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

Chapter 1.
Polylactide Based Bio-Materials
latter case P D LA is used as a nucleating agent, <strong>the</strong>reby increasing <strong>the</strong> crystallization rate. Due to <strong>the</strong> higher
crystallinity <strong>of</strong> this stereo-complex, <strong>the</strong> biodegradability will become slower. The interesting feature is that
<strong>the</strong> polymer blend remains transparent.
Even when burned, PLA produces no nitrogen oxide gases and only one-third <strong>of</strong> <strong>the</strong> combustion
heat generated by polyolefins; it does not damage <strong>the</strong> incinerator and provides significant energy savings.
The increasing appreciation <strong>of</strong> <strong>the</strong> various intrinsic properties <strong>of</strong> PLA, coupled with knowledge <strong>of</strong> how such
properties can be improved to achieve compatibility with <strong>the</strong>rmoplastics processing, manufacturing, and
end-use requirements, has fuelled technological and commercial interest in PLA. Over <strong>the</strong> last few years, a
wealth <strong>of</strong> investigations have been undertaken to enhance <strong>the</strong> mechanical properties and <strong>the</strong> impact
resistance <strong>of</strong> PLA. It can <strong>the</strong>refore compete with o<strong>the</strong>r low-cost biodegradable/biocompatible or commodity
polymers.
These efforts have made use <strong>of</strong> biodegradable and non-biodegradable fillers and plasticizers or
blending <strong>of</strong> PLA with o<strong>the</strong>r polymers [Martin and Avérous, 2001]. In recent years <strong>the</strong> nano-scale has
afforded unique opportunities to create revolutionary material combinations. Nano-structured materials or
nano-composites based on polymers have been an area <strong>of</strong> intense industrial and academic research over <strong>the</strong>
past one and a half decades [Sinha Ray and Okamoto, 2003; Biswas and Ray, 2001; Alexandre and Dubois,
2000; Zanetti et al., 2000; LeBaron et al., 1999]. In principle, nano-composites are an extreme case <strong>of</strong>
composite materials in which interfacial interactions between two phases are maximized. In <strong>the</strong> literature,
<strong>the</strong> term nano-composite is generally used for polymers with submicrometer dispersions. In polymer-based
nano-composites, nanometer-sized particles <strong>of</strong> inorganic or organic-materials are homogeneously dispersed
as separate particles in a polymer matrix. This is one way <strong>of</strong> characterizing this type <strong>of</strong> material. There is, in
fact, a wide variety <strong>of</strong> nano-particles and <strong>of</strong> ways to differentiate <strong>the</strong>m and to classify <strong>the</strong>m by <strong>the</strong> number <strong>of</strong>
dimensions <strong>the</strong>y possess. Their shape varies and includes:
i. needlelike or tubelike structures regarded as one-dimensional particles (for example,
inorganic nano-tubes, carbon nano-tubes, or sepiolites);
ii.
iii.
two-dimensional platelet structures (for example, layered silicates); and
spheroidal three-dimensional structures (for example, silica or zinc oxide).
To date, various types <strong>of</strong> nano-reinforcements such as nano-clay, cellulose nano-whiskers,
ultrafine layered titanate, nano-alumina, and carbon nano-tubes have been used for <strong>the</strong> preparation <strong>of</strong> nanocomposites
with PLA [Yu, 2009; Mark, 2006; Kim et al., 2006; Nishida et al., 2005; Nazhat et al., 2001;
Dumitriu, 1994].
2.2 Poly(lactide-co-glycolide acid) (PLGA)
2.2.1 General Structures <strong>of</strong> PLGA Copolymers
Glycolic acid is present in small amounts in a wide variety <strong>of</strong> fruits and vegetables. It accumulates
during photosyn<strong>the</strong>sis in a side path <strong>of</strong> <strong>the</strong> Krebs cycle. So far, economically viable methods to produce
glycolic acid in photosyn<strong>the</strong>tic biological systems do not exist. At an industrial scale, carbon monoxide,
formaldehyde and water are reacted at elevated temperature and pressure to produce glycolic acid. PLGA is
syn<strong>the</strong>sized by means <strong>of</strong> random ring-opening co-polymerization <strong>of</strong> <strong>the</strong> two different monomers, <strong>the</strong> LA and
<strong>the</strong> GA (cf. Figure 1.5).
Common catalysts used in <strong>the</strong> preparation <strong>of</strong> this polymer include Tin (II) 2-Ethylhexanoate, Tin
(II) Alkoxides or aluminum isopropoxide. During polymerization, successive monomeric units (<strong>of</strong> glycolic
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