Spherulitic morphology of isotactic polypropylene investigated by scanning electron microscopy

The aim of this work is to investigate the complex spherulitic structure of bulk polypropylene samples from direct scanning electron microscopy (SEM) observations of etched surfaces. Thick plates of isotactic polypropylene were moulded by intrusion. Preliminary characterization, involving X-ray diffraction, microdensitometry and differential scanning calorimetry (d.s.c.), showed that the slow solidification process develops a variable proportion of the monoclinic (α) and hexagonal (β) phases, ranging from 0% of β-crystals at the surface to 60 vol% of this phase at the core. In addition, samples were cut across the thickness of the plates, finely polished and then etched with an appropriate reagent which preferentially attacks the amorphous fraction of the polymer. SEM examination of such samples revealed two populations of spherulites with quite different contrasts, which were unambiguously associated with the two crystalline structures. The α-spherulites have a dark aspect while the β-ones are very bright. These contrast effects are discussed in terms of the topology of the etched surface. For the α-spherulites, whose lamellae are straight and finely interlocked along the radial and tangential directions, the etched sections are very smooth and consequently the lateral diffusion of normal incident electrons is weak. In contrast, the β-spherulites are characterized by curved lamellae and sheaf-like structures, thus making the surface rougher after etching, and which contribute to the emission of more secondary electrons to the detector. This interpretation is confirmed by the corresponding contrast observed in metallographic microscopy using low-angle illumination.