Page 77 - Carrahers_Polymer_Chemistry,_Eighth_Edition

40 Carraher’s Polymer Chemistry

FIGURE 2.17 Crystalline polymer structures formed under applied tension including flow conditions. Middle

shows the tertiary monofibrilar structure including platelets and the left shows these monofi brilar structures

bundled together forming a quaternary structure fibril. Right shows the distorted shish kebab formed with
more rapid fl ow.
structures that are important in developing the characteristic good toughness found in semicrystal-
line polymers. They act to tie together the entire assembly of spherulites into a more or less coherent
“package.”
Depending upon the particular conditions of crystallization, a number of secondary and tertiary
structures can be formed. In most cases, crystalline polymers attempt to form crystalline platelets.
Under little or no externally applied stress, these platelets organize themselves in spherulites as pic-
tured in Figures 2.15 and 2.16. They start by a nucleating process and begin to radiate outward from
the central nucleating site. Amorphous chain segments get trapped between the forming crystalline
platelet combinations, giving a kind of fuzzy or frayed exterior. These platelets are generally either
planar, as shown in Figure 2.17, or they can be helical or twisted. The platelets continue to grow
until they butt up against other spherulites.

Under externally applied stress, including simple melt flow, the tertiary structure can approach a
shish kebab arrangement where there are planes of platelets separated by areas where there exists both
crystalline and amorphous regions as pictured in Figure 2.17, left. These shish kebab structures often
organize into quaternary structures consisting of bundles of shish kebab single-strand fi laments forming

fibrils. Under more rapid flow conditions the shish kebab itself become distorted (Figure 2.17, right).

Interestingly, the amorphous regions within the spherulite confer onto the material some fl exi-
bility while the crystalline platelets give the material strength, just as in the case with largely amor-
phous materials. This theme of amorphous fl exibility and crystalline strength (and brittleness) is a
central idea in polymer structure–property relationships.
It must be remembered that the secondary structure of both the amorphous and crystalline
regions typically tend toward a helical arrangement of the backbone for most polymers but not poly-
ethylene, which forms a crank-shaft structure because of the lack of steric restraints (i.e., lack of
pendant groups off the backbone).
The kind, amount, and distribution of polymer chain order/disorder (crystalline/amorphous) is
driven by the processing (including pre- and post-) conditions, and thus it is possible to vary the
polymer properties through a knowledge of and the ability to control the molecular-level structures.
The crystalline regions may be disrupted by processing techniques such as thermoforming and

extrusion of plastics and drawing of fibers. In the last process, which is descriptive of the others, the

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