Abstract
Raman active longitudinal acoustic modes ( LAM) have been observed in the perfluoro n-alkanes and in a random copolymer of tetrafluoroethylene and hexafluoropropylene (TFE-HFP). Low frequency bands were found in both the melt and solid phases of the oligomers, and their comparison indicates that regular extended helical conformations exist in the melt phase of perfluoro n-alkanes below n-C 12F 26. Good agreement is found between the lamellar long spacings of the copolymer and that calculated from the LAM frequencies. Normal mode calculations, using available force fields, have been performed on the infinite homopolymer (PTFE) and n-C 16F 34. It was found that the helix reversal type of chain defect does not affect LAM frequencies and intensities.
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