Humid ageing of polyetherimide: Chemical and physical interactions with water - Polyimides and Other High Temperature Polymers - page 261

-

Η
RT

p

(7)

p = p exp

o

with H p = 43 kJ.mol -1 .
The combination of equations (5), (6) and (7) gives:

Η
RT

-

c

C = C exp

(8)

o

with C o = S o p o and H c = (H s + H p ).
All the experimental results can be explained if H s is negative and close to H P
in absolute value. Thus, C will increase or decrease with temperature depending
on whether H s is lower or higher than H P in absolute value, as previously estab-
lished in recently published works [10, 11, 13].
3.2.2. Diffusion
Diffusivity is analyzed assuming a Fickian mechanism. The coefficient of diffu-
sion D was determined from the classical relationship:

2

π
16

m
m

e
t

2

D = for m t ≤ 0.6 m ∞ (9)

∞

where e is the sample thickness.
The Arrhenius plot for diffusion coefficient (Fig. 5) displays a non-linear shape
suggesting two possible ways of interpretation. The first explanation considers

Figure 5. Variation of water diffusivity in PEI with temperature.