Effect of Crystallinity on PLA’s Microbiological Behaviour

Abstract

Recent work describes changes in polylactic acid samples with different crystallinity during microbiological degradation. We treated PLA at 93°C for different periods of time, which yielded samples with particular crystallinity. The fraction of crystalline phase was determined by differential scanning calorimetry, and the visual effect of crystallinity was measured by colorimetric method with black and white backgrounds. The medium for biological degradation process was living sludge under room temperature and normal atmospheric pressure. Furthermore, the change in mass was also measured. The results show that increased crystallinity reduces the rate of mass lost. The volumetric proportion of crystallinity is in direct correlation with opacity, so checking transparency is also a suitable possibility for estimating crystallinity. DSC, colorimetric method and visual observation experiments confirm that crystallinity has increased proportionally by the time of heat treatment and caused opacity. The experiments show that water uptake happened faster and in much higher volume in polymers having dominantly amorphous structure than in the case of samples with higher crystallinity. In the case of materials with only 2.43% crystallinity, weight lost began later because they had a greater water uptake during the first 7-12 days, while this period took only 7 days with a very low water uptake for samples containing approximately 35% crystalline phase. After swelling, weight loss of the crystalline samples was much slower than that of samples containing more amorphous parts, because crystalline phases inhibit the diffusion of small water molecules and the microbes with it.