Homopolymers, crystallization kinetic range

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 overall crystallization kinetics of copolymers and their blends have been studied by DSC in the temperature range 1(X)-132°C [69]. For all examined blend compositions a single crystallization exotherm was observed at each T, whereas crystallization of mechanical mixtures of the copolymers showed separated exotherms of each component, thus supporting that the crystallization of melt mixed blends occurred from a homogeneous melt. The overall crystallization rate of copolymers was found to be affected by the copolymer structure and lower than that of PP homopolymer (BP < EP < PP), while the crystallization rate of the blends was intermediate between those of pure components (Fig. 10.12). The kinetics were analyzed by means of the Avrami equation (Eq. 10.14) the calculated values of the Avrami exponent for the blends, with average values of n from... 

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