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Co-assembly of double hydrophilic copolymers with amphiphilic molecules.

Karel Procházka Charles University in Prague, Faculty of Science, Department of Physical and Macromolecular Chemistry

Complexes of polymers and polyelectrolytes with low-molar-mass compounds, e.g., with surfactants and other amphiphilic molecules have attracted a wide interest of scientists because of their rich and interesting behavior and a number of potential applications. Besides an important interplay of electrostatic and hydrophobic interactions, the co-assembly is driven by entropy increase due to liberation of small counter-ions in the bulk solution and changes of water structure. To design materials with required properties, a number of parameters, such as polyelectrolyte and/or surfactant chemical structure, architecture, molar mass, charge density, etc. have to be taken into account. A high number of parameters that can be varied allows one, on one hand, to tune the structure and stimuli-responsive properties of prepared nanoparticles with a high efficiency, but on the other hand, it requires complex and lengthy studies of potentially applicable systems.
We have recently studied complexes of poly(methacrylic acid)-b-poly(ethylene oxide), PMA-PEO, with (i) N-dodecylpyridinium chloride and (ii) [3-cobalt(III) bis(1,2-dicarbollide)]- anion "COSAN" by a combination of static and dynamic light scattering, small angle neutron and X-ray scattering, NMR, microscopy (AFM and cryo-TEM), ITC and other techniques.
The studies of the co-assembly of PMA-PEO with very high molar mass in a broad range of molar ratios of the surfactant - copolymer, Z, allowed us to indentify four different regimes of the self-assembling behavior : (i) coexistence of loose aggregates of electrostatically bound surfactants to PMA block with free and almost unperturbed copolymer coils for Z << 1 ; (ii) formation of aggregates with ill-defined cores formed by DPCl micelles attached to coiled PMA chains (beads-on-a-string nanoparticles) in the range 0.38 < Z < 1 (they coexist either with free copolymer chains and swollen nanoparticles decorated by surfactant micelles - for lower Z, or with core-shell nanoparticles - for higher Z) ; (iii) formation of compact core-shell particles with cores formed by ordered (crystalline) DPCl micelles for 1 < Z < 3.6 and (iv) the coexistence region of core-shell nanoparticles with free DPCl micelles.
The PMA-PEO co-assembly with COSAN surprisingly yields nanoparticles containing insoluble cores formed by PEO-COSAN complex (stabilized, e.g., by interesting dihydrogen bonds) and soluble PMA shells. Since boron-containing compounds find application in mediciene, e.g., as "boron neutron capture therapy" agents and COSAN-based compounds have been recently discovered as potent inhibitors of HIV protease, the nanoparticles have been studied as promissing drug carriers.