Modern Polymer Spect..

I94
3 k'ibrcitionul Specfra CIS a Probe of Structural Order
a
ORTH
a
\N\ 3 ~
y.\\\ \ \ '
b
Figure 3-58. Sketch of the mechanism of phase transition of n-nonadecaiie based on detailed
spectroscopic evidence. (a) Schematic overall view; (b) details on a molecular level.
Let us take two samples of crystalline orthorhombic n-alkanes with an odd
number of carbon atoms. They show properties identical to those observed for C19.
Let us make by suitable methods a mechanical mixture say of C21 and C23. The
infrared spectra of the mechanical mixtures at low temperature are just the superposition
of the spectra of the two independent components. Let us focus on the
crystal field splitting, say of the doublets near 940 cn-' (C23) and 925 cin- ' (C21 j
(Figure 3-59). If the mechanical mixture is brought near the solid-solid phase transition
the infrared spectrum evolves with time. Time- and temperature-depeiideiit
spectra show that the doublets become singlets thus showing that chains have
moved (Figure 3-59). The final product is a mixed crystal of C21 and C23 as also
proved by parallel DSC studies. These studies were first reported by Ungar and
Keller [ 1561 and were re-examined and interpreted as reported in [ 1551.