Effect of crystallinity and loading-rate on mode I fracture behavior of poly(lactic acid)
Introduction
Poly(lactic acid) (PLA), a biodegradable polymer, has widely been used in industrial applications such as interior parts of automobiles, exterior parts of electric devices and food packages. Recently, PLA is also considered to be used as a biomaterial mainly due to its bioabsorbability and biocompatibility. For example, it has been used for bone fixation devices in orthopedic and oral surgeries. Since bioabsorbability is one of the key factors for PLA medical devices, some of the studies have mainly focused on the degradation behavior in vivo and in vitro [1], [2], [3], [4], [5]. Some fundamental studies were also performed to characterize the crystallization behavior and mechanical properties of thin PLA films [6], [7], [8], [9]. Although such PLA implants are in bulk forms such as plate, rod and screw, a few studies have been performed to characterize the mechanical behavior of PLA solids [10], [11]. In addition, it is reported that fracture of PLA implants sometimes occurs in medical application, while the fracture behavior of PLA has not sufficiently been understood yet.
The aim of this study is to characterize the effects of crystallization and loading-rate on the mode I fracture toughness and mechanism of PLA. Annealing was performed to control the microstructure of the PLA samples. Microstructures were then observed using a polarizing microscope (POM). Thermal and dynamic mechanical properties were measured by a differential scanning calorimetry (DSC) and a dynamic mechanical analyser (DMA), respectively. Mode I fracture tests were performed to measure the critical strain energy release rate as a mode I fracture toughness under static and impact loading-rates. Crack growth behaviors and fracture surfaces were also observed using POM and a scanning electron microscope (SEM) to characterize the fracture mechanisms, and the macroscopic fracture toughness values were then correlated with the microscopic mechanisms.
Section snippets
Material and specimen
PLA pellets (Lacty#9030) were supplied by Shimadzu Co., Ltd as test sample. The content of L-form and the weight average molecular weight are 95% and 140,000 g mol−1, respectively. Thermal properties of the pellets such as the glass transition, the melting and the crystallization temperature are 64, 167 and 123 °C, respectively. Plates of 5 mm thick were fabricated using a hot press installed with a water cooling system. Those Pellets were heated and melted at 180 °C, pressed for 30 min at 30 MPa and
Crystallinity and spherulite size
Effects of annealing conditions, temperature and time, on the crystallinity, Xc, evaluated using Eq. (1) are shown in Fig. 2. The lowest value of Xc is about 2.7% for the quenched sample, and Xc of the annealed samples increases with increase of annealing time and temperature. The highest Xc is about 56% for the 100 °C–24 h sample.
POM microphotographs of the microstructures are shown in Fig. 3. It is clearly observed that the density and size of spherulites increase with increase of annealing
Conclusions
PLA samples were annealed under several conditions to obtain different microstructures with varying spherulite size and density. Dynamic mechanical analysis was then performed to assess the effect of crystallization on dynamic viscoelastic properties. Mode I fracture testing of the PLA specimens was carried out to measure the critical strain energy release rate GIC as the mode I fracture toughness. Polarizing and scanning electron microscopies were also conducted to characterize the fracture
References (25)
- et al.
Polymer
(1981) - et al.
Biomaterials
(1992) - et al.
Biomaterials
(1997) - et al.
Polymer
(1980) - et al.
Polymer
(1995) - et al.
Polymer
(1998) Pure Appl Chem
(1974)- et al.
J Mater Sci, Mater Med
(1990) - et al.
Biomaterials
(1993) - et al.
Macromolecules
(1990)