**3. Fracture behavior of PLA/PCL blend**

Blending with ductile biodegradable polymers such as PCL (Broz, 2003; Dell'Erba, 2001; Chen, 2003; Todo, 2007; Tsuji, 1996, 1998, 2001, 2003), poly(butylene succinate-co-εcaprolactone) (PBSC) (Vannaladsaysy, 2010) and poly (butylene succinate-co-L-lactate) (PBSL) (Shibata, 2006, 2007; Vannaladsaysy, 2009; Vilay, 2009) has extensively been investigated in order to improve the fracture energy of PLA. Amoung of them, PCL is known to be bioabsorbable and bioaabsorbable, therefore has been applied in medical fields. In this paragraph, the Mode I fracture behavior of PLA/PCL blend is discussed.

FE-SEM micrographs of the cryo-fracture surfaces of PLA/PCL blends are shown in Fig.8. Phase separations indicated as spherulites of PCL are clearly observed. It is obvious that the size of the PCL spherulites increases with increase of PCL content. It is also seen that voids are created as a result of removal of the dispersed PCL droplets. In general, a blend of immiscible polymers such as PLLA and PCL creates macro-phase separation of the two components due to difference of solubility parameter. This kind of phase separation dramatically affects the physical and mechanical properties of the blend (Dell'Erba et al., 2001; Maglio et al., 1999; Tsuji et al., 2003).

Dependence of PCL content on the critical energy release rate at crack initiation, *Gin*, is shown in Fig.9. *Gin* increases with increase of PCL content up to 5wt%, and *Gin* becomes about 1.5 times greater than that of PLA. *Gin* slightly decreases as PCL content increases above 5wt%; however, *Gin* values of the blends with 10 and 15wt% of PCL are still higher than that of PLA.

 (a)PCL:5wt% (b)PCL: 15wt% Fig. 8 Morphology of PLA/PCL blends

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

In this paragraph, the Mode I fracture behavior of PLA/PCL blend is discussed.

(a)PCL:5wt% (b)PCL: 15wt%

Blending with ductile biodegradable polymers such as PCL (Broz, 2003; Dell'Erba, 2001; Chen, 2003; Todo, 2007; Tsuji, 1996, 1998, 2001, 2003), poly(butylene succinate-co-εcaprolactone) (PBSC) (Vannaladsaysy, 2010) and poly (butylene succinate-co-L-lactate) (PBSL) (Shibata, 2006, 2007; Vannaladsaysy, 2009; Vilay, 2009) has extensively been investigated in order to improve the fracture energy of PLA. Amoung of them, PCL is known to be bioabsorbable and bioaabsorbable, therefore has been applied in medical fields.

FE-SEM micrographs of the cryo-fracture surfaces of PLA/PCL blends are shown in Fig.8. Phase separations indicated as spherulites of PCL are clearly observed. It is obvious that the size of the PCL spherulites increases with increase of PCL content. It is also seen that voids are created as a result of removal of the dispersed PCL droplets. In general, a blend of immiscible polymers such as PLLA and PCL creates macro-phase separation of the two components due to difference of solubility parameter. This kind of phase separation dramatically affects the physical and mechanical properties of the blend (Dell'Erba et al.,

Dependence of PCL content on the critical energy release rate at crack initiation, *Gin*, is shown in Fig.9. *Gin* increases with increase of PCL content up to 5wt%, and *Gin* becomes about 1.5 times greater than that of PLA. *Gin* slightly decreases as PCL content increases above 5wt%; however, *Gin* values of the blends with 10 and 15wt% of PCL are still higher

Fig. 7. Poralized micrographs of damage zones.

**3. Fracture behavior of PLA/PCL blend** 

2001; Maglio et al., 1999; Tsuji et al., 2003).

Fig. 8 Morphology of PLA/PCL blends

than that of PLA.

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

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 removal of PCL droplets.

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

Fracture Mechanisms of Biodegradable PLA and PLA/PCL Blends 383

to PLA/PCL blends effectively improves their immiscibility (Takayama, 2006a, 2006b;

FE-SEM micrographs of cryo-fractured surfaces of PLA/PCL and PLA/PCL/LTI are shown in Fig.13. The content of PCL was 15wt% in these materials. Spherical features appeared on the micrograph are thought to be PCL-rich phases. These micrographs clearly showed that the size of the PCL-rich phase dramatically decreases by LTI addition. It is thus presumed that LTI addition effectively improves the miscibility of PLA and PCL. This is thought to be related to the following chemical reaction, that is, the hydroxyl group of PLA and the

HO-R' + R-N=C=O → R-NHCOO-R'

**0 0.5 1 1.5 2 LTI content [phr]**

Dependence of LTI content on the molecular weight, *M*w, is shown in Fig.14.For comparison, *M*w of neat PLA is also shown in the figure. *M*w values of the blends tend to

**PLA**

Fig. 13. FE-SEM micrographs of cryo-fracture surfaces of PLA/PCL and PLA/PCL/LTI.

(a)PLA/PCL (b)PLA/PCL/LTI

Harada, 2007, 2008) and therefore the fracture energy (Takayama, 2006a, 2006b).

isocyanate group of LTI creates urethane bond:

**0**

Fig. 14. Dependence of LTI content on the molecular weight.

**1**

*M*

**X105**

**[gmol-1]**

*w*

**2**

**3**

**4**

Fig. 11. FE-SEM micrographs of crack-tip region of PLA/PCL.

Fig. 12. FE-SEM micrograph of fracture surface of PLA/PCL.

In summary, it was shown that the fracture energy of PLA can be improved by blending with PCL with unchanged biocompatible and bioabsorbable characteristics. This improvement is considered to be achieved by stress relaxation and energy dissipation mechanisms such as extensive multiple craze formation of continuous phase and creation of extended fibril structures of dispersed phase. It is important to note that PLA/PCL exhibited phase separation due to incompatibility of two components, and created voids owing to removal of dispersed PCL phase. Those voids increased with increase of PCL content. PLA/PCL exhibited craze-like deformation of continuous phase similar to neat PLA during mode I fracture process, however, the size of the damage zone was much larger than the PLA, corresponding to the higher *Gin*. PLA/PCL also showed creation of voids by PCL phase separation within the fracture process region, and these voids were likely to be extended at lower stress level, and therefore, decrease *Gin* due to local stress concentration.
