Modern Polymer Spect..

3.18 Bmd Brourleiziriy md Cmforrnational Flrxibilitjj 175
IL-180°C
-1
cm
3050 3000 2950 2900 2850 2800 2750
Figure 3-45 Raman spectrum of C~jH52 in urea clathrate at 3°C and -180°C. Notice that for
d-At,,,,2 = 6 cn-' at T= -180°C and Ai~,p = 28 cm-' at T= 25°C; Ai~,p of d' remains practically
constant at the same two temperatures.
narrow at low temperatures and broadens substantially at higher temperatures
(Figure 3-45).
3.18.1 Principles
In Section 3.17, experiments have been presented showing that in the case of polyinethylene
molecules (including polyethylene), in going from the solid all-tlvrizs to
the liquid phases, the pattern of Raman scattering changes dramatically but in
a consistent and characteristic way. We focus our attention on the temperaturedependent
broadening observed for the d- and 8 modes; in contrast, the width and
shape of d+ modes are practically temperature-independent. In Section 3.17.2 it was
suggested that one possible explanation of the band broadening of d- could be a
convolution of bands associated with d- modes in different quasi g~iziclie conformations
existing in the liquid or 'amorphous' material.
This interpretation may be only partial since Snyder has shown that band
broadening of d- and 6 modes is certainly observed also for hexadecane in uiea
clathrates where the molecule is believed to be (on average) in the tu.m~-planar