Humid ageing of polyetherimide: Chemical and physical interactions with water - Polyimides and Other High Temperature Polymers - page 260

Figure 4. Variation of water solubility in PEI with temperature.

m ∞ being the equilibrium mass gain), showing that the diffusion process is appar-
ently Fickian.
3.2.1. Solubility: effect of temperature
Since, in the studied case, the concentration remains low ( water mass fraction, w
≤ 0.07) and the polymer remains in the same state, one can, in a first approxima-

tion, consider that the equilibrium water concentration C obeys the Henry's law:
C = Sp (5)
where S is the solubility coefficient and p the water vapor pressure. S varies with
the temperature according to Arrhenius law:

Η
RT

-

s

S = S exp

(6)

o

with H s : heat of dissolution.
S o : pre-exponential factor of solubility.
Figure 4 shows the Arrhenius plot for solubility. The heat of dissolution: H s =
-43 kJ.mol -1 .
However, the sorption tests are made in saturated vapor pressure at various tem-
peratures. Between 20 and 100°C the vapor pressure also obeys Arrhenius law: