Modern Polymer Spect..

1.2 Bcrckgroinizd 7
where AA (i~) is the iilti/ncrte dichroic d~juwrce which is related to D, ( v) by
The ~ t r ~ ~ absorbcince, r ~ w d A,( if) = [A (1)) + 2AI (v)]/3, is believed to be independent
of the degree of molecular orientation as long as the system remains symmetric
with respect to the optical axis.
If one accepts that the transition dipole orientation angle ci is a fixed molecular
constant unaffected by the conformation of polymer chains, then D, and AAz
should also be independent of the state of chain orientation. The classical theory
of IR dichroisin for polymers thus makes an interesting assertion, according to Eq.
(1-7), that IR dichroic difference AA(v) is alivays linearly proportional to the average
orientation Pz(0) of polymer chains regardless of the IR wavenumber v. In
other words, one should be able to determine the state of orientation of the entire
polymer chain by simply observing the local orientation of a transition dipole
associated with the molecular vibration of any arbitrarily chosen local functional
group, as long as ci is known.
1.2.4 Breakdown of the Classical Theory
An interesting observation often made during the DIRLD measurement of polymers
is that the phase angle p(~) between the applied strain and dynamic IR
dichroic diff‘erence is strongly dependent on the IR wavenumber [3, 33-25]. Timedependent
dichroism intensities measured at certain wavenumbers change much
faster than others. Individual dynamic IR dichroisin signals thus become out of
phase with each other as depicted in Figure 1-8. As already demonstrated in Figure
1-6, the shape of the in-phase spectrum also becomes quite different from that of the
quadrature spectrum. The relative amount of dynamic dichroism signal appearing
in the in-phase and quadrature components varies considerably as a function of
wavenumber.
t
Figure 1-8. Tinie-dependent sinusoidal u
DIRLD signals from an atactic poly- 5
(methyl methacrylate) sample detected at 0”
different IR wavenumbers. Signals are out
of phase with each other.
a:
(2952 cm-1)
r
Time, t