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5.3 Kinetics of Crystal Nucleation 117
Onset time of crystal nucleation (s) 10 4 2 0
10
10
25 50 T g 75 100 125
Temperature (°C)
Figure 5.7 Onset time of crystal nucle- sets collected to estimate instrumental or
ation of PLLA as a function of temperature. preparative uncertainties. Adapted from Ref.
The temperature range of the glass tran- [51], Copyright (2013), with permission from
sition is indicated by the gray shaded bar. Elsevier.
The various symbols represent different data
∘
overlapping with crystal growth on annealing at 73 C, whereas annealing at
∘
53 C resulted in less nucleation.
For demonstration of the effect of formation of crystal nuclei in the glassy state
on the acceleration of crystallization at elevated temperature, in Figure 5.8 are
shown POM images obtained on initially fully amorphous samples of PLA, which
∘
were annealed in the glassy state and then cold-crystallized at 120 Cfor aperiod
of 10 min [52]. Images shown in the various rows and columns were obtained on
samples which were annealed for different time and at different temperatures,
∘
respectively, as indicated. Annealing at 50 C less than about 100 min is not
connected with nuclei formation as it is concluded from the constant low number
∘
of spherulites growing at 120 C. Only if the annealing time exceeds 100 min, an
∘
increase of the spherulite density on crystallization is observed at 120 C owing
∘
to the formation of nuclei at 50 C. With increasing annealing temperature, an
∘
increased spherulite number evolving at 120 C is observed on annealing for
∘
shorter periods of time. For example, aging at 60 C leads to a distinct increase of
∘
the nuclei density after 30 min, while nuclei formation at 70 Cisdetectedafter
annealing for only 2 min.
Effective routes for acceleration of the inherent slow crystallization rate of PLA
are the increase of the nucleation density by addition of heterogeneous nucle-
ators and the increase of the chain mobility by addition of plasticizers [14, 57].
Chemical nucleation agents such as organic salts of sodium, which successfully
were employed to accelerate the crystallization of poly(ethylene terephthalate) or
polycarbonate, failed in the specific case of PLA [14, 57, 58]. Physical nucleation
agents, in contrast, have been shown to be effective to enhance the crystallization
of PLA. Among these, talc has been proven superior, most effective, and cost
