Chemistry Reference
In-Depth Information
that between the polymer chain substituents and is taken here to be zero when the
two chain segments are as far as possible from each other.
When the two chain segments would be visually one behind the other if
viewed along the polymer backbone, the conformations are said to be eclipsed.
The other extreme conformations shown are ones in which the chain substitu-
ents are staggered. The latter are lower energy conformations than eclipsed forms
because the substituents on adjacent main chain carbons are further removed from
each other. The lowest energy form in polyethylene is the staggered trans confor-
mation. This corresponds to the planar zigzag form shown in another projection
in
Fig. 1.6
. It is also called an all-trans conformation. This is the shape of the
macromolecule in crystalline regions of polyethylene.
The conformation of a polymer in its crystals will generally be that with the
lowest energy consistent with a regular placement of structural units in the unit
cell. It can be predicted from a knowledge of the polymer configuration and the
van der Waals radii of the chain substituents. (These radii are deduced from the
distances observed between different molecules in crystal lattices.) Thus, the
radius of fluorine atoms is slightly greater than that of hydrogen, and the all-trans
crystal conformation of polyethylene is too crowded for poly(tetrafluoroethylene)
which crystallizes instead in a very extended 13
1
helix form. Helices are character-
ized by a number of f
j
, where f is the number of monomer units per j complete
turns of the helix. Polyethylene could be characterized as a 1
1
helix in its unit cell.
Helical conformations occur frequently in macromolecular crystals. Isotactic
polypropylene crystallizes as a 3
1
helix because the bulky methyl substituents on
every second carbon atom in the polymer backbone force the molecule from a
trans/trans/trans
conformation into a trans/gauche/trans/gauche
sequence
...
...
with angles of rotation of 0
(trans) followed by a 120
(gauche) twist.
In syndiotactic polymers, the substituents are farther apart because the config-
urations of successive chiral carbons alternate (cf.
Fig. 1.5
). The trans/trans/trans
...
planar zigzag conformation is generally the lowest energy form and is
observed in crystals of syndiotactic 1,2-poly(butadiene) and poly(vinyl chloride).
Syndiotactic polypropylene can also crystallize in this conformation but a trans/
trans/gauche/gauche
sequence is slightly favored energetically.
Polyamides are an important example of polymers that do not contain pseudo-
asymmetric atoms in their main chains. The chain conformation and crystal struc-
ture of such polymers is influenced by the hydrogen bonds between the carbonyls
and NH groups of neighboring chains. Polyamides crystallize in the form of sheets,
with the macromolecules themselves packed in planar zigzag conformations.
The difference between the energy minima in the trans and gauche staggered
conformations is labeled
...
Δ
e in
Fig. 1.6
. When this energy is less than the thermal
energy RT/L provided by collisions of segments, none of the three possible stag-
gered forms will be preferred. If this occurs, the overall conformation of an iso-
lated macromolecule will be a random coil. When
RT/L, there will be a
preference for the trans state. We have seen that this is the only form in the poly-
ethylene crystallite.
Δ
e
.
