MORPHOLOGICAL-STUDIES OF ORDERED, SOLID POLYMERS BY SCANNING FORCE MICROSCOPY

Contact mode atomic force microscopy (AFM) and lateral force microscopy (LFM) were used to image morphological features of solid polymers. The micro-structures discussed include solution-grown lamellar crystals of polyethylene (PE) and polyoxymethylene (POM), the lamellar morphology of the alpha-spherulites of melt-crystallized, isotactic polypropylene (i-PP), microfibrils of oriented POM, and the structure of lyotropic poly(p-phenylene terephthalamide)(PPTA) fibers. Micrographs of lamellar structures show chain-folding domains in PE and in hexagonal POM, as well as spiral overgrowth, growth twin and crazes in collapsed PE pyramids. Results include the demonstration of the directional dependence of the frictional force measured by LFM in the different chain-folding domains of hexagonal POM lamellae. This indicates that directionally-ordered chain folds exist at the free surface of the POM crystals in the different chain-folding domains. Images of the terraces of left and right-handed dislocations in POM were also captured. The lamellar thickness and its distribution in alpha-spherulites of i-PP crystallized from the melt at different temperatures were quantitatively studied. The average lamellar thickness as a function of the reciprocal degree of supercooling satisfied the Thomson-Gibbs equation. The distribution of the lamellar thickness was quantitatively described by a two-parameter log-normal distribution. POM fibers studied included extended-chain crystals obtained in topotactic, solid-state polymerization of trioxane, and mechanically drawn, uniaxially oriented samples. Extended-chain crystals of POM consisted of ordered microfibrils with an axis aligned in the crystallographic c (or drawing) direction. Microfibrils in POM obtained in the topotactic reactions showed better order when compared with the morphology shown by the mechanically drawn POM samples. The latter showed imperfect alignment of twisted microfibrils and microfibrillar splitting. PPTA fibers were prepared using the dry-jet wet-spinning technique, followed by annealing. A skin-core differentiation across the fibers and a microfibrillar morphology with a microfibrillar diameter varying in the range of 50 +/- 20 nm was observed.