Imaging and quantifying the different crystalline structures of polypropylene with the atomic force microscope
Thesis (MSc (Chemistry and Polymer Science))--University of Stellenbosch, 2006.
Isotactic polypropylene (iPP) exists in four different crystalline phases: α-, β-, γ- and the mesomorphic (smectic) form. The α-phase is probably the most predominant form of iPP with a monoclinic unit cell, whereas the unit cell of the γ-phase is orthorhombic. The aim of this study was to examine the ability of AFM to identify the α and γ crystalline forms of polypropylene and polypropylene copolymers that were not especially treated. Spherulites imaged on propylene-ethylene random copolymer surfaces had diameters ranging from 10-30μm and predominantly radial lamellae. The cross-hatching phenomenon of isotactic polypropylene was observed for samples containing both the α- and γ-phase. Two types of γ-phase spherulites were observed, namely a featherlike form for samples with a high content of the γ-phase and a bundle-like form with varying sizes, which is typical for the mixed structure of both α- and γ-forms. In some images of polypropylene-ethylene copolymer sample lozenge-shaped structures could be observed between the γ-spherulites, which might be single polyethylene crystals. Both the α- and γ-phase of iPP were successfully imaged and identified with AFM without prior treatment of the surface. The degree of crystallinity obtained from AFM images was calculated as a percentage of the surface area exhibiting regular structures. The results compared well to X-ray results.