Conformation measurement vibrational selection rules

Information concerning conformation, tacticity and crystallinity may also be obtained (1). Vibrational transitions accesssible to IR spectroscopy are governed by the selection rule that there must be a change in dipole moment during excitation of the polymer vibrations. Thus symmetric vibrations which are detected by Raman spectroscopy are inaccessible to IR absorption measurements. 

Since two resonance lines at 39.0 and 47.7 ppm that correspond to those observed in the ttgg form and a resonance line at 49.0 ppm that corresponds to that in the tttt form are recognized in the gel spectrum, a coexistence of these two forms in the gel might be supposed. In an attempt to determine the possibility of the coexistence of the two forms in the gel, we measured the IR spectrum that is sensitive to the molecular conformation. The number of normal vibrational modes depends sensitively on the molecular conformation based on the selection rule of the symmetry species. Kobayashi et al. confirmed the vibrational modes assignable to the ttgg conformation in the IR spectrum for the gel from a sPP/carbon disulfide system [117]. However, since we used o-dichlorobenzene as solvent, we examined whether the gel structure depends on the solvent. 

The Raman spectrum of a polymer is sensitive to the chain conformation since the latter determines the vibrational properties. One expects characteristic differences between a crystalline sequence with a unique regular helical conformation and the wide distribution of different conformations typical for a melt. Figure 5.16 displays spectra measured for polyethylene, in part (a) the spectrum of a crystal, measured for a specially prepared sample of extended chain polyethylene with a crystallinity close to 100%, and in part (b) the spectrum of the melt. The third spectrum (part (c)) was obtained for a partially crystalline sample. The spectrum of the crystal shows sharp bands because restrictive selection rules apply in order to be Raman active, all monomers have to move in-phase. This contrasts with the disordered liquid state, where in the absence of a structural symmetry, selection rules are relaxed and the Raman bands are broadened. On first view, the spectrum of the semicrystalline sample looks like a superposition of the elementary spectra (a) and (b) however. 

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