Page 103 - Biodegradable Polyesters

4.3 Properties of Poly(propylene alkylenedicarboxylates) 81

2 W g −1
0.5 W g −1
PPSebacate PPSebacate
Normalized heat flow (W g −1 ) endo up PPSuberate Normalized heat flow (W g −1 ) endo up PPSuberate
PPAzelate
PPAzelate

PPPimelate
PPPimelate
PPAdipate
PPAdipate
PPGlutarate
PPGlutarate
PPSuccinate PPSuccinate

0 20 40 60 80 100 −80 −60 −40 −20 0 20 40 60 80 100
(a) Temperature (°C) (b) Temperature (°C)
Figure 4.2 DSC traces of as-received samples of poly(propylene alkylene dicarboxylate)s
∘
recorded at a heating rate of 20 Cmin −1 (a) and of the melt-quenched samples, recorded
∘
at a heating rate of 2.5 Cmin −1 (b) [9].
unexpected behavior is recorded because the samples are semicrystalline and
cannot be taken to be completely amorphous after quenching from their melt.
As can be seen in Figure 4.2b, where the thermograms of aliphatic polyesters
∘
after quenching are recorded with 2.5 C heating rate, PPSub was not completely
amorphous as a small, cold crystallization peak was recorded after the glass
transition temperature. On the contrary, PPSu and PPGlu can be taken to be
completely amorphous and it is characteristic that even during heating they
cannot be crystallized. In the case of PPAz and PPSeb, such cold crystallization
was not detected and only a small exothermic peak was recorded just before their
melting points, indicating that both polyesters have been crystallized significantly
during quenching and showed no cold crystallization on subsequent heating. This
is because of the high numbers of methylene groups that both polyesters have and
their high flexibility. Owing to this flexibility, their cold crystallization could be at
much lower temperatures. As can be seen, the cold crystallization temperatures
are shifted progressively to lower temperatures from PPAd to PPSub.
Since all the synthesized polyesters are semicrystalline materials, it is interest-
0
ing to find their equilibrium melting points (T ). There are already some studies
m
0
in the literature and for PPSu, PPGlu, PPAd, and PPAz, the T values were found
m
∘
0
at 61, 67, 56, and 70 C, respectively [10]. Similar T values were reported for
m
∘
∘
0
PPAz (72.2 C) and PPSeb (69.9 C) [26, 27], while for PPSu the T was found at
m