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900 Macromolecules, Structure

FIGURE 46 Banded spherulitic structure in polyethylene (mag-
nification: 700×).
FIGURE 47 Electron micrographs of a 70/30 mol ratio
polystyrene–polybutadiene AB block copolymer: (a) microtomed
normal to the long dimensions and (b) microtomed along the long
chain direction. The radiating fibrils of the spherulite are dimension.
actually elongated, chain-folded lamellae. The concentric
bands, when present, arise from the fact that the lamellae
twist about the radius like ribbons and do so in a regular greater than true crystalline dimensions. In (a) the polybu-
and cooperative manner. The period of twist corresponds tadiene segments are actually rods, but appear as circles
to the separation of bands seen in the optical microscope. because the sample has been microtomed normal to their
When the bands are absent the structure is essentially the long dimensions. In (b) the direction of microtoming is
same but without the twisting. that of their long dimension and their elongated form is
evident.
B. Block Copolymer Morphology
We have seen (Section III.E) that an important class of
copolymers, both scientifically and technologically, are SEE ALSO THE FOLLOWING ARTICLES
those of the block type. Usually, the synthesis of block
copolymers is designed to produce macromolecules of ei- BIOPOLYMERS • INFRARED SPECTROSCOPY • MICROAN-
ther the AB or the ABA type, where the letters designate ALYTICAL ASSAYS • NUCLEAR MAGNETIC RESONANCE
different kinds of chains. A much studied class of block • POLYMER PROCESSING • POLYMERS,SYNTHESIS
copolymers is that containing polystyrene and polybutadi- • RAMAN SPECTROSCOPY • RUBBER,NATURAL •
ene sections. It is a general rule that polymers of different RUBBER,SYNTHETIC
structure, even though closely related, will not mix. The
fundamental reason is that such large molecules are neces-
sarily few in number, and the entropy gain on mixing them BIBLIOGRAPHY
is negligibly small (Section II). As a consequence, the A
and B segments cannot exist in a molecularly dispersed
Allcock, H. R. (1990). “Contemporary Polymer Chemistry,” Prentice-
state but must form small separate domains, the covalent
Hall, Upper Sadde River, NJ.
bonding preventing separation into macroscopic phases. Bovey, F. A. (1982). “Chain Structure and Conformation of Macro-
The domains may take the form of spheres, rods, or lamel- molecules,” Academic Press, New York.
lae, the dimensions of which depend on the end-to-end Bovey, F. A., and Mirau, P. A. (1996). “NMR of Polymers,” Academic
lengthofthesegments(usuallyclosetotheunperturbeddi- Press, New York.
deGennes, P. G. (1979). “Scaling Concepts in Polymer Physics,” Cornell
mensions) (Section II) and the interfacial tension between
University Press, Ithaca, NY.
the domains. The segments present in greater volume frac- Elias, H. G. (1997). “Introduction to Polymer Science,” John Wiley and
tion will generally form a continuous phase in which the Sons, New York.
lesserphaseexistsasseparatemicrophases.InFig.47elec- “Encyclopedia of Polymer Science and Engineering” (1988). Wiley
tron micrographs of a 70/30 polystyrene–polybutadiene Interscience, New York.
Flory, P. J.(1953).“Principles of Polymer Chemistry.”Cornell University
(mole ratio) AB block copolymer are shown. Although the
Press, Ithaca, NY.
appearance of the domains is of almost crystalline regu- Flory, P. J. (1969). “Statistical Mechanics of Chain Molecules,” Wiley
larity, the dimensions are at least an order of magnitude Interscience, New York.