The spin probe technique
was used to study graphite oxide (GO)
powders swelled in polar liquids (CH3CN, CH3OH, and H2O) and liquid-free GO membranes (GOM). The nitroxide
radicals TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl)
and TEMPOL (4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl) readily penetrated into the interplane space of GO from the
solution. Electron paramagnetic resonance (EPR) spectra of these radical
probes were sensitive to molecular mobility and orientation ordering
within the internal space of GO. The radicals embedded in swelled
GO were in two states with different rotational mobilities. The small
fraction of radicals located in the interplane space of GO and detected
in the broad range of temperatures was in the state of fast rotation,
similar to the same radicals dissolved in bulk liquids, thus providing
experimental evidence of formation of a liquid-like media within the
interplane space of GO. Such mobile media may be responsible for the
unusual permeation properties of GOM, which is reported in the literature.
Second, less-mobile fraction of radicals was found to be immobilized
at the internal surface of GO and was sensitive to phase transformations
in the swelled GO structures. The transformations were detected as
anomalies at temperature dependences of rotational mobility of radicals.
The detected dependence of EPR spectra of probe radicals on orientation
of GOM, relative to the direction of magnetic field in the EPR spectrometer,
was used for quantitative characterization of orientation alignment
of GO planes within the membranes. Such an approach may serve as an
elegant method to estimate the relative quality of membranes and other
GO-layered structures.
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