QUASI-HARMONIC TREATMENT OF INFRARED AND RAMAN VIBRATIONAL FREQUENCY-SHIFTS INDUCED BY TENSILE DEFORMATION OF POLYMER-CHAINS .2. APPLICATION TO THE POLYOXYMETHYLENE AND ISOTACTIC POLYPROPYLENE SINGLE CHAINS AND THE 3-DIMENSIONAL ORTHORHOMBIC POLYETHYLENE CRYSTAL

Quasiharmonic equations are derived for stress-induced vibrational frequency shifts in the infrared and Raman spectra of polymer chains subjected to a tensile stress. The expressions are applied to the helical chains of polyoxymethylene and isotactic polypropylene. Observed frequency shifts can be reproduced well by using reasonable anharmonic force constants. A semiquantitative interpretation is given for the close relationship between stress-induced vibrational frequency shifts and the deformation mechanism of the polymer chains. Stress-induced frequency shifts are also calculated for an orthorhombic polyethylene crystal subjected to uniaxial tension along the chain axis or to hydrostatic pressure. The results consistently and reasonably reproduce observed data, not only for the intramolecular vibrational modes but also for the external lattice modes.