Oxygen K Edge Scattering from Bulk Comb Diblock Copolymer Reveals Extended, Ordered Backbones above Lamellar...

The evolution of molecular morphology in bulk samples of comb diblock copolymer pNdc₁₂-b-pNte₂₁ across the lamellar order-disorder transition (ODT) is studied using resonant X-ray scattering at the oxygen K edge with the goal of determining whether the molecules remain extended or collapse above the ODT. The distinct spectral resonances of carbonyl oxygen on the backbone and ether oxygen in the pNte side chains combine with their different site symmetry within the molecule to yield strong differences in bulk structural sensitivity at all temperatures. Comparison with simple models for the disordered phase clearly reveals that disordering at the ODT corresponds to loss of positional order of molecules with extended backbones that retain orientational order, rather than backbone collapse into a locally isotropic disordered phase. This conclusion is facilitated directly by the distinct structural sensitivity at the two resonances. The roles of depolarized scattering in enhancing this sensitivity, and background fluorescence in limiting dynamic range in oxygen resonant scattering are discussed.