Modern Polymer Spect..

3.18 Band Broadening and Confornmtioiud Flesibility 179
molecule is embedded in a medium which transforms the free rotation into a librational
motion d- and the libration can, in principle, couple because they belong
to the same symmetry species. A similar observation was made by Snyder for the
CH2 rocking mode 11461.
The contribution toward the understanding of chain flexibility comes from the
following experiments and calculations which are described in detail in 11371 and
[145].
1. The first experiment has been that of recording the Rainan spectra of long
n-alkanes in urea clathrates [ 1451 and in another host lattice (perhydrotriphenylene)
where each guest n-alkane molecule sits in a channel, isolated from
the other chains and retaining, on average, the all tram-planar structure. Similar
experiments are also reported in [146] and [149]. By changing the temperature,
the host lattice changes the average diameter of the columns where the guest
molecule sits, thus allowing a varying degree of librational freedom for the guest
molecule. While at low temperature the librational motion is strongly hindered,
at relatively higher temperatures the lattice expands anisotropically allowing the
guest molecule to perform librational motions of large amplitude.
We expect the band of the d- mode to be narrow at low temperatures and to
broaden at higher temperatures. Figure 3-45 shows that this is indeed the case.
The band not only broadens but its frequency also shifts upward by a few
wavenumbers. It is concluded that the almost rigid (and possibly small amplitude)
collective librations about the chain axis at low temperature evolve with
increasing temperature towards a collective libro-torsional (or libro-twisting)
motion with nonnegligible angular distortions more or less hindered by the
environment.
2. It must be pointed out that the broadening and upward frequency shift of the d-
mode cannot be ascribed to the generation of gauche defects inside the host lattice.
It has been verified experimentally that the frequencies of LAMl, LAM3,
and LAM5 of the guest molecule embedded in the host lattice are identical to
those observed for the same n-alkane in the crystalline state [145]. Hence, the
molecule does not change its length from that of the tvaizs structure [145].
3. In going from the solid to ‘softer’ systems, certainly the frequency of the d-
mode shifts upward by a few wavenumbers. No shifting (nor broadening) is
observed for crystalline n-alkanes until the so-called soft ‘rotatory phase’ is
reached before melting. In the so-called a phase (see Section 3.19) some shifting
and broadening is observed.
Levin [150] and Gaber and Peticolas [151] have shown that the upper shift
and broadening occurs also in bioinembrane materials. Such a shift has been
used by Gaber and Peticolas to monitor the ‘lateral interactions’ in phospholipids.
We notice that for these phospholipid-type material the long-chain alkyl
residues are fixed at one end to polar head groups and thus cannot perform rigid
librational motions about their axes. Forcefully they can only perform librotwisting
or libro-torsional motions.
4. The last step is to account for the dynamical origin of the fact that when the long
polyniethylene chain increases its libro-torsional flexibility the frequency of d-