POLYMER DRYING .6. EPR SPIN-PROBE STUDIES ON SWELLED POLY(STYRENE-CO-DIVINYLBENZENE)

The electron paramagnetic resonance (EPR) signal and the residual number (alpha(t)) of volatile molecules per phenyl group of poly (styrene-co-divinylbenzene) in samples that had been swelled to saturation in a dilute solution of a nitroxide spin-probe (TEMPO or 4-oxo-TEMPO), dissolved in a volatile liquid, were monitored simultaneously as the system containing excess liquid was allowed to evaporate to dryness. The results showed that the characteristic motionally narrowed three-line EPR spectrum began to change when alpha(t) became equal to alpha(G) (the number of sorbed molecules per phenyl group of polymer at liquid-saturation). The ratio of the intensity of the low-field and high-field hyperfine peaks relative to the middle peak decreased monotonically to an asymptotic limit that was attained when alpha(t) became equal to alpha(g) (the number of residual absorbed molecules per phenyl group of polymer at completion of the transition from the rubbery state to the glassy state). The EPR hyperfine pattern, from which the rotational correlation times were estimated, changed most significantly as alpha(t) decreased from alpha(G) to alpha(g) while exhibiting inflections at about alpha'(s) and alpha'(g), the compositions that mark, respectively, incipient desorption of adsorbed molecules and incipient transition from the rubbery state to the glassy state. The pattern between these inflections points, however, varied with the affinity of the solvent for the polymer.