Dry Casting Research Articles

Anomalous reinforcing effects in cellulose gel-based polymeric nanocomposites

Cellulose-synthetic polymer nanocomposite films were prepared by immersion of cellulose gel in polymer solutions followed by dry casting. The cellulose hydrogel was prepared from aqueous alkali-urea solution. As the synthetic polymer, polystyrene (PS) and poly(methyl methacrylate) (PMMA) were used. The polymer content could be changed between 10 and 80% by changing polymer concentration of immersing solution. While the mechanical properties of the cellulose-PMMA composite films showed a nearly linear dependence on PMMA content, those of cellulose-PS composites showed an anomalous behavior; both tensile strength and Young’s modulus showed prominent maxima at 15–30 wt% PS contents. This anomaly may have resulted from the specific interaction between the aromatic ring of PS and the hydrophobic plane of the glucopyranoside. Both PMMA and PS composite films showed significant improvements in dimensional thermal stability; up to 25 wt% synthetic polymer content, the coefficient of thermal expansion (CTE) was as low as ca. 30 ppm/K, about 1/3 of the pure polymers. This indicates that the regenerated cellulose network is effective in suppressing thermal expansion of the synthetic polymers.

The interplay of membrane formation and drug release in solution-cast films of polylactide polymers

The interplay of phase inversion and drug release has been studied for films of several biodegradable polylactide polymers cast from solutions containing polymer, solvent, and drug (naproxen). Variables studied included polymer type and concentration, solvent type, and film casting conditions (i.e. free or forced convection, humidity). Film morphologies and thermal properties indicate that reduction of the T g of the amorphous poly (lactide- co-glycolide) (PLGA) and poly ( d, l-lactide) (PDLLA) systems caused by the drug, inhibits stabilization of a porous, structure, regardless of dry casting conditions and drug loads. Porous membranes could be formed by wet casting; however, drug loss during casting, makes this a non-viable process. For semi-crystalline PLLA, membrane morphologies could be varied by controlling the mass transfer path to form a single-phase dense film by polymer crystallization or a liquid–liquid two-phase structure followed by locking-in by polymer crystallization. However, the lack of drug solubility in the crystalline phase leads to unfavorable drug distributions most often leading to a burst release. Release profiles for all three polymers were found to follow a two-stage release model, with a first stage diffusive release followed by zero-order release in the second stage due to polymer erosion.

Electrostatic properties of maghemite (γ-Fe2O3) nanocrystalline quantum dots determined by electrophoretic deposition

We applied a DC electric field between two flat electrodes to attract thermally charged maghemite(γ-Fe2O3) nanocrystalline quantum dots dissolved in hexane to form smooth, robust, large area andapparently identical films of equal thickness on both electrodes. Visible microscopy,scanning electron microscopy, atomic force microscopy and profilometry showed that theelectrophoretically deposited dot films were very smooth with an rms roughness of∼10 nmfor ∼0.2 µm thick films. The films were of high quality. They did not re-dissolve in hexane (as do thoseformed by dry casting), which is a good solvent for these dots, or in common cleaningsolvents such as water, alcohols and acetone. The deposition on both electrodes impliesthere are both positively and negatively thermally charged dots, unlike conventionalelectrophoretic deposition. We used simple thermodynamics to explain the results ofelectrophoretic deposition macroscopically. To connect the macroscopic nature of thedeposition to the microscopic nature of the dots we performed electrophoretic mobilitymeasurements of the dots and the results seem to complement the thermodynamictreatment.

Preparation of a polymeric membrane with a fine porous structure by dry casting

AbstractWe prepared a polymeric membrane with a fine porous structure from polystyrene (PS), poly(ethylene glycol) (PEG), and solvent solutions by exploiting the phase separation induced in the course of dry casting. To determine the effect of the drying rate and phase separation on the developed porous structure, six different solvents, including toluene, chlorobenzene, tetrahydrofuran, methyl ethyl ketone, 1,4‐dioxane, and chloroform, were used. The pore size and density drastically changed with the different solvents and drying conditions. The polymer concentration at the onset of the phase separation into PEG‐rich and PS‐rich phases also strongly affected the cellular structure. The solubility of PEG into PS and the solvent solutions changed the concentration, which corresponded to the viscosity of the PS‐rich solution at the onset of the phase separation. The higher solubility of PEG in the solutions delayed the onset of phase separation during drying and increased the viscosity. The higher viscosity and the higher drying rate prevented the phase‐separated PEG domains from coalescing and made the resulting pore size smaller and the pore density larger. The finest porous structure, with a pore size of approximately 1 μm and a pore density of 0.08 1/μm2, was prepared from PS/PEG and a 90 wt % chloroform solution. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Microstructure evolution in dry cast cellulose acetate membranes by cryo-SEM

The development of microstructure during drying-induced phase inversion or dry casting of homogeneous water/acetone/cellulose acetate coatings, which evolve into asymmetric separation membranes, was witnessed using ‘time-sectioning’ cryogenic scanning electron microscopy (cryo-SEM). Coating specimens were prepared via the following sequential steps: uniformly coating or casting the polymer solution onto a substrate, drying with environmental control for a specific time, rapidly freezing the specimen in liquid cryogen, fracturing to reveal the coating cross-section, subliming briefly for topographical contrast, sputter-coating to prevent charging and cryo-SEM imaging. Specimens were created with different drying times and hence each specimen is called a ‘time-section’. The earliest time-section, one from a coating soon after deposition, shows a featureless specimen, as expected for a homogeneous polymer solution. As drying proceeds, time-sectioning reveals first the nucleation of polymer-lean droplets dispersed within a polymer-rich matrix across a region bound by the free surface above and a phase separation front below. On further drying, this front travels down to the substrate; the polymer-lean droplets grow and coalesce, forming a smoothly interconnected phase, which eventually becomes the pore space of a honeycomb-like structure as drying progresses. Meanwhile at the free surface, a seemingly dense skin develops on drying, while a nodular intermediate layer appears between the thinner skin and the thicker honeycomb-like substructure. The images are analyzed with composition paths derived from theoretical modeling to elucidate the fundamentals of microstructure development in asymmetric membranes.

Poly(caprolactone triol) as plasticizer agent for cellulose acetate films: influence of the preparation procedure and plasticizer content on the physico‐chemical properties

AbstractThe influence of the plasticizer content and film preparation procedure on the morphology, density, thermal and mechanical properties of cellulose acetate (CA) films plasticized with poly‐(caprolactone triol) (PCL‐T), were studied. Differential scanning calorimetry (DSC), thermal mechanical analysis (TMA), scanning electron microscopy (SEM), wide‐angle X‐ray diffraction (WAXD) and infrared spectroscopy (FT‐IR) techniques were used. The films were prepared by dry‐casting CA and CA/PCL‐T in acetone or acetone/water solutions, which produced transparent and opaque films, respectively. In contrast to the transparent films, which were dense, the opaque films presented a porous morphology. However, the presence of PCL‐T reduced the opaque film porosity, increasing, in consequence, its bulk density. The TMA results revealed that PCL‐T reduced the glass transition temperature more significantly in the transparent than in opaque films. Only the transparent CA/PCL‐T films presented a melting temperature, that reduced with higher concentrations of PCL‐T, suggesting a higher ordering (crystallinity) when the films were prepared in the absence of water, as observed from WAXD curves. The mechanical properties also showed that the transparent films were more soft and tough than the opaque films. In summary, PCL‐T was a good plasticizer agent for CA films due to the presence of hydrogen bonds between the components (FT‐IR spectra). The presence of water in the dry casting process has a significant effect mainly on film morphology and mechanical properties. Copyright © 2004 John Wiley & Sons, Ltd.

Finite element simulation of solidification of aluminium casting alloy LM 25

AbstractA two-dimensional axisymmetric finite element analysis is applied to the simulation of the solidification of the aluminium casting alloy LM 25. Solute redistribution during primary phase freezing is described using the Schell equation, and the two-phase eutectic is assumed to solidify over a temperature range of 2 K. Excellent agreement is found between the computed and experimental thermal analysis data for a dry sand axisymmetric casting. The model predictions are used to discuss the observed variation of casting density with distance from the feeder in unmodified dry sand castings with and without a chill, and a relationship is obtained for casting density at any location as a function of the computed value of the distance between the 90 and 100% solid isotherms.MST /942

Conventional and Chemical Processing of High Tc Superconductors

ABSTRACTConventional and chemical processing of the superconducting YBa2Cu3Ox ceramic powders are reviewed. Conditions for calcination, sintering and microstructural development are shown to be important considerations for the superconducting properties of YBa2Cu3Ox ceramics. We examine different forming techniques, e.g. dry pressing, hot pressing, tape casting and screen printing, to prepare superconducting components with different sizes, shapes and configurations. We also identify and discuss problem areas related to incomplete decomposition of BaCo3 precursor; oxidation of YBa2Cu3Ox ceramics; sensitivity of the YBa2Cu3Ox phase to water; reactivity of the YBa2Cu3Ox phase with substrates and co-firing materials; low critical current density in bulk ceramics of YBa2Cu3Ox and low mechanical strength of YBa2Cu3Ox ceramics.

Ionically crosslinked poly(acrylic acid) membranes. III. Reverse osmosis results for dry cast membranes

AbstractThe reverse osmosis properties of ionically crosslinked polyacrylic acid membranes were investigated in terms of the salt separation of a 0.1% NaCl solution and water flux. The membranes were synthesized and crosslinked with the metal ion Al3+ via the dry casting technique described in a previous paper. The effect of such variables as the polymer concentration in the casting solution, the ratios of solvents used (DMF/H2O), the ratio of monomer to the crosslinking agent (AA/Al), the evaporation time and temperature, and the nonsolvent nature and treatment times were studied in some detail. The most important variable was found to be the length and nature of the treatment in the nonsolvents acetone and methanol. In the best series of the membranes that were synthesized, fluxes of more than 3.0 gfd, with salt separations at the 80%–85% level, were obtained.

Polyamides from unsaturated acids. III. Properties of polyfumaramides from diamines with short aliphatic chains and from piperazines

AbstractProperties of polyfumaramides from liner unsubstituted aliphatic diamines. from piperazines and substituted piperazines were determined.Polyfumaramides from linear unsubstituted aliphatic diamines show a high water sensitivity, and a low range of solubility.Polyfumaramides from piperazines and substituted piperazines show a high water sensitivity, and a good solubility in organic solvents.Films from substituted piperazines polyfumaramides were prepared by dry casting. These films show high tensile properties, high resistence to temperature and good optical properties. By physical treatment as drawing, films properties may be changed, water sensitivity is reduced and tensile strength increased.Correlation between chemical structure and properties of polymers and films are discussed.

The dry casting of viscose film

AbstractRegenerated cellulose film prepared by a dry‐cast method has demonstrated outstanding superiority over similar film produced by the conventional wet‐cast process. Factors contributing to the superior quality of film prepared by the dry cast process are: (1) physical properties several times greater than for wet‐cast film of corresponding thickness; (2) high durability at conditions of low temperature and low humidity even without the addition of softening agents, and (3) outstanding dimensional stability when exposed to conditions of varying temperature and humidity. The dry‐cast cellophane differs structurally from the commercial product in having a lower degree of order, a lower degree of swelling in the gel form, and minor differences in molecular orientation. A commercial process for producing dry cast cellophane may be envisioned as a heated, polished support for drying the viscose film; a regenerating system of alcoholic hydrochloric acid, hot glycerine, or a hot, oxygenfree gas; a water both for purifying the film; and a conventional cellophane dryer. The ideal viscose composition for producing dry‐cast cellophane is high cellulose content, moderately high DP, sufficient degree of substitution on the cellulose to yield a high degree of dispersion, and low caustic content.