Core-shell morphologies

When monomers of drastically different solubiUty (39) or hydrophobicity are used or when staged polymerizations (40,41) are carried out, core—shell morphologies are possible. A wide variety of core—shell latices have found appHcation ia paints, impact modifiers, and as carriers for biomolecules. In staged polymerizations, spherical core—shell particles are made when polymer made from the first monomer is more hydrophobic than polymer made from the second monomer (42). When the first polymer made is less hydrophobic then the second, complex morphologies are possible including voids and half-moons (43), although spherical particles stiU occur (44). 

Abstract A convenient method to synthesize metal nanoparticles with unique properties is highly desirable for many applications. The sonochemical reduction of metal ions has been found to be useful for synthesizing nanoparticles of desired size range. In addition, bimetallic alloys or particles with core-shell morphology can also be synthesized depending upon the experimental conditions used during the sonochemical preparation process. The photocatalytic efficiency of semiconductor particles can be improved by simultaneous reduction and loading of metal nanoparticles on the surface of semiconductor particles. The current review focuses on the recent developments in the sonochemical synthesis of monometallic and bimetallic metal nanoparticles and metal-loaded semiconductor nanoparticles. 

The appearance and persistence of core-shell structures as well as the occurrence of phase separation are attributed to a small asymmetry in the X -parameters (xPS-pi = 0.06, xpi-pdms = 0.09 and xps-pdms = 0.20). Hence, a PDMS core surrounded by a PI shell embedded in a PS matrix results in a smaller inner diameter interfacial area, relative to that for the PS-PI case. In a blend of a PS-fo-PI-fc-PDMS triblock with PS and PDMS homopolymers, more PS homopolymer is expected to be found in the corona of the PS block than PDMS homopolymer in the corona of the PDMS block because the penalty for contact between the PI block and PDMS homopolymer is larger. In consequence, the distribution of homopolymers favours an expanded PS-PI interface, making the core-shell morphologies, gyroid and cylinder, more prevalent. 

Thermoplastic elastomers (TPE), 9 565-566, 24 695-720 applications for, 24 709-717 based on block copolymers, 24 697t based on graft copolymers, ionomers, and structures with core-shell morphologies, 24 699 based on hard polymer/elastomer combinations, 24 699t based on silicone rubber blends, 24 700 commercial production of, 24 705-708 economic aspects of, 24 708-709 elastomer phase in, 24 703 glass-transition and crystal melting temperatures of, 24 702t hard phase in, 24 703-704 health and safety factors related to, 24 717-718... 

Loading of guests within the SCKs (for potential delivery) is modeled after lipoproteins, which are composites of cholesterol, cholesteryl ester, phospholipids, and protein forming biological structures of core-shell morphology... 

Figure 9.3 Morphology control in the preparation of arborescent copolymers (a) core-shell morphology from short side chains, and (b) star-like morphology from long side chains...

A new class of metal-loaded nanoparticles was developed by Reynolds et al. (95). These materials have a core-shell morphology, where the core is a functionalized polymer with a high affinity to the Gd(III) ions. The core polymer contained monomers with carboxylate pendant arms, such as ethylacrylate, methacrylate, butylacrylate or allylmethacrylate. The shell consisted of a... 

In another method, VBC is added to a highly polymerised HIPE of a styrene/ DVB mixture in water, followed by complete polymerisation. This gave an agglomerated material with a core-shell morphology. The outer, poly(VBC)... 

Other polymer materials which can be prepared include latexes, or particle agglomerates, by dispersed phase polymerisation. These can be either hydrophilic or hydrophobic in nature, or may have core-shell morphologies. They can be employed as support materials for a number of catalyst systems. Polymerisation of both phases of the emulsions produces composite materials, which have found use as selective membranes for the separation of mixtures of liquids with similar physical properties. 

In the earliest studies, so-called core-shell morphologies were identified, where the core of the minority end-component is separated from the other endblock by a shell of the midblock (B). Spherical microdomains surrounded by a shell of the midblock have been observed using TEM for a number of polymers (Arai et al. 1980 Riess et al. 1989). Mogi et al. (1992a) studied a series of PI-PS-P2VP triblocks where the volume fraction of the PS middle block was varied from 0.3 to 0.8, whilst the volume fractions of the endblocks were kept equal to each other. On increasing the PS volume fraction a core-shell lamellar... 

Transmission and scanning electron microscopy, differential scanning calorimetry and minimum film temperature analysis supports a core/shell morphology for the two-stage latex polymers, consisting predominantly of a polystyrene rich core surrounded by a soft acrylic rich shell. 

Experimentally, the described synthetic strategy was first realized by Lozinsky et al. [80,81], who studied the redox-initiated free-radical copolymerization of thermosensitive N-vinylcaprolac lam with hydrophilic N-vinyl-imidazole at different temperatures. These and other experimental studies [82 - 84] showed the universality of this approach of obtaining copolymers capable of forming nanostructures with a core-shell morphology. 

This size is larger than the size of the pure PB2 estimated from the AFM observations (i.e., the height of the particles because the width is systematically overestimated by the tip-broadening effect). This difference is consistent with the embedding of the PB2 in the P2VP layer. The core-shell morphology of these particles was confirmed by high resolution TEM (HR TEM). As seen from the Fig. 6,... 

Fig. 2. Diagram of nanoparticle structure (core-shell morphology)...

Temperature- and pH-sensitive core-shell microgels consisting of a PNIPAAm core crosslinked with BIS and a polyvinylamine (PVAm) shell were synthesized by graft copolymerization in the absence of surfactant and stabilizer [106] The core-shell morphology of the microgels was confirmed by TEM and zeta-potential measurements. Other examples of core-shell microgel systems are PNIPAAm-g-P(NIPAM-co-styrene) colloids [107] or PS(core)-g-PNIPAAm (shell) particles [108],... 

It can be shown theoretically that the relative amounts of the two monomers in a two-stage emulsion polymerization can affect the final particle structure [ 19], with core-shell morphology being favored thermodynamically, as the amount of second-stage polymer is increased. 

American Chemical Society [36].) Inset in (b) a close-up view showing the core-shell morphology of similar fibers. (Reprinted with permission from Chem. Eur. J., 1999, 5, 2740 [37].)... 

Sanchez-Solis, A. Estrada, M.R. Cruz, J. Manero, O. On the properties and processing of polyethylene terephthalate/styrene-butadiene rubber blend. Polym. Eng. Sci. 2000,40 (5), 1216-1225. Luzinov, I. Xi, K. Pagnoulle, C. Huynh-Ba, G. Jerome, R. Composition effect on the core-shell morphology and mechanical properties of ternary polystyrene/styrene butadiene rubber polyethylene blends. Polymer 1999, 40 (10), 2511-2520. 

---