Fracture micromechanisms of bioabsorbable PLLA/PCL polymer blends

doi:10.1016/j.engfracmech.2006.05.021

Engineering Fracture Mechanics

Volume 74, Issue 12, August 2007, Pages 1872-1883

https://doi.org/10.1016/j.engfracmech.2006.05.021 Get rights and content

Abstract

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(ε-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, G_in, was then measured to assess the effect of PCL content. It was shown that G_in is dramatically improved by blending PCL with PLLA, and the maximum 51% of increase of G_in 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.

Introduction

Poly(l-lactide) (PLLA) has successfully been used as a polymeric biomaterial for bone fixation devices in oral and orthopedic surgeries, mainly owing to its bioabsorbability and biocompatibility. As the clinical application of such absorbable devices enlarges, it has been reported that sudden fracture of the devices often takes place under unexpected high stress condition in the human body [1], [2]. Recently, fracture properties and mechanisms of PLLA have extensively been investigated by the authors, and the effects of microstructure, loading-rate, hydrolysis and drawing have clarified [3], [4], [5], [6], [7].

Many attempts have recently been made to improve the mechanical properties of PLLA and polylactide (PLA) through fabrication of blends with other polymers and composite materials with clay or natural fibers [8], [9], [10], [11], [12], [13], [14], [15], [16]. For example, Yuan and Ruckenstein found that toughness of PLA can be improved by blending a proper amount of polyurethane and a proper extent of cross-linking [8]. Park and Im suggested that toughness of PLLA/starch blend can be improved by using gelatinized starch as dispersed phases mainly owing to improvement of the interfacial adhesion [10]. Anderson et al. were investigated blends of PLLA with linear low-density polyethylene (LLDPE) and PLA/LLDPE, and found that amorphous PLA/LLDPE can be toughened by using PLLA-PE block copolymer for compatibilization; on the other hand, for semicrystalline PLLA/LLDPE, such compatibilizer is not necessary for toughness improvement [12].

Poly(ε-caprolactone) (PCL) is another bioabsorbable polymer, and more ductile with low glass transition temperature, −60 °C, than PLLA. Polymer Blends of PLLA and PCL have been considered to improve the physical properties of PLLA, and then, the thermal and mechanical properties and morphologies of PLLA/PCL blends have been characterized [17], [18], [19], [20]. However, their fracture properties and fracture mechanisms were not dealt in these studies. Recently, it was shown that the blend of PCL is an effective way to improve the fracture toughness of PLLA [21].

In this study, PLLA/PCL blends with different PCL contents were fabricated to investigate the effects of PCL content on the fracture property, mechanism and microstructure of PLLA/PCL. Mode I fracture testing was performed to measure the mode I fracture property, and then, fracture mechanisms were characterized by polarizing optical microscopy (POM) and scanning electron microscopy (SEM). The macroscopic fracture property was then correlated with the microscopic structures and fracture mechanisms.

Section snippets

Materials

PLLA pellets of medical grade (Lacty^®#5000, Shimadzu Co., Ltd.) and PCL pellets (Celgreen PH7, Daicel Chemistry Industries Ltd.) were used for blending. The weight average molecular weights of the PLLA and PCL are 200,000 and 120,000 gmol⁻¹, the glass transition temperatures, 60 and −60 °C, and the melting temperatures 178 and 60 °C, respectively. These pellets were held into a desiccator to keep them dry and prevent from degradation due to hydrolysis due to moisture.

Mixtures of the PLLA and PCL

Crystallinity of PLLA

DSC thermograms of neat PLLA and PLLA/PCL blends are shown in Fig. 4. The peak around 70 °C observed in PLLA/PCL blends is recognized as the melting point of PCL. The glass transition of PLLA is also included in this region. The peak around 90 °C corresponds to the crystallization of PLLA. Enlargement of the peak in PLLA/PCL blends suggests that the crystallization of PLLA is activated by PCL blending. The peak around 180 °C corresponds to the melting point of PLLA. Crystallinity of PLLA was

Conclusions

PLLA/PCL polymer blend was developed to improve the initiation toughness of fracture of brittle PLLA, and fracture behavior and mechanisms of PLLA/PCL were studied by polarizing optical and scanning electron microscopies. The results obtained are as follows:

(1)
The energy release rate at crack initiation, G_in, of PLLA/PCL can be optimized with 5 wt% of PCL.
(2)
Morphological study showed that phase separation takes place due to the incompatibility of the two components, and spherical structures are

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Fracture micromechanisms of bioabsorbable PLLA/PCL polymer blends

Abstract

Introduction

Section snippets

Materials

Crystallinity of PLLA

Conclusions

J Arthrosc Relat Surg

J Cranio-Maxill Surg

Biomaterials

Polymer

Biomaterials

Polymer

Effects of Isothermal crystallization on fracture toughness and crack growth behavior of poly(lactic acid)

J Mater Sci

Effect of annealing on fracture mechanism of biodegradable poly(lactic acid)

Key Engng Mater

Effect of annealing on the fracture toughness of poly(lactic acid)