Steric stabilized copolymer latexes

Table II. Compositions of the Steric Stabilized Copolymer Latexes...

Amalvy, J. L, Unali, G. F., Li, Y, Granger-Bevan, S., Aimes, S. P, Binks, B. P, Rodrigues, J. A., and Whitby, C. P. 2004. Synthesis of sterically stabilized polystyrene latex particles using cationic block copolymers and macromonomers and their application as stimulus-responsive particulate emulsifiers for oil-in-water emulsions. Langmuir 20 4345-4354. 

Figure 9 The schematical representation of dispersion polymerization process, (a) initially homogeneous dispersion medium (b) particle formation and stabilizer adsorption onto the nucleated macroradicals (c) capturing of radicals generated in the continuous medium by the forming particles and monomer diffusion to the forming particles (d) polymerization within the monomer swollen latex particles, (e) latex particle stabilized by steric stabilizer and graft copolymer molecules (f) list of symbols.

The stabilizing of aqueous latexes succeeded by using emulsifiers (anionic, nonionic) and/or their mixture, steric stabilizators (polyvinyl alcohol (PVOH), hydroxyethyl cellulose, polyethylene glycol, new protective colloids etc.), and polymerizable surfaces active agents, in general. Vinyl acetate (VAc) emulsion homopolymers and copolymers (latexes) are widely used as binders in water-based interior and exterior architectural paints, coatings, and adhesives, since they have higher mechanical and water resistance properties than the homopolymers of both monomers [2, 4, 7]. 

The results showed that all batch polymerizations gave a two-peaked copolymer compositional distribution, a butyl acrylate-rich fraction, which varied according to the monomer ratio, and polyvinyl acetate. All starved semi-continuous polymerizations gave a single-peaked copolymer compositional distribution which corresponded to the monomer ratio. The latex particle sizes and type and concentration of surface groups were correlated with the conditions of polymerization. The stability of the latex to added electrolyte showed that particles were stabilized by both electrostatic and steric stabilization with the steric stabilization groups provided by surface hydrolysis of vinyl acetate units in the polymer chain. The extent of this surface hydrolysis was greater for the starved semi-continuous sample than for the batch sample. 

Dispersion polymerization is defined as a type of precipitation polymerization by which polymeric microspheres are formed in the presence of a suitable steric stabilizer from an initially homogeneous reaction mixture. Under favorable circumstances, this polymerization can yield, in a batch process, monodisperse, or nearly monodisperse, latex particles with a relatively large diameter (up to 15 pm) [103]. The solvent selected as the reaction medium is a good solvent for both the monomer and the steric stabilizer, but a non-solvent for the polymer being formed and therefore a selective solvent for the graft copolymer. This restriction on the choice of solvent means that these reactions can be carried out... 

PCA may be utilized to produce stable PMMA polymer latexes in CO2 by spraying the polymer solution in the presence of a steric stabilizer, introduced in either the organic or the CO2 phase. One of the most successful stabilizers is poly(l,l-dihydroperfiuorooctyl acrylate) or copolymers con-... 

Two representative types of steric stabilizer have been studied by Cosgrove et al. (1982). The first was a partially hydrolyzed (88%) sample of poIy(vinyl alcohol). It was, of course, a copolymer containing short blocks (e.g. of average length 6 repeat units) of poly(vinyl acetate). The second sample was poly(oxyethylene) that was terminally anchored to the surface of the latex particles. The normalized segment density distribution functions for these two samples are shown in Fig. 12.1a and 12.1b. 

The best steric stabilizers are amphipathic block or graft copolymers such as poly(oxyethylene lauryl ether) (Mr 1200). Commercial nonionic surfactants are classified according to the hydrophilic-lipophilic balance (HLB), which scales the relative solubilities of the two components in aqueous and nonaqueous media. The need for the anchor part of the stabilizing molecule can be eliminated if the stabilizing moieties can be covalently bonded to the latex particles. 

In addition to the electrostatic stabilization mechanism, latex particles can be stabilized by adsorption of hydrophilic polymer chains on their particle surfaces. The physically adsorbed or chemically grafted polymer chains surrounding the colloidal particles and extending themselves into the continuous aqueous phase serve as a steric barrier against the close approach of the pair of particles. In this manner, coagulation of latex particles can be prevented via the steric stabilization mechanism [17,18]. Typical nonionic surfactants and surface-active, nonionic block copolymers are quite effective in imparting such a steric stabilization effect to colloidal dispersions. 

An analysis presented of the forces contributing to the attraction and repulsion interactions between macromolecules in acrylate latices. The electrostatic repulsion forces, enthalpy and entropy effects, and the attraction forces from the expanded Hamaker equation are analysed. The influence of the structure of copolymers consisting of monomeric units of alkyl acrylate or methacrylate (methyl to n-butyl) and acryhc or methacryhc acid on the physico-chemical properties of the latices and their stabihty were determined. On the basis of experiments and calculations it was established that the stability of latices is decided by two mechanisms. The first (ionic stabilisation) consists in adsorption of anionic emulsifier particles, and the second (ionic-steric stabilisation) involves adsorption of such an emulsifier on an adsorption layer formed by the polymer macromolecules forming the latex. 25 refs. Articles from this journal can be requested for translation by subscribers to the Rapra produced International Polymer Science and Technology. 

It is noteworthy that a basic assumption made in the derivation of the free radical desorption rate constant is that the adsorbed layer of surfactant or stabilizer surrounding the particle does not act as a barrier against the molecular diffusion of free radicals out of the particle. Nevertheless, a significant reduction (one order of magnitude) in the free radical desorption rate constant can happen in the emulsion polymerization of styrene stabilized by a polymeric surfactant [42]. This can be attributed to the steric barrier established by the adsorbed polymeric surfactant molecules on the particle surface, which retards the desorption of free radicals out of the particle. Coen et al. [70] studied the reaction kinetics of the seeded emulsion polymerization of styrene. The polystyrene seed latex particles were stabilized by the anionic random copolymer of styrene and acrylic acid. For reference, the polystyrene seed latex particles stabilized by a conventional anionic surfactant were also included in this study. The electrosteric effect of the latex particle surface layer containing the polyelectrolyte is the greatly reduced rate of desorption of free radicals out of the particle as compared to the counterpart associated with a simple... 

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