Hollow polymeric microspheres

The silica microspheres provide some diversity but not enough for many complex discrimination tasks. To introduce more sensor variety, hollow polymeric microspheres have been fabricated8. The preparation of these hollow microspheres involves coating silica microspheres by living radical polymerization, using the surface as the initiation site. Once the polymer layer forms on the silica microbead surface, the silica core is removed by chemical etching. These hollow spheres can be derivatized with the dye of interest. The main advantage of these polymer microspheres is the variety of monomers that can be employed in their fabrication to produce sensors with many different surface functionalities and polymer compositions. 

Mandal T. K., Fleming M. S., Walt D. R., Production of hollow polymeric microspheres by surface-confined living radical polymerization on silica templates, Chem. Mater. 2000 12 3481-7. 

The dual-responsive hollow polymeric microspheres, which could be responsive simultaneously to two environmental stimuli, are expected to be more suitable for the controlled release of drugs. For example, the pH and temperature dual-responsive hollow polymeric microspheres could be responsive to the pH and temperature changes in their environment. Temperature-responsive outer shell and pH-responsive inner shell that encapsulate magnetic nanoparticles are widely used for the delivery of therapeutic compounds to the targeting specific sites. 

Pengcheng D., Tingmei W., and Peng L. Double-walled hollow polymeric microspheres with independent pH and temperature dual-responsive and magnetic-targeting function from onion-shaped coreshell structures. Colloids Surf. B 102 (2013) 1-8. 

Silica microspheres ( 3 /tm) with initiating moiety (S-8) induced the copper-catalyzed radical polymerization of benzyl methacrylate to form polymer layers on the surface.459 The thickness of polymer shell can be increased to 550—600 nm, where the Mn and MWDs of the arm polymers were 26500 and 1.26, respectively. Removal of the core silica by chemical etching gave uniform hollow polymer microspheres. 

An improved polymerization-induced colloid aggregation (im-PlCA) method was developed to prepare zeolite microspheres with hierarchical porous stractures and a uniform size, which could easily be carried out by adding urea and formaldehyde to an acidic pH precontrolled colloidal solution, as obtained from a hydrothermal crystallization process. After removing the polymeric component, solid and hollow zeolite microspheres can be obtained under different preparation conditions [172]. 

Hollow microsphere (diameter 360-1200 nm) of PANI-NSA has been fabricated using an emulsion template method at low temperature [284]. In this template method, the target material is precipitated or polymerized on the surface of the template, which results in a core-shell structure. On removing the template, hollow microsphere can be obtained. However, the removal of the template often affects the spherical structure, especially for hollow polymer microsphere. Therefore, they select the emulsion template method as the emulsion can be readily removed through dissolution or evaporation after polymerization. 

Syntactic foamed plastics (from the Greek ovvxa C, to put together) or spheroplastics are a special kind of gas filled polymeric material. They consist of a polymer matrix, called the binder, and a filler of hollow spherical particles, called microspheres, microcapsules, or microballoons, distributed within the binder. Expoxy and phenolic resins, polyesters, silicones, polyurethanes, and several other polymers and oligomers are used as binders, while the fillers have been made of glass, carbon, metal, ceramics, polymers, and resins. The foamed plastic is formed by the microcapsular method, i.e. the gas-filled particles are inserted into the polymer binder1,2). 

Polymeric 47,61), inorganic, and carbon 37) hollow macrospheres are used too. They have densities of 200-500 kg/m3 and are larger than 1 mm. By using macrospheres and microspheres together the apparent density of a syntactic foam can be reduced, however its specific strength is also less than that of a foam made only with microspheres 18,62). 

The materials employed for making hollow microspheres include inorganic materials such as glass and silica, and polymeric materials such as epoxy resin, unsaturated polyester resin, silicone resin, phenolics, polyvinyl alcohol, polyvinyl chloride, polyjM-opylene and polystyrene, among others, commercial jx oducts available are glass, silica, phenolics, epoxy resin, silicones, etc. Table 36 shows low-density hollow spheres. Table 37 shows physical properties of glass microspheres, and Table 38 shows comparison of some fillers on the physical properties of resulting foams (10). 

On the other hand, thermoplastic-based hollow microspheres can be prepared by heating thermoplastics containing low-boiling-point solvents. One example is polystyrene hollow microspheres. In the first stage, expandable polystyrene powder is prepared, e.g., polystyrene powder containing propane, butane or pentane is prepared by emulsion polymerization. The powder is then exposed to steam for expansion to form hollow microspheres. 

Room temperature ionic liquids (RTILs) are molten salts whose melting points are below room temperature. RTILs are formed when the constituent ions are sterically mismatched, thereby hindering crystal formation [17]. As polar solvents, RTILs have unique applications as tunable and environmentally benign solvents with very low volatility, high fire resistance, excellent chemical and thermal stability and wide liquid temperature range and electrochemical windows [17-19]. Solvent applications of RTILs include, for example, organic synthesis [17,20, 21], separations [22, 23], storage and transportation of hazardous chemicals [24], polymeric electrolytes [25, 26], dissolution of natural products [27] and synthesis of hollow metal oxide microspheres [28]. 

Liu et at fabricated the well-defined carboxylated CNTs/PPy composite hollow microspheres with near xmiform particle size of 1.4 pm via chemical oxidative interfacial polymerization of Py in the presence of the carboxylated carbon nanotubes (CNT-COOH) for the first time (Figure 8.1) [33]. It was found that the presence of the carboxylated CNTs greatly improved their morphological, thermal, and electrical conductive properties. The cycling stability as electrode materials for supercapacitors had been evidently improved by introducing the CNT-COOH, although the presence of the CNT-COOH had slightly enhanced their SC. 

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