Double Melting Behavior and Plasticization of Polylactic acid – a TOPEM and FTIR Study

The multiple melting behavior is a common phenomenon observed by differential scanning calorimetry (DSC) for polymorphic polymers. In the case of poly(lactic acid) (PLA) this is detected as double melting peaks, which have been proposed to arise from α- and α’- crystalline forms of the polymer. Various explanations have been presented for the mechanism behind this behavior, namely melt-recrystallization, multiple lamellae population and multiple crystal structure, but so far none of them has been fully confirmed. Determination of the thermal properties of PLA is crucial, because the polymer is used as a material for medical applications alongside food packaging. PLA is known to degrade via hydrolysis under certain thermal and humidity conditions, for which reason the degradation time and rate should be taken into account. At body temperature the plasticizing effect of water can be utilized in the activation of the shape-memory of amorphous poly(D,L-lactic acid) (PDLLA). In this thesis, an investigation of the double melting behavior of the poly(L,D-lactic acid) (P(L/D)LA) samples with two different D-contents by a new temperature-modulated differential scanning calorimetry (TMDSC) method, TOPEM is presented. The aim of the study is to determine the suitable parameters for TOPEM studies in the melting region and establish, whether this method is convenient to measure the phenomenon in question. The other part of the study concentrates on the examination of the plasticizing effect of water by Fourier transform infrared (FTIR) spectroscopy. In addition, the effect of gamma sterilization on the dry and wet samples is investigated. The results obtained by TOPEM correlate well with the ones attained by the conventional DSC method. The required crystallization temperature and time for the appearance of double melting peaks in the DSC curves are different for the samples with different D-contents. In both cases, the major part of melting is observed to be non-reversing. This is explained by superheating of the samples, which is assumed to be due to the slow melting kinetics during TOPEM measurements. Reversible melting in turn occurs simultaneously with the crystallization process, which is detected in the non-reversing heat flow curve. This supports the melting-recrystallization model suggested in previous studies. The plasticizing effect of water is observed clearly in certain IR-absorption bands in the region of 3800–3400 cm-1. A new band appearing in the region of 1700–1500 cm-1 is assigned to the bending mode of free water present in the samples. The changes in the spectra indicate that PLA degrades because of water treatment. Gamma sterilization, in turn, is not observed to have significant effect on the bands. Further investigation would be required to determine, whether this is due to alternating quality and thickness of the samples