Novel Structure Change in Nonequimolar Complexes of Linear Poly(ethylenimine) and Octadecanoic Acid: Effects of Composition

Abstract

Polymer-amphiphile complexes based on specific noncovalent interactions have received considerable attention in recent years due to its simple preparation and interesting properties. However, little attention has been paid to the complex composition effect on the crystal and mesomorphous structure of amphiphile side chains in nonequimolar polymer-amphiphile complexes. The structure of a series of nonequimolar linear poly(ethylenimine)-octadecanoic acid complexes, lPEI-OA-x, with the molar ratio x of OA to the lPEI amino group ranging from 0.66 to 1.45, was investigated with WAXD, SAXS, FTIR, DSC, fluorescence spectrum, and polarized optical microscope. Two crystalline modifications ‚O (‚- orthorhombic) and ‚T (‚-triclinic) of OA side chains have been found to coexist in the complexes varying with x, and only 8-10 CH2 groups in an OA molecule participate in the crystallization. The crystalline OA tails with amorphous lPEI form the lamella stacking structure. The complexes of x > 1.0 are predominant with ‚O form and stacked into an end-to-end bilayer lamella with the long period of 5.6 nm. While for the complexes of x < 1.0, the ‚T form is dominantly stacked in the interdigitating monolayer structure with the long period of 2.8 nm. The fluorescence emission from pyrene-doped complexes indicates a decrease in the microenvironment polarity with increasing x. The thermotropic liquid crystal state has been observed from the complexes of x < 1.0 at temperatures above the melting point of the OA tail crystal. This study demonstrates that the crystalline and mesomorphous structures of polyelectrolyte- amphiphile complexes can be effectively tuned by changing the relative amount of bound amphiphiles. In other words, the stacking structure of amphiphile molecules depends on the amount of added binding polyelectrolytes.