Structural Properties of Bulk and Aqueous Systems of PEO-PIB-PEO Triblock Copolymers as Studied by Small-Angle Neutron Scattering and Cryo-Transmission Electron Microscopy

The phase behavior of a low molecular weight (M-w = 6000) symmetric triblock copolymer of poly(ethylene oxide) and poly(isobutylene), PEO-PIB-PEO, in the bulk as well in aqueous, D2O, solutions has been studied using small-angle neutron scattering and cryo-transmission electron microscopy. In aqueous solutions PEO-PIB-PEO self-associates into micelles. At low polymer concentration, the micelles predominantly have threadlike form, with lengths of typically 1-2000 Angstrom. Those coexist, however, with spheroidal micelles of similar diameter. For a polymer concentration above roughly 20% the aggregates probably have a more disclike shape, as the micelles organize in lamellar structure. The 30% solution forms a bulk lamellar structure which, upon shear, organizes in a monodomain crystal. The bulk, PEO-PIB-PEO block copolymer forms at low temperatures a lamellar ordered phase induced by the PEO crystallization into lamellar sheets of PEO chains, presumably in helical form with a single fold. In a temperature regime near the transition temperature of T-c approximate to 45 degrees C, the PEO chains unfold, giving rise to significant swelling of the lamellar. Above T-c approximate to 45 degrees C, a strong correlation peak is observed corresponding to that observed in amorphous block copolymer systems, but it is still not clear whether this peak reflects strong concentration fluctuations of a disordered phase or the Bragg scattering of an ordered mesophase of amorphous blocks.