Brush Segment Research Articles

Towards the description of adsorption of water in slit-like pores with walls covered by molecular brushes.

The density profiles, adsorption isotherms, and phase behavior of a water model in a slit-like pore with walls modified by pre-adsorbed tethered chain molecules have been studied in the framework of a density functional theory. Each chain is bonded to the surface by its terminal segment, and the surface density of grafted chains is the same for each wall. The model for water taken from the work of Clark et al. [Mol. Phys. 104, 3561 (2006)] reproduces successfully a bulk equation of state. The mean field approach has been used to describe the effects of attractive interactions. The chemical association effects are taken into account by using the first-order thermodynamic perturbation theory of Wertheim. We have found that the presence of molecular brushes on the pore walls has important consequences for the adsorption and phase behavior of confined water. If the brush segments do not attract water molecules strongly, the vapor-liquid coexistence envelope shrinks upon increasing brush density, but the critical temperature is weakly affected. Alteration from capillary condensation to evaporation is observed with changes in the brush density, number of segments of tethered chains, and/or chemical identity of segments. The crossover temperature is affected by all these factors. Moreover, we have shown that affinity of water to segments of tethers is an important factor determining adsorption of water vapor and the entire phase diagram.

Synthesis and Self-Assembly of Toothbrush-like Block Copolymers

Block copolymers (BCPs) can self-assemble in selective solvents to form morphologies based on their chemistries and architectures. In this study, we investigate BCPs having both brush and linear segments in what is referred to as “toothbrush” architecture. Using anionic polymerization, we have synthesized a linear poly(styrene-block-isoprene) followed by growing poly(ethylene oxide) grafts from the isoprene block to form a brush segment. A combination of nonselective and selective solvents is used to drive the self-assembly of the toothbrush polymers. Analysis by dynamic light scattering and transmission electron microscopy show that the self-assembly is highly influenced by the density of grafts in the brush segment, with densely grafted polymers forming only spherical micelles and sparsely grafted brushes forming higher order structures including cylindrical micelles and vesicles. Additionally, calculations of micelle corona graft density and corona thickness indicate that the corona of the micelles for...

Tandem Catalysis for the Preparation of Cylindrical Polypeptide Brushes

Here, we report a method for synthesis of cylindrical copolypeptide brushes via N-carboxyanhydride (NCA) polymerization utilizing a new tandem catalysis approach that allows preparation of brushes with controlled segment lengths in a straightforward, one-pot procedure requiring no intermediate isolation or purification steps. To obtain high-density brush copolypeptides, we used a "grafting from" approach where alloc-α-aminoamide groups were installed onto the side chains of NCAs to serve as masked initiators. These groups were inert during cobalt-initiated NCA polymerization and gave allyloxycarbonyl-α-aminoamide-substituted polypeptide main chains. The alloc-α-aminoamide groups were then activated in situ using nickel to generate initiators for growth of side-chain brush segments. This use of stepwise tandem cobalt and nickel catalysis was found to be an efficient method for preparation of high-chain-density, cylindrical copolypeptide brushes, where both the main chains and side chains can be prepared with controlled segment lengths.

Interfacial properties of statistical copolymer brushes in contact with homopolymer melts

We use polymer self-consistent field theory to quantify the interfacial properties of random copolymer brushes (AB) in contact with a homopolymer melt chemically identical to one of the blocks (A). We calculate the interfacial widths and interfacial energies between the melt and the brush as a function of the relative chain sizes, grafting densities, compositions of the random copolymer in the brush, and degree of chemical incompatibility between the A and B species. Our results indicate that the interfacial energies between the melt and the brush increase (signifying expulsion of the free chains from the brush) with increasing grafting density, chemical incompatibility between A and B components, and size of the free chains relative to the grafted chains. We also compare the interfacial energies of random copolymers of different sequence characteristics and find that, except for the case of very blocky or proteinlike chains, blockiness of the copolymer has only little effect on interfacial properties. Our results for interfacial energies are rationalized based on the concept of an "effective volume fraction" of the brush copolymers, f(eff), which quantifies the chemical composition of the brush segments in the interfacial zone between the brush and melt copolymers. Using this concept, we modify the strong-stretching theory of brush-melt interfaces to arrive at a simple model whose results qualitatively agree with our results from self-consistent field theory. We discuss the ramifications of our results for the design of neutral surfaces.

Synthesis and self‐assembly behavior of amphiphilic polypeptide‐based brush‐coil block copolymers

AbstractSynthesis and self‐assembly behavior of a novel amphiphilic brush‐coil block copolymer bearing hydrophilic poly(ethylene glycol) segment and hydrophobic polypeptide brush segment were presented in this work. The poly(γ‐benzyl‐L‐glutamate) (PBLG) brush is synthesized through “grafting from” strategy by ring‐opening polymerization of γ‐benzyl‐L‐glutamate‐N‐carboxyanhydride (BLG‐NCA) initiated by the flanking terminal primary amino group of macroinitiator. The copolymers were characterized by 1H NMR, gel permeation chromatography, Fourier transform infrared, circular dichroism spectrum, and differential scanning calorimetry. The self‐assembly behavior of the brush‐coil block copolymers in aqueous solution was investigated by means of transmission electron microscopy, scanning electron microscopy, atomic force microscopy, and laser light scattering. Spherical micelles were observed when the length of PBLG brush is shorter. The aggregate morphology transforms to spindle‐like micelles and then to rod‐like micelles, as the length of polypeptide brush increases. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5967–5978, 2009

Molecular Dynamics Simulations of Capillary Rise Experiments in Nanotubes Coated with Polymer Brushes

The capillary filling of a nanotube coated with a polymer brush is studied by molecular dynamics simulations of a coarse-grained model, assuming various conditions for the fluid-wall and fluid-brush interactions. Whereas the fluid is modeled by simple point particles interacting with Lennard-Jones forces, the (end-grafted, fully flexible) polymers that form the brush coating are described by a standard bead-spring model. Our experiments reveal that capillary filling is observed even for walls that would not be wetted by the fluid, provided the polymer brush coating itself wets. Generally, it is found that the capillary rise always proceeds through a t1/2 law with time t while the underlying molecular mechanism differs for wettable and nonwettable walls. For wettable walls, fluid imbibition is compatible with the Lucas-Washburn mechanism whereby the total influx of matter drops steadily with growing chain length N and the meniscus speed goes through a minimum at intermediate chain lengths. Moreover, because of flow, the polymer brush reorganizes its structure by forming a dense plug of chain segments under the meniscus that follows the meniscus in its motion. When the tube wall does not wet, one observes no meniscus formation for short chains although the fluid seeps through the wet brush. For a brush coating with longer chains, axial segregation between the brush segments and the fluid occurs by a kind of diffusive spreading, reminiscent of invasion percolation transport in a random medium, leading to the formation of a moving meniscus. For even longer chains that reach the tube axis, the rise of a meniscus with vanishing curvature-like imbibition in a porous medium is observed to take place.

The kinetics of polymer brush penetration in to a high molecular weight matrix

Neutron reflectivity has been used to measure concentration profiles of polymer brushes as they penetrate a high molecular weight polymer matrix. The brushes consist of end-functionalized deuterated polystyrene strongly grafted to a silicon substrate, in a melt matrix of (protonated) polystyrene. Initially the brush is totally excluded from the matrix. The rate at which the brush chains penetrate the matrix was investigated by measuring the brush segment density profiles after annealing the samples above the glass transition temperature for a range of times. We find that when the initial interface between the brush and the matrix is sharp, there is at first a rapid penetration of the brush into the matrix. This is followed by a slow approach to equilibrium, which is not achieved after annealing for a day. These observations are consistent with the predictions of a recent analytical theory and computer simulations. In contrast, samples with an initially diffuse interface reach equilibrium quickly. Two different matrix polymers were used to investigate the effect of the matrix molecular weight. Comparison has been made with a mean field model for the equilibrium brush profile.