Summary

     The purpose of this PhD thesis is to study the effect of thermal, biological and photo degradation on polylactide (PLA) to characterize the changes occurring under different conditions during its life cycle. This biodegradable polymer is obtained from renewable resources and is considered an excellent candidate to substitute other polymeric materials with scarce degradability. The decrease on the polylactide molar mass was monitored by Gel Permeation Chromatography (GPC) and Viscometry. Additionally, Fourier Transform Infrared Spectroscopy (FTIR) was used to determine the degradation mechanisms and their effect on the chemical structure of PLA. Moreover, the impact of each type of degradation on the morphology and the thermal and viscoelastic properties of PLA was also determined. Thermogravimetric Analysis (TGA) was applied to monitor changes in the thermal stability of the materials caused by the different degradation types, by using several parameters such as the maximum thermal degradation rate or the activation energy. The effect of bio and photo degradation on the material surface was evaluated by Scanning Electron Microscopy (SEM), revealing variations exclusively caused by biological degradation. The thermal and viscoelastic properties were measured by Dynamic Mechanical Thermal Analysis (DMTA), Differential Scanning Calorimetry (DSC) and Optical Microscopy (OM). The decrease of molar mass with biodegradation time follows a first order process (Mn = Mno e-kt) while the molar mass of specimens tested during thermal and photo degradation follows a second order law (1/Mn = (1/Mno)+k·t). Several characteristic parameters were obtained to determine their variation with degradation and molar mass and the effect of each type of degradation. The dependence of the molar mass with the spherulites growth rate is especially acute in bio and photo degraded samples, following an exponential law typical of semicrystalline polymers. The results have shown that each degradation is controlled by several factor that affect differently the morphological, thermal and mechanical properties of PLA, highlighting that degradation cannot be explained by a solely effect of chains breakage and molar mass reduction. The appearance of new functional groups is fundamental to control the crystallization of the material. Furthermore the formation and size of crystals are the most significant parameters to describe the macroscopic properties, and therefore establish the PLA degradation. In particular, the results obtained are of great interest to design a controlled management of PLA disposal and to estimate the time to reach considerable reduced molar masses. This study offers a complete characterisation of each degradation type, through the combination of all the techniques under use, by creating a methodology capable to predict the behaviour of polymers potentially degradable at long exposure times.