The effects of nanosilica (SiO

2

) on crystallization and thermal aging behaviors of polyethylene terephthalate (PET) have been studied using differential scanning calorimetry (DSC) and polarized optical microscopy (POM)

viscometry

tensile testing and scanning electron microscopy (SEM). For non-isothermal and isothermal crystallizations

the crystallization rate of PET increases considerably with increasing content of SiO

2

providing a large number of nucleation sites

but the relative crystallinity of the nanocomposites has little differences with that of neat PET. According to POM observation

the nucleation of PET becomes faster and the nucleation density increases significantly with increasing SiO

2

content. For PET and its nanocomposites thermally aged at 190 °C

the results of intrinsic viscosity

carboxyl content and tensile test show that the degradation rate of PET is reduced with the addition of a small content of SiO

2

but the degradation rate increases with further addition of SiO

2

owing to the dual effect of SiO

2

on PET degradation.