Different penetrants and their effects on the physical properties of PLLA
Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
Poly (L-lactic acid) (PLLA) is a biodegradable material used in various applications, such as food packaging and medical implants. PLLA is hard and brittle by nature; hence, modification of the material may be necessary to obtain desirable properties. In this work, the physical changes occurring in amorphous and semi-crystalline PLLA caused by different penetrants were investigated. Water, ethanol and methanol were used as penetrants, where the sorption kinetics was studied by soaking the samples at 37 °C. The sorption studies where followed by desorption studies by drying the samples in an auto-desiccator. Furthermore, the change in fictive temperature and change in crystallinity where also studied by using differential scanning calorimetry (DSC) and Wide Angle X-ray Scattering (WAXS) respectively. Sorption of water obeyed Fickian (Case I) diffusion with constant diffusion coefficient in both amorphous and semi-crystalline PLLA samples. DSC showed a depressed fictive temperature and minor change in crystallinity for amorphous PLLA samples. For semi-crystalline PLLA the change in water sorption did not show any significant change in fictive temperature. Ethanol sorption of amorphous PLLA samples showed Fickian (Case I) diffusion, with a concentration dependent diffusion coefficient while semi-crystalline PLLA samples showed non-Fickian diffusion. Furthermore, ethanol soaked samples showed a depressed fictive temperature and change in crystallinity for both amorphous and semi-crystalline PLLA samples. Amorphous PLLA samples soaked in methanol showed two-stage diffusion with a fast initial sorption that changed to a significantly slower sorption, indicating a transition from a material with low crystallinity to a material with high crystallinity. Sorption of methanol in semi-crystalline PLLA samples showed a fast initial uptake and a decrease in methanol uptake with time after reaching saturation. The possible reason for this unusual behavior may be due to induced crystallization or that residual molecules are being leached out. DSC data och methanol soaked samples showed a depressed fictive temperature of both amorphous and semi-crystalline PLLA samples. Furthermore, it also showed that methanol was the most efficient penetrant to depress the fictive temperature on both amorphous and semi-crystalline structure of PLLA. Overall the amorphous PLLA samples showed a higher uptake of penetrants than the semi-crystalline PLLA samples, which is no surprise, because it is easier for molecules to penetrate a random amorphous structure than a dense packed crystalline structure. Desorption of methanol in expanded material indicated a soaking leaching of material, whereas the other desorption experiments did not reached their initial weight due to extended time frame of these experiments. Ethanol and methanol induced crystallinity, whereas water did not have any impact on the crystallinity or the crystal structure. In expanded material ethanol induced a higher degree of crystallinity and therefore having the biggest spherical shaped crystal size whereas methanol having second biggest spherical shaped crystal size, and water the smallest size.
Place, publisher, year, edition, pages
poly(lactic acid), penetrants, diffsuion, physical ageing, crystallisation
Engineering and Technology
IdentifiersURN: urn:nbn:se:kth:diva-145866OAI: oai:DiVA.org:kth-145866DiVA: diva2:720769