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

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Chapter 1. Polylactide Based Bio-Materials Figure 1.3: Ring opening polymerization <strong>of</strong> lactide to polylactide. Lactic acid is obtained by <strong>the</strong> fermentation <strong>of</strong> engineered microbes <strong>of</strong> <strong>the</strong> genus Lactobacilli. These microorganisms are highly efficient sources <strong>of</strong> lactic acid. Lactobacilli can be subdivided into strains that produce ei<strong>the</strong>r <strong>the</strong> L (+) or <strong>the</strong> D (-) isomer. A variety <strong>of</strong> different sugars is used as carbon sources in <strong>the</strong> fermentation process. These sugars are ei<strong>the</strong>r specifically prepared enzymatically from starch for lactic acid production, or are by products from fruit processing. In addition, lactose, a by-product in <strong>the</strong> cheese industry, can also be used as a carbon source for lactic acid production. In all cases sugars from renewable resources are transformed into a value-added product by an enzymatic whole-cell catalysis process. The subsequent steps in <strong>the</strong> production <strong>of</strong> PLA are based on syn<strong>the</strong>tic chemistry and involve <strong>the</strong> formation <strong>of</strong> <strong>the</strong> dimer by a self-condensation reaction that results in a low molecular weight prepolymer. Depolymerization <strong>of</strong> <strong>the</strong> prepolymer gives lactide, which is <strong>the</strong>n polymerized through ring-opening polymerization. The prepolymer can also be polymerized into high molecular weight PLA by <strong>the</strong> action <strong>of</strong> chain coupling agents. In contrast to lactic acid, glycolic acid is produced in an industrial scale by a chemical process. The different ways <strong>of</strong> producing PLA are ga<strong>the</strong>red in Figure 1.4. Figure 1.4: Different ways <strong>of</strong> producing PLA. [Garlotta, 2001] - 14 -
Chapter 1. Polylactide Based Bio-Materials 2.1.3 Properties <strong>of</strong> Polylactic Acid Commercially available PLA grades are copolymers <strong>of</strong> poly(L-lactide) with meso-lactide or D- lactide. The D/L ratio is known to affect <strong>the</strong> properties <strong>of</strong> PLA (melting temperature, degree <strong>of</strong> crystallinity and so on). PLA has a balance <strong>of</strong> mechanical properties, <strong>the</strong>rmal plasticity, and biodegradability and is readily processed [Fang and Hanna, 1999]. Different properties <strong>of</strong> PLA are summarized in Tables 1.1. The homopolymer <strong>of</strong> L-lactide (P L LA) is a semi-crystalline polymer. These types <strong>of</strong> materials exhibit high tensile strength and low elongation and consequently have a high modulus that makes <strong>the</strong>m more suitable for load-bearing applications such as in orthopaedic fixation and sutures [Van de Velde and Kiekens, 2002]. The racemic P D,L LA is an amorphous polymer exhibiting a random distribution <strong>of</strong> both isomeric forms <strong>of</strong> lactic acid, and accordingly is unable to arrange into an organized crystalline structure. This material has lower tensile strength, higher elongation, and a much more rapid degradation time, making it more attractive as a drug delivery system. Poly(L-lactide) is about 37% crystalline, with a melting point <strong>of</strong> 173−178°C and a glass-transition temperature <strong>of</strong> 60−65°C. The degradation time <strong>of</strong> P L LA is much slower than that <strong>of</strong> P D,L LA, requiring more than 2 years to be completely absorbed. Copolymers <strong>of</strong> L-lactide and D,L-lactide have been prepared to disrupt <strong>the</strong> crystallinity <strong>of</strong> L-lactide and accelerate <strong>the</strong> degradation process [Middleton and Tipton, 1998]. The densities <strong>of</strong> P L LA and P D,L LA are reported in Table 1.2. They are mostly based on standards such as ASTM D792. Density can be a very important design parameter since elevated density values imply high transportation costs. Density is <strong>of</strong>ten used for <strong>the</strong> calculation <strong>of</strong> ‘specific properties’, i.e. dividing mechanical properties by <strong>the</strong> appropriate density. All o<strong>the</strong>r properties can be compared among each o<strong>the</strong>r. Tensile properties are clearly best for <strong>the</strong> densest reported polymers. Varying <strong>the</strong> molecular weight from 50 over 150 to 200 kDaltons will increase tensile strengths for P L LA <strong>of</strong> 15.5, 80 and 150 MPa, respectively [Van de Velde and Kiekens, 2002]. Table 1.2: Main physical properties <strong>of</strong> different PLAs. [Van de Velde and Kiekens, 2002] Properties Unit P L LA P D,L LA Molecular weight (M w ) Dalton 100–300 − Density ρ - 1.24−1.30 1.25−1.27 Tensile strength σ MPa 15.5−150 27.6−50 Specific tensile strength S* Nm/g 40−66.8 22.1−39.4 Tensile modulus E GPa 2.7−4.1 1−3.5 Specific tensile modulus E* kNm/g 2.2−3.8 0.8−2.4 Strength at break MPa 44−66 − Flexural strength MPa 88−119 − Ultimate strain ε % 3−6 2−10 Elongation at break % 100–180 − Glass transition temperature T g °C 60−65 55−60 Melting point T m °C 173−178 amorphous Heat <strong>of</strong> melting H m J/g 8.1–93.1 − Degree <strong>of</strong> crystallinity, % 10–40 − Surface energy J/m 2 38 − Solubility parameter δ H (J/ml) 1/2 19–20.5 − Polylactic acid can be processed like most <strong>the</strong>rmoplastics into fibre (for example by using conventional melt spinning processes) and film. The melting temperature <strong>of</strong> P L LA can be increased 40–50°C by physically blending <strong>the</strong> polymer with P D LA. P L LA and P D LA are known to form a highly regular stereocomplex with increased crystallinity. The maximum effect in temperature stability is achieved when a 50-50 blend is used, but even at lower concentrations <strong>of</strong> 3–10% <strong>of</strong> P D LA a substantial effect is achieved. In <strong>the</strong> - 15 -
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