Molecular deformation of polypropylene

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Abstract
Molecular deformation of polypropylene (PP) was investigated by stress infrared experiments on ultraoriented isotactic PP samples, and the results were compared with single chain deformation calculations. A nonlinear stress‐strain curve of PP was determined in a strain range of 8% which resulted in an average chain modulus, Ec ?40 GPa, and an initial Ec ?30 GPa. PP chain stretching was accompanied by considerable C‐C bond stretching and valence angle bending in addition to torsional angle changes. Vibrational frequency shifts Δν with stress were calculated using both harmonic and anharmonic potential energy functions for C‐C stretching. The harmonic results incorrectly predicted a considerable number of Δν’s to higher frequencies in addition to lower frequencies. Experimentally, only one band shifted to higher frequencies, and shifts to lower frequencies generally increased with increasing temperature. These results are in agreement with the anharmonic calculation in which the majority of vibrations shifted to lower frequencies. Detailed valence coordinate changes and vibrational frequency shifts with stress and temperature are presented and discussed.