Kinetically frozen micelles

In our computer studies of the conformational behavior of the shell-forming chains, we used MC simulations [91, 95] on a simple cubic lattice and studied the shell behavior of a single micelle only. Because we modeled the behavior of shells of kinetically frozen micelles, we simulated a spherical polymer brush tethered to the surface of a hydrophobic spherical core. The association number was taken from the experiment. The size of the core, lattice constant (i.e., the size of the lattice Kuhn segment ) and the effective chain length were recalculated from experimental values on the basis of the coarse graining parameterization [95]. 

Lejeune et al. [153] employed a chemical approach to lowering of interfacial tension in poly( -butyl acrylate)-(polyacrylic acid) (PnBA-PAA). PnBA-PAA forms kinetically frozen micelles in water that are not able to reorganize over a month. By statistical incorporation of hydrophilic acrylic acid (AA) units into the hydrophobic PnBA block, P(nBA5o%-stat-AA5o%)-PAA, they could moderate the hydrophobicity of the core block such that unimer exchange was promoted and thermodynamic equilibrium was reached at shorter times. 

Linear amphiphilic block copolymers have been assembled in a selective solvent with a great diversity of kinetically frozen micelles. When a perfect control of nanostructure size and shape was required, brush copolymers were found to be the preferred templates to prepare cylindrical nanoparticles. Wooley et al. 

Block copolymers of polystyrene (PS) and poly(methacrylic acid) (PMA) form spherical micelles in water, with a glassy core of PS blocks and extended PMA block chains forming a corona [16-18]. However, the PS-PMA block copolymers cannot be dissolved directly into water. In a dioxane-water (80 20, v/v) mixture, the PS-PMA block copolymers undergo closed association to form equilibrium micelles. A stepwise dialysis of the dioxane-water solution of the PS-PMA block copolymers, with a gradual increase in water content, eventually allows one to obtain an optically clear aqueous solution of micelles, which are referred to as kinetically frozen micelles [18]. 

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... 

The resulting micellar aggregates resemble, in most of their aspects, those obtained with classical low molecular weight surfactants, but the nonergodicity of BCs allows the preparation of many different kinetically frozen morphologies. From the initial basic observations of micelle formation by Merret in 1954 [24] to the last structures of living micelles obtained by Winnik and co-workers in 2007... 

Recently, a versatile class of poly(ethylene propylene)/poly(ethylene oxide) block copolymer micelles were introduced they were stable due to a combination of high block incompatibility, kinetically frozen core, and high interfacial tension between core and solvent [53, 58]. Moreover, by using a co-solvent of varying composition, the aggregation number was controlled and soft spheres from star-like to micelle-like could be obtained. Another way is core stabilization via chemical crosslinking, say by UV radiation [59-64]. 

The salt-controlled behavior of PE coronae of kinetically frozen star-like micelles was examined experimentally [52, 58]. A good correspondence between the theoretical (—1/5) and the observed (—0.18 in [52], and —0.2 in [58]) values of the exponent was found. 

The studied triblock copolymer PS-PVP-PEO was purchased from Polymer Source (Dorval, Canada). The number-average molar masses of PS, PVP, and PEO blocks were 2.1 x 10 , 1.2 x 10 , and 3.5 x 10 g mol , respectively, and the poly-dispersity index of the sample was 1.10. The copolymer is insoluble in aqueous media, but the micelles can be prepared indirectly both in acidic and alkaline aqueous solutions by dialysis from 1,4-dioxane-methanol mixtures [88]. The micelles can be transferred from acidic to alkaline alkaline solutions and vice versa, but the addition of a base together with intense stirring promotes aggregation. Two factors contribute to the destabilization of micelles after the pH increase (a) In alkaline media, the PVP blocks become insoluble, collapse and form an upper layer of the core. Since the cores of micelles are kinetically frozen, the association number does not change. The mass of insoluble cores increases, while the length of soluble shellforming chains decreases, which results in a deteriorated thermodynamic stability of micellar solutions, (b) The PVP middle layer shrinks and PEO chains come close to each other, which worsens the solubility due to insufficient solvation of PEO blocks. 

The main part is then devoted to the equilibrium exchange kinetics of selected PEP-PEO micellar systems. We report on TR-SANS measurements in pure water that, independently of block copolymer molecular weight, composition, and temperature, revealed frozen micelles. This review further concerns the effect of tuning the kinetics by addition of co-solvents, i.e., reduction of y. The relaxation behavior of some selected systems revealing chain exchange dynamics that can be resolved by TR-SANS wfll be presented, followed by a discussion of the main observation, namely, the unexpected appearance of a pseudo-logarithmic time decay of the relaxation function. 

Equilibrium Kinetics in Pure Water Frozen Micelles... 

When applied to the precipitation of amphiphilic block copolymers, the nanoprecipitation is often described in terms of self-assembling the precipitated particle is seen as a micelle, which is built by diffusion-limited aggregation of single molecules of the polymer [48]. Different from the usual micellization processes, which take place near the critical micelle concentration and are reversibile, in this case the operation takes place at much larger concentration and the resulting micelles are kinetically frozen... 

If the solution is cooled slowly (8 C/mln to 1350 C/min), Sol 2 micelles appear. If, however, cooling is too rapid ( b2,000 C/min), a continuous lace-like, noncellular polymer network is apparent in photomicrographs. This lace-like network is the frozen Sol 1 structure which, for kinetic reasons, is unable to assume the Sol 2 configuration before it becomes immobilized. 

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