Chemistry Reference
In-Depth Information
Fig. 2.1
Intramolecular
forces
O
O
O
O
O
R'
R'
R
O
O
R
O
O
R
O
O
O
O
O
R'
R'
R
O
O
R
O
O
R
2.1.2
Induction Forces in Polymers
Electrostatic forces also result from slight displacement of electrons and nuclei in covalent molecules
from proximity to electrostatic fields associated with the dipoles from other molecules. These are
induced dipoles
. The displacements cause interactions between the
induced dipoles
and the
perma-
nent dipoles
creating forces of attraction. The energy of the induction forces, however, is small and
not temperature-dependent.
There are additional attraction forces that result from different instantaneous configurations of
the electrons and nuclei about the bonds of the polymeric chains. These are time varying dipoles
that average out to zero. They are polarizations arising from molecular motions. The total bond
energy of all the secondary bond forces combined, including hydrogen bonding, ranges between
2 and 10 kcal/mole. Of these, however, hydrogen bonding takes up the greatest share of the bond
strength. In Table
2.1
are listed the intramolecular forces of some linear polymers [
2
,
3
].
As can be seen in Table
2.1
., polyethylene possesses much less cohesive energy than does a
polyamide. This difference is primarily due to hydrogen bonding. A good illustration is a comparison
of molecules of a polyamide, like nylon 11, with linear polyethylene. Both have similar chemical
structures, but the difference is that nylon 11 has in its structure periodic amide linkages after every
tenth carbon, while such linkages are absent in polyethylene. The amide linkages participate in
hydrogen bonding with neighboring chains. This is illustrated in Fig.
2.2
.
Due to this hydrogen bonding, nylon 11melts at 184-187
Cand is soluble only in very strong solvents.
Linear polyethylene, on the other hand, melts at 130-134
C and is soluble in hot aromatic solvents.
The energy of dipole interactions, (
Є
k
) can be calculated from the equation [
1
]:
4
RTÞr
6
C
k
¼ð
2
m
=
3
where,
m
represents the dipole moment of the polarized section of the molecule,
r
is the distance
T
R
between the dipoles,
is the ideal gas constant.
Intermolecular forces affect the rigidity of all polymers. Should these forces be weak, because the
cohesive energy is low (1-2 kcal/mole), the polymeric chains tend to be flexible. Such chains respond
readily to applied stresses and can exhibit typical properties of elastomers. High cohesive energy, on
the other hand, (5 kcal/mole or higher) causes the materials to be strong and tough. These polymers
exhibit resistance to applied stresses and usually possess good mechanical properties. The
temperatures and the flexibility of polymeric molecules govern both the sizes of molecular segments,
is the temperature in Kelvin, and
