SURFACE SEGREGATION IN MODEL SYMMETRICAL POLYOLEFIN DIBLOCK COPOLYMERMELTS

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.