Gabriel Luna-Bárcenas

Microcellular microspheres and microballoons by precipitation with a vapour-liquid compressed fluid antisolvent

Abstract A new type of precipitation with a compressed fluid antisolvent (PCA) is demonstrated for the formation of porous polymeric microspheres and microballoons (hollow microspheres). The antisolvent is composed of pure saturated vapour over saturated liquid CO2. A polystyrene (PS) in toluene solution is sprayed through a capillary into CO2 vapour to form droplets, which fall into liquid CO2 where they are rapidly dried and vitrified. Both the thickness and porosity of the microcellular shells can be controlled by changing the initial solution composition. The thickness is inversely proportional to the initial PS concentration. As the concentration is increased there is a transition from porous microballoons to porous microspheres. The cell sizes and surface areas of the microspheres are approximately 1–20 μm and 3–40 m2g−1, respectively. The mass transfer pathway may be altered by addition of CO2 to the polymer solution before spraying, resulting in greater and more uniform porosity. Compared with methanol as an antisolvent, CO2 produces more porous and spherical microspheres, with 7–14 times faster precipitation.

Semicrystalline microfibrils and hollow fibres by precipitation with a compressed-fluid antisolvent

Solutions of polyacrylonitrile (PAN) in dimethylformamide (DMF) sprayed into supercritical fluid carbon dioxide form hollow fibres and highly oriented microfibrils (< 1 μm diameter). In the dilute region, microfibrils are produced with diameters as low as 100 nm due to the dipole-dipole forces, in contrast with microspheres produced from solutions of polystyrene (PS) in toluene. For PAN microfibrils, orientation increases with shear, then goes through a maximum and eventually decreases at higher flow rates due to an expanding jet. The concentration for the transition from microfibrils to a single hollow fibre is in agreement with the calculated transition concentration from the dilute to semidilute region, C∗. In the semidilute region, the morphology changes from hollow fibres to highly oriented fibrils with an increase in flow rate. The increase in turbulence enhances convective mass transport, leading to more uniform nucleation throughout the cross-section of the jet, favouring the highly oriented fibrils. The enhanced transport of CO2 into the jet lowers the solvent quality, raising C∗, which further favours fibril formation. For both PAN-DMF and PS-toluene solutions, the transition from highly oriented microfibrils to hollow fibres occurs at about 3C∗ (in a good solvent), suggesting some similarities in the mass-transfer pathways in each system.

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...

Synthesis of macroporous poly(acrylic acid)–carbon nanotube composites by frontal polymerization in deep-eutectic solvents

Deep Eutectic Solvents (DESs) formed between Acrylic Acid (AA) and Choline Chloride (CCl) exhibit certain properties of ionic liquids (e.g. high viscosity) that make them suitable for frontal polymerization (FP). The use of DESs not only as a monomer but also as the solvent prevents the use of additional solvents (i.e. typically of organic nature) and offers a green tool for the synthesis of functional composites. We have recently explored this approach for the preparation of poly(acrylic acid) (PAA) and poly(methacrylic acid). In this work, we have taken advantage of the outstanding capability of DESs to solubilize and/or disperse a number of substances to incorporate – in a homogeneous fashion – carbon nanotubes (in this particular case, N-doped MWCNT – CNxMWCNTs) in the polymerizable DES. Interestingly, CNxMWCNTs also played the role of an inert filler in FP. The resulting PAA–CNxMWCNT composites exhibited some distinct features as compared to previous PAA also obtained via DES-assisted FP. For instance, PAA–CNxMWCNT composites can undergo swelling depending on the pH, as bare PAA. However, the presence of CNxMWCNTs allows the formation of a macroporous structure after submission to a freeze-drying process, the achievement of which was not possible in bare PAA. The combination of structural (e.g. macroporosity) and functional (e.g. stimuli responsive) properties exhibited by these materials besides an eventually high biocompatibility – coming from the green character of the DES-assisted synthesis – should make the resulting macroporous PAA–CNxMWCNT composites excellent candidates for their future application as biomaterials.

Phase behavior of poly(1,1-dihydroperfluorooctylacrylate) in supercritical carbon dioxide

Abstract Liquid–fluid phase equilibria data are reported for the poly(1,1-dihydroperfluorooctylacrylate) (poly(FOA))–CO2 system at sub- and supercritical fluid conditions and modeled with the statistical associated fluid theory (SAFT). Lower critical solution temperature (LCST) phase behavior is observed with a critical concentration between 1.0 and 2.0 wt.% poly(FOA). The high solubility of poly(FOA) is consistent with its low cohesive energy density and the weak van der Waals forces of CO2. To aid this analysis, pressure–volume–temperature (PVT) data are reported for pure poly(FOA) and correlated with lattice-fluid and SAFT theory.

Monte Carlo simulation of polymer chain collapse in athermal solvents

By computer simulation, Dijkstra et al. [M. Dijkstra, D. Frenkel, and J. P. Hansen, J. Chem. Phys. 101, 3179 (1994)] reported the first entropy‐driven polymer chain collapse in an athermal solvent. To gain a better understanding of chain collapse physics in the absence of attractive interactions, we performed on‐lattice NVT Monte Carlo simulations on a single polymer chain immersed in a hard‐core solvent of variable size, shape, and density. In general, solvent quality decreases with increasing solvent density and incipient chain collapse occurs at a unique critical density for a given solvent size and shape. The critical density is smaller for large solvent molecules, but solvent shape also plays a role. Unfavorable solvent‐chain excluded volume (EV) interactions drive the collapse transition. The EV interaction is reduced and the solvent entropy increases when the chain collapses, but there is an accompanying and unfavorable loss of chain conformational entropy. At the transition density these opposing ...

SIMULATION OF PHASE EQUILIBRIA FOR POLYMER-SUPERCRITICAL SOLVENT MIXTURES

Phase equilibria for mixtures of a polymer and a supercritical solvent are investigated by means of expanded Gibbs ensemble simulations. Both lower and upper critical solution temperature (LCST and UCST) phenomena are observed for such systems. A closed-loop phase diagram is observed for systems with no specific interactions. The results of our simulations for Lennard-Jones polymer–solvent mixtures are in qualitative agreement with experimental data for polymers in supercritical solvents.

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.

New insights into the bactericidal activity of chitosan-Ag bionanocomposite: the role of the electrical conductivity.

The relationship between electrical conductivity, structure and antibacterial properties of chitosan-silver nanoparticles (CS/AgnP) biocomposites has been analyzed. To test the films antimicrobial activity, Gram-positive and Gram-negative bacteria were studied. The interactions between silver nanoparticles with chitosan suggest the formation of silver ions which plays a major role in nanocomposites bactericidal potency. In CS/AgnP biocomposites, the bactericide effectiveness increases by increasing AgnP concentrations up to 3 wt%, which is close to the electrical percolation threshold of ca. 3 wt%. As the AgnP concentration increases above this threshold, the bactericidal potency is greatly diminished. The elucidated correlation between electrical conductivity and antibacterial activity could be useful in the design of other nanocomposites that involve polymeric-based matrices.

Nonequilibrium molecular dynamics of the rheological and structural properties of linear and branched molecules. Simple shear and poiseuille flows; instabilities and slip

Nonequilibrium molecular-dynamics simulations are performed for linear and branched chain molecules to study their rheological and structural properties under simple shear and Poiseuille flows. Molecules are described by a spring-monomer model with a given intermolecular potential. The equations of motion are solved for shear and Poiseuille flows with Lees and Edwards [A. W. Lees and S. F. Edwards, J. Phys. C 5, 1921 (1972)] periodic boundary conditions. A multiple time-scale algorithm extended to nonequilibrium situations is used as the integration method, and the simulations are performed at constant temperature using Nose-Hoover [S. Nose, J. Chem. Phys. 81, 511 (1984)] dynamics. In simple shear, molecules with flow-induced ellipsoidal shape, having significant segment concentrations along the gradient and neutral directions, exhibit substantial flow resistance. Linear molecules have larger zero-shear-rate viscosity than that of branched molecules, however, this behavior reverses as the shear rate is increased. The relaxation time of the molecules is associated with segment concentrations directed along the gradient and neutral directions, and hence it depends on structure and molecular weight. The results of this study are in qualitative agreement with other simulation studies and with experimental data. The pressure (Poiseuille) flow is induced by an external force F(e) simulated by confining the molecules in the region between surfaces which have attractive forces. Conditions at the boundary strongly influence the type of the slip flow predicted. A parabolic velocity profile with apparent slip on the wall is predicted under weakly attractive wall conditions, independent of molecular structure. In the case of strongly attractive walls, a layer of adhered molecules to the wall produces an abrupt distortion of the velocity profile which leads to slip between fluid layers with magnitude that depends on the molecular structure. Finally, the molecular deformation under flow depends on the attractive force of the wall, in such a way that molecules are highly deformed in the case of strong attracting walls.