Modern Polymer Spect..

146 3 Vibratioiiul Sycctm CIS (I Probe of Structural Ordcr
3-25a); (ii) palinitic acid ?-Dz (Figure 3-25b); and (iii) pahnitic acid 3-D2 (Figure
3-25~1. The clear band which appears near 620 cn-' is the gap-niode indicating
unquestionably the existence of the local conformation CH?(T)-CD?IT)-CH? both
in positions 3 and 4. Identical spectra have been observed along the whole chain,
thus confirming the whole lrms structure of palinitic acid in the solid state [107,
110, 1111.
3.14.2 Molecular Mobility and Phase Transitions in Therrnotropic
Liquid Crystals from the Spectroscopy of Defect Modes
The issue is the description of the molecular changes accompanying the phase
transitions in systems which show liquid crystalline phases. While much is known
on the morphological changes, little is known on what happens to the molecular
structure through the various phase transitions.
We use defect mode spectroscopy for aiming at the structural changes of prototypical
thermotropic liquid crystals such as alkylcyanobiphenyl and polyesters.
3.14.2.1 Case 1: Cyano-alkyl biphenyls
Let us first consider dodecyl-cyanobyphenyl which shows two phase transitions at
48 "C (crystalline-semectic A) and at 58.5 "C (smectic A-isotropic). The analysis
of the whole spectrum is reported in the original work [112] and we focus here only
on the variation with temperature of the conforiiiational structure of the dodecyl
side chain which we hope to reveal with the study of the temperature-dependent
vibrational spectrum in the 1420-1280 cm-' range where defect modes are expected
to occur.
Figure 3-26 reports such spectra, together with the difference spectra obtained in
the following way. Let s(J + 1) be the spectrum recorded at temperature T(J + 1)
and s(j) the spectrum taken at temperature T(j). Then AT = T(J + 1) - T(j) is the
temperature gradient in our experiment. From Figure 3-26, while drastic changes
are observed from the crystalline to smectic A phase, the spectra do not change
much from sniectic A to isotropic. The difference spectra highlight such changes
and allow the defect modes GTG', GTG, GG, and end-TG to be identified. The
following conclusion can be easily reached: (i) the trans-planar chain exists up to the
crystal-smectic A transition; (ii) at the crystal-smectic A transition the trans-planar
chain collapses in a conforinationally distorted geometry, as indicated by the appearance
of all defect modes listed above; (iii) the observed upward shift of the
umbrella deformation mode of the CH3 group from 1376.5 to 1378 cm-l indicates
that the environment at the end of the chain is changed; and (iv) no great conformational
changes occur from the sniectic A to the isotropic phase; the cyanobiphenyl
core must then supervise this phase transition (as also shown by the
vibratioiial spectrum). Another case treated is that of 4-cyano, 4'octyloxy biphenyl
[113].