Water in Local Free Volumes of Polyimides: A Positron Lifetime Study

Positron annihilation lifetime (PAL) spectroscopy was used to study the effect of water uptake on the free volume in 6 FDA-ODA polyimide (PI2566, DuPont R ). Ortho- positronium (o-Ps) lifetime (τ3) and intensity (I3) data were analyzed assuming a Gaussian size distribution o f pre-existing (excess free-volume) holes, and further assuming that holes occupied b y water molecules would no t be detected in the measurement. When exposed to humidity, bo th the mean ho le volume a nd the number of holes not occupied b y water molecules decreases with increasing relative humidity, as indicated b y decreasing τ3 and I3. Our PAL results correlated with hu midity-induced mass uptake a nd vo lume e xpansion support a model according to which water absorption in polyimides occurs in two stages. At relative humidities s maller than about 30 %, water is absorbed mostly in large, p re-existing holes, with each hole typically occupied by a single water molecule. At larger humidities, an increasing fraction o f the sorbed water molecules will occupy sites other than p re-existing empty holes. A multiple occupation o f larger holes by water molecules is discussed as a possible mechanism.