Chemistry Reference
In-Depth Information
The time required for the transition between trans and gauche states will
depend on the height of the energy barrier
RT/L, the bar-
rier height is not significant and trans/gauche isomerizations will take place in
times of the order of 10
2
11
sec. When a macromolecule with small
Δ
E in
Fig. 1.6
.If
Δ
E
,
Δ
E is
stretched into an extended form, the majority of successive carbon
carbon links
will be trans, but gauche conformations will be formed rapidly when the mole-
cule is permitted to relax again. As a result, the overall molecular shape will
change rapidly from an extended form to a coiled, ball shape. This is the basis of
the ideal elastic behavior outlined in more detail in
Section 4.5
. Note that a
stretched polymer molecule will recoil rapidly to a random coil shape only if (1)
there is no strong preference for any staggered conformation over another (
e is
small; there is little difference between the energy minima) and (2) if the rotation
about carbon
Δ
E is small; the energy
barriers between staggered forms are small). If condition (1) holds but (2) does
not, the polymer sample will respond sluggishly when the force holding it in an
extended conformation is removed.
The trans staggered conformation is a lower energy form than either of the
gauche staggered forms of polyethylene. The difference is much less for polyiso-
butene, however, as illustrated in
Fig. 1.7
. Here the chain substituent on the rear
carbon shown is either between a methyl and polymer chain or between two
methyl groups on the other chain carbon atom. Since no conformation is favored,
this polymer tends to assume a random coil conformation. The polymer is elasto-
meric and can be caused to crystallize only by stretching. However, rotations
between staggered conformations require sufficient energy for the chain to over-
come the high barrier represented by crowded eclipsed forms, and polyisobutene
does not retain its elastic character at temperatures as low as those at which more
resilient rubbers can be used.
carbon bonds in the main chain is rapid (
Δ
CH
2
CH
2
CH
2
CH
2
H
CH
2
H
H
H
H
3
C
CH
3
H
3
C
CH
3
H
3
C
CH
3
H
CH
2
H
gauche
−
trans
FIGURE 1.7
Newman projections of staggered conformations of adjacent carbons in the main chain of
polyisobutene.
