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Time-dependent failure of amorphous polylactides in static loading conditions.

Engels TA, Söntjens SH, Smit TH, Govaert LE - J Mater Sci Mater Med (2010)

Bottom Line: The phenomenon is common to all polymers, and finds its origin in stress-activated segmental molecular mobility leading to a steady rate of plastic flow.The stress-dependence of this flow-rate is well captured by Eyring's theory of absolute rates, as demonstrated on three amorphous polylactides of different stereoregularity.We show that the kinetics of the three materials are comparable and can be well described using the proposed modeling framework.The main conclusion is that knowledge of the instantaneous strength of a polymeric material is insufficient to predict its long-term performance.

View Article: PubMed Central - PubMed

Affiliation: Section Materials Technology (MaTe), Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands. l.e.govaert@tue.nl

ABSTRACT
Polylactides are commonly praised for their excellent mechanical properties (e.g. a high modulus and yield strength). In combination with their bioresorbability and biocompatibility, they are considered prime candidates for application in load-bearing biomedical implants. Unfortunately, however, their long-term performance under static load is far from impressive. In a previous in vivo study on degradable polylactide spinal cages in a goat model it was observed that, although short-term mechanical and real-time degradation experiments predicted otherwise, the implants failed prematurely under the specified loads. In this study we demonstrate that this premature failure is attributed to the time-dependent character of the material used. The phenomenon is common to all polymers, and finds its origin in stress-activated segmental molecular mobility leading to a steady rate of plastic flow. The stress-dependence of this flow-rate is well captured by Eyring's theory of absolute rates, as demonstrated on three amorphous polylactides of different stereoregularity.We show that the kinetics of the three materials are comparable and can be well described using the proposed modeling framework. The main conclusion is that knowledge of the instantaneous strength of a polymeric material is insufficient to predict its long-term performance.

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Thermal analysis of the polylactides investigated. Left: DSC. Right: DMTA
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Fig1: Thermal analysis of the polylactides investigated. Left: DSC. Right: DMTA

Mentions: Before discussing the phenomenology and modeling of the mechanical behavior of the amorphous polylactides, it is confirmed that the materials are fully amorphous by means of Differential Scanning Calorimetry (DSC) and Dynamical Mechanical Thermal Analysis (DMTA). Figure 1 (left) shows DSC heating traces measured at 10°C/min. The PLDLLA and PDLLA show no crystalline behavior at all and can be regarded 100% amorphous, which is as expected since the D-enantiomer content is 15% or more [20]. The stereoregular PLLA by contrast shows marked exo- and endo-thermic peaks indicating that crystallization does occur. The net value of the heat lost during crystallization and the heat regained during melting accounts to zero, indicating that no, or at least a negligible, crystalline fraction is present in the initial material and that the crystallization phenomena observed in the DSC experiments can be attributed to the experimental routine itself. This is confirmed by the DMTA experiments, performed only on the PLLA and PLDLLA, which show that the initial moduli of the two materials are identical within experimental uncertainty and the drop in moduli, upon passing the glass transition, are similar. Small fractions of crystallinity would be evidenced by an increase in the modulus both in the glassy and in the rubbery state [33]. Moreover the material was as transparent as the PLDLLA and PDLLA samples at the start of any experiment, another indication that the material is amorphous after compression molding. In accordance with the DSC experiments the glass transition temperature found by DMTA is higher for the PLLA, see Table 1. The upswing in modulus observed for the PLLA above its glass transition temperature (Tg) is a result of cold crystallization induced by the measurement itself and reported in literature [34, 35]. The higher glass transition temperature of the PLLA in comparison to the PLDLLA and PDLLA can be explained by an increased mobility of the PLDLLA chains due to their less stereo-regular buildup [34, 36].Fig. 1


Time-dependent failure of amorphous polylactides in static loading conditions.

Engels TA, Söntjens SH, Smit TH, Govaert LE - J Mater Sci Mater Med (2010)

Thermal analysis of the polylactides investigated. Left: DSC. Right: DMTA
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2805790&req=5

Fig1: Thermal analysis of the polylactides investigated. Left: DSC. Right: DMTA
Mentions: Before discussing the phenomenology and modeling of the mechanical behavior of the amorphous polylactides, it is confirmed that the materials are fully amorphous by means of Differential Scanning Calorimetry (DSC) and Dynamical Mechanical Thermal Analysis (DMTA). Figure 1 (left) shows DSC heating traces measured at 10°C/min. The PLDLLA and PDLLA show no crystalline behavior at all and can be regarded 100% amorphous, which is as expected since the D-enantiomer content is 15% or more [20]. The stereoregular PLLA by contrast shows marked exo- and endo-thermic peaks indicating that crystallization does occur. The net value of the heat lost during crystallization and the heat regained during melting accounts to zero, indicating that no, or at least a negligible, crystalline fraction is present in the initial material and that the crystallization phenomena observed in the DSC experiments can be attributed to the experimental routine itself. This is confirmed by the DMTA experiments, performed only on the PLLA and PLDLLA, which show that the initial moduli of the two materials are identical within experimental uncertainty and the drop in moduli, upon passing the glass transition, are similar. Small fractions of crystallinity would be evidenced by an increase in the modulus both in the glassy and in the rubbery state [33]. Moreover the material was as transparent as the PLDLLA and PDLLA samples at the start of any experiment, another indication that the material is amorphous after compression molding. In accordance with the DSC experiments the glass transition temperature found by DMTA is higher for the PLLA, see Table 1. The upswing in modulus observed for the PLLA above its glass transition temperature (Tg) is a result of cold crystallization induced by the measurement itself and reported in literature [34, 35]. The higher glass transition temperature of the PLLA in comparison to the PLDLLA and PDLLA can be explained by an increased mobility of the PLDLLA chains due to their less stereo-regular buildup [34, 36].Fig. 1

Bottom Line: The phenomenon is common to all polymers, and finds its origin in stress-activated segmental molecular mobility leading to a steady rate of plastic flow.The stress-dependence of this flow-rate is well captured by Eyring's theory of absolute rates, as demonstrated on three amorphous polylactides of different stereoregularity.We show that the kinetics of the three materials are comparable and can be well described using the proposed modeling framework.The main conclusion is that knowledge of the instantaneous strength of a polymeric material is insufficient to predict its long-term performance.

View Article: PubMed Central - PubMed

Affiliation: Section Materials Technology (MaTe), Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands. l.e.govaert@tue.nl

ABSTRACT
Polylactides are commonly praised for their excellent mechanical properties (e.g. a high modulus and yield strength). In combination with their bioresorbability and biocompatibility, they are considered prime candidates for application in load-bearing biomedical implants. Unfortunately, however, their long-term performance under static load is far from impressive. In a previous in vivo study on degradable polylactide spinal cages in a goat model it was observed that, although short-term mechanical and real-time degradation experiments predicted otherwise, the implants failed prematurely under the specified loads. In this study we demonstrate that this premature failure is attributed to the time-dependent character of the material used. The phenomenon is common to all polymers, and finds its origin in stress-activated segmental molecular mobility leading to a steady rate of plastic flow. The stress-dependence of this flow-rate is well captured by Eyring's theory of absolute rates, as demonstrated on three amorphous polylactides of different stereoregularity.We show that the kinetics of the three materials are comparable and can be well described using the proposed modeling framework. The main conclusion is that knowledge of the instantaneous strength of a polymeric material is insufficient to predict its long-term performance.

Show MeSH
Related in: MedlinePlus