Polystyrene latex system

Figure 10. Electron micrograph of composite silica-polystyrene latex system,SPL(-), prepared by using bare silica particles as the seed.

Figure 12. Gel permeation chromatogram of latex polymer separated from composite silica-polystyrene latex system, SPL(HPC).

Figure 9.15 Critical flocculation temperature values and 9 temj>eratures of polyethylene oxide (PEO) for polystyrene latex systems stabilized by adsorbed Pluronic in the presence of varying concentrations of KCl. Pluronic concentration, 200 ppm. From Tadros and Vincent [35]. 

Calculation of material halances of the polystyrene latexes through these LEG systems showed significant retention of the sample within the column. Percent recoveries of polystyrene latexes calculated at 2 k nm for the columns oj Set II are given in Table IV. The small particle sizes (<1000 A) are completely recovered, while significant loss of sample is seen for the larger particle sizes. 

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). 

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). 

In this paper we report some rheological studies of aqueous concentrated polystyrene latex dispersions, in the presence of physically adsorbed poly(vinyl alcohol). This system has been chosen in view of its relevance to many practical systems and since many of the parameters needed for interpretation of the rheological results are available (15-18). The viscoelastic properties of a 20% w/w latex dispersion were investigated as a function of polymer coverage, using creep measurements. 

Figure 8.14 CLSM images showing the initial development of the microstructure of a phase-separated mixed biopolymer system (25.5 wt% sugar, 31.4 wt% glucose syrup, 7 wt% gelatin, and 4 wt% oxidized starch pH = 5.2, low ionic strength) containing 0.7 wt% polystyrene latex particles (d32 = 0.3 pm). The sample was quenched from 90 to 1 °C, held at 1 °C for 10 min, heated to 40 °C at 6 °C min-1, and observed at 40 °C for various times (a) 2 min, (b) 4 min, (c) 8 min, and (d) 16 min. White regions are rich in colloidal particles. Reproduced from Firoozmand et ai (2009) with permission.

Monodisperse spheres are not only uniquely easy to characterize, but also very rarely encountered. Polymerization under carefully controlled conditions allows the preparation of the polystyrene latex shown in Figure 1.8. Latexes of this sort are used as standards for the size calibration of optical and electron micrographs (also see Section 1.5a.3). However, in the majority of colloidal systems, the particles are neither spherical nor monodisperse, but it is often useful to define convenient effective linear dimensions that are representative of the sizes and shapes of the particles. There are many ways of doing this, and whether they are appropriate or not depends on the use of such dimensions in practice. There are excellent books devoted to this topic (see, for example, Allen 1990) and, therefore, we consider only a few examples here for the purpose of illustration. 

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. 

An illustration of the effect is shown in Figure 14 where the change in stability of a polystyrene latex with a diameter of 52.7 nm is shown as a function of the ratio N-/ (N+ + N ) where N = the number of negatively charged particles per unit volume and N+ = the number of positively charged particles per unit volume. As can be seen, the system becomes completely unstable when this ratio reaches ca. 0.25. 

There are special problems that occur when the particle diameter Is large relative to the wavelength of Incident light. This Is of Interest since many latex systems have particles with diameters of 300 to 1000 nm and a large ratio of particle to fluid refractive Index (1.2 for polystyrene latex). The most common wavelengths for lasers used In light scattering are on the order of 500 nm. 

The raw data trace for a mixture of 6 standard polystyrene latex microspheres is shown in Figure 2. This separation was done in 20 minutes at 10,450 rpm. While particle size data in the first few minutes is difficult to quantitate accurately with the DCP, this separation demonstrates the resolution capability of the instrument. Figures 3-7 show typical raw data, and number, surface and weight differential and cumulative distribution plots produced by the data system along with the corresponding report. 

As pointed out in Chapter 8, the forces of centrifugation are too weak to influence the distribution of small molecules. The molecular weight M of species must be 106 in order to generate the necessary force in SdFFF. However for M > 106, there are many important separation problems involving polymers, biological macromolecules (such as DNAs), subcellular particles, emulsions, and a great variety of natural and industrial colloids. SdFFF has been applied to many such systems [10-12,16]. An example of the separation of colloidal polystyrene latex microspheres is shown in Figure 9.9,. 

Comparison of calculated values (present work) of the limiting volume fraction of free polymer 0, at which instability sets in, with experimental values. System Polyethylene oxide-stabilized polystyrene latex with polyethylene oxide as the free polymer at 298 K, with a = 85 nm, Xi = 0.466, xs = 0.31 and fi = 5 nm... 

The ODN adsorption onto cationic polystyrene latexes as a function of NaCl concentration and at acidic pH was investigated [24] and found to be slightly influenced by the salinity as given in Fig. 5. At acidic pH, the adsorbed amount of ODN decreased markedly as the salinity increases, compared to the adsorption at basic pH. For such a highly charged colloidal system, the effect of salt was attributed to the reduction in the attractive electrostatic interaction. 

Lee and Lightfoot [229] developed the theoretical basis of Fl-FFF. This theory has been confirmed by numerous works on the fractionation of model systems, including monodisperse spherical polystyrene latexes and a number of proteins [41,228,229,240], some polydextrans [229], viruses [241], and other spherical particles and macromolecules [242,243]. 

Many papers report the fractionation of polystyrene latexes or mixtures thereof, as such commonly available spherical latex standards are an ideal system to test FFF setups or evaluations (for an example, see [362,401]). Recent coupling of Fl-FFF to MALLS enables a very high precision in particle size determinations. One example is shown in Fig. 31, where two Duke standard latex batches of a nominal size of 100 nm were investigated by Fl-FFF/M ALLS, underlining both separation power and resolution. Using traditional techniques such as photon correlation spectroscopy (PCS) and classic Fl-FFF detection, these samples seem to be identical. However, with Fl-FFF/MALLS, the batches could be separated as two discrete size distributions with a peak size that differed by 3 nm. However, it is not stated if a precise temperature control was maintained so that, critically considered, the observed differences could also have their origin in slight temperature... 

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