Equilibrium short-range order in the isotropic phase of a poly(ester-imide) with a C22H44 alkane spacer

Small-angle X-ray scattering (SAXS) was performed on a sample of poly(4,4'- phthaloimidobenzoyldoeicosamethyleneoxycarbonyl) (PEIM-22) as a function o f temperature. Wide-angle X-ray diffraction and differential scanning calor imetry were used to follow the isotropization of the crystalline PEIM-22. T he crystals of PEIM-22 consist of biphasic layers up to the isotropization temperature. A series of SAXS peaks are observed for the crystals between t heta = 0.3 and 3.5 degrees. The width of these peaks indicates the formatio n of a smectic-like, crystalline layer structure of a coherently scattering domain size of only 3-4 repeating units. In the isotropic phase, a single, broader peak remained at a spacing of approximate to 2.6 nm, suggesting ev en at high temperature the existence of equilibrium, short-range, local ord er. The SAXS profiles were calculated based on a model of alternating layer s of a linear, paracrystalline lattice. The results were discussed together with similar data on model compounds in the literature, and it is suggeste d that the short-range order in the isotropic phase is due to a nanometer-s cale separation of the polar, aromatic phthaloimidobenzoyl from the flexibl e doeicosamethyleneoxycarbonyl. (C) 2000 John Wiley & Sons, Inc.