Fourier-Transform Infrared Studies of Polypropylene During Mechanical and Thermal Deformations

Abstract

Infrared spectroscopy, which looks at short range order, provides information on the strength of molecular vibrational frequencies and intensities. The measurement of changes in vibrational frequency, (nu), and in the vibrational intensity, which are related to variations in the intramolecular and intermolecular environment should provide a useful view of molecular processes. Quantitative FTIR measurements of frequency shifts, (DELTA)(nu), intensity changes and absorbance profile asymmetry are reported for various polypropylene samples as functions of uniaxial stress, (sigma), and temperature. Generally, it was found that stress- and temperature-induced frequency shift coefficients, (alpha)(,x) and (beta)(,x), depended on the stress rate (sigma), draw ratio (lamda), molecular orientation f, and the annealing conditions. With increasing (sigma), generally (alpha)(,x) increased to an apparent asymptotic limit. With increasing (lamda), f, (alpha)(,x) also increased to an asymptotic value corresponding to a single chain value (alpha)(,c) (1168) of -0.26 and Young's modulus E(,c) of 36.8 GPa for isotactic polypropylene. The observed variations in (alpha)(,x) and (beta)(,x) have been interpreted in terms of the stress distribution function F((sigma)), chain orientation distribution function a((gamma),(phi)), and time-temperature dependent molecular rearrangement of the internal structure therefore affecting the oscillator distribution. The molecular stress distribution has been obtained from the infrared absorption band using Ergun's deconvolution method, while the spatial orientation distribution has been obtained by coupling the experimentally observed absorption with Kratky's model of a distribution function.