Modern Polymer Spect..

j&890 860 830
29,JT
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Figure 3-54. Details of the Raman spectrum of n-nonadecane
in the orthorhombic (0°C) and c( phases 179.7‘C). The line
near 890 cn-’ indicates that all chain ends are in the TT confomiations.
In the u phase a small poptilation of chaiii~ has
tilted their heads in TG arid GT conlormations.
(w) At TI, in the CH stretching range the line at 3882 cin-’ is submerged by the
broad scattering near 3000 cn-’ from which two peaks emerge at 2850 (d’)
and 2875 cm-‘ (d-). The typical Raman spectrum of the “melt” is observed. It
should be compared with Figure 3-43.
(x) At T, near 720 cmP1 a clear stepwise increase of the diffuse broad background
scattering is observed.
(y) At T, the line near 1090 cin-’ becomes strong at the expenses of the k = 0
singularities of the infinite trans-planar chain at 1135 and 1074 cm-’ in the
orthorhombic and a phases. This line is characteristic of the stretching of the
C-C bonds in gauche conformation (Figure 3-51c).
Conclusioii no. 3
At the orthorhombic to a phase transition the disappearance of the factor group
splitting (observations (a) and (n)) indicates that the orthorhombic lattice with two
molecules per units cells disappears and is replaced either by a unit cell with one
molecule per unit cell or by a lattice where the tridimensional order is practically
lost.
Chains, however, remain mostly straight tmns-planar (observations if), (in),(o),
(p) and (9)). Perturbations appear at chain ends (observations (c), (g) and (o)),
indicating that the medium surrounding the CH3 group has changed, in particular
the inter-lamellar forces have weakened (observation (0)). The following structures
appear at the end of the molecule -CH?(T)-CH2(G)-CH?-CH3 and
-CH?(G)-CH~IT)-CHI-CH~(observation (s)). This accounts for the lowering of
the overall symmetry which causes the appearance of a few of the coinponents of
the P modes inactive for the orthorhombic stiucture (observation (e)). It seems
likely that some sort of disordering at the surface of the lamella takes place with
some chains having their ends tilted in gauche conformations (observation (r)). The
occurrence of a few GTG, GTG’ and GG conformational defects (or unspecified
gauche defects) is indicated (observations (11) and (1)); the topology of these defects
along the inoIecuIes cannot, so far, be defined.