Page 33 - Biodegradable Polyesters

1.3 Biodegradable Polyesters 11

PLA is the product resulting from polymerization of L,L-lactide. PLLA has a
∘
crystallinity of around 37%, a glass transition temperature between 50 and 65 C,
∘
a melting temperature between 173 and 178 C and a tensile modulus between
2.7 and 16 GPa [33, 58]. PLLA can be processed like most thermoplastics into
∘
fiber and film. The melting temperature of PLLA can be increased by 40–50 C
∘
and its heat deflection temperature can be increased from approximately 60 Cto
∘
up to 190 C by physically blending a PLLA with PDLA. PDLA and PLLA form a
highly regular stereocomplex with increased crystallinity. The temperature stabil-
ity is enhanced when a 50 : 50 blend is used, but even at lower concentrations of
3–10% of PDLA, there is still a significant improvement [50, 57, 59]. PDLA acts
as a nucleating agent in a blend of PLLA in the formation of crystalline structure
[60, 61]. Thus PDLA helps in increasing the crystallization rate. Biodegradation of
PDLA is slower than for PLA because the former has the higher crystallinity. The
differences in the degradation behavior of the amorphous and crystalline PLAs
can be explained by assuming a simple hydrolysis as the main degradation mech-
anism [62]. PDLA is optically transparent and this is very useful in poly(lactide)
blends [63, 64]. PLDLLA has been used as scaffold for bone engineering [65].

Applications Polylactide-based polymers are available for controlled drug
releases, implantable composites, bone fixation parts, packaging, and paper
coatings, sustained release systems for pesticides and fertilizers, and compost
bags [66]. Histological studies indicate that the PLA is nontoxic, nontissue
reactive, and biodegradable, and neither the polymer nor its degradation prod-
ucts are retained in any of the vital organs of the animals. PLA is suitable for
sutures, vascular grafts, and other surgical implants. The polymer implant,
however, degrades slowly in vivo, losing 12–14% in three months. Kulkarni et al.
[35] reported that high-molecular-weight PLA made from the cyclic lactide
intermediate is suitable for casting films or spinning fibers. The films are quite
permeable to water vapor and can soften in the presence of water. Sinclair and
Gynn [67] prepared polymers/copolymers using glycolic and lactic acid-based
compound for implant devices in managing maxillofacial trauma. In 1973, Sin-
clair [68] extended polymers of lactic and glycolic acids as ecologically beneficial,
biodegradable polyesters encapsulating materials for slow release of herbicide in
soil. PLA prepared from D,L-lactic acid via a D,L-lactide intermediate was mixed
∘
with ground urea, and compression-molded at 130 C into pellets containing 25%
urea. In sand, the pellets showed a slow biodegradation of the polymer to lactic
acid and a urea-release rate 0.1–1% per day.
Fibrillated and self-reinforced (SR) poly(L-lactide) rods 4.5 mm in diameter were
used for fixing right femoral, cortical bone osteotomies of rabbits and observed
for3–48 weeks.Noneofthe rods brokeduring this period.Therodswerealso
tested mechanically. About 25% of the initial 136 MPa shear-strength of the rods
was left after 24 weeks. The results show that fibrillated SR-PLLA rods are strong
enough to be used in intramedullary nailing of femoral cortical bone osteotomies
in rabbits [69].