VIBRATIONAL-SPECTRA AND THEORETICAL 3-DIMENSIONAL ELASTIC-CONSTANTS OF ISOTACTIC POLYPROPYLENE CRYSTAL - AN IMPORTANT ROLE OF ANHARMONIC VIBRATIONS

Calculation of normal mode frequencies and three-dimensional elastic constants is made for the first time for isotactic polypropylene crystal on the basis of lattice dynamical theory. The vibrational frequencies are found in relatively good agreement with the observed values for both the internal and external modes. The calculated Young's modulus along the chain axis is 40.1 GPa which agrees well with the X-ray observed crystallite modulus ca. 40 GPa. The anisotropic curves of Young's modulus and linear compressibility in the plane perpendicular to the chain axis are also calculated but agreement with the values observed at room temperature is not good. This discrepancy is erased successfully by considering a large anharmonic effect of methyl torsional modes.