Transport of water in high T polymers: A comparison between interacting and non-interacting systems - Polyimides and Other High Temperature Polymers - page 274

resin; this effect, which is likely due to differences in the dielectric constant of the
matrices, confirms the common origin of the peaks in the two systems. On the
other hand, the lower frequency component for PMDA-ODA is displaced by at
least 50 cm -1 compared to its counterpart in the epoxy: this indicates that the water
absorbed in the polyimide forms hydrogen bonding interactions of a lower
strength as compared to those formed in the epoxy matrix.
In the case of the epoxy resin, the water species not interacting with the net-
work (S 0 ) are expected to be characterised by high molecular mobility and limited
plasticizing efficiency. These species correspond to free water molecules confined
into nanopores, according to the physical picture put forward by Soles and co-
workers [1-3], or to molecularly dispersed water with no H-bonding interactions,
located in pockets surrounded by hydrophobic moieties, where the likelihood of
hydrogen bonding is low, according to Mijovic and coworkers [12]. The latter
species are those responsible for γ -relaxations observed through DRS in epoxy

systems and in BMI [11, 12].
Conversely, S 2 molecules are firmly bound to specific sites along the polymer
network, thus exhibiting a much lower mobility and a higher plasticizing effi-
ciency.
With regard to the nature of the S 1 species, as already pointed out they can ei-
ther represent dimers or water molecules interacting with the polymer network
through a hydrogen bond which involves only one hydrogen atom of the water
molecule. These two adducts cannot be distinguished on the basis of the experi-
mental evidence provided by FTIR spectroscopy. Since these different kinds of
interactions are expected to promote different plasticizing effects on the matrix,
no conclusion can be made about the mobility and plasticizing efficiency of the S 1
water species.
This scenario is somehow different from the one proposed by Zhou and Lucas
[4] which assumes that two types of absorbed water molecules exist, both of them
residing on specific interaction sites: type I molecules, interacting with only one
site of the epoxy network and type II molecules, forming interconnective hydro-
gen bonds with two different sites on the epoxy network. The latter lead to a sec-
ondary physical crosslink network, contributing an antiplasticization effect [5]. In
this view, water molecules may act as proton donors as well as acceptors. In our
opinion, at least for TGDDM-DDS system, this physical picture is unlikely. In
fact, the above model does not consider the presence of free water, which is
clearly identified in both MIR and NIR spectra. Moreover, FTIR spectroscopy in-
dicates that most of the interacting water molecules are S 2 species: molecular ge-
ometry considerations suggest that intramolecular S 2 adducts are formed at single
sites along the network, which are characterized by the presence of two closely
located proton acceptors (e.g. amino-alcohol moieties or sulphone groups). Possi-
ble structures of these complexes formed by hydrogen-bond associations are re-
ported in ref. [14]. This hypothesis is also supported by the results of molecular
dynamics simulations reported by Mijovic et al. [11].