J. Carson Meredith

LCST Phase Separation in Biodegradable Polymer Blends: Poly(D,L-lactide) and Poly(ε -caprolactone)

A lower critical solution temperature (LCST) phase transition is reported for blends of the biodegradable polymers poly(D,L-lactide (PDLA) and poly(e-caprolactone) (PCL). From light scattering measurements the cloud point curve is determined to have a critical temperature of 86°C and a critical concentration of mass fraction 36 wt.-% PCL. Optical microscopy of phase-separated films indicates a spinodal morphology at the critical concentration, and droplet phases at off-critical concentrations. After quenching phase separated blends below the melting temperature of PCL (60°C), the crystallization of PCL is used to positively identify PCL-rich and PDLA-rich phases. When cystallization of PCL follows LCST phase separation, the separation, the size, shape, and distribution of crystalline regions can be adjusted by the degree of PCL/PDLA phase seperation. Thus, the LCST phase separation offers a novel method to control microphase structure in biodegradable materials. Applications to control of mechanical and physical properties in tissue engineering scaffolds are discussed in light of the results.

Facile Preparation of Highly-Scattering Metal Nanoparticle-Coated Polymer Microbeads and Their Surface Plasmon Resonance

We report on the facile preparation of highly scattering metal-coated polystyrene (PS) latex beads by using solvent-controlled heterocoagulation. Starting with an aqueous dispersion of PS beads and poly(vinyl pyrrolidone)-capped metal nanoparticles (NPs), homogeneous and dense metal coatings were obtained by the controlled addition and removal of tetrahydrofuran (THF). Different sizes (30, 60, and 80 nm), chemistries (gold and silver), and shapes (sphere and cube) of NPs were successfully incorporated on commercially available PS beads. The resulting metal coated-PS microspheres exhibited highly enhanced scattering and tunable optical characteristics useful for biomedical imaging, sensors, and opto-electronic devices. The fabricated composite beads were stable, with no loss of metal coating, during long-term water storage. The morphology and coverage of the metal coating, and the beads optical properties, were controllable over a wide range by the concentration of THF and metal NP, and NP size, shape, and chemistry.

Relationship between polymer chain conformation and phase boundaries in a supercritical fluid

We investigate the solvent density driven changes in polymer conformation and phase behavior that occur in a supercritical fluid, with a particular emphasis on conditions near the lower critical solution temperature (LCST) phase boundary. Using continuous space Monte Carlo simulations, the mean square end-to-end distance (R) and radius of gyration (Rg) are calculated for a single chain with 20 Lennard-Jones segments in a monomeric solvent over a broad range of densities and temperatures. The chains collapse as temperature increases at constant pressure, or as density decreases at constant temperature. A minimum in R and Rg occurs at a temperature slightly above the coil-to-globule transition temperature (C-GTT), where the chain adopts a quasi-ideal conformation, defined by the balance of binary attractive and repulsive interactions. Expanded ensemble simulations of finite-concentration polymer–solvent mixtures reveal that the LCST phase boundary correlates well with the single chain C-GTT. At temperatures...

High-throughput screening of metal-organic frameworks for CO2 separation.

A parallel high-throughput sorption methodology is described for screening CO(2) and N(2) adsorption and diffusion selectivity in metal organic frameworks, before and after exposure to water vapor and acid gases. We illustrate this approach by simultaneously investigating 8 candidate Metal-Organic Framework (MOF) materials, of which the best material was found to have a CO(2)/N(2) membrane selectivity of 152 and a CO(2) permeability of 60 barrer for Co-NIC. This approach provides a significant increase in efficiency of obtaining the separation properties of MOFs. While we describe here the identification of novel materials for CO(2) capture, the methodology enables exploration of the performance and stability of novel porous materials for a wide range of applications.

Combinatorial investigation of dewetting: polystyrene thin films on gradient hydrophilic surfaces

Film stability and dewetting is important to control for applications in coatings such as photoresists, paints, adhesives, lubricants, and biomaterials. We demonstrate the use of 2D combinatorial libraries to investigate thin film dewetting. Substrate libraries with gradients in contact angle ðuÞ were prepared by immersing Si ‐ H passivated Si in a Piranha solution (H2SO4/H2O2/H2O) at a controlled rate. Libraries of thin films of polystyrene on gradient etched silicon substrates containing orthogonal continuous variation of thickness were screened for dewetting behavior using automated optical microscopy. After comparing the high-throughput screening method to conventional studies of thickness effect on dewetting, a detailed morphological phase-map of the effects of contact angle on dewetting of polystyrene film was generated. Dewetting trends were visibly apparent. The number of polygons of dewetted polymer is sensitive to surface hydrophilicity as characterized by contact angle studies. q 2002 Elsevier Science Ltd. All rights reserved.

Advances in combinatorial and high-throughput screening of biofunctional polymers for gene delivery, tissue engineering and anti-fouling coatings

The past ten years have witnessed the emergence of a new set of tools, combinatorial and high-throughput screening, in polymeric biomaterials development. These tools, developed initially in the drug-discovery industry, and later applied to catalysts and inorganic materials, allow orders of magnitude increases in the rate of exploration and characterization of new materials. This feature article covers recent examples of high-throughput and combinatorial studies of biofunctional polymers. Biofunctional polymers exhibit chemistry or physical properties specifically designed to function in a biological context. Examples include polymers with unique binding affinities or surface physical properties for gene and drug delivery, tissue engineering scaffolds, anti-fouling and anti-bacterial coatings, and biocatalytic applications.

POLYMER CHAIN COLLAPSE NEAR THE LOWER CRITICAL SOLUTION TEMPERATURE

Abstract We report for the first time by computer simulations, evidence of polymer chain collapse near a lower critical solution temperature (LCST). Continuous space, Monte Carlo simulations have been performed on freely jointed, Lennard-Jones chains in a LJ monomeric solvent with symmetric energetics. The LCST phase boundary and associated chain collapse in the supercritical solvent region has been established for chains of size 20. This Letter focuses on why the chain collapses near a LCST.

Dye-labeled polystyrene latex microspheres prepared via a combined swelling-diffusion technique

A series of water-insoluble, biologically compatible dyes, meso-tetraphenylchlorin, meso-tetraphenylporphyrin and chlorophyll-a, were successfully incorporated into beads composed of linear polystyrene (PS) via a tunable combined swelling-diffusion process. Dyed PS beads were prepared by the addition of a dye solution in tetrahydrofuran to an aqueous suspension of 10 μm PS beads in the presence of a poly((ethylene glycol)-b-(propylene glycol)-b-(ethylene glycol)) block copolymer surfactant. The presence of surfactant was found to be beneficial to prevent particle aggregation, especially at tetrahydrofuran contents above 30%. Dye loading was shown to be tunable by simple adjustments in dye composition. Confocal fluorescence microscopy indicated that dyes were distributed uniformly throughout the entire PS bead, but heterogeneously with ~500 nm diameter droplets, indicative of a separate dye phase within the PS matrix. The stability of dyed beads, indicated by resistance to dye leaching in solvent, was found to be governed by the degree of swelling of PS in the solvent medium. Hence, no leaching was observed even when a good solvent for the dye was used (ethanol), as long as that solvent did not swell the carrier particle, PS. No leaching of dyes from the beads was observed during long-term (2 years) storage in water.

Measuring the influence of solution chemistry on the adhesion of Au nanoparticles to mica using colloid probe atomic force microscopy

Engineered nanoparticles are used increasingly in numerous commercial products, leading to concerns over their environmental fate and ecotoxicity. We report the adaptation of colloid probe atomic force microscopy (AFM) to quantitatively determine the adhesive behavior of gold nanoparticles (Au NPs) with mica, chosen as a model for sand, in various water chemistries. Au NP-covered polystyrene (PS) beads were prepared by a combined swelling-heteroaggregation (CSH) technique prior to attachment to tipless AFM cantilevers. Force measurements were performed over a range of solution conditions (pH, ionic strength (IS), and natural organic matter (NOM) content). Plain PS beads with no Au NPs were used as controls. In general, adhesion of Au NP-PS beads to mica were found to increase as IS increased while a rise in pH led to a decrease in adhesion. Plain PS beads were not observed to adhere to mica in any of the experimental solution conditions, and the PS force curves were unaffected by changes in the pH and electrolyte concentrations. In the presence of NOM, pull-off forces for Au NP-PS beads increased in magnitude when NaCl was added. In addition, the experimental approach force curves were not successfully described by the classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory of colloidal stability. To reconcile the discrepancy between theory and experiment, an extended DLVO (xDLVO) empirical model was used to account for the contribution of non-DLVO interactions (known collectively as structural forces) between the Au NPs and mica surfaces.

Simulation of structure and interaction forces for surfaces coated with grafted chains in a compressible solvent

Lennard-Jones chains grafted to solid surfaces in a supercritical solvent are simulated with a continuum grand canonical Monte Carlo method. The force of interaction between two surfaces is calculated as a function of solvent density and temperature and analyzed as a function of the conformational properties of the grafted chains. At high, liquidlike bulk solvent densities, the chains are solvated and the interaction forces are repulsive. As the solvent density is lowered, the chains collapse, and the surfaces become attractive, indicating flocculation. The critical flocculation density coincides with the critical solution density for a bulk mixture of chains and solvent (corrected for local density enhancement). The bulk critical solution density, in turn, corresponds to the coil-to-globule transition of a single chain in bulk solution. The predicted correspondence between these properties agrees with results from lattice-fluid self-consistent field theory and colloid stability experiments. In good and p...