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 tr iblock copolymer of poly(ethylene oxide) and poly(isobutylene), PEO-PI B-PEO, in the bulk as well in aqueous, D2O, solutions has been studied using small-angle neutron scattering and cryo-transmission electron m icroscopy. In aqueous solutions PEO-PIB-PEO self-associates into micel les. At low polymer concentration, the micelles predominantly have thr eadlike 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 d isclike 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 crysta llization into lamellar sheets of PEO chains, presumably in helical fo rm with a single fold. In a temperature regime near the transition tem perature of T-c approximate to 45 degrees C, the PEO chains unfold, gi ving rise to significant swelling of the lamellar. Above T-c approxima te to 45 degrees C, a strong correlation peak is observed correspondin g to that observed in amorphous block copolymer systems, but it is sti ll not clear whether this peak reflects strong concentration fluctuati ons of a disordered phase or the Bragg scattering of an ordered mesoph ase of amorphous blocks.