Modern Polymer Spect..

3.19 A Worked E.utinple 181
pre-melting state for n-alkanes; just a few degrees above ToLx the crystal melts. The
temperature range of existence for the t( phase decreases with increasing the length
of the chain and is also pressure-dependent. Even polyethylene seems to show a
similar pre-melting state just a few degrees before melting.
The generality of the phenomenon to be understood was the reason which has
convinced us of the need to perfom a detailed spectroscopic study on the simplest
system, n-nonadecane which shows the largest temperature range for the existence
of the a phase.
We have studied the following molecules: n-nonadecane, n-nonadecane-2D2 and
n-nonadecane- 1 OD? (hereafter referred to as C19, 2D2 and 1 OD2 respectively).
The known structural features of C19 are the following: crystalline C19 below
22.8 "C crystallizes in the orthorhonibic lattice with two molecules per unit cell
[152]. Chains are trans-planar and perpendicular to the plane formed by the methyl
groups, thus being perpendicular to the lamella surface. At 22.8 "C a phase transition
is observed to the a phase and melting occurs at 32 "C. The measured heat of
transition is 3300 cal/mol, while the heat of melting is 10950 cal/mol [153]. The
volume changes determined with a dilatometer at the solid-solid phase transition
and melting are 2.5% and 10.5% respectively. The structure of the r phase below
melting is reported to be 'hexagonal' with some indication of rigid rotations of the
chains [154].
We add here inforination for the two selectively deuterated materials which were
prepared in the authors' laboratory. There is no reason to think that 2D10 may
not have the same structure as the parent molecule. The perturbations by the two
deuterium atoms should be negligible regarding the static structure while they will
affect the vibrational spectra. As a result of the 'regular' packing, the lamellae
formed by these all-trans 10D2 molecules contain precisely in the middle a 'layer' of
CD2 groups. From the viewpoint of vibrations we expect normal crystal field splittings
for the vibrations of the CD2 since intermolecular phase coupling within the
lattice may occur regularly. In contrast, 2D2 molecules in the crystal, while retaining
the orthorhombic structure, are arranged at random with the heads labeled
with CD2 randomly distributed up or down within the crystal. Within Borii-
Oppenheimer approximation the intermolecular potential cannot distinguish between
deuterated or undeuterated ends of the hydrocarbon chain. However, because
of the random mechanical perturbations of the vibrations at the site of chain heads
in this molecule, crystal field splitting of the CD2 vibrations is not expected (in
spite of the orthorhombic packing). These predictions are indeed verified in the infrared
and Raman spectra of both lOD2 and 2D2.
3.19.2 Vibrational Spectra of pure n-Nonadecane
N = 59 is the number of atoms in the C19 molecule. The symmetry point group of the
single trcms-planar chain of C19 is C?" and its 59 x 3 - 6 = 171 vibrations can be
classified in the following irreducible representations with their spectroscopic activity: