**4. Toughness improvement of PLA/PCL blend**

#### **4.1 Effect of LTI**

As shown in the above section, blending with PCL successfully improved the fracture energy of brittle PLA. It was, however, also found that the immiscibility of PLA and PCL causes phase separation, and tends to lower the fracture energy especially when PCL content increases. It has recently been found that the addition of lysine tri-isocyanate (LTI)

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.

As shown in the above section, blending with PCL successfully improved the fracture energy of brittle PLA. It was, however, also found that the immiscibility of PLA and PCL causes phase separation, and tends to lower the fracture energy especially when PCL content increases. It has recently been found that the addition of lysine tri-isocyanate (LTI)

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

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

**4. Toughness improvement of PLA/PCL blend** 

**4.1 Effect of LTI** 

to PLA/PCL blends effectively improves their immiscibility (Takayama, 2006a, 2006b; Harada, 2007, 2008) and therefore the fracture energy (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 isocyanate group of LTI creates urethane bond:

#### HO-R' + R-N=C=O → R-NHCOO-R'

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

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

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

Fracture Mechanisms of Biodegradable PLA and PLA/PCL Blends 385

**0 0.5 1 1.5 2**

**PLA**

**LTI content [phr]**

(a)LTI: 0.5 phr

FE-SEM micrographs of fracture surfaces of PLA, PLA/PCL and PLA/PCL/LTI are shown in Fig. 18. The fracture surface of PLA is very smooth, corresponding to a brittle fracture behavior with low fracture energy. The surface roughness increases with the existence of elongated PCL and cavities by PCL blending. These cavities are thought to be created by debonding of the PCL-rich phases from the surrounding PLA matrix phase and usually cause local stress concentration in the surrounding regions. Thus, this kind of cavitation tends to lower the fracture energy because of the local stress concentration, and

 (b)LTI:1 phr (c)LTI:2phr Fig. 17. Poralized micrographs of crack growth behaviors in PLA/PCL/LTI.

**0**

Fig. 16. Dependent of LTI content on the fracture energy, *J*in.

**2**

**4**

*Jin* **[kJ/m2**

**]**

**6**

**8**

keep unchanged up to 1phr of LTI content, and then rapidly increase up to 2phr, suggesting that the chemical reaction between the hydroxyl groups of PLA and PCL and the isocyanate groups of LTI was promoted by addition of LTI more than 1 phr. This microstructural change in molecular level due to LTI addition strongly support the macroscopic improvement of the fracture energy.

Dependence of LTI content on the crystallinity of PLA, *x*c,PLA, is shownIn Fig.15..*x*c,PLA of PLA/PCL and PLA/PCL/LTI with 0.5phr of LTI are higher than that of PLA..It is considered that the crystallization of PLA in the blends was progressed actively more than in neat PLA. With increase of LTI content, *x*c,PLA decreases rapidly at 1 phr and this value is slightly lower than that of neat PLA. These results support that the phase separation between PLA and PCL is improved dramatically at 1 phr. It is presumed that the chemical reation between LTI and PLA/PCL during melt-mixing process results in the improvement of miscibility, and therefore the mobility of PLA and PCL molecules during solidification in cooling process is reduced, resulting in the reduction of crystallization.

Fig. 15. Dependence of LTI content on crystallinity of PLA in PLA/PCL and PLA/PCL/LTI.

Dependence of LTI content on *Jin* is shown in Fig. 16. It is seen that *Jin* of PLA/PCL is a little larger than that of PLA, indicating the effectiveness of PCL blend on *Jin* is very low. *Jin* of PLA/PCL is effectively improved by LTI addition, and *Jin* increases with increase of LTI content up to 1.5 phr. There is no difference of *Jin* between 2 phr and 1 phr of LTI addition, suggesting that the improvement of *Jin* is saturated with about 1.5 phr of LTI.

Poralized micrographs of crack growth behaviors in PLA/PCL/LTI are shown in Fig.17. In PLA/PCL/LTI with 0.5 phr of LTI, craze-like features are still seen in the crack-tip region as also seen in neat PLA (Fig.7(a)) and PLA/PCL blend (Fig.10). The number of the craze-lines is obviously decreased due to LTI addition. With higher content of LTI, such craze-like feature is no longer generated, and instead, the crack-tip region is plastically deformed, very similar to the crack-tip deformation in ductile plastics and metal. It is known that this kind of plastic deformation dissipates more energy than the craze-like damage, resulting in the greater fracture energy. It is therefore thought that LTI addition to PLA/PCL dramatically changes the crack-tip deformation mechanism; as a result, *Jin* is greatly improved.

keep unchanged up to 1phr of LTI content, and then rapidly increase up to 2phr, suggesting that the chemical reaction between the hydroxyl groups of PLA and PCL and the isocyanate groups of LTI was promoted by addition of LTI more than 1 phr. This microstructural change in molecular level due to LTI addition strongly support the macroscopic

Dependence of LTI content on the crystallinity of PLA, *x*c,PLA, is shownIn Fig.15..*x*c,PLA of PLA/PCL and PLA/PCL/LTI with 0.5phr of LTI are higher than that of PLA..It is considered that the crystallization of PLA in the blends was progressed actively more than in neat PLA. With increase of LTI content, *x*c,PLA decreases rapidly at 1 phr and this value is slightly lower than that of neat PLA. These results support that the phase separation between PLA and PCL is improved dramatically at 1 phr. It is presumed that the chemical reation between LTI and PLA/PCL during melt-mixing process results in the improvement of miscibility, and therefore the mobility of PLA and PCL molecules during solidification in

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

Fig. 15. Dependence of LTI content on crystallinity of PLA in PLA/PCL and PLA/PCL/LTI. Dependence of LTI content on *Jin* is shown in Fig. 16. It is seen that *Jin* of PLA/PCL is a little larger than that of PLA, indicating the effectiveness of PCL blend on *Jin* is very low. *Jin* of PLA/PCL is effectively improved by LTI addition, and *Jin* increases with increase of LTI content up to 1.5 phr. There is no difference of *Jin* between 2 phr and 1 phr of LTI addition,

Poralized micrographs of crack growth behaviors in PLA/PCL/LTI are shown in Fig.17. In PLA/PCL/LTI with 0.5 phr of LTI, craze-like features are still seen in the crack-tip region as also seen in neat PLA (Fig.7(a)) and PLA/PCL blend (Fig.10). The number of the craze-lines is obviously decreased due to LTI addition. With higher content of LTI, such craze-like feature is no longer generated, and instead, the crack-tip region is plastically deformed, very similar to the crack-tip deformation in ductile plastics and metal. It is known that this kind of plastic deformation dissipates more energy than the craze-like damage, resulting in the greater fracture energy. It is therefore thought that LTI addition to PLA/PCL dramatically

suggesting that the improvement of *Jin* is saturated with about 1.5 phr of LTI.

changes the crack-tip deformation mechanism; as a result, *Jin* is greatly improved.

**PLA** 

cooling process is reduced, resulting in the reduction of crystallization.

**0**

**5**

**10**

*xc,PLA* **[%]**

**15**

improvement of the fracture energy.

Fig. 16. Dependent of LTI content on the fracture energy, *J*in.

Fig. 17. Poralized micrographs of crack growth behaviors in PLA/PCL/LTI.

FE-SEM micrographs of fracture surfaces of PLA, PLA/PCL and PLA/PCL/LTI are shown in Fig. 18. The fracture surface of PLA is very smooth, corresponding to a brittle fracture behavior with low fracture energy. The surface roughness increases with the existence of elongated PCL and cavities by PCL blending. These cavities are thought to be created by debonding of the PCL-rich phases from the surrounding PLA matrix phase and usually cause local stress concentration in the surrounding regions. Thus, this kind of cavitation tends to lower the fracture energy because of the local stress concentration, and

Fracture Mechanisms of Biodegradable PLA and PLA/PCL Blends 387

effectively improved. However, blending of ductile PCL with PLA degrades another mechanical properties such as strength and elastic modulus of the base polymer PLA. Recently, Tsuji et al. found that the mechanical properties such as elastic modulus and tensile strength of PLA could be improved by annealing (Tsuji et al., 1995). It is thus expected that annealing process may also affect on the mechanical properties of

FE-SEM micrographs of cryo-fracture surfaces of quenched and annealed PLA/PCL and PLA/PCL/LTI are shown in Fig.19. Both the quenched and the annealed PLA/PCL show spherical structures of PCL. It is obviously seen that PCL spherulites in PLA/PCL/LTI are much smaller than those in PLA/PCL. The annealed blends exhibit rougher surfaces with spherical structures than the quenched samples. These spherical structures are thought to be

annealing on the crystallinity, *xc,PLA*, and the molecular weight, *Mw*, are also shown in Table 1. It is clearly seen that *xc,PLA* dramatically increases due to annealing. Thus, increasing *xc,PLA* is likely to strengthen the structure of the polymer, resulting in the increase of *E* and

of PLA/PCL increases slightly due to LTI, suggesting that LTI is thought to generate

σ

σ

*<sup>f</sup>* increase due to annealing. Effects of

*<sup>f</sup>*, under three-point

σ*<sup>f</sup>*. *Mw*

 (c) Quenched PLA/PCL/LTI (d) Annealed PLA/PCL/LTI Fig. 19. FE-SEM micrographs of microstructures of PLA/PCL and PLA/PCL/LTI.

Effects of annealing on the elastic modulus, *E*, and the strength,

bending condition are shown in Fig.20. Both *E* and

the spherulites of PLA generated by crystallization during annealing process.

(a) Quenched PLA/PCL (b) Annealed PLA/PCL

PLA/PCL/LTI blend.

compensates to the increase of fracture energy due to the ductile deformation of PCL. This is the reason for the slight improvement of *Jin* in PLA/PCL shown in Fig. 16. It is clearly seen from Fig. 18(c) that cavities do not exist on the fracture surface of PLA/PCL/LTI, indicating that the miscibility of PLA and PCL improves due to LTI addition. In addition, elongated structures are more on PLA/PCL/LTI than PLA/PCL. Thus, extensive ductile deformation associated with disappearance of cavitation is the primary mechanism of the dramatic improvement of *Jin*.

Fig. 18. FE-SEM microgprahs of fracture surfaces of PLA, PLA/PCL and PLA/PCL/LTI.

In summary, the miscibility between PLA and PCL is dramatically improved by introducing LTI as an additive. The increase of molecular weight and the decrease of crystallinity with increase of LTI content clearly indicate that crosslinks are generated by urethane bonds in which the hydroxyl groups at the ends of PLA and PCL molecules react with the isocyanate groups of LTI during molding process. Such microstructural modification results in the dramatic improvement of the macroscopic fracture property, *Jin*.

### **4.2 Effect of annealing process on PLA/PCL/LTI**

As described in the previous section, the immiscibility of PLA/PCL can be improved by adding LTI as a compatibilizer, and as a result, the fracture energy of PLA/PCL is

compensates to the increase of fracture energy due to the ductile deformation of PCL. This is the reason for the slight improvement of *Jin* in PLA/PCL shown in Fig. 16. It is clearly seen from Fig. 18(c) that cavities do not exist on the fracture surface of PLA/PCL/LTI, indicating that the miscibility of PLA and PCL improves due to LTI addition. In addition, elongated structures are more on PLA/PCL/LTI than PLA/PCL. Thus, extensive ductile deformation associated with disappearance of cavitation is the primary mechanism of the dramatic

(c)PLA/PCL/LTI Fig. 18. FE-SEM microgprahs of fracture surfaces of PLA, PLA/PCL and PLA/PCL/LTI.

In summary, the miscibility between PLA and PCL is dramatically improved by introducing LTI as an additive. The increase of molecular weight and the decrease of crystallinity with increase of LTI content clearly indicate that crosslinks are generated by urethane bonds in which the hydroxyl groups at the ends of PLA and PCL molecules react with the isocyanate groups of LTI during molding process. Such microstructural modification results in the

As described in the previous section, the immiscibility of PLA/PCL can be improved by adding LTI as a compatibilizer, and as a result, the fracture energy of PLA/PCL is

(a)PLA (b)PLA/PCL

dramatic improvement of the macroscopic fracture property, *Jin*.

**4.2 Effect of annealing process on PLA/PCL/LTI** 

improvement of *Jin*.

effectively improved. However, blending of ductile PCL with PLA degrades another mechanical properties such as strength and elastic modulus of the base polymer PLA. Recently, Tsuji et al. found that the mechanical properties such as elastic modulus and tensile strength of PLA could be improved by annealing (Tsuji et al., 1995). It is thus expected that annealing process may also affect on the mechanical properties of PLA/PCL/LTI blend.

FE-SEM micrographs of cryo-fracture surfaces of quenched and annealed PLA/PCL and PLA/PCL/LTI are shown in Fig.19. Both the quenched and the annealed PLA/PCL show spherical structures of PCL. It is obviously seen that PCL spherulites in PLA/PCL/LTI are much smaller than those in PLA/PCL. The annealed blends exhibit rougher surfaces with spherical structures than the quenched samples. These spherical structures are thought to be the spherulites of PLA generated by crystallization during annealing process.

(a) Quenched PLA/PCL (b) Annealed PLA/PCL

Fig. 19. FE-SEM micrographs of microstructures of PLA/PCL and PLA/PCL/LTI.

Effects of annealing on the elastic modulus, *E*, and the strength, σ*<sup>f</sup>*, under three-point bending condition are shown in Fig.20. Both *E* and σ*<sup>f</sup>* increase due to annealing. Effects of annealing on the crystallinity, *xc,PLA*, and the molecular weight, *Mw*, are also shown in Table 1. It is clearly seen that *xc,PLA* dramatically increases due to annealing. Thus, increasing *xc,PLA* is likely to strengthen the structure of the polymer, resulting in the increase of *E* and σ*<sup>f</sup>*. *Mw* of PLA/PCL increases slightly due to LTI, suggesting that LTI is thought to generate

Fracture Mechanisms of Biodegradable PLA and PLA/PCL Blends 389

suppression of ductile deformation of the PCL spherulites. This is considered to be the

It is clearly seen from Figs.22(c) and (d) that in the PLA/PCL/LTI blends, cavity formation is totally suppressed and as a result, ductile deformation is expanded due to the improved miscibility of PLA and PCL by LTI addition. This implies that the miscibility of PLA and PCL is improved by crosslinking of PLA and PCL macromolecules induced by the chemical reaction between the hydroxyl group of PLA and PCL and the isocyanate group of LTI. FE-SEM micrographs at higher magnification show that for both the quenched and annealed PLA/PCL/LTI blends, entangled fibril structures of PLA and PCL are observed. It is thus considered that this kind of structural transformation due to polymerization by LTI blending results in strengthening the structure of the PLA/PCL blends. The microstructure of PLA/PCL/LTI is thought to be further strengthened due to crystallization of PLA by annealing, resulting in the dramatic improvement of the mode I fracture energy *Jin* as shown

primary reason for the degradation of *Jin* as shown in Fig.21.

Fig. 21. Effects of annealing on the critical J-integral at crack intiation, *Jin*.

 (a) Quenched PLA/PCL

in Fig.22.

additional polymerization with hydroxyl and carboxyl groups lying in the ends of PLA or PCL molecules. This polymerization results in the improvement of miscibility of PLA and PCL as shown in Fig.19(c). On the other hand, *Mw* of PLA/PCL decreases slightly due to annealing, indicating progression of thermal degradation in this blend. On the contrary, *Mw* of PLA/PCL/LTI slightly increases, suggesting that additional polymerization take place during annealing process.

Fig. 20. Effects of annealing on bending mechanical properties.


Table 1. Effects of annealing on the crystallinity and molecular weight.

Effects of annealing on the critical J-integral at crack initiation, *Jin*, are shown in Fig.21. It is clearly seen that *Jin* of PLA/PCL/LTI effectively increases due to annealing; on the contrary, PLA/PCL exhibites decrease of *Jin*.

FE-SEM micrographs of the fracture surfaces of the mode I fracture specimens are shown in Fig.22. By comparing Figs.22(a) and (b), it is clearly seen that ductile deformation of spherical PCL phase is suppressed by annealing. Cavities are also observed on the surface of PLA/PCL, as a result of removal of the spherical PCL phases. FE-SEM micrographs at higher magnification show that elongated structures of the spherical PCL phases are observed in the quenched PLA/PCL, while ruptured PLA fibrils and undeformed PCL spherulites are observed in the annealed PLA/PCL. It is also interesting to see in Fig.22(a) that some PCL fibrils are penetrated into the PLA phase and seem to be entangled with PLA fibrils. It is thought that the PLA phase creates a firm structure due to crystallization by annealing and therefore, entangled PCL fibrils with PLA fibrils are firmly trapped in the PLA phase. The PCL spherulites are thus surrounded by such firm crystallized PLA phase with entanglement of PLA and PCL fibrils, resulting in the rupture of the PCL fibrils and the

additional polymerization with hydroxyl and carboxyl groups lying in the ends of PLA or PCL molecules. This polymerization results in the improvement of miscibility of PLA and PCL as shown in Fig.19(c). On the other hand, *Mw* of PLA/PCL decreases slightly due to annealing, indicating progression of thermal degradation in this blend. On the contrary, *Mw* of PLA/PCL/LTI slightly increases, suggesting that additional polymerization take place

(a)Elastic modulus (b)Strength

Blend *Xc,PLA* (%) *Mw* (g/mol) Quenched PLA/PCL 11.4 1.03 x 105 Annealed PLA/PCL 45.6 9.08 x 104 Quenched PLA/PCL/LTI 4.8 1.13 x 105 Annealed PLA/PCL/LTI 36.6 1.52 x 105

Effects of annealing on the critical J-integral at crack initiation, *Jin*, are shown in Fig.21. It is clearly seen that *Jin* of PLA/PCL/LTI effectively increases due to annealing; on the contrary,

FE-SEM micrographs of the fracture surfaces of the mode I fracture specimens are shown in Fig.22. By comparing Figs.22(a) and (b), it is clearly seen that ductile deformation of spherical PCL phase is suppressed by annealing. Cavities are also observed on the surface of PLA/PCL, as a result of removal of the spherical PCL phases. FE-SEM micrographs at higher magnification show that elongated structures of the spherical PCL phases are observed in the quenched PLA/PCL, while ruptured PLA fibrils and undeformed PCL spherulites are observed in the annealed PLA/PCL. It is also interesting to see in Fig.22(a) that some PCL fibrils are penetrated into the PLA phase and seem to be entangled with PLA fibrils. It is thought that the PLA phase creates a firm structure due to crystallization by annealing and therefore, entangled PCL fibrils with PLA fibrils are firmly trapped in the PLA phase. The PCL spherulites are thus surrounded by such firm crystallized PLA phase with entanglement of PLA and PCL fibrils, resulting in the rupture of the PCL fibrils and the

Fig. 20. Effects of annealing on bending mechanical properties.

Table 1. Effects of annealing on the crystallinity and molecular weight.

PLA/PCL exhibites decrease of *Jin*.

during annealing process.

suppression of ductile deformation of the PCL spherulites. This is considered to be the primary reason for the degradation of *Jin* as shown in Fig.21.

It is clearly seen from Figs.22(c) and (d) that in the PLA/PCL/LTI blends, cavity formation is totally suppressed and as a result, ductile deformation is expanded due to the improved miscibility of PLA and PCL by LTI addition. This implies that the miscibility of PLA and PCL is improved by crosslinking of PLA and PCL macromolecules induced by the chemical reaction between the hydroxyl group of PLA and PCL and the isocyanate group of LTI. FE-SEM micrographs at higher magnification show that for both the quenched and annealed PLA/PCL/LTI blends, entangled fibril structures of PLA and PCL are observed. It is thus considered that this kind of structural transformation due to polymerization by LTI blending results in strengthening the structure of the PLA/PCL blends. The microstructure of PLA/PCL/LTI is thought to be further strengthened due to crystallization of PLA by annealing, resulting in the dramatic improvement of the mode I fracture energy *Jin* as shown in Fig.22.

Fig. 21. Effects of annealing on the critical J-integral at crack intiation, *Jin*.

(a) Quenched PLA/PCL

Fracture Mechanisms of Biodegradable PLA and PLA/PCL Blends 391

strengthen the microstructure, resulting in the dramatic improvement of the mode I fracture

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

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energy.

**5. Conclusion** 

**6. References** 

276.

4190.

7840.

degrade the fracture energy of PLA/PCL blends.

(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 strengthen the microstructure, resulting in the dramatic improvement of the mode I fracture energy.
