INFRARED AND RAMAN-SPECTRA OF MACROCYCLIC POLY(OXYMETHYLENE)

Infrared and Raman spectra of macrocyclic poly(oxymethylene) (MC-POM) samples having M(n)BAR = 1090 have been investigated in relation to the unusually large and specific frequency shifts of the A2 infrared bands of linear POM accompanied by the change in morphology from the extended-chain crystal (ECC) to the folded-chain crystal (FCC). The MC-POM sample deposited from solution gives rise to an IR spectrum having the A2 bands at frequencies very close to those of FCC of linear POM. The A2 bands of melt-crystallized MC-POM are red-shifted compared to those of the solution-deposited sample in a similar manner, but with smaller band shifts. The E1 IR bands as well as the Raman-active A1 and E2 bands of MC-POM are essentially the same as those of linear POM (both ECC and FCC) regardless of the sampling process, indicating that the stems of the macrocyclic molecule assume the same (9/5)-helical conformation as that of linear POM. The LAM-1 Raman band of MC-POM is observed around 30 cm-1 as anticipated from the stem length (evaluated from M(n)BAR) and the Young's modulus of the (9/5) helix (estimated by a spectroscopic method). The remarkably large dependence of the A2 frequencies of MC-POM on the sampling process has been interpreted by the difference in the stacking mode of molecules among the samples. On compression, MC-POM is only weakly transformed to the metastable orthorhombic modification compared with linear POM.