Research Paper
Mechanical characterization and modelling of the temperature-dependent impact behaviour of a biocompatible poly(L-lactide)/poly(ε-caprolactone) polymer blend

https://doi.org/10.1016/j.jmbbm.2015.07.007Get rights and content

Highlights

  • The dynamic mechanical properties of PLLA/PCL have been studied at three temperatures.

  • A strain-rate and temperature dependent constitutive model is presented.

  • A validated numerical model for PLLA/PCL is provided.

Abstract

Poly(ε-caprolactone) (PCL) is a ductile, bioabsorbable polymer that has been employed as a blend partner for poly(L-lactic acid) (PLLA). An improvement of the material strength and impact resistance of PLLA/PCL polymer blends compared to pure PLLA has been shown previously. To use numerical simulations in the design process of new components composed of the PLLA/PCL blend, a constitutive model for the material has to be established. In this work, a constitutive model for a PLLA/PCL polymer blend is established from the results of compressive tests at high and low strain rates at three different temperatures, including the body temperature. Finite element simulations of the split Hopkinson pressure bar test using the established constitutive model are carried out under the same condition as the experiments. During the experiments, the changes in the diameter and thickness of the specimens are captured by a high-speed video camera. The accuracy of the numerical model is tested by comparing the simulation results, such as the stress, strain, thickness and diameter histories of the specimens, with those measured in the experiments. The numerical model is also validated against an impact test of non-homogenous strains and strain rates. The results of this study provide a validated numerical model for a PLLA/PCL polymer blend at strain rates of up to 1800 s−1 in the temperature range between 22 °C and 50 °C.

Introduction

For many applications, the use of environmentally friendly polymers is of great interest especially for biomechanical applications where there is a desire to use environmentally friendly and biocompatible materials with good mechanical properties. Introducing a new material usually requires that the material can be simulated numerically and that the mechanical behaviour can be described accurately by a constitutive model. The need for accurate models of biodegradable polymers is discussed in Vieira et al. (2013). A good constitutive model can be of great assistance in the design of new products and process simulations.

Poly(L-lactic acid) (PLLA) shows good biocompatibility and is decomposed and absorbed within living organisms. PLLA is widely used in the fields of orthopaedic and oral surgeries for bone fixation devices because they do not require a second surgery for removal (Mohanty et al., 2000). To overcome the brittleness and low impact strength of PLLA, its mechanical properties have been improved by blending with ductile polymers or reinforcing with natural fibres. Among them, poly(ε-caprolactone) (PCL) has been employed to be a prospective blend partner for PLLA because it is a ductile bio-absorbable polymer. Improvements in the material strength and impact resistance of PLLA/PCL polymer blends have been studied previously by Tsuji and Ikada (1996), Tsuji et al. (2003), Chen et al. (2003), Takayama and Todo (2006), Todo et al. (2007) and Takayama et al. (2011). Although good results regarding the impact resistance have been reported, in most cases, the impact resistances of PLLA/PCL polymer blends are only based on the experimental results of Izod impact strength tests or Charpy impact strength tests.

Constitutive modelling and finite element models of PLLA and PCL polymers can be found in e.g. (Douglas et al., 2015, Hwang and Todo, 2012, Muliana and Rajagopal, 2012). There are few studies regarding the basic mechanical properties of these polymer blends at high strain rates and body temperature. In a previous study, the strain rate dependence of PLLA/PCL polymer blend specimens was studied at room temperature (Nishida et al., 2009). To the authors׳ knowledge, there have been no constitutive models established for the actual PLLA/PCL polymer blend that take the temperature and strain rate in to account.

In the present study, specimens of a PLLA/PCL polymer blend with a mixing ratio (mass fraction) of 80:20 were used. To use numerical simulations in the future design of components, a constitutive model for the polymer blend was established. The stress–strain relations of the polymer blend at high and low strain rates were measured at 22 °C, 36 °C and 50 °C. The coefficients of a constitutive model for PLLA/PCL=80:20 in a modified Cowper–Symonds constitutive equation with strain rate sensitive and temperature dependent parameters were determined using the results of compressive tests at high and low strain rates. The finite element method (FEM) was used for simulations of the split Hopkinson pressure bar (Kolsky bar) experiments using the determined coefficients. The simulations were carried out under the same condition as the experiments. The stress, strain, thickness and diameter histories of the specimens in the simulations at high strain rates were compared with those in the experiments. In addition, an impact test with a non-homogenous strain rate was carried out to validate the numerical model.

Section snippets

Materials

Polymer blends of PLLA and PCL were prepared using PLLA from the Toyota Motor Co. (Eco plastic U׳zB-3) and PCL from Daicel Chemical Co. (Celgreen P-H7). Because PCL is a ductile, biodegradable polymer, there are many possibilities for good mechanical properties (for example, high impact strength) in polymer blends with PLLA. The chemical structural formulas of PLLA and PCL are shown in Fig. 1. The molecular weight (Mw) and PDI (Mw/Mn) of PLLA and PCL were 1.9×105, 2.0 and 1.2×105, 2.0

Material characterization

Material parameters for the strain rate and temperature-dependent Cowper–Symonds equation (3), are determined from the experimental tests. The true stress versus true strain curves for PLLA/PCL=80:20 at the three different temperatures are evaluated from the experiments by Eqs. (1), (2) and are shown in Fig. 4.

The flow stress increases with an increasing strain rate, as is commonly observed in most engineering polymers. The flow stress decreases with increasing temperature, as is also commonly

Conclusions

  • In the present work, the mechanical behaviour at both a low and high strain rate of a PLLA/PCL polymer blend with a mixing ratio 80:20 was investigated at three different temperatures.

  • The finite element method was used for the simulations of the SPHB (Kolsky bar) experiments together with the calibrated constitutive model for PLLA/PCL=80:20 mix.

  • A good agreement between the simulated stress, strain, thickness and diameter histories of the specimens with those measured in the experiments was

Acknowledgements

The authors are greatly indebted to nac Imaging Technology, Inc., for its help with taking images using a high-speed camera.

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