FROM LITTLE MOLECULES TO BIG MOLECULES
The behavior of polymers represents a continuation of the behavior of smaller
molecules at the limit of very high molecular weight. As a simple example,
consider the normal alkane hydrocarbon series
These compounds have the general structure
where the number of —CH2— groups, n, is allowed to increase up to several
thousand. The progression of their state and properties is shown in Table 1.1.
At room temperature, the first four members of the series are gases.
n-Pentane boils at 36.1°C and is a low-viscosity liquid.As the molecular weight
of the series increases, the viscosity of the members increases. Although commercial
gasolines contain many branched-chain materials and aromatics as
well as straight-chain alkanes, the viscosity of gasoline is markedly lower than
that of kerosene, motor oil, and grease because of its lower average chain
length.
These latter materials are usually mixtures of several molecular species,
although they are easily separable and identifiable. This point is important.
Properties of the alkane/polyethylene series
because most polymers are also “mixtures”; that is, they have a molecular
weight distribution. In high polymers, however, it becomes difficult to separate
each of the molecular species, and people talk about molecular weight
averages.
Compositions of normal alkanes averaging more than about 20 to 25 carbon
atoms are crystalline at room temperature. These are simple solids known as
wax. It must be emphasized that at up to 50 carbon atoms the material is far
from being polymeric in the ordinary sense of the term.
The polymeric alkanes with no side groups that contain 1000 to 3000 carbon
atoms are known as polyethylenes. Polyethylene has the chemical structure
which originates from the structure of the monomer ethylene, CH2=CH2.The
quantity n is the number of mers—or monomeric units in the chain. In some
places the structure is written
or polymethylene. (Then n’ = 2n.) The relationship of the latter structure to
the alkane series is clearer.While true alkanes have CH3— as end groups, most
polyethylenes have initiator residues.
Even at a chain length of thousands of carbons, the melting point of polyethylene
is still slightly molecular-weight-dependent, but most linear polyethylenes
have melting or fusion temperatures, Tf, near 140°C. The approach to
the theoretical asymptote of about 145°C at infinite molecular weight (1) is
illustrated schematically in Figure 1.1.
The greatest differences between polyethylene and wax lie in their mechanical
behavior, however. While wax is a brittle solid, polyethylene is a tough
plastic. Comparing resistance to break of a child’s birthday candle with a wash
bottle tip, both of about the same diameter, shows that the wash bottle tip can
be repeatedly bent whereas the candle breaks on the first deformation.
