Diblock copolymers micellar solutions

The PPO block is temperature-sensitive, with an LCST (cloud point) of around 15 °C, whereas the DEA block is pH-responsive. At pH 6.5 and 5°C, both blocks are solvated and the copolymer is molecularly dissolved in aqueous solution. On adjusting the solution pH to pH 8.5, the DEA block becomes deprotonated and hence hydrophobic, leading to the formation of DEA-core micelles at 5 °C. On the other hand, if the pH is held constant at pH 6.5 and the temperature is raised above the LCST of the PPO block, PPO-core micelles are obtained with the weakly cationic DEA chains forming the micelle coronas. Since these new diblock copolymers can exist in two micellar states we have christened them schizophrenic copolymers. ... 

Figure 4 Summary of aqueous solution conditions for the two micellar forms of the schizophrenic PPOj -DEA diblock copolymer...

For block copolymers with a polyacid or polybase block, the structure and properties of micellar solutions depend on the pH. For example, Morishima et al. (1982b) found that for a poly(9-vinylphenanthrene)-poly(methacrylic acid) (PVPT-PMA) diblock in water, the rate constant for the fluorescence quenching of phenanthrene groups by oxidative non-ionic quencher is pH dependent. These authors suggested that at low pH the polyacid units are not fully ionized and may participate in the formation of hydrophobic domains, cooperatively with PVPT. An alternative explanation is that the PM A chains are less solvated when... 

This chapter is concerned with experiments and theory for semidilute and concentrated block copolymer solutions.The focus is on the thermodynamics, i.e. the phase behaviour of both micellar solutions and non-micellar (e.g. swollen lamellar) phases. The chapter is organized very simply Section 4.2 contains a general account of gelation in block copolymer solutions. Section 4.3 is concerned with the solution phase behaviour of poly(oxyethylene)-containing diblocks and tri-blocks. The phase behaviour of styrenic block copolymers in selective solvents is discussed in Section 4.4. Section 4.5 is then concerned with theories for ordered block copolymer solutions, including both non-micellar phases in semidilute solutions and micellar gels. There has been little work on the dynamics of semidilute and concentrated block copolymer solutions, and this is reflected by the limited discussion of this subject in this chapter. 

So far, micelles and vesicles of amphiphilic block copolymers with two different blocks have been described. In this section the work on amphiphilic block copolymers and block copolyampholytes composed of three different blocks will be reviewed. Much less work has been carried out on these systems and there are less systematic studies available. Focus will be laid on block copolymers with at least one polyelectrolyte block. While in the case of amphiphilic diblock copolymers questions like the influence of block lengths on the size of micellar aggregates have been studied in great detail, in ternary block copolyampholytes other properties have attracted greater interest, such as the influence of the block sequence on the solution properties and aggregate formation. 

This chapter deals almost exclusively with neat, or pure, diblock copolymer melts. Polymer blends are discussed in Chapter 9, micellar solutions in Chapter 12, and stabilized suspensions in Chapter 6. In the following, Section 13.2 briefly reviews the thermodynamics of block copolymers, and Section 13.3 describes the rheological properties and flow alignment of lamellae, cylinders, and sphere-forming mesophases of block copolymers. More thorough reviews of the thermodynamics and dynamics of block copolymers in the liquid state have been written by Bates and Fredrickson (1990 Fredrickson and Bates 1996). The processing of block copolymers and mechanical properties of the solid-state structures formed by them are covered in Folkes (1985). Biological applications are discussed in Alexandridis (1996). 

Yu et al.180 have synthesized cyclic diblock copolymers of ethylene oxide and propylene oxide. They polymerized sequentially propylene oxide and ethylene oxide by using the difunctional initiator (I) shown in Scheme 89. The cyclization reaction of the a,a>-hydroxyl-ended diblock macromolecules was carried out under Williamson conditions. A solution of the triblock precursor in a mixture of dichloromethane and hexane 65 35 (v/v) was added to a stirred suspension of powdered KOH (85% w/v) in the same dichloromethane/hexane mixture (Scheme 89). After separation and evaporation of the organic phase, the cyclic diblocks were isolated by fractional precipitation. The dilute solution properties of the cyclics and the corresponding linear triblock and diblock copolymers with the same composition and total molecular weight were compared. By examining the micellar behavior in water they found that the aggregation numbers were on the order Nj < Nc < No, where N Nq, and No, are the aggregation numbers of the triblocks, cyclic, and diblocks, respectively. 

There continues to be extensive interest in latexes and micellar systems. The structure of acrylic latex particles has been investigated by non-radiative energy transfer by labelling the co-monomers with fluorescent acceptor-donor systems. Phase separations could also be measured in this way. Excimer fluorescence has been used to measure the critical micelle temperature in diblock copolymers of polystyrene with ethylene-propylene and the results agree well with dynamic light scattering measurements. Fluorescence anisotropy has been used to measure adsorption isotherms of labelled polymers to silica as well as segmental relaxation processes in solutions of acrylic polymers. In the latter case unusual interactions were indicated between the polymers and chlorinated hydrocarbon solvents. Fluorescence analysis of hydrophobically modifled cellulose have shown the operation of slow dynamic processes while fluorescence... 

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 pH-induced micellization of a DMAEMA-6-DEAEMA diblock copolymer has been studied in detail using dynamic light scattering, small-angle neutron scattering, and fluorescence spectroscopy [166], The DMAEMA constitutes the corona of the micelle, whereas the DEAEMA forms the core. Pyrene was used as a probe to determine the nature of the DEAEMA blocks. It was shown that the hydrophobicity of the micellar cores increased progressively as the solution pH was adjusted from pH 7 to 9. In the presence of an electrolyte, it was possible to observe both individual chains (unimers) and micelles under certain conditions. The critical micellization pH depended on both the copolymer concentration and also the background electrolyte concentration. 

KEL Kelarakis, A., Ming, X.-T., Yuan, X.-F., and Booth, C., Aqueous micellar solutions of mixed triblock and diblock copolymers studied using oscillatory shear, Langmuir, 20, 2036, 2004. 

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