Dielectric relaxation and deuteron NMR of water in polyimide films

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Abstract
Polyimide films containing up to 3.2 wt. % water (H2 O or D2 O) have been studied by dielectric relaxation (DR) and 2H nuclear magnetic resonance (NMR) spectroscopies. Dielectric loss measurements show the presence of a double peak near 200 K due to water, with two components designated as γ1 and γ2 . The γ2 peak, which appears at the lower temperature, is only present for the interior portion of thick (>25 μm) films and rapidly decreases as the moisture content decreases. Correspondingly, deuteron NMR measurements show the presence of narrow and broad lines, which are correlated, respectively, with the γ1 and γ2 DR peaks. The spin‐spin relaxation time T2, is much longer for the narrow line than for the broad line. The γ1 DR peak and the narrow NMR line are attributed to sites where isolated water molecules are only loosely coupled to the polymer chains. The γ2 and broad NMR lines involve small clusters of water molecules with associated nuclear magnetic dipole‐dipole coupling. Additional measurements on polyimides containing methanol and acetic acid absorbents are also reported.