Copolymer unimers

Copolymer unimers assembled in a bilayer around an aqueous reservoir... 

Fig. 3.23 Copolymer unimers assembled in a bilayer around an aqueous reservoir. (From ref. [112])...

Size 100-300 nm Copolymer unimers surrounding a drug reservoir or oily core... 

Octadecyidimethylsilane SIO6619.0 32426-11-2 Quaternium-24 32440-50-9 Antaron V-216 Ganex V-216 PVP/hexadecene copolymer Unimer U-151 32503-27-8... 

The critical concentration at which the first micelle forms is called the critical micelle concentration, or CMC. As the concentration of block copolymer chains increases in the solution, more micelles are formed while the concentration of nonassociated chains, called unimers, remains constant and is equal to the value of the CMC. This ideal situation corresponds to a system at thermodynamic equilibrium. However, experimental investigations on the CMC have revealed that its value depends on the method used for its determination. Therefore, it seems more reasonable to define phenomenologically the CMC as the concentration at which a sufficient number of micelles is formed to be detected by a given method [16]. In practical terms, the CMC is often determined from plots of the surface tension as a function of the logarithm of the concentration. The CMC is then defined as the concentration at which the surface tension stops decreasing and reaches a plateau value. 

Size-exclusion chromatography (SEC) has been used to characterize the unimer-micelle distribution. However, SEC is not an absolute method and thus requires calibration. Since it is practically impossible to calibrate a SEC apparatus for the unimers and micelles formed by a block copolymer, only indicative MW values can be obtained. Moreover, several authors have noted a strong perturbation of the unimer-micelle equilibrium during SEC experiments even when interaction of the material with the SEC column was minimized [4,61,62],... 

Stop-flow experiments have been performed in order to study the kinetics of micellization, as illustrated by the work of Tuzar and coworkers on PS-PB diblocks and the parent PS-PB-PS triblocks [63]. In these experiments, the block copolymers are initially dissolved as unimers in a nonselective mixed solvent. The composition of the mixed solvent is then changed in order to trigger micellization, and the scattered light intensity is recorded as a function of time. The experiment is repeated in the reverse order, i.e., starting from the block copolymer micelles that are then disassembled by a change in the mixed solvent composition. The analysis of the experimental results revealed two distinct processes assigned as unimer-micelle equilibration at constant micelle concentration (fast process) and association-dissociation equilibration, accompanied by changes in micellar concentration (slow process). 

Exchange of unimers between two different types of block copolymer micelles has often been referred to as hybridization. This situation is more complex than for the case described above because thermodynamic parameters now come into play in addition to the kinetic ones. A typical example of such hybridization is related to the mixing of micelles formed by two different copolymers of the same chemical nature but with different composition and/or length for the constituent blocks. Tuzar et al. [41] studied the mixing of PS-PMAA micelles with different sizes in water-dioxane mixtures by sedimentation velocity measurements. These authors concluded that the different chains were mixing with time, the driving force being to reach the maximum entropy. 

It has been shown that block copolymer micelles are dynamic structures, although they can be kinetically frozen. Unimers can thus escape from micelles and be exchanged with other micelles or be adsorbed on another interface... 

An early illustration of pH-responsive micellization can be found in P2VP-PEO copolymers that exist as unimers at pH < 5 and form micelles at higher pH values, as demonstrated by Webber et al. [165]. Recently, we investigated a P2VP-PEO copolymer with a larger P2VP block [ 166]. Although this copoly-... 

Stimulus-responsive ABC triblock micelles were also investigated by Patrickios et al. for copolymers containing insoluble PEVE blocks, thermore-sponsive PMVE midblocks, and water-soluble PMTEGVE outer blocks with varying block sequences [277]. While in aqueous solutions only unimers were found, and the addition of salt led to aggregates. 

In a solution containing both unimer and micelles, Ma, which is by definition more sensitive to low molar mass particles, is always less than the weight-average molecular weight, Mv. Further details on osmometry can be found in the review by Adams (1989), whilst examples of its application to micellar block copolymer solutions are given in Chapter 3. 

Sedimentation in a centrifugal field has been demonstrated to be a useful tool in the investigation of micellization in block copolymers, as first demonstrated by Tuzar etal. (1974). In this method, the velocity at which a solute species (unimer or micelle) is displaced under the influence of a strong centrifugal force is measured (Brown et al. 1995). For a given centrifugal force, the sedimentation velocity of a solute depends on its molecular weight, its buoyancy and friction... 

Micelles are formed by association of molecules in a selective solvent above a critical micelle concentration (one). Since micelles are a thermodynamically stable system at equilibrium, it has been suggested (Chu and Zhou 1996) that association is a more appropriate term than aggregation, which usually refers to the non-equilibrium growth of colloidal particles into clusters. There are two possible models for the association of molecules into micelles (Elias 1972,1973 Tuzar and Kratochvil 1976). In the first, termed open association, there is a continuous distribution of micelles containing 1,2,3,..., n molecules, with an associated continuous series of equilibrium constants. However, the model of open association does not lead to a cmc. Since a cmc is observed for block copolymer micelles, the model of closed association is applicable. However, as pointed out by Elias (1973), the cmc does not correspond to a thermodynamic property of the system, it can simply be defined phenomenologically as the concentration at which a sufficient number of micelles is formed to be detected by a given method. Thermodynamically, closed association corresponds to an equilibrium between molecules (unimers), A, and micelles, Ap, containingp molecules ... 

Fig. 3.35 Time dependence of the intensity of scattered light, / after mixing 1% micellar solutions of a PS-PB-PS triblock (A/ = 105 kg mol1, 40% PS) in 1,4-dioxane with an equal amount of heptane ( ) 1 % unimer solution of the same copolymer in 1,4-dioxane/40 vol% heptane with an equal amount of 1.4-dioxane (O) (Bednar et til. 1988).

The observation by Price and co-workers (Booth et al. 1978 Price et al. 1982 Teo et al. 1986) of a loss of copolymer during elution through a GPC column was ascribed to adsorption of copolymer by the gel in the column. Spacek (1986) proposed the alternative explanation that some of the micelles, formed inside the gel particles, cannot leave narrow pores permeable only for the unimer. A simplified diffusion model was used to estimate the time needed for the formation of micelles under the conditions of the GPC experiment. The value obtained was in reasonable agreement with results from a stopped-flow experiment (Bednar et al. 1988). On the other hand, the experimental fact that the major part of the... 

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