Crystallization of Polymers. Volume 1, Equilibrium Concepts

1.4 Morphological features 21
Fig. 1.12 Electron micrograph of linear polyethylene (M v = 50 000) isothermally crystallized
at 89 ◦ C from a dilute tetralin solution. (Courtesy of Dr. R. V. Rice)
Most interesting is the fact that chain axes are preferentially ordered normal or nearly
normal to the basal planes of the lamellae. The lamellar habit is typical of crystallites
formed by homopolymers. Such structures are central to the understanding of
molecular morphology and properties. Of particular interest and importance is the
nature of the interphase between the crystalline and noncrystalline regions. This
problem will be discussed in detail in Volume 3.
When homopolymers are crystallized from very dilute solutions, either
lozenge-shaped platelets or crystals that possess a dendritic habit are formed.
Some typical electron micrographs of the crystals precipitated from dilute solution
are shown in Figs. 1.12 and 1.13. The crystal habit that is observed depends on
the molecular weight of the polymer and the crystallization conditions, such as
the temperature and the nature of the solvent. A very striking feature is that the
platelets are only about 100 to 200 A ❛
thick. In conjunction with selected-area
electron diffraction studies, it is shown that the chains are again preferentially
oriented normal, or nearly so, to the basal plane of the platelet. Considering the