Page 336 - Biodegradable Polyesters

314 12 Biodegradable Polyesters Polymer–Polymer Composites

12.6.2
Mechanical Properties of PLA/PGA Nano-/Microfibrillar Polymer–Polymer Nanofibrillar
Composites

The PLLA/PGA MFCs’ properties were compared with those of neat PLLA
and the PLLA/PGA blends from which the MFCs were derived, as shown in
Table 12.4. Young’s moduli of PLLA/PGA (80/20 by wt%) and PLLA/PGA (70/30
by wt%) are 5% and 16% higher than that of neat PLLA but these blends are very
brittle, both with mean strain at break of 2.2%. The MFCs, PLLA/PGA MFC
(80/20 by wt%) and PLLA/PGA MFC (70/30 by wt%) exhibited Young’s moduli
15% and 36% higher than that of neat PLLA – clearly showing the enhancement
of stiffness brought about by fibrillizing the PGA, despite poor fibrillization
in PLLA/PGA MFC (80/20 by wt%). The toughness improvement from fibril-
lization is clear:The mean breaking strains of PLLA/PGA MFC (80/20 by wt%)
and PLLA/PGA MFC (70/30 by wt%) were 15.3% and 13.7%, respectively – a
huge improvement over the simple blends. PLLA/PGA MFC (70/30 by wt%)
exhibits significant strength improvement too, with a strength of 88.1 MPa:
84% and 69% higher than those of neat PLLA and PLLA/PGA (70/30 by wt%),
respectively. The lack of strength improvement in the case of PLLA/PGA MFC
(80/20 by wt%) can be attributed to poor fibrillization as observed by SEM
(Figure 12.10f).
12.6.3
Viscoelastic Behavior of PLLA/PGA Nano-/Microfibrillar Polymer–Polymer Composites

From Figure 12.11, it is clear that both MFCs relax quicker than neat PLLA – from
an initial stress of 20 MPa their stresses all reduced to 10–11 MPa, while the

Table 12.4 Tensile strengths, Young’s moduli, and strains at break of PLLA, PLLA/PGA
blends, and their corresponding MFCs [47].

Material Young’s Improvement Tensile Improvement Strain at
modulus versus neat strength versus neat break (%)
(GPa) PLLA (%) (MPa) PLLA (%)

PLLA 3.00 ± 0.06 — 48.0 ± 3.8 — 12.6 a)
PLLA/PGA 3.16 ± 0.07 5 51.5 ± 1.6 7 2.2 ± 0.2
(80/20 by wt%)
PLLA/PGA 3.47 ± 0.11 16 52.0 ± 1.8 8 2.2 ± 0.3
(70/30 by wt%)
PLLA/PGA MFC 3.44 ± 0.14 15 52.5 ± 0.6 9 15.3 ± 1.1
(80/20 by wt%)
PLLA/PGA MFC 4.08 ± 0.14 36 88.1 ± 6.8 84 13.7 ± 1.2
(70/30 by wt%)

a) Strain at break of the one PLLA specimen which failed within the gauge length, others failed at
or near the grips.