Surface segregation in model symmetric polyolefin diblock copolymer melts

Abstract

Surface segregation in thin films of model symmetric polyolefin diblock copolymers exhibiting a lamellar morphology has been investigated. Films were spin coated on a variety of substrates and the resulting microstructure analyzed using neutron reflection, X-ray reflection, ellipsometry~ light microscopy and secondary ion mass spectrometry. Each of the block copolymers is sufficiently close to the order-disorder transition (CDT) so that the composition profile within the lamellar domains can be represented by a nearly sinusoidal function. In all cases, the same block was found to enrich both the polymer/solid and the polymerlair 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 instances, we find that the conforrrationally smaller block is the surface-active one. We propose that an entropic driving force, attributable to conformational asymmetry, plays an important role in the near-surface behavior of block copolymer melts.