"structurally Neutral" Densely Packed Homopolymer-Adsorbed Chains for Directed Self-Assembly of Block Copolymer Thin Films

We here report that adsorbed chains composed of one of the constituent blocks can be used as a new surface modification approach to induce perpendicularly oriented lamellar microdomains in block copolymer thin films. A nearly symmetric polystyrene-block-poly(methyl methacrylate) (PS-block-PMMA) diblock copolymer was used as a model. Densely packed PS- or PMMA-adsorbed chains of about 2-3 nm in thickness ("polymer nanocoatings") were deposited on silicon (Si) substrates using a solvent-rinsing approach. Spin-cast films of 40 or 60 nm-thick PS-block-PMMA (equivalent to two or three interdomain spacings) were subsequently deposited onto the PS or PMMA nanocoatings. Grazing incidence small-angle X-ray scattering experiments revealed the formation of perpendicularly oriented lamellar microdomains within the entire films at 200 °C, where balanced interfacial interactions at the polymer-air interface were achieved. Additionally, X-ray photon correlation spectroscopy studies demonstrated the dynamics of the fully standing lamellar microdomains in the melt, which are coupled to cooperative interdomain movement. We demonstrate that the "neutrality" of the nanocoating is attributed to its noninteractive property against both blocks. This "structurally neutral" property prevents adsorption of the PS-block-PMMA chains on the bare Si substrate that causes the undesirable substrate field effect.