Basic Nature
Of Polymers
CHEMICAL
COMPOSITION
The term polymer
denotes a molecule
that is made up of
many(poly) parts(mers). The mer ending
represents the simplest repeating chemical
structural unit from
which the polymer
is composed. Thus poly(methy1 methacrylate) is a polymer
having chemical structural units derived from
methyl methacrylate, as indicated
by the simplified reaction and
structural formula I.
The molecules from
which the polymer
is constructed are called
monomers (one part). Polymer molecules may be prepared from
a mixture of different
types of monomers.
They are called copolymers if
they contain two
or more different chemical units and
telpolymers if they contain
three different units, as indicated by
the structural formulas 11 and 111.
As a
convenience in expressing the structural
formulas of polymers, the mer units are
enclosed in brackets, and subscripts such as n,
m, and p represent the average
number of the various mer units that
make up the polymer molecules. Notice that
in normal polymers
the mer units
are spaced in a random orientation
along the polymer chain. It is possible, however, to
produce copolymers with mer
units arranged so
that a large number of one mer type are connected to a large number
of another mer type. This
special type of polymer is called
a blockpolymer. It also is possible to produce polymers having mer units with a
special spatial arrangement with respect to the adjacent units; these are called stereospeczfic polymers.
MOLECULAR
WEIGHT
The molecular weight of
the polymer molecule, which equals the molecular weight of the various mers multiplied by the number
of the mers, may range from
thousands to millions
of molecular weight units,
depending on the preparation conditions. The higher the molecular weight of the
polymer made from a single monomer, the higher the degree of polymerization. The term polymerization is
often used in a qualitative sense, but the degree of
polymerization is defined
as the total number
of mers in a polymer molecule. In general, the
molecular weight of
a polymer is reported
as the average molecular weight because the number of repeating units may
vary greatly from one molecule to another. As would be
expected, the fraction of low-, medium-, and high- molecular-weight
molecules in a
material or, in other words,
the molecular weight
distribution, has as pronounced an
effect on the
physical properties as the average molecular weight does.
Therefore two poly(methy1
methacrylate) samples can have
the same chemical composition but greatly different physical properties because
one of the samples
has a high
percentage of low- molecular-weight molecules, whereas
the other has a high percentage
of high-molecular weight molecules. Variation in the molecular
weight distribution may be
obtained by altering
the polymerization procedure. These materials therefore do not
possess any precise
physical constants, such as
melting point, as
ordinary small molecules do. For example, the higher the
molecular weight, the higher
the softening and
melting points and the stiffer
the plastic.
SPATIAL STRUCTURE
In addition to
chemical composition and molecular weight, the
physical or spatial
structure of the polymer
molecules is also
important in determining
the properties of
the polymer. There are three
basic types of structures: linear, branched, and cross-linked. They are illustrated in
Figure as segments of linear, branched, and cross-linked polymers. The linear
homopolymer has mer units of the same
type, and the random copolymer of the
linear type has
the two mer units randomly distributed
along the chain. The linear block
copolymer has segments, or blocks, along the chain where the mer units
are the same. The branched homopolymer
again consists of the
same mer units, whereas the graft-branched copolymer consists of one
type of mer unit on the main chain and
another mer for
the branches. The cross-linked
polymer shown is
made up of a
homopolymer cross-linked with
a single crosslinking agent.
The linear and branched molecules are separate and discrete, whereas
the cross-linked molecules are a network
structure that may result in the polymer's becoming one giant molecule. The spatial structure
of polymers affects
their flow properties, but
generalizations are difficult
to make because either
the interaction between linear polymer
molecules or the
length of the branches
on the branched
molecules may be more important in a particular example. In
general, however, the cross-linked
polymers flow at higher
temperatures than linear
or branched polymers. Another
distinguishing feature of some
cross-linked polymers is
that they do not absorb
liquids as readily
as either the linear or branched
materials.
An additional method of
classifying polymers other than by
their spatial structure is according to
whether they are
thermoplastic or thermosetting. The
term thermoplastic refers
to polymers that may be softened
by heating and solidify on cooling,
the process being
repeatable.
Typical examples of
polymers of this
type are poly(methy1 methacrylate),
polyethylene- polyvinylacetate, and polystyrene. The term thermosetting refers
to plastics that
solidify during fabrication but
cannot be softened by reheating. These
polymers generally become
nonfusible because of a
crosslinking reaction and the
formation of a spacial structure. Typical dental examples are
cross-linked poly(methy1 methacrylate), silicones, cis-polyisoprene, and bisphenol A-diacrylates.
Polymers as a
class have unique
properties, and by varying
the chemical composition,
molecular weight, molecular-weight
distribution, or spatial arrangement of the mer units, the physical and
mechanical properties of polymers
may be altered.
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