Adsorption of block copolymers

Adsorption of block copolymers onto a surface is another pathway for surface functionalization. Block copolymers in solution of selective solvent afford the possibility to both self-assemble and adsorb onto a surface. The adsorption behavior is governed mostly by the interaction between the polymers and the solvent, but also by the size and the conformation of the polymer chains and by the interfacial contact energy of the polymer chains with the substrate [115-119], Indeed, in a selective solvent, one of the blocks is in a good solvent it swells and does not adsorb to the surface while the other block, which is in a poor solvent, will adsorb strongly to the surface to minimize its contact with the solvent. There have been a considerable number of studies dedicated to the adsorption of block copolymers to flat or curved surfaces, including adsorption of poly(/cr/-butylstyrcnc)-ft/od -sodium poly(styrenesulfonate) onto silica surfaces [120], polystyrene-Woc -poly(acrylic acid) onto weak polyelectrolyte multilayer surfaces [121], polyethylene-Wocfc-poly(ethylene oxide) on alkanethiol-patterned gold surfaces [122], or poly(ethylene oxide)-Woc -poly(lactide) onto colloidal polystyrene particles [123], 

The adsorption of block copolymers can be controlled by different stimuli, in particular by the pH since most of the brushes formed by block copolymers adsorption are polyelectrolyte brushes [129, 130], The group of Armes, for instance, studied the pH-controlled adsorption of a series of block copolymers [131, 132], In the case of copolymers bearing hydrophobic 2-(diethylamino)ethyl methacrylate groups (DEA) and a water-soluble zwiterionic poly(2-methacryloyl phosphoryl-choline) (MPC) block, they showed that at low pH the cationic DEA flatted to the anionic silicon surface while the MPC was in contact with the solution [132], At around neutral pH, micelles were formed in solution and adsorbed onto the surface because the DEA core was still weakly cationic. The MPC block formed the micelle coronas. Nevertheless, at higher pH the micelles became less cationic and the adsorption rate decreased. 

In general, A and B subchains in an AB- or an ABA-type block copolymer have different solubilities or affinities for a solvent or other polymers. Therefore, it is expected that a block copolymer is surface-active when dissolved in a suitable solvent or mixed in polymer melts108. This property of block copolymers is now utilized to stabilize or flocculate colloidal dispersions. Blocks A, which are insoluble in a given solvent, are anchored in an insoluble polymer particle, and blocks B, which are soluble in the solvent, form a surface layer around the particle. 

Dawkins and Taylor109 dispersed poly(methyl methacrylate) (PMMA) or polystyrene (PS) particles in n-alkanes stabilized by AB block copolymers of styrene and dimethyl-siloxane. In these cases, styrene blocks act as anchors and dimethylsiloxane blocks give a surface layer. The thickness 6 of the dimethylsiloxane layer was determined by viscosity measurements as a function of the molecular weight of dimethylsiloxane blocks. 

In Fig. 23, d is plotted against MB, the molecular weight of dimethylsiloxane blocks, for various dispersions. As can be seen, above MB = 10 x 103 d falls between the two limiting lines corresponding to the fully stretched chain model and the random coil model, while below MB = 10 x 103, 6 is closer to the former model than to the latter. 

From the adsorbance of dimethylsiloxane blocks, the mean separation y between adjacent dimethylsiloxane chains was calculated and found to be almost equal to the radius of gyration (s2)1,2 of the dimethylsiloxane chain calculated from intrinsic viscosity. 

Since n-alkanes are good solvents for poly (dimethylsiloxane), the adjacent dimethylsiloxane chains may not interpenetrate, as a result of excluded volume effect. Thus, the shape of a dimethylsiloxane chain may be represented by a prolate ellipsoid with y as the minor axis. The major axis h may be calculated by equating the volume of ellipsoid to (y/2)2(h/2) to (s2)3/2. The h value so calculated agreed reasonably well with the value of 5. Therefore, the shape of the stabilizing dimethylsiloxane chain should be close to an ellipsoid. 

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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. 

There is a substantial body of theoretical work on micellization in block copolymers. The simplest approaches are the scaling theories, which account quite successfully for the scaling of block copolymer dimensions with length of the constituent blocks. Rather detailed mean field theories have also been developed, of which the most advanced at present is the self-consistent field theory, in its lattice and continuum guises. These theories are reviewed in depth in Chapter 3. A limited amount of work has been performed on the kinetics of micellization, although this is largely an unexplored field. Micelle formation at the liquid-air interface has been investigated experimentally, and a number of types of surface micelles have been identified. In addition, adsorption of block copolymers at liquid interfaces has attracted considerable attention. This work is also summarized in Chapter 3. 

In this chapter, the focus is largely on experimental and theoretical studies of micellization in a range of solutions of model block copolymers prepared by anionic polymerization. A discussion of both neutral and ionic block copolymers is included, and features specific to the latter type are detailed. The adsorption of block copolymers at the liquid interface is also considered in this chapter. Recent experiments on copolymer monolayers absorbed at liquid-air and liquid-liquid interfaces are summarized, and recent observations of surface micelles outlined. Thus this chapter is concerned both with bulk micellization and the surface properties of dilute copolymer solutions. 

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. 

The adsorption of block copolymers from a selective solvent was considered by Ligoure (1991). He predicted the existence of surface micelles (see Fig. 3.22) in the case when the block interacting unfavourably with the solvent only partially wets the surface. The model predicts a critical surface micellar concentration (csmc) that differs from the bulk cmc. When the contact angle, which characterizes the interfacial interactions between the copolymer, adsorbing surface, and solvent is lower than some universal value, surface micelles were predicted to appear at a lower copolymer concentration than bulk ones. Experimental results on surfaces are discussed in Section 3.8.4. 

Fig. 3.27 Schematic of the Monle Carlo moves in the lattice simulations of micellization and adsorption of block copolymers by Mattice and co-workers, (a) Brownian moton of a chain (b) end flip of an end bead (c) two types of kink jump (d) reptalion of a chain (Zhan et at. 1993d).

Marques, C. M., J. F. Joanny, and L. Leibler. 1988. Adsorption of block copolymers in selective solvents. Macromoleculefil 1051-1059. 

Fig. 14. Schematic representation of different chromatographic situations in liquid chromatography at the critical point of adsorption of block copolymers. For explanation of 1-4 see text...

In a similar manner, several nanoparticles have been produced in the presence of block copolymers in selective solvents so as to form micelles that encapsulate particles such as metal salts. Consequently, these micelles are chemically converted to finely disperse colloidal hybrid polymer/metal particles with interesting catalytic, non-linear optic, semiconductor and magnetic properties [1, 20]. Finally, another area of potential application of amphiphilic block copolymers is that involving surface modification through the adsorption of block copolymer micelles or film formation. The use of a suitable micellar system allows for the alteration of specific surface characteristics, such as wetting and biocompatibility, or even enables the dispersion and stabilisation of solid pigment particles in a liquid or solid phase [1, 178]. 

Bijsterbosch, H. D., Cohen Stuart, M. A., and Rleer, G. J. 1998. Nonselective adsorption of block copolymers and the effect of block incompatibility. Macromolecules 31 7436-7444. 

Two methods for surface modification could be applied (i) Adsorption of block copolymers of PEO-PPO-PEO, namely Poloxamers, or Poloxamines that are made of polyethylene diamine with four branches of PEG chains. The molecular weight of the... 

Nativ-Roth, E., et al. Physical adsorption of block copolymers to SWNT and MWNT a nonwrapping mechmismMacmmolecules.200lJ, 40(10), 3676 3685. 

The statistical thermodynamics of block copolymer adsorption was considered elsewhere.Many theories attempt to characterize adsorption by smface density, block segment distribution profile, and the thickness of adsorbed layer. As a rule, an adsorbed diblock copolymer has one block adsorbed on the surface in a rather flat conformation, whereas the other block, having a lower surface activity, forms dangling tails. Because of their freely dangling blocks, adsorbed diblock copolymers are often interpenetrated. The adsorption of block copolymers leads to the segregation of blocks in the adsorption layer. It was found that both kinetic and equilibrium features of the block copolymer adsorption are intimately related to the phase behavior of the block copolymer solution. In particular, a very strong increase in the adsorbed amount is observed when the system approaches the phase boundary. As a consequence, a partial phase separation phenomenon may proceed in the surface zone. 

A very interesting problem is the adsorption of block copolymers at the interface between incompatible homopolymers,which lower the interfacial tension and therefore act as compatibilizing agents in such blends. This phenomenon has been studied theoretically (e.g.. Refs 200-203, 207), experimentally (e.g.. Ref. 206), and by Monte Carlo simulation. In the last work the A, B homopolymers are not included explicitly in the simulation, however, and their existence shows up only indirectly via suitable energy parameters which differ in the A-phase (for z > T/2) from those in the B-phase (for z < Ljl). The A-B interface hence is sharp on the scale of the lattice spacing and treated as strictly localized. Wang et al treat L lattices with lattice sizes up to = 50 and up to 400 chains of composition Na= Nb = ox variable / with N = 10 up to /= 3/4, and discuss the description of the block copolymer adsorption at the A-B interface in terms of Langmuir-type isotherms. 

Block copolymers containing both hydrophilic and hydrophobic components exhibit typical surfactant behaviour in aqueous media. Adsorption of block copolymers at organic/air interfaces can also give measurable reductions in surface tension. 

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