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Lysine tri-isocyanate (LTI) was added to a biodegradable polymer blend of poly(lactic acid) (PLA) and poly (e-caprolacton) (PCL) under melt-mixing condition in order to improve the miscibility of PLA and PCL. It was found that the mode I fracture energy of the polymer blend effectively increases with increase of LTI content. Microscopic examination also showed that the size of PCL phase decreases due to LTI addition, leading to the reduction of void formation and suppression of local stress concentration, thus the improvement of the fracture energy. The improved miscibility also contributes to the ductility enhancement, which further increases the fracture energy.

The effects of the additive amount of a chain extender, with a weight ratio of 3–5 %, on stress-strain curves of poly(lactic acid) (PCL) and poly(butylene succinate-co-l-lactate) (PBSL) polymer blends were examined with a mixing ratio of PLA to PBSL of 70/30. The additive increased the elongation at break, and the strain rate decreased elongation at break.

PLA/PCL and PLA/PCL/LTI blends were developed to improve the fracture properties of biodegradable PLA. LTI was blended to improve the miscibility of PLA and PCL. It was shown that the fracture toughness values were dramatically increased due to LTI addition. SEM results also exhibited that PCL spherulites decreases due to LTI addition and therefore, void formation is reduced and local stress concentration is suppressed, resulting in the improvement of the toughness values. The improved miscibility is also closely related to the enhancement of ductile deformation; as a result, the fracture toughness is increased.

Poly( l -lactide) (PLLA) has actively been used as a biomaterial for resorbable bone fixation devices for use in orthopedic and oral surgeries. Recently, in order to improve the fracture properties of brittle PLLA, polymer blends of PLLA and a ductile bioabsorbable polymer, poly(e-caprolactone) (PCL), have been developed. The aim of the present study is to elucidate details of the fracture behavior and toughening mechanisms of PLLA/PCL blends. PLLA/PCL blends with different PCL contents were developed, and the critical energy release rate at crack initiation, Gin, was then measured to assess the effect of PCL content. It was shown that Gin is dramatically improved by blending PCL with PLLA, and the maximum 51% of increase of Gin is acheived with 5 wt% of PCL. Polarizing optical microscopy (POM) and scanning electron microscopy (SEM) of crack growth behavior were also performed to characterize the fracture mechanism. PLLA/PCL showed multiple craze formation in the crack-tip region, and elongated fibrils and voids construct the crazes. SEM of fracture surface also indicated that stretched fibril structures are formed on the surface as a result of elongation of PCL spherulites under high tensile stress condition in the crack-tip region. Thus, these damage formations are considered to be the primary energy dissipation mechanisms that resulted in the improvement of fracture energy.

Lysine triisocyanate (LTI) was used to blends of poly(lactide) (PLA) with poly(e-caprolactone) (PCL) to improve the immiscibility of the two different kinds of biodegradable polymers. Fracture properties such as the J-integral at initiation, Jin, and the total fracture energy, Jf, of PLA/PCL and PLA/PCL/LTI were evaluated, and compared the results to assess the effectiveness of LTI additive on improvement of the fracture properties. Fracture surfaces were also examined using a scanning electron microscope (SEM) to characterize fracture mechanism. The results of the SEM observation exhibited that the size of PCL spherulites became smaller by adding LTI, and therefore void formation was inhibited, resulting in improvement of the fracture properties.

P(LLA-CL) copolymer was additionally blended to PLLA/PCL polymer blend to improve the immiscibility between PLLA and PCL. Mechanical properties such as the bending modulus, strength and the maximum J-integral were then measured and the phase morphology and fracture surface were observed using FE-SEM. Crystallinity values of PLLA and PCL were also evaluated using DSC. It is seen that those mechanical properties successfully increased due to P(LLA-CL) addition. The results of FE-SEM observation and crystallinity measurement suggests that the immiscibility is improved by P(LLA-CL) blending and as a result, spherulites of PCL dispersed in the blend are reduced, resulting in effective increase of ductile deformation. It is thought that this kind of phase structural change causes the improvement of the mechanical properties.

Attempts have been made to improve the impact resistance of biodegradable thermoplastic polymer, PLA. A ductile biodegradable polymer, PCL, has been used to improve such property of PLA by using blending technique. Details of the impact fracture properties and mechanisms of PLA/PCL blends, however, have not fully been understood yet. Recently, it was also found that LTI can improve the immiscibility between PLA and PCL. In this study, PLA/PCL and PLA/PCL/LTI blends were prepared, and their impact fracture toughness values were measuredto assess the effect of PCL content and LTI addition on the impact resistance. Fracture mechanisms of the polymer blends were also characterized by scanning electron microscopy.

Recent years, considerable attention has been paid to biodegradable polymers, mainly owing to increasing interest for preservation of environment and substitution of petrochemical polymers. Poly (lactic acid) (PLA), one of typical biodegradable polymers, is being considered for use in a variety of industrial fields including car, computer and electric appliances. PLA is also utilized as a biomaterial in medical fields such as orthopedics and oral surgery [1, 2], due to bioabsorbability and biocompatibility. Fracture properties and fracture behavior of PLA have been studied, and it was found that crazes are formed in crack-tip region prior to crack initiation, that is similar to the fracture behavior of brittle polymers such as PP and PS [3–7]. Improvement of the fracture properties of such brittle polymers can generally be achieved by blending a ductile secondary phase to the base polymer. Poly (e-caprolacton) (PCL), a ductile biodegradable polymer, has been chosen as a blending partner for PLA [8–14], and the fracture properties of PLA/PCL was found to be greater than those of neat PLA [9]. However, it was also found that the immiscibility of PLA and PCL causes phase separation, and tends to lower the fracture properties especially when PCL content increases. In the present study, lysine triisocyanate (LTI) was used as an additive for PLA/PCL blend to improve such immiscibility. Impact fracture properties of PLA/PCL/LTI blend was evaluated and compared to those of PLA/PCL blend to assess the effectiveness of LTI addition. Effect of LTI addition on fracture micromechanism was also investigated by observing their fracture surfaces using a scanning electron microscope (SEM). PLA/PCL and PLA/PCL/LTI blends were fabricated from PLA pellets (Lacty #9030, Shimadzu Co. Ltd), PCL pellets (CelgreenH7, Daicel Chemistry Industries Co.) and LTI (Kyowa Co. Ltd) by melt-mixing in a conventional melt-mixer at 180 C for 20 min at a rotor speed of 50 rpm. The mixing ratio of PLA and PCL was fixed at 85:15 in weight fraction, and the LTI content was chosen to be 1 wt% in this study. Plates of 2-mm thick were then fabricated from the mixtures using a hot press attached with water-cooling system. These mixtures were melted at 180 C and pressed at 30 MPa, and then quenched to room temperature using the cooling system. Single-edge-notchbend (SENB) specimens were prepared from these PLA/ PCL and PLA/PCL/LTI plates. The J-integral at crack initiation, Jin, and the average fracture energy, Jf, were evaluated at an impact rate of 1.4 m/s using an instrumented drop weight impact testing system with dynamic displacement measuring apparatus [15, 16]. Jin and Jf were calculated using the following formulae

ポリ乳酸 (PLA) とポリ (e-カプロラクトン) (PCL) のブレンドを溶融混合法により作製し, 混合時にリジントリイソシアネート (LTI) を添加することで相構造を制御し破壊特性の改善を試みた. 破壊特性としてき裂進展開始時のエネルギー解放率Jinを計測し, またPLAの結晶化度 (xc, PLA) と重量平均分子量 (Mw) を測定した. LTI添加量の増加とともにJinとMwは増加し, xc, PLAは低下する傾向にあった. LTIをPLA/PCLブレンドに添加すると架橋反応が生じてMwが増大する一方で結晶化は抑制されxc, PLAは減少すると理解された. また, 巨視的な破壊エネルギーを決定する因子として球状PCLのサイズが考えられ, LTIを添加することによってPCLのサイズが減少し, 結果としてJinが向上したと考えられた.

Phase morphology of blends of biodegradable polymers, PLA and PCL, was controlled by introducing an additive, LTI, to improve the mechanical properties of the mixture. Fracture properties such as the J-integral at initiation Jin, and the total fracture energy Jf, of PLA/PCL blends with different LTI contents were evaluated, and were compared to assess the effect of LTI content on these properties. Both Jin and Jf values were dramatically improved by LTI addition. Fracture surfaces were also examined using SEM to characterize the effect of LTI addition on the fracture mechanism. It was confirmed that the size of PCL spherulites became smaller due to the activation of LTI as a surface-active agent. As a result, void formation was inhibited, and plastic deformation to be larger, resulting the improvement of fracture properties.

Poly (lactic acid) (PLA), made from natural resources such as starch of plants, is one of typical biodegradable thermoplastic polymers and has extensively been used in medical fields such as orthopedics, neurosurgery and oral surgery as bone fixation devices mainly due to biocompatibility and bioabsorbability (Higashi et al., 1986; Ikada et al., 1996; Middleton & Tipton, 2000; Mohanty, 2000). Its importance has led to many studies on its mechanical properties and fracture behavior which found that the mode I fracture behavior of PLA is relatively brittle in nature (Todo et al., 2002; Park et al., 2004, 2005, 2006). Therefore, blending with a ductile biodegradable and bioabsorbable polymer such as poly (e-caprolacton) (PCL) has been adopted to improve the fracture energy of brittle PLA (Broz et al., 2003; Dell’Erba et al., 2001; Chen et al., 2003; Todo et al., 2007; Tsuji & Ikada, 1996, 1998; Tsuji & Ishizuka, 2001; Tsuji et al., 2003); however, it was also found that phase separation originated by immiscibility of PLA and PCL tends to degrade the mechanical properties of PLA/PCL blends (Todo et al., 2007). It has recently been found that such phase separation can dramatically be improved by using an isocyanate group, lysine tri-isocyanate (LTI) (Takayama et al., 2006; Takayama & Todo, 2006; Harada et al., 2007, 2008), and the fracture properties of PLA/PCL/LTI are much higher than those of PLA/PCL. In this chapter, firstly the fracture behavior and micromechanism of pure PLA are summarized (Park et al., 2004, 2005, 2006; Todo et al., 2002). Effects of crystallization behavior and loading-rate on the mode I fracture behavior are discussed. Effect of unidirectional drawing on the fracture energy is also presented as one of the effective ways to improve the brittleness of PLA (Todo, 2007). Secondly, the fracture behavior of PLA/PCL blends is discussed on the basis of the relationship between the microstructure and the fracture property (Todo et al., 2007b). In the third section, improvement of microstructural morphology of PLA/PCL by using LTI is discussed (Takayama et al., 2006; Takayama & Todo, 2006; Todo & Takayama, 2007; Todo et al., 2007a). It has been found that addition of LTI effectively improves the phase morphology of PLA/PCL, resulting in dramatic improvement of fracture energy. Effects of annealing on the mechanical properties of PLA/PCL/LTI blend are discussed in the last section (Takayama et al., 2011). It has been found that a thermal annealing process can effectively improve the mechanical properties of the polymer blend, as a result of strengthened structures due to crystallization of PLA.

The effects of annealing on the mechanical properties of polymer blends of poly(lactic acid) (PLA) and poly(e-caprolactone) (PCL) were investigated. The bending strength and modulus of PLA/PCL tend to increase due to crystallization of the PLA phase by annealing. The mode I fracture energy, J(in), of PLA/PCL decreases dramatically due to the suppression of the ductile deformation of the spherical PCL phase by annealing. The immiscibility of PLA and PCL can be improved by adding lysine triisocyanate (LTI) as a result of additional polymerization. The phase transformation due to LTI addition reduces the size of the spherical PCL phase, resulting in higher fracture energy. An annealing process applied to PLA/PCL/LTI further strengthens the microstructure, resulting in effective improvement of the fracture energy.

Poly(lactic acid) (PLA) has been used as a biomaterial for bone fixation devices in oral and orthopedic surgery because of good biocompatibility and bioabsorbability. Because Poly(e-Caprolactone) (PCL) is a ductile, bioabsorbable and biodegradable polymer, many types of PLA/ PCL polymer blends have been developed to improve its material strength and impact resistance. The stress-strain curves of PLA and PLA/PCL polymer blends were measured using a split Hopkinson pressure bar (Kolsky bar) method and a universal testing machine. The effect of PCL content on Young's modulus and yield stress was examined. The values of constants in Cowper-Symonds equation with respect to yield stress were determined for PLA and PLA/PCL polymer blend specimens. PLA/PCL specimens were observed using a scanning electron microscope. Godkänd; 2009; 20090912 (parj)