Anchoring of diblock copolymers

Anchoring of Diblock Copolymers of Polystyrene and Poly(dimethyl siloxane) on Polymer Particles... 

Much work on the preparation of nonaqueous polymer dispersions has involved the radical polymerization of acrylic monomers in the presence of copolymers having the A block the same as the acrylic polymer in the particle core 2). The preparation of polymer dispersions other than polystyrene in the presence of a PS-PDMS diblock copolymer is of interest because effective anchoring of the copolymer may be influenced by the degree of compatibility between the PS anchor block and the polymer molecules in the particle core. The present paper describes the interpretation of experimental studies performed with the aim of determining the mode of anchoring of PS blocks to polystyrene, poly(methyl methacrylate), and poly(vinyl acetate) (PVA) particles. 

Section 4 focuses on the segregation of diblock copolymers admixed to homopolymer matrices. The copolymers form brush-like layers composed of chains attached by their anchor blocks to the external interface. Similar dou-... 

The adsorption of block and graft copolymers is more complex, as the intimate structure of the chain will determine the extent of adsorption [37]. Random copolymers adsorb in an intermediate fashion compared to that of the corresponding homopolymers. Block copolymers retain the adsorption preference of the individual blocks. The hydrophilic block (e.g., PEO the buoy) extends away from the particle surface into the bulk solution, while the hydrophobic anchor block (e.g., PS or PPO) provides a firm attachment to the surface. Figure 6.14 shows the theoretical prediction of diblock copolymer adsorption according to SF theory. In this case, the surface density cr was plotted versus the fraction of anchor segments v, and adsorption was shown to depend on the anchor/buoy composition. 

The amount of adsorption was greater than for homopolymers, and the adsorbed layer thickness was more extended and dense. For a triblock copolymer A-B-A, with two buoy chains and one anchor chain, the behaviour was similar to that of diblock copolymers this is shown in Figure 6.15 for the PEO-PPO-PEO block (Pluronic). 

Figure 7 Predictions of Scheutjens and Fleer theory for the adsorption of diblock copolymers surface density o versus fraction of anchor segment v. (Reproduced with permission from Ref. 16.)... 

One may wonder whether adsorption from selective solvents leads to more dense brushes. As was pointed out above, this implies that the anchoring blocks attract each other so that they are prone to form self-assembled structures. One therefore has to bear in mind that not only does adsorption take place from a micellar solution, but also that there is a good chance that these micelles, once they are on the surface, remain micelles so that inhomogeneous adsorption layers are produced. Examples of adsorption studies of diblock copolymers from selective solvents are those by Marra and Hair [46] (polystyrene-poly(ethylene oxide) from heptane/toluene on mica). 

From the above it is clear that to get high grafting densities from the adsorption of diblock copolymers one needs to meet three requirements (1) a strong attraction between anchor block and surface, (2) avoidance of crowding of the anchor blocks at the surface as a limiting factor (keep out of the anchor regime), and (3) no high kinetic barrier built up for the polymers to reach the surface. 

FIG. 6 Prediction of Scheutjens and Fleer s theory for adsorption (surface density a) (a) and adsorbed layer thickness 6 (b) of diblock copolymers vs. fraction of anchor segment Ua-... 

A variant on the grafting method is to use a diblock copolymer made out of two distinct blocks, as shown in Fig. 3c. The first block is insoluble and is attracted to the substrate, and thus acts as an anchor fixing the chain to the surface it is drawn as a thick line in Fig. 3c. It should be long enough to cause irreversible fixation on the surface. The other block is a soluble one (the buoy ), forming the bmsh layer. For example, fixation on hydrophobic surfaces from a water solution can be made using a polystyrene-poly(ethylene oxide) (PS-PEO) diblock copolymer. The PS block is insoluble in water and is attracted towards the substrate, whereas the PEO forms the brush layer. The process of diblock copolymer fixation... 

Fig. 8. General structures of polymeric dispersants (a) liomopolymer, (b) random copolymer, (c) diblock copolymer, and (d) comb polymer, where A = anchor group, B = soluble repeat unit, and C = repeat unit with solubility different from B. The repeat units may occur in sequences hundreds of...

Diblock copolymers consist of one sequence of anchor segments and a second sequence of backbone segments. The relative lengths of the two sequences can be controlled to provide a wide variety of adsorption and barrier characteristics. Typical commercial dispersants may use alkane... 

AB diblock copolymers in the presence of a selective surface can form an adsorbed layer, which is a planar form of aggregation or self-assembly. This is very useful in the manipulation of the surface properties of solid surfaces, especially those that are employed in liquid media. Several situations have been studied both theoretically and experimentally, among them the case of a selective surface but a nonselective solvent [75] which results in swelling of both the anchor and the buoy layers. However, we concentrate on the situation most closely related to the micelle conditions just discussed, namely, adsorption from a selective solvent. Our theoretical discussion is adapted and abbreviated from that of Marques et al. [76], who considered many features not discussed here. They began their analysis from the grand canonical free energy of a block copolymer layer in equilibrium with a reservoir containing soluble block copolymer at chemical potential peK. They also considered the possible effects of micellization in solution on the adsorption process [61]. We assume in this presentation that the anchor layer is in a solvent-free, melt state above Tg. The anchor layer is assumed to be thin and smooth, with a sharp interface between it and the solvent swollen buoy layer. 

The interfacial properties of chain-like molecules in many polymeric and colloidal systems are dependent on the conformation of the chains adsorbed at the interface (.1). Chains adsorbed at the solid-liquid interface may be produced by anchoring diblock copolymers to particles in a polymer dispersion. Such dispersions are conveniently prepared by polymerizing in the presence of a preformed AB diblock copolymer a monomer dissolved in a diluent which is a precipitant for the polymer. The A block which is... 

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