Transport of water in high T polymers: A comparison between interacting and non-interacting systems - Polyimides and Other High Temperature Polymers - page 275

Furthermore, Zhou and Lucas [5] argue that a proof for the existence of water
molecules forming multi-site interconnective bonds is the increase of T g of the
water/epoxy system for long exposure time in sorption experiments, which fol-
lows the initial abrupt T g depression related to the so-called type I water mole-
cules. Actually, this T g increase can be alternatively accounted for by invoking
standard free volume arguments [25]. The addition of a penetrant increases the
free volume, lowering the T g but, at the same time, reduces the excess free vol-
ume associated with the glassy state. If the addition of free volume from the
penetrant does not outweighs the loss of excess free volume associated with low-
ering of T g (as is the case for molecules that significantly relax the glassy matrix
structure) a progressive densification sets in, promoting an antiplasticization ef-
fect.
In the case of PMDA-ODA, most of the water molecules are either confined
in microvoids and/or molecularly dispersed with no H-bonding interaction with
the polymer backbone. Also in this case, a lower mobility is expected for S 2
species; however, the origin of this effect is now related to the increased volume
of the clusters or, alternatively, to the increase of activation energy due to a dif-
fusive jump (detachment of a single water molecule from the cluster) [26]. On
the basis of the above considerations, a lower plasticizing efficiency of absorbed
water molecules is expected in the case of the polyimide as compared to the ep-
oxy.
The relative contributions, at sorption equilibrium, of the different water spe-
cies as derived by the curve fitting results of the subtraction profiles, are reported
as a function of water vapour activity in Figure 7 for the epoxy and Figure 8 for
the polyimide.
In the case of the epoxy resin, the relative amounts of S 0 and S 2 species de-
crease with water vapour activity, while that of S 1 increases. Although these ab-
sorbance data cannot be directly transformed into relative concentrations, due to
the dependence of molar absorptivity on frequency [14], nevertheless they pro-
vide useful qualitative information. The above findings are consistent with the
physical picture of a system where the relative population of interacting water
species at equilibrium changes as a function of the external water activity and
hence of the total content of absorbed water. In fact, at low activities, strong inter-
actions are more likely to develop for enthalpic reasons, while, as the activity in-
creases, strongly interacting sites get closer to saturation thereby favouring the
formation of weakly interacting or dimeric species.
In the case of the PMDA-ODA matrix, an opposite behaviour was observed:
the relative contributions of both S 0 and S 1 species decreased with water activity
while the contribution of S 2 species increased. This trend is likely related to the
tendency of forming water clusters, which increases with the concentration of ab-
sorbed molecules [27].
The absorbance-concentration curves for the two matrices relative to the ν OH

band are compared in Figure 9. In agreement with the Beer-Lambert relationship,