Accelerated in vitro degradation properties of polylactic acid/phosphate glass fibre composites
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Degradation properties were studied for polylactic acid (PLA) and phosphate glass fibre (40P2O5–24MgO–16CaO–16Na2O–4Fe2O3, denoted as P40) reinforced unidirectional (UD) and randomly mat (RM) PLA composites using phosphate buffer saline (PBS) media over a range of temperatures from 21 to 85 °C. Glass transition and melting temperatures for PLA decreased from 61.3 and 167.4 to 52.7 and 151.6 °C, respectively, and crystallinity increased from 9.2 to 58.3 % during 3 days of degradation period in PBS media at 85 °C. Appearance of sharp crystalline peaks after degradation at higher temperatures which was confirmed via X-ray diffraction analysis was also indicative of increase in crystallinity. However, flexural strength decreased by approximately 20 % (for PLA) and by around 50 % (P40 RM and P40 UD composites) of the initial strength after degradation in PBS at 37 °C. No significant changes in mechanical properties were observed before and after degradation of composites at 21 °C for 56 days. Monomodal molecular weight distribution for the PLA before and after degradation in PBS at 37 °C was replaced by bimodal after degradation at higher temperatures. Arrhenius equation applied for the change in molecular weight of the polymer and composite samples and the obtained degradation activation energies were 85.4, 78.7 and 74.1 kJ mol−1 for PLA within PLA alone, P40 RM and P40 UD composites, respectively. Time prediction was applied to correlate short-term degradation (at elevated temperatures) to the long-term effects (at 37 °C) using both ‘tipping point’ and molecular weight as co-ordinates.
KeywordsPLLA Flexural Strength Molecular Weight Distribution Degradation Temperature Accelerate Degradation
The authors would also like to thank Dr. Derek Irvine and Ms. Natasha Birkin for their help with the GPC.
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