Modern Polymer Spect..

3 9 Mouiiiq ToIvmds Rerrlitv: From Order to Disordrr 123
I " ~ ' " " l " " l ' ' ' '
I 1
Figure 3-13. Inelastic neutron scat- 1 , , , , I , , , , I , , , I , , , ,
teriiig (INS) spectrum of polyethyl- 0 1000 2000 3000 4000
ene at 30 K (from [86]).
wavenumbedcm -'
The ideal infinite polymer chain iiz ziucuo has allowed to consti-uct dynamical
theories which describe ideal spectroscopic properties of the polymer determined
through strict optical selection rules and obtained by ideally clean experiments. In
reality, polymerization reactions never yield perfect chemical linking and perfect
stereo-regularity; these chemical and stereochemical defects necessarily introduce
conformational distortions as to avoid steric encumbrance. Moreover, the complex
processes of crystallization from melt or solution of a very long and flexible object
implies the generation of various kinds of morphologies which reflect particular
packing of 'conformationally' ordered chains. New concepts such as chain folding,
looping, tie molecules, fringed micelles, etc., are necessarily introduced for the description
of the building up of the supermolecular structure of a polymer chain. It
follows that any polymer material which has been allowed to crystallize consists of
a fraction of matter crystallized in a 3D-lattice and a fraction of 'conformationally
irregular' material [46, 871.
In polymer material science it is essential to know the detailed independent
structural characteristics (and the relative concentrations) of the crystalline and
'amorphous' (or irregular) fractions of a polymer sample. All mechanical, viscoelastic
or, in general, the physical properties of real polymeric materials depend on
the relative fraction of crystalline versus non crystalline material.
Since the spectroscopic manifestations of molecular disorder are many and very
characteristic, the vibrational spectrum has become a usefyl probe for molecular
order-disorder which can be described at the level of a few Angstroms.
Spectroscopists are asked to find ways to account for the new spectral features
associated to disorder, thus making vibrational spectra useful for the practical
structural characterization of a real polymer material. Obviously, the theories presented
in the previous sections of this chapter must be adapted or re-worked in
order to understand the spectra of disordered organic polymers.
The prototypical case of an experimental observation to be accounted for is the
spectrum of a sample of orthorhombic polyethylene [88] (Figure 3-14). Let us con-