Page 76 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Polymer Structure (Morphology) 39

(a) (b)

FIGURE 2.15 Structure of a spherulite from the bulk. (a) shows a slice of a simple spherulite. As further
growth occurs, filling in, branch points, and so on occur as shown above (b). The contour lines are simply the

hairpin turning points for the folded chains.

(a) (b)
FIGURE 2.16 Spherulite structure showing the molecular-level lamellar chain-folded platelets and tie and
frayed chain arrangements (a) and a more complete model of two sets of three lamellar chain-folded platelets
formed from polyethylene (b). Each platelet contains about 850 ethylene units as shown here.

turned” methylene units as pictured in Figures 2.15 and 2.16. The polymer chain axes are perpen-

dicular to the large flat crystal faces. A single polymer chain with 1,000 ethylene (2,000 methylene)
units might undergo on the order of 50 of these hairpin turns on the top surface and another 50 turns
on the bottom face with about 20 ethylene units between the two surfaces.
Many polymers form more complex single crystals when crystallized from dilute solution,
including hollow pyramids that often collapse on drying. As the polymer concentration increases,
other structures occur, including twins, spirals, and multilayer dendritic structures, with the main
structure being spherulites.
When polymer solids are produced from their melts, the most common structures are these
spherulites that can be seen by the naked eye and can be viewed as Maltese cross-like structures
with polarized light and crossed Nicol prisms under a microscope.
For linear PE, the initial structure formed is a single crystal with folded chain lamellae. These
quickly lead to the formation of sheaf-like structures called axialites or hedrites. As growth pro-
ceeds, the lamellae develop on either side of a central reference point. The lamellae continue to fan
out, occupying increasing volume sections through the formation of additional lamellae at appropri-
ate branch points. The result is the formation of spherulites as pictured in Figures 2.15 and 2.16.
While the lamellar structures present in spherulites are similar to those present in polymer sin-
gle crystals, the folding of chains in spherulites is less organized. Further, the structures that exist
between these lamellar structures are generally occupied by amorphous structures, including atactic
chain segments, low molecular weight chains, and impurities.
The individual spherulite lamellae are bound together by “tie” molecules that are present in more
than one spherulite. Sometimes these tie segments form intercrystalline links, which are threadlike

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