Page 138 - Biodegradable Polyesters

116 5 Crystallization of Poly(lactic acid)

(a) (d)

(b) (e)

(c) (f)

∘ ∘ ∘ ∘
Figure 5.6 POM images of PLA crystallized at (a) 81 C, (b) 91 C, (c) 111 C, (d) 116 C,
∘
∘
(e) 121 C, and (f) 126 C. Adapted from Ref. [48], Copyright (2013), with permission from
Elsevier.
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of melt-crystallization at 110 C decreased from about 155 to 105 min on cold-
∘
crystallization after earlier cooling the relaxed melt at a rate of 10 Cmin −1 to
∘
∘
25 C and immediate reheating at identical rate to 110 C. This observation has
been explained by a difference of the nucleation density, that is, by additional
∘
nuclei formed during cooling to 25 C. In a different study [56], PLA samples were
∘
cooled at different rates between 5 and 500 Cmin −1 to obtain fully amorphous
samples, and then cold-crystallized on reheating. It has been shown that cooling
at moderate rates resulted in formation of a larger number of nuclei compared
to samples cooled at higher rate. Also, the effect of the residence time on the
nucleation kinetics was estimated from the shift of the cold-crystallization
exotherm to lower temperatures on subsequent heating [55]. Nucleation was