Modern Polymer Spect..

150 3 T’ibratioiinl Spectra L ~S u Probe oj StrzictLir.al Order.
intensity of the GTG’ defect inodes increases sharply at the K-S transition, remains
on a smooth platem until the S-I transition occurs, and increases steeply in the
I phase. It has to be noted that even in the I phase the GG defect mode is still
practically not observable.
The mechanism on a molecular level which is derived from the vibrational spectra
can be suniinarized as follows: at room temperature the sample contains a sizable
concentration of ordered material in which the decamethylene spacers are
predominantly in the trms-planar structure. When the I(-S transition is
approached the main spectroscopic signal observed is that associated to the GTC’
modes. It is known that the GTG’ defect introduces a kink in the chain but does not
change the trajectory of the polyniethylene sequence keeping both arms at either
side of the defect parallel. The overall effect is that the length of the decaniethylene
segment simply shrinks. It follows that in the S phase the spacers shrink because of
the introduction of conformational kinks, inducing a sort of shearing motion between
the large aromatic mesogenic groups. The lack of any sizable concentration
of GG structures even in the I phase leads to the interesting conclusion that the
molecular structure in the I phase is not much different from that in the S phase. No
indication is found of ‘liquid-like structures’ observed in other polyniethylene systems
in the liquid phase. It has been proposed [114] that one way to reconcile
the observation of macroscopically isotropic phase with the observation on the
molecular level is to envisage that the isotropic phase consists of a collection of
microdomains whose directors are randomly oriented. However, inside the microdomains
polymer molecules are organized as they are in the S phase.
3.14.2.3 Case 3: Chain Folding in Polyethylene Single Crystals
It is known that chain folding occurs in polyethylene crystals; the fold structure,
however, has been a subject of interest and controversy [105]. It was obvious for
polymer spectroscopists to use vibrational infrared and/or Raman spectra to try to
contribute to the understanding of the shape of the fold (i.e., the molecular conformation)
within the fold at the surface of polyethylene single crystals.
The large amount of spectroscopic work carried out by many authors is
sumarized in [115]. Here, we wish to focus specifically on the contribution given by
defect mode spectroscopy considering both calculations and experiments. The issue
is whether the surface of polyethylene single crystals consists mostly of loosely
looped folds or also of a fraction of chains with a tight adjacent re-entry. In the
former case, ‘liquid like’ defects such as GG, GTG, GTG’ etc. are expected and can
be recognized in the vibrational spectrum. If tight fold re-entry exists defects of the
type GGTGG must occur at the surface of the lainellar single crystal.
The first question which needed to be answered in a less qualitative way was
where to locate the vibrations localized on the GGTGG defect.
Calculations have been carried out on a long polymethylene all-tivm host molecule
containing evenly spaced GGTGG defects. NET and IIM and the ‘island
analysis’ were applied [99]. The most meaningful results are presented below.
Two highly localized modes are calculated to lie near 1361 and 1358 cin-’ and