Biomedical Engineering Reference
In-Depth Information
strengthen the microstructure, resulting in the dramatic improvement of the mode I fracture
energy.
5. Conclusion
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.
6. References
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