Polymer latices, particle size

The above latices were prepared by batch precipitation polymerisation of N-isopropylmethacrylamide using methylenebisacrylamide, as crosslinker, and potassium persulphate, as polymerisation initiator. The effect of the crosslinker on total conversion of polymer, latex particle size and morphological properties and colloidal properties of the final microgel particles were investigated. The relationship between the amount of water-soluble polymer and amount of crosslinker and the influence of temperature on the electrophoretic mobility of the latex are considered. 11 refs. 

Some modifications to the microsuspension process have been developed such as a seeded process (12) and an extension to this where an emulsion polymer latex, particle size sO.lpm, is added to the seed latex and monomer at the beginning of the polymerisation (13). In the latter process it is claimed that by... 

In monomer-starved conditions, particles are not saturated with monomer and grow at a rate controlled by the rate of monomer addition. In this case, the nucleation period is prolonged by a slower depletion of emulsifier micelles. Thus, lower monomer addition rates led to a decrease in the polymer latex particle size, but favored the increase in the number of the particles produced. 

PVA and TaM -for the 88%-hydrolyzed PVA. The same dependence was found for the adsorbed layer thickness measured by viscosity and photon correlation spectroscopy. Extension of the adsorption isotherms to higher concentrations gave a second rise in surface concentration, which was attributed to multilayer adsorption and incipient phase separation at the interface. The latex particle size had no effect on the adsorption density however, the thickness of the adsorbed layer increased with increasing particle size, which was attributed to changes in the configuration of the adsorbed polymer molecules. The electrolyte stability of the bare and PVA-covered particles showed that the bare particles coagulated in the primary minimum and the PVA-covered particles flocculated in the secondary minimum and the larger particles were less stable than the smaller particles. 

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. 

The polystyrene seed latex was monodispersed. Even after several grow-ups (polymerizations) the final 1650 A latex was monodispersed. Hydrodynamic chromatography on the 1650 A latex gave a mean diameter of 1660 a with a size variance as small as for normal polystyrene latex standards (typical standard of 1760 8 with a standard deviation of 23 a). The final latex particle size could be accurately predicted from the initial particle size and the total amounts of monomer and polymer used. 

In the methodology developed by us [24], the incompatibility of the two polymers was exploited in a positive way. The composites were obtained using a two-step method. In the first step, hydrophilic (hydrophobic) polymer latex particles were prepared using the concentrated emulsion method. The monomer-precursor of the continuous phase of the composite or water, when that monomer was hydrophilic, was selected as the continuous phase of the emulsion. In the second step, the emulsion whose dispersed phase was polymerized was dispersed in the continuous-phase monomer of the composite or its solution in water when the monomer was hydrophilic, after a suitable initiator was introduced in the continuous phase. The submicrometer size hydrophilic (hydrophobic) latexes were thus dispersed in the hydrophobic (hydrophilic) continuous phase without the addition of a dispersant. The experimental observations indicated that the above colloidal dispersions remained stable. The stability is due to both the dispersant introduced in the first step and the presence of the films of the continuous phase of the concentrated emulsion around the latex particles. These films consist of either the monomer-precursor of the continuous phase of the composite or water when the monomer-precursor is hydrophilic. This ensured the compatibility of the particles with the continuous phase. The preparation of poly(styrenesulfonic acid) salt latexes dispersed in cross-linked polystyrene matrices as well as of polystyrene latexes dispersed in crosslinked polyacrylamide matrices is described below. The two-step method is compared to the single-step ones based on concentrated emulsions or microemulsions. 

We used a matrix copolymer system consisting of methyl methacrylate (MMA) and styrene (St) grafted on polybutadiene rubber. The variables investigated were latex particle size (360, 2000, and 5000 A), degree of grafting, rubber content, and the degree of particle dispersion. The following variables must be considered when a transparent impact polymer is prepared. 

Fig- 4, The effect of electrolyte and particle size on the properties of polymer latex particles irv an aqueous medium. (Reproduced with permission of Cham. iird. London)). 

The influence of particle size on MWD has been investigated by Morton et al. (1954). They showed that the average molecular weight of the polymer produced was insensitive to the latex particle size. This is consistent with the molecular weight being dominated by chain transfer to monomer, a conclusion that holds irrespective of any differential swelLng of particles of different sizes. 

Polymer latex particles[169] in the range from 20 to 400 nm (or larger) with different surface functionalities can be employed as templates for the synthesis of macroporous materials. The route of templating polymer dispersions is complementary to the synthesis in lyotropic liquid-crystalline phases, leading to a bimodal size distribution of the pores. 

Polymer latex particles play a major role in coatings and paint industry. The size distributions in multicomponent formulations as well as the drying of paints and the coalescence of particles into a continuous protective film are topics that have been frequently investigated by AFM approaches. AFM provides direct access to the visualization down to the individual particle level and, as discussed in Sect. 4.3 in Chap. 4, to the assessment of the mechanical properties. 

The polymer latex stability obtained from the mini-emulsion polymerization with various ratios of SDS/CA decreases in the series l/3>l/10>l/l>l/6>l/0, which is consistent with the stability of monomer droplets reported by Ugelstad (l/3>l/2>l/l>l/6>l/0) [106]. The latex particle size decreases with increasing CA concentration. Furthermore, a two-dimensional hexagonal packing of surface-active molecules has been reported to be formed at a molar ratio of SDS/CA=l/3 in the colloidal system [107]. The good packing of the oil-water interfacial zone leads to satisfactory stability of monomer droplets, and it remains intact throughout the polymerization. 

In HEC-thickened formulations, low-shear-rate viscosities increase with decreasing latex particle size. This effect has been a major limitation in formulating small-particle latices. The phenomenon appears to arise from electro viscous, hydration, or flocculation effects, not a depletion layer mechanism. Associative thickeners achieve efficient viscosity in coating formulations via participation in synthesis and formulation surfactant micelles to form pseudo macromolecules and via an ion-dipole interaction between the cations of surface carboxylate groups on the latex and the ether linkages of the associative thickener. Generally, an excess of synthesis surfactant is found in the production of small-particle latices. The achievement of lower viscosities in small-particle ( 100 nm) latex coatings thickened with associative thickener appears to occur by extensive disruption of the polymer hydrophobe s participation in intermicellar networks. 

Studies on the formation of polymer latex particles have provided an alternative mechanism whereby uniform particles can result from a homogeneous nucleation-precipitation reaction. In emulsion polymerization, an insoluble monomer is mixed with water and a water-soluble free radical initiator is added. Final particle size depends on... 

Table 4.3 Effect of Latex Particle Size on Chloride Ion Permeability of SBR-Modified Mortars with a Polymer-Cement Ratio of 15% by ASTM C 1202.

Polymerization mechanism Monomer Initiator (I), catalyst (C), enzyme (E) or oxid. agent (OA) Polymer Surfactant Latex particle size, nm Ref. 

The fact that these polymers do not dissolve in flieir own monomers leads to complex phase separation at veiy low conversions during the polymerization process. The phase separation is even more complex than in a conventional emulsion polymerization since, not only is there phase separation between the newly formed polymer and the continuous wat phase, but also betweoi the polymer and monomer present. Due to this phase s aradon the locus of polymerization is not the interior of the polymer particles, simply because there is no monomer in the interiors. Free radicals formed in the water phase toid to precipitate or adsorb at the surface of existing particles r idly after their fonnation. Thus the major part of the propagation process will take place at the surface of the polymer latex particles [5,6]. Evidence for this is that the rate of polymerization is proportional to the total surface area of the latex particles this bend has been determined by varying both the latex particle concentration and size and observing the polymerization rate [5]. 

In 1962 Rinehart et al. [30] and Canale et al. [31] independently reported on the rhodium-catalysed emulsion polymerization of butadiene to a high trans-, A polymer utilizing sodium dodecylbenzenesulfonate as an emulsifer. More than 10 years later Entezami et al. [32] showed that trans- and cis-l,3-pentadiene can be polymerized in emulsion using RhCls as a catalyst and sodium dodecylbenzenesulfonate as a surfactant. Unfortunately, these authors did not mention latex particle sizes. Finally, ethylene can also be polymerized in emulsion [33] using a rhodium-based catalyst. 

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