Adsorption from block copolymer solutions

Balazs and Lewandowski (1990) have performed simulations of the adsorption of triblock copolymers onto a planar surface, and examined the conformations of the adsorbed chains. Monte Carlo simulations were performed of the motion of hydrophilic-hydrophobic chains on a cubic lattice. These simulations revealed a complex structure in the interfacial region due to the self-assembly of chains, driven by the solvent-incompatible block, reducing adsorption onto the surface. The influence on the surface coverage of length of the hydrophilic segement, polymer concentration, interaction energy between hydrophilic block and the 

However, the surface coverage is the same for both copolymers when weakly adsorbed to the surface. Surface density profiles were also compared. Finally, scaling relationships for triblock copolymer adsorption under weak adsorption conditions were derived (Haliloglu et al. 1997). In a related paper (Nguyen-Misra et al. 1996), adsorption and bridging of triblock copolymers in an athermal solvent and confined between two parallel flat surfaces were studied, and the dynamic response of the system to sinusoidal and step shear was examined. 

2 Surface force experiments on adsorbed block copolymers 

PS-PEO diblocks adsorbed on glass plates. Adsorption from toluene led to preferential adsorption of PEO however, due to the very high asymmetry of the copolymers studied, and the favourable adsorption energy, PS was also adsorbed. On the approach of a second coated plate, the PS volume fraction increased near the interface, indicative of strong interlayer repulsion (Cosgrove et al. 1994). 

3 Neutron and X-ray reflectivity studies of adsorbed block copolymers 

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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. 

This chapter is organized as follows. The thermodynamics of the critical micelle concentration are considered in Section 3.2. Section 3.3 is concerned with a summary of experiments characterizing micellization in block copolymers, and tables are used to provide a summary of some of the studies from the vast literature. Theories for dilute block copolymer solutions are described in Section 3.4, including both scaling models and mean field theories. Computer simulations of block copolymer micelles are discussed in Section 3.5. Micellization of ionic block copolymers is described in Section 3.6. Several methods for the study of dynamics in block copolymer solutions are sketched in Section 3.7. Finally, Section 3.8 is concerned with adsorption of block copolymers at the liquid interface. 

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. 

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. 

Marques et al. [116] have studied the adsorption of an A - B diblock copolymer from a dilute solution onto a solid surface that attracts the A block and repels the B block in a nonselective solvent, good for both blocks. 

Bijsterbosch, H.D. Stuart, M.A.C. Fleer, G.J. Adsorption kinetics of diblock copolymers from a micellar solution on silica and titania. Macromolecules 1998, 31, 9281-9294. lijima, M. Nagasaki, Y. Okada, T. Kato, M. Kataoka, K. Core-polymerized reactive micellesm from heterotelechelic amphiphilic block copolymers. Macromolecules 1999, 32, 1140-1146. 

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. 

Monograph from basic physical chemistry to applications theory, modelling and experiment dilute and concentrated solution neutral and polyelectrolyte block copolymers variety of phase transitions phase diagrams adsorption applications... 

The fraction of immobilised segments was further used to investigate the adsorption from a mixture of two immiscible polymers from solution [64]. A IH decoupled 13C spectrum was employed to identify the block of a block-copolymer, which was adsorbed in trains, by detection of the mobile segments [65]. 

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). 

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