Page 207 - Carrahers_Polymer_Chemistry,_Eighth_Edition

170 Carraher’s Polymer Chemistry

FIGURE 5.3 Space-filling structure of a portion of a linear amorphous polyethylene region.

science, once the critical parameters are known on a macrolevel, we are able to better understand
them on a molecular and conceptual level. An example that illustrates this involves the processing
of annealed PE. In general, for most linear PE, micelles and associated spherulites are formed when
it is melted and then slowly cooled. If no force is applied during the annealing process (simple melt
crystallization), a high amount of force and large deformation is required to breakdown the initial
spherulite structures with reformation occurring along the axis of the pull when high-strength PE
rod, film, and sheet is produced. However, if the PE is crystallized under pressure applied in one

direction, less energy and lower deformation is required to align the PE spherulites since the spheru-
lites are already partly aligned. In both cases, stretching of the molecular network is required. For
the simple melt crystallized PE, the original spherulite structure is destroyed during the deforma-

tion followed by the formation of new fibrillar structures. For the pressure-associated annealing
process, elongated micelles are formed that largely remain after the deformation process.
LDPE films are nearly clear even though they contain a mixture of crystalline and amorphous

regions. This is because the crystalline portions are space filling and not isolated spherulites allow-
ing a largely homogeneous structure with respect to refractive index resulting in a material that is
transparent. In fact, the major reason that LDPE fi lms appear hazy or not completely transparent is
because of the roughness of the surface and is not due to the light scattering of the interior material.

Space-filling models of amorphous and crystalline linear PE are given in Figures 5.3 and 5.4.
Typical uses of the various PEs include the following:

1. UHMWPE-battery separators, light-weight fibers, permanent solid lubricant materials
in railcar manufacture, automobile parts, truck liners; liners to hoppers, bins, and chutes;
farm machinery as sprockets, idlers, wear plates, and wear shoes; moving parts of weaving
machines and can and bottle handling machines, artificial joints, including hip and knee

replacements, gears, butcher’s chopping boards, sewage-treatment-bearings, sprockets, wear
shoes; lumbering-chute, sluice, and chain-drag liners; neutron shields; also as components in
bullet-resistant wear.
2. “Typical” HDPE-blow-molded products—bottles, cans, trays, drums, tanks, and pails; injec-
tion-molded products—housewares, toys, food containers, cases, pails, and crates; fi lms,

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