Core-shell latexes

Figure 6.11. Idealized image intensities for transmission electron microscopy of latex particles (a) homogeneous latex, (b) core-shell latex, (c) ultramicrotomed thin section through core-shell latex. Core/shell ratio, calculated assuming 50 wt % PVC/50 wt % P(B-co-AN) and polymer densities of 1.39 and 1.06 g/cm, respectively, gives T2 = 1.31ri. ...

The classification scheme illustrated in Figure 13.1 contains many possible connections, not shown. For example, the latex core/shell materials and the latex IPNs differ principally by the presence of cross-links in the latter, but significant morphological and mechanical differences may also occur. The mechanochemical blends are synthesized when a polymer 1/monomer 2 mix is masticated with sufficient shear to degrade polymer 1 to a certain extent. The various free radicals, anions and cations formed then initiate the... 

TABLE 1 Characteristic Properties of Core-Shell Latex Particles with Polystyrene Core... 

Core-shell nanocomposite of Mg(OH)2/PMMA with an average particle size of ca. 500nm where Mg(OH)2 is the core and PMMA is the shell was successfidly prepared by the emulsion polymerization of MMA in the presence of surface modified Mj OH)2. The grapelike ( re-shell microspheres with PMMA nodules could he obtained as stable latex. 

Similar structures using a silica-in-carbon core-shell structure have also been synthesized [76], which afford new possibilities for nanoencapsulation. Carbon can be removed by calcination, leaving silica, and (if the positions of silica and carbon are reversed) a carbon shell can be created using NH4OH solution to dissolve the silica. The formation of silica microparticles using silanol-functionalized polystyrene latexes proceeds along similar lines (Scheme 5.16) [77]. 

Various novel imprinting techniques have also been presented recently. For instance, latex particles surfaces were imprinted with a cholesterol derivative in a core-shell emulsion polymerization. This was performed in a two-step procedure starting with polymerizing DVB over a polystyrene core followed by a second polymerization with a vinyl surfactant and a surfactant/cholesterol-hybrid molecule as monomer and template, respectively. The submicrometer particles did bind cholesterol in a mixture of 2-propanol (60%) and water [134]. Also new is a technique for the orientated immobilization of templates on silica surfaces [ 135]. Molecular imprinting was performed in this case by generating a polymer covering the silica as well as templates. This step was followed by the dissolution of the silica support with hydrofluoric acid. Theophylline selective MIP were obtained. 

Carty, P. White, S. Price, D. Lu, L. (1999) Smoke suppression in plasticised chlorinated poly(vinyl) chloride (CPVC). Polymer Degradation Stability 63 465-468 Caruso, F. Susha, A.S. Giersig, M. Moh-wald, H. (1999) Magnetic core-shell particles Preparation of magnetite multilayers in polymer latex microspheres. Adv. Mater. 11 950-952... 

Fig. 2.20. AFM pictures of (left) a porous crystal made from a core-shell latex the crystalline lattice of the Si02 appears grey while the nanopores are black right) surface of a dried core-shell latex...

Seeded polymerization using a slight amount of monomer leads to the surface modification without changing particle size. The resulting particles are a kind of core-shell particles or, more exactly, core-skin particles (Fig. 12.2.4C). Seeded polymerization of sugar-units-containing styrene derivative on polystyrene seed particle was carried out to obtain latex particles covered with sugar units (17). A necessary condition for this is that the monomer is more hydrophilic than the seed polymer. If not, the monomer permeates into the seed particle and only a small fraction remains on the... 

Composite core-shell type microspheres were prepared by in situ heterogeneous polymerization on monodispersed seed latex particles suspended in an aqueous magnetite dispersion stabilized with sodium oleate (58). 

Another aspect of seeded polymerization is that a seed may be formed from one monomer composition whereas the added monomer may be of a different composition. This may lead to core-shell latex particles. Such copolymers may have substantially different properties then regular copolymers. 

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 agglomeration step, the latexes are partially agglomerated using a core/shell agglomerating agent latex, which consists of an elastomeric 1,3-butadiene/slyrene copolymer core and an ethyl acrylate/methacrylic acid copolymer shell. This partial agglomeration operation should not be confused with a coagulation operation where the emulsion is fully destabilized (13). 

Core-shell polymers were commercially introduced as impact modifiers for poly(vinyl chloride) PVC, in the 1960s. They are produced by a two-stage latex emulsion polymerization technique (Cruz-Ramos, 2000). The core is a graftable elastomeric material, usually crosslinked, that is insoluble in the thermoset precursors. Typical elastomers used for these purposes are crosslinked poly(butadiene), random copolymers of styrene and butadiene,... 

Wide angle light scattering is used as the principal probe to examine the core-shell structure proposed for certain acrylic acid acrylate ester copolymer latexes. Additional techniques were sedimentation and photon correlation spectroscopy. 

The work represents an application of core-shell light scattering theory to polymer latex suspensions and addresses the separate identification of light scattering by dust, latex particles and low molecular weight solutes. 

The typical results of the wide-angle light-scattering (WALS) analysis on the latex particles that were presumed to be cores are given in Table I where the theoretical model to be fitted was either S = sphere or CS = core-shell, the variance measured the goodness of fit, the modal size parameter is given by aM = 27rr/A, aQ is the log normal breadth parameter, DM is the core diameter in ym, mi the relative refractive index of the core and m2 the relative refractive index of the shell of indicated thickness. 

Theoretical calculations have demonstrated the feasibility of distinguishing a core-shell latex from a homogeneous latex. 

The morphology of two-stage (styrene//styrene-butadiene) and (styrene-butadiene//styrene) latex particles was found to vary from a core-shell structure to a complete phase separation with various two-phase structures in between, depending on polymerization sequence, polymerization conditions, polymer compatibility, molecular weights, polymer phase ratio, etc. 

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. 

Dufour, M.G. and Guyot, A, (2003) Nonionic reactive surfactants. Part 2. Core-shell latexes from emulsion polymerization. Colloid Polym. Sci, 285, 105-12. 

Abstract. An overview of the synthesis and applications of microgels and coreshell particles is provided, with emphasis on work originating from the author s laboratory. Microgels, which are cross-linked polymer latex particles, can be used for selective uptake of ions or polymers, or the controlled release of various compounds. Various methods for the synthesis of core-shell particles are described such as interfacial polymerization, layer-by-layer deposition, colloidosomes , internal phase separation, and silica shells. The release kinetics for controlled (sustained or triggered) release purposes is discussed. 

The copolymer composition in miniemulsion copolymerization of vinyl acetate and butyl acrylate during the initial 70% conversion was found to be less rich in vinyl acetate monomer units [34]. Miniemulsion polymerization also allowed the synthesis of particles in which butyl acrylate and a PMMA macromonomer [83, 84] or styrene and a PMMA macromonomer [85] were copolymerized. The macromonomer acts as compatibilizing agent for the preparation of core/shell PBA/PMMA particles. The degree of phase separation between the two polymers in the composite particles is affected by the amount of macromonomer used in the seed latex preparation. 

Emulsion polymerization was successfully employed for the preparation of nano-scale MIPs by synthesizing core-shell latexes with an imprinted shell. The use of a template with surfactant properties led to enhanced surface imprinting. Magnetic cores were synthesized to render MIPs which could be manipulated by magnetic fields in suspension, thereby facilitating the separation of the colloidal solid phase from the suspending solution. 

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