A morphological study of linear and branched poly(ethylene terephthalate) using transmission electron microscopy and small-angle X-ray scattering

Small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) were used to probe the final morphology of linear and novel branched poly(ethylene terephthalate) (PET) samples having undergone shear-induced crystallization. The branched PET samples were produced via a reactive extrusion process designed to increase the melt strength of PET and to broaden the processing options available to include extrusion blow molding and thermoforming. The highest molar mass, and, therefore, the most branched samples exhibited highly anisotropic final SAXS morphologies indicating that the semicrystalline lamellas were preferentially oriented perpendicular to the flow direction. The lower molar mass samples all displayed isotropic final SAXS morphologies indicating that chain relaxation had occurred prior to crystallization. For the high molar mass samples, chain entanglements slow the relaxation time after exposure to shear and the chain orientation induced by the shear produced an enhanced nucleation effect for the subsequent crystallization leading to faster crystallization kinetics. Ruthenium tetroxide (RuO2 ) was employed successfully to provide a lamellar scale contrast for analyzing PET crystals by TEM. Long string-like crystals were observed for all of the samples regardless of their orientation with respect to the flow direction. These crystals often displayed a significant degree of orientation over the short range. Analysis using Fourier transforms on filtered TEM images produced d-spacings generally comparable to those obtained from the SAXS analysis, in the 100-200 angstrom range.