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This paper reports an efficient method of preparing porous polymeric microspheres by solvent evaporation in foam phase, in which phase separation between polymer and porogen occurs in foam phase instead of that in water phase by using the traditional solvent evaporation method. The method provides outstanding features, including being time-saving, of high-yield and able for continuous production, in which formation of porous polymeric microspheres finished within 3 min with a high production yield up to approximate 95 wt% and the process was able to be developed into a continuous process for production of porous polymeric microspheres. It was also universal to non-crosslinked polymers since the method is a development on the traditional emulsion solvent evaporation method. The new method is efficient and can be used potentially on the industrial scale for continuous production of porous polymeric microspheres.

The aim of this work was to study an improved solvent evaporation method to prepare poly (lactic acid) (PLA) microspheres via foam-transfer. Since the foaming process and its transfer were critical to the improved method, they have been studied. Additionally, the delivery capability of foams was studied as a function of the oil/water ratio, the stirring rate, the concentration of polyvinyl alcohol (PVA) and ethanol (EtOH) in the aqueous phase (ωPVA, ωEtOH). It was found that foaming varied during the preparation process and it influenced the properties of PLA microspheres. When the oil/water ratio (w/w) ≥ 3:10, stirring rate ≥ 600 r/min, ωPVA ≥ 1 wt%, and ωEtOH = 0 wt%, solvent evaporation was able to produce enough foams for foam-transfer, which helped to deliver more than 89 wt% PLA microspheres to the receiving vessel. However, ωPVA ≤ 0.3 wt% and ωEtOH = 20 wt% were unfavorable for maintaining the spherical shape of PLA microspheres and caused the aggregation. The methodology was further used to prepare azoxystrobin-loaded PLA microspheres successfully with a high encapsulation efficiency of 86.54%. This work is meaningful since it enables an efficient and continuous route to prepare functional biodegradable polymer microspheres.

In this study, vacuum low-pressure carburizing heat treatments were carried out on 18Cr2Ni4WA case-carburized alloy steel. The evolution and phase transformation mechanism of the microstructure of the carburized layer during low-temperature tempering and its effect on the surface hardness were studied. The results showed that the carburized layer of the 18Cr2Ni4WA steel was composed of a large quantity of martensite and retained austenite. The type of martensite matrix changed from acicular martensite to lath martensite from the surface to the core. The hardness of the carburized layer gradually decreased as the carbon content decreased. A thermodynamic model was used to show that the low-carbon retained austenite was easier to transform into martensite at lower temperatures, since the high-carbon retained austenite was more thermally stable than the low-carbon retained austenite. The mechanical stability&mdash, not the thermal stability&mdash, of the retained austenite in the carburized layer dominated after carburizing and quenching, and cryogenic treatment had a limited effect on promoting the martensite formation. During low-temperature tempering, the solid-solution carbon content of the martensite decreased, the compressive stress on the retained austenite was reduced and the mechanical stability of the retained austenite decreased. Therefore, during cooling after low-temperature tempering, the low-carbon retained austenite transformed into martensite, whereas the high-carbon retained austenite still remained in the microstructure. The changes in the martensite matrix hardness had a far greater effect than the transformation of the retained austenite to martensite on the case hardness of the carburized layer.

A solvent evaporation self-motivated synthetic methodology based on foaming-transfer has been developed to fabricate porous polymer microspheres. The novel method is a technical improvement for the traditional solvent evaporation method, in which solvent evaporation enables foam formation and foaming-transfer. Foaming-transfer helps deliver oil droplets from the aqueous phase to the receiving vessel, while phase separation takes place in oil droplets between polymer and porogen to produce porous polymer microspheres. Because of foaming-transfer, the novel method enables a continuous production of polymer microspheres via continuous feeding. Additionally, high efficiency has been observed since solvent evaporation in foams is much quicker than that in the aqueous phase. The methodology was further developed to prepare versatile porous polymer microspheres by introducing extra organic solvents, including ethanol (EtOH), acetone and ethyl acetate. Solvents like EtOH and acetone that have relatively poor compatibilities with oil phase were able to control structure, surface morphology and particle size of polymer microspheres when varying the concentration of solvents; while introducing solvents like ethyl acetate having good miscibility with oil phase hardly changed these properties. It was mainly because the difference on the compatibilities led to different distributions of solvent molecules in the system, which altered the evaporation process of solvent and the surface tension of the system, leading to the variation on structure, particle size and surface roughness. This work is meaningful since it developed an efficient, continuous and versatile method for the production of porous polymer microspheres.

This study has developed a method to produce porous polymeric microspheres in a foam phase. And, the foam phase was a mixture of polymer, organic solvent, porogen and emulsion stabilizer. The process was comprised of a series of constant steps homogenization of the materials, growth of the foam phase and collection of the products. The growth of the foam phase was the key which combined mechanical agitation, solvent evaporation and oil droplets development under increasing temperature. The method has been seen time-saving, high yield and continuous process and it could be useful for industrial production of porous polymer microspheres in a continuous process.

Polymeric microspheres with different internal structures have been widely used because of their characteristics in the structures. This paper reports a method of controlling the internal structures of polymeric microspheres via the introduction of a water-soluble organic solvent to the continuous phase in the foam phase preparation of porous polymeric microspheres. The introduction of a water-soluble organic solvent enables the control of polymeric microspheres’ internal structures, from porous to hollow. Because a water-soluble organic solvent is introduced, the organic solvent may be diffused toward the interface because of the affinity between the organic solvent and the oil droplets, resulting an accumulation of organic solvent molecules at the interface to form an organic solvent layer. The presence of this layer may decrease the evaporation rate of the internal organic solvent in an oil droplet, which extends the time for the mingling of porogen droplets to form a few large pores or even an extremely large single pore inside. This method is also capable of altering the thickness of hollow microspheres’ shells in a desired way, with improved efficiency, yield and the capacity for continuous use on an industrial scale.

In this study, formulation of latex with methyl methacrylate (MMA) as a function of surfactant concentration ([S]) under critical micelle concentration (CMC) was investigated to explore the nucleation mechanism and the key to control the particle number (N p) of latex in emulsion polymerization. Influences of surfactant concentration [S] on the conversion of monomer, particle size (PS), N p, and surfactant surface coverage (θ) were studied, as well. Flocculation nucleation was considered as the main mechanism to generate particle nuclei when [S] was much less than CMC. The nucleation mechanism experienced an evolution of a flocculation nucleation-dominated region ([S] 3.3 mmol L−1). The contribution ratio of the flocculation nucleation was 86 % compared with the micellar nucleation. The particle number (N p) was found to be controlled by the combination of micellar and flocculation nucleation-dominated regions under CMC, and the regulation range was about 1–6. The results can be used to guide the preparation of a bimodal high solid content (HSC) latex by creating a population of small particles in the presence of an initial population of larger ones in a controllable way. In this way, a controllable amount of large latex particles were prepared using a high emulsifier concentration, at first, and then the second population of small particles was formulated by secondary nucleation at a proper time through controlling surfactant concentration at a low level, followed by the growth of the two groups of the particles, and then the bimodal HSC latex can be prepared at low viscosity.