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

Chapter 1.
Polylactide Based Bio-Materials
or lactic acid) are linked toge<strong>the</strong>r in PLGA by ester linkages, thus resulting in linear, aliphatic polyester as a
product [Middleton and Tipton, 1998]. Using <strong>the</strong> polyglycolide and polylactide properties as a starting point,
it is possible to copolymerize <strong>the</strong> two monomers to extend <strong>the</strong> range <strong>of</strong> homopolymer properties.
Copolymers <strong>of</strong> glycolide with both L-lactide and D,L-lactide have been developed for both device and drug
delivery applications.
Figure 1.5: Schemaic syn<strong>the</strong>sis <strong>of</strong> poly(lactide-co-glycolide).
[Middleton and Tipton, 1998]
To tailor <strong>the</strong> processability and to enhance biodegradation, <strong>the</strong>se copolymers are fur<strong>the</strong>r modified
by copolymerizing with linear dicarboxylic acids (e.g. adipic acid) and glycol components with more than
four methylene groups (e.g. hexanediol) [Sublett, 1983]. To enhance <strong>the</strong> environmentally benevolent aspect
<strong>of</strong> <strong>the</strong>se materials and to broaden <strong>the</strong> range <strong>of</strong> <strong>the</strong>ir use, aliphatic-aromatic co-polyesters blended with
cellulose esters have been processed into useful fibres, films and moulded objects [Buchanan et al., 1994,
1995].
2.2.2 Properties <strong>of</strong> PLGA Copolymers
Physical properties <strong>of</strong> poly(glycolic acid), as well as different PLGAs, are ga<strong>the</strong>red in Table 1.3.
Table 1.3: Main physical properties <strong>of</strong> PGA and several PLGAs.
Properties Unit PGA P D,L LGA 50:50 P D,L LGA 75:25 P D,L LGA 85:15
Density ρ gm/cm 3 1.5−1.7 1.3−1.4 1.3 1.25
Tensile strength σ MPa 60−99.7 41.4−55.2 41.4−55.2 45−52
Tensile modulus E GPa 6−7 1−4.3 1.4−4.1 2.0*
Ultimate strain ε % 1.5−20 2−10 2.5−10 −
Specific tensile strength S* Nm/g 40−45.1 30.9−41.2 31.8−42.5 −
Specific tensile modulus E* kNm/g 4−4.5 8−2.1 1.1−2.1 −
Glass transition temperature T g °C 35−40 45−50 50−55 55−55
Melting point T m °C 225−230 amorphous amorphous amorphous
[Van de Velde and Kiekens, 2002]
Polyglycolide has a glass transition temperature between 35 and 45°C and its melting point is
reported to be in <strong>the</strong> range <strong>of</strong> 225-230°C. PGA also exhibits a higher degree <strong>of</strong> crystallinity than PLA (~ 45-
55 %), thus resulting in better mechanical properties but insolubility in water [Middleton and Tipton, 1998].
The solubility <strong>of</strong> this polyester is somewhat unique, in that its high molecular weight form is insoluble in
almost all common organic solvents (acetone, dichloromethane, chlor<strong>of</strong>orm, ethyl acetate, tetrahydr<strong>of</strong>uran).
The exceptions are highly fluorinated organics such as HFIP (hexafluoroisopropanol) while low molecular
weight oligomers sufficiently differ in <strong>the</strong>ir physical properties to be more soluble. Sutures <strong>of</strong> PGA lose
about 50% <strong>of</strong> <strong>the</strong>ir strength after 2 weeks and 100% at 4 weeks, and are completely absorbed in 4~6 months.
Glycolide has been copolymerized with o<strong>the</strong>r monomers to reduce <strong>the</strong> stiffness <strong>of</strong> <strong>the</strong> resulting fibres
[Middleton and Tipton, 1998].
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