Page 87 - Carrahers_Polymer_Chemistry,_Eighth_Edition

50 Carraher’s Polymer Chemistry

12. There are three types of cross-linking. These are chemical and two physical types of cross-
linking. Physical cross-linking results from formation of crystalline regions within polymer
structures and from chain entanglement. Cross-linked materials have good dimensional mem-
ory. Chemically cross-linked materials do not dissolve and do not melt.

GLOSSARY
Amorphous: Noncrystalline polymer or region in a polymer.
Atactic: Polymer in which there is a random arrangement of pendant groups on each side of
the chain.
Backbone: Principle chain in a polymer.
Branched polymer: Polymer having extensions attached to the polymer backbone. Not pen-
dant groups.
Bulky groups: Sterically large groups.
Chiral center: Asymmetric center such as a carbon atom with four different groups attached
to it.

Cold drawing: Stretching a fiber under room temperature.
Configurations: Related chemical structures produced by the breaking and remaking of pri-

mary covalent bonds.
Conformations: Various shapes of polymer chains resulting from the rotation about single
bonds in the polymer chain.
Conformer: Shape produced by a change in the conformation of a polymer.
Contour length: Fully extended length of a polymer chain; equal to the product of the length
of a repeat unit times the number of units or mers.
Critical chain length (z): Minimum chain length required for entanglement of polymer
chains.
Cross-linked density: Measure of the relative degree of cross-linking.
Crystalline polymer: Polymer with ordered structure.
Crystallites: Regions of crystallinity.
Degree of polymerization: Number of repeat units in a chain.
Dipole–dipole interactions: Moderate secondary forces between polar groups in different or
the same polymer chain.
Dispersion forces: Low-energy secondary forces due to the creation of momentary induced
dipoles; also known as London forces.
Glass transition temperature (T ): Temperature range where a polymer gains local or segmen-
g
tal mobility.
Glassy state: Hard, brittle state; below T .
g
Gutta percha: Naturally occurring trans isomer of polyisoprene.

Head-to-tail configuration: Normal sequence of mers in which the pendant groups are regu-
larly spaced; for PVC, the chlorine atom appears on every other carbon.
Intermolecular forces: Secondary forces between different molecules.
Intramolecular forces: Secondary forces within the same molecule.
Isotactic: Polymer where the geometry of the pendant groups are all on the same side of the
polymer backbone.
Lamellar: Plate-like or planar (flat) in shape.

Linear polymer: Polymer without chains extending off the backbone.
Low-density polyethylene (LDPE): A branched form of PE produced at high pressure by the
free–radical-initiated polymerization of ethylene.
Maltese cross: Cross with arms like arrowheads pointing inward.
Melting point (T ): First-order transition when the solid and liquid phases are in
m
equilibrium.

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