China
Africa
Explore chapters and articles related to this topic
Recent Advances and Future Perspectives in Heterophase Polymerization
Published in Hugo Hernandez, Klaus Tauer, Heterophase Polymerization, 2021
Cho et al. [127] observed that the particle size obtained by dispersion polymerization can be controlled during polymerization by adjusting the reaction temperature and the composition of the continuous phase. By feeding water to the original ethanol medium, the polymerization may eventually switch from dispersion to emulsion polymerization. They also found that larger clusters of particles can be obtained by emulsifying the dispersion in hexadecane. A different alternative for controlling particle size was proposed by Nauman et al. [128] for the miniemulsion polymerization of methyl methacrylate. They used a porous glass membrane with different pore sizes for adjusting the final polymer particle size. Hydrophobic initiators are required for minimizing secondary particle nucleation in the continuous phase.
Introduction and Basics of Nanotechnology
Published in Rakesh K. Sindhu, Mansi Chitkara, Inderjeet Singh Sandhu, Nanotechnology, 2021
Anjali Saharan, Pooja Mittal, Kashish Wilson, Inderjeet Verma
This method involves polymerization of monomers in an aqueous solution. For the preparation of aqueous solution, two different techniques are used: (a) emulsion polymerization—emulsification of monomer in nonsolvent phase and (b) dispersion polymerization-dispersion of monomer in nonsolvent phase. The drug is incorporated in the nanoparticle either by dissolution of drug in polymerization medium or by adsorption on the nanoparticle. This suspension of nanoparticles contains excipients like surfactants and stabilizers, which are finally removed by the ultracentrifugation method and the resulting suspension is suspended in an isotonic medium that is surfactant free. The nanoparticles produced by this method are poly butyl cyanoacrylate or poly(alkylcyanoacrylate). The process variables of particle size are concentration of stabilizer and surfactant involved in preparation.
P
Published in Joseph C. Salamone, Polymeric Materials Encyclopedia, 2020
Dispersion polymerization is actually a precipitation polymerization carried out in the presence of a suitable steric stabilizer in order to control the particle size. Although the process has been mainly studied in radical polymerization, a few experiments have been done in polycondensation for the production of polyesters or polyurethanes in the presence of specially designed stabilizers carrying either cationic groups or epoxy for ionic or covalent grafting onto the particles. References are given in the review of Arshady and Georges.8 More recently, dispersion polymerization has been applied to anionic polymerization in the condition of living polymerization. The team of Schneider and Mulhaupt reported the synthesis of polystyrene particles with a narrow particle-size distribution around 2.1 microns and a narrow molecular weight distribution (Mw/Mn = 1.05) upon polymerization of styrene in hexane, initiated by butyllithium and carried out in the presence of a block copolymer of styrene and trimethylsilylstyrene as steric stabilizer.10 A similar system is reported by Awan et al using styrene-butadiene block copolymers of different structures as stabilizers.11 They again achieve narrow molecular-weight distribution, but they observed that the particle-size distribution was dependent on the structure of the stabilizer (its type—AB being better than ABA) and its composition (the polystyrene block should be long enough).
Preparation and characterization of CdS/ZnS core-shell nanoparticles
Published in Journal of Dispersion Science and Technology, 2020
Zhenqian Zhang, Yongzhou Lin, Fang Liu
The core-shell nanoparticles, which break through the simple superposition in different functional substances, have attracted much attention as new types of materials in recent years.[2–5] The preparation methods of core-shell nanoparticles are mainly divided into physical adsorption method and monomer polymerization method.[17–23] Physical adsorption refers to a method that mixing prepares nanocrystals and coated materials, which attach or assemble semiconductor nanocrystals into carrier materials. Although the preparation process is simple, there are also some disadvantages such as the inefficiency diffusion and adsorption efficiency of nanocrystals. In addition, the polymer network structure of the shell is loose; the core layer is tight; and the scope of application is limited. The monomer polymerization methods include microemulsion polymerization, emulsion polymerization, miniemulsion polymerization, suspension polymerization and dispersion polymerization. The nanocrystals prepared by microemulsion have good monodisperse and interfacial properties, but it has many shortcomings such as excessive raw materials demand, complex processes and severe control conditions.