 20, 60, 120 and 240 minutes have been performed. As shown in
Figure 5.6-(A), <strong>the</strong> sorption curve is reaching to a plateau after 60 minutes <strong>of</strong> processing which means that
<strong>the</strong> polymer can be considered to be quiet saturated by CO 2 after 60 minutes at 125 bars and 36.5°C. In
Figure 5.6-(B), M t denotes <strong>the</strong> weight <strong>of</strong> CO 2 inside <strong>the</strong> polymer at time t, and M ∞ is <strong>the</strong> maximum sorbed
amount <strong>of</strong> CO 2 . Here, all M t values are <strong>the</strong> extrapolated data (to t = 0) <strong>of</strong> <strong>the</strong> initial linear parts <strong>of</strong> <strong>the</strong>
desorption curves as presented in Chapter 3, section 4. Each M t value is analyzed after different experiments.
M ∞ has been taken as <strong>the</strong> value <strong>of</strong> 240 minutes, where <strong>the</strong> equilibrium is supposed to be completely reached.
The sorption behaviour is modelled using one dimensional diffusion equation from a plane sheet
(Eq.2,2), chapter 2. The Minerr function <strong>of</strong> Mathcad has been used to optimize <strong>the</strong> modelling and to
calculate <strong>the</strong> average sorption-diffusion coefficient. Thus, <strong>the</strong> sorption diffusion coefficient was found as
3.64 × 10 -10 m 2 s -1 for P sat = 125 bars, T sat = 36.5°C and t sat = 120 min. Indeed, <strong>the</strong> capacity <strong>of</strong> sorption <strong>of</strong>
CO 2 , thus <strong>the</strong> diffusion coefficient must increase with <strong>the</strong> increasing amount <strong>of</strong> CO 2 solubilized in <strong>the</strong>
polymer, but to simplify <strong>the</strong> calculations, we have considered an average sorption-diffusion coefficient all
across <strong>the</strong> time interval. Mathcad programs for diffusion are presented in Annex A-2. The <strong>the</strong> model is in
very good agreement with experimental data as show in Figure 5.6-(A).
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