The order-disorder transition and the disordered micelle regime for poly(ethylenepropylene-b-dimethylsiloxane) spheres

Two asymmetric poly(ethylenepropylene-b-dimethylsiloxane) diblock copolymer melts have been examined by rheology, small-angle X-ray scattering (SAXS), and small-angle neutron scattering (SANS). The two copolymers have total molecular weights of 24 400 and 23 900 and PDMS volume fractions of 0.12 and 0.10, respectively. The latter copolymer was obtained by extensive fractionation of the former, the difference in composition arising primarily from the removal of traces of PDMS homopolymer. The order-disorder transition (ODT) is unambiguously identified by rheology and SAXS. Above the ODT the SANS and rheological properties are consistent with the recently proposed disordered micelle regime, whereby the copolymers are associated in micellar aggregates that are not ordered on a lattice. The viscosity of the melt in this regime is greater than that of a poly(ethylenepropylene) homopolymer of the same total molecular weight. The SANS structure factors were analyzed by a generalized inverse Fourier transform and by direct fitting to a model based on the Percus-Yevick structure factor and either a spherical or an ellipsoidal form factor. Both approaches give comparable results, and the average micelle core radius, effective hard-sphere radius, and volume fraction were extracted as a function of temperature. The fitting suggests that as temperature increases the micelles remain relatively unchanged, but their number density and volume fraction decrease steadily. The fractionated copolymer has a significantly smaller micellar core than its precursor and therefore a significantly smaller aggregation number. These differences are attributable to the partitioning of PDMS homopolymer into the micellar cores. For these samples the critical micelle temperature (cmt), where the volume fraction of micelles should become negligible, was not accessed, even 100 deg above the ODT.