Transport of water in high T polymers: A comparison between interacting and non-interacting systems - Polyimides and Other High Temperature Polymers - page 278

Figure 10. Sorption isotherms in terms of water sorbed amount vs. water vapour activity for epoxy
matrix (at 24°C) and polyimide matrix (30°C).
ples of these curves are shown in Figure 11a for the epoxy and in Figure 11b for
the polyimide. For both matrices, at all the investigated activities, desorption ki-
netics is slower than sorption kinetics, which is a typical feature of Fickian sys-
tems where mutual diffusivity is an increasing function of penetrant concentration
[29]. In the case of epoxy this effect is more pronounced and is related to the pro-
gressive saturation of absorption capacity of microvoids and of specific interac-
tion sites of the network. In fact, both of these adsorption phenomena slow down
the diffusion process [26]. On the other hand, for the polyimide, only the absorp-
tion in microvoids plays a role, while clustering, which occurs at higher activities,
has an opposite effect (i.e. depression of mutual diffusivity). Therefore, it is rea-
sonable to expect that the occurrence of the two opposing effects produces a neg-
ligible dependence of diffusivity upon concentration.
When mutual diffusivity increases with concentration, the shape of sorption
curves does not depend significantly on the functional dependence of the diffu-
sion coefficient [29] and is not substantially different from the case of constant