Cubic gels and lamellar crystals in concentrated solutions of an amphiphilic diblock copolymer

The phase behaviour of the alcohol ethoxylate diblock copolymer C17E40 (C = methylene, E = oxyethylene and the subscripts denote the number of repeats) in aqueous solutions has been investigated using small-angle X-ray scattering (SAXS) and rheology. The sol-gel boundary was located from flow experiments. The structure of the optically isotropic hard gel was determined using SAXS to be body-centred cubic over a concentration range from similar to 15 to similar to 70 wt% copolymer. At high concentrations (greater than or similar to 60 wt% diblock), crystallization of poly(oxyethylene) was found to lead to phase separation into a biphasic structure in which semicrystalline lamellae coexist with a bcc phase. This phase separation is believed to result from the inability of the crystallized poly(oxyethylene) chains to pack in bcc structure, as evidenced by the much larger domain spacing of the lamellar structure. (C) 1998 Elsevier Science B.V. All rights reserved.