Accordion bands

The Raman spectra of n-hexyl, n-octyl, n-decyl, and n-dodecyl trimethyl-ammonium bromides in the 150-600 cm" region are shown in Fig. 1. Strong Raman bands are observed for all of these in the 160-300 cm region, and these shift markedly to a lower frequency with an increase in chain length. We may assign the bands observed at 251, 208, 179, and 164 cm to the accordion modes of the n-hexyl, n-octyl, n-decyl, and n-dodecyl chains, respectively. Figure 2 shows the linear relationship between the accordion band frequency and the reciprocal number of carbon atoms (1/Cn) for the fom n-alkyltrimethylammonium bromides. We may assume that the accordion band frequencies of the n-octyl, n-decyl, and n-dodecyl chains,... 

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In crystalline solid polymers, not only do the chains vibrate in the normal manner which we associate with infrared or Raman modes, but it is also possible for the all-tew.s section of a chain such as polyethylene to undergo an accordion type of motion. The element which is involved in the motion is constrained by the size of the crystal lamellae described in Chapter 6. The motion is not a relaxation and it gives rise to a resonance observed in the far infrared or Raman spectrum. Typically, crystalline polyethylene will possess a vibration band at about 120 cm" which is associated with this accordion motion. The precise position depends on the length of the lamellae, the shorter lamellae having a higher frequency of resonance. This collective vibration of the chains is quantised, and is a p/jowow. These well defined acoustic vibrations are very important in understanding the temperature dependent dynamic behaviour of crystalline solid polymers. 

The Raman spectra of the spacer-related compounds TMB, HMB, OMB, and DMB have also been examined. The crystal structures of hexa- and decamethonimn bromides [7] were elucidated by a single-crystal X-ray diffraction analysis, which showed that the molecular chain, which included the two nitrogen atoms and one CH3 carbon atom of the terminal N(CH2>3 groups, was in a fully extended form. Thus, the methonium bromide chains should provide evidence for the accordion vibrational bands. 

Because the plots of the band frequencies of the methonium bromides against the reciprocal number of carbon atoms are linear, these bands for TMB, HMB, OMB, and DMB are assigned to the accordion modes of the extended molecular chains [4,5]. 

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