**5. Conclusion**

390 Biomaterials – Physics and Chemistry

 (b) Annealed PLA/PCL

 (c) Quenched PLA/PCL/LTI

 (d) Annealed PLA/PCL/LTI

In summary, the bending modulus and strength of both PLA/PCL and PLA/PCL/LTI are effectively improved by annealing. Crystallization of the PLA phase by annealing is thought to strengthen the structure of the PLA/PCL blend, resulting in increase of these properties. The mode I fracture energy of PLA/PCL significantly decreases by annealing mainly owing to embrittlement of the PLA phase. For the case of PLA/PCL/LTI, the structural transformation due to polymerization by LTI addition and crystallization by annealing

Fig. 22. FE-SEM micrographs of fracture surfaces of quenched and annealed samples.

In this chapter, the fundamental fracture characteristics of bioabsorbable PLA were firstly discussed, and then as examples of toughening, effects of unidirectional drawing and blending with PCL on the fracture behavior were presented. Finally, microstructural modification for PLA/PCL blends using LTI additive was discussed. Thermal processes have great influences on the microstructure and the mechanical properties of PLA mainly due to crystallization behaviour during the heating process. Highly crystallized PLA tends to exhibit very brittle fracture behavior with low fracture energy. Amorphous PLA can generate multiple crazes at crack-tip region to dissipate more energy during fracture process than crystallized materials in which craze formation is suppressed. Drawing process can arrange molecules in one direction so that the fracture resistance in the perpendicular to the drawing direction is greatly improved, while the resistance in the drawing direction tends to degrade. Another effective way to improve the fracture energy is blending with ductile polymer such as PCL. PLA/PCL blends show higher fracture energy with extensive damage formation in crack-tip regions than neat PLA; however, the immiscibility of PLA and PCL results in phase separation morphology in which spherulites of PCL are dispersed in PLA matrix. Such morphological problem can effectively be improved by using LTI as an additive. The phase separation is almost disappeared and the fracture energy is greatly improved. The fracture micromechanism is changed from multiple craze-like damage formation to plastic deformation in crack-tip region. Furthermore, the mechanical properties including elastic modulus, strength and fracture energy of PLA/PCL/LTI blends can effectively be improved by introducing annealing process, although such process tends to degrade the fracture energy of PLA/PCL blends.
