Dipole alignment via phase separation of amphiphilic side chains

Polymeric 2nd-order nonlinear optical (NLO) materials require a nonisotropic array of noncentrosymmetric molecules. This is achieved in practice by aligning molecules with strong dipoles in a matrix, usually by poling using an electric field. The high molecular dipole moment hinders macroscopic alignment of chromophores, because electrostatic intermolecular interactions can extend over considerable distances (>1nm) and favor the formation of antiparallel alignment of dipoles. Electrostatic interaction energies can exceed thermal energies to favor an overall centrosymmetric ordering of chromophores.; We investigated whether the tendency for phase separation seen in diblock oligomers can be used to align chromophores and produce thermodynamically stable 2nd-order NLO materials. We synthesized a series of exact length diblock oligomers with alkyl and ethylene oxide chains attached at the one and four positions of a phenyl ring. DSC, Raman and FT-IR experiments indicated a stepwise crystallization mechanism for these series of compounds. Powder X-ray diffraction data show that they adopt a lamellar structure with the benzene rings aligned in a planar array at the interface due to phase separation of the alkyl and ethylene oxide chains. The lamellar structure changes upon annealing at higher temperatures, often forming interdigitated structures.; NLO chromophores could be aligned by the same approach. The benzene ring of the oligomers was replaced by p-nitroaniline since its size is close to that of benzene. The molecule s crystallized in a head-to-tail array of chains due to strong H-bonding between amino hydrogens and the neighboring oxygens of the nitro group. For the case of short chains, the molecules pack in layers in an antiparallel fashion due to the strong dipoledipole interaction, effectively canceling the net dipole moment of the crystal. Electrostatic effects favor pairing of the dipole moments, while phase separation and crystallization of the ethylene oxide and alkyl blocks favors alignment of the dipoles. The competition of these two energy terms determines the final structures of the compounds. When the amphiphilic chains are sufficiently long, the effects of phase separation dominate and the dipole moments are aligned at the interface between the side chains. Thus, manipulating the phase behavior of the block copolymers opens a new route to materials with ordered dipoles.