Buoy block

Thus, the spacing of the chains relative to the neutral, free, swollen size of the buoy blocks is, for a given chemical system and temperature, a unique function of the solvent-enhanced size asymmetry of the diblock polymer and a weak function of the effective Hamaker constant for adsorption. The degree of crowding of the nonadsorbing blocks, measured by a decrease in the left-hand side of Eq. 28, increases with increasing asymmetry of the block copolymer. 

Diblock copolymers A-N immersed in a homopolymer P matrix segregate to its interfaces. One of the copolymer blocks ( anchor moiety A) selectively attaches to the interface while the other ( buoy block N) dangles out to form a brush like layer, providing a simple means for the realization of polymer brushes (see Fig. 33). 

Figure 6.18 shows the hydrodynamic thickness versus fraction of anchor segment for an ABA block copolymer of PEO-PPO-PEO [37]. The theoretical (SF) predictions of adsorbed amount and layer thickness versus fraction of anchor segment are shown in the inserts of Figure 6.18. In the presence of two buoy blocks and a central anchor block (as in the above example), the A-B-A block showed a similar...

A peculiar example of a diblock copolymer is a high molecular weight surfactant whose head group, if it has an affinity for the surface, can be likened to the anchor block, whilst the tail corresponds to the buoy block. Thus, for aqueous media, non-ionic surfactants, such as the oxyethylated alkylphenols or fatty alcohols, are classic stabilisers with the general formula... 

Fig. 3.15. (a) A layer of polymer chains grafted at one end onto an interface. For high grafting densities, the chains are uncoiled, (b) Diblock copolymer adsorbed onto an interface. The black sequence (or anchor block) has a strong affinity for the surface, whereas the white sequence (or buoy block) is repelled by the surface and stretches out into the liquid... 

Chain architecture also plays a role in determining the adsorption characteristics of copolymers. For instance, if we consider triblock ABA-type copolymers the relative positions of the anchor and buoy blocks become important. When there are two buoy blocks and a central anchor block, the copolymers show diblock AB-type behavior (see Fig. 9 and 10). If, however, there are two anchor blocks and a central buoy block, surface precipitation of the polymer molecule at the particle surface is generally observed. This precipitation (or multilayer formation) process is due to strong interaction between the anchor blocks themselves and manifests itself in the form of an ever-increasing adsorption isotherm (i.e., there is no plateau) of the type shown in Figure 11. When compared with... 

ABA triblock copolymers that have two buoy blocks, these molecules have higher adsorbed amounts and layer thickness for a given molecular weight. 

Chain architecture also plays a role in determining the adsorption characteristics of block copolymers. For example, for an A-B-A triblock the relative position of the anchor and buoy blocks becomes important. This is illustrated below in the experimental section on polymer adsorption. 

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. 

Figure 7 shows the results of measurements of adsorption density by Parsonage, etal. [77] on a series of eighteen block copolymers, with poly(2-vinylpyridine) [PVP] anchors and polystyrene [PS] buoys, adsorbed from toluene (selective for PS) of variable molecular weight in each block. The results are presented as the reciprocal square of Eq. 28, that is, as a dimensionless number density of chains ct (d/Rg A)-2. For all but the copolymers of highest asymmetry, Eq. 28 is in good agreement with the data of Fig. 7. The high asymmetry copolymers are in the regime of the data of curves (a) and (c) of Fig. 3 where the large relative size...

The steric stabilization, which is imparted by polymer molecules grafted onto the colloidal particles, is extensively employed.3 Amphiphilic block copolymers are widely used as steric stabilizers. The solvent-incompatible moieties of the block copolymer provide anchors for the polymer molecules that are adsorbed onto the surface of the colloidal particles, and the solvent-compatible (buoy) moieties extend into the solvent phase. When two particles with block copolymers on their surface approach each other, a steric repulsion is generated bet ween the two particles as soon as the tips of the buoy moieties begin to contact, and this repulsion increases the stability of the colloidal system.4-6 Polymers can also induce aggregation due to either depletion 7-11 or bridging interactions.12 15... 

The most widely used applications reported in 1973 included rib structures in the fabrication of lightweight reinforced-plastic parts and as a core material in sandwich construction, bonded to metal, wood, or glass. Other applications are as reinforcements for aircraft-control surfaces, radome housings, fUler blocks under fuel cells, tank floats for indicating devices, and ribs, posts, and framing in houses and shelters. Due to its buoyancy characteristics cellulose acetate foam has been used in lifeboats, buoys, and other flotation devices (21). 

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). 

In many cases the anchoring point is provided not by a single chemical group (in the experiments described above this was a zwitterionic end-group) but by one half of a block copolymer, which may be much less soluble in the solvent than is the half that forms the brush. In this situation the less soluble part of the diblock that adsorbs on the wall is known as the anchor , while the soluble part that forms the brush is known as the buoy . The amount of adsorbed... 

A sophisticated discussion of the surface pressure isotherms for spread films of amphiphilic block copolymers on liquid surfaces has been developed for the situation in which the solvated block forms a brush layer at the air/liquid interface that is anchored to the interface by the insoluble block, analogous to the anchor-buoy systems discussed for solid/liquid interfaces in section 6.1. If the anchoring block is sufficiently small then it makes no contribution to the surface pressure. The surface pressure can be written in terms of the grafting density o at the surface as... 

Figures 7 and 8 show the theoretical adsorption and hydrodynamic layer thickness behavior of an AB block copolymer, but as can be seen from these figures, fhey depend on the anchor/buoy composition. In general, the adsorbed amounts are higher than is the case for homopolymers and the adsorbed layer more extended and more dense. Maximum levels of adsorption are achieved when around 10% of the segments are anchors. In principle, then, these materials are better steric stabilizers than homopolymers.

In block copolymers (qv), the block that is preferentially adsorbed is called the anchor while the nonadsorbing block is called the buoy. In this case the adsorbed structure is affected by the nature of the solvent and the relative sizes of the blocks. 

In a solvent that selectively solubilizes one of the blocks, when the chain adsorbs, the anchor collapses to form a dense layer, while the buoy stretches out to form a brush-like structure. Adsorption in this case is decided by the competition between van der Waals attraction between the anchor and the substrate and repulsive interactions between the buoys. This is referred to as the van der Waals brush regime (37). 

If the solvent is nonselective, the anchor has a swollen structure similar to an adsorbed homopolymer. In the case of adsorption of an AB block copol5mier from nonselective solvents there are two possible cases (38) The first is referred to as the buoy regime, in which the anchor are small and adsorption is dominated by repulsive interactions between the buoys. Increasing the length of the anchor will increase the amount adsorbed. The second case is called the anchor regime, in which the anchor layer is saturated and the buoys are small in size. Here, increasing the length of the anchor block will not increase the adsorbed amount. 

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