Structure within thin epoxy films is investigated by neutron reflectivity (
NR) as a function of resin/cross-linker composition and cure temperature. V
ariation in the cross-link density normal to the substrate surface is exami
ned by swelling the films with the good solvent d-nitrobenzene (d-NB). The
principal observation is a large excess of d-NB near the air surface. This
is not a wetting layer, but rather indicates a lower cross-link density in
the near-surface region. This effect is due to preferential segregation of
the cross-linker to the air surface, driven by the lower surface tension of
the cross-linker relative to the epoxide oligomers. The magnitude of the e
ffect is a function of composition and cure temperature. Exclusion of d-NB
from the region immediately adjacent to the substrate surface is also obser
ved, possibly indicating a tightly bound layer of epoxy. Regarding swelling
in the bulk of the films, the behavior is nonsymmetric with departure from
the stoichiometric ratio. The films deficient in curing agent show greater
equilibrium swelling and faster swelling kinetics than the films with an e
xcess of curing agent.