Transport of water in high T polymers: A comparison between interacting and non-interacting systems - Polyimides and Other High Temperature Polymers - page 266

when exposed to humid environment, absorb significant amounts of water which
adversely affects their physical-mechanical properties.
The main documented effects of water on polymer matrices are, among others,
plasticization, which occurs by different mechanisms depending on the level of
interaction of sorbed water molecules with the matrix; changes of physical
properties, i.e. decrease of elastic moduli, decrease of yield strength, change of
yield/deformation mechanisms; hygrothermal degradation, i.e., microcracks, age-
ing, chain scission through hydrolysis, degradation of fibre/matrix interface in
composites and swelling stresses.
To explain the anomalous sorption features of some glassy polymers several
authors have proposed that the water is made up of two species, i.e., those form-
ing a molecular solution and those confined into areas of abnormally large free
volume, often referred to as microvoids.
Recently, direct evidence of the presence of nanopores by Positron Annihila-
tion Life-time Spectroscopy (PALS) has been reported by Soles and coworkers
for a series of epoxy systems [1-3]. It has been found that these nanopores have
an average diameter of around 5-6 Å and can form a continuous network of chan-
nels across the polymer matrix. Water traverses the epoxy through the network of
nanopores until it reaches polar hydroxyl and amine groups where it forms strong
hydrogen bonding interactions. Thus, the nanopores provide access routes to the
water molecules to reach the specific interaction sites. In this description the to-
pology, polarity and molecular motions combine to control transport. Aging ef-
fects on water sorption equilibrium and kinetics observed in the investigated sam-
ples have been interpreted by assuming that sorbed water is capable of
reorganizing the molecular topology of the matrix in a way that reveals more po-
tentially interacting polar sites.
Also Zhou and Lucas [4, 5] have invoked the interplay between network topo-
logical modification and molecular interactions to interpret the water sorption re-
sults. They assumed the existence of two types of molecular interactions on the
basis of solid state NMR, gravimetric measurements, and calorimetry.
With respect to the issue of molecular interactions, an earlier NMR investiga-
tion by Jelinski and coworkers [6, 7] concluded that i) water was impeded in its
movements, jumping from one site to another with a characteristic residence time
of 7·10 -10 s; ii) no free water existed, iii) there was no evidence of tightly bound
water, and iv) the disruption of the secondary hydrogen-bonded network in the
epoxy matrix by sorbed water was unlikely.
Furthermore, several investigations on difunctional epoxies based on dielectric
relaxation spectroscopy (DRS) support the idea of water molecules partly bound
to polymer chains and partly clustered into microvoids [8-10]. Grave et al. [10]
were able to calculate the relative amounts of bound and free water from the mag-
nitude of the relaxation at 10 5 Hz and 10 9 Hz, respectively. It is worth noting that
the relaxations at 10 9 Hz are in good agreement with the characteristic jumping
time for the water molecules to move from one site to the other as determined by
NMR analysis [6, 7].