Comparison of original and cross-linked wormlike micelles of poly(ethylene oxide-b-butadiene) in water: Rheological properties and effects of poly(ethylene oxide) addition

Chemical fixation can convert self-assembled amphiphilic aggregates into covalently bonded giant macromolecules that can have properties that are fundamentally different from the unreacted precursors. Following up our previous report, we here extend the comparison between pristine and cross-linked wormlike micelles prepared from a cross-linkable poly(ethylene oxide-b-butadiene) diblock copolymer. Despite retention of the overall morphology, the cross-linked wormlike micelles exhibit unusual linear and nonlinear flow properties that presumably reflect the micelle stiffening upon cross-linking. Chemical fixation also influences their responses to changes in thermodynamic conditions. To explore this point, we added nonadsorbing homo-poly(ethylene oxide) (PEO) into the otherwise stable micelle solutions as a means for creating depletion effects (that is, attractive interactions between the micelles). Experimental studies of the phase behavior and attendant macroscopic properties of the mixtures of the wormlike micelles and PEO in water indicate that, when mixed with homo-PEO, the unreacted micelles phase separate to form hexagonal arrays, due to the depletion interactions. In contrast, the cross-linked analogues, under comparable conditions, remain homogeneously dispersed, and instead form physical gels. Such gels are characterized by time-dependent rheological responses upon large deformation and no flow under small deformations. These results suggest that the addition of PEO produces depletion-induced demixing of the cross-linked wormlike micelles leading to a nonequilibrium physical gel.