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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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PLDLLA. Left: Yield stress versus applied strain rate. Right: Applied stress versus time-to-failure. Lines are drawn using Eqs. 3 and 4, with the parameters listed in Table 2
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Fig7: PLDLLA. Left: Yield stress versus applied strain rate. Right: Applied stress versus time-to-failure. Lines are drawn using Eqs. 3 and 4, with the parameters listed in Table 2

Mentions: Figures 6, 7 and 8 show the yield stress versus the applied strain rate (left) and applied stress versus the time-to-failure (right) for all three polylactides at different temperatures. The lines drawn in these figures are obtained by fitting Eqs. 3 and 4 to the data of all three polylactides simultaneously, by means of a least squares approach, resulting in a single parameter set, see Table 2. The only parameter allowed to vary for each material is the absolute reference strain rate, , which depends on the thermomechanical history of the material and the relative undercooling with respect to Tg [19]. The resulting values for are 2.33 × 1022, 1.64 × 1023 and 4.69 × 1022 s−1 for the PLLA, PLDLLA and PDLLA, respectively. The resulting fits are found to describe the experimental data quite well. The fact that the rate determining parameters for these materials are, within experimental error, the same, corresponds well to observations in other studies in which the apparent activation energy of the glass transition temperature is found to be the same [35, 47]. Yielding of polymers is often regarded as the mechanical equivalent of the glass transition, i.e. the influence of physical aging on the increase in yield stress and increase in enthalpy recovery have been shown to be proportionally related [48, 49], and the kinetics of both processes may, therefore, be expected to at least show the same qualitative behavior.Fig. 6


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

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

PLDLLA. Left: Yield stress versus applied strain rate. Right: Applied stress versus time-to-failure. Lines are drawn using Eqs. 3 and 4, with the parameters listed in Table 2
© Copyright Policy
Related In: Results  -  Collection

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

Fig7: PLDLLA. Left: Yield stress versus applied strain rate. Right: Applied stress versus time-to-failure. Lines are drawn using Eqs. 3 and 4, with the parameters listed in Table 2
Mentions: Figures 6, 7 and 8 show the yield stress versus the applied strain rate (left) and applied stress versus the time-to-failure (right) for all three polylactides at different temperatures. The lines drawn in these figures are obtained by fitting Eqs. 3 and 4 to the data of all three polylactides simultaneously, by means of a least squares approach, resulting in a single parameter set, see Table 2. The only parameter allowed to vary for each material is the absolute reference strain rate, , which depends on the thermomechanical history of the material and the relative undercooling with respect to Tg [19]. The resulting values for are 2.33 × 1022, 1.64 × 1023 and 4.69 × 1022 s−1 for the PLLA, PLDLLA and PDLLA, respectively. The resulting fits are found to describe the experimental data quite well. The fact that the rate determining parameters for these materials are, within experimental error, the same, corresponds well to observations in other studies in which the apparent activation energy of the glass transition temperature is found to be the same [35, 47]. Yielding of polymers is often regarded as the mechanical equivalent of the glass transition, i.e. the influence of physical aging on the increase in yield stress and increase in enthalpy recovery have been shown to be proportionally related [48, 49], and the kinetics of both processes may, therefore, be expected to at least show the same qualitative behavior.Fig. 6

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