Modern Polymer Spect..

3.14 Case Studies 157
the molecular level. Many authors have focused on the vibrational infrared and
Raman spectra as probes for understanding the structure and dynamics of these
systems.
We have tried to analyze a few cases with our spectroscopy of defect modes and
mention here some of the systems studied.
1. Fattj, acids. Generally, fatty acid crystallize as dimers held together by hydrogen
bonds between the two carboxyl groups which face each other in a head-to-head
arrangement. In other sections of this chapter the contributions to the understanding
of the 'static' structure given by the spectroscopy of mass defects and by
LAM spectroscopy have been already presented. The case of fatty acids with
long polymethylene chains has been studied [ 1241 focusing on the conformational
evolution which originates the phase transitions. FTIR spectra clearly and
distinctly show that when thermal energy is provided to stearic acid in the crystalline
state first interlamellar CH? ...CH3 distances increase (as seen in the case
of n-nonadecane; see Section 3.19) and hydrogen bonds then relax. Simultaneously,
intermolecular distances increase with resulting lateral expansion of the
lattice. These phenomena become relevant approximately 10 " below melting.
Few confonnational kinks are generated below melting and only close to the
melting the concentration of GTG, GTG', and GG defects increases rapidly
bringing the system to a conformational collapse in the liquid phase [124].
However, most of the conformational disorder occurs on the lamella surface,
thus adding more evidence to the existence of a general phenomenon of 'surface
melting' which emerges from our studies on many chain molecules [125]. These
results give a detailed description of the mechanism of phase transition in fatty
acids which was vaguely hinted by NMR experiments [ 1261.
2. Bilajw systenis. The bilayer organic perowskytes were obviously used as simple
models of biological bilayered phospholipids and biomeinbrane systems. We
have studied [127] the system [n-CH3( CH2)"NH3]2MnC14 (hereafter referred to
as C14Mn) which in the solid forms two coexisting, but parallel, layers; a similar
molecule with Zn (C14Zn) in place of Mn forins two interpenetrating or 'intercalated'
layers. The conformational flexibility and phase transitions of these two
classes of molecules turns out to be different. DSC data indicate for C14Mn a
main phase transition in the range 74-85" which is described by our conformational
studies as involving the formation exclusively of GTG' kinks which do
not alter the conformational trajectory of each alkyl stem. No GG defects are
observed and clusters of ordered and disordered chains coexist over a large
temperature range (67-80°C). By simulation of the spectra it is found that, on
the average, at 78°C only one GTG' kink per chain is formed; between 65 and
78 "C chains with only one kink are formed and arrange themselves in clusters. It
follows that disorder proceeds cooperatively throughout the system.
The behavior with temperature of C14Zn is totally different [127]. The main
phase transition at 100°C is accompanied by an abrupt generation of conformational
defects at 99 "C. Above melting, chains have reached almost a liquidlike
structure. Correlations between chains do not exist. However, from the
temperature dependence of the factor group splitting of the CH2 bending and