Ned B. Bowden

Spontaneous formation of ordered structures in thin films of metals supported on an elastomeric polymer

Spontaneous generation of complex order in apparently simple systems is both arresting and potentially useful. Here we describe the appearance of complex, ordered structures induced by the buckling of thin metal films owing to thermal contraction of an underlying substrate. We deposit the films from the vapour phase on a thermally expanded polymer (polydimethylsiloxane, PDMS). Subsequent cooling of the polymer creates compressive stress in the metal film that is relieved by buckling with a uniform wavelength of 20–50 micrometres. The waves can be controlled and orientated by relief structures in the surface of the polymer, which can set up intricate, ordered patterns over large areas. We can account qualitatively for the size and form of the patterned features in terms of the non-uniform stresses developed in the film near steps on the polymer substrate. This patterning process may find applications in optical devices such as diffraction gratings and optical sensors, and as the basis for methods of strain analysis in materials.

The controlled formation of ordered, sinusoidal structures by plasma oxidation of an elastomeric polymer

This letter describes a technique for generating waves on polydimethylsiloxane (PDMS) patterned in bas-relief. The PDMS is heated, and its surface oxidized in an oxygen plasma; this oxidation forms a thin, stiff silicate layer on the surface. When the PDMS cools, it contracts and places the silicate layer under compressive stress. This stress is relieved by buckling to form patterns of waves with wavelengths from 0.5 to 10 μm. The waves are locally ordered near a step or edge in the PDMS. The wavelength, amplitude, and pattern of the waves can be controlled by controlling the temperature of the PDMS and the duration of the oxidation. The mechanism for the formation and order of the waves is described.

Autonomous Movement and Self-Assembly

The artificial millimeter-scale “autonomous movers” glide across the surface of a liquid without an external power source. This system is based on a combination of two processes: Motion of individual objects powered by the catalytic decomposition of hydrogen peroxide, and relative motion (self-assembly) caused by capillary interactions at the fluid/air interface. The picture shows the rotational/translational motion of a single object; the motion of a pair of these object depends on their chirality.

Hierarchically Organized Structures Engineered from Controlled Evaporative Self-Assembly

By constraining an asymmetric comb block copolymer (CBCP) toluene solution to evaporate in a wedge-on-Si geometry composed of a wedge lens situated on a Si substrate, gradient concentric stripelike surface patterns of CBCP at the microscopic scale were yielded as a direct consequence of controlled evaporative self-assembly of CBCP. The formation of either straight stripes or jagged stripes was dictated by the height of the wedge. Upon subsequent solvent vapor annealing, hierarchically organized structures of CBCP were produced, resulting from the interplay of solvent vapor assisted, unfavorable interfacial interaction driven destabilization of CBCP from the Si substrate at the microscopic scale and the solvent vapor promoted reconstruction of CBCP nanodomains within the stripes at the nanometer scale. This facile approach of combining controlled evaporative self-assembly with subsequent solvent vapor annealing offers a new platform to rationally design and engineer self-assembling building blocks into functional materials and devices in a simple, cost-effective manner.

MACROSCOPIC, HIERARCHICAL, TWO-DIMENSIONAL SELF-ASSEMBLY

By tailoring capillary interactions at a fluid-fluid interface, a hierarchical two-dimensional self-assembly of hexagonal millimeter-sized poly(dimethylsiloxane) plates has been demonstrated (see picture). The strength and direction of capillary forces between plates was controlled by patterning of the surfaces of the plates to be hydophobic or hydrophilic. The thick lines indicate hydrophobic faces whose mutual attraction forms the basis of capillarity.

A Materials Approach to Site-Isolation of Grubbs Catalysts from Incompatible Solvents and m-Chloroperoxybenzoic Acid

The development of a method for site-isolation of Grubbs second-generation catalyst from MCPBA is described. In these reactions, Grubbs catalyst was dissolved in a solvent consisting of a mixture (1:1 v/v) of 1-butyl-3-methylimidazolium hexafluorophosphate and methylene chloride and completely encapsulated within a thimble fabricated from polydimethylsiloxane (PDMS). A series of molecules that react by cross metathesis or ring-closing metathesis were added to the interior of the thimble and allowed to react. In the last step, m-chloroperoxybenzoic acid (MCPBA) dissolved in MeOH/H(2)O (1:1 v/v) was added to the exterior of the PDMS thimble. Small organic molecules diffused through the PDMS to react with MCPBA to form epoxides, but the Grubbs catalyst remained encapsulated. This result is important because Grubbs catalyst catalytically decomposes MCPBA at ratios of MCPBA to Grubbs of 3000 to 1. The yields for this two-step cascade sequence ranged from 67 to 83 %. The concept behind this sequence is that small organic molecules have high flux through PDMS but large molecules--such as Grubbs catalyst--and ionic reagents--such as MCPBA--have much lower flux through PDMS. Small molecules can thus react both outside and inside PDMS thimbles, whereas incompatible catalysts and reagents remain site-isolated from each other. This method does not require alteration of structures of the catalysts or reagents, so it may be applied to a wide range of homogeneous catalysts and reagents. To demonstrate further that the catalyst was encapsulated, the Grubbs catalyst was successfully recycled within the cascade sequence.

Hierarchically Ordered Structures Enabled by Controlled Evaporative Self‐Assembly

Hierarchical structures are common in both nature and technology. In the latter context, controlling the spatial arrangement of components over multiple length scales (i.e., forming hierarchically ordered structures) is highly desirable for many applications, such as lab-on-a-chip devices, integrated circuits, and microelectromechanical systems (MEMSs). [ 1 ] Most hierarchically ordered structures are fabricated by very costly, complicated, and destructive surface patterning techniques that rely heavily on lithographic conditions (e.g., electron-beam (e-beam) or deep-UV), the use of additional external forces (e.g., electric or magnetic fi elds), and the use of sophisticated multistep processing. By contrast, a simple yet promising strategy to create complex, hierarchically ordered structures is the combination of top-down controlled evaporative self-assembly with bottom-up molecular selfassembly. Evaporative self-assembly of nonvolatile solutes (e.g., polymers, viruses, DNA, latex particles, nanocrystals, and carbon nanotubes) from a sessile droplet yields a rich family of intriguing structures [ 2–4 ] possessing submicrometer dimensions and larger. These structures, however, often lack regularity. In order to take advantage of the extreme simplicity of this top-down technique and to exploit its full potential in many applications, control of the evaporative fl ux, solution concentration, and interfacial interactions of the solvent, solute, and substrate is necessary to produce highly ordered, complex structures. In a number of studies that elegantly demonstrated precise control over droplet evaporation, [ 5–15 ] including the controlled anisotropic wetting/ Hierarchically Ordered Structures Enabled by Controlled Evaporative Self-Assembly

Sequential reactions with Grubbs catalyst and AD-mix-alpha/beta using PDMS thimbles

Incompatible Grubbs catalyst and an osmium dihydroxylation catalyst were site-isolated from each other using polydimethylsiloxane thimbles. The Grubbs catalyst was added to the interior of the thimbles, and AD-mix-alpha/beta was added to the exterior. Organic substrates readily fluxed through the walls of the thimbles and reacted with each catalyst. A series of cascade reactions were developed including those with intermediates possessing low boiling points or that were foul smelling.

Epoxidation of the surface of polydicyclopentadiene for the self-assembly of organic monolayers

Polydicyclopentadiene was reacted with m-chloroperoxybenzoic acid to yield a surface that was terminated with epoxides. The X-ray photoelectron spectrum (XPS) of the sample demonstrated that the top ten nm of the surface had been oxidized. The grazing angle attenuated total reflection-infrared spectrum of this surface was unchanged from that of native PDCPD which demonstrated that the oxidation was only on the surface and that the bulk PDCPD was unchanged. The PDCPD–epoxide surface was then reacted with two different amines that possessed F or Cl atoms to study the ring opening reaction between surface epoxides and amines. This reaction was rapid and completed within an hour. The method of Tougaard was applied to the F and Cl peaks in the XPS to investigate their locations and whether the amines were uniformly distributed in the top ten nm or were localized at the surface. This analysis clearly described the amines as being present only on the surface. The PDCPD–epoxide surface was also reacted with poly(ethylene imine) to generate a surface that exposed numerous amines. The amines bonded to Cu such that this metal did not flux through PCPCD while the flux of 4-nitrobenzaldehyde was unaffected.

Selective flux of organic liquids and solids using nanoporous membranes of polydicyclopentadiene

Membranes were fabricated from the ring opening metathesis polymerization of dicyclopentadiene with the Grubbs first generation catalyst, and the permeability of twenty-one molecules through them was studied. Both polar and apolar molecules with molecular weights from 101 to 583 g mol−1 permeated these membranes with values for flux of 10−5 to 10−6 mol cm−2 h−1 but selected molecules did not permeate them and had flux 104 to 105 times slower. The difference in flux was large between molecules that permeated and those that did not permeate, but no trend was observed that correlated flux with molecular weight or hydrophobicity. Rather, molecules that did not permeate the membranes had large cross-sectional areas that led to low rates of diffusion within the highly cross-linked polydicyclopentadiene membranes. The degree of cross-linking within the polydicyclopentadiene membranes was measured using infrared spectroscopy and approximately 84% of the dicyclopentadiene monomer had reacted to form cross-links. These are the first organic solvent nanofiltration membranes that separate molecules with molecular weights from 100 to 600 g mol−1 based on cross-sectional areas.