Water absorption and transport in bis-silane films

Water (D2O) ingress in bis-amino silane and bis-sulfur silane films was studied by in situ neutron reflectivity. Bis-amino silane film absorbs substantially more D2O (33 vol%) than the bis-sulfur silane film (4.6 vol%) at equilibrium. The volume increase (swelling) of both films, however, is much smaller than the total volume of D2O absorbed in the films. The results suggest that the absorbed water exists in two populations: one is dissolved in the polymer matrix (Henry's mode) while the other occupies unrelaxed free volume existing in the polymer (Langmuir mode). The dominance of Langmuir mode accounts for the small film thickness change during the water absorption. Dual-mode sorption is also consistent with the observed two-stage swelling process whereby an initial rapid increase in film thickness is followed by a slower process extending over 11 h.