Page 102 - Biodegradable Polyesters

80 4 Synthesis, Properties, and Mathematical Modeling of Biodegradable Aliphatic Polyesters

In the case that aliphatic acids cannot be produced in high purity (>99.5%) their
diesters with methanol can be also used as monomers. In this case, the synthesis
of polyesters follows the same procedure with only slight differences, mainly the
removal of methanol instead of water as a by-product.

4.3
Properties of Poly(propylene alkylenedicarboxylates)

Aliphatic polyesters, because of their favorable features of biodegradability and
biocompatibility, are one of the most important classes of synthetic biodegrad-
able polymers and are nowadays available commercially in a variety of types. In
recent years, biodegradable polymers have attracted considerable attention as
green materials and biomaterials in pharmaceutical, medical, and biomedical
engineering applications, including drug delivery systems, artificial implants, and
functional materials in tissue engineering.
Some pioneering works have been already mentioned concerning the syn-
thesis and comparable studies of aliphatic polyesters prepared from 1,3-PD
and different aliphatic acids [9–11]. However, it is expected such research will
increase progressively because markets are always looking for new materials
and owing to the appearance of 1,3-PD in large amounts. A complete series
of seven aliphatic polyesters from 1,3-PD and aliphatic diacids with increas-
ing number of methylene units (x) such as succinic, glutaric, adipic, pimelic,
suberic, azelaic, or sebacic acid have been prepared by melt polycondensa-
tion and characterized as poly(propylene succinate) (PPSu), poly(propylene
glutarate) (PPGlu), poly(propylene adipate) (PPAd), poly(propylene pimalate)
(PPPim), poly(propylene suberate) (PPSub), poly(propylene azelate) (PPAz), and
poly(propylene sebacate) (PPSeb), respectively [9]. At room temperature, all the
polyesters appeared as semicrystalline materials with variation of softness. Their
softness is dependent on their molecular weight, the degree of crystallinity, and
their melting points. From the differential scanning calorimetry (DSC) traces
of the as received samples shown in Figure 4.2a, it can be seen that the melting
∘
points decreases from x = 2to x = 4, that is, from PPSu (T = 49.3 C) to PPAd
m
∘
(T = 43.3 C), and then increases again up to x = 7 for PPAz. The melting point
m ∘ ∘
of PPSeb (x = 8) is 56.8 C, which is very close to that of PPAz (57.1 C). These
∘
low melting points are close to the melting point of PCL (about 60 C), much
∘
lower than the melting point of PBSu (about 110 C), and quite far from the
melting point of PLA, which are the three most used aliphatic polyesters. In
contrast to T values, the glass transition temperatures (T )ofthe polyesters
m g
are expected to decrease monotonically with increasing number of methylene
units owing to the increase in chain flexibility. However, this is not in case. The
∘
glass transition of PPSu was −34.5 C, while the T values of PPGlu and PPAd
g
∘
∘
were −53.3 and −58.8 C, respectively. PPPim has the lowest T (−63 C) among
g
all synthesized polyesters and next to that, the T values of PPSub, PPAz, and
g ∘
PPSeb were found to increase to −57.8, −56.6, and −53.1 Crespectively. This