Self-assembly of polystyrene-block-poly(ethylene oxide) copolymers at the air-water interface: is dewetting the genesis of surface aggregate formation?

Block copolymer self-assembly at the air-water interface is commonly regarded as a two-dimensional counterpart of equilibrium block copolymer self-assembly in solution and in the bulk; however, the present analysis of atomic force microscopy (AFM) and isotherm data at different spreading concentrations suggests a nonequilibrium mechanism for the formation of various polystyrene-b-poly(ethylene oxide) (PS-b-PEO) aggregates (spaghetti, dots, rings, and chainlike aggregates) at the air-water interface starting with an initial dewetting of the copolymer spreading solution from the water surface. We show that different spreading concentrations provide kinetic snapshots of various stages of self-assembly at the air-water interface as a result of different degrees of PS chain entanglements in the spreading solution. Two block copolymers are investigated: MW = 141k (11.4 wt % PEO) and MW = 185k (18.9 wt % PEO). Langmuir compression isotherms for the 185k sample deposited from a range of spreading concentrations (0.1-2.0 mg/mL) indicate less dense packing of copolymer chains within aggregate cores formed at lower spreading concentrations due to a competition between the interfacial adsorption of PEO blocks and the kinetic restrictions of PS chain entanglements. From AFM analysis of the transferred Langmuir-Blodgett films, it is clear that PS chain entanglements in the spreading solution also affect the morphological evolution of surface aggregates for both samples, with earlier structures being trapped at higher concentrations. At the highest spreading concentration for the 141k copolymer, the coexistence of long spaghetti aggregates with cellular arrays of holes, along with various transition structures, indicates that various surface aggregates evolve from networks of rims formed as a result of dewetting of the evaporating spreading solution from the water surface.     

Tandem interactions in the self-assembly of ionic block copolymers

The aim of the present review is to show how the phenomena of block copolymer self-assembly and interactions of ionic (or ionizable) groups in polymer systems can be combined to produce materials with versatile and unique behavior. In our discussion, we consider two classes of tandem interactions. First, block copolymers containing short ionic blocks and long nonionic blocks are investigated in organic media. In systems of this type, block copolymer self-assembly and short-range electrostatic interactions act in tandem, forming regular and highly-stable spherical structures. Next, we consider ionic (or ionizable) block copolymers dissolved in aqueous media. In this case, block copolymer self-assembly acts in tandem with the hydrophilic nature of the soluble blocks, resulting in a wide range of unique morphologies.     

Multiscale Control of Hierarchical Structure in Crystalline Block Copolymer Nanoparticles Using Microfluidics

Hierarchical semicrystalline block copolymer nanoparticles are produced in a segmented gas‐liquid microfluidic reactor with top‐down control of multiscale structural features, including nanoparticle morphologies, sizes, and internal crystallinities. Control of multiscale structure on disparate length scales by a single control variable (flow rate) enables tailoring of drug delivery nanoparticle function including release rates.

     

Enhanced fluorescence from arrays of nanoholes in a gold film

Arrays of sub-wavelength holes (nanoholes) in gold films were used as a substrate for enhanced fluorescence spectroscopy. Seven arrays of nanoholes with distinct periodicities (distances between the holes) were fabricated. The arrays were then spin-coated with polystyrene films containing different concentrations of the fluorescent dye oxazine 720. The dye was excited via resonant extraordinary transmission of the laser source through the nanoholes. Enhanced fluorescence was observed when the geometric characteristics of the arrays allowed for an enhancement in the transmitted excitation. This enhancement occurred via surface plasmon excitation by the laser and a consequential increase in the local electromagnetic field in a sub-wavelength region at the metal-film interface. It was demonstrated that the sensitivity of the fluorescence measurement (change in signal vs change in dye concentration in the polymer film) is significantly larger at the surface plasmon resonance conditions than that obtained from equivalent films on glass substrates. Enhancement factors for the fluorescence emission were calculated for each array, with a maximum enhancement of close to 2 orders of magnitude as compared to the emission of films on glass. The results presented here indicate that arrays of nanoholes are interesting substrates for the development of fluorescence sensors based on surface plasmon resonance, as they provide a platform that allows both spatial confinement and enhancement of excitation light. Moreover, the collinear characteristics of the present optical setup, due to the resonant extraordinary transmission through the nanohole arrays, are more conducive to miniaturization and chip integration than more traditional experimental geometries.     

Optimal rectangle packing

We consider the NP-complete problem of finding an enclosing rectangle of minimum area that will contain a given a set of rectangles. We present two different constraint-satisfaction formulations of this problem. The first searches a space of absolute placements of rectangles in the enclosing rectangle, while the other searches a space of relative placements between pairs of rectangles. Both approaches dramatically outperform previous approaches to optimal rectangle packing. For problems where the rectangle dimensions have low precision, such as small integers, absolute placement is generally more efficient, whereas for rectangles with high-precision dimensions, relative placement will be more effective. In two sets of experiments, we find both the smallest rectangles and squares that can contain the set of squares of size 1×1, 2×2,…,N×N, for N up to 27. In addition, we solve an open problem dating to 1966, concerning packing the set of consecutive squares up to 24×24 in a square of size 70×70. Finally, we find the smallest enclosing rectangles that can contain a set of unoriented rectangles of size 1×2, 2×3, 3×4,…,N×(N+1), for N up to 25.     

Formation and shear-induced processing of quantum dot colloidal assemblies in a multiphase microfluidic chip

The controlled self-assembly of polymer-stabilized quantum dots (QDs) into mesoscale aqueous spherical assemblies termed quantum dot compound micelles (QDCMs) using a two-phase gas-segmented microfluidic reactor is described. Self-assembly is initiated by the fast mixing of water (approximately 1 s) with a blend solution of polystyrene-coated QDs and amphiphilic polystyrene-block-poly(acrylic acid) stabilizing chains via chaotic advection within liquid plugs moving through a sinusoidal channel. Subsequent recirculating flow within a post-formation channel subjects the dynamic QDCMs to shear-induced processing, controlled via the flow rate and channel length, before a final quench into pure water. During processing, larger QDCMs within the initial population undergo breakup into smaller particles, resulting in smaller mean particle sizes, smaller relative standard deviations, and more skewed distribution shapes, as the overall shear exposure is increased. For these cases, shear-induced size reduction is sufficient to dominate surface tension-driven growth.     

A hierarchical self-assembly route to three-dimensional polymer-quantum dot photonic arrays

We demonstrate a new hierarchical self-assembly strategy for the formation of photonic arrays containing quantum dots (QDs), in which sequential self-assembly steps introduce organization on progressively longer length scales, ranging from the nanoscale to the microscale regimes. The first step in this approach is the self-assembly of diblock copolymers to form block ionomer reverse micelles (SA1); within each micelle core, a single CdS QD is synthesized to yield the hybrid building block BC-QD. Once SA1 is completed, the hydrophobic BD-QD building blocks are blended with amphiphilic block copolymer stabilizing chains in an organic solvent; water addition induces secondary self-assembly (SA2) to form quantum dot compound micelles (QDCMs). Finally, aqueous dispersions of QDCMs are slowly evaporated to induce the formation of three-dimensional (3D) close-packed arrays in a tertiary self-assembly step (SA3). The resulting hierarchical assemblies, consisting of a periodic array of hybrid spheres each containing multiple CdS QDs, exhibit the collective property of a photonic stop band, along with photoluminescence arising from the constituent QDs. A high degree of structural control is possible at each level of organization by judicious selection of experimental variables, allowing various parameters governing the collective optical properties, including QD size, nanoparticle spacing, and mesocale periodicity, to be independently tuned. The resulting control over optical properties via successive self-assembly steps should provide new opportunities for hierarchical materials for QD lasers and all-optical switching.     

DNA based molecular motors

Most of the essential cellular processes such as polymerisation reactions, gene expression and regulation are governed by mechanical processes. Controlled mechanical investigations of these processes are therefore required in order to take our understanding of molecular biology to the next level. Single-molecule manipulation and force spectroscopy have over the last 15 years been developed into extremely powerful techniques. Applying these techniques to the investigation of proteins and DNA molecules has led to a mechanistic understanding of protein function on the level of single molecules. As examples for DNA based molecular machines we will describe single-molecule experiments on RNA polymerases as well as on the packaging of DNA into a viral capsid—a process that is driven by one of the most powerful molecular motors.     

Hierarchical nanoparticle/block copolymer surface features via synergistic self-assembly at the air-water interface

This work demonstrates the first example of the controlled organization of semiconducting nanoparticles (NPs) using amphiphilic block copolymer self-assembly at the air-water interface. Preferential interactions between polystyrene-functionalized NPs and the polystyrene block of an amphiphilic polystyrene-b-poly(ethylene oxide) block copolymer result in synergistic self-assembly at the air-water interface, forming a range of highly stable one-dimensional NP/polymer surface features, including branched nanowires, nanocables up to 100 microm in length, and nanowires with nanoring connectors. This strategy offers new routes to hierarchical hybrid assemblies with potential photonics applications because the nanoscale organization of NPs is coupled to features with dimensions that are commensurate with optical wavelengths.     

ESR Study of Trimethylsilane Plasma Polymer Part II; Effect of Consecutive Treatments and Mixed Gases

An ESR study has indicated that a second plasma treatment on plasma deposited films from trimethylsilane (TMS) monomer gas has the ability to modify the characteristics of the primary plasma polymer significantly in a favorable manner for many applications. The effect of the second plasma polymerization on the primary plasma polymer of TMS depends on the nature of the second monomer. Plasma of F-containing monomer, hexafluoroethane (HFE) and perfluoromethane (CF₄), decreases the ESR signal of TMS and no detectable signal due to F-containing monomer was found. The decay rate of the signal decreased significantly. In contrast to this situation, CH₄ plasma treatment yields an ESR signal that is a composite of that observed from TMS and CH₄ films individually. The overall signal increased in this instance, but didn't show appreciable decay in 24 hr period. A second treatment by nonpolymer forming plasmas also decreased the ESR signal of TMS, and decreased the decay rate, indicating that the second gas plasma treatment yields a somewhat similar effect found with the HFE plasma treatment. Plasma polymerization of mixtures of TMS and nonpolymer-forming gases increased the ESR signal but decreased the decay rate, except in the case of oxygen. A mixture of (TMS + O₂) behaved as a completely different monomer. No ESR signal was found in this system. The ESR analysis was supported by XPS data and an insight into the mechanisms occurring in these thin films are discussed.