Investigation of the Low-Frequency Motions in Isotactic and Atactic Polypropylene by Neutron Inelastic Scattering

The vibrational spectra between 1000 and 30 cm—1, derived from measurements of the inelastic scattering of ``cold'' neutrons, are presented for isotactic polypropylene at sample temperatures below and above both the glass transition and the melting point. A tentative assignment of the observed modes is suggested by comparison with recent calculations by Miyazawa, Ideguchi, and Fukushima of the optically active phases of the fundamental vibrations for an isotactic helical polypropylene chain and with the neutron measurements of the low-frequency modes in highly crystalline polyethylene. In addition to the skeletal optical modes, the neutron spectra for isotactic polypropylene indicate the existence of two acoustic modes—skeletal deformation and skeletal torsion—with high-frequency limits at 620±50 cm—1 and 110±10 cm—1, respectively. As in polyethylene, these modes appear to be strongly influenced by the presence and phase of the disorder in the sample. Similar spectra for atactic polypropylene above and below the glass transition show a much less pronounced structure, although weak bands are observed which correlate well with the skeletal optical modes observed in the isotactic polymer.