Spherulites induction periods

As mentioned above, PLA should be addressed as a random copolymer rather than as a homopolymer the properties of the former depend on the ratio between L-lactic acid and D-lactic acid units. A few studies describe the influence of the concentration of D-lactic acid co-units in the PLLA macromolecule on the crystallization kinetics [15, 37, 77-79]. The incorporation of D-lactic acid co-units reduces the radial growth rate of spherulites and increases the induction period of spherulite formation, as is typical for random copolymers. In a recent work, the influence of the chain structure on the crystal polymorphism of PL A was detailed [15], with the results summarized in Figure 5.13. It shows the influence of D-lactic acid units on spherulite growth rates and crystal polymorphism of PLA for two selected molar mass ranges. 

The first part of the curve is an induction period in which time is required for the formation of nuclei. The steepest part is related to the growth of the spheruhtes. The final part is when the spherulites begin to touch each other and the rate slows down. When the crystallisation temperature is increased, the shape... 

Figure 16.9 Radial growth rates of spherulites and the induction periods for spherulite growth of pure phase and blends of stereocomplexationable polymers as estimated from the respective POM images. Reprinted with permission from Ref [79] 2011, Nature Publishing Group.

Parameters for the simulations are presented in Table 1. Figure 8 presents the view of the sample by transmission, i.e., the projected view of the sample parallel to the light beam, for four different times during the solidification (5, 35, 65, and 125 s). Origin of time f = 0 was chosen to be when nucleation starts, once the induction period has elapsed. In the simulations, liquid is represented as black, solid as white. In simulation SI, all the isolated spherulites have the same size, because they nucleate at the same time and the growth rate is constant. This is not the case for the other simulations, where there is a size distribution due to nucleation throughout solidification. 

Nucleus formation can be followed throu the development and propagation of spherulites. When polyfdecamethylene terephthalate) is crystallized, an induction or nudeation period first occurs (Figure 10-7). During this period, either homogeneous or weak-interaction hetert eneous stable nuclei are formed. 

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