Los resultados que se analizan en este trabajo proceden fundamentalmente de dos polímeros (PET, PEN) y hemos utilizado esencialmente la técnica TSC centrándonos en el comportamiento de los mecanismos de conducción ligados a las correspondientes relajaciones en función de la cristalización fría y del envejecimiento físico de los materiales.Los capítulos 1, 2 y 3 describen las características básicas de los polímeros y de los electretes y presentan los fundamentos teóricos de las técnicas que se han utilizado.El capítulo 4 se dedica a exponer el montaje experimental que hemos realizado de los dispositivos utilizados en las medidas. Nuestro trabajo experimental queda condensado en los capítulos 5, 6, 7 y 8 cuyos resultados están basados en diversos trabajos de investigación que se han presentado en diferentes congresos nacionales e internacionales, y que han dado lugar a diversas publicaciones. En el capítulo 5 [1, 2] se comparan los resultados obtenidos a partir de electretes polarizados convencionalmente con los obtenidos por ventanas térmicas. El capítulo 6 [2,3] muestra los efectos que causan la cristalización fría realizada por etapas sobre las relajaciones dieléctricas y de carga libre. El capítulo 7 [4, 5] estudia los efectos del envejecimiento sobre PET amorfo analizado con y sin carga en corona. El capítulo 8 [6] estudia el comportamiento del PET con diferentes grados de cristalinidad y sometido a diferentes tiempos de envejecimiento. / A comparative study of the different relaxations present in amorphous and partially crystallized poly(ethylene terephthalate) (PET) and poly(ethylene-2,6-naphthalene dicarboxylate) (PEN) have been carried out by thermally stimulated depolarization currents (TSDC), thermally stimulated polarization currents (TSPC), differential scanning calorimetry (DSC) and dynamic electric analysis (DEA). Amorphous film (PET and PEN) has been crystallized by the thermal step stimulation method. DSC measurements of these samples show the appearance of a small endothermic recrystallization peak once the crystallization of the sample is completed. This recrystallization peak increases and shifts towards higher temperatures as the sample is further heat treated. Associated with the appearance of this endothermic peak, the  relaxation passes through a maximum with a sharp decrease with further heat treatment. -relaxation shifts to high temperature and broadens with the increase of degree of crystallinity. Physical ageing in (PET) has been studied by (DSC), (TSDC) and (TSPC) as a function of the material crystallinity degree. In first case physical ageing was carried out annealing the amorphous sample at 60ºC for several ageing times (from ta=0 to 264h). Structural relaxation has been studied by DSC through the analysis of the endothermic peak superposed to the glass transition, which increases and shifts towards higher temperatures as the annealing time increases. Current measurements spectrums of annealed samples show the appearance of a relaxation peak which we named αa . The evolution of this peak with the annealing time is presented, and it has been related directly to the endothermic peak obtained by DSC. Experiences carried out show the dipolar origin of the αa peak, related to the polarization of chains within the material when mobility is recovered above Tg. Calorimetric measurements confirm the presence of two separated glass transitions in partially crystallized samples (xc=27%), associated with the presence of two different amorphous regions. TSPC measurements on aged samples show a current peak, which is directly related to the polarization of the amorphous regions when mobility is recovered above the glass transition. The study of these peaks in aged partially crystallized samples shows evidences of a double glass transition associated with the interspherulitic and interlamelae amorphous regions.