748 Bravard and Boyd
Macromolecules, Vol. 36, No. 3, 2003
hypothesis that the â* relaxation in PEN is related to
the â3 component in PET, but the former is shifted to
lower frequency isothermally or higher temperature
isochronally compared to the latter.
C-O bond has the lowest one.25,26 Molecular dynamics
(MD) simulations of PET show that dipolar relaxation
responds to the bond types in this fashion.26 The slowest
component of relaxation is associated with the CA-C
bonds, the next with the C-C bond, and the fastest
component with the C-O bonds. It is also found in MD
simulations that dipolar relaxation in PEN is slower
than in PET a circumstance only attributable to the
hindrance induced by the bulky naphthoic units. Al-
though the situation is no doubt more complex, it seems
to be useful to think of the three relaxation components
as associated with the bonds as â1 ∼ C-O; â2 ∼C-C;
â*, â3 ∼ CA-C.
There are two simple scenarios as to the nature of
the shifting process from â* to â3 as the PET content
increases. The first would be that the process continu-
ously moves to higher frequency or lower temperature.
The other would be that the â* process disappears at
low frequency while the â3 process grows at high
frequency without a frequency shift. The first scenario
would imply on the molecular scale that the â* ∼ â3
mechanism involves some degree of cooperativity in
groups of bond rotations or conformational jumps. The
second scenario would be more appropriate to a highly
localized association of the process to specific bond
rearrangements.
The experimental data do not totally distinguish
between the two scenarios above. As noted (section PET/
PEN Copolymers: Subglass â* Process), because of the
interference of the R process, the phenomenological
analysis of the â* process was incomplete in that only
log fmax behavior was available for the lowest PEN
content copolymer (2PET/1PEN). It does seem clear,
however, that there is an actual shift in the â* location
with increasing PET content (Figure 13) with a decrease
in activation energy toward that of the â3 component
in PET (Figure 7). It also seems clear that there is no
appearance of the â3 component at the position observed
in PET homopolymer as PET content increases in the
copolymers studied (Figures 11 and 14). These observa-
tions are consistent with the first scenario. However,
the degree of shift in position over the compositions of
the first two copolymers is rather slight (Figure 10).
Thus, it appears that the shifting scenario is basically
correct. However, the slowing of the chain dynamics
associated with the â* process seems to be sensitive to
naphthoic unit content in a manner that would indicate
interaction over several monomeric units. That is,
keeping in mind that the conformationally mobile bonds
are in the ester moieties spanning two aromatic rings,
suppose that the dynamics were sensitive to the com-
position of a pair of monomers units NN vs NT vs TT
(where N ) naphthoic, T ) terephthalic) so that the
dynamics were slowed by the N unit in either NN or
NT pairs. Then it would follow that the relaxation would
tend not to respond to T content until high T content,
i.e., a predominance of T-T pairs.
Ack n ow led gm en t. The authors are indebted to the
Polymers Program, National Science Foundation, for
financial support of this work. They are grateful to Dr.
J . C. Coburn of the DuPont Co. for furnishing samples
of amorphous PET and PEN films.
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Finally, it worth speculating on the occurrence of
three subglass components (â1, â2, â3 ∼ â*). It is
tempting but perhaps to some degree naive to associate
these with the three conformationally flexible bonds in
the structures that allow dipolar relaxation. Keeping in
mind that the dipole is effectively directed along the
ester carbonyl bond and that the ester unit is effectively
conformationally rigid, the mobile bonds are the aro-
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