Morphologies diblock copolymer micelles

The influence of micellar morphology on the exchange kinetics in diblock copolymer micelles has been investigated by Lund et al. [104]. The studied system was a short chain PEPl-PEOl copolymer with symmetric block composition in water/ DMF mixtures as selective solvents for PEO. The morphological behavior of this system has already been described. The main features are illustrated in Eig. 20d. 

Microphase separation with three phases can be obtained for an ABC triblock copolymer. The morphology of an ABC triblock copolymer has been suggested by Riess et al. [40]. An interesting morphology of microphase separation with three phases has been reported for ABC triblock copolymers [41-43]. On the other hand, we reported microphase separation with three phases in the blend system of AB and AC diblock copolymer micelles [44]. In that blend, two B and C spherical microdomains were dispersed at random in an A matrix. 

The vast majority of block copolymer micelles has been constructed from AB diblock copolymers. However, ABC triblock copolymers have attracted a great deal of interest due to the huge number of different morphologies that have been observed so far in bulk and because the introduction of a third block may introduce interesting new functionalities. Although many investigations have... 

Fig.23. Schematic illustration of wetting geometries expected for ultra-thin films of diblock copolymers a - parallel lamellae, b - surface (pinned) micelles, c - perpendicular lamellae. L corresponds to the equilibrium period of the lamellar morphology...

Since they act as surfactants, copolymers are added in only small amounts, typically from a thousandth parts to a few hundredth parts. Theoretically, Leibler [30] showed that only 2% of a diblock copolymer may thermodynamically stabilize an 80%/20% incompatible blend with an optimum morphology (submicronic droplets). However, in practice kinetic control and micelle formation interfere in this best-case scenario. To a some extent, compatibilization increases with copolymer concentration [8,31,32], Beyond a critical concentration (critical micellar concentration cmc) little or no improvement is observed (moreover, for high amounts, the copolymer can act as a plasticizer). Copolymer molecular weight influence is similar to that of the concentration effect. For example, in a PS/PDMS system [8,31,32], when the copolymer molecular weight increases, domain size decreases to a certain extent. Hu et al. [31] correlated their experimental results with theoretical prediction of the Leibler s brush theory [30]. Leibler distinguishes two regimes to characterize the behaviour of the copolymer at the interface... 

It is an important aspect that block-copolymer micelles are characterized by much longer relaxation times than compared to low molecular surfactants. Non-equilibrium morphologies can easily be obtained in a vitrified state due to the efficient suppression of structural reorganization, because of the corresponding very slow response of the micelles to changes of temperature, solvent and concentration. In the case of a block-ionomer, i.e. a diblock copolymer where one block consists of ionic units, it was observed that micelles which formed in non-polar solution needed weeks to re-equilibrate after dilution of the solvent [226-228]. 

Fig. 4 Schematic representation of an island (ribbon) morphology formed by diblock copolymers on the surface Z = 0. Adsorbed A chains form a 2D melt on the surface. The island (ribbon) composed of B blocks is described as part of a sphere (cylinder) of radius Rs, H and R are the height and the radius of the surface micelle, respectively yi, y2> Ks are the surface tension coefficients of the air/B, the air/A, and A/B interfaces, respectively (Reproduced with permission from [36]. Copyright 1999 American Chemical Society)...

Abstract We present an overview of statistical thermodynamic theories that describe the self-assembly of amphiphilic ionic/hydrophobic diblock copolymers in dilute solution. Block copolymers with both strongly and weakly dissociating (pH-sensitive) ionic blocks are considered. We focus mostly on structural and morphological transitions that occur in self-assembled aggregates as a response to varied environmental conditions (ionic strength and pH in the solution). Analytical theory is complemented by a numerical self-consistent field approach. Theoretical predictions are compared to selected experimental data on micellization of ionic/hydrophobic diblock copolymers in aqueous solutions. 

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