Modern Polymer Spect..

154 3 Vihtional Spectvu as a Probe of Structurd Order
1500 1460 1440 1420 1 uoo
Wa v e n urn be r s
Figure 3-31. Infrared absorption spectrum of a coininercial film of polyethylene before and after
subtraction ofthe spectrum of the defect mode component near 1440 cm-'. After subtraction of the
defect mode the ratio between compoiient a and b of the correlation field splitting approached the
ideal value for a setting angle of42" (see text).
polyethylene [122]. Certainly the wing at the higher frequency side of the defect
mode overlaps with at least line b of the doublet, thus adding more intensity to line
b and removing any physical meaning to be observed intensity ratio between the
two components. However, by difference spectroscopy it is possible to remove the
absorption due to the conformationally irregular fraction.
This observation is the basis for the determination of the amount of material with
pzdw structure in any sample consisting of polymetliylene chains partially crystalline
and partially conformationally disordered amorphous.
This method has been applied to experiments of multiple internal reflection infrared
spectroscopy on thin films of commercial polyethylene [120]. It is known that
'optical microtomy' can be achieved by changing the incidence angle of the inconiing
beam. Structural depth profiling has then be performed thus allowing to probe
the ordered-disordered structure of such films (Figure 3-32).