Solute diffusion in associative copolymer solutions

Fluorescence recovery after photobleaching was used to study the diffusion of water-soluble solutes in aqueous solutions of an associative polymer, po ly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide)-g-poly(ac rylic acid) (Pluronic-PAA). Above the critical micellization temperature (C MT), the Pluronic-PAA polymers form micellar aggregates that serve as cross -links for gelation of the polymer solution at these temperatures; the tran sition from no gel to complete gelation occurs over a 10 degreesC temperatu re:range beginning at the CMT. The apparent diffusion coefficients of small hydrophobic fluorescent dyes decreased over the gel transition region as s t portion of the dye present was immobilized within the hydrophobic cores o f the micellelike aggregates, whose concentration increased with increasing temperature. A small effect on hydrophilic probes was observed in the tran sition region, but the diffusion characteristics once the gel. was formed w ere those that would-have been anticipated had there been no gelling. Diffu sion of rigid spheres (proteins) depended,on the Pluronic-PAA solution stru cture and could be explained qualitatively by the obstruction theory relati ng motion of the proteins in the interstices between impenetrable micellar aggregates to the aggregate volume fraction Diffusion of flexible coils (de xtrans) in Pluronic-PAA undergoing sol-gel transition corresponded to the, simple scaling prediction D similar to N-0(-1/2) ( where No is the number o f monomer units) afforded by the Zimm model for solutes of radius of gyrati on much smaller than the effective mean pore size in the gels.