Modern Polymer Spect..

3.14 Case Studies 15 1
correspond to the out-of-phase motions of the two CH, groups located between two
C-C bonds in gcruchc conformation. These two modes are largely localized at the
defect while the other portion of the chain remains practically still. It is interesting
to note that the defect modes in the CH2 wagging region near 1350 cm-' show that
in this spectral range the GGTGG defect behaves as if it were made up of GG and
GTG defects moving almost independently.
In the lower-frequency region defect modes are calculated near 1105, 900, and
714 cm-'. The mode near 714 cni-' is of particular interest since it lies just below
the limiting k = 0 mode located at 720 cn-l of the CH2 rocking dispersion curve
and is just at the edge of a large energy gap which extends as far as the cut-off of the
branch cu5 (see Figure 3-3). From IIM, we learn that the defect mode near 714 cmP1
originates from a cluster of several CH, groups, including the defect, which is performing
mainly a rocking motion. From the shape of the nornial mode this nearthe-edge
mode cannot yet be considered a localized gap mode since it shows a
nonnegligible degree of cooperativity. Similar cases of near-the-edge resonances
were found for polyethylene in the lower frequency range and for polyvinylchloride
containing head-to-head defects [90].
The modes calculated near 1105 and 900 cm-l are the results of a complicated
mixture of several internal coordinates, but can be described mainly as C-C resonance
modes. They are likely to be observed in the Ranian spectra.
The results of the calculations were checked with the experimental infrared and
Raman spectra of the cyclic hydrocarbon C34H68 whose structure determined by
Newiiian and Kay [116] indicates that two segments of 12 CH? groups in a transplanar
zig-zag conforination are joined by two GGTGG defects. Coincidences
between experiments and calculations are striking. However, a critical analysis
needed to be made ([99, 1151) on whether these signals are characteristic only of the
GGTGG defect and may allow such a defect to be disentangled from the signals of
other conforniational defects possibly existing in a realistic polyethylene single
crystal. The conclusion reached is that calculations on several model systems and
experiments on C34H68 suggest [99] that a simultaneous appearance in the infrared
spectrum of two bands near 1342 cm-' and one near 700 cm-' (certainly below the
in-phase limiting CH2 rocking mode of the tram-planar chain) can be taken as evidence
for the existence of GGTGG defects in polyethylene single crystals. The other
calculated modes occur at frequencies where other modes of the host lattice may
also occur. Calculations were also extended to predict the intensity pattern for
the defect modes in the CH2 wagging range [117, 1181. Figure 3-28 compares
the experimental spectrum with that calculated using the force field proposed by
Shimanouchi [ 1 191.
Experiments of difference spectroscopy combined with band deconvolution processes
were presented and conclusions were not unambiguous [ 1 151. On the other
hand, the arbitrary decisions taken when applying band deconvolution techniques
sometimes make issue more complex. We have tried to avoid band deconvolution
and proceeded directly to careflil experiments of difference spectroscopy on sectored
polyethylene single crystals. The principle on which the experiments was based is
the following. From the study of sectored polyethylene single crystals it is known
that sectors correspond to regions where molecular chains are crystallized in the