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

Chapter 5.
Characterization <strong>of</strong> Scaffolds for Connective Tissue Engineering
1.1.4 Discussion on <strong>the</strong> Transitions
The parameters <strong>of</strong> <strong>the</strong> observed glass transition measured by DSC are reported on Table 5.5,
determined by <strong>the</strong> DSC technique.
Table 5.5: Glass transitions parameters <strong>of</strong> <strong>the</strong> various polylactides.
T Onset
(°C)
T Midpoint
(°C)
Polylactide
Cp
(J/g.K)
P L,D LA (PABR L 68) 54.2 55.6 0.421 54.9
g (°C)
(supplier)
P L,DL LA (Resomer ® LR 704) 59.8 60.1 0.347 56−62
PLGA 85:15 (Resomer ® RG 858S) 41.8 42.7 0.388 43
PLGA 85:15 (DL-PLG) 52.2 53.1 0.451 50−55
PLGA 50:50 (PLG 8523) 57.4 59.5 0.465 55−60
PLGA 85:15 (PLG 8531) 56.1 57.4 0.462 55−60
PLGA 85:15 (LG 857 S) 59.3 61.4 0.392 57−63
PLGA 50:50 (Resomer ® RG 504) 47.1 49.2 0.499 46−50
PLGA 50:50 (PDLG 5010) 47.7 49.1 0.524 46−50
Polymer molecules are <strong>of</strong>ten partially crystalline, with crystalline regions dispersed within
amorphous material. Chain disorder or misalignment, which is common, leads to amorphous material since
twisting, kinking and coiling prevent strict ordering required in <strong>the</strong> crystalline state. Thus, linear polymers
with small side groups, which are not too long form crystalline regions easier than branched, network,
atactic polymers, random copolymers, or polymers with bulky side groups. Crystalline polymers are denser
than amorphous polymers, so <strong>the</strong> degree <strong>of</strong> crystallinity can be obtained from <strong>the</strong> measurement <strong>of</strong> density.
Crystallinity is indicative <strong>of</strong> amount <strong>of</strong> crystalline region in polymer with respect to amorphous
content. Crystallinity influences many <strong>of</strong> <strong>the</strong> polymer properties such as hardness, modulus, tensile,
stiffness, and melting point. The ratio <strong>of</strong> glycolide to lactide at different compositions allows control <strong>of</strong> <strong>the</strong>
degree <strong>of</strong> crystallinity <strong>of</strong> <strong>the</strong> polymers [Park et al., 1995; Cohn et al., 1987]. When <strong>the</strong> crystalline PGA is
co-polymerized with PLA, <strong>the</strong> degree <strong>of</strong> crystallinity is reduced and, as a result, this leads to increase in
rates <strong>of</strong> hydration and hydrolysis. It can <strong>the</strong>refore be concluded that <strong>the</strong> degradation time <strong>of</strong> <strong>the</strong> copolymer is
related to <strong>the</strong> ratio <strong>of</strong> monomers used in syn<strong>the</strong>sis. In general, <strong>the</strong> higher <strong>the</strong> glycolide content, <strong>the</strong> quicker
<strong>the</strong> rate <strong>of</strong> degradation has been observed [Park, 1995].
1.1.4.1 Effect <strong>of</strong> L and DL Ratio on Thermal Property <strong>of</strong> Polylactide Acid
For <strong>the</strong>rmal properties, it can be observed that P L , DL LA copolymers showed T g glass transition
temperature ranging 55−60°C. However, only P L,D LA copolymers containing 10 mol % D,LLA showed T g
<strong>of</strong> 54°C; similarly <strong>the</strong> T m melting point <strong>of</strong> <strong>the</strong>se two polymers were also at 154 and 143°C, respectively.
These copolymers had lower degree <strong>of</strong> crystallinity than that <strong>of</strong> P L LA homopolymer (~50%). Buchatip et al.
[2008] also produced similar type <strong>of</strong> results explaining that T g glass transition, T m melting temperature as
well as crystallinity <strong>of</strong> <strong>the</strong> copolymers decreased as mol% <strong>of</strong> D,LLA comonomer increased. Melting peak
and crystallinity can not be observed in P D,L LA homopolymer and copolymers with more than 10 mol % <strong>of</strong>
D,LLA suggesting <strong>the</strong> amorphous nature <strong>of</strong> <strong>the</strong>se polymers.
Apart from this discussion, it may be noted that <strong>the</strong> different processes used for <strong>the</strong>
copolymerization (polycondensation/open ring polymerization/copolymerization) and <strong>the</strong> nature <strong>of</strong> catalyst
used also plays an important role in <strong>the</strong> crystallinity and o<strong>the</strong>r properties <strong>of</strong> <strong>the</strong> final copolymer.
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