Surface-directed patterning of polymer/nanoparticle assemblies on microcontact-printed substrates

Date

2012-01-18

Authors

Harirchian-Saei, Saman

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Abstract

Two different strategies for producing hierarchical polymer/nanoparticle (NP) patterned structures are presented in this work. The first strategy combines self-assembly of amphiphilic block copolymers at the air-water interface with microscale template assembly of the resulting aggregates on chemically-patterned substrates. Aggregates are formed via interfacial self-assembly of 141k polystyrene-block-poly (ethylene oxide) (PS-b-PEO, Mw=141 k) or a blend of PS-b-PEO (Mw=185 k) and PS-coated CdS (PS-CdS) quantum dots (QDs), to form aggregates of copolymer or copolymer/NP. Using Langmuir-Blodgett (LB) technique, the formed aggregates are then transferred to patterned substrates with alternating hydrophilic/hydrophobic stripes, obtained by microcontact printing (µCP) octadecyltrichlorosilane (OTS) on glass. The effect of different parameters including surface pressure, orientation of the patterned substrate respect to the air-water interface, and withdrawal speed was studied. Successful aggregate transfer to the hydrophilic domains of the patterned hydrophilic/hydrophobic substrate is achieved when patterned stripes are oriented perpendicular to the water surface during LB transfer and when substrates are withdrawn at low speed and low compression pressure. The second strategy combines the phase-separation of immiscible polymer blends during spin-coating with µCP. We show the surface-directed patterning of a phase-separating polymer blend on optically-transparent (OTS)-patterned glass substrate obtained via µCP. First, morphologies and pattern registration of thin spin-coated films of PS (Mw=131 k)/ poly(methyl methacrylate) (PMMA, Mw= 120 k) blends on patterned glass with alternating hydrophilic/hydrophobic stripes is studied for a range of experimental conditions including polymer concentration, blend composition, solvent, and spin rate. Good registration of polar PMMA to hydrophilic glass surface and non-polar PS to hydrophobic OTS lines is found under conditions, where polymer domain sizes are commensurate with the pattern periodicity. Next, we apply this method to pattern NPs using blends of PMMA and PS-CdS QDs via spin-coating onto OTS-patterned glass. Ultimately the method was extended to simultaneously pattern multi-NP functional assemblies using PS-CdS and a sample of PMMA-coated silver NP (PMMA-Ag). The specific interest in patterns of Ag NPs and CdS QDs is to provide a suitable proof-of-concept system for simultaneous multi-NP patterning. However, this system also has some interesting optical behaviour as a result of QD-surface plasmon interactions that is investigated in details. The challenge in PS-CdS/PMMA-Ag NPs patterning is the gelation as the solvent evaporates during spin-coating that restricts the NPs mobility and constraints their phase-separation. We show that adding homopolymers to the NPs blends prevents the overlap of approaching NP brushes and prevents the resulting gelation. Feature sizes were then fine-tuned by changing solution concentration and spin rate, in order to obtain NPs domains which can be surface-directed on OTS-patterned glass substrates.

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Keywords

Microcontact printing, polymers, nanoparticles

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