FURTHER EPR STUDIES OF MOLECULAR MOTIONS IN POLYMER PLASTICIZER MIXTURES/

Molecular motions in mixtures of the side chain polymer-poly(vinyl ace tate) and dibuthyl phthalate were studied as a function of polymer con centration and temperature using the technique of paramagnetic resonan ce (EPR). When the small spherical probe tempol (TPL) was used, we wer e able to approximate the observed EPR spectrum with a simulation usin g a single rotational correlation time tau. The previously developed G rest-Cohen all-temperature model matched the Arrhenius plots. The EPR spectra from a cigar-shaped cholestane (COL) probe could not be adequa tely matched by a single tau simulation when the polymer was at temper atures somewhat above the glass to rubber transition temperature (T-g) . Points corresponding to these temperatures were left off of the Arrh enius plot and a discontinuity was observed where the gap in the data occurred. As the concentration of plasticizer was increased, we found that the discontinuity became less steep, but the tau at which the gap occurs was always approximate to 10(-8) s. The spectra observed at th e temperature region of the gap were approximately 50-50 composites of experimental spectra observed at +/-10 K. In both the TPL and COL cas es, there was evidence of the existence of multiple correlation times. Preliminary studies of other polymers, both with and without side cha ins, also indicated the existence of the gap when COL is used as the p robe. (C) 1993 John Wiley & Sons, Inc.