Inverted core-shell particles

Second, the reduced free energy change for the formation of inverted core shell particles (a) with the lyophohic liquid 3 and the lyophilic liquid 1 in the shell and the core, respectively, leads to equation 47. Note that the same reference state is assumed as above. 

For composite particles in water, the three extreme morphologies may be considered as core-shell, with the more hydrophilic component interfacing with water, inverted core-shell, with the more hydrophobic component interfacing with water, and separated particles. 

DMS [Hu et al., 1997b], Baselines were used for subtraction, as is common in spectroscopy calculations [Fay etal., 1991], The IPN core/shell particles showed the highest damping, comparing with separate and multilayered core/shell particles. Also, normal synthesized (rubbery polymer was synthesized first) and inverted synthesized... 

In the case of PMMA/PS composite particles, where the equilibrium morphology was an inverted core-shell structure, employing a chain transfer agent (CQaBr), even under conditions favoring a core-shell structure (Le. a very low rate of monomer addition), resulted in the formation of particles where the core contained a number of PS domains distributed throughout the entire FMMA [ ase [57]. This indicated that shorter PS chains have higher diffusivities in the highly viscous core particles. 

Practical methods can be used to ensure that the required particle morphology is obtained. A high Tg or molar mass for the hrst-formed polymer forces the second-formed polymer to reside on its surface due to kinetic effects. The use of an inherently more water-soluble second polymer as compared to the first will also contribute to obtaining the desired morphology. Inverted core-shell latexes are most readily obtained when a more hydrophobic monomer for the second-formed polymer is polymerized in the presence of a less hydrophobic polymer latex. An alternative is to render the second monomer coUmdaUy unstable ufien polymerized, forcing it to reside in the core of the first formed polymer. 

The above computations for the fiee energy changes for composite latex particles have shown that only core-shell, inverted core-shell and hemispherical particles are stable in a thermodynamic sense. All of the other reported morphologies (e.g. sandwich-like , raspberry or confetti-like particles or occluded domains) are non-equilibrium, kinetically controlled structures, prepared... 

Intentional structure in latexes is traditionally obtained by polymerizing a second monomer over a first-formed latex polymer to form a core-shell latex (see Chapter 9). A more recent development allows the production of a core-shell latex using an inversion process. In this so-called inverted core-shell process the second monomer is polymerized in the presence of a first-formed latex but forms the core of the core-shell latex particle. In both the core-shell and inverted core-shell procedures the final morphology is governed by the change in 6ee... 

As mentioned before, hybrid latex particles are usually prepared by seeded emulsion polymerization. In the first stage, well-defined inorganic or organic particles are prepared, while in the second stage a monomer is polymerized in the presence of these well-defined particles. Multistage emulsion polymerization produces structures such as core-shell, inverted core-shell structures, and phase-separated structures such as sandwich structures, hemispheres, raspberry-like and void... 

The above synthetic strategy leads to easy generation of [R(Ag°)(Cu )] H and [R(Au°)(Pd°)] Cl nanocomposites with their inverted structures. The order of deposition of the bimetallic shells on the polystyrene beads can be altered by the successive immobilization of their corresponding precursors. Matrixes such as [R (Pd°)(Pt°)] Cl and [R(Ag°)(Au°)]+Cl were also synthesized from their corresponding metal chloride precursors. The layer-by-layer deposition technique has been widely used to fabricate core-shell particles because of its convenience to tailor the thickness and composition of the shells. The thickness can be controlled by varying the number of cycles of operation immobilization and subsequent reduction. In this way, we can deposit more than two metals on any kind of charged polystyrene bead. 

If the seed latex particles can barely be swollen by the second-stage monomer and a water-soluble initiator is used, then the subsequent seeded emulsion polymerization will be localized near the particle surface layer. Thus, the postformed polymer tends to form a surface layer around the seed latex particle. An example of this kind of morphological structure of latex particles is the seeded emulsion polymerization of methyl methacrylate in the presence of a polyvinylidene chloride seed latex. On the other hand, free radical polymerization can take place inside the seed latex particles. In this manner, various morphological structures of latex particles such as the perfect core/shell, inverted core/shell, dumbbell-shaped, and occluded structures can be achieved, depending on various physical parameters and polymerization conditions. 

Cho and Lee [6] used three different initiators, potassium persulfate, 2,2 -azobisisobutyronitrile, and 4,4 -azobis(4-cyanovaleric acid) (water-soluble, but less hydrophilic than potassium persulfate) to investigate their effects on the emulsion polymerization of styrene in the presence of polymethyl methacrylate seed latex particles. Inverted core/shell latex particles were observed when 2,2 -azobisisobutyronitrile or 4,4 -azobis(4-cyanovaleric acid) was used to initiate free radical polymerization. The use of potassium persulfate resulted in various morphological structures of latex particles, which were largely determined by the initiator concentration and polymerization temperature. 

Recently, Durant et al. [55] developed a mechanistic model based on the classic Smith-Ewart theory [48] for the two-phase emulsion polymerization kinetics. This model, which takes into consideration complete kinetic events associated with free radicals, provides a delicate procedure to calculate the polymerization rate for latex particles with two distinct polymer phases. It allows the calculation of the average number of free radicals for each polymer phase and collapses to the correct solutions when applied to single-phase latex particles. Several examples were described for latex particles with core-shell, inverted core-shell, and hemispherical structures, in which the polymer glass transition temperature, monomer concentration and free radical entry rate were varied. This work illustrates the important fact that morphology development and polymerization kinetics are coupled processes and need to be treated simultaneously in order to develop a more realistic model for two-phase emulsion polymerization systems. More efforts are required to advance our knowledge in this research field. 

The shape or form of the particles is referred to as the particle morphology. Particles may be uniformly spherical, have core-shell morphologies (51, 263), be hemispherical (382), have domains or inclusions (349), be nonspherical or irregular in shape, or may be inverted (in which the core and shell compositions are reversed). Figure 5 illustrates some of the possible particle morphologies. The particular morphology is determined by thermodynamic (equilibrium) (67,101) and kinetic (rate of phase separation versus rate of polymerisahon) considerations. In some cases, latex... 

---