Reversible film formation from nano-sized PNIPAM particles below glass transition

Reversible film formation process from nano-sized Poly(N-isopropylacrylamide) (PNIPAM) microgel particles were studied during heating-cooling cycles at various rates. Photon transmission technique was used and transmitted photon intensity I _tr was monitored during heating–cooling cycles. The increase and decrease in I _tr during heating and cooling was explained with the void closure and void reconstruction processes, and the corresponding activation energies were measured. It was observed that PNIPAM microgels required less energy during reconstruction of voids than their closure.     

A photon transmission study for film formation from poly(vinyl acetate) latex particles with different molecular weights

The photon transmission technique was used to monitor the temperature evolution of film formation from poly(vinyl acetate) (PVAc) latex particles with two different molecular weights. Two sets of latex films were prepared below the glass transition temperature (T_g) of PVAc, which are named as low (LM) and high molecular weight (HM) films. These films were annealed at elevated temperatures above the T_g of PVAc for various time intervals. It is observed that transmitted photon intensity (I_tr) from these films increased as the annealing temperature was increased. Onset temperatures (T_H) at given times (τ_H) for starting the optical clarity of LM and HM films were measured and used to calculate the healing activation energies (ΔH) for the PVAc minor chains, and found to be as 28.1 kcal/mol and 27.7 kcal/mol, respectively. The increase in the transmitted photon intensity, I_tr above T_H was attributed to the increase in the number of disappeared interfaces between the deformed latex particles. Prager–Tirrell (PT) model was employed to interpret the increase in the crossing density of chains at the junction surfaces. The interdiffusion (backbone) activation energies (ΔE) were measured and found to be 177.5 kcal/mol and 210.7 kcal/mol for a diffusing PVAc chains across the junction surface of LM and HM latex films, respectively. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2918–2925, 2007     

Time evolution of film formation from polystyrene particles: a percolation approach

This work reports the film formation process from surfactant-free polystyrene (PS) latex particles. Steady state fluorescence and photon transmission techniques were used to study the evolution of film formation. The films were prepared from fluorescein (F)-labeled PS latex particles at room temperature and annealed in 2.5-min-time intervals above the glass transition temperature (T _g) of PS. Fluorescence intensity (I _F) from F was measured after each annealing step to monitor the stages of film formation. Evolution of transparency of latex films was monitored by using the photon transmission intensity, I _tr. Drastic increase in I _tr and I _F above the critical annealing times, t _r and t _c, respectively, were attributed to the percolation behavior of the PS material. Critical exponents, β, of percolation clusters were measured and found to be around 0.31 and 0.37 for I _tr and I _F measurement, respectively, which were attributed to the site percolation model.     

Hydroplasticization effect in structured latex particles film formation

Series of emulsion copolymers with structured particles were synthesized comprising copolymerized acrylic or methacrylic acid in the outer layer. All samples were based on particles, containing identical cores slightly crosslinked by allyl methacrylate and variable shells, weight ratio core/shell being constant 1/1. Each sample contained 10 wt% HEMA in the shell to achieve the film crosslinkability. In both series samples with different hardness and polarity (varible styrene/butyl acrylate ratio) of the shell layer were prepared. It was shown that the extent of particle swelling and hydroplasticization depends not only on the content of dissociated carboxylic groups, but also on the composition and crosslinking of the rest of polymer chain i.e. on its polarity and rigidity and on the origin of carboxylic groups. The effect of dissociated carboxylic groups on lowering the minimum film forming temperature was much more pronounced if the polymer chains were more polar.     

Study of the formation of biodegradable polycaprolactone particles using solvent evaporation method

Poly(ε-caprolactone) (PCL), an aliphatic polyester paved the way in the formation of biodegradable and biocompatible polymer particles in tissue engineering and drug delivery applications. The factors affecting in the preparation of PCL particles such as influences of the concentration of PCL, molecular weight and concentration of poly(vinylpyrrolidone) (PVP) stabilizer, and the reaction time on the characteristics of PCL particles were investigated. In the fabrication of PCL particles, size, morphology and stability of the polymer particles were tailored by varying the type and concentration of the stabilizer to the polymer, reaction time and stirring speed of the reaction. Additionally, formation of particles was also related to the amount of PCL and PVP concentration. The PCL particles possessed a spherical shape with the size ranges of 5-20 micron and stability increases with higher concentration of PVP while coagulation observed when PCL particles prepared with the low concentration of PVP. The change in the morphology and sizes of the PCL particle at different reaction conditions were evaluated by scanning electron microscopy (SEM) analysis.     

Phospholipid-assisted synthesis of stable F-containing colloidal particles and their film formation

This letter illustrates for the first time the preparation of p-methyl methacrylate/n-butyl acrylate/heptadecafluorodecyl methacrylate (p-MMA/nBA/FMA) colloidal dispersions containing up to 15% w/w FMA, which is accomplished by the utilization of biologically active phospholipids (PLs) and ionic surfactants. The use of monomer-starved conditions during emulsion polymerization and the utilization of 1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC), sodium dodecyl sulfate (SDS), and phosphoric acid bis(tridecafluoro-octyl) ester ammonium salt (FSP) as surfactants, which function as transfer and dispersing agents, facilitate a suitable environment for the polymerization of p-MMA/nBA/FMA colloidal dispersions that exhibit nonspherical particle morphologies. Such nonspherical particles upon coalescence form phase-separated films with unique surface properties.     

Void closure and interdiffusion processes during latex film formation from surfactant-free polystyrene particles: a fluorescence study

This study reports a steady state fluorescence (SSF) technique for studying film formation from surfactant-free polystyrene (PS) latex particles. The latex films were prepared from pyrene (P)-labeled PS particles at room temperature and annealed at elevated temperatures for 5-, 10-, 20-, and 30-min time intervals above the glass transition (T(g)) temperature of polystyrene. During the annealing processes, the transparency of the film changed considerably. Scattered light (I(sc)) and fluorescence intensity (I(0P)) from P were measured after each annealing step to monitor the stages of film formation. Evolution of transparency of latex films were monitored using photon transmission intensity, I(tr). Scanning electron microscopy (SEM) was used to detect variation in the physical structure of annealed films. Onset temperature for film formation, T(0), void closure, T(v), and healing temperatures, T(h), were determined and corresponding activation energies were measured. Void closure and interdiffusion stages were modeled and related activation energies were determined.     

Spontaneous film formation from a polymer solution

An unusual continuous film formation process of lateral pentyloxy substituted poly(p-phenylene terephthalate)s (s-PPPT) and poly(carbonate) (PC) is observed. A liquid film of polymer solution creeps over the surface of water dropped into the polymer solution. By vaporization of the solvent a solid polymer film is formed on the water surface and can be removed. The driving force for the film formation mechanism is assumed by the reduction of the surface tension of water. Experiments verify this mechanism by increasing the film formation speed using a gas stream, by reducing the formation speed through lowering the surface tension by rinsing agents, and by lowering the solubility of the polymer. As expected, no effects are found by variation of the pH-value of water. Necessary conditions for the film formation process are: good solubility of the polar polymers in organic solvents having a high vapor pressure, complete phase separation, solution density higher than water density, and a surrounding gas phase unsaturated with solvent vapor.The thickness of the mechanically stable films is less than 0.5 m. The films are amorphous by microscopical, FT-IR, x-ray, and DTA investigations.     

Skim and cream natural rubber particles: colloidal properties, coalescence and film formation

Cream and skim fractions of freshly tapped natural rubber latex have been studied using atomic force microscopy and scanning electric potential microscopy to elucidate the topology and charge properties in film formation. Elemental distribution maps of the particles have also been obtained using electron energy-loss imaging in a low-energy transmission electron microscope. The two rubber fractions are obtained by centrifugation. The cream fraction is stable while rapid coagulation occurs in the skim fraction. After removal of the coagulum, no further coagulation occurs and the remaining skim rubber particles are stable. The rubber particles from the cream rubber particles contain higher amount of adsorbed protein-phospholipid materials compared to those in the "self-cleaned" skim fraction. This difference in membrane property has a significant impact on the spreading of the cis-1,4-polyisoprene cores, their coalescence and film formation behavior. Coalescence of cream particles appears to be hindered by the membrane materials, forming a rough film that retains the topology of individual particles. Skim particles coalesce more readily, forming relatively smooth films.     

Molecular weight effect on latex film formation induced by solvent vapor: an optical transmission study

Vesicular electrokinetic chromatography was used to investigate solute partitioning from the aqueous phase into dihexadecyl hydrogen phosphate (DHP) vesicles. Retention factors of neutral solutes are related to their partition coefficients between the aqueous phase and vesicles (K(vw)). The K(vw) of the aromatic test solutes were readily obtained from the slopes of the linear relationships between retention factors versus DHP concentrations. The technique offers the advantages of speed, automation, and small sample size for determination of partition coefficients. The K(vw) values of 43 uncharged solutes were measured at below as well as above the phase transition temperatures. The logarithms of partition coefficients (log K(vw)) of solutes at 71 degrees C (above T(c)) were slightly higher than those at 36 degrees C (below T(c)). The solvation characteristics of DHP were also studied using linear solvation energy relationships at the two temperatures.     

Preparation of spheroidal and ellipsoidal particles from spherical polymer particles by extension of polymer film

Generation of particles of various sizes and shapes can be of great interest to scientists and engineers. We have developed a new and robust apparatus which can generate oblate spheroids from spherical polymer particles. A sheet of film dispersed with spherical particles was held by an eight-jaw extensional apparatus. The jaws were positioned in the edges of a regular octagon and moved radially to induce biaxial extension in an oil bath above the glass transition temperatures of the film and particles. We have demonstrated that polystyrene particles dispersed in a polyvinyl alcohol film can be stretched to generate various shapes by this method. The microscopic studies show that the oblate spheroids obtained by this method are virtually exact spheroids without showing knife edges. Also depending on positions with regard to holders, ellipsoids and even prolate spheroids can be obtained. The method has been found to be robust in that the deformation is always reproducible regardless of film thickness and very small deformation can be applied for nearly spherical particles. We have confirmed that this method can be applied for particles of submicrons to 10?μm in diameter or even larger ones. It is expected that the spheroids and ellipsoids obtained by this method can be of help in many studies including colloids, suspension rheology, electrophoresis, printed electronics, and pharmaceutical science.     

Effect of solvent on single crystal formation from dilute polyethylene solution

Summary To investigate the effect of solvent on polyethylene single crystal formation, the end surface free energy of lamellae was estimated from the dissolution temperature of single crystal in solvent from which single crystals were formed, by the equation based on the thermodynamic consideration. The density measurements of the single crystal were also carried out in a density gradient column, further to obtain informations about solvent effect on single crystal formation. From the results on the end surface free energy and the density of single crystals, the conclusion obtained in the preceding paper, that more regular surface may be formed with solvent having larger thermodynamic interaction parameter and larger molar volume, was confirmed.

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Zusammenfassung Die freie Oberflächenenergie von Polyäthylenkristallen wurde aus der Auflösungstemperatur der Kristalle in dem Lösungsmittel, aus dem sie gebildet wurden, über thermodynamische Betrachtungen abgeschätzt.Aus der Größe der freien Oberflächenenergie und der Dichte der Einkristalle wurde die in der vorhergehenden Veröffentlichung angegebene Folgerung bestätigt, daß die Oberfläche um so regelmäßiger ausgebildet wird, je größer der thermodynamische Wechselwirkungsparameter und das Molvolumen des Lösungsmittels sind.

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The authors wish to express thanks to ProfessorB. Wunderlich for giving them the polyethylene sample. Thanks are also due to ProfessorI. Sakurada for the facilities in the low-angle X-ray measurements.     

A dynamic density functional theory for particles in a flowing solvent

We present a dynamic density functional theory (dDFT) which takes into account the advection of the particles by a flowing solvent. For potential flows, we can use the same closure as in the absence of solvent flow. The structure of the resulting advected dDFT suggests that it could be used for nonpotential flows as well. We apply this dDFT to Brownian particles (e.g., polymer coils) in a solvent flowing around a spherical obstacle (e.g., a colloid) and compare the results with direct simulations of the underlying Brownian dynamics. Although numerical limitations do not allow for an accurate quantitative check of the advected dDFT both show the same qualitative features. In contrast to previous works which neglected the deformation of the flow by the obstacle, we find that the bow wave in the density distribution of particles in front of the obstacle as well as the wake behind it are reduced dramatically. As a consequence, the friction force exerted by the (polymer) particles on the colloid can be reduced drastically.     

One-step fabrication of nanostructured Ni film with lotus effect from deep eutectic solvent

We report a procedure to fabricate nanostructured Ni films via programmed electrochemical deposition from a choline-chloride-based ionic liquid at a high temperature of 90 °C. Three electrodeposition modes using constant voltage, pulse voltage, and reverse pulse voltage produce a variety of nanostructured Ni films with micro/nanobinary surface architectures, such as nanosheets, aligned nanostrips, and hierarchical flowers. The nanostructured Ni films possess face-centered cubic crystal structure. Amazingly, it is found that the electrodeposited Ni films deliver the superhydrophobic surfaces without any further modifications by low surface-energy materials, which might be attributed to the vigorous micro/nanobinary architectures and the surface chemical composition. The electrochemical measurements reveal that the superhydrophobic Ni film exhibit an obvious passivation phenomenon, which could provide enhanced corrosion resistance for the substrate in the aqueous solutions.     

The generally useful estimate of solvent systems method facilitates off-line two-dimensional countercurrent chromatography for isolating compositions from Siraitia grosvenorii roots

Solvent system selection is a crucial and the most time-consuming step for successful countercurrent chromatography separation. A thin-layer chromatography-based generally useful estimate of solvent systems method has been developed to simplify the solvent system selection. We herein utilized the method to select a solvent system for off-line two-dimensional countercurrent chromatography to separate chemical compositions from a complex fraction of the Siraitia grosvenorii root extract. The first-dimensional countercurrent separation using chloroform/methanol/water (10:5.5:4.5, v/v/v) yielded four compounds with high purity and three mixture fractions (Fr I, III, and VII). The second-dimensional countercurrent separation conducted on Fr I, III, and VII using the hexane/ethyl acetate/methanol/water (4:6:6:4, 3:7:3:7, v/v/v) and chloroform/methanol/water (10:9:6, v/v/v) solvent systems, respectively, produced another four compounds. Four triterpenoids and four lignans were finally isolated, including two novel compounds. Hence, the generally useful estimate of solvent systems method is a feasible and efficient approach for selecting an applicable solvent system for separating complex samples. In addition, the off-line two-dimensional countercurrent chromatography method can improve both the peak resolution and the capacity of countercurrent chromatography.     

Effective interaction of particles immersed in a solvent

Additional structural forces arise from superposition of the perturbations produced in the medium by solute molecules. The potentials of the structural forces are related to the activity coefficient of the solute. Exact integrodifferential equations are derived for these potentials, and solutions are obtained for the cases of weak and strong solvation. It is found that the structural forces differ from vacuum interactions in including contributions from the coordinates of two or more molecules. The radius of interaction between particles is found to be increased and is determined by the rate of decay of the correlations in the solvent.I am indebted to V. M. Vdovenko, associate Member, Academy of Sciences of the USSR, and to F. M. Kunin, for assistance in this work.     

Film formation from nano‐sized polystyrene latex particles

This work reports on the steady state fluorescence (SSF) technique for studying film formation from surfactant‐free, nano‐sized polystyrene (PS) latex particles prepared via emulsion polymerization. The latex films were prepared from pyrene (P)‐labeled PS particles at room temperature and annealed at elevated temperatures in 5, 10, 15, 20 and 30 min time intervals above the glass transition temperature (T_g) of PS. During the annealing processes, the transparency of the film was improved considerably. Monomer and excimer fluorescence intensities, I_P and I_E respectively, from P were measured after each annealing step to monitor the stages of film formation. Evolution of transparency of the latex films was monitored by using photon transmission intensity, I_tr. Void closure and interdiffusion stages were modeled and related activation energies were determined and found to be 10.3 and 50.3 kJ mol⁻¹. Void closure temperatures, T_v, were determined from the minima of I_tr value. Copyright © 2005 John Wiley & Sons, Ltd.     

Optically transparent conductive network formation induced by solvent evaporation from tin-oxide-nanoparticle suspensions

This investigation describes an optically transparent antistatic film composed of antimony-doped tin oxide (ATO) nanoparticles dispersed in a polymer matrix, with remarkably improved electrical and optical properties. The film is fabricated on the basis of a synergistic interaction between self-assembling nanoparticles and self-organizing matrix materials. The antistatic property of the film is obtained at ATO concentrations above a threshold value. A scaling analysis of the data yields an extremely low critical concentration (0.0020 volume fraction), which is considerably lower than the value predicted by percolation theory. Microscopic observations of the film have revealed a characteristic microstructure: "single-stranded" chainlike (linear form or fibrous) aggregates consisting of ATO nanoparticles and large ATO-depleted areas. The experiment results suggest that the high optical transparency and the low critical concentration are derived from the characteristic microstructures of the film.