Emulsion polymerization core-shell’ structure

The theory also has relevance to the so-called seeded " emulsion polymerization reactioas- In these reactions, polymerization is initial in the presence of a seed latex under conditions such that new particles are unlikely to form. The loci for the compartmentalized free-radical polymerization that occurs are therefore provided principally by the particles of the initial seed latex. Such reactions are of interest for the preparation of latices whose particles have, for instance, a core-shell" structure. They are also of great interest for investigating the fondamentals of compartmentalized free-radical polymerization processes. In this latter connection it is important to note that, in principle, measurements of conversion as a function of time during nonsteady-state polymerizations in seeded systems offer the possibility of access to certain fundamental properties of reaction systems not otherwise available. As in the case of free-radical polymerization reactions that occur in homogeneous media, investigation of the reaction during the nonsteady state can provide information of a fundamental nature not available through measurements made on the same reaction system in the steady state. 

Non-radiative direct energy transfer (DET) is the transfer of the exited state energy from a donor molecule to an acceptor molecule. This transfer occurs without the appearance of a photon, and is primarily a result of dipole-dipole interactions between the donor and acceptor. DET between phenanthrene and anthracene chromophores has been successfully employed to investigate the morphologies of PMMA and PS labelled homopolymer latex particles prepared by seeded emulsion polymerization, as well as PMMA/PS and PS/PMMA composite particles [85]. The results tend to confirm the existence of a core-shell structure of the latex particles, but more important, provide deeper insights into the interfacial structures in the particles. There is a limitation in the quantitative interpretation of the data due to the overall extent of energy transfer which is still small, even when there is substantial mixing nevertheless, trends are apparent. 

Some encapsulation processes have limited variability with regards to payload. For example, the payload capacity of molecular encapsulation or complexation in cyclodextrins is limited by affinity equilibrium of the active molecule to the host molecule. Conversely, the payload for some of the common emulsion-based processes, such as interfacial polymerization, will remain high due to the inability to increase shell thickness set by the diffusion limits of the reactive monomers used to form the shell. While liposomes can also have a core-shell structure, their formation process and structure severely limit payload. Lipophilic active ingredients can be entrapped within the lipid bilayer of the liposome but are limited to low percentages to avoid disrupting the bilayer structure. Hydrophilic active ingredients can be entrapped in the core of the liposome, but payload is again limited by their solubility or concentration in the inner aqueous environment. 

Zhang K, Wu W, Meng H, Guo K, Chen J-F. Pickering emulsion polymerization Preparation of polystyrene/nano-SiOj composite microspheres with core-shell structure. Powder Technology. 2009 190(3) 393-400. 

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... 

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. 

Another effective method is to achieve core-shell structured nanocomposites. The shells can be C and SiO. In the case of Carbon-coated nano-Si composite, the preparation is as follows as anode material for LIBs. At first, PAN-coated Si nanoparticles are formed by emulsion polymerization and the PAN-coated precursor is heat-treated under Ar to generate a Si-C core-shell nanocomposite (Fig. 5.4a). The conductive carbon shell envelops the Si nanoparticles and suppresses the aggregation of the Si nanoparticles during electrochemical cycling, subsequently ameliorating the capacity retention of this composite anode material (Fig. 5.4b). 

A core-shell structured nano Ti02-C can be prepared as Scheme 5.2. The initial Ti02 is anatase, PAN was coated on the surface of nano-Ti02 particles with emulsion polymerization, after calcination at 800°C, nano Ti02-C composites were obtained... 

Very little has been reported about the use of spectroscopic methods for monitoring and control of other polymerization systems. Lenzi et al. [191] reported that the NIR spectra collected in a dispersive instrument with a transflectance probe may contain very useful information about the structure of core-shell polystyrene beads produced through simultaneous semibatch emulsion/suspension polymerizations. Lenzi et al. [192] developed a polymerization technique that combines recipes of typical emulsion and suspension polymerizations to produce core-shell polymer beads. More interesting, the appearance of the core-shell structure always led to qualitatively different NIR spectra that could not have been obtained with polymer suspensions, polymer emulsions, or mixtures of polymer suspensions and emulsions. As described by Lenzi et al. [191], different spectral peaks could be detected in the wavelength region constrained between 1700 and 1900nm when the core-shell structure developed. 

The production of such particles usually results fi-om the emulsion copolymerization of a hydrophobic monomer, such as styrene with a water-soluble monomer, such as acrylic acid. Differences in water solubility of the two monomers along with disparate reactivity ratios led to the preparation of particles having a core-shell structure with the hydro-phobic polymer in the core and the water-soluble polymer in the shell layer. It was also found that precipitation polymerization of alkyl(meth)acrylamide, such as N-isopropyl... 

RAFT polymerization is suited to aqueous solutions and emulsion polymerization, and thus has been widely used to prepare SPB (Bernadette et al., 2010 Edmondson et al., 2004 McCormick et al., 2006). The core—shell structure of amphiphilic block copolymers via RAFT-mediated polymerization is given in Fig. 4.7 (Liu et al., 2011). 

Emulsion polymerization is one of the most common techniques used to prepare polymer nanocomposites and hybrid nanoparticles with a core-shell structure as it is environmentally friendly and economically feasible on an industrial scale. There are three general well-studied processes to prepare polymer-MH nanocomposites, i.e. in situ monomer-nano-MH emulsion polymerization, an in situ combined process of precipitation and emulsion polymerization and surface-initiated in situ emulsion polymerization. Each of these methods is described below. 

Recently core shell microspheres of NIPAM copolymer gels with sty-rene(St) have been prepared [33]. Because St-NIPAM copolymer microspheres prepared by soap-free emulsion polymerization have an imperfect core shell structure, seeded polymerization of NIPAM was carried out using the St-NIPAM microspheres as seeds to prepare uniform core shell microspheres. The particles obtained are thermosensitive gels and the adhesion between these particles and leukocytes has been investigated. Adsorption of proteins on the particles was also studied using PNIPAM gels [34]. 

ASA structural latexes have been synthesized in a two stage seeded emulsion polymerization. In the first stage, partially crosslinked poly(n-butyl acrylate) and poly( -butyl acrylate-sfaf-2-ethylhexyl acrylate) rubber cores are synthesized. In the second stage, a hard styrene acrylonitrile copolymer (SAN) shell is grafted onto the rubber seeds (16). 

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