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

Chemical fixation can convert self-assembled amphiphilic aggregates into co valently bonded giant macromolecules that can have properties that are fund amentally different from the unreacted precursors. Following up our previou s report, we here extend the comparison between pristine and cross-linked w ormlike micelles prepared from a cross-linkable poly(ethylene oxide-b-butad iene) diblock copolymer. Despite retention of the overall morphology, the c ross-linked wormlike micelles exhibit unusual linear and nonlinear flow pro perties that presumably reflect the micelle stiffening upon cross-linking. Chemical fixation also influences their responses to changes in thermodynam ic conditions, To explore this point, we added nonadsorbing homopoly(ethyle ne oxide) (PEO) into the otherwise stable micelle solutions as a means for creating depletion effects (that is, attractive interactions between the mi celles). Experimental studies of the phase behavior and attendant macroscop ic properties of the mixtures of the worn-dike micelles and PEO in water in dicate that, when mixed with homo-PEO, the unreacted micelles phase separat e to form hexagonal arrays, due to the depletion interactions. In contrast, the cross-linked analogues, under comparable conditions, remain homogeneou sly 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 pr oduces depletion-induced demixing of the cross-linked wormlike micelles lea ding to a nonequilibrium physical gel.