9 dl/g. The iv level was 0.65?0.55 dl/g when about 50% of selleck products the initial bending strength was left, indicating that the bending strength dropped at higher iv levels than did the shear strength. We may speculate that in order to resist 3-point bending stress a polymer needs to have longer molecular chains in its structure than is the case in shear stress. Figure 1. In vitro occurring strength retention of ?1.1?1.2 mm oriented PLLA, 96L/4D PLA, and 80L/20D,L PLA samples plotted against determined inherent viscosity value measured from the same specimens; shear strength (A) and bending … The results showed that for both the mechanical testing types, PLLA started to show strength loss at higher iv levels than the studied stereocopolymers.
However, we found that there were not enough data points where the rapid shear strength loss occurred at the inherent viscosity range of 0.75�C0.5 dl/g (equal to ca. 36�C44 weeks in vitro) to fully analyze the behavior of the PLLA specimens, particularly in the case of the shear strength values, (Fig. 1A). The observed difference most likely stems from differences between the morphologies of the PLLA and the PLA stereocopolymers. The more regular polymer chains of the homopolymer PLLA may also withstand mechanical stresses in a slightly different manner than those in the less regular stereocopolymer structures. The effect of the specimen��s inherent viscosity on strength retention behavior The effect that the initial iv of the processed specimens had on the strength retention time was analyzed using oriented ?4.0 mm 96L/4D PLA and 80L/20D,L PLA rods.
The analysis was conducted by comparing the shear strengths of high iv (EtO-sterilized) and low iv (gamma sterilized) samples derived from the same manufacturing lots, and thus the same raw material batch. Plotting the measured shear strength values as a function of degradation time (Fig. 2A) showed that the sterilization method had no noticeable effect on the initial shear strength levels of the analyzed sample types. All the samples retained their initial shear strength levels throughout the first 38 weeks of the degradation period (i.e., 9 mo), after which low iv (gamma sterilized) samples showed signs of decline in shear strength. This decline was visualized by plotting the shear strength (% of initial) as a function of inherent viscosity (Fig.
2B), but the shear strength values of the low iv 96L/4D PLA and 80L/20D,L PLA were still at least 90% of the initial stage at 52-week follow-up point. Figure 2. Effect AV-951 of the polymers initial iv on shear strength retention of ?4.0 mm oriented 96L/4D PLA and 80L/20D,L PLA samples. Shear strength as a function of degradation time (A) and inherent viscosity (B). Values are averages of the … The effect of the sample diameter on strength retention behavior The effect of the sample diameter on the strength retention dynamics was analyzed by comparing the oriented ? 1.2 mm and ? 4.