Multi scale structure evolution of pet/mmt and PET/SiO2 nanocomposites upon solid state uniaxial deformation : an in situ WAXS/SAXS study

This work reports an investigation on the solid state uniaxial deformation of neat poly(ethylene terephthalate), PET, and its nanocomposites with montmorillonite, MMT, and nanosilica, SiO2, followed by in situ WAXS/SAXS (under X-ray synchrotron source radiation). The nanocomposites were produced via direct melt blending with addition of 0.3wt% of nanoparticles. Structural characterization by TEM revealed relatively good dispersion of both types nanofillers, namely intercalated morphology for the PET/MMT nanocomposite and agglomerate sizes (c.a. 80 nm) for the PET/ SiO2 nanocomposites. Despite the type of nanoreinforcements, three common stages of multiscale structure evolution were indentified: - Stage I, before necking, is characterized by a small amount of amorphous phase evolving into mesophase at almost constant molecular orientation level; - Stage II, at neck propagation, where a rapid increase of polymer molecular orientation is accompanied by a sharp increase of the mesophase and by the formation of a periodical mesophase; it is also observed the appearance of crazes in the polymer matrix, and voids within the nanoparticles agglomerates 1-4. - Stage III, during necking, corresponds to the transformation of crazes and voids into micro-voids, at a plateau of average molecular orientation. The highest periodical mesophase content is achieved together with a slight increment of mesophase1-4. In comparison to the neat PET structure evolution, all kind of nanocomposites showed: i) improved amount of mesophase and maximum periodical mesophase formed at earlier deformations; ii) retarded crazes widening/growing within the polymer bulk, and iii) similar maximum orientation level are achieved. Multiscale structures modelling are suggested based on the results obtained