Structure-directed self-assembly of alkyl-aryl-ethylene oxide amphiphiles

Polyethylene/poly(ethylene oxide) diblock oligomers strongly phase separate and crystallize in lamellar structures. We synthesized exact length analogues that have a 1,4-disubstituted benzene ring inserted at the junction between the alkyl and PEO chains, and determined their solid-state structures. The n-alkyl chains are 14, 16, 18 or 20 carbons in length and the poly(ethylene oxide) chains 0 to 7 repeat units and are capped with methyl groups. Powder X-ray diffraction data indicate phase separation of the alkyl and ethylene oxide chains into lamellar structures with the benzene rings aligned in a planar array at the interface between the two blocks. For some compounds, an initially formed bilayer motif is transformed to an interdigitated structure upon annealing at higher temperatures. Scanning calorimetry and vibrational spectroscopy support independent crystallization of the two blocks. On cooling from the melt state, the alkyl chains crystallize first, followed by the poly(ethylene oxide) segments. Alignment of the aromatic ring via crystallization of the two blocks suggests a general approach for aligning polar molecules in nanometre-scale materials.