ABSTRACT

The effect of the thermal oxidation on the molecular mobility in isotactic polypropylene has been examined between 300 and 410 K by the Electron Spin Resonance (ESR) spin-probe method using the 2,2,5,5 tetramethyl-1-oxyl-3-pyrolidine-3-carboxy-4-aminobenzene (TMOPCA) nitroxide radical.

The reorientation correlation times τ_R have been obtained from the automated fit of spectra.  It appears that in a wide temperature range, the motion of the spin probe has to be expressed by a distribution of correlation times relevant to the slow motional regime (τ_R > 2 ns).  Several distributions functions proposed in previous magnetic resonance studies on the motion of polymer chains have been considered.  Tentatively applied to the present system, they have been found to be valid only in narrow temperature ranges.  A model free approach has therefore been adopted which consists in computing ESR spectra in a wide range of correlation times (1 to 70 ns) and seeking for the weighted average of spectra which yields the best fit of experimental ones.  This has been performed using the Levenverg-Marquardt’s optimization algorithm taking as adjustable parameters the statistical weights of spectra computed as functions of τ_R, i.e. the sought distribution function Φ(τ_R).  From Φ(τ_R) one obtains the temperature dependence of the average correlation time < τ_R > and of the standard deviation Δτ about it.  The oxidation of the polymer results in a bimodal distribution Φ(τ) and a marked increase of < τ_R > and Δτ resulting possibly from interactions between polar groups formed in the  chains.