Surface Modification with Cross-Linked Random Copolymers: Minimum Effective Thickness.
Ultrathin films of benzocyclobutene (BCB)-functionalized random copolymer with styrene and methyl methacrylate, P(S-r-BCB-r-MMA), with thicknesses ranging from 0 to 10.5 nm, were thermally cross-linked on Si substrates. The penetration of deuterated PMMA (dPMMA) into the P(S-r-BCB-r-MMA) films and the microdomain orientation of PS-b-PMMA diblock copolymers on P(S-r-BCB-r-MMA) coated substrates were investigated by neutron reflectivity (NR) and scanning force microscopy (SFM), respectively. NR measurements on bilayers of dPMMA on cross-linked P(S-r-BCB-r-MMA) showed that the neutron scattering length density (SLD) at the substrate was equal to that of P(S-r-BCB-r-MMA) if the P(S-r-BCB-r-MMA) film was thicker than 5.5 nm. With decreasing thickness of the P(S-r-BCB-r-MMA) film, the SLD at the substrate increased, characteristic of an increasing penetration of the dPMMA. When thin films of PS-b-PMMA diblock copolymer having cylindrical microdomains are placed on surfaces modified with cross-linked films of P(S-r-BCB-r-MMA) thinner than 5.5 nm, the cylindrical microdomains orient parallel to the surface, whereas for thicker films, the microdomains orient normal to the surface. Both of these results indicate that interfacial interactions are screened when the P(S-r-BCB-r-MMA) film is thicker than 5.5 nm.