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hydrophilicity compared to other members of the polyimide family. Values close
to 1.5% were, for instance, reported for the equilibrium mass gain in saturated
conditions [7]. As it has been shown in a previous paper [8], such water concen-
trations are, however, sufficient to induce significant changes in the mechanical
behaviour. This is the reason why it was interesting for us to study in detail the
water absorption in saturated conditions in PEI samples in the 20-100°C tempera-
ture interval. On the other hand, the PEI chemical structure, in particular the pres-
ence of imide functions sensitive to hydrolysis, allows us to study the eventual
PEI-water chemical interaction.
2. EXPERIMENTAL
2.1. Material
The material examined in this study was an extruded plaque (3 mm thick) of
thermoplastic polyetherimide (Ultem 1000) supplied by General Electric (number
average molar mass 20 kg.mol
-1
, glass transition temperature in dry state 217°C
and density 1.27 g.cm
-3
).
2.2. Ageing tests
Ageing tests were performed in a thermostated (
0.1°C) bath with distilled water
at 100°C, in acidic solution (pH 2) at 100°C, and into hermetically sealed reaction
vessels at 170°C and 180°C.
In order to avoid any other polyimide degradation process than hydrolysis, es-
pecially oxidation, oxygen was carefully removed from the reaction vessel using
30 min nitrogen bubbling before heating.
±
2.3. Size exclusion chromatography (SEC)
SEC measurements were carried out in chloroform solution, at a 0.5 ml.min
-1
flow rate. The calibration was made with polystyrene standards.
2.4. Infrared spectroscopy
IR spectroscopic measurements were made on a Brücker IFS 28 spectrophotome-
ter at a resolution of 4 cm
-1
.
2.5. Sorption tests and gravimetric study
After drying at 50°C under vacuum until constant weight, the samples were im-
mersed in a thermostated (
0.1°C) bath with distilled water at temperatures rang-
ing between 20 and 100°C. They were periodically taken out and weighed with an
electronic microbalance (Mettler Mark 3) with relative accuracy of 10
-4
.
The average duration of weighing operation was less than 3 min, during which
the sample was exposed to ambient atmosphere (temperature: 20°C, relative hu-
±
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