SPHERULITIC MORPHOLOGY OF ISOTACTIC POLYPROPYLENE INVESTIGATED BY SCANNING ELECTRON-MICROSCOPY

The aim of this work is to investigate the complex spherulitic structu re of bulk polypropylene samples from direct scanning electron microsc opy (SEM) observations of etched surfaces. Thick plates of isotactic p olypropylene were moulded by intrusion. Preliminary characterization, involving X-ray diffraction, microdensitometry and differential scanni ng calorimetry (d.s.c.), showed that the slow solidification process d evelops a variable proportion of the monoclinic (alpha) and hexagonal (beta) phases, ranging from 0% of beta-crystals at the surface to 60 v ol% of this phase at the core. In addition, samples were cut across th e thickness of the plates, finely polished and then etched with an app ropriate reagent which preferentially attacks the amorphous fraction o f the polymer. SEM examination of such samples revealed two population s of spherulites with quite different contrasts, which were unambiguou sly associated with the two crystalline structures. The alpha-spheruli tes have a dark aspect while the beta-ones are very bright. These cont rast effects are discussed in terms of the topology of the etched surf ace. For the alpha-spherulites, whose lamellae are straight and finely interlocked along the radial and tangential directions, the etched se ctions are very smooth and consequently the lateral diffusion of norma l incident electrons is weak. In contrast, the beta-spherulites are ch aracterized by curved lamellae and sheaf-like structures, thus making the surface rougher after etching, and which contribute to the emissio n of more secondary electrons to the detector. This interpretation is confirmed by the corresponding contrast observed in metallographic mic roscopy using low-angle illumination.