Nanopatterning
Techniques for fabricating on sub-micron length scales span a wide range, from sophisticated lithographic methods that have their origins in the semiconductor industry to more recent materials and chemical advances that rely on self-organization. For delineating patterns below 100 nm, several approaches have been proposed (and indeed demonstrated). These include nano-imprint lithography (including micro-contact printing, mold-assisted lithography, and hot embossing lithography), near-field optical lithography, direct patterning on a nanometer scale with scanning-probe microscopes, self-assembly of monolayers, pattern formation based on phase separation of polymers, etc. The search is on for nonphotolithographic methods that could provide technologically simpler and cheaper nanofabrication strategies. Some of these approaches are better suited for producing individual nano-structures for the investigation of nanometer-scale devices; the throughput is likely to always remain impracticably low for commercial application. Others such as nanoimprint lithography have the potential of high throughput due to parallel processing, do not require sophisticated tools, and allow nanoscale replication for data storage.
The natural length scales of polymer chains and their morphologies in the bulk, which lie in the nanometer domain, make polymers ideal building blocks for nanopatterning. Recent developments in the use of polymers for the fabrication of nanostructures via lithographic and self-assembling strategies have been reviewed.
Nanopatterning via Phase Separation of Polymers
Block copolymers of flexible, chemically incompatible, and dissimilar blocks can microphase separate into a variety of morphologies with nanometer scale dimensions. This self-assembly process is driven by an unfavorable mixing enthalpy and a small mixing entropy, while the covalent bond between the two blocks prevents macrophase separation. When the microphase separated morphology can be controlled and turned into a useful structure, phase separation of block copolymers can be a powerful tool for fabricating nanostructures without additional lithography and processing steps. In addition, to block copolymers copolymers, random copolymers comprising sufficiently dissimilar components based on size and chemical nature, for e.g., SSQ-MMA copolymer, SSQ/Polypropylene blend, or physical blends of hydrophobic and hydrophilic polymers driven by an underlying pattern of incompatibility have been shown to yield functional and arbitrary patterns. The use of surfactants in such blends may be used to accentuate the areas of different surface tension.
Soft lithographic approaches have been combined with surfactant and particulate templating procedures to create to create hierarchically ordered oxides. A recent report combines molecular scale, evaporation-induced self-assembly (EISA) of organically modified mesophases with macroscopic, evaporative printing procedures. This allows the rapid fabrication of hierarchical structures exhibiting form and function on multiple length scales and at multiple locations. The formulation of the “ink” in such hierarchial assembly strategies is complex and may use a variety of surfactants.
Self-Assembled Monolayer Systems (SAMS)
Polyelectrolytes are defined as materials for which the solution properties in solvents of high dielectric constant are governed by electrostatic interactions over distances larger than typical molecular dimensions. These materials are widely used in industrial applications as dispersants in aqueous media, flocculating agents to coagulate slurries and industrial wastes, for sizing in textile and paper manufacture, and as conditioning additives to drilling muds and soil to prevent abrasive damage. More recently, they have been applied in molecular self-assembly techniques for thin film deposition of electrically conducting polymers, conjugated polymers for light emitting devices, nanoparticles,and noncentrosymmetric-ordered second order nonlinear optical (NLO) devices.
The technique of Self-Assembled Monolayers or SAMs is an ingeniously simple, yet powerful nanoscale approach for the fabrication of functional supramolecular assemblies for various device applications. It involves the alternate adsorption of anionic and cationic polyelectrolytes onto a suitable substrate. Typically, only one of these is the active layer, while the other enables the composite multilayered film to be bound by electrostatic attraction. Alternatively, the oppositely charged polyelectrolytes may serve as a barrier for the sustained release of an active core. Controlled formation of highly ordered, three-dimensional, multifunctional, reactive, thin films containing biological molecules is seeing widespread application in the areas of biotechnology and biomaterials science.
Of the potential polyanionic candidates, poly(styrenesulfonic acid) (PSSA) and its salts have been used extensively. Why? Excellent adsorption properties, water solubility, smooth films with easily controllable thickness, controlled level of loading/penetration of active component—all properties essential in realizing high performance devices based on supramolecular structures in terms of selectivity, sensitivity, response time, and stability. In addition, PSSA has been used to dope thiophene-based conjugated polymers to make them conducting, e.g., PEDOT/PSS Aldrich products. Post treatment of polyaniline (emeraldine salt), grafted to lignin (see Aldrich products , with PSSA may also be used to enhance electrical conductivity.
Norbornadiene
A scan of the scientific and patent literature reveals that this versatile monomer has been applied over a vast spectrum of high technology applications, in fields ranging from materials science, ag-related, and pharmaceutical, to being used as a model system in fundamental research activity (for example, in testing new nanocatalysts – in single-step hydrogenations, tandem cycloaddition reactions, Pauson-Khand annelation as well as in comprehensive theoretical studies. The unique set of properties offered by Norbornadiene along with its transformations driving some of the reported applications are highlighted in the following table.
Scheme 1. Conversion between norbornadiene (NBD) and quadricylane (QC).
Table 1. Properties of norbornadiene.
| NBD Property | Transformation | Application of NBD |
| Bicyclic, strained system | Undergoes photochemical valence isomerization to Quadricyclane (QC), converting solar energy to strain energy (See Scheme 1) |
Solar energy storage system8 |
| Bicyclic, strained system | Strain energy released gradually as heat upon conversion back to NBD (See Scheme 1) |
Microheater; Energetic binder for solid rocket propellants10 |
| Diolefin | Starter for norbornenediol derivative to synthesize stereo-controlled ROMP-derived precursor polymer, which upon thermal elimination results in a special form of polyacetylene | Synthon for Conductive Polymer, polyacetylene |
| Cycloaliphatic Monomer | Controlled cyclopolymerization of bulky ester derivatives of NBD to afford regioregular 2-alkoxycarbonyl nortricyclene polymers | New resist materials based on cycloaliphatic polymers for imaging with 193 nm lasers |
| Rigid, 3-D, crosslinking agent | Synthesis of soybean oilstyrene-NBD thermosetting copolymers | Shape memory polymer with Tg well above ambient |
| NBD | ROMP synthesized fluoropolymer, poly[2,3-bis (trifluoromethyl)NBD] having usefully high values of pyroelectric properties combined with low dielectric loss at RT | Pyroelectric transducer with a figure of merit comparable or better than that of PVDF |
| NBD | Controlled selectivity and reaction rate for cyclic dimerization, co-dimerization, isomerization, and allylation of NBD | As a universal substrate for organic and petrochemical synthesis of wide range of rare polycyclic hydrocarbons |
| NBD | Inhibited ethylene production and RNAase induction in apple pulp tissue | Growth regulator in fruit production |
| Highly strained NBD | Reagent in the Pd-catalyzed conversion of hydrosilanes to alkoxysilanes | Efficient hydrogen scavenger |
| Bicyclic NB | Starter in a fifteen step synthesis of (+)-Sparteine | Synthon in the first asymmetric total synthesis of (+)-Sparteine |
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I. Introduction
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