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 of temperature. Wide-angle X-ray diffraction and differential scanning calorimetry were used to follow the isotropization of the crystalline PEIM-22. The crystals of PEIM-22 consist of biphasic layers up to the isotropization temperature. A series of SAXS peaks are observed for the crystals between θ = 0.3 and 3.5°. The width of these peaks indicates the formation 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 ≈2.6 nm, suggesting even at high temperature the existence of equilibrium, short-range, local order. The SAXS profiles were calculated based on a model of alternating layers of a linear, paracrystalline lattice. The results were discussed together with similar data on model compounds in the literature, and it is suggested that the short-range order in the isotropic phase is due to a nanometer-scale separation of the polar, aromatic phthaloimidobenzoyl from the flexible doeicosamethyleneoxycarbonyl.