Latex particle number

Dependence of latex particle number on emulsifier and initiator concentrations... 

R. L. Zollars, Turbidimetric Method for On-Line Determination of Latex Particle Number and Particle Size Distribution. J. Coll. Int. Sci. (1980), 74, 163-172. 

The final latexes were characterized by their polymer solid content r (g/L) measured by gravimetry and by the average particles radius (R) (nm) obtained by dynamic LLS in HCl aqueous solution (pH 3.2). The final latex particle number (Np) (L ) was calculated according to... 

Table 3-3 The Number of Latex Particles at Various Heights in gm Above a Reference Point...

The AeroSizer, manufactured by Amherst Process Instmments Inc. (Hadley, Massachusetts), is equipped with a special device called the AeroDisperser for ensuring efficient dispersal of the powders to be inspected. The disperser and the measurement instmment are shown schematically in Figure 13. The aerosol particles to be characterized are sucked into the inspection zone which operates at a partial vacuum. As the air leaves the nozzle at near sonic velocities, the particles in the stream are accelerated across an inspection zone where they cross two laser beams. The time of flight between the two laser beams is used to deduce the size of the particles. The instmment is caUbrated with latex particles of known size. A stream of clean air confines the aerosol stream to the measurement zone. This technique is known as hydrodynamic focusing. A computer correlation estabUshes which peak in the second laser inspection matches the initiation of action from the first laser beam. The equipment can measure particles at a rate of 10,000/s. The output from the AeroSizer can either be displayed as a number count or a volume percentage count. 

Soap. A critical ingredient for emulsion polymerization is the soap (qv), which performs a number of key roles, including production of oil (monomer) in water emulsion, provision of the loci for polymerization (micelle), stabilization of the latex particle, and impartation of characteristics to the finished polymer. 

The rate of termination reaction is slower than that observed in the homogenous bulk or solution polymerization since the limited number of free radicals exists in the polymerization loci having a reasonably small volume (i.e., monomer swollen forming latex particle). Higher degree of polymerizations can be achieved in an emulsion system relative to the homogenous polymerization due to the existence of this limitation. 

Based on the Smith-Ewart theory, the number of latex particles formed and the rate of polymerization in Interval II is proportional with the 0,6 power of the emulsifier concentration. This relation was also observed experimentally for the emulsion polymerization of styrene by Bartholomeet al. [51], Dunn and Al-Shahib [52] demonstrated that when the concentrations of the different emulsifiers were selected so that the micellar concentrations were equal, the same number of particles having the same size could be obtained by the same polymerization rates in Interval II in the existence of different emulsifiers [52], The number of micelles formed initially in the polymerization medium increases with the increasing emulsifier concentration. This leads to an increase in the total amount of monomer solubilized by micelles. However, the number of emulsifier molecules in one micelle is constant for a certain type of emulsifier and does not change with the emulsifier concentration. The monomer is distributed into more micelles and thus, the... 

Medvedev et al. [57] extensively studied the use of nonionic emulsifiers in emulsion polymerization. The emulsion polymerizations in the presence of nonionic emulsifiers exhibited some differences relative to those carried out with the ionic ones. Medvedev et al, [57] proposed that the size of latex particles remained constant during the reaction period, but their number increased continually with the increasing monomer conversion. The use of nonionic emulsifiers in emulsion polymerization usually results in larger sizes relative to those obtained by the ionic emulsifiers. It is possible to reach a final size value of 250 nm by the use of nonionic emulsifiers in the emulsion polymerization of styrene [58]. 

Polymeric particles can be constructed from a number of different monomers or copolymer combinations. Some of the more common ones include polystyrene (traditional latex particles), poly(styrene/divinylbenzene) copolymers, poly(styrene/acrylate) copolymers, polymethylmethacrylate (PMMA), poly(hydroxyethyl methacrylate) (pHEMA), poly(vinyltoluene), poly(styrene/butadiene) copolymers, and poly(styrene/vinyltoluene) copolymers. In addition, by mixing into the polymerization reaction combinations of functional monomers, one can create reactive or functional groups on the particle surface for subsequent coupling to affinity ligands. One example of this is a poly(styrene/acrylate) copolymer particle, which creates carboxylate groups within the polymer structure, the number of which is dependent on the ratio of monomers used in the polymerization process. 

To illustrate how the effect of the adsorption on the modulus of the filled gel may be modelled we consider the interaction of the same HEUR polymer as described above but in this case filled with poly(ethylmetha-crylate) latex particles. In this case the particle surface is not so hydrophobic but adsorption of the poly (ethylene oxide) backbone is possible. Note that if a terminal hydrophobe of a chain is detached from a micellar cluster and is adsorbed onto the surface, there is no net change in the number of network links and hence the only change in modulus would be due to the volume fraction of the filler. It is only if the backbone is adsorbed that an increase in the number density of network links is produced. As the particles are relatively large compared to the chain dimensions, each adsorption site leads to one additional link. The situation is shown schematically in Figure 2.13. If the number density of additional network links is JVL, we may now write the relative modulus Gr — G/Gf as... 

The surfaces of colloidal particles are often charged and these changes can arise from a number of sources. Chemically bound ionogenic species may be found on the surface of particles such as rubber or paint latex particles. Charged species may be physically adsorbed if ionic surface active materials, for example, have been added. A charged surface may occur on a crystal lattice. An example is the isomorphous substitution of lower valency cations such as aluminium for silicon in the lattice structure of clays. A further example is the adsorption of lattice ions... 

The micelles are present at a concentration of about lO per ml of liquor and each micelle contains around 100 monomer molecules. In contrast, the number of monomer droplets is only about 10 ° per ml. Thus, despite the larger volume of monomer droplets, the micelles offer a very much larger surface area. A radical formed in the aqueous phase will thus encounter a monomer-filled micelle much more often than a monomer droplet. Therefore, the polymerization takes place practically only in the micelles and not in the monomer droplets. The monomer consumed in the micelles is replaced by diffusion from the monomer droplets through the aqueous phase. According to the theories of Harkins and of Smith and Ewart, the kinetic course of an emulsion polymerization is divided into three intervals At first some of the micelles increase rapidly in size as the polymerization advances and are transformed into so-called latex particles, containing both monomer and polymer. These are still very much smaller than the monomer droplets and have an initial diameter of about 20-40 pm, corresponding to about... 

From this point onwards, the polymerization occurs only in the latex particles, whose number, however, remains constant. The monomer droplets still have the function of a reservoir that delivers the monomer to the latex particles, so that the concentration of monomer in the latex particles and therefore the rate of polymerization remains constant. During this second period, the reaction is thus of zero order with respect to the monomer. 

In the preparation of a polymer latex, the initial relationship of water, surfactant, and monomer concentration determines the number of particles present in the reaction vessel. Once the process is underway, further addition of monomer does not change the number of latex particles. If such additional... 

Spherical latex particles with a reasonably well-defined number of charges per particle can be synthesized and used to study the non-Newtonian behavior of charged dispersions and related electroviscous phenomena (described in Chapter 4). The surface... 

Polystyrene latex particles were coagulated by the addition of Ba(N03)2. The number of dispersed particles deposited onto a planar polystyrene surface was determined 15 min after the addition of salt by optical microscopy. The light microscope does not permit the aggregation of the deposited particles to be determined subsequent examination by the electron microscope gives this information. Clint et al. obtained the following results ... 

We have to ask, however, how large an error is introduced when the excluded volume effect is neglected. Before considering this question, a recent result of cluster size dis-tritution for antigen coated latex spheres which were cross-linked by antibodies, may be discussed. Schulthess et al.178) measured this distribution as a function of the mean number of bonds per latex particle, b = af, where f is the number of antigens bound per latex particle and a the extent of reaction. 

Spherical particles of known diameter (e.g., 5% to 20% of the diameter of the aperture in the glass tube) are used to calibrate the electrical pulse counting instrument. The particles are suspended to an appropriate concentration in electrolyte solution (see recipe). Monodisperse latex particles are commercially available, which can be used for this purpose. Particle size calibration standards can be obtained from a number of chemical suppliers or from the National Institute of Standards and Technology (e.g., NBS 1003b). Lines (1996) lists a number of standards that are appropriate for this purpose. 

It is usual to consider the course of emulsion polymerization to proceed through three intervals [16,17]. The particle number increases with time in Interval I, where latex particles are being formed, and then remains constant during Intervals II and II. The monomer concentration in particles is in equilibrium with a monomer saturated aqueous solution. Swelling is limited only by the opposite force of the particle surface/water tension. Hence, the concentration of monomer in the particles is usually taken as constant up to the point where free monomer droplets disappear. In Intervals I and II, the monomer concentration... 

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