Bead Morphology Research Articles

The change of bead morphology formed on electrospun polystyrene fibers

Polystyrene (PS) dissolved in the mixture of tetrahydrofuran (THF) and N, N-dimethyl formamide (DMF) was electrospun to prepare fibers of sub-micron in diameters. Electropinning parameters such as polymer concentration, applied voltage and tip-to-collector distance were controlled. From these parameters it was determined that while the surface tension of polymer solution had linear correlation with the critical voltage, throughput was dependent on electric conductivity. The electrospun PS fibers produced contained irregular beads and electrospinning certainly was enhanced with increasing DMF content. The bead concentration was also controlled by DMF content. The aspect ratio of the formed beads and the diameter of fibers were increased with increasing solution concentration. When PS was dissolved in only THF, an unexpected half hollow spheres (HHS) structure appeared. Also, different shape forms of PS non-woven mats have been prepared by controlling electrospinning parameters.

Polymer-dispersed Liquid Crystals: Effect of Partial Matrix Fluorination on Polymer Bead-based Morphology

The morphology and electrooptical properties of polymer-dispersed liquid crystals (PDLCs) with partially fluorinated polymer matrices are investigated. Films were prepared via photo-induced free-radical polymerization of an initially isotropic solution comprised of pentafunctional (5 reactive groups) acrylate monomer and nematic liquid crystal molecules. Phase separation of discrete domains of LC is induced by the photopolymerization. The film morphologies consisted of aggregated polymeric particles dispersed throughout a continuous liquid crystal medium. Changes in morphology and electrooptical properties were observed as trifluoroethyl and hexafluoroisopropyl methacrylate were partially substituted for the multifunctional acrylate monomer. Methyl methacrylate was used in control films due to the chemical similarities to fluorinated monomers. The incorporation of fluorinated monomers resulted in better definition of the polymer bead morphology and improvement in contrast ratio, while control films displayed less pronounced changes in optical properties. Real-time transmittance monitoring and polarized optical microscopy (POM) revealed that the monofunctional monomer addition resulted in a delay of the LC phase appearence with increasing comonomer concentration. However, a LC phase appeared earlier for fluorinated systems as compared to nonfluorinated control films of similar comonomer concentration. This was interpreted as an indication of lower LC solubility in the semifluorinated polymer matrix.

Factors Controlling the Magnesium Weld Morphology in Deep Penetration Welding by a CO<SUB>2</SUB> Laser

In laser welding with power density beyond 104 W · mm−2, the formation of plasma cavities, commonly referred to as keyholes, leads to deep penetration welds with high aspect ratios. In this paper, the morphologies of keyhole welds produced with a 6 kW CW CO2 laser on two die-cast magnesium alloys, AZ91 and AM50, are compared. It was found that the two magnesium alloys responded differently to laser welding. Though irregular weld cross-section profiles were consistently observed on each materials, bead dimensions often varied with the welding variables in contrasting ways. For both alloys, important characteristics of the weld beads such as depth, width, crown height (hump), and surface ripples were analyzed as a function of the welding parameters, most particularly the heat input. Results show that the use of heat input, a variable grouping two welding parameters into one, was often inadequate in characterizing the bead morphology. Several explanations are given, including base metal vaporization, but the process of bremsstralung absorption explains it well and rationalizes many observed characteristics of laser weld morphology.

Drug particle size influence on enteric beads produced by a droplet extrusion/precipitation method

The influence of drug particle size on the production of enteric beads by a polymer precipitation technique was investigated. Drug particle dimensions are known to play an important role in most microencapsulation techniques. Bead morphology was greatly influenced by drug particle size, and spherical shaped beads could only be obtained after size reduction of nimesulide crystals. This is confirmed by the angle of repose measurements, which show a significant decrease in theta values when beads are formulated with smaller drug particles. Furthermore, results show that drug encapsulation efficiency and in vitro drug release rates are also greatly dependent on both drug particle size and drug/polymer ratio in the initial suspension. Preparations containing 10.2 microm drug particles show a two-fold increase in the release rates when compared to those prepared with 40 microm particles.

Investigation of the in vitro release of gentamicin from a polyanhydride matrix

Septacin™ is a sustained release formulation consisting of gentamicin sulfate dispersed in a biodegradable polyanhydride matrix. The polyanhydride matrix is a copolymer of erucic acid dimer (EAD) and sebacic acid in a 1:1 weight ratio. In vitro drug release was performed in both water and pH 7.4 phosphate buffer. The drug release in water was faster than that in the buffer, which was the opposite of what would be expected based upon a faster polymer hydrolysis rate in the buffer. Theoretical treatment of the data using the Peppas model revealed that release in water was anomalous, while the release in pH 7.4 phosphate buffer was diffusion-controlled. Profound bead morphology differences were observed between beads in these two in vitro release media. Cracking was observed in beads in water and swelling with no apparent cracking was seen in beads in buffer. Concurrent monitoring of drug and sebacic acid release indicated that drug release is not via surface erosion. Osmotic effects were found to play little role in the in vitro drug release. There was no spectroscopic evidence of amide formation between the drug and copolymer. Sulfate release was monitored along with drug release and the results indicate that there is ion-exchange occurring during the pH 7.4 in vitro release. It was subsequently demonstrated that gentamicin can form an insoluble salt with EAD. This salt formation explains the slower drug release in pH 7.4 phosphate buffer.

Crystallization kinetics for semicrystalline random copolymers of vinylidene chloride (VDC) with methyl acrylate (MA), and the effects on the internal morphology of the resin particles formed during synthesis

AbstractIsothermal crystallization kinetics for random copolymers of vinylidene chloride (VDC) with methyl acrylate (MA) is reported. Syntheses of many semicrystalline polymers follow heterogeneous reaction paths in which the macromolecule chains phase separate from the reaction mixtures. The internal particle morphology (the internal structure of the resin bead) from this type of reaction is granular and porous, as a result of the demixing processes accompanying polymer formation. Demixing in these polymers involves either liquid‐liquid (L‐L) phase separation followed by liquid‐solid (L‐S) transformation (crystallization) or L‐S transformation alone. Crystallization (L‐S transformation) must be an indispensable part of the process if a porous granular structure is to be expected. This is because L‐S transformation is the most probable means by which the demixed structure can be stabilized against complete coalescence or agglomeration, which would lead to totally fused bead internal structure. This is particularly true if the glass transition temperature (Tg) is lower than the polymerization temperatures, as is the case with the VDC‐MA copolymers. Copolymers that crystallize the fastest will have the finest (most porous) resin bead morphology. The result of this work is consistent with expectation. The homopolymer (PVDC) that crystallizes the fastest has the finest resin bead internal morphology. The copolymers show slower crystallization rates with increasing noncrystallizable MA content. Correspondingly, resin morphology measured by specific surface area decreased with increasing amounts of the noncrystallizable (MA) comonomer unit in the copolymer. This is clearly seen in SEM photographs of the internal bead structures of these copolymers. ©1995 John Wiley & Sons, Inc.

Physical and Chemical Behavior of Welding Fluxes

Shielded-metal arc, submerged arc, and flux-cored arc welding processes (1) all rely on a flux to achieve an acceptable weld deposit. The welding flux for each of these processes must meet specific physical and chemical requirements to perform properly (2). A flux must have a melting range such that the weld metal solidifies before the molten slag does, and the resulting slag must have a density such that it floats to the top of the weld pool and collects there. The specific ranges of melting and density vary depending on the alloy being welded. The flux must also assist in main­ taining the arc plasma, in producing a shielding gas to protect the weld deposit from the atmosphere, and in limiting the amount of splatter. The flux viscosity is important because it controls the extent to which the flux can chemically alter the weld-metal composition, and it influences weld­ metal porosity, bead morphology, and the ability to make out-of-position weld deposits (3, 4). The flux must contain specific chemical additions to influence the weld metal's composition, microstructure, and properties.

Morphology of suspension polymerized polyacrylamide

In the course of studies on the characterization of commercial polyacrylamides used in tertiary oil recovery, samples of BASF Sepaflood suspension polymerized polyacrylamide with varying degrees of hydrolysis were observed using a Hitachi S-450 scanning electron microscope. Figs ! and 2 show an overall view of a nonhydrolysed (CE 5093) and highly hydrolysed (CE 5095) Sepaflood sample, respectively. Although in both cases the expected bead morphology of a suspension polymer is observed, the photographs reveal the presence of certain individual beads with differing morphologies. Hollow shells (Fig. 1) and nested spheres within the beads (Fig. 2) can be seen in the samples. After having observed many samples, the following conclusions were drawn: 1. The majority of the beads are either solid (Fig. 3) or hollow with shells of different thicknesses (Fig. 4). Figure 2 Overall view of sample CE 5095. Scale bar is 620/tm. (Arrows point to nested spheres in beads).

Particle structure of suspension poly(vinyl chloride) and its origin in the polymerization process

AbstractThe initial stage of the suspension polymerization of poly(vinyl chloride) (PVC) is characterized by the formation of colloidally stable micron‐sized grains of PVC inside the polymerizing ca. 150 μm vinyl chloride droplets. The fate of these micron‐sized PVC grains depends upon the agitation conditions. If no agitation is employed, they serve as growth centers for further polymerization to give a final particle possessing a uniform internal bead morphology. In agitated systems, these grains coagulate early in the conversion to give a more irregular structure in the interior of the PVC particle. The formation of these stable growth centers appears to be unique to PVC. The polymerization of acrylonitrile, also insoluble in its monomer, is characterized by rapid agglomeration of the precipitated polymer throughout the polymerization. In PVC, the colloidal stability of the polymerizing grains is demonstrated to be electrical in nature. A pericellular membrane or skin formed by polymerization in both the water and vinyl phase completely surrounds the polymerizing droplet after about (1–2)% conversion. This skin is responsible for the charge retention of the PVC grains inside the polymerizing monomer droplets.

Extensional flow induced crystallization from solution

An experimental apparatus and technique were developed for studying the crystallization of polyethylene from solutions undergoing an oscillatory, constant stretch rate, extensional flow. Investigation of the resulting fiber morphology, obtained at various temperatures and stretch rates, shows that the fibers exhibit a bead and stringlike appearance. Study of the melting, acid etching, and selective dissolution behavior of the fibers leads to the conclusion that fiber growth occurs in a fashion which directly results in the bead and string morphology and that the morphology is not a result of secondary, epitaxial crystal growth as previously believed. A model for the growth process, as suggested by the data, is presented. Evidence is also presented to suggest that at certain temperatures and stretch rates, a triclinic phase transformation occurs during the flow process. The effect is to produce two separate melting entities which appear to be stretch rate sensitive, in line with previously published studies on the effects of stretching on the behavior of bulk polymer.