Modern Polymer Spect..

3.19 A Worked E.xuniple 191
Figure 3-55. Sketch of assumed
locations of selectively deuterated
chains of n-nonadecane (2D2 arid
10D2, see text) in the orthorhombic
crystal.
-
-5Hz- $ D z - ~ H ~
Conclusion no. 4
The process of melting can be described in the following way: the tvuizs-planar
structure of the fraiu chains collapses (observations (i), (j) and (t)) and conformational
disorder takes place accompanied by the appearance of the following
conformational defects: GG, GTG, GTG’ (observations (j) and (l)), end-gauche
(observation (u)) and gauche in general (observations (b), (t), (y), (v), (w), and (x)).
In practice we find characteristic signals of liquid-like structures.
With the aid of the selectively deuterated species 2D2 and 10D2 further details on
the processes of solid-solid phase transition and melting can be derived.
First, we consider the 2D2 molecule. Of necessity, the process of packing of traizsplanar
chains to forni an orthorhonibic crystalline lamella cannot distinguish
deuterated and undeuterated chain ends; the resulting lamella can be described by
a random distribution of ‘up’ or ‘down’ chains (Figure 3-55). With such chains we
can distinguish the conformational behavior of the heads and/or the tails of the
chains. It becomes thus possible to be sure of the origins of the signals coming from
conformational defects.
The most direct information comes from the frequencies of the CD? rocking
which have been calculated with normal coordinate calculations and are reported in
Table 3.1.
The infrared spectrum of 2D2 in the 670-600 cm-’ range shows only a band
near 620 cm-’ below To (Figure 3-56). The band is not split, as mentioned above,
because the precise phase coupling is lost by the random distribution of ‘up’ and
‘down’ chains. At the solid-solid transition a very weak additional band appears
near 655 cm-’ (Figure 3-56) indicating the formation of gauche conformations at
the deuterated chain ends, thus reiterating what has been already learned from C19.
From a quantitative measure of the absorption intensities of the these modes it
transpires that approximately 25% of chain ends are conformationally distorted.
The infrared spectrum of 10D2 provides further information (Figure 3-57]. For
the orthorhonibic phase the line near 620 c1n-l is split because in this case the phase
coupling is achieved since translational symmetry is obtained within the crystal. At