Electrokinetic molecular separation in nanoscale fluidic channels

This report presents a study of electrokinetic transport in a series of integrated macro- to nano-fluidic chips that allow for controlled injection of molecular mixtures into high-density arrays of nanochannels. The high-aspect-ratio nanochannels were fabricated on a Si wafer using interferometric lithography and standard semiconductor industry processes, and are capped with a transparent Pyrex cover slip to allow for experimental observations. Confocal laser scanning microscopy was used to examine the electrokinetic transport of a negatively charged dye (Alexa 488) and a neutral dye (rhodamine B) within nanochannels that varied in width from 35 to 200 nm with electric field strengths equal to or below 2000 V m-1. In the negatively charged channels, nanoconfinement and interactions between the respective solutes and channel walls give rise to higher electroosmotic velocities for the negatively charged dye than for the neutral dye, towards the negative electrode, resulting in an anomalous separation that occurs over a relatively short distance (<1 mm). Increasing the channel widths leads to a switch in the electroosmotic transport behavior observed in microscale channels, where neutral molecules move faster because the negatively charged molecules are slowed by the electrophoretic drag. Thus a clear distinction between "nano-" and "microfluidic" regimes is established. We present an analytical model that accounts for the electrokinetic transport and adsorption (of the neutral dye) at the channel walls, and is in good agreement with the experimental data. The observed effects have potential for use in new nano-separation technologies.     

Adsorptive separation using self-assembly on graphite: from nanoscale to bulk processes

Adsorptive separation is a promising lower-energy alternative for traditional industrial separation processes. While carbon-based materials have a long history in adsorptive removal of organic contaminants from solution or gas mixtures, separation using an adsorption/desorption protocol is rarely considered. The main drawbacks are the limited control in bulk adsorption experiments, as often all organic molecules are adsorbed, and lack of desorption methods to retrieve the adsorbed molecules. Using high-resolution on-surface characterization with scanning tunneling microscopy (STM), an increased understanding of the on-surface adsorption behavior under different conditions was obtained. The insight obtained from the nanoscale experiments was used to develop a highly selective separation method using adsorption and desorption on graphite, which was tested for the separation of quinonoid zwitterions. These experiments on adsorptive separation using self-assembly on graphite show its potential and demonstrate the advantage of combining surface characterization techniques with bulk experiments to exploit different possible applications of carbon-based materials.

Insights from high-resolution on-surface characterization techniques are used to improve the control over adsorption and desorption on graphite in bulk adsorptive separation processes.     

Macro- to nanoscale wear prevention via molecular adsorption

As the size of mechanical systems shrinks from macro- to nanoscales, surface phenomena such as adhesion, friction, and wear become increasingly significant. This paper demonstrates the use of alcohol adsorption as a means of continuously replenishing the lubricating layer on the working device surfaces and elucidates the tribochemical reaction products formed in the sliding contact region. Friction and wear of native silicon oxide were studied over a wide range of length scales from macro- to nanoscales using a ball-on-flat tribometer (millimeter scale), sidewall microelectromechanical system (MEMS) tribometer (micrometer scale), and atomic force microscopy (nanometer scale). In all cases, the alcohol vapor adsorption successfully lubricated and prevented wear. Imaging time-of-flight secondary ion mass spectrometry analysis of the sliding contact region revealed that high molecular weight oligomeric species were formed via tribochemical reactions of the adsorbed linear alcohol molecules. These tribochemical products seemed to enhance the lubrication and wear prevention. In the case of sidewall MEMS tests, the lifetime of the MEMS device was radically increased via vapor-phase lubrication with alcohol.     

Electrochemical detection of nanoscale phase separation in binary self-assembled monolayers

Developing methods to probe the nature and structure of nanoscale environments continues to be a challenge in nanoscience. We report a cyclic voltammetry investigation of the internal, hydrogen-bond-driven phase separation of amide-containing thiols and alkanethiols. Amide-containing thiols with a terminal ferrocene carboxylate functional group were investigated in two binary monolayers, one homogeneously mixed and the other phase separated. The electrochemical response of the ferrocene probe was used to monitor adsorbate coverage, environment, and phase separation within each of these monolayers. The results demonstrate that the behavior of ferrocene-containing monolayers can be used to probe nanoscale organization.     

Effect of molecular weight on reaction-induced phase separation of epoxy resin modified with fluorocarbon chain terminated polyetherimide

Epoxy resins are currently used for many important applications such as adhesives, encapsulates and ad-vanced composite matrixes. However, the further use of epoxies is limited because of their inherent brittle-ness. Thus, the modifications of epoxy resin…     

Light-directed mesoscale phase separation via holographic polymerization

Holographic polymerization (HP) is a simple, fast, and attractive technique to fabricate one-, two- and three-dimensional complex and functional nanostructures. Not only does the coupling of photopolymerization and light-directed phase separation HP process render rich polymer physics to the latter, it also leads to profound morphology-sensitive properties of HP structures, ranging from nano- to mesoscales. The past two decades witnessed tremendous progress in the field and in this review, we will probe the fundamental characteristics and parameters of HP, exemplify the versatility of this nanofabrication technique by presenting a diverse selection of HP patterned soft materials, and discuss some unique applications of such HP structures. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 232–250     

Supermolecular self organization via molecular aggregation. Non-discotic three chain diols displaying a hexagonal columnar phase

A new class of mesogen formed by three chain diols is described. These compounds consist of a central part to which three aliphatic chains of various length (n = 4-14) are attached at one end and two hydroxy groups at the other. X-ray scattering, dilatometric measurements, dielectric and Kerr relaxation studies were carried out. Both X-ray investigations, dielectric and electro-optic studies revealed that the molecules are aggregated in the isotropic fluid state and that hydrogen bonding and dipole correlation contribute to this aggregation. Columnar assemblies of up to several hundred molecules are orientationally correlated in the isotropic fluid phase. Correlated reorientations occur in the presence of external electric fields. The X-ray data show that the aggregates pack in a hexagonal way at lower temperatures giving a structure which strongly resembles that of the hexagonal disordered columnar phase. This anisotropic phase can be quenched to a glassy liquid-crystalline state. It is evident that the mesogenic properties of this new class of compounds are a function of the disc-like shape of the aggregates rather than the shapes of the individual molecules.     

Template-assisted patterning of nanoscale self-assembled monolayer arrays on surfaces

We report a general, simple, and inexpensive approach to pattern features of self-assembled monolayers (SAMs) on silicon and gold surfaces using porous anodic alumina films as templates. The SAM patterns, with feature sizes down to 30 nm and densities higher than 10(10)/cm(2), can be prepared over large areas (>5 cm(2)). The feature dimensions can be tuned by controlling the alumina template structure. These SAM patterns have been successfully used as resists for fabricating gold and silicon nanoparticle arrays on substrates by wet-chemical etching. In addition, we show that arrays of gold features can be patterned with 10-nm gaps between the dots.     

Preparation of epoxy monoliths via chemically induced phase separation

Epoxy porous monoliths were prepared from a commercial epoxy resin, D.E.R. 331, that cured with a tertiary amine, 2,4,6-tris-(dimethylaminomethyl) phenol, in the presence of a solvent, diisobutyl ketone (DIBK). During the curing process, polymers were formed and a decrease in its solubility in DIBK; the solution thus phase-separated, usually referred to as chemically induced phase separation. The phase separation formed interconnected polymer-poor phase that then became interconnected pores after the removal of DIBK. By varying the content of DIBK from 32 to 40 vol.%, epoxy monoliths with interconnected pores were prepared, with surface pore size ranging from 0.20 to 2.33 μm, overall porosity from 0.41 to 0.60, and ethanol permeability from 10 to 4,717 L/(m²?h^?1?bar^?1). The glass transition temperatures of the epoxy monoliths, measured with differential scanning calorimetry, were all higher than 100 °C, and temperatures of 5 % weight loss, analyzed by thermal gravimetry, were higher than 350 °C, evidencing the monoliths’ high thermal stability. Also, the monolith morphology was found to be strongly related to the reaction mechanism of polymerization. The results indicate that the mechanism of chain initiation and propagation associated with the tertiary amine can effectively form monoliths with interconnected pores, which cannot be easily prepared with a stepwise polymerization mechanism associated with using primary amine as the curing agent.     

Sorption and separation of CO2 via nanoscale AlO(OH) hollow spheres

The CO(2) uptake on nanoscale AlO(OH) hollow spheres (260 mg g(-1)) as a new material is comparable to that on many metal-organic frameworks although their specific surface area is much lower (530 m(2) g(?1)versus 1500-6000 m(2) g(?1)). Suited temperature-pressure cycles allow for reversible storage and separation of CO(2) while the CO(2) uptake is 4.3-times higher as compared to N(2).     

A model for isotope separation via molecular multiphoton photodissociation

Multiphoton photofragmentation of an “isolated” molecule on the lowest potential surface is considered in terms of a truncated anharmonic oscillator driven by a pulsed, intense laser field. Intramolecular vibrational relaxation and indirect photodissociation are accounted for by assignment of decay widths to the appropriate levels. The effective-hamiltonian formalism is applied to derive explicit expressions for the photofragmentation yield and for the isotopic separation factor.     

Self-assembled two-dimensional ordered arrays of tripod-type molecules on graphite

Tripod-type molecules with long alkyl chains, 1,1,1-tris(4-alkoxyphenyl)ethanes with octadecyloxy or docosyloxy chains, self-assemble into two-dimensional crystallites on drop-casting onto the surface of highly oriented pyrolytic graphite. In the two-dimensional crystalline domain, the molecules are organized in a mortise-and-tenon motif, as revealed by scanning tunneling microscopy. The time evolution of the crystallite formation has been followed by the dynamic force mode atomic force microscopy. The tripods may be used as a basis for the extension of a two-dimensional order into three-dimensional molecular architectures.     

Intramolecular phase separation of a copolymer chain with mobile primary structure

The phenomenon of intramolecular phase separation of a single copolymer chain with mobile primary structure (i.e., a polymer chain with reversibly adsorbable ligands) in dilute solution is investigated on the basis of a Flory-type interpolation theory of the coil-globule phase transition. It is shown that under certain critical conditions the stability of the homogeneous primary structure is violated and intramolecular phase separation takes place. Phase diagrams for the cases of rigid and flexible chains are calculated. The main characteristics of a polymer chain in the two-phase state (such as fraction of monomeric units, swelling coefficient and fraction of units with ligands for coexisting intramolecular phases) are investigated.     

Self-assembly of patterned monolayers with nanometer features: molecular selection based on dipole interactions and chain length

Patterned cocrystal monolayers self-assemble on HOPG in contact with solutions containing complementary pairs of 1,5-chain-substituted anthracene derivatives. Monolayer unit cells containing three or four molecules and spanning 9-11 nm are generated. The monolayers consist of alternating aromatic and aliphatic columns. The designs and dimensions of the cocrystal patterns (unit cells) are determined by (i) the preferred packing alignment of identical length side chains, (ii) the selectivity of each side chain for neighboring chains, (iii) the identities of the two side chains on each anthracene, and (iv) the 2D-chirality of 1,5-substituted anthracenes. The aliphatic columns form by interdigitation of identical length side chains arrayed in an antiparallel alignment, with the nth heavy atom of one side chain in registration with the (omega+2-n)th heavy atom of two adjacent chains ((omega <--> 2) packing). Adjacent side chains are attached, alternately, to anthracenes in one of the two flanking aromatic columns. The preference for (omega <--> 2) packing optimizes side-chain van der Waals interactions. The composition and fidelity of patterning in the cocrystal monolayers requires an additional source of "molecular recognition" in addition to side-chain length. Dipolar interactions, both attractive and repulsive, between ether groups in neighboring, (omega <--> 2) packed side chains, constitute a second recognition element needed for cocrystal self-assembly.     

Isolated and linear arrays of surfactant-encapsulated polyoxometalate clusters on graphite

We report on the self-assembly of several surfactant-encapsulated clusters (SECs) on the basal plane of graphite consisting of the doughnut-shaped tungstophosphate anion [Na(H2O)P5W30O110] covered by a hydrophobic shell of surfactants. Well-ordered rodlike structures are observed using scanning force microscopy. No such ordering is observed if the surfactant methyltrioctadecylammonium is used for encapsulation, suggesting that the density of alkyl chains around the polyoxometalate cluster is an important factor in determining the order of SEC assemblies on graphite. Coadsorption of tetratetracontane (n-C44H90) and (DODA)14[Na(H2O)P5W30O110] results in single, isolated SECs on a buffer layer of tetratetracontane, as determined by scanning tunneling microscopy.     

ToF-SIMS imaging of the nanoscale phase separation in polymeric light emitting diodes: effect of nanostructure on device efficiency

The nanostructure of the light emissive layer (EL) of polymer light emitting diodes (PLEDs) was investigated using force modulation microscopy (FMM) and scanning time-of-flight secondary ion mass spectrometry (ToF-SIMS) excited with focused Bi(3)(2+) primary beam. Three-dimensional nanostructures were reconstructed from high resolution ToF-SIMS images acquired with different C(60)(+) sputtering times. The observed nanostructure is related to the efficiency of the PLED. In poly(9-vinyl-carbazole) (PVK) based EL, a high processing temperature (60 °C) yielded less nanoscale phase separation than a low processing temperature (30 °C). This nanostructure can be further suppressed by replacing the host polymer with poly[oxy(3-(9H-9-carbazol-9-ilmethyl-2-methyltrimethylene)] (SL74) and poly[3-(carbazol-9-ylmethyl)-3-methyloxetane] (RS12), which have similar chemical structures and energy levels as PVK. The device efficiency increases when the phase separation inside the EL is suppressed. While the spontaneous formation of a bicontinuous nanostructure inside the active layer is known to provide a path for charge carrier transportation and to be the key to highly efficient polymeric solar cells, these nanostructures are less efficient for trapping the carrier inside the EL and thus lower the power conversion efficiency of the PLED devices.     

Sensitive phase separation behavior of ultra-high molecular weight polyethylene in polybutene

Thermally induced phase separation (TIPS) has prompted a great deal of interest, especially as an effective approach to fabricate ultra-high molecular weight polyethylene (UHMWPE) microporous membranes. However, the existing utilized diluents for the TIPS process of UHMWPE suffer from environmental and health issues. Herein, we utilized low molecular weight polybutene (PB) bearing similar structure with liquid paraffin (LP) but inferior miscibility with UHMWPE relative to UHMWPE/LP blend, as a diluent for the TIPS process of UHMWPE. The phase separation behavior of UHMWPE/PB blends were investigated by the combination of rheological measurements, optical microscopy as well as differential scanning calorimeter (DSC). The results suggest that PB is fully miscible with UHMWPE at elevated temperature, but yielding a more sensitive phase separation behavior in respect to LP in TIPS process, because PB has weaker interaction with UHMWPE. The Jeziorny method analysis indicates that the crystallization mechanism of UHMWPE/LP blends is in line with that of UHMWPE/PB blends, which includes nucleation and growth as well as their dynamic competition. Moreover, compared to those of UHMWPE/LP blends, UHMWPE/PB blends display higher TIPS temperature and faster TIPS rate along with faster overall crystallization rate, further demonstrating that PB can accelerate phase separation rates and enhance the efficiency of TIPS process.     

Formation of microporous Teflon PFA membranes via thermally induced phase separation

Poly(tetrafluoroethylene-co-perfluoro-(propyl vinyl ether)) (Teflon^® PFA) membranes of a variety of structures have been produced through thermally induced phase separation of Teflon^® PFA-chlorotrifluoroethylene melt-blends of different compositions. A phase diagram of the two component system was constructed, and electron microscopy was used to characterize the structures of membranes produced. The morphological characteristics of the Teflon^® PFA membranes have been explained on the basis of equilibrium driving forces for liquid-liquid and solid-liquid phase separations.     

Configuration mixing effects on molecular dipole transition moments

The behavior of dipole transition matrix elements between adiabatic eigenstates of diatomic molecules is explored in the region of large configuration mixing. It is demonstrated that the matrix element has a lorentzian shape as a function of internuclear distance. The effect of this shape on the spectrum of LiH is investigated. The strong enhancement of the dipole transition moment in the interaction region is suggested as a source for nonthermal population distributions and laser radiation in alkali diatomics and hydrides.     

Effects of polymer chain length and stiffness on phase separation dynamics of semidilute polymer solution

Three-dimensional dissipative particle dynamics (DPD) simulations were performed to investigate the phase separation dynamics of semidilute polymer solutions with different polymer chain length and stiffness. For the polymer solution composed of shorter and more flexible chains, a crossover of the domain growth exponent from 1/3 to 2/3 was observed during the course of phase separation, indicating that the growth mechanism altered from diffusion to interfacial-tension driven flow. When the chain flexibility was kept the same but the chain was lengthened to allow for the chain entanglement to occur, the growth exponent changed to 1/4 in the diffusion-dominating coarsening regime while the growth exponent remained 2/3 in the flow-dominating regime. When the chain length was kept short but the stiffness was increased, the growth exponent became 1/6 in the diffusion-dominating regime and little effect was observed in the flow-dominating coarsening regime. The slow down of the phase separation dynamics in the diffusion-dominating coarsening could be explained by that the polymer chains could only perform wormlike movement when chain entanglements occurred or when the chain motion was limited by chain stiffness during phase separation. Moreover, when both the effects of chain length and stiffness were enhanced, polymer networks composed of longer and stiffer chains appeared and imposed an energy barrier for phase separation to occur. As a result, the polymer solution with stiffer and longer chains required a larger quench depth to initiate the phase separation and caused the delay in crossover of the coarsening mechanism from diffusion to flow.