Modern Polymer Spect..

158 3 Plhrutional Spectru CIS Q Probe qf'StructLirri1 Order
CH2 rocking inodes it is observed that the transition is 'prepared' since the
intermolecular forces between chains in C14Zn [weaker than in C14Mn) quickly
weaken and the thermal expansion gives more freedom for torsional fluctuations
of the alkyl chains, thus leading to conformational collapse.
Two classes of mechanisms which lead molecules to phase transition and melting
have been found, namely: (i) nucleation of conforniational disorder from clusters
whose size continuously increases, thus affecting later all chains; and (ii) cooperative
expansion of the whole lattice caused by an increase with temperature of the mean
amplitudes of torsional fluctuations which reaches a threshold level after which
conformational collapse occurs.
In nature, many other polymethylene systems-also of practical relevance-can be
found which show phase transitions that need to be accounted for. More theoretical
and experimental spectroscopic work needs to be done. We have studied the spectra
of intercalated layered alkylaininonium zirconium phosphates [ 128) and of the solutions
of decylammonium chloride [ 1291. The spectroscopic data present U~~LISLI~~
features with temperature which need further theoretical and experimental studies
using the techniques presented in this chapter.
3.15 Simultaneous Configurational and Conformational
Disorder. The Case of Polyvinylchloride
The cases treated in the previous sections were restricted to the study of the few
cases of mass defects and of the very many cases of conformational defects. Most of
the relevant polymers, however, may contain configurational disorder which carries
as a consequence also some conformational disorder. This case has been considered
within the theoretical approach discussed in this chapter.
The mathematical aspects for a general case of polymers with defects in tacticity
were first worked out, formulae are available and can be applied to any case. We
have applied these mathematical techniques to the case of polyvinylcloride (PVC)
which is known to be strongly configurationally disordered. Our work was done
when the problem of the structure of PVC was highly controversial.
First, the dynamics of ordered PVC was treated in terms of the three energetically
most likely ordered chain structures [go], namely: (i) planar extended syndiotactic
with (-TTTT-) conformation; (ii) helical isotactic, threefold helical structure
(-GTGTGT-); and (iii) folded syndiotactic (-TTGGTTGG-) (Figure 3-12). Phonon
dispersion curves and density of vibrational states were calculated for the three
models using the most reliable force fields available at that time (Figure 3-12). The
infrared and Raman activity of the k = 0 phonons for each perfect chain are the
following: model (i), z = 2, t = 1, 12 atoms per crystallographic unit cell, $(n, d/2),
point group, 9A1 (ir 11, R) +7A2 (R) t9B1 (ir I, R) +7B? (ir I, R); model (iii
z = 3, t = 1, 18 atoms per unit cell, $(2x/3,d/3), point group, 16A (ir 11, R) +