Interfacial tension, latex

This paper compares the swelling of monodisperse polystyrene and polymethyl methacrylate latexes with their monomers and the estimated particle-water interfacial tensions with the theoretical curves from Morton s equation. A new model which takes into account the effect of water dissolved in the swollen particles and in the monomer phase on the swelling of relatively hydrophilic systems is presented. 

The interfacial tension between a swollen particle and the aqueous phase can be approximated by the interfacial tension between the monomer phase and the swollen latex dispersion. The latter was measured by the drop volume method. The swollen latex dispersion was dropped into the monomer phase using a microsyringe with a thin-wall needle the end of which had been filed flat. The drop volume and density data were then converted to interfacial tension. 

Swelling of polystyrene latex particles with styrene. The swelling ratios and the corresponding interfacial tensions for the different-size latexes with added anionic surfactants Aerosol MA and sodium dodecyl sulfate are listed in Table II. Those values obtained with added nonionic surfactant Triton X-100 and polymeric surfactant polyvinyl pyrrolidone are listed in Table III. Figure 1 compares theoretical curves from Model I with all of the experimental data. It is found that a curve corresponding to Xmp = 0.35 fits the data best. Therefore, a semi-empirical... 

Swelling of polymethyl methacrylate latex particles with methyl methacrylate. Table IV lists the swelling ratios and interfacial tensions for the different-size polymethyl methacrylate latexes with added Aerosol MA and sodium dodecyl sulfate emulsifiers. Comparison of the data with the theoretical curves from Model I (Figure 2) defines an apparent interaction parameter of 0.45 and the semi-empirical equation ... 

In summary, the thermodynamic Model I based on Morton s theory has been used to successfully fit experimental data and obtain semi-empirical equations for the swelling of polystyrene and polymethyl methacrylate latexes. The semi-empirical equations offer a quick method for estimating swelling ratio from particle size and interfacial tension. The generalized form of Model II might prove to be more suitable for describing the swelling phenomena of relatively hydrophilic systems. ... 

Mixed emulsifiers are commonly used in combination with electrolytes to attain oil/water interfacial tensions substantially less than 1 dyne/cm, eg. 10 1 to 10 dynes/cm (31). The stability of the resulting microemulsions is usually attributed to the formation of an interfacial film (32,33). Even though the mechanism of stabilization has not yet been resolved, the excess surfactant used in microemulsions usually assures good stability. However, due to the very low mixed emulsifier concentrations used in miniemulsions, an understanding of the interactions between mixed emulsifier molecules at oil/water interfaces should greatly facilitate the development of miniemulsion and mini-latex formulations to achieve good stability. 

A variety of organic colloids including emulsions and polymer latexes have been dispersed in carbon dioxide in the presence of surfactants (3,13). In most cases, owing to the lower interfacial tension of the former as explained shortly it is easier to form organic-in-C02 emulsions than water-in-C02, emulsions. Sterically stabilized colloids are stable above the critical flocculation density (CFD) and precipitate below this density. In some cases the CFD occurs at the upper critical solution density of the steric stabilizer, that is, the density at which the stabilizer phase separates from CO2, as has been shown by theory (14,15) and experiment (16). So-called ambidextrous surfactants have been designed to allow polymer latexes produced in CO2 to be transferred to an aqueous solution to form a dispersion (17,18). 

Aqueous viscosity was measured at room temperature with a Brookfield and Ubbelohde viscometer. Intrinsic viscosities were measured by a five-point dilution method no shear-rate corrections were made for the data. Interfacial tension was measured with a DuNouy ring tensiometer against toluene at various polymer concentrations. The formulations used to evaluate the HMHECs for latex paints have been described elsewhere (5). 

Let the radius of a latex particle swollen to equilibrium be r. If dr is the increase in radius caused by the imbibition of dn, moles of monomer, the cMie-sponding increase in interfacial area is Snrdr, causing an increase in the Gibbs interfacial energy of the particle AGj = Sjrrdry, where y is the interfacial tension. The increase in the volume of the particle is... 

The negatively charged hydrophilic headgroup of the anionic surfactants may comprise sulfate, sulfonate, sulfosuccinate or phosphate groups attached to an extended hydrophobic backbone [82]. The nature of the hydrophilic group will influence the extent of electrostatic stabilization, the behaviour of the surfactant as a fiinction of pH, the degree of hydrolysis, and the variation of latex stability with time, electrolyte and temperature conditions. The nature of the backbone hydrophobe will influence the adsorption behaviour of the surfactant onto the latex particle surface, its cmc value, the interfacial tension (which affects monomer emulsification), and the extent of steric stabilization, among other factors. 

Complete description of the thermodynamics of partitioning of monomers between the aqueous phase, monomer droplets and latex particles obviously is more complex and requires knowledge of many quantities that are difficult to measure, such as interaction paramet and interfacial tensions [6,7]. As a consequence, there have been essentially two types of approach used to account for... 

Chen et al. [36] presented a conclusive comparistMi of the latex particle surface polarities (as determined by contact angle measuremoits) to the measured interfacial tensions of various polymer phases dissolved in the second-stage monomer against the aqueous phase. They prepared PS/PEMA composite particles using two different monodisperse PS seed latexes (produced by the Dow Chemical Co. as model colloids). The results of the interfacial tension measurements for each polymer phase, i.e. PS core particles and PEMA shell polymer dissolved in EMA monomer, showed that the interfacial tension first decreased and then remained constant as the polymer concentration was increased. This demonstrated that the polarity of the polymer-monomer solution interface with the aqueous phase increased until reaching a certain equilibrium point, which depended on the amount and nature of the polar components of the polymer. 

The data presented in Table 9.3 show that particles with lower surface polarities (ATP) cause the polymer dissolved in the second stage monomer to have relatively higher interfacial tension values (y iyn)./aq.phase) against the aqueous phase. The PEMA (or PS) latexes produced using AffiN initiator, which contains only weak polar surface groups, exhibited low surface polarities and higher interfacial tensions as compared to both Dow PS seed particles. These results showed that the surface polarity of the particles is primarily influenced by the functional groups attached to the polymer chain ends, rather than the bulk hydrophilicity of the monomers employed. 

The surface polarity of latex pWticles is a key parameter which influences which phase would be inside or outside of the final composite particles. Polymers with the highest surface polarities and lowest interfacial tensions will tend to be located at the outer particle surface. Based on the data presented in Table 9.3, one can expect that the final morphology of the composite latex PS-300/PEMA-AIBN... 

The relative hydrophilicity of the comonomers used in the synthesis of composite latexes has been considered to be the main polymoization parameter which influences particle morphology [48,51]. In the literature, the surface polarity of a latex particle has often been erroneously related to the bulk polymer polarity [6,45]. The most important propaty of an interface is its interfacial tension, which depends on the molecular interactions between the boundary polymer phase which is swollen with the second-stage monomer. The outermost layer of a latex particle is different from its interior (core), being enriched with p lar groups of different origins (e.g., S04 . OH, COOH, CN) which are better solvated by water, and thus tend to concentrate near the wato phase. 

Table 9.3 Latex surface polarity and polymer/aqueous phase interfacial tensions...

The interfacial tension between the various polymer phases in the composite latex particles is of paramount importance in determining their final morphology. Other factors, such as the mode of monomer addition, the surface polarity of the polymer particles, initiator type, surfactant type, the presence or absence of crosslinking of one or more of the polymer phases, and the presence of chain transfer agents will also strongly influence the final particle structure. In addition, a survey of methods used to characterize the composite latex particles has been given. 

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