Adsorption, polymer critical

The data, when compared with a model, suggested that bare areas of polymer exist after adsorption (i.e., the protein film forms in "islands ") and that the degree of coverage of the polymer surface with adsorbed protein increases with increasing polymer critical surface tension. 

The surface properties of polymers are important in technology of plastics, coatings, textiles, films, and adhesives through their role in processes of wetting, adsorption, and adhesion. We will discuss only surface tensions of polymer melts that can be measured directly by reversible deformation or can be inferred from drop shapes. Those inferred from contact angles of liquids on solid polymers ( critical surface tension of wetting ) are excluded, as their relations to surface tensions are uncertain. 

Poly(methyl methacrylate) and polytetrahydrofuran polymers were studied at the critical point of adsorption. This critical point of adsorption occurs where the retention of a given polymer is governed strictly hy the number and types of functional groups on the polymer [858]. The authors show plots of log MW vs. retention time for various mobile phase compositions on a given column. The critical point is reached when the retention time becomes independent of the molecular weight of the polymer. For poly(methyl methacrylate) that point was reached on a silica column (RI detector) with a 73/27 methyl ethyl ketone/cyclohexane mobile phase. For polytetrahydrofuran, the silica column and a 95/5 acetone/hexane mobile phase created the critical conditions. This approach has enabled the individual blocks within the co-polymer to be studied (i.e., the portion of the polymer that can make contact with the support surface). 

By 1980, research and development shifted from relatively inexpensive surfactants such as petroleum sulfonates to more cosdy but more effective surfactants tailored to reservoir and cmde oil properties. Critical surfactant issues are performance in saline injection waters, adsorption on reservoir rock, partitioning into reservoir cmde oil, chemical stabiUty in the reservoir, interactions with the mobiUty control polymer, and production problems caused by resultant emulsions. Reservoir heterogeneity can also greatly reduce process effectiveness. The decline in oil prices in the early 1980s halted much of the work because of the relatively high cost of micellar processes. 

The elution behavior of various polymers near their critical adsorption point with silica gel packings and various eluents has been studied (12). It was of interest to apply hybrid column systems composed of active ( critical ) packings (silica gels) in combination with nonactive (nonadsorptive) PS/DVB and DVB-based gels. Some PS/DVB and DVB gels exhibited rather strong... 

It is evident from these results that the interactive properties of the investigated SEC PS/DVB or DVB gels are very different. Because polar electroneutral macromolecules of PMMA were more retained from a nonpolar solvent (toluene) than from polar ones (THF, chloroform), we conclude that the dipol-dipol interactions were operative. Columns No. 1 and No. 2 were very interactive and can be applied successfully to LC techniques that combine exclusion and interaction (adsorption) mechanisms. These emerging techniques are LC at the critical adsorption point (18), the already mentioned LC under limiting conditions of adsorption (15,18), and LC under limiting conditions of desorption (16). In these cases, the adsorptivity of the SEC columns may even be advantageous. In most conventional SEC applications, however, the interactive properties of columns may cause important problems. In any case, interactive properties of SEC columns should be considered when applying the universal calibration, especially for medium polar and polar polymers. It is therefore advisable to check the elution properties of SEC columns before use with the... 

The elution of such gels is an example not of size exclusion but rather of hydrodynamic fractionation (HDF). However, it must be remembered that merely being able to physically fit an insoluble material through the column interstices is not the only criterion for whether the GPC/HDF analysis of an insoluble material will be successful. A well-designed HDF packing and eluant combination will often elute up to the estimated radius in Eq. (5), but adsorption can drastically limit this upper analysis radius. For example, work in our laboratory using an 8-mm-bead-diameter Polymer Laboratories aqueous GPC column for HDF found that that column could not elute 204 nM pSty particles, even though Eq. (5) estimates a critical radius of —1.5 jam. 

Electrostatic and adsorption effects conspire to make aqueous GPC more likely to be nonideal than organic solvent GPC. Thus, universal calibration is often not obeyed in aqueous systems. Elence, it is much more critical that the standard chosen for calibration share with the polymer being analyzed chemical characteristics that affect these interactions. Because standards that meet this criterion are often not available, it is prudent to include in each analysis set a sample of a secondary standard of the same composition and molecular weight as the sample. Thus, changes in the chromatography of the analyte relative to the standards will be detected. 

As has been pointed out, both entropic and enthalpic interactions affect the chromatographic behavior of macromolecules. They are adjusted to the required type of separation by selecting appropriate stationary and mobile phases. In a third mode of liquid chromatography of polymers, liquid chromatography at the critical condition (LCCC) (Entelis etal., 1985,1986 Pasch, 1997), the adsorptive interactions are fully compensated by entropic interactions. This mode is also referred to as liquid chromatography at the critical point of adsorption. Hence, TAS is equal to AH and therefore, AG becomes zero. K is 1 irrespective of molar mass and, consequently, homopolymer molecules of different molar masses coelute in one chromatographic... 

In the case of heterogeneous polymers the experimental methods need to be refined. In order to analyze those polymers it is necessary to determine a set of functions / (M), which describe the distribution for each kind of heterogeneity i This could be the mass distributions of the blocks in a diblock copolymer. The standard SEC methods fail here and one needs to refine the method, e.g., by performing liquid chromatography at the critical point of adsorption [59] or combine SEC with methods, which are, for instance, sensitive to the chemical structure, e.g., high-pressure liquid chromatography (HPLC), infrared (IR), or nuclear magnetic resonance spectroscopy (NMR) [57],... 

As with experimental work on polymer adsorption, experiments in the area of dispersion stability in the presence of polymers require detailed characterisation of the systems under study and the various controlling parameters (discussed above) to be varied in a systematic way. One should seek the answer to several questions. Is the system (thermodynamically) stable If not, what is the nature of the equilibrium state and what are the kinetics of flocculation If it is stable, under what critical conditions ( s, T, x> p etc.) can flocculation be induced ... 

We investigate theoretically how the adsorption of the polymer varies with the displacer concentration. A simple analytical expression for the critical displacer concentration is derived, which is found to agree very well with numerical results from recent polymer adsorption theory. One of the applications of this expression is the determination of segmental adsorption energies from experimental desorption conditions and the adsorption energy of the displacer. Illustrative experiments and other applications are briefly discussed. 

An adsorption-desorption transition is illustrated schematically in Figure 1, where we plot a displacement isotherm, i.e. the adsorbed amount of a polymer as a function of the composition of a mixture of solvent and displacer. At the left in Figure 1, where the concentration of displacer is low, the polymer surface excess is positive. As we increase the proportion of displacer in the mixture, we observe a decrease in the adsorbed amount. At a certain composition the adsorbed amount of polymer becomes zero. The concentration at which the polymer surface excess just vanishes will be denoted as the critical displacer concentration cr. Beyond 4>cr, the surface excess of the polymer is negative (and very small if the polymer concentration is low). 

Recent polymer adsorption theories, such as those of Roe (3) and of Scheutjens and Fleer (h) allow the calculation of displacement isotherms, so that we could study the dependency of these isotherms on various parameters by numerical methods. However, all the essential features of displacement can also be demonstrated by means of a simple analytical expression for the critical point, which can be derived in a straightforward way. 

The theory of polymer adsorption is complicated for most situations, because in general the free energy of adsorption is determined by contributions from each layer i where the segment density is different from that in the bulk solution. However, at the critical point the situation is much simpler since the segment density profile is essentially flat. Only the layer immedia-... 

The critical adsorption energy. A critical adsorption energy is predicted by many theories (2-6). Its value is dependent on conformational properties of the polymer and usually estimated as a few tenths ofkT (7). Yet, a method to determine x c experimentally has never been suggested. Displacement studies provide such a method. Inspection of Equation 5 bears out that Xsc is obtained from a displacement isotherm, provided that Xgd and the solvency terms vanish. This condition is met by taking as the displacer a molecule which is (nearly) identical to the repeating unit of the polymer, i.e. the polymer is displaced by its own monomer. Such... 

Polymer Adsorption at the Lower Critical Solution Temperature and Its Effect on Colloid Stability... 

Adsorption behavior and the effect on colloid stability of water soluble polymers with a lower critical solution temperature(LCST) have been studied using polystyrene latices plus hydroxy propyl cellulose(HPC). Saturated adsorption(As) of HPC depended significantly on the adsorption temperature and the As obtained at the LCST was 1.5 times as large as the value at room temperature. The high As value obtained at the LCST remained for a long time at room temperature, and the dense adsorption layer formed on the latex particles showed strong protective action against salt and temperature. Furthermore, the dense adsorption layer of HPC on silica particles was very effective in the encapsulation process with polystyrene via emulsion polymerization in which the HPC-coated silica particles were used as seed. 

In this study, adsorption behavior of water soluble polymers and their effect on colloid stability have been studied using polystyrene latices plus cellulose derivatives. As the aqueous solution of hydroxy propyl cellulose(HPC) has a lower critical solution temperature(LCST), near 50 °C(6 ), an increased adsorption and strong protection can be expected by treating the latices with HPC at the LCST. 

As another criterion of stability, a critical flocculation temperature(OFT) was measured. The measurement of CFT was carried out as follows the bare latex suspension was mixed with the polymer solution of various concentrations at 1+8 °C by the same procedure as in the adsorption experiments. Then, the mixture in a Pyrex tube(8 ml, U.0 wt %) was warmed slowly in a water bath and the critical temperature at which the dispersion becomes suddenly cloudy was measured with the naked eye. 

In common with some other authors (18-20), Napper removed excess stabilizer from the dispersion medium so as to give the dispersed particles full surface coverage, leaving negligible amounts of free polymer in solution. As the solvency was worsened, no more polymer could be adsorbed, so that critical flocculation conditions do not necessarily correspond to surface saturation. In the present work, which may refer more closely with some practical applications, the stabilizer is kept at the plateau adsorption level but at the expense of complicating the system by the presence of free polymer. Clarke and Vincent (21) have reported on the effect of free polystyrene on the stability of silica with terminally-attached sytrene chains, but the very considerable differences to our studies make an assessment of the possible role played by unadsorbed polymer unproductive. 

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