Polystyrene latex layers

Paine et al. [99] tried different stabilizers [i.e., hydroxy propylcellulose, poly(N-vinylpyrollidone), and poly(acrylic acid)] in the dispersion polymerization of styrene initiated with AIBN in the ethanol medium. The direct observation of the stained thin sections of the particles by transmission electron microscopy showed the existence of stabilizer layer in 10-20 nm thickness on the surface of the polystyrene particles. When the polystyrene latexes were dissolved in dioxane and precipitated with methanol, new latex particles with a similar surface stabilizer morphology were obtained. These results supported the grafting mechanism of stabilization during dispersion polymerization of styrene in polar solvents. 

A very similar effect of the surface concentration on the conformation of adsorbed macromolecules was observed by Cohen Stuart et al. [25] who studied the diffusion of the polystyrene latex particles in aqueous solutions of PEO by photon-correlation spectroscopy. The thickness of the hydrodynamic layer 8 (nm) calculated from the loss of the particle diffusivity was low at low coverage but showed a steep increase as the adsorbed amount exceeded a certain threshold. Concretely, 8 increased from 40 to 170 nm when the surface concentration of PEO rose from 1.0 to 1.5 mg/m2. This character of the dependence is consistent with the calculations made by the authors [25] according to the theory developed by Scheutjens and Fleer [10,12] which predicts a similar variation of the hydrodynamic layer thickness of adsorbed polymer with coverage. The dominant contribution to this thickness comes from long tails which extend far into the solution. 

Several experimental parameters have been used to describe the conformation of a polymer adsorbed at the solid-solution interface these include the thickness of the adsorbed layer (photon correlation spectroscopy(J ) (p.c.s.), small angle neutron scattering (2) (s.a.n.s.), ellipsometry (3) and force-distance measurements between adsorbed layers (A), and the surface bound fraction (e.s.r. (5), n.m.r. ( 6), calorimetry (7) and i.r. (8)). However, it is very difficult to describe the adsorbed layer with a single parameter and ideally the segment density profile of the adsorbed chain is required. Recently s.a.n.s. (9) has been used to obtain segment density profiles for polyethylene oxide (PEO) and partially hydrolysed polyvinyl alcohol adsorbed on polystyrene latex. For PEO, two types of system were examined one where the chains were terminally-anchored and the other where the polymer was physically adsorbed from solution. The profiles for these two... 

In this paper we present results for a series of PEO fractions physically adsorbed on per-deutero polystyrene latex (PSL) in the plateau region of the adsorption isotherm. Hydro-dynamic and adsorption measurements have also been made on this system. Using a porous layer theory developed recently by Cohen Stuart (10) we have calculated the hydrodynamic thickness of these adsorbed polymers directly from the experimental density profiles. The results are then compared with model calculations based on density profiles obtained from the Scheutjens and Fleer (SF) layer model of polymer adsorption (11). 

Rheological Studies of Aqueous Concentrated Polystyrene Latex Dispersions with Adsorbed Poly(vinyl alcohol) Layers... 

Any fundamental study of the rheology of concentrated suspensions necessitates the use of simple systems of well-defined geometry and where the surface characteristics of the particles are well established. For that purpose well-characterized polymer particles of narrow size distribution are used in aqueous or non-aqueous systems. For interpretation of the rheological results, the inter-particle pair-potential must be well-defined and theories must be available for its calculation. The simplest system to consider is that where the pair potential may be represented by a hard sphere model. This, for example, is the case for polystyrene latex dispersions in organic solvents such as benzyl alcohol or cresol, whereby electrostatic interactions are well screened (1). Concentrated dispersions in non-polar media in which the particles are stabilized by a "built-in" stabilizer layer, may also be used, since the pair-potential can be represented by a hard-sphere interaction, where the hard sphere radius is given by the particles radius plus the adsorbed layer thickness. Systems of this type have been recently studied by Croucher and coworkers. (10,11) and Strivens (12). 

Ahmed et al.t measured t]red/(i> for polystyrene latexes with adsorbed layers of commercial poly(vinyl alcohol) (PVA) samples of different molecular weights. The latex particles were 190 nm in diameter and the limiting values of as - 0 had the following values for PVA samples of the indicated molecular weight ... 

Garvey et al.85) made a similar sedimentation study on poly(vinyl alcohol) adsorbed on polystyrene latex particles. Adsorbance of the polymer was also measured. Both the thickness of the adsorbed layer and the adsorbance increased linearly with the square root of the molecular weight. The volume occupied by a polymer molecule in the adsorbed layer was approximately equal to that of the effective hydrodynamic sphere in bulk solution. However, the measured values of LH were greater than the hydrodynamic diameters of the polymer coils in solution. Thus, it may be concluded that adsorbed poly(vinyl alcohol) assumes a conformation elongated in the direction normal to the surface. 

Particle electrophoresis studies have proved to be useful in the investigation of model systems (e.g. silver halide sols and polystyrene latex dispersions) and practical situations (e.g. clay suspensions, water purification, paper-making and detergency) where colloid stability is involved. In estimating the double-layer repulsive forces between particles, it is usually assumed that /rd is the operative potential and that tf/d and (calculated from electrophoretic mobilities) are identical. 

Earlier work (3) has shown that cleaned monodisperse polystyrene latexes stabilized with surface sulfate (and perhaps a few hydroxyl) groups an be used as model colloids. For example, the distribution of H ions in the electric double layer as determined by conductometric titration has been correlated with the particle diameter determined by ultracentrifugation (3). The conductometric titration gives two measures of the concentration of H+ ions the initial conductance of the latex and the amount of base required for neutralization. The number of H+ ions determined by conductance is always smaller than the number determined by titration. This difference is attributed to the distribution of the H+ ions in the electric double layer those closest to the particle surface contribute least to the overall conductance. This distribution is expressed as the apparent degree of dissociation a, which is defined as the ratio H+ ions... 

Hull and Kitchener (2) measured the rate of deposition of 0.3- an-diameter polystyrene latex particles onto a rotating disk coated with a film of polyvinyl formaldehyde. In electrolytes of high ionic strength (where the double-layer repulsion is negligible), they found close agreement between experiments and the prediction of Levich s boundary-layer analysis (Eq. 3]), indicating that a diffusion boundary layer exists and that its thickness is large compared to the domain of van der Waais and hydrodynamic interactions. These are neces-... 

In order to test the model used here, calculated values of the limiting free polymer concentration 0 at which phase separation occurs are compared with the experimental data [6] on the aqueous dispersions of polystyrene latex particles with adsorbed polyethylene oxide and with polyethylene oxide as the free polymer. Since no information is available regarding the thickness of the adsorbed layer, the values used by Vincent et al. [6] in their theoretical calculations are adopted. Table 1 compares the experimental values of the limiting volume fraction of the free polymer with our calculated values for two different molecular weights of the free polymer. The simple model used here gives reasonably good agreement with the experimental values. 

The values thus estimated (Table 3.4) are in reasonable agreement with the hydrodynamic layer thicknesses on polystyrene latex measured by photon correlation spectrometry and ultracentrifugation [224]. 

Similar results to those obtained here by the stability measurements have been reported by Roe and Brass (7.8) They studied polystyrene latex stabilized by potassium palmitate. The analysis supplied by these authors shows that the order of magnitude of the slope of the stability curves can be accounted for as an entropic effect of crowding of adsorbed molecules during an encounter between two particles. They pointed this out as a possible explanation as the amount of emulsifier adsorbed strongly affects the stability without altering the electrophoreti-cally derived double-layer potential. 

The effect, on dispersion and de-agglomeration in water, of electrostatic repulsion force arising from the surface potential and the double layer 1/x around particles has been investigate. Several suspensions of polystyrene latex in an agglomerated state were prepared where j/ and 1 x were controlled by the pH and electrolyte concentration respectively. These were accelerated in a convergent nozzle to give an external force and the resulting dispersions were examined by optical microscopy. It was found that the dispersion was enhanced with an increase in y/and 1/x. 

Several investigations have determined tbe absorption behavior of surfactant adsorption on particles of aqueous polymer dispersions by adsorption titration. The results have been similar to those observed by Wolfram for adsorption on a planar polymer surface determined from the wettipg angle. Thus, Paxton (1969) established that the area occupied by a sodium dodecylbenzylsulfonate molecule in a saturated adsorption layer (ylsiim) the surface of PMMA latex particles is 1.31 nm, whereas on the surface of polystyrene latex particles it is only 0.53 nm. The author considers that previous studies of adsorption of this emulsifier, which gave adsorption area Msitm) were carried out on interfaces with similar adsotption... 

As previously explained, it can be advantageons to generate extraporosity at a larger scale in the separative layer. The main condition that has to be respected is that the additional porosity mnst not be directly interconnected in order to preserve the entoff fixed by the porosity of the continuous phase. Templating by polystyrene latex was nsed to prodnee individnal macropores inside the silica layer (Figure 25.25). This route can be applied to prepare membranes of other oxides with varions possible strategies in terms of the synthesis process (Figure 25.26). In addition, the presence of dispersed micron-size or submicron-size... 

FIGURE 25.25 SEM image of the cross-section of a silica layer with spherical macropores resulting from the thermal degradation of polystyrene latex. 

The magnetic polystyrene latex particles with diameter of 120 nm were covered by PNIPA gel layer. The mPS latex prepared according to the procedure described previously was strongly stirred at 60 °C and kept under N2 atmosphere for 1 h. Then, 0.05 g APS and 0.5 mL 1 M NIPA solution were added to the mPS latex and the reaction mixture was stirred at 60 °C for more than 1 h. Then, 0.5 mL 1 M NIPA solution and 0.36 mL 0.1 M BA solution were added. After 2 h, 0.5 mL 1 M NIPA solution and 0.36 mL 0.1 M BA solution were added again to the mixture. This mixture was stirred 60 °C for more than 2 h under N2 atmosphere. Figure 9 shows the structure of the core-shell microsphere in dry state. 

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