**2.1 Effect of crystallization**

The microstructure of crystalline PLA can be changed through annealing process from amorphous to highly crystallized states as shown in Fig.1. 70C-3h indicates an annealing process that the specimens are kept in an oven at 70C for three hours.

The annealing process of 70C-3h results in an amorphous state, on the contrary, the 100C-3h and 100C-24h processes create highly crystallized states and longer annealing time tends to increase the size of crystals.

Fig. 1. Polarized micrographs of microstructures of PLA.

It has been found that such microstructure dramatically affects the fracture behavior of PLA. As an example, the critical energy release rate, *GIC*, measured at a quasi-static rate, 1 mm/min, and an impact rate, 1 m/s, of loading is shown in Fig.2. as a function of crystallinity. At the quasi-static rate, *GIC* slightly decreases with increase of crystallinity up to 11.6%, and kept constant up to 48.3%. Above 48.3%, *GIC* rapidly decrease. On the other hand, at the impact rate, *GIC* tends to increase with increase of crystallinity. As a result, the static *GIC* is greater than the impact value up to 48.3%, and above 48.3%, on the contrary, the impact value becomes higher than the static value. This result suggests that the fracture mechanism at the static rate is different from that at the impact rate.

Fig. 3 shows polarized microphotographs of arrested cracks in the PLA specimens prepared under different annealing conditions, and tested under static and impact loading rates. For the amorphous specimen with the crystallinity, *Xc*=2.7%, under the static loading-rate (Fig.3(a)), extensive multiple crazes were generated in the crack-tip region, while only a few crazes were observed under the impact loading-rate (Fig.3(b)). This kind of craze formation in crack-tip region is usually observed in amorphous polymers such as polystyrene in which craze formation is dominant rather than shear plastic deformation (Botosis, 1987). Disappearance of multiple craze formation observed at the impact rate corresponds to the reduction of additional energy dissipation in the crack-tip region compared to the static case where multiple crazes are formed, and therefore results in the decrease of *GIC* as shown in Fig.2. On the contrary, for the highly crystallized specimen with *Xc*=55.8% tested at the static rate (Fig.3(c)), a straight single crack without craze formation in the surroundings is observed. This type of crack growth usually corresponds to brittle fracture and lower *GIC* than the amorphous dominant samples in which crazes are generated in crack-tip region. At the impact rate (Fig.3(d)), the main crack tends to be distorted and branched. These behaviors may be related to the increase of *GIC* at the impact rate, although the detail of the mechanism is still unclear, and further study will be performed to elucidate such mechanism.

The microstructure of crystalline PLA can be changed through annealing process from amorphous to highly crystallized states as shown in Fig.1. 70C-3h indicates an annealing

The annealing process of 70C-3h results in an amorphous state, on the contrary, the 100C-3h and 100C-24h processes create highly crystallized states and longer annealing time tends

(a) 70C-3h (b) 100C-3h (c) 100C-24h

It has been found that such microstructure dramatically affects the fracture behavior of PLA. As an example, the critical energy release rate, *GIC*, measured at a quasi-static rate, 1 mm/min, and an impact rate, 1 m/s, of loading is shown in Fig.2. as a function of crystallinity. At the quasi-static rate, *GIC* slightly decreases with increase of crystallinity up to 11.6%, and kept constant up to 48.3%. Above 48.3%, *GIC* rapidly decrease. On the other hand, at the impact rate, *GIC* tends to increase with increase of crystallinity. As a result, the static *GIC* is greater than the impact value up to 48.3%, and above 48.3%, on the contrary, the impact value becomes higher than the static value. This result suggests that the fracture

Fig. 3 shows polarized microphotographs of arrested cracks in the PLA specimens prepared under different annealing conditions, and tested under static and impact loading rates. For the amorphous specimen with the crystallinity, *Xc*=2.7%, under the static loading-rate (Fig.3(a)), extensive multiple crazes were generated in the crack-tip region, while only a few crazes were observed under the impact loading-rate (Fig.3(b)). This kind of craze formation in crack-tip region is usually observed in amorphous polymers such as polystyrene in which craze formation is dominant rather than shear plastic deformation (Botosis, 1987). Disappearance of multiple craze formation observed at the impact rate corresponds to the reduction of additional energy dissipation in the crack-tip region compared to the static case where multiple crazes are formed, and therefore results in the decrease of *GIC* as shown in Fig.2. On the contrary, for the highly crystallized specimen with *Xc*=55.8% tested at the static rate (Fig.3(c)), a straight single crack without craze formation in the surroundings is observed. This type of crack growth usually corresponds to brittle fracture and lower *GIC* than the amorphous dominant samples in which crazes are generated in crack-tip region. At the impact rate (Fig.3(d)), the main crack tends to be distorted and branched. These behaviors may be related to the increase of *GIC* at the impact rate, although the detail of the mechanism is still unclear, and further study will be performed to elucidate such

Fig. 1. Polarized micrographs of microstructures of PLA.

mechanism at the static rate is different from that at the impact rate.

process that the specimens are kept in an oven at 70C for three hours.

**2. Fracture behavior of PLA** 

**2.1 Effect of crystallization**

to increase the size of crystals.

mechanism.

Fig. 2. Dependence of crystallinity on the critical energy release rate under a quasi-static and an impact loading conditions.

(c) *Xc*=55.8%, static (d) *Xc*=55.8%, impact

Fig. 3. Polarized micrographs of crack growth behavior.

Fig.4 shows FE-SEM micrographs of the fracture surfaces of the PLA samples. For the amorphous sample tested at the static rate, the fracture surface exhibits deep concavities and hackles due to multiple craze formation (Fig.4(a)). The fracture surfaces of the crystallized samples (Fig.4(c)) appears to be smoother than the amorphous one, corresponding to the decrease of the toughness values. The impact fracture surface of the amorphous sample (Fig.4(b)) is obviously smoother than the static one, corresponding to the decrease of *GIC*. It is noted that drawing fibrils are also observed on the impact fracture surface, suggesting that effect of high strain-rate exists. Roughness of the impact fracture surface appears to increase with increase of crystallinity comparing the surfaces shown in Figs.4(b) and (d). For the impact surface of the highly crystallized sample (Fig.4(d)), relatively fine roughness exists suggesting the increase of *GIC* as crystallinity increases.

Fracture Mechanisms of Biodegradable PLA and PLA/PCL Blends 379

drawing direction. It is thus thought that the ductile deformation due to elongation of the oriented molecules and the transverse crack formation are primary mechanisms of toughening in draw-processed PLA. On the other hand, the fracture surface of the parallel were much smoother than that of the original, corresponding to the lower *Jin* value. *Jin* is contributed by energy dissipation through not only creation of fracture surface but also development of process zone. Poralized micrographs of notch-tip regions of the original and the perpendicular are shown in Fig.7. In the original, multiple crazes forming a fan shape were observed. They were initiated from the initial notch-tip and propagated almost perpendicularly to the tensile direction. For the perpendicular with draw ratio 2.5, crazes were much denser and the width of the damage region was much wider than the original. Transverse cracks generated in the drawing direction are observed, and these obviously correspond to the crevices observed on the fracture surfaces as shown in Fig.6(b). Larger damage region consisting of crazes and transverse cracks generated in crack-tip region indicates larger energy dissipation under crack initiation and propagation processes, and

(a)Perpendicular direction (b) Parallel direction

(a)Original (b)Drawed, perpendicular (c)Drawed, parallel

In summary, it was shown that the crystallization behavior greatly affects the fracture behavior of PLA. Microstructure of PLA can easily be changed through annealing process by changing temperature and heating time. The static fracture energy tends to decrease as

Fig. 5. Dependence of draw ratio on the critical *J*-integral at crack initation.

crystallinity increases, while the impact fracture energy increases.

Fig. 6. FE-SEM micrographs of fracture surfaces (draw ratio=2.5).

therefore, greater *Jin*.

Fig. 4. FE-SEM micrographs of fracture surfaces.

#### **2.2 Effect of unidirectional drawing process**

Drawing process is known to be an effective way to improve the mechanical properties of thermoplastics, and effects of drawing on tensile and fracture properties of thermoplastics such as polypropylene (Mohanraj et al., 2003a, 2003b; Uehara et al., 1996), poly(acrylonitrile) (Sawai et al., 1999; Yamane et al., 1997) and PLA (Todo, 2007) have been studied. PLA is usually draw-processed when it is used for bone fixation devices, and therefore, fundamental effect of drawing on its fracture behavior needs to be characterized. As an example, dependence of draw ratio on the critical *J*-integral at crack initiation, *Jin*, is shown in Fig.5. In the fracture specimens, the initial notches were introduced in the direction perpendicular or parallel to the drawing direction. Therefore, the two different types of specimens are denoted as 'perpendicular' and 'parallel'. For the parallel, *Jin* decreased with increase of draw ratio, and *Jin* for draw ratio of 2.5 became about one fifth of the original. On the contrary, for the perpendicular, *Jin* increased as draw ratio increased, and *Jin* for draw ratio of 2.5 became five times greater than that of the original. Thus, greater energy is needed for crack propagation in the perpendicular than in the parallel. This is easily understood by considering the effect of drawing on the micromechanism of fracture. In draw-processed polymer, molecules are reoriented in the drawing direction. Therefore, energy dissipation during crack growth by elongation and scission of such oriented molecules is much greater in the perpendicular direction than in the parallel direction where such elongation and scission processes obviously decrease.

FE-SEM micrographs of fracture surfaces are shown in Fig. 6. The perpendicular with draw ratio 2.5 exhibited rougher surface with ductile deformation than the original (without drawing). It is interesting to note that crevices existed on the fracture surfaces that were thought to be cracks transversely propagated between the parallel fibrils reoriented in the

Drawing process is known to be an effective way to improve the mechanical properties of thermoplastics, and effects of drawing on tensile and fracture properties of thermoplastics such as polypropylene (Mohanraj et al., 2003a, 2003b; Uehara et al., 1996), poly(acrylonitrile) (Sawai et al., 1999; Yamane et al., 1997) and PLA (Todo, 2007) have been studied. PLA is usually draw-processed when it is used for bone fixation devices, and therefore, fundamental effect of drawing on its fracture behavior needs to be characterized. As an example, dependence of draw ratio on the critical *J*-integral at crack initiation, *Jin*, is shown in Fig.5. In the fracture specimens, the initial notches were introduced in the direction perpendicular or parallel to the drawing direction. Therefore, the two different types of specimens are denoted as 'perpendicular' and 'parallel'. For the parallel, *Jin* decreased with increase of draw ratio, and *Jin* for draw ratio of 2.5 became about one fifth of the original. On the contrary, for the perpendicular, *Jin* increased as draw ratio increased, and *Jin* for draw ratio of 2.5 became five times greater than that of the original. Thus, greater energy is needed for crack propagation in the perpendicular than in the parallel. This is easily understood by considering the effect of drawing on the micromechanism of fracture. In draw-processed polymer, molecules are reoriented in the drawing direction. Therefore, energy dissipation during crack growth by elongation and scission of such oriented molecules is much greater in the perpendicular direction than in the parallel direction where

FE-SEM micrographs of fracture surfaces are shown in Fig. 6. The perpendicular with draw ratio 2.5 exhibited rougher surface with ductile deformation than the original (without drawing). It is interesting to note that crevices existed on the fracture surfaces that were thought to be cracks transversely propagated between the parallel fibrils reoriented in the

(a) *Xc*=2.7%, static (b) *Xc*=2.7%, impact

(c) *Xc*=55.8%, static (d) *Xc*=55.8%, impact

Fig. 4. FE-SEM micrographs of fracture surfaces.

**2.2 Effect of unidirectional drawing process** 

such elongation and scission processes obviously decrease.

drawing direction. It is thus thought that the ductile deformation due to elongation of the oriented molecules and the transverse crack formation are primary mechanisms of toughening in draw-processed PLA. On the other hand, the fracture surface of the parallel were much smoother than that of the original, corresponding to the lower *Jin* value. *Jin* is contributed by energy dissipation through not only creation of fracture surface but also development of process zone. Poralized micrographs of notch-tip regions of the original and the perpendicular are shown in Fig.7. In the original, multiple crazes forming a fan shape were observed. They were initiated from the initial notch-tip and propagated almost perpendicularly to the tensile direction. For the perpendicular with draw ratio 2.5, crazes were much denser and the width of the damage region was much wider than the original. Transverse cracks generated in the drawing direction are observed, and these obviously correspond to the crevices observed on the fracture surfaces as shown in Fig.6(b). Larger damage region consisting of crazes and transverse cracks generated in crack-tip region indicates larger energy dissipation under crack initiation and propagation processes, and therefore, greater *Jin*.

Fig. 5. Dependence of draw ratio on the critical *J*-integral at crack initation.

In summary, it was shown that the crystallization behavior greatly affects the fracture behavior of PLA. Microstructure of PLA can easily be changed through annealing process by changing temperature and heating time. The static fracture energy tends to decrease as crystallinity increases, while the impact fracture energy increases.

Fig. 6. FE-SEM micrographs of fracture surfaces (draw ratio=2.5).

Fracture Mechanisms of Biodegradable PLA and PLA/PCL Blends 381

**0 5 10 15 PCL content (wt%)**

Fig. 9. Dependence of PCL content on the critical energy release rate at crack initiation.

Fig. 10. Polarized micrograph of crack growth behavior in PLA/PCL.

Polarized micrograph of crack-tip region of PLA/PCL (PCL:15wt%) is shown in Fig.10. Craze-like damages similar to neat PLA shown in Fig.7(a) are created in the crack-tip region, and the size of the damage zone is much larger than that of PLA. FE-SEM micrographs of surface on the crack-tip region are also shown in Fig.11. The right-hand figure is a micrograph of the craze-like damages at higher magnification. The micrograph clearly indicates a typical structure of craze, consisting of voids and fibrils. The spherulites of PCL are also seen. The extended fibrils of the matrix PLA were found to be much longer than those of the neat PLA, suggesting that the existence of the dispersed PCL spherulites in PLA tends to enhance ductile deformation of PLA fibrils. It is thus clear that the dispersed PCL droplets play an important role in the formation of the crazelike damages in PLA/PCL blend. FE-SEM micrograph of the fracture surface is shown in Fig.12. Increased ductile deformation of the matrix PLA with appearance of porous structures is clearly observed on the surface. These holes are thought to be created by

**0**

**2**

**4**

*Gin* **(KJ/m2**

removal of PCL droplets.

**)**

**6**

**8**

(a)Original (b)draw ratio=2.5, perpendicular
