Homopolymers adsorption

Polymer adsorption from solution is a very large subject and it is difficult to provide an exhaustive treatment. We will try to describe the scaling and self- consistent field descriptions of homopolymer adsorption, together with experimental data selected to illustrate the important aspects. 

In this section we give a selection of theoretical and experimental results for homopolymer adsorption. For a meaningful comparison between theory and experiment it is mandatory that the experimental system Is well defined, with as many parameters known as possible (chain length and chain-length distribution, solvency, adsorbent properties, etc.). Wherever feasible, we shall discuss theoretical predictions In combination with experimental data. However, this correspondence cannot be malnteiined in all cases there are useful theoretical predictions that, as yet, cannot be checked experimentally (for example, the relative contributions of loops and tails), whereas for some measurable quantities no quantitative theory has yet been developed (for example, most kinetic data). 

For these copolymers, a distinction has to be made between random and block copolymers. These two classes may differ considerably in their Interfacial behaviour. Most of the trends discussed for homopolymer adsorption apply also to random copolymers, so that the insights presented in this chapter can still serve as a guide when dealing with practical systems in which, so far, mostly random copolymers are used. Block copolymers are potentially very interesting for many speciflc applications but are not yet applied on a large scale. We return to these more sophisticated systems in Volume V. 

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). 

The motivation to study such terminally attached polymers lies in their enhanced power to stabilize particles and surfaces against flocculation. Attaching a polymer by one end to the surface opens up a much more effective route to stable siufaces. Bridging and creation of polymer loops on the same surface, as encountered in the case of homopolymer adsorption, do not occur if the main-polymer section is chosen such that it does not adsorb to the surface. 

A. N. Semenov, J. F. Joanny. Kinetics of adsorption of linear homopolymers onto flat surfaces Rouse dynamics. J Physique II 5 859, 1995. 

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... 

The theoretical and (model) experimental work referred to above has largely been concerned with linear homopolymers adsorbed on regular surfaces. However, there is a vast literature of experimental studies on more complex systems. Unfortunately, in many cases the systems are either ill-defined and/or only adsorption isotherms have been established for drawing general conclusions or comparison with theory such studies are of little use. On the theoretical side, clearly the work needs to be extended towards these more complex systems. In particular, developments are required in the following areas (starts have already been made in some cases) ... 

For h < 26, the situation is much more complex. One not only needs to know 4>(z) for each layer, but how 4>(z) changes as the two particles approach, i.e. 4>(z,h) this may well depend on the time-scale of the approach, i.e. the equilibrium path may not be followed. Scheutjens and Fleer (25) in an extension of their model for polymer adsorption have analysed the situation for two interacting uncharged parallel, flat plates carrying adsorbed, neutral homopolymer, interacting under equilibrium conditions. Only a semi-quantitative picture will be presented here. 

So far, we dealt with the displacement of simple homopolymers. However, the phenomenon of displacement is by no means restricted to such simple macromolecules. Copolymers have been succesfully eluted by means of adsorption chromatography (13). 

PVA Particles. Dispersions were prepared in order to examine stabilization for a core polymer having a glass transition temperature below the dispersion polymerization temperature. PVA particles prepared with a block copolymer having M PS) x 10000 showed a tendency to flocculate at ambient temperature during redispersion cycles to remove excess block copolymer, particularly if the dispersion polymerization had not proceeded to 100 conversion of monomer. It is well documented that on mixing solutions of polystyrene and poly(vinyl acetate) homopolymers phase separation tends to occur (10,11), and solubility studies (12) of PS in n-heptane suggest that PS blocks with Mn(PS) 10000 will be close to dissolution when dispersion polymerizations are performed at 3 +3 K. Consequently, we may postulate that for soft polymer particles the block copolymer is rejected from the particle because of an incompatibility effect and is adsorbed at the particle surface. If the block copolymer desorbs from the particle surface, then particle agglomeration will occur unless rapid adsorption of other copolymer molecules occurs from a reservoir of excess block copolymer. 

The adsorption of block and random copolymers of styrene and methyl methacrylate on to silica from their solutions in carbon tetrachloride/n-heptane, and the resulting dispersion stability, has been investigated. Theta-conditions for the homopolymers and analogous critical non-solvent volume fractions for random copolymers were determined by cloud-point titration. The adsorption of block copolymers varied steadily with the non-solvent content, whilst that of the random copolymers became progressively more dependent on solvent quality only as theta-conditions and phase separation were approached. 

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... 

FIGURE 16.13 Schematic representation of separation of a block copolymer poly(A)-block-poly(B) from its parent homopolymers poly(A) and poly(B). The elnent promotes free SEC elntion of all distinct constitnents of mixtnre. The LC LCD procednre with two local barriers is applied. Poly(A) is not adsorptive and it is not retained within colnmn by any component of mobile phase and barrier(s). At least one component of barrier(s) promotes adsorption of both the homopolymer poly(B) and the block copolymer that contains poly(B) blocks, (a) Sitnation in the moment of sample introdnction Barrier 1 has been injected as first. It is more efficient and decelerates elntion of block copolymer. After certain time delay, barrier 2 has been introdnced. It exhibits decreased blocking (adsorption promoting) efficacy. Barrier 2 allows the breakthrongh and the SEC elution of block copolymer but it hinders fast elution of more adsorptive homopolymer poly(B). The time delay 1 between sample and barrier 1 determines retention volume of block copolymer while the time delay 2 between sample and barrier 2 controls retention volume of homopolymer poly(B). (b) Situation after about 20 percent of total elution time. The non retained polymer poly(X) elutes as first. It is followed with the block copolymer, later with the adsorptive homopolymer poly(B), and finally with the non retained low-molar-mass or oligomeric admixture. Notice that the peak position has an opposite sign compared to retention time or retention volume Tr. 

The following plain example might demonstrate the usefulness of the e° data. In benzene (e° = 0.32) polystyrene samples are eluted from a silica column, whereas polymethylmethacrylates and its copolymers are not. In THF (0.57) even PMMA homopolymers leave the column. Hence, THF is strong enough to prevent PMMA from adsorption. In chloroform (0.40) random copolymers with no more than 50%... 

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 two extremes on the styrene-butadiene block copolymer composition scale are homopolymers of butadiene or styrene, respectively. To test the usefulness of homopolymers as dispersants, polybutadiene (PB) was carboxylated by adding thioglycolic acid, and polystyrene (PS) having carboxylic groups was prepared by copolymerizing small amounts of acrylic acid (AA) into the styrene chain. Adsorption experiments with these carboxylated homopolymers are listed in Table V. In the first... 

It is well known (3,5,6) that sodium lauryl sulfate interacts with some polymers such as polyvinyl acetate causing solubilization of the insoluble polymer leading to an increase in viscosity. In Figure 3, viscosity of the homopolymer and 70/30 VA/BA at various NaLS/polymer ratio is shown. It is seen that the viscosity of the 2% latex dispersion increases with increase in NaLS/polymer ratio. Similar visoosity data for the 85/15 VA/BA was intermediate between the homopolymer and 70/30 VA/BA latexes. Surfactants that showed a normal saturation type adsorption behavior did not show any significant visoosity increase of the latex. 

Polarity of Vinyl Acrylic Latex and Surfactant Adsorption Contact angle measurements, dispersion and polar contribution to latex film surface tension and polarity of polymer calculated according to the method of Kaelble (10) of the three latex films are whown in Table V. It is seen that the polarity of the latex film decreases with increase in butyl acrylate content of the vinyl acrylic co-polymer. The polarity of the 70/30 (VA/BA) latex is very similar to that of the polybutyl acrylate homopolymer estimated to be about 0.21 (1). ... 

The interaction parameter, as expected, decreases with increase in polarity of the latex surface (12). It shows that at saturation adsorption, the extent of interaction of Igepal CO-630 with the PVAC homopolymer and the two VA/BA co-polymer latexes is 29%, 49%, and 57% respectively of the theoretical limit corresponding to a close packed monolayer adsorption. 

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 . 

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