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>Vibrational Spectroscopic Study on the Solid-state Phase Transition of Polyoxymethylene II. Pressure-induced Phase Transition of Polyoxymethylene from the Trigonal to the Orthorhombic Form
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Vibrational Spectroscopic Study on the Solid-state Phase Transition of Polyoxymethylene II. Pressure-induced Phase Transition of Polyoxymethylene from the Trigonal to the Orthorhombic Form
Pressure-induced solid-state phase traition of polyoxymethlene from the trigonal (t-POM) to the orthorhombic (o-POM) form was investigated by means of infrared and Raman spectroscopies. Morphologically, t-POM is known to appear in various structures between two extreme structures of typical extended chain crystal (ECC) and typical folded chain crystal (FCC). Two samples of ECC and FCC t-POM were used as the starting materials. Each sample, in a mixture with KBr powder, was pressed in a pistoncylinder cell at a pressure up to 2.5GPa. After removal of pressure, the pellets of the samples were subjected to the spectral measurements. In the infrared spectrum of o-POM produced from the FCC sample, only the B1 bands corresponding to the A2 bands of t- POM1-3) shifted towards the high-frequency side compared with those of o-POM produced from the ECC sample of t-POM. It is clear that the morphology of the starting sample remains unchanged throughout the pressure-induced transition, and the normal frequencies of the B1 modes belonging to a particular symmetry species having the transition dipole parallel to the chain axis are strongly influenced by the morphology in orthorhombic form as well as in the A2 modes of t-POM.
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