Abstract: Ab initio molecular orbital calculation was employed to optimized the geometry structure of poly (vinylidine fluoride) (PVDF) all-trans molecule. The calculated result is consistent with that of experiment. In our calculation, two different values of bond angle C－C－C were found, i.e.112.8° and 113.3°.The infrared intensity spectrum was obtained from frequencies calculation of the all-trans molecule. In the range of 400～4 000 cm－1,the IR spectra of polarized and unpolarized β-phase PVDF film (30 μm thick) have been measured. The calculated IR intensity spectrum is in good agreement with observed IR spectrum. The vibrational modes in the range of 400～4 000 cm－1 were divided into six series, series Ⅰ(544～415 cm－1), series Ⅱ(913～792 cm－1), series Ⅲ(1 353～998 cm－1), series Ⅳ(1 458～1 361 cm－1), seriesⅤand Ⅵ. Only series Ⅲ of all the vibrational modes causes the change of the dipole moment of PVDF molecule, and is related to the spontaneous polarization.

Key words: IR spectrum, PVDF, Vibrational modes, Ab initio molecular orbital calculation