Latex surface layer

Figure 4. Schematic of the surface layer of a latex particle...

Sedimentation. The sedimentation method, described previously 5), is based on the relative sedimentation rates of swollen and unswollen latex particles. Starting with the Stokes expression for centrifugal sedimentation, an equation can be developed for the ratio of the sedimentation coefficient of a particle, S, to the sedimentation coefficient, S, of the same particle having a surface layer ... 

A quantitative comparison of particle expansion determined by the three methods is given in Table I. The particle diamete of the standard acrylic latex was determined by PCS to be 1120 A. This value was used in the calculation of the increase in particle radius at maximum expansion in each case. The sedimentation method yielded the largest increase in radius, 302 A, followed by the viscometric value of 240 K. Possibly the shear involved in the latter method resulted in a partial collapse of the surface layer. The value determined by PCS was found to be approximately half that determined by sedimentation. Since the PCS determination is presumed to be free of particle interactions at a concentration of 5 X 10 4%, we must conclude that the other two methods (at 1% solids) exhibit such interactions. As a result, the charged particles settle slower (19) and yield a higher viscosity than in the absence of these (repulsive) interactions. 

The detectable amount of adsorbed species can be extremely low. A retention time shift of Atr=0.310 at a modest G (103 g) with w=250 pm results in only -10 17gof adsorbed mass (density 1.4 g/cm3). This mass corresponds to a very small layer, only 0.6 A thick on a 0.2 pm sphere [186]. The above approach has been used to measure protein adsorbed on latex surfaces [186-188], which is relevant to immunodiagnostic assays and biomedical implants. Complete adsorption isotherms can be measured [186] and antigen-antibody binding ratios determined [187]. 

The hairy particles stabilized by non-ionic emulsifier (electrosteric or steric stabilization) enhance the barrier for entering radicals and differ from the polymer particles stabilized by ionic emulsifier [35]. For example, the polymer lattices with the hairy interface have much smaller values of both the radical entry (p) and exit (kdes) rate coefficients as compared to the thin particle surface layer of the same size [128,129]. The decrease of p in the electrosterically stabilized lattices is ascribed to a hairy layer which reduces the diffusion of oligomeric radicals, so that these radicals may be terminated prior to actual entry. For the electrostatically stabilized lattices with the thin interfacial layer, exit of radicals occurs by the chain transfer reaction [35]. This chain transfer reaction results in a monomeric radical which then exits out of the particle by diffusing through the aqueous phase and this event is competing with the propagation reaction in the particle [130]. The decrease of kdes in the electrosterically stabilized latex... 

Studies of typical nanomaterials (soil mineral components, adsorbents, silica gels with deposited proteins, so called smart surfaces, latexes, synthetic zeolites modified by ions, MCM-41 molecular sieves) were made earlier by the author of this chapter [11-17]. At present our research focuses on studies of surface properties (e.g. adsorption capacity), total heterogeneity (energetic and geometrical) of surface layers, as well as structures and phase transformations of... 

A different situation arises when studying PMMA-latexes swollen by a nonpolar monomer like styrene which exhibits at ambient temperature a much lower solubihty in water (0.2 g/1) than MMA (15.9 g/1) [55]. Styrene has a very low electron density (see Table 1) in comparison to soUd PMMA and both an enrichment or a depletion of this monomer in the surface layer are easily discernible in a SAXS-experiment [55]. In comparison to the above system PMMA/MMA a critical test of the influence of entropic versus enthalpic forces becomes possible if the entropic wall-repulsion effect prevails styrene should be enriched in a surface layer. Because of the lower electron density of styrene this surface layer must exhibit a lower electron density than the core of the particle. If, on the other hand, the unfavorable enthalpic interactions between styrene and water are decisive, the more polar polymeric component PMMA should be enriched in a surface layer. In that case a surface layer with an enhanced electron density is expected. 

In the course of the SAXS-studies of a PMMA latex swollen with MMA [52] or the unpolar styrene [55] it became obvious that both a depletion as well as an enrichment of the polymer in the surface layer maybe observed Fig. 21 displays the SAXS-scattering intensities obtained from a PMMA latex swollen by MMA [52]. 

Thus, adsorption of small amounts of these surfactants can be monitored conveniently by SAXS because in the case of PS particles the main scattering intensity arises from the surface layer. In consequence, the scattering intensity of PS-particles covered by surfactant molecules strongly increases compared to the uncovered particles. Therefore PS latexes present ideal model systems for studying the adsorption of surfactants and polymers on colloidal particles from solution [53]. 

Although the actual diameter of a polymeric particle can be measured by microscopic or other methods, the effective diameter for hydrodynamic puiposes, and hence the effective volume fraction, may be considerably larger. Surface layers can significantly increase the effective volume of latex particles. Such layers may be due to adsrxbed surfactants, adsrabed or reacted polymeric stabilizers such as poly(vinyl alcohol), hydroxyethyl cellulose or poly(ethylene oxide), and surface charges on the polymer particle. The smaller the particle size, the greater will be the contribution a surface layer (rf given thickness to the effective volume of flie particle. 

Layer-by-Layer Modification of Latex 8.5.5.1 Latex surface charge excess... 

Sample Latex Surface Area/m Theoretical PPy Loading (mass%) Actual PPy Loading (mass%) Calculated PPy Layer Thickness/nm Colloid Stability of PPy-Coated Latex cP /S cm ... 

C. Kruger, H. W. Spiess, U. Jonas Controlled Assembly of Carboxylated Latex Particles on Patterned Surface Layers , Proceedings PARTEC 2001, International Congress for Particle Technology 2WS1,17/2,1-8. 

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