Significant structure change in nonequimolar complexes of poly(ethylenimine) and octadecanoic acid induced by polymer backbone branching

To illustrate the effect of polymer backbone branching on the crystal and mesomorphous structures of polyelectrolyte- amphiphile complexes, the complex of branched poly( ethylenimine) ( bPEI) and octadecanoic acid ( OA), bPEI- OA- x, where x is the mole ratio of OA to amino group, was investigated with WAXD, SAXS, FTIR, DSC, and polarized optical microscope. The corresponding results from the complexes of linear poly( ethylenimine) ( lPEI) and OA, lPEI- OA- x with x < 1 were taken as the reference. The bound OA alkyl tails form the hexagonal crystal phase (RH) in the bPEI- OA complex as opposed to the less symmetric triclinic phase (, T) in the lPEI- OA complex. The alkyl chains of bound OA molecules in both crystal phases were interdigitated and the long period of the mesomorphous layer was 3.1 nm for the bPEI- OA, longer than 2.8 nm for the lPEIOA owing to less dense stacking in the former. Thermal stability of all the complexes was enhanced compared with the pure OA due to the strong electrostatic attraction between - COO- from OA and - NH2+ or - NH3+ from PEI. An ionic thermotropic liquid crystal phase of smectic B was found at temperatures above the melting point of the OA tail crystal, but the isotropic transition temperature T-i of the bPEI- OA complexes was higher by ca. 20 degrees C than that of the lPEI- OA complexes due to higher symmetry of the RH phase in the bPEI-OA tail chain crystal. The present results have demonstrated for the first time that the branching of polyelectrolyte backbone can tune the crystal and mesomorphous structures of polyelectrolyte- amphiphile complexes in addition to the complex composition.