Functionalized homopolymers, adsorption

As heterogeneous polymers are distributed in more than one molecular parameter, more than one chromatographic separation technique must be used. For functional homopolymers evidence is first obtained that the optimum separation protocol includes liquid chromatography at the critical point of adsorption as the first dimension of separation, yielding fractions which are homogeneous in functionality. When these fractions are subjected to any molar mass sensitive separation technique, MMD for each functionality fraction, and therefore the complete FTD-MMD relationship, is obtained. Two-dimensional separations of this type are very much susceptible to automation, as has been shown by Much et al. [88] and Kilz and coworkers [89-91]. 

An example of adsorption of this kind is the adsorption occurring at the oil-water interface. The driving force for adsorption in this case is the minimization of the interfacial tension between the two interfaces. Typically, random copolymers or block copolymers, in which the monomeric imits are preferentially solvated in either of the two phases, adsorb readily at the interface. In the case of homopolymers, adsorption occurs either if the polymer is soluble in both the phases or if the polymer has functional groups that can reduce the interfacial tension. Thus, both polyCethylene oxide) and poly(methyl methaciylate) readily adsorb at the toluene-water interface. The former is soluble in both the phases while the latter has polar side groups that effectively screen the interactions between toluene and water. However, because of its hydrophobic nature polyst3U ene does not adsorb at the same interface (50). 

Qualitative and quantitative elemental analysis of polymers can be carried out by the conventional methods used for low-molecular-weight compounds. So a detailed description is not needed here. Elemental analysis or determination of functional groups is especially valuable for copolymers or chemically modified polymers. For homopolymers where the elemental analysis should agree with that of the monomer, deviations from the theoretical values are an indication of side reactions during polymerization. However, they can also sometimes be caused by inclusion or adsorption of solvent or precipitant, or, in commercial polymers, to the presence of added stabilizers. The preparation of the sample for... 

The competitive adsorption of a short symmetric PS-PI diblocks or a long asymmetric PS-PI diblock to the surface of a PS homopolymer was examined by Budkowski etal. (1995).They used nuclear reaction analysis (Section 1.4.18) with labelled diblocks to determine the concentration of deuterium atoms as a function of depth, and hence the volume fraction of labelled chains. It was thus found that the shorter diblock tends to adsorb preferentially to the interface. The surface excess of PS and its interfacial density were compared to a theory for bidisperse brushes, a generalization of the model due to Leibler (1988). Excellent quantitative agreement was found, with no adjustable parameters. 

The simplest example of this kind is connected to the conformation of a homopolymer partly adsorbed onto a flat substrate (Fig. 9). Let us assume that the chain segments being in direct contact with the surface in some typical instant conformation (Fig. 9a) are chemically modified (Fig. 9b). This can take place when the surface catalyzes some chemical transformation of the adsorbed segments. One can expect that after desorption (Fig. 9c), such a copolymer will have special functional properties it will be tuned to adsorption . 

Note that x does not explicitly enter [3.4.56]. The effect of x indirectly accounted for through its influence on the profile 0 z). As a matter of fact, [3.4.56] is not an exclusive result of the SF-theoiy earlier polymer adsorption models such as those of Roe ) and Helfemd ) give exactly the same form for x as a functional of the profile, although these theories predict a different profile as a function of x cmd X (and, hence, lead to different numerical results for tc). Equation [3.4.56] may be considered as the lattice version of a density functioned. This equation does not only apply to adsorbed homopolymer layers, but it is also valid for brushes (where... 

Whereas SEC, well-known for the determination of molar mass distribution, has been used since 1959, LACCC was first mentioned by Belenkii et al. in 1976 and then by Entelis et al. in 1986. The theory and experimental details for the determination of critical conditions have been described by Pasch. " The separation of polymers under critical conditions of adsorption allows, for example, the elution of homopolymers with the same repeat unit irrespective of their molar mass, according to the number and nature of functional groups. 

FIG. 3 The different adsorption mechanisms discussed in this chapter, (a) Adsorption of a homopolymer, where each monomer has the same interaction with the substrate the tail, train, and loop sections of the adsorbing chain are shown, (b) Grafting of an end-functionalized polymer via a chemical or a physical bond, (c) Adsorption of a diblock copolymer where one of the two blocks is attached to the substrate surface, while the other is not. 

Figure 6.5 Schematic adsorption isotherm for a homopolymer on latex particles (a) adsorbed amount (P) versus the equilibrium polymer concentration (Cp ) (b) adsorbed layer thickness S) as a function of the initial polymer concentration (C ,").

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