The behavior of poly(ethylene oxide)-block-poly(N,N-dimethylacrylamide) (PEO-b-PDMA) in aqueous solution at dilute concentrations was studied by laser light scattering. Even though water was a nonselective solvent for both PEO and PDMA blocks, PEO-b-PDMA formed a certain loose associate in aqueous solutions. Different from the micellization of block copolymers in selective solvents, the association of PEO-b-PDMA in aqueous solution showed weak concentration and temperature dependence as well as opposite salt effect. At 4.0 M NaCl, the association was completely suppressed and PEO-b-PDMA existed as individual polymer chains. This opposite “salting out” effect revealed the association mechanism of block copolymers in nonselective solvent: the solubility of PEO block in aqueous solution was unique, and it exhibited a better hydrophilicity and thick hydration shell than other water-soluble polymers. This inequality or weak incompatibility drove the block copolymer to associate into loose structures containing multidomains rich of either blocks. The dependence of the association on the block ratio also supported our conclusions.
Poor compatibility was the major drawback of polymer mixtures when used as DNA separation media. Using poly(ethylene oxide)-b-poly(N, N-dimethylacrylamide) (PEO(44)-b-PDMA(88)) and PEO (M(w): 1.3 MDa) as an example, we demonstrated the concept that the compatibility was significantly improved when mixing a homopolymer with its copolymer. Laser light scattering indicated that the major interaction between PEO and PEO-b-PDMA in dilute solution was the weak hydrodynamic interaction, which showed almost no effect on the viscosity and the static scattering pattern. In semidilute or concentrated solution, viscosity measurement also suggested good compatibility between the two components. When tested as DNA separation medium by CE, the viscosity of the mixture was extremely low, only 5 cP for 5.0 m/v% PEO-b-PDMA+0.1 m/v% PEO at 25 degrees C. The performance on DNA separation could be tuned by varying the concentration of each component as well as the component ratio. Good separation on both dsDNA and ssDNA was achieved.
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