Water sorption of polycarbonate and its effect on the polymer’s dielectric behavior

Polycarbonate absorbs water in two stages: in the initial period of absorption at an elevated temperature all of the water was found in an unassociated state when cooled to room temperature; and, in the second stage, most of the additional water absorbed by the polymer was identified in a separate liquid phase (clustered water). From water sorption studies on polycarbonate bars, we advance the following model for the formation of clusters. After the polymer is saturated with water at an elevated temperature and cooled, its solubility is lowered and water condenses in the form of microscopic water-filled cavities, providing the internal pressure which is generated by the excess water exceeds the strength of the polymer. Below Tg the clusters are formed only after the polycarbonate (Mw=26 600) has been hydrolytically degraded; whereas, above Tg, clusters can be formed without degradation. Cluster regions were found by SEM to be spherical cavities, usually measuring from 0.1 to 1.0 μ across. In addition, the glass temperature of polycarbonate was lowered as the molecular weight decreased due to hydrolysis and the amount of water in the polymer increased. Unassociated water was found to enhance the low-temperature dielectric loss of polycarbonate. Frozen clustered water produced an additional secondary loss peak about 40 °C below the β transition. In addition, the liquid clustered water produced a loss mechanism in the kHz region that was interpreted as a Maxwell-Wagner effect.