Page 43 - Academic Press Encyclopedia of Physical Science and Technology 3rd Polymer

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Encyclopedia of Physical Science and Technology EN008C-602 July 25, 2001 20:31

858 Macromolecules, Structure

chromophore and a chromophore in the ground state, polymer–polymer and polymer–solvent interactions
the emission of which can be used to monitor spatial are exactly balanced.
proximityandtherebyestablishpolymerconformation. Tacticity Specification of the spatial arrangement of
Freely jointed chain Model for a polymer chain in which sidechains with respect to the zigzag plane of the poly-
there are no restrictions on the bond angles between mer chain. Common types of tacticity are atactic, syn-
monomer units. diotactic, and isotactic.
Geometrical isomers Isomers existing at carbon–carbon
double bonds or other bonds that are not free to rotate.
Graft copolymer Homopolymer or copolymer onto MACROMOLECULES are very large molecules. The
which are covalently attached polymer chains of an- word polymer is derived from the Greek words meaning
other type. “many parts” and is often used as an alternative term, one
Heterotactic Triad configurational sequence containing which emphasizes the nature of macromolecules as being
one isotactic and one syndiotactic dyad, symbolized by builtupbythejoiningofmanysmallchemicalunitsknown
m and r, respectively. as monomers (“single parts”). Polymers may be either
Isotactic Arrangement of successive units of a polymer of biological origin (e.g., proteins, cellulose, DNA, and
chain so that all of the sidechain substituents are located RNA) or synthetic origin (e.g., polyethylene, polystyrene,
on the same side of the zigzag plane. nylon, and polyesters). We shall deal in this article mainly
Meso (m) Arrangement of two adjacent side-chains on with the latter.
the same side of a vinyl polymer chain in planar zigzag Macromolecules have structure on several levels. At the
(all-trans) form (see isotactic). chemical level they are repeating sequences of a particular
Microstructure Precise way that monomer units are type of monomer unit covalently joined. Thus, units of the
joined together to form a polymer. monomer styrene may be linked to form polystyrene:
Monomer Small chemical unit from which polymers are
CH CH 2 CH CH 2 CH CH 2 CH CH 2 CH CH 2
synthesized.
Morphology Describes the supramolecular organization
of molecules.
STYRENE POLYSTYRENE
Polydispersity Characterization of the molecular weight
¯
¯
distribution of a polymer, given by M w /M n . The microstructural level refers to the precise way that
these units are joined together. They are represented above
Polymer From the Greek words meaning “many parts,”
as connected head-to-tail, but they may also be connected
used as an alternative term for macromolecule.
head-to-head : tail-to-tail and other options also exist. The
Polymerization Process of synthesizing a polymer.
conformation of a macromolecule describes the spatial
Racemic (r) Arrangement of two adjacent sidechains on
arrangement of the polymer along its contour and may be
opposite sides of a vinyl polymer chain in planar zigzag
altered by rotations about
(all trans) form (see syndiotactic).
2 1/2
Radius of gyration Root mean square distance s of CH 2 CH CH CH 2 CH 2 CH CH CH 2
the units of a chain from its center of gravity.
Regioregularity Pertains to the occurrence of head-to-
head: tail-to-tail defects in a linear polymer chain.
bonds without alteration of the covalent structure. Many
Rotationalisomericstate Modelforthetheoreticaltreat-
polymers are partially crystalline, and the chain confor-
ment of polymer chains in which the main chain va-
mation is markedly influenced and conditioned by the re-
lence angles are fixed and the chain rotation is limited
quirements of the crystal packing of the chains. Finally,
to a few states.
the morphology of a macromolecular material refers to
Spherulite Aggregate of crystals with a radiating fibrillar
thesupramolecularorganizationofthemolecules,whether
structure, a common habit in polymer crystallization.
crystalline or amorphous, in the solid state.
Step polymerization See condensation polymerization.
Syndiotactic Arrangement of successive units of a poly-
mer chain so that all of the side-chain substituents
I. THE NATURE OF MACROMOLECULES
are located on regularly alternating sides of the zigzag
plane.
A. The Macromolecular Concept
Θ solvent Solvent in which a particular polymer adopts
its unperturbed dimensions. The utility of natural rubber was recognized two centuries
Θ temperature Temperature at which a particular poly- ago, and cellulose esters were articles of commerce at the
mer adopts its unperturbed dimensions because the end of the nineteenth century. Leo Baekeland invented the