Easy Orientation of Diblock Copolymers on Self-Assembled Monolayers Using UV Irradiation

Oriented thick films of block copolymer made by multiple successive coatings: perforated lamellae versus oriented lamellae

Building 3D ordered nanostructures by copolymer deposition on a substrate implies a full control beyond the thin film regime. We have used here block copolymers (BCPs) forming bulk lamellar phases to form thick, i.e. much thicker than the lamellar period, structured films on a substrate. Films are formed by a simple method of multiple successive coatings. The film structure is controlled using the combined action of surface templating and annealing time. Sections of the thick layers were characterized by scanning electron microscopy (SEM) after etching of one of the BCP moieties. We show that perfect hexagonally perforated films (HPL) with lamellae parallel to the substrate are formed for a wide thickness range up to 300 nm. Grazing incidence small angle X-ray scattering (GISAXS) confirms such an organization by revealing that perforations sit on a hexagonal lattice. A lamellar organization perpendicular to the substrate is shown to take over for thicker films. A scenario consistent with our observations is proposed, where the sequence of phases results from the balance between surface and stretching energy effects.

Effects of the Density of Chemical Cross-links and Physical Entanglements of Ultraviolet-Irradiated Polystyrene Chains on Domain Orientation and Spatial Order of Polystyrene-block-Poly(methyl methacrylate) Nano-Domains

Ultraviolet irradiation (UVI) of varied duration caused cross-linking and neutralization of polystyrene (PS) homopolymers of molar mass (M_n) from 6 to 290 kg mol^-1 on a silicon-oxide surface. An optimal neutral skin layer on the surface of the PS was obtained via brief UVI in air (UVIA), by which the PS had no preferential interaction with either block in the copolymer. UVI in an inert environment (gaseous dinitrogen) (UVIN) stabilized the PS layers via cross-linking and enabled the PS networks to have an effective adhesive contact with the underlying substrate. Thorough examination of domain orientations and spatial orders of a series of block copolymer, polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA), thin films deposited on these UVI-treated PS support layers yielded clear evidence that a dense layer of neutralized PS chains was required for the perpendicular orientation of PS-b-PMMA nanodomains. In particular, in addition to neutralization, two factors-the densities of physical entanglements and of chemical crosslinks-both in UVI-treated PS should be considered for the perpendicular orientation of nanolamellae and nanocylinders in symmetric and asymmetric PS-b-PMMA thin films. The density of physical entanglement in PS depends intrinsically on M_n of the PS, whereas the density of chemical cross-links was controlled with a varied duration of UVIN. Sufficiently large densities of physical entanglements and chemical cross-links can prevent PS-b-PMMA chains from penetrating through the neutral skin layer. The total density of physical entanglements and chemical cross-links required for the perpendicular orientation is correlated with the dimensions of the PS-b-PMMA chains.

Tuning polymer-surface chemistries and interfacial interactions with UV irradiated polystyrene chains to control domain orientations in thin films of PS-b-PMMA

We demonstrate a simple, rapid, cost-effective and robust approach to modify the surface of a solid substrate, based on a UV-irradiated film of a general plastic polymer. Thin films of homopolymer polystyrene (PS) of controlled thickness were spin-coated on diverse metal, semiconductor and polymeric surfaces. Specific surface chemistry was tuned with UV irradiation in air (UVIA); interactions at the PS/substrate interface were enhanced with UV irradiation in nitrogen (UVIN). Oxidized and cross-linked PS served as a neutral surface on various metal, quartz, semiconductor and polymeric substrates to induce perpendicularly oriented cylinders or lamellae in a self-assembled block copolymer.

Molecularly functionalized silicon substrates for orientation control of the microphase separation of PS-b-PMMA and PS-b-PDMS block copolymer systems

The use of block copolymer (BCP) thin films to generate nanostructured surfaces for device and other applications requires precise control of interfacial energies to achieve the desired domain orientation. Usually, the surface chemistry is engineered through the use of homo- or random copolymer brushes grown or attached to the surface. Herein, we demonstrate a facile, rapid, and tunable approach to surface functionalization using a molecular approach based on ethylene glycol attachment to the surface. The effectiveness of the molecular approach is demonstrated for the microphase separation of PS-b-PMMA and PS-b-PDMS BCPs in thin films and the development of nanoscale features at the substrate.

Tailoring nanostructures using copolymer nanoimprint lithography

The generation of defect-free polymer nanostructures by nanoimprinting methods is described. Long-range nanorheology and shorter-range surface energy effects can be efficiently combined to provide alignment of copolymer lamellae over several micrometers. As an example, a perpendicular organization with respect to circular tracks is shown, demonstrating the possibility of writing ordered radial nanostructures over large distances.

Amphiphilic behavior and membrane solubility of a dicholesteryl-cyclodextrin

Amphiphilic cyclodextrins (CDs) are good candidates to functionalize natural membranes as well as synthetic vesicles. In this paper, we provide a full description of the interfacial behavior of pure 6I,6IV-(β-cholesteryl)succinylamido-6I,6IV-(6-deoxy-per-(2,3,6-O-methyl))cycloheptaose (TBdSC) and how it inserts in dipalmitoyl-l-α-phosphatidylcholine (DPPC) monolayers as a membrane model. Langmuir isotherms of pure TBdSC suggest a reorganization upon compression, which could be clarified using X-ray reflectivity. The CD head can adjust its conformation to the available area per molecule. A compatible model involving a rotation around a horizontal axis defined by the two selectively substituted glucose units is proposed. The in-plane structure is characterized at all scales by Brewster angle microscopy (BAM) on the water surface and atomic force microscopy (AFM) on monolayers deposited on solid substrates. The same tools are used for its mixtures with DPPC. We show in particular that TBdSC seems to be soluble in the liquid-expanded DPPC. However, phase segregation occurs at higher pressure, allowing for sequentially liquid-condensed DPPC and high-pressure conformation of TBdSC. This gives rise to a remarkable contrast inversion in both imaging methods.

Polymeric cross-linked surface treatments for controlling block copolymer orientation in thin films

The orientation of cylinder-forming poly(styrene-block-methyl methacrylate) [P(S-b-MMA)] was investigated on two sets of polymeric surface treatments: 10 para-substituted polystyrene derivatives with <10 mol % poly(4-vinylbenzyl azide) and a series of poly(styrene-random-4-vinylbenzyl azide) [P(S-r-VBzAz)] copolymers with 5-100 mol % poly(4-vinylbenzyl azide). The copolymers were spin-coated to form thin films and then cross-linked by heating. The resulting films exhibited a range of surface tensions from 21 to 45 dyn/cm. Perpendicular orientation of P(S-b-MMA) cylinders was achieved with poly(p-bromostyrene) and all the [P(S-r-VBzAz)] copolymer surface treatments, most notably the homopolymer of poly(4-vinylbenzyl azide). Films made from these simple copolymers are as effective as random terpolymer alignment layers commonly made from both block monomers and a cross-linkable monomer.