Page 106 - Biodegradable Polyesters

84 4 Synthesis, Properties, and Mathematical Modeling of Biodegradable Aliphatic Polyesters

did not show significant weight loss in presence of enzyme which may be due
to its higher degree of crystallinity and melting point compared to the PPSu,
PPAd, and copolyesters. However, according to the work of Herzog et al. [29],
the enzymatic hydrolysis should increase by increasing the chain length of the
diacid as this also increases the chain flexibility. In this case, PPSeb and PPAz
should have the highest mass loss rates. This does not happen, which may be
due to the higher crystallinity of these polyesters. However, all polyesters have
much higher enzymatic hydrolysis rates than PCL, which is one of the most used
aliphatic polyesters and was included for comparison purposes. appears that
these polyesters degrade much faster than the already extensively used polyesters
and this is very important for a lot of applications.
Considering the effect of the glycol used on the mass loss of aliphatic
polyester films during enzymatic hydrolysis with Rhizopus delemar lipase at
∘
pH = 7.2and 30 ± 1 C, it was found that PPSu has highest hydrolysis rates
compared to PESu and PBSu, following the trend PPSu > PESu > PBSu. This is
because PBSu has the highest melting point and degree of crystallinity, while
PPSu has the lowest [30].
An additional factor that could also influence the enzymatic hydrolysis of
aliphatic polyesters is the addition of nanofillers. In a recent study, PPSeb
nanocomposites containing 2 wt% of either fumed silica nanoparticles (SiO ),
2
multi-walled carbon nanotubes (MWCNTs), or montmorillonite were prepared
by in situ polymerization [31]. From the measurement of mechanical proper-
ties, it was found that tensile strength and Young’s moduli were significantly
increased in nanocomposites owing to the addition of nanoparticles. It is also
very interesting to study the effect of nanoparticles on enzymatic hydrolysis of
PPSeb (Figure 4.5). Mass loss measurements showed that, when compared to
neat PPSeb, the presence of nanoparticles results in reduced enzymatic hydrolysis
rates. This is due to the hindering effect of nanoparticles on the action of the
enzymes because the former reduce the available surface area for hydrolysis, but
also due to the interactions taking place between the nanoparticles and PPSeb
matrix [32].
The mechanism of PPSeb enzymatic hydrolysis was investigated by molecular
weight variation and LC-MS analysis of the soluble by-products. It was found that
PPSeb and its nanocomposites have identical hydrolysis mechanisms even though
the nanocomposites have lower hydrolysis rates. From the identification of the
produced fragments with mass spectra (MS), it was found that molar masses
less than 443 are formed in all samples after 24 h of enzymatic hydrolysis. These
fragments are attributed to sebacic acid, monomers between sebacic acid and
1,3-PD, and dimers such as PD–Seb–PD–Seb. In such a case, macromolecules
degrade from their ends and as a result water-soluble oligomers are formed
and then removed from the film surface. Such water-soluble monomers and
co-oligomers produced by hydrolysis were extensively studied in the hydrol-
ysis of poly(butylene succinate-co-butylene sebacate)s, P(BS-co-BSe)s, and
poly(butylene succinate-co-butylene adipate)s, P(BS-co-BA)s by using Mucor
miehei and R. arrhizus [33].