Surface segregation in thin films of model symmetric polyolefin dibloc
k copolymers exhibiting a lamellar morphology has been investigated. F
ilms were spin coated on a variety of substrates and the resulting mic
rostructure analyzed using neutron reflection, X-ray reflection, ellip
sometry, light microscopy and secondary ion mass spectrometry. Each of
the block copolymers is sufficiently close to the order-disorder tran
sition (ODT) so that the composition profile within the lamellar domai
ns can be represented by a nearly sinusoidal function. In all cases, t
he same block was found to enrich both the polymer/solid and the polym
er/air interfaces, even when the surface energy of the solid substrate
was higher than that of either block of the copolymer. These results
cannot be reconciled with preferential enrichment based solely on the
difference in the dispersive surface tension between blocks. In all in
stances, we find that the conformationally smaller block is the surfac
e-active one. We propose that an entropic driving force, attributable
to conformational asymmetry, plays an important role in the near-surfa
ce behavior of block copolymer melts.