Polymer Bound Fraction

Adsorbed Layer Thickness S and Segment Density Distribution / (z) 

Three direct methods can be applied for determination of adsorbed layer thickness ellipsometry, attenuated total reflection (ATR) and neutron scattering. The first two [38] depend on the difference between refractive indices between the substrate, the adsorbed layer and bulk solution and require a flat reflecting surface. Ellipsometry [38] is based on the principle that light undergoes a change in polarizability when it is reflected at a flat surface (whether covered or uncovered with a polymer layer). 

The results show a monotonic decay of p(z) with distance z from the surface and several regions may be distinguished. Close to the surface (0 z 3 nm), the decay in p z) is rapid, and, assuming a thickness of 1.3 nm for the bound layer, p was 

Experimental Methods for Measurement of Adsorption Parameters for Polymeric Surfactants  

Hydrodynamic thickness versus fraction of anchor segment va pfO-PPO-PEO block copolymer. Insert theoretical predictions. 

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Pioneers in measuring polymer bound fractions by IR were Fontana and Thomas 1), who studied poly(alkyl methacrylates) in n-dodecane on silica. Upon adsorption, part of the carbonyl stretching bemd (originally at 1736 cm ) shifted to a lower frequency (1714 cm ) due to formation of bound carbonyls. Fontana and Thomas also observed shifts in the vibration of silanol groups from the substrate and used these to obtain the train density. The IR technique has been exploited by several authors using different poljrmer systems, mostly with non-aqueous media and silica or tltanla as the substrates ll. The ATR approach has been used in several more recent studies ). 

The polymer bound fraction, p, can be directly determined using spectroscopic methods such as NMR. The method depends on the reduction in the mobility of the segments that are in close contact with the surface. By using a pulsed NMR technique, one can estimatep. An indirect method for estimation of p is to use microcalorimetry. Basically one compares the enthalpy of adsorption per molecule with that per segment [9]. The latter may be obtained by using small molecules of similar structure to a polymer segment. 

An illustrative example is the work of Clark et al, on the conformation of poly(vinyl pyrrolidone) (PVP) adsorbed on silica 0). These authors determined bound fractions from magnetic resonance experiments. In one instance they added acetone to an aqueous solution of PVP in order to achieve theta conditions for this polymer. They expected to observe an increase in the bound fraction on the basis of solvency effects as predicted by all modern polymer adsorption theory (2-6), but found exactly the opposite effect. Their explanation was plausible, namely that acetone, with ability to adsorb strongly on silica due to its carbonyl group, would be able to partially displace the polymer by competing for the available surface sites. 

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... 

Figure 19-13 illustrates the principle of an enzyme-linked immunosorbent assay, abbreviated ELISA in biochemical literature. Antibody 1, which is specific for the analyte of interest (the antigen), is bound to a polymeric support. In steps 1 and 2, analyte is incubated with the polymer-bound antibody to form a complex. The fraction of antibody sites that bind analyte is proportional to the concentration of analyte in the unknown. The surface is then... 

The extracted samples (in mobile phase acetonitrile 0.001M phosphate buffer pH 3.4, 70 30) were injected (20 (A per sample, 1 ml/min flow rate) onto the clenbuterol-MIP and blank polymer packed HPLC columns. The bound fraction was eluted in the mobile phase and the elution profile monitored at 208-300 nm. 

Lattice models play a central role in the description of polymer solutions as well as adsorbed polymer layers. All of the adsorption models reviewed so far assume a one-to-one correspondence between lattice random-walks and polymer configurations. In particular, the general scheme was to postulate the train-loop or train-loop—tail architecture, formulate the partition function, and then calculate the equilibrium statistics, e.g., bound fraction, average loop... 

It is not always possible to measure c(z) in full detail so that introduction of some average properties remains useful. One such parameter is the train fraction or bound fraction p. In terms of fig. 5.6 it is the amount of polymer within the thickness f of a segment on the surface, divided by the total adsorbed amount. Spectroscopic and other techniques can often be successfully applied to determine p. The general trends found support the picture sketched above, i.e., p Is close to unity when the adsorbed amount is low, and it decreases as the pseudo-plateau of the adsorption Isotherm (see fig. 5.7) is approached. Reported bound fractions vary widely, depending on the system and method chosen. It should be said that bound fraction measurements leave much room for differences in Interpretation some results are still not well understood. We will briefly discuss the methods in sec. 5.6, and give some exjjerimental data in secs. 5.7 and 5.8. 

For the determination of the fraction of segments in trains or, equivalently, the bound fraction p, one relies on features which allow differentiation between free (non-adsorbed) segments and segments in contact with the surface. In some cases, it is also possible to discriminate between parts of the solid substrate covered with polymer segments, and bare surface parts. One can then also determine the train density. 

We divide this section into three main themes structure, adsorbed amount, and polydispersity. In sec. 5.7a we discuss the structure of the adsorbed layer, paying attention to the volume fraction profile, the composition in terms of loops and tails, the bound fraction, and the layer thickness. Section 5.7b deals with the adsorbed amount as a function of the polymer concentration, the chain length, the solvency and the surface affinity. These two sections are mainly concerned with monodisperse polymer. Polydispersity, where competition between different chain lengths shows up, is treated In sec. 5.7c. 

Territorially bound fraction of total amount of bound A ion Degree of polymerization of polyphosphate (number of monomer units per polymer molecule)... 

Separation of the signal corresponding to either the bound or free-labeled analyte from that of the total labeled analyte population can be accomplished in two ways. The first involves physical separation of the protein (antibody) bound fraction from the free fraction of labeled analyte. This can be accomplished by a salting-out procedure, using a salt such as ammonium sulfate or a polymer such as polyethylene... 

Analysis of the scattering data yielded information about the structure of the polymer layer at the interface and, in particular, allowed determination of the polymer volume fraction profile as a function of distance from the oil-water interface. The profiles obtained were consistent with the hydrophobic poly (propylene oxide) blocks bound to the surface of the droplet while the hydrophilic poly(ethylene oxide) blocks were largely present as tails. The authors also determined the effect of salt on the conformation of the adsorbed polymer layer. 

Distribution between trains and loops in molecules adsorbed at solid/Iiquid interfaces is also possible and has been shown to occur for flexible polymers. There are some indications that protein molecules at solid/liquid interfaces do not always undergo the drastic conformational changes that occur at fluid/fluid interfaces. At a solid/liquid interface, an adsorbing molecule cannot penetrate the solid phase. Furthermore, adsorption may be confined to sites and thus be localized. Using infrared difference spectroscopy, Morrissey and Stromberg (1974) found a bound fraction (number of carbonyl surface... 

Figure 1. Variation in configurational entropy, S, with fraction of dimerized reagent, Ft, for dissolved and polymer-bound cases.

Testing procedure. Small pieces of uncured rubber are immersed for several days at room temperature in a large excess of good solvent such as toluene. The sample in contact with solvent becomes divided into three parts polymer solution, mpi, solvent-dispersed filler particles with absorbed polymer chains, mpu, and solvent-swollen gel of filler particles connected through polymer chains, mpm. The fraction of polymer bound to filler is determined from the equation B = ( m 4- m m p where mp is total mass of polymer. The fraction of polymer not dispersed by solvent is given by the following equation G = m / m p. 

Polymer bound Ru(III)-DAP complexes were found to be stable upto 100°C. These catalysts were found to be effective for oxidation of cyclohexane under mild operating conditions. The rate of reaction was studied by varying different parameters and the order of reaction with respect to [catalyst] as well as [substrate] was found to be fractional for both the catalysts. This... 

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