Modern Polymer Spect..

1380 1340 1302
Figure 3-28. Coinparison between the calculated and esperimental
infrared spectrum in the CH2 wagging region of the
GGTGG conformational defect. Calculations have been made
the with force field by Shiinanouchi [I 191. The intensity
cm-'
paraiiieters were developed in the laboratory of the author.
{ 1 lo} and { 100) crystallographic planes. By changing the crystallization conditions,
sectored crystals with varying ratios between the { 110) to {loo} sectors can
be obtained. Chain-folding studies has also indicated that tight-fold re-entry with
GGTGG defects can only occur when chain crystallize in the {loo} planes.
Sectored crystals were kindly provided to us by Professor A. Keller with the following
% of { loo} faces: 0, 31, 35,45, and 55. The experiment of difference infrared
spectroscopy we attempted is sketched Figure 3-29. We aimed at isolating the absorption
to be associated with the GGTGG possibly existing on the surface of the
{loo} fraction of the sectored crystals. The experiments were not easy and after
many attempts the final spectrum obtained (Figure 3-30) is compared with that of
the cyclic hydrocarbon C34H60. The matching of the two spectra is striking and does
suggest that, in agreement with calculations, on the surface of polyethylene single
crystals obtained in particular crystallization conditions tight fold re-entry with
GGTGG defects may indeed occur.
3.14.2.4 Case 4: The Structure of the Skin and Core in Polyethylene
Films (Normal and Ultradrawn)
In addition to the defect modes presented in the previous sections it has been shown
recently by both calculation and experiments that generally when yazrclze defects
exist in polyethylene a broad band arises in the CH2 bending range near 1440 cm-'
[ 1201. The band is very broad, possibly because it originates from a distribution
of nonequilibrium gauche structures constrained by the sample morphology (Figure
3-31). Close to such absorption one observes for crystalline orthorhombic n-alkanes
and polyethylene the doublet due to crystal field splitting (see Section 3.6). Let a
and b label the two components observed near 1473 and 1463 cm-' respectively
(Figure 3-31). It is known that the setting angle B between the planes defined by the
tr.arzs-planar chain skeleton in the lattice is approximately 42" [121]. Such value of
0 determines the intensity ratio IJIb = 1.2333 between the two components of the
crystal doublet of the CH2 bending and CH2 rocking in orthorhombic n-alkane and