Hollow polymer structure

It was found that the nanocapsules are formed in a miniemulsion process by a variety of monomers in the presence of larger amounts of a hydrophobic oil. Hydrophobic oil and monomer form a common miniemulsion before polymerization, whereas the polymer is immiscible with the oil and phase-separates throughout polymerization to form particles with a morphology consisting of a hollow polymer structure surrounding the oil. The differences in the hydro-philicity of the oil and the polymer turned out to be the driving force for the formation of nano capsules. 

Figure 4.11 Electron micrographs of polyethylene crystals, (a) Dark-field illumination shows crystals to have a hollow pyramid structure. (Reprinted with permission from P. H. Geil, Polymer Single Crystals, Interscience, New York, 1963.) (b) Transmission micrograph in which contrast is enhanced by shadow casting [Reprinted with permission from D. H. Reneker and P. H. Geil, /. Appl. Phys. 31 1916 (I960).]...

The most versatile method to prepare such hollow capsules is self-assembly [203-205, 214, 215]. Owing to their amphiphilic nature and molecular geometry, lipid-based amphiphiles can aggregate into spherical closed bilayer structures in water so-called liposomes. It is quite reasonable that the hollow sphere structure of liposomes makes them suitable as precursors for the preparation of more functional capsules via modification of the surfaces with polymers and ligand molecules [205, 216, 217]. Indeed, numerous studies based on liposomes in this context have been performed [205, 209, 213]. 

It should be noted that the development of such polymer systems is stimulated by existing experimental works. In particular, the experimental methods of preparation of nanometer-sized hollow-sphere structures have been suggested [58-63] because of their possible usage for encapsulation of molecules or colloidal particles. The preparation of hollow-sphere structures, generally, is based on self-assembling properties of block copolymers in a selective solvent, i.e., on the formation of polymer micelles with a nanometersized diameter. Further cross-finking of the shell of the micelle and photodegradation [64] of the core part produce nanometer-sized hollow cross-linked micelles. 

Fig. 10 Stages of preparation of a copolymer envelope a adsorption of homopolymer chain on a colloidal particle b coloring of the polymer chain (blue corresponds to chemically modified monomer units and red to adsorbed units) and introduction of crosslinks (shown as green sticks) to stabilize the hollow-spherical structure c elimination of the core particle. Adapted from [57]...

Gratson CM, Garcia-Santamaria E, Lousse V, Xu M, Ean S, Lewis JA, Braun PV. (2006) Direct-write assembly of three-dimensional photonic crystals Conversion of polymer scaffolds to silicon hollow-woodpile structures. Adv Mater 18 461—465. 

At least five groups have attempted polymerization in vesicles [7-11]. Pertinent experimental conditions, results, and appropriate references are summarized in Table 1. The polymerized morphologies observed are hollow polymer shells, parachutes , matrioshka structures, and necklaces as shown in Fig. 3. ( Matrioshka refers to the concentric sphere morphology that resembles the nesting Russian dolls bearing the same name.) Structures of the surfactants and monomers used in the experiments are shown in Fig. 4. 

Fig. 3. Polymer morphologies obtained by vesicle templating Hollow polymer spheres (top left) [24], parachutes (top right) [18], necklaces (bottom left) [17], and matrioshka structures (bottom right) [17], Adapted with permission from [17,18,24]...

Styrene, DVB, and surfactants A1,A2,A1/A2 1 2-2 1 Extrusion DMPA, UV lamp or pulsed laser Hollow polymer spheres and phase separated structures Cryo TEM, QELS, CPC, stability tests Jung et al. (2000) [15]... 

Kadali et al. demonstrated another useful application of NACs prepared by tandem assembly in the formation of catalyst support materials. NACs can be calcined to remove the polymer without collapsing the hollow sphere structure (Fig. 10). Such stability is more difficult to achieve with LBL-assembled capsules. On calcination,... 

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. 

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