Simultaneous patterning of two different types of nanoparticles into alternating domains of a striped array of a polymer blend in a single spin-casting step.

A fast and convenient method is developed for simultaneously patterning inorganic nanoparticles with different optical, electronic or magnetic functionality to specific surface regions, by spin-casting onto microcontact printed substrates blend solutions in which the two nanoparticle types are functionalized with surface polymer brush layers of different surface energies. The process is based on phase separation of different nanoparticles based on their immiscible brush layers during spin-casting, with the underlying surface energy heterogeneity of the patterned substrate directing the different NP types to domains of different surface energies. Here, we specifically demonstrate the simultaneous localization of cadmium sulfide quantum dots (CdS QDs), addressed with a surface layer of polystyrene (PS), and silver nanoparticles (Ag NPs), addressed with a surface layer of poly(methyl methacrylate) (PMMA), onto the non-polar and polar surface domains, respectively, of hydrophilic glass patterned with hydrophobic octadecyltrichlorosilane (OTS) stripe arrays with micron-scale periodicities. In order to prevent gelation of solvent-swollen polymer-brush coated NPs during spin casting, which effects strong kinetic constraints on phase separation and localization, PS, PMMA or PS/PMMA homopolymer blends of sufficiently high Mw were added to the NP blends to increase the free volume between approaching NPs. The process parameters were fine-tuned to obtain control over defects in the obtained patterns.

Directed polystyrene/poly(methyl methacrylate) phase separation and nanoparticle ordering on transparent chemically patterned substrates.

We investigate the surface-directed phase separation of spin-coated polystyrene/poly(methyl methacrylate) (PS/PMMA) blends on prepatterned octadecyltrichlorosilane (OTS)-glass substrates under various experimental conditions. As a result of tandem processes of spinodal decomposition and selective wetting of polymer components during spin-coating, low-energy OTS stripes and high-energy glass surfaces laterally arrange the phase-separated polymers according to the chemical pattern on the substrate. Optimal pattern replication was achieved when the length scale of phase separation, controlled via the polymer concentration of the spin-coating solution, matched the smallest feature dimension in a striped chemical pattern possessing two alternating distances between stripes. It was also shown that polymer blend patterns were most closely registered with the underlying substrate when the PS/PMMA composition ratio (30/70, w/w) matched the areal fraction of OTS on the glass surface (∼30%). The influence of solvents demonstrated that a solvent with a relatively low volatility, such toluene, was required for patterning so that domain feature sizes were able to coarsen to the size of the patterned features before film vitrification. As well, we showed that the technique and optimized conditions developed in this study could be applied to pattern photoluminescent CdS quantum dots into microscale arrays of parallel lines via spin-coating onto transparent OTS-glass substrates.

Block copolymer mimetic self-assembly of inorganic nanoparticles.

Emerging strategies for assembling inorganic nanoparticles into ensembles with multiscale organization are establishing a new paradigm for the synthesis of devices and functional materials with applications ranging from drug delivery to photonics. In this work, the solution self-assembly of amphiphilic ionic block copolymers into morphologically tunable aggregates provides the inspiration and design strategy for nanoparticle building blocks with the essential chemical and conformational features of ionic block copolymer chains in aqueous media. We produce inorganic nanoparticles with surface-tethered mixed brushes of hydrophobic and chargeable hydrophilic chains which self-assemble in polar solvent mixtures into unprecedented hierarchical superstructures analogous to known ionic block copolymer aggregates but with complex organizations of nanoparticles in three dimensions. Electrostatic repulsion between hydrophilic chains forces nonequilibrium pathways to variable kinetic structures with internal lamellar organization of nanoparticles; however, decreasing electrostatic interactions through salt or acid addition allows tunable equilibrium assemblies, including supermicelles and bilayer vesicles of nanoparticles, to be formed. The application of ionic block copolymer assembly principles and mechanisms opens a new chemical toolbox for the organization of nanoparticles into functional assemblies.

Strands, networks, and continents from polystyrene dewetting at the air-water interface: implications for amphiphilic block copolymer self-assembly.

We demonstrate that nanoscale aggregates similar to those formed via amphiphilic block copolymer self-assembly at the air-water interface, including strands, networks, and continents, can be generated by the simple spreading of PS homopolymer solutions on water. Two different PS homopolymers of different molecular weight (PS-405k, M(n) = 405 000 g mol(-1) and PS-33k, M(n) = 33 000 g mol(-1)) are spread at the air-water interface at various spreading concentrations ranging from 0.25 to 3.0 mg/mL. Aggregate formation is driven by PS dewetting from water as the spreading solvent evaporates. We propose that a high spreading concentration or a high molecular weight lead to chain entanglements that restrict macromolecular mobility in the solution, enabling the kinetic trapping of nanostructures associated with early and intermediate stages of PS dewetting. Comparison of PS-405k with a mainly hydrophobic PS-b-PEO block copolymer of similar molecular weight (PSEO-392k, M(n) = 392 000 g mol(-1), 2.0 wt % PEO) allows the effect of a relatively short surface active block on aggregate formation to be investigated. We show that whereas the PEO block is not a required component for the formation of strands and other nonglobular aggregates, it does increase the number of these aggregates at a given spreading concentration and decreases the minimum spreading concentration at which these aggregates are observed, along with decreasing the dimensions and polydispersity of specific surface features. The results provide supporting evidence for the role of PS dewetting in the generation of multiple PS-b-PEO aggregate morphologies at the air-water interface, as originally described in earlier paper from our group.

Patterning block copolymer aggregates via Langmuir-Blodgett transfer to microcontact-printed substrates.

We demonstrate a new strategy for producing hierarchical polymer nanostructures, which combines nanoscale self-assembly of amphiphilic block copolymers at the air-water interface with microscale templated assembly of the resulting aggregates on chemically patterned substrates. Aggregates are formed via interfacial self-assembly of 141k polystyrene-b-poly(ethylene oxide) (PS-b-PEO, MW = 141k, 11.4 wt % PEO) or a blend of 185k PS-b-PEO (MW = 185k, 18.9 wt % PEO) and PS-coated CdS nanoparticles to form strandlike copolymer or copolymer-nanoparticle aggregates. Using Langmuir-Blodgett (LB) techniques, the aggregates are then transferred to patterned substrates possessing alternating hydrophilic/hydrophobic stripes, obtained by microcontact printing octadecyltrichlorosilane (OTS) on glass. The aggregates are transferred under various conditions of surface pressure, orientation of the patterned substrate, and withdrawal speed. Templated assembly of aggregates into the hydrophilic substrate domains is achieved when the hydrophilic/hydrophobic stripes are oriented perpendicular to the water surface during LB transfer; this is explained by surface energy heterogeneities along the subphase-substrate contact line, which induce selective dewetting and concomitant monolayer rearrangement at the drying front. In contrast, parallel orientation of stripes results in nonselective transfer of the monolayer without registration to the underlying surface pattern. By studying the effect of surface pressure, we show that packing constraints imposed by compression of aggregates to high surface densities prevent the formation of patterned LB films that match the established periodicity of the OTS-patterned glass. As well, it is shown that efficient transfer of aggregates to the patterned glass requires slower substrate withdrawal speeds compared to transfer to unpatterned hydrophilic glass.